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Advances in stable and flexible perovskite solar cells
Qamar Wali, Faiza Jan Iftikhar, Naveen Kumar Elumalai, Yaseen Iqbal, Sidra Yousaf,
Shahid Iqbal, Rajan Jose
PII:
S1567-1739(20)30061-4
DOI:
https://doi.org/10.1016/j.cap.2020.03.007
Reference:
CAP 5173
To appear in:
Current Applied Physics
Received Date: 7 August 2019
Revised Date:
3 February 2020
Accepted Date: 9 March 2020
Please cite this article as: Q. Wali, F.J. Iftikhar, N.K. Elumalai, Y. Iqbal, S. Yousaf, S. Iqbal, R. Jose,
Advances in stable and flexible perovskite solar cells, Current Applied Physics, https://doi.org/10.1016/
j.cap.2020.03.007.
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Advances in stable and flexible perovskite solar cells
Qamar Walia*, Faiza Jan Iftikhara, Naveen Kumar Elumalaib, Yaseen Iqbalc Sidra Yousafc, Shahid Iqbala, and Rajan
Josed
a
School of Applied Sciences & Humanities, National University of Technology, Islamabad, 42000, Pakistan,
Faculty of Engineering and Science, Curtin University, Sarawak Malaysia, 98009 Miri, Sarawak, Malaysia,
c
Materials Research Laboratory, Department of Physics, University of Peshawar, 25120, Pakistan, dNanostructured
Renewable Energy Materials Laboratory, Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang
26300, Malaysia
b
*
Correspondence: qamar@nutech.edu.pk
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Abstract
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Roll-to-roll (R2R) production is an innovative approach and is fast becoming a very popular
14
industrial method for high throughput and mass production of solar cells. Replacement of costly
15
indium tin oxide (ITO), which conventionally has served as the transparent electrode would be a
16
great approach for roll to roll production of flexible cost effective solar cells. Indium tin oxide
17
(ITO) and fluorine-doped tin oxide (FTO) are brittle and ultimately limit the device flexibility.
18
Perovskite solar cells (PSCs) have been the centre of photovoltaic research community during
19
the recent years owing to its exceptional performance and economical prices. The best reported
20
PSCs fabricated by employing mesoporous TiO2 layers require elevated temperatures in the
21
range of 400-500°C which limits its applications to solely glass substrates. In such a scenario
22
developing flexible PSCs technology can be considered a suitable and exciting arena from the
23
application point of view, them being flexible, lightweight, portable, and easy to integrate over
24
both small, large and curved surfaces.
25
Keywords: Renewable energy; lightweight device; portable device; flexible perovskite solar
26
cell; fibre shaped solar cells;
1
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1.
Introduction
28
Solar-to-electrical energy conversion lies at the forefront of human advancements for an
29
energy-secured world with a clean environment. Consequently, improving the performance and
30
lowering the cost of the current solar cell technologies as well as searching for new solar energy
31
conversion protocols have become a dynamic area for clean and sustainable energy research.
32
Achievement of these objectives needs optimization of solar cell materials and configuration so
33
that, (i) the photoactive materials have a high photon absorption cross-section, (ii) photo-
34
generated electrons are long-lived so that a high-volume fraction of these electrons can be
35
transported within the device over an appreciable length scale (of the order of few hundred
36
micrometres), and (iii) high hole mobility so that the electron loss in the photoactive material can
37
be quickly compensated. The presence of these characteristics in the first and second generation
38
solar cells enabled their commercial availability; yet the cost of electricity which has indeed,
39
significantly reduced over the past decade is still pricier than that from the conventional fossil
40
fuels and hence is an area where scope of ample research is evident.
41
It is imperative for adopting adaptable technological applications that solar cells are highly
42
efficient, stable and economical supported by transparent flexible electrodes and
43
favourable operating conditions. Moreover, the first three conditions were successfully
44
realized by the first two generation solar cells however the third generation solar cell
45
offered additional benefits such as being flexible, transparent and solution processable.
46
However, soon the interest waned due to failure of earlier third generation solar cells to
47
accomplish efficiency, stability and cost effectiveness. Nonetheless, the advent of hybrid
48
organic-inorganic PSCs has witnessed a revival of confidence on the third-generation solar
49
cells in terms of boosting the efficiency and stability with economical cost. The fascinating
2
50
features that have captured attention of scientists towards PSC are their extraordinary
51
charge mobilities due to their efficient light absorbing property that is responsible for
52
creation of charge carriers [1-5].
53
Many organometallic hybrid perovskite compounds have been tested in PSCs and show
54
dramatic performance at laboratory scale: from a photo-conversion efficiency (η) of merely
55
~3.8% (the first architecture of PSCs reported in 2009) to a record η = ~25.2[6]%, exceeding
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the η of the present photovoltaic (PV) cells based on polycrystalline silicon, consuming ~1000
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times less light harvesting material than silicon [7]. Additionally, perovskite devices are
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fabricated employing relatively simpler low-cost solution processing methods without the need
59
for vacuum steps or elevated temperatures. PSCs have remained at par with the efficient copper
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indium gallium di-selenide (CIGS) solar cells with an η =23.4%[6] since their inception [8].
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Substrates coated with metal oxides such as TiO2, ZnO, SnO2 and other similar materials play a
62
vital role in PSCs because they not only support the perovskite molecules, but also accept and
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transport photoinduced-electrons[9, 10]. The substrates must possess the following features; (i)
64
high specific surface area for anchoring large amount of perovskite molecule, (ii) a suitable
65
morphology to support electrons’ diffusion and additionally scatter light and (iii) a favourable
66
conduction band alignment (The conduction band of the metal oxide must be lower than the dye
67
lowest unoccupied molecular orbitals in order to favour easy electrons injections) with respect to
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the lowest unoccupied molecular orbitals of the perovskite molecule[9-13]. Various substrates
69
have been employed so far in PSCs owing to their unique characteristic. For instance, the
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commonly used flexible substrates such as poly(ethylene terephthalate) (PET) could not endure
71
high processing temperature significantly higher than >200 °C. Hence, most of the PSCs
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fabricated are being reported on either glass or PET substrates however a new avenue in this
3
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direction has been to use paper as a substrate because of it being easily available, flexible and
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disposable along with being cost effective. Hence the first ever paper based PSC device with
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architecture:
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reported with a power efficiency of 2.7% steering future research towards sustainable solutions
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of using such a substrates[14].
Paper/Au/SnO2/mesoTiO2/CH3NH3PbI3/Spiro-OMeTAD/MoOx/Au/MoOx
is
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For scalable roll to roll (R2R) production of truly flexible, cost effective solar cells,
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alternatives are needed to replace indium tin oxide (ITO), which conventionally serves as the
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transparent electrode. Flexible, thin film and lightweighted PSCs based on plastic substrates
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can be utilized in niche applications for instance, electronic textiles, portable electronic
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chargers, large scale industrial roofing and electronic textile industry[15]. Employing an
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extremely thin (~1 µm) light absorber of organometal halide layer in PSCs delineates itself as
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the most promising candidate for flexible thin film solar cells. The solution processable and
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low temperature fabrication of perovskite substrate layers in PSCs presents itself as a great
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prospect to realize its importance as flexible devices. The high performance of PSCs arises
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mostly from a photoactive perovskite comprised of organic-inorganic hybrid lead halide
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(ABX3) as shown in Figure 1. In this ABX3-type structure, A is an organic monovalent cation
89
(methyl ammonium (MA+, CH3NH3+)), formamidinium (FA+, CH(NH2)2+ [16-19], B is a metal
90
cation such as lead (Pb2+), germanium (Ge2+) or tin (Sn2+) [20-23] and X is a halogen anion such
91
as iodine (I−), chlorine (Cl−), bromine (Br−), hexafluoroborate (BF6-) or tetrafluoroborate (BF4-)
92
[24-28]. Similarly, modifying the chemistry of perovskites via suitable substitutions or exploring
93
different configurations as well as tailoring various interfacial layers and microstructures have
94
notable effects on the performance and stability of the PSCs devices [29-37].
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Figure 1: (a) Crystal structure of organic-inorganic perovskite, where central big atom is the methyl
ammonium while the Pb atom is surrounded by six halogen atoms and (b) A cation coordinated by X ions
in12-fold cuboctahedral symmetry.
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Perovskite acts as a light absorber as well as provides a transport medium for
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photoinduced charge species (holes and electrons). The concept of PSCs originated from the dye-
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sensitized solar cells (DSSCs) where a thick (10 − 20 µm) mesoporous-TiO2 (m-TiO2) scaffold
103
was required to anchor sufficient dye and enable complete light absorption [38]. During 2009,
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Miyasaka et al., [39] employed CH3NH3PbI3 and CH3NH3PbBr3 as sensitizers in a solar cell
105
device with architecture similar to DSSCs which resulted in η ≈3.81% and a significant high
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photovoltage (VOC = 0.96 V). In a similar device architecture, Park and co-workers [40]
107
optimized the m-TiO2 layer’s thickness and treated it with Pb(NO2)2 prior to the deposition
108
which yielded η ≈ 6.51%. However, these devices were unstable due to rapid dissolution of the
109
perovskite layer by the liquid electrolyte.
110
The issue of rapid and abrupt degradation of perovskite layer in PSCs by liquid
111
electrolyte was partially solved with the inclusion of a novel hole transport layer (HTL) named
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as 2,2',7,7'-tetrakis (N,N-p-dimethoxy-phenylamino)-9,9'-spirobifluorene (spiro-OMeTAD) into
5
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the device. The newly developed PSC employed a HTL Spiro-OMeTAD accompanied with
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halides in the form of CH3NH3PbI3 and mixed halides CH3NH3PbI3-xClx as a light absorber
115
coated with a very thin ≈0.6 µm m-TiO2 scaffold layer yielding η ≈9.7%. The replacement of
116
liquid electrolyte by spiro-OMeTAD significantly improved the device stability under ambient
117
conditions for about 500 h. Unfortunately, these devices were unstable for long term use [18].
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Despite a number of attempts to eliminate m-TiO2, this class of PSCs has shown the highest
119
η >20% so far in a device structure similar to DSSCs (with a significant reduction in the
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thickness of m-TiO2 layer from 10 µm to 300 nm).
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Device designs for PSC have been further explored by changing the orientation and/or tailoring
122
the material’s thickness in relation to TiO2 with meso-porous framework (m-TiO2) with η > 21%
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[41], regular planar (η ≈ 20.44%) [42], inverted planar (η ≈19.9%) [43], meso- superstructure
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TiO2 (η ≈15.9%) [44], flexible devices (η ≈18%) [45-48], HTL free (η ≈16.0%) [49], electron
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transport layer free (ETL) (η ≈14.14%) [50], and fibre-shaped PSC architecture (η ≈5.3%) [14,
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51]. Figure 2 shows how efficiencies have improved over the same period for both systems and
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it is clear that in recent years, the efficiencies have seen a tremendous surge in flexible PSC
128
devices versus the normal PSC devices.
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Figure 2: Comparison of progress made in efficiency in normal and flexible perovskite solar
cells. Scopus (14.10.2019).
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As illustrated in Figure 3(a-b), the number of published articles on PSC (according to Scopus)
134
has exponentially increased since 2013 and a similar trend can be seen with flexible PSC devices
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since 2012 with 2018 heralding the emergence of new, implementable and rational innovations
136
in PSCs.
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Figure 3: Progress have been made (a) PSCs and (b) flexible PSCs. Data was taken from Scopus
(14th October, 2019)
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This review will briefly elaborate upon PSCs and perovskite crystal structures highlighting the
143
progress that has been made in flexible PSCs. In addition, stability and large scale production has
144
also been deliberated on.
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2.
Progress made in flexible PSCs
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Flexible and light-weight solar cells have considerable significance in terms of supplying
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power to wearable and portable devices as well as reducing the cost of solar panels. R2R
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manufacturing of PSCs has proven itself as a fast and economical method which is ideal for mass
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production. PSCs with different architectures have been explored and investigated in detail since
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their inception. These include, mesoporous, planar (regular and inverted), HTL-free and ETL-
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free PSCs constructs. Among these, the inverted class of PSCs possessing a p-i-n structure is an
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excellent choice for flexible devices than the regular planar devices owing to its lower hysteresis
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and fabrication at a low temperature through an in-situ solution process assisted by organic
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vapours. In high performing planar PSCs, the TiO2 layer is usually deposited by spray pyrolysis
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or spin coating which requires very high sintering temperatures >450 oC, [44, 52] in order to
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obtain highly dense structure and crystallinity. This high sintering temperature is not compatible
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with most of the polymeric substrates and therefore complicates the fabrication process.
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Most PSCs are made on ITO or FTO transparent conductive electrodes which have
160
several disadvantages. The scarcity and costliness of Florine or Indium leads to an increase in the
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overall cost of PSCs production [53, 54]. Additionally, a 50% decrease in power conversion
162
efficiency η at low bending cycles is reported when ITO is used as the transparent conducting
163
oxide attributed to mechanical deformity produced in ITO. On the other hand, a very high
164
temperature is required for indium-free substrate such as FTO which is unsuitable for plastic
165
substrates. Properties such as mechanical flexibility, high optical transmittance, spatially
166
homogenous and low sheet resistance are a prerequisite to employ flexible substrates for PSCs
167
[55, 56], yet very few materials exhibit all these features of the substrate, such as metal film
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typically possesses low transmittance when it is thick while thinner films lead to poor
169
conductivity.
170
(styrenesulfonate) (HC-PEDOT:PSS) electrode could be fabricated with a low sheet resistance,
171
and high mechanical flexibility [57-59].
Similarly,
Highly
conductive
poly(3,4ethylenedioxythiophene):
poly
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In order to investigate ITO-free transparent conducting electrode TCE, Yoon et al., [47]
173
employed graphene as a TCE on top of a thin film of polyethylene naphthalate (PEN) in flexible
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PSCs which was reported to exhibit η ~16.8% without showing any hysteresis. At the same time,
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the inferior conductivity of the deposited graphene layer on PEN was significantly improved by
176
introducing a molybdenum trioxide (MoO3) layer. A schematic of the flexible PSCs based on
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graphene as an anode is depicted in Figure 4a, with device architecture represented as
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PEN/graphene-MoO3/PEDOT:PSS/MAPbI3/C60/BCP/LiF/Al where the HTL, ETL and hole
179
blocking layers are represented as PEDOT:PSS, C60 and BCP (bathocuproine) respectively. The
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annealing temperature for all these layers was kept below 110 oC during fabrication process as
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the glass transition temperature of PEN is about 120 oC. The as fabricated flexible devices
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comprised of PEN substrate with Graphene doped MoO3 electrode Gr-Mo/PEN and ITO based
183
electrode ITO/PEN were compared with each other and their J-V characteristics as concluded are
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shown in Figure 4b-c. The figure shows similar JSC curves for both the devices; however, the
185
device based on Gr-Mo/PEN resulted in a lower fill factor FF ~0.78 than that of ITO/PEN (0.83)
186
owing to the relatively higher sheet resistance of the former.
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These devices were scanned in both the reverse and forward directions and gave a
188
significantly high η= ~17.3 for ITO/PEN based device and η= ~16.8% for the Gr-Mo/PEN based
189
device with nominal hysteresis. Bending stability was also checked for the ITO/PEN and Gr-
190
Mo/PEN devices for its practical applications. The devices were bent with radius (R = ∞, 6, 4,
191
and 2 mm) for 1000 bending cycles. Figure 4d shows the relationship of normalized η of the
192
ITO/PEN and Gr-Mo/PEN based devices. The latter has maintained 90% of its original
193
performance when bended at R = 6 mm, 4 mm, and 2 mm while the performance of the
194
ITO/PEN based device deteriorated to 60% (for R= 6 mm) and 25% (for R= 4 mm) from its
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original values. Figure 4e shows the relationship between η and bending cycles for both the
196
devices particularly at R = 4 mm. One can see a similar trend for both the devices at the
197
beginning of the bending upto 250 cycles; however, there was a sharp drop in η for the ITO/PEN
198
device when bending increased. In stark contrast, the Gr-Mo/PEN based device showed a stable
199
performance over bending for 1000 cycles at R = 4 mm. Such an outstanding performance of the
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Gr-Mo/PEN based PSCs against the mechanical distortion paves the way for these devices to be
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realized as potential candidates for new foldable PV applications.
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ITO has been the photoanode material of choice because it offers high conductivity and
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transparency on both flexible plastic and rigid glass substrates. [60] However it has its own
204
limitation of being expensive and brittle and hence affects flexibility of the device [61, 62], yet it
205
is reported to improve bending by using with configuration as ITO/PET and ITO/PEN. [63]
206
Besides Kaltenbrunner et al., have employed PEDOT: PSS in place of ITO as promising
207
candidate in flexible PSCs with a thickness of only 3 µm exhibiting an η value of 12.5%. [64]
208
Metal foil has also been use as a flexible valid robust substrate for PSC fabrication that has high
209
electrical conductance and thermal endurance with low rate of water transmission as compared to
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plastic substrates. Thus Ti foils have been selected for flexible SCs because of their excellent
211
resistance to corrosion as well excellent ratio of strength to weight. However the opaque nature
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of metals needs to be countered by using a light transparent top counter electrode. [60] It also
213
poses difficulty in fabricating solar modules connected in series unless cell is divided. Hence a
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physical separation is required to avoid short circuiting which is in contrast to the glass
215
substrates where laser etching helps to prevent short circuiting. Cu and stainless steel foils apart
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from Ti foil have been used as substrates [65, 66] in flexible PSCs while fibrous shaped metal
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photoanode has also been investigated and found to be flexible enough to be integrated with
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wearable electronic gadgets. [67] While currently flexible PSCs on metal substrates are restricted
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to development of single cells where the metal foil is used as carrier and bottom electrode only.
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Additionally, the roughness needs to be dealt with by pretreating the metal foil with electro-
221
polishing. [68]
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Most of the metal based PSCs report using a mesoporous TiO2 layer with an n-i-p architecture
223
and hence titanium foil is the most exhaustively employed substrate. [68-70] A most efficient
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PSC obtained by anodizing a TiOx layer to nanotube TiO2 used as the ETL with η of 8.3% is
225
reported. [67] In another study, spray pyrolysis was used to obtain a compact TiO2 layer along
226
with perovskite layer and demonstrated an η value of 11% where the top electrode was
227
comprised of Ag/ITO layer that resulted in improved performance. [68] However metal film may
228
confer poor conductivity to the flexible PSC which can be overcome by using a mesh of metal
229
together with conducting polymer as a hybrid counter electrode. Al2O3 scaffold was reported
230
with lamination of a transparent electrode composed of Ni mesh impregnated onto a
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polyethylene phthalate (PET) foil covered with PEDOT: PSS with an adhesive that ensured good
232
electrical conductivity. The η value was demonstrated to be 10.3%. [71] Ag mesh deposited
233
through spray pyrolysis and Cu foil employed as substrate was reported by [65]. Ag nanowires as
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metal fibers with highly efficient flexible PSCs have been reported. Ag nanowires can be an
235
effective alternative to transparent conducting oxides where a top electrode of Ag nanowires was
236
used in flexible PSCs in the form of fibers.[72] Nevertheless the fabrication procedure as
237
employed was different from the ones used for glass or flexible substrates. Electro deposition can
238
be an effective method to deposit Pb salt e.g., by a two-step synthesis of PSCs which leads to an
239
increase in performance. The result portrays versatility in fabrication process adopted by a
240
variety of substrates for designing high performance PSCs.
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Flexible PSCs printed on a low priced metal foil is reported. R2R high purity PSC fabrication
242
offers an alternative route to a high heat treatment process in the halide perovskite processing.
243
Doojin Vak working on development of protocol and instrument for R2R fabrication of flexible
244
PSCs on plastic substrates has also discussed the heat treatment issue and have used a slot-die
245
coating method instead of spin coating one to avoid rapid evaporation of solvent. Hence
246
extensive work in this domain will lead to advancement of the film fabrication process on a
247
suitable substrate for its large scale production.[73] Due to high conductivity and flexibility,
248
carbon nanotubes (CNT) and graphene have also been used as photoanodes. Hence CNT
249
deposited on PET substrate was reported to show an efficiency of 5.38%. [74] Graphene
250
modified with MoO3 to reduce its hydrophobicity has also been introduced on PET substrate in
251
flexible PSCs with a higher efficiency of 16.8% than CNT modified PET. [47] It showed even
252
higher flexibility and transmittance than ITO films.
253
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Figure 4: (a) Basic architecture of graphene-based flexible perovskite solar cell where the inset image is the
complete photograph of the device, (b&c) J-V curves of the prepared flexible devices based on Gr-Mo/PEN and
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257
258
259
260
ITO/PEN with respect to both scan directions, (d) normalized efficiency after 1000 bending cycles with various
bending radii: flat, 6, 4, and 2 mm, and (e) normalized efficiency vs bending cycles with a fixed radius 4 mm of the
respective devices (reprinted from ref.[47]) Copyright 2017 RSC.
261
nanoparticles synthesized at temperatures <100 oC for developing a flexible PSC [75]. The
262
inclusion of n-type Zn2SnO4 layer significantly enhanced the transmittance performance of
263
flexible polyethylenenaphthalate/indium-doped tin oxide (PEN/ITO) up to 75 – 90% over the
264
entire wavelength’s range of light as compared to PEN/ITO as a bare flexible electrode.
Recently, a high η >15% has been reported in a study utilizing metal oxide Zn2SnO4
265
In another similar study on flexible devices, a very condensed amorphous TiO2 (am-
266
TiO2) film was prepared by magnetron sputtering process at room temperature which was
267
employed as ETL in flexible PSCs [76]. The synthesized am-TiO2 dense layer was highly
268
transparent across the solar spectrum and yielded an impressive η >15% with reasonable values
269
for key photovoltaic PV parameters as shown in Table 1. The J-V curve of the various flexible
270
devices are shown in Figure 5 with inset showing the bent devices. The performance of the
271
flexible device showed no degradation even after mechanical bending at 65º for 100 times, thus,
272
assuring mechanical stability (Figure 5a). Similarly, highly bendable flexible PSCs based on
273
amorphous am-TiO2 with a bending diameter of ≈1mm has shown significant η= ≈12.2% based
274
on PEN/ITO substrate [62] with the corresponding J-V curve shown in Figure 5b. A thick
275
amorphous TiOx (≈20 nm) layer was deposited by plasma enhanced atomic layer deposition at a
276
very low temperature (80 oC). The stability and durability of the fabricated devices were
277
investigated through bending tests. Three different radii of curvature were chosen i.e. 400 mm,
278
10 mm, and 4mm (R400, R10, and R4, respectively). The devices’ performance with R400 and R10
279
maintained the quality without any significant reduction in η during 1000 bending cycles
15
280
whereas for the device with R4, η was maintained at 90% after 25 cycles, however at the end of
281
1000 cycles, the device lost 50% of the initial η.
282
Zhang et al [77] fabricated a novel flexible HTL-free PSC with notable performance of
283
η ≈9.7% and 12.5% with flexible PET/ITO and glass/ITO substrate using Fast deposition
284
crystallization (FDC) technique for perovskite deposition as shown in Figure 5c. The low
285
performance regarding η of PET/ITO based PSC may be attributed to high resistance of ITO on
286
PET. The HTL-free device was tested for stability in comparison to the most commonly
287
employed PEDOS: PSS HTL under (30 ± 10%) humid environment at room temperature. The
288
device with PEDOT: PSS as HTL lost 99% of the original η in just 50 min while the same
289
reduction of the HTL-free device occurred in 300 min. Chen et al., [78] employed a layer by
290
layer approach for mixed perovskite CH3NH3PbI3-xClx deposition on ITO-coated PET flexible
291
substrate. The J-V curve of the flexible device based on CH3NH3PbI3-xClx shown in Figure 5d
292
resulted in η= ~12.25%. The flexible device bent repeatedly at 60o for 1000 times without
293
showing any drastic change in its performance.
16
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295
296
297
298
Figure 5: J-V curve and bent photographs of the best flexible PSCs (a)PET/ITO/am-TiO2/ CH3NH3PbI3-xClx /spiroOMeTAD/Au [76] (b) PET/ITO/MAPbI3/PC61BM/Al [62], (c)PEN/ITO/am-TiO2/ CH3NH3PbI3-xClx/spiroOMeTAD/Ag [77] (d) PEN/ITO/PEDOT:PSS/CH3NH3PbI3-xClx/PCBM/Ca/Al [78]. Reproduced from ref. 77, 62, 78
and 79.
299
300
Jin et al., [79] employed ZnO as ETL in flexible PSCs and compared its performance
301
with a TiO2-based device. Figure 6(a-b) illustrate a schematic energy band diagram of ZnO and
302
TiO2 with respect to MAPbI3 perovskite. Upon light illumination, the photoinduced charge
303
carriers are transported from MAPbI3 layer into ZnO or TiO2 as ETL and Polytriarylamine
304
(PTAA) as HTL, respectively. It has been reported that TiO2 as ETL in planar PSCs possesses
305
hysteresis with respect to the scan direction owing to its lower electron conductivity (≈6×10–5
306
mS.cm–1) than the hole conductivity of PTAA (≈3.4×10–2 mS.cm–1) and is responsible for
17
307
accumulation of electron at the TiO2/perovskite interface [80]. On the other hand, ZnO as ETL
308
could minimize the hysteresis due to its high electron conductivity. Moreover, ZnO conducting
309
layer can be formed at room temperature by sol gel method so that it is more feasible for flexible
310
PSCs. The SEM cross sectional micrograph of the ZnO (~40 nm) as ETL based flexible PSCs is
311
shown in Figure 6c.
312
The J-V characteristic curves of ZnO and TiO2 ETL based devices with respect to
313
direction under 200 ms of delay time (scan rate = 10 mV.200 ms-1) are shown in Figure 6(d-e).
314
It is evident from the figure that the device based on ZnO ETL showed less hysteresis with
315
respect to the scan direction as well as delivered improved VOC in comparison to its TiO2 ETL
316
counterpart. The maximum η= ~17.7% (in reverse direction) and 17.5% (in forward direction)
317
were achieved for ZnO ETL based device in comparison to 17.3% and 16.8% for TiO2 ETL-
318
based device, respectively. The other PV parameters are listed in Table 1. The external quantum
319
efficiency (EQE) of both the devices were almost the same and the calculated JSC using EQE
320
(20.0 mA.cm−2 for TiO2 and 20.1 mA.cm−2 for ZnO) were in close agreement with the directly
321
measured J-V data. Moreover, similar values for EQE curves were expected because the UV-Vis
322
absorbance of FTO/TiO2/MAPbI3 and FTO/ZnO/MAPbI3 films showed similar absorbance over
323
the same spectral range of light.
324
The reproducibility of the devices based on ZnO and TiO2 ETLs is shown in histograms
325
(Figure 6f). The figure shows deviation in average value of η for each of the 40 devices where
326
the ZnO based device has slightly higher value of η (15.96 ± 1.07%) than its TiO2 counterpart
327
(15.20 ± 1.23%). The better performance of ZnO ETL based device was further investigated by
328
studying the I-V characteristics of the electrodes: ITO/TiO2/Au and ITO/ZnO/Au in sandwich
329
cells using the relation I = σοAd-1V, where A is the active area (0.16 cm2) and d is film thickness
18
330
(50 nm) of the sample. The calculated conductivity of the ZnO was 0.0031 mS.cm-1 which was
331
about 50-folds higher than that of TiO2 (0.00006 mS.cm-1). With ZnO ETL employed on
332
PEN/ITO, the planar flexible device yielded η= ~15.4% in the forward direction while 15.6% in
333
the reverse direction as shown in Figure 6g. The other PV parameters are listed in Table 1.
334
Regarding the mechanical durability, the flexible device based on ZnO ETL showed no visible
335
change in η when bent into curvature of different radii.
336
337
338
339
340
341
342
343
Figure 6: Schematic band energy illustration of (a) TiO2 and (b) ZnO ETL with (c) SEM cross sectional image of
representative devices, (d)J-V of TiO2 and (e) ZnO ETL based devices with respect to scan direction, (d), (f)
efficiency deviation of 40 sample of TiO2 and ZnO, (g) J-V curve of the flexible device based on ZnO ETL along
with bended flexible cell photographs shown in inset (reproduced with permission from Ref. [79]). Copyright of
RSC.
19
344
Yang et al., [81] employed solid-state ionic-liquids (ss-IL) as ETL in flexible PSCs with a device
345
architecture shown in Figure 7a along with energy band diagram. The ss-IL is known for its
346
unique characteristics (such as high electrical conductivity, high carriers’ mobility, better thermal
347
stability) substantiating its superior performance in batteries, supercapacitors, actuators, and
348
organic solar cells [82, 83]. Figure 7b shows the SEM cross sectional micrograph of the full
349
device while the upper-right inset displays the photograph of the flexible device where the
350
thickness of the ss-IL was only about 10 nm. The flexible device based on ss-IL ETL exhibited
351
no hysteresis with respect to scan direction (Figure 7c).
352
Owing to the outstanding properties of ss-IL (such as wide bandgap, anti-reflection, high
353
electron mobility, and suitable work function WF), the fabricated device exhibited no hysteresis.
354
The device without ss-IL showed inferior performance due to maximal recombination of charge
355
carries in comparison to devices based on ss-IL as reported earlier. The insertion of a thin ss-IL
356
layer between ITO and MAPbI3 significantly enhanced the performance of the cell and the η lept
357
from 8.19% (without ss-IL) to an impressive η ≈16.09% (with ss-IL) which has been reported as
358
the highest value achieved so far for such devices. The Incident photon to current conversion
359
efficiency (IPCE) of ss-IL based device was notably higher than the device without ss-IL
360
(Figure 7d). Additionally the integrated JSC deduced from IPCE was in close agreement with the
361
J-V profile. The fabricated device was bent at curvatures of 12, 7, and 5 mm and flexed for 300
362
cycles which showed no notable change in η value (Figure 7e). In comparison to ITO/MAPbI3
363
electrode, the Photoluminescence (PL) intensity of ITO/ss-IL/MAPbI3 significantly quenched
364
showing excellent charge extraction capability due to its high electron mobility and reasonable
365
WF.
366
20
367
368
369
370
371
372
373
Figure 7:(a) Schematic architecture along with energy band diagram of flexible PSC, (b) SEM colourful cross
section and the upper-right inset shows the photograph of the flexible bended PSCs, (c) J-V profile of flexible PSC
with ss-IL with respect to scan direction and without, (d) IPCE and integrated JSC, and (d) J-V profile of bending at
curvature radius of 12, 7, and 5 mm and flexed for 300 cycles (reproduced with permission from Ref.[81]).
Copyright of John Willy & Sons.
374
375
Li et al., [84] fabricated flexible PSCs based on p-i-n architecture with significantly high
376
value of η ≈14%. The J-V curve (Figure 8a) showed a small hysteresis with respect to the scan
377
directions. For mechanical flexibility and durability the inverted flexible device was bent into
378
five curvatures of radii r = ∞, 7, 5, 3.5, and 2 mm in one bending cycle. The resulting η showed a
379
stable trend for all the radii and therefore validated the superior flexibility of PET/Ag-
380
mesh/PH1000 electrode. Yin et al., [85] employed solution derived NiOx as HTL in a flexible
381
device which yielded a decent η= ~13.43%. Figure 8b shows the J-V curve and its comparison
21
382
with the device based on ITO glass. However, η (≈12.63%) slightly dropped for the reverse scan
383
direction (VOC →JSC).
384
Jung et al., [86] employed a solvent engineering approach for perovskite film deposition
385
at a very low annealing temperature of 70oC and fabricated inverted planar flexible PSCs which
386
delivered η ≈9.43%. The J-V curve with the inset showing the photograph for the flexible device
387
as seen in Figure 8c exhibits a high FF ≈75%. After bending 20 times, the device still exhibited
388
η ≈8.42%.
389
Yoon et al., [87] reported fabrication of highly efficient, hysteresis free and stable
390
flexible PSCs with η ≈16%. The J-V profile (Figure 8d) exhibited no hysteresis with respect to
391
the scan direction. Its mechanical stability was investigated by bending test where 1000
392
consecutive bending cycles at different radii of curvatures (10 mm and 5 mm) were employed for
393
its evaluation. The results revealed that bending showed almost no loss of η for 10 mm curvature
394
while a 20% decrease in η was observed after 1000 bending cycles. Xi et al., [88] reported η
395
≈13.03% in a flexible device employing α-FAPbI3 (formamidinium lead iodide) perovskite as a
396
light absorber. Figure 8e depicts the representative J-V profile along with photographs of bent
397
flexible device in the inset. As can be seen from the figure, negligible dependence was observed
398
with respect to the scan direction i.e., from FB-SC (VOC→JSC) and from SC-FB (JSC→VOC). In
399
addition, the J-V profile exhibited reduction of 0.18%, 0.61% and 0.81% in η accompanied at
400
bending angles of 15°,30°and 45° after 1000 times, respectively, showing excellent working
401
ductility. Hu et al., [89] employed FAPbI3 as light absorber in flexible PSCs and reported an η of
402
≈12.70%. The FAPbI3 film was directly grown onto an ITO with a simplified device
403
architecture. Its J-V curve is shown in Figure 8f. The bending test was performed in order to
22
404
examine the robustness of the device which showed a slight effect when the device was bent 20
405
times.
406
407
408
409
410
411
412
413
414
Figure 8: J-V characteristic with inset photographs of the best flexible PSC (a) p-i-n based architecture PET/Agmesh/PH1000/PEDOT:PSS(35 nm)/MAPbI3(≈280 nm)PCBM(≈60 nm)/Al(100 nm) hysteresis free η≈14%[84], (b)
ITO/NiOx/MAPbI3/PCBM/Ag device[85](c)PET/PEDOT:PSS/MAPbI3(300 nm) /PC61BM (50 nm) /Bis-C60(10
nm)/Ag(150 nm)[86](d)PEN/ITO/C60/Perov/Spiro/Au, J-V curve with inset shows a digital camera image of the
flexible device [87], (e)ITO/PEDOT:PSS/α-FAPbI3/PCBM/BCP/Ag[88](f) ITO(PET)/Cl-FAPbI3(350 nm)/spiroOMeTAD(150 nm)/Au(100 nm) η ≈12.70% [89].
415
performance of flexible PSCs with a p-i-n structure as compared to glass/ITO is lower largely
416
due to its surface roughness leading to significant leakage of current. Hence to improve η values
417
for flexible PSCs, PEDOT: PSS was deposited onto ITO surface with NiO film immobilized
418
over the bare ITO along with 1,8-diiodooctane (DIO) and H2O added as dual additives to the
419
precursor solution. This improved the uniformity and grain size with a high flexibility and high
420
value of 18% for η.
The η value of flexible PSCs has indeed reached from 6.4% to 18% as reported by [90]. The
23
421
Solar cells with PET/ITO architecture and integrated with a wide variety of metal oxides as
422
flexible
423
perovskite/mAl2O3/N2 plasma SnO2/ITO/PET have displayed an outstanding η of 18.1% with 90
424
% performance from its initial value after 1000 bending cycles. [91] It is shown that the UV
425
radiation with low media frequency N2 and N2O plasma emission results in good quality film of
426
SnO2 at low enough temperatures and the film retains same properties as of thermally annealed
427
SnO2 as a compact layer. Flexible solar cells have long been pursued due to extensive advantages
428
as compared to rigid substrates. However oxide layer deposition as a charge extraction layer is
429
shown to be deposited by an energy intensive process through traditional thermal treatment.
430
Co doped NiOx NPs for inverted planar PSCs as HTL have been employed at different
431
concentrations of Co and have demonstrated a η of 14.5% as compared to pristine NiO-based
432
HTL processed at low temperature that shows limited conductivity while high temperature Li
433
Mg doped NiOx based HTL reporting a η of 16.2% is disadvantaged with respect to the high
434
pyrolysis process used for its fabrication.[92] Therefore alternatives in the form of Co-NiOx
435
nanostructured HTL is sought. Whereas Fe-doped NiOx based planar PSC improved η by
436
17.57% compared to pure NiOx at 15.41 %, the flexible PSC have shown a η of 14.42% for the
437
doped HTL as compared to 13.37% for NiOx /perovskite delineating promising candidature for
438
HTL for constructing flexible PSCs in future. [93] Sr-NiOx film as HTL in comparison to
439
pristine NiOx at different concentrations of Sr exhibits η at 20.07% retaining 60% of original
440
η for 100 days.[94] This is attributed to reduction of carrier recombination, matching of energy
441
levels with perovskite layers and improvement of conductivity by doping NiOx with Sr. Cs
442
doped NiO2 have shown a remarkable η of 19.35% used as HTL at a relatively low annealing
443
temperature for inverted planar PSCs. [95] A 2-step process comprising of a hydrothermal
substrates
have
demonstrated
interesting
results.
Thus
Au/spiro-OMeTAD/
24
444
process followed by an in situ hydrolysis and oxidation of SnCl2 for synthesis of SnO2 NPs as
445
ETL for flexible PSC (ITO/PEN) is reported in [96]. SnO2 NPs is reported to have improved the
446
crystallinity of the PSC layer with an η of 15.21%. Zn2SO4 as ETL was developed as a nanoink
447
on perovskite based CH3NH3Pb(I0.9Br0.1)3 achieving a η of 16.5% for low temperature flexible
448
PSCs with regular architecture by improving the transport ability of photogenerated electrons in
449
the energy levels with inverted flexible PSCs. [97] Yoon et al., reported an η of 16.8% with high
450
efficiency and flexibility constructed from graphene as the anode deposited by CVD having the
451
architecture
452
photoanode counterpart deposited with graphene and 2nm MoO3 to overcome graphene
453
hydrophobicity showed an η of 17.3% which is the highest so far and is expected to improve
454
further. [47]
455
Exposure of spin coated metal oxide salts (SnO2) deposited on FTO to deep UV (DUV)
456
irradiation as a low temperature alternative to conventional annealing step was used as an ETL
457
with MAPbI3-xClx as the absorber resulting in 16.2% of η while photonically annealed
458
counterpart resulted in 15% of η at low substrate temperatures for planar PSCs. The device
459
parameters are lower than the thermally annealed SnO2. However using thin film of metal oxide
460
at low enough temperatures seems a plausible solution to high temperature based flexible
461
polymer PSCs. [98]
462
Yang et al [48] employed Ethylene diamine tetraacetic acid (EDTA) complexed tin oxide (SnO2)
463
as electron-transport layer
464
transport layer is better matched with perovskite that resulted in higher open circuit voltage. A η
465
of 18.28% has been achieved using EDTA –SnO2 complex as ETL in F-PSCs. Feng et al[46]
466
employed a novel dimethyl sulfide (DS) additive and effectively improved the performance of
467
the F-PSCs. Such an effective additive technique helps tremendously to control the perovskite
as
PEN/Graphene-MO/PEDOT:PSS/CH3NH3I3/C60/BCP/LiF/Al.
Its
ITO
in PSCs. The fermi level of EDTA complexed SnO2 electron-
25
468
morphology i.e., the grain size and crystallinity improves and the trap state density in the
469
resultant perovskite film is reduced on flexible substrates for F-PSCs. The schematic diagram of
470
the device is shown in Figure 9 a-b. The η of 18.40% has been achieved for flexible devices
471
with good mechanical tolerance, the highest reported so far for the F-PSCs. In addition, the
472
environmental stability of the F-PSCs significantly enhances by 1.72 times compared to the
473
device without the additive, likely due to the large grain size that suppresses perovskite
474
degradation at grain boundaries (Figure 9 c-e).
475
476
26
477
478
479
Figure 9: (a -b) schematic illustration of the F-PSC structure, cross-sectional SEM image of a MAPbI3–
DS absorber, (c-d) J–V curves of F-PSCs of pure and DS additive MAPbI3 and (e) The η of flexible
device based on MAPbI3–DS at different bending curvature radii after 5000 flexing cycles.
480
481
482
483
Cost analysis
484
to low cost, high efficiency and stability together with additional features such as flexibility and
485
easy tandem with other technologies. [5] Si solar cells have proven to exhibit long lifetime and
486
higher efficiency than PSCs (20-25%) however are rigid, hefty and expensive though the price
487
has seen a fall since their inception. New generation organic photovoltaics OPV and DSSC
488
suffer from low efficiencies and short lifetimes yet exhibit interesting and remarkable
489
functionalities including flexibility and transparency. At the same time, the PSCs have shown to
490
demonstrate similar features concurrently with a comparable η of 21% with Si and an easiness to
491
be printed on flexible substrates. It is estimated that the cost of flexible substances such as
492
PET/PEN is much lower (∼30%) as compared to when glass conducting surfaces are used. [99]
493
The cost breakdown for PSCs includes fabrication method chosen and its architecture with
494
relation to substrate and other components such as sealants and electrical connections,labor,
495
overhead cost and capital investment for its large scale production. [60] Flexible PSCs have the
496
characteristics of OPVs with the fabrication method also mimicking the OPV method of
497
fabrication and hence similar in investment. The cost effectiveness for flexible device can be
498
calculated as in [100] while prices of materials used for fabrication of flexible PSC can be taken
499
from the local market. It is however worth mentioning here that commercialization of PSCs
500
requires installation of facilities to fabricate it at a large scale which is still not available. Unlike
501
organic SCs, PSCs can be constructed in different architectures with a number of technologies.
To allow PSCs technology to overwhelm the market trend, it needs to be considered with regards
27
502
Among the materials used for preparing PSCs, substrates are considered to leap the fabrication
503
costs up. Hence the cost will increase to ∼40% for ITO/glass ∼36% for PET/PEN and ∼5% for
504
metallic substrates and this area largely affects the economics of its fabrication. R2R processing
505
at low temperature for flexible substrate such as PET/PEN holds great promise to reduce labor
506
cost for its fabrication as compared to glass based counterparts. The cost economics for
507
fabrication of solar cells depends on its different applications and hence cost needs to be tailored
508
according to it.
509
The quest to attain par excellence in domotics by investing on light harvesting technology using
510
ambient or poor indoor light conditions has been the topic of much discourse now [101, 102].
511
Harvesting energy provides smart solutions to make our wireless sensing and buildings efficient
512
in term of energy use. Hence intelligent sensors that are self-powered, low maintenance, are
513
flexible, ensure easy instalment for managing building systems without the need to change the
514
older infrastructure have attracted a lot of attention. It is also noteworthy to mention that due to
515
low cost of materials used, cost of fabrication and the efficiency of the device, photovoltaics
516
based on perovskites are the chosen candidates for harvesting light in the contemporary era as
517
compared to the more efficient organic and inorganic solar cells. Planar PSC utilizing composite
518
layer of SnO2/MgO as ETL is shown to demonstrate efficiencies of 25% at 200 lux and 26.9% at
519
400 lux as compared to a SnO2 counterpart PSCs. The MgO overlayer is shown to exhibit better
520
stability and can better resist recombination of charges at the interface leading to better
521
efficiencies[103]. Milder indoor condition in comparison to harsh outdoor conditions calls for a
522
more focus on stabilities for modulating the devices to be used outdoor for commercial outlets.
523
Additionally, small area PSCs using inverted ITO/PEDOT:PSS/oragno-perovskite/ETL/1,3,5-
524
tri(m-pyrid-3-yl-phenyl)benzene/Ag have demonstrated an η of 27.4% without an hysteresis
28
525
effect under fluorescent illumination at 100 lux while large area device has shown an η of 20.4%
526
with very long lasting stability[104]. In the study, [6,6]-phenyl-C 61 -butyric acid methyl ester,
527
C60 and C70 were used as different ETLs. However, flexible in comparison to rigid devices are
528
lightweight, economical, thin and adapt effectively to the substrate which are important
529
properties for integrating them seamlessly in indoor environments. Flexible photovoltaics
530
especially amorphous silicon, DSSC, organic voltaic cell and PSCs have been the candidates of
531
choice to accomplish the aim to develop buildings that are smarter especially if ambient light
532
could be used[104-107]. These harvester technologies have a huge potential to replace older
533
batteries and/or extend shelf life of rechargeable batteries for electronics. This is bound to
534
provide a growing platform for developing sustainable solutions through the internet of
535
things[108]. Flexible planar and mesoscopic PSC for harvesting artificial light are reported for
536
device
537
xClx/spiro-MeOTAD/Au
538
compared with planar PSC based solar cells fabricated[109]. It was found that the mesoporous
539
flexible PSCs exceeded the performed of DSSC and flexible amorphous Si based solar cell under
540
illumination of LED with η = 10.8% for 200 lux and 12.1% for 400 lux. Yet, there is scope of
541
improvement to reach the best values of rigid substrates standing at η= 24% with LED light
542
illumination of 400 lux. It was established that planar devices show poor efficiencies with
543
mesoscopic cells using atomic layer deposition of c-TiO2 layer and device architecture as
544
PET/ITO/c-TiO2/CH3NH3PbI3–xClx/spiro-MeOTAD/Au. Furthermore, mesoscopic devices based
545
on a sol gel deposited TiO2 compact layer also exhibited poor performance efficiencies at
546
standard testing conditions in indoor conditions. Similarly Flexible PSCs were fabricated using
547
solution processible SnO2 and m-TiO2 as the ETL and it was found out that the cell with device
configuration
PET/ITO/atomic
layer
deposited-c-TiO2/meso-TiO2/CH3NH3PbI3–
by illuminating it with artificial white LED lamp at 200 and 400 lux
29
548
configuration PET/ITO/SnO2/m-TiO2/CH3NH3PbI3/Spiro-MeOTAD/Au delivered an efficiency
549
of 14.8% under light source of AM1.5G which was better than the PSCs using only SnO2 as the
550
ETL. The cell were illuminated with 200 and 400 lux artificial indoor light and exhibited an
551
η=12.8% and η= 13.3% which is the highest so far achieved in comparison to PSC with SnO2 as
552
ETL highlighting its importance and feasibility for commercial applications. This was scaled up
553
to be used as a highly efficient PSC module that was fabricated with an active area of 12 cm2
554
with laser scribing for the first time ever and showed η of 8.8% illuminated under 1 sun[110]
555
which was better than the efficiency reported in [111]. While, previously a MAPbI3 based PSC
556
module on a glass substrate with an active area of 10 cm2 showed an η of 13-14% [112]. Hence
557
the results on flexible PSC for module fabrication demonstrate its potential to be used to further
558
boost the efficiency of flexible perovskite by using its intrinsic qualities of being flexible and
559
lightweight.
560
561
562
563
Table 1: Available data in the literature for flexible and fibre PSCs, device architecture, PV parameters JSC (mA.cm2
), VOC (V), FF (%) and η(%) along with perovskite deposition method
Regular device architecture/PSCs
Deposition
Method
JSC
2
(mA/cm )
VOC
FF
(V)
(%)
η
Ref
(%)
PEN/ITO/ZnO/ MAPbI3/PTAA/Au (RS)
One step SC
18.7
1.10
76
15.6
[79]
PEN/ITO/ZnO/ MAPbI3/PTAA/Au (FS)
One step SC
18.7
1.10
75
15.4
-----
ITO/Zn2SnO4/MAPbI3/PTAA/Au
Two step SC
21.6
1.05
67
15.3
[75]
PET/Ag//PH1000/PEDOT:PSS/MAPbI3/PCBM/Al (RS)
Two step SC
19.5
0.91
80
14.2
[84]
PET/Ag//PH1000/PEDOT:PSS/MAPbI3/PCBM/Al (FS)
Two step SC
19.3
0.90
79
13.7
-----
ITO/Li-SnO2/perovskite/Spiro/Au
One step SC
20.57
1.06
67.27
14.78
[113]
PEN/ITO/PEDOT:PSS/Perovskite/PC60BM/Ag
One step SC
PEN/ITO/PhNa-1T/Perovskite/PC60BM/Ag
14.4
0.88
66.2
8.4
[114]
18.4
1.03
77.4
14.7
[114]
ITO/PEDOT/Hybrid Perv/PCBM/BCP/Ag (RS)
Two step SC
18.93
0.994
69.2
13.03
[88]
ITO/PEDOT/Hybrid Perv/PCBM/BCP/Ag (FS)
Two step SC
18.89
0.994
69.3
13.02
[88]
PEN/ITO /NiOx/MAPbI3/PCBM/Ag (FS)
One step SC
18.74
1.04
68.9
13.43
[85]
PEN/ITO /NiOx/MAPbI3/PCBM/Ag (RS)
--------------
19.37
1.03
63.3
12.63
----
PET/ITO/TiO2/MAPbIx-3Clx/PTTA/Au
One step SC
21.3
0.91
69
13.5
[115]
PET/ITO/am-TiO2/ pervoskite/Au
Thermal Eva2
20.90
1.03
72
15.07
[76]
30
PEN/ITO/TiOx/MAPbI3-xClx/spiro-OMeTAD/Ag
One step SC
21.4
0.95
60
12.2
[62]
PDMS/ITO/ZnO/Perovskite/Spiro/Au w/o nanocone
Two step SC
17.7
0.97
70
12.06
[116]
PDMS/ITO/ZnO/Perovskite/Spiro/Au with nanocone
Two step SC
19.3
980
69
13.14
----
IZO/PET/TiO2/MAPbI3/Spiro-OMeTAD/Au (RS)
One step SC
18.2
1049
69
13.2
[117]
IZO/PET/TiO2/MAPbI3/Spiro-OMeTAD/Au (FS)
One step SC
17.7
1031
56
10.2
----
PET/ITO/TiO2/MAPbI3-xClx/Spiro-OMeTAD/Ag
Ultra- spray
20.6
1.03
61.5
13.0
[118]
IZO/PET/TiO2/MAPbI3/Spiro-OMeTAD/Ag(RS)
One step SC
16.8
1020
71
12.2
[119]
IZO/PET/TiO2/MAPbI3/Spiro-OMeTAD/Ag(FS)
One step SC
17.0
1010
63
10.8
-----
Epoxy/ITO/ZnO/MAPbI3/Spiro-OMeTAD/Au
Two step SC
19.2
0.87
67.6
11.29
[120]
17.0
983
61
10.3
[69]
PET/PEDOT:PSS/Spiro/MAPbI3/Al2O3/bl/Ti
Ti/TiO2 NTs/MAPbI3/Spiro-OMeTAD/CNTs
Two step SC
14.36
0.99
68
8.31
[67]
PET/ITO/TiO2 ALD/TiO2-MAPbI3-xCl/Spiro/Au
One step SC
12.6
0.828
71
7.4
[121]
16.5
0.996
55
9.1
[68]
ITO/Spiro-OMeTAD/TiO2-MAPbI3/bl/Ti
564
2
565
3.
PET/ITO/ MAPbI3/PC61BM/Al
FDC
14.8
0.96
68.1
9.7
[77]
PET/AZO/Ag/AZO/PEDOT:PSS/MAPbI3/PCBM/Au
One step SC
14.3
1.04
47
7.0
[122]
PET/PEDOT:PSS/MAPbI3/PCBM/PFN-P1/Ag
One step SC
12.73
0.94
66.24
7.92
[123]
AgNWs/Spiro-OMeTAD/MAPbI3/Ti
One step SC
16.48
0.918
49.0
7.45
[124]
PEN/ITO/C60/Perovskite/Spiro/Au
SC(R)
23.21
1.02
67.33
16.00
[87]
PEN/ITO/C60/Perovskite/Spiro/Au
SC(F)
23.45
1.03
63.80
15.45
[87]
PEN/ITO/ZnO/Perovskite/Spiro/Au
Two step SC
21.92
0.90
62.67
12.34
[125]
Thermal evaporation system
Fibre shaped PSCs (FPSCs)
566
Fibre-shaped solar cells (FPSCs) are linear in shape and can harvest solar energy from 3D
567
space. Due to their symmetrical structure and adjustable aspect ratio, FPSCs can be potential
568
flexible power sources [126]. The importance of flexible fibre-shaped devices had been realized
569
for DSSC and organic solar cells by considering their η ≈8% reported in [127-130]. Hu et al.,
570
[51] fabricated PSCs into a fibre-shaped device and achieved η ≈5.35%. The device architecture
571
comprised of Ti/c-TiO2/m-TiO2/perovskite/Spiro-OMeTAD/Au is shown in Figure 10a. The c-
572
TiO2 layer was coated in situ on a titanium (Ti) wire through electric heating followed by coating
573
with mesoporous-TiO2 (m-TiO2) via a dipping process. Each of these layers in flexible PSCs can
574
be seen in SEM cross sectional view (Figure 10b). The typical device micrograph is shown in
575
Figure 10c where a 25 µm thick Au layer was wrapped around as a lead electrode. The
31
576
perovskite and HTL layers were obtained via a dipping solution process while Au electrode of
577
nanoparticle size dimensions was produced by magnetron sputtering as a semi-transparent
578
electrode. Figure 10(d-e) presents the J-V characteristics of the best device with respect to scan
579
direction. After optimization of each layer, the best performing fibre-shaped device yielded
580
η ≈5.35% with JSC ≈12.32 mA.cm-2, VOC ≈0.714 V, and FF ≈60.9%.
581
582
583
584
585
586
Figure 10: (a) Schematic of the fibre-shaped PSC structure. (b) SEM cross-sectional micrograph, (c) Image of
typical device, (d) J-V of the best fibre PSCs, and (e) J-V curve with respect for scan direction (reproduced with
permission from Ref.[51]). Copyright of Royal Society of Chemistry.
587
Deng et al., [131] reported FPSCs with η= ~4.81 – 5.22% with schematics of its
588
preparation shown in Figure 11a. Similarly, another report by Qui et al., [132] reported
589
fabrication of FPSCs using cathodic deposition solution process for perovskite layers with high
590
coverage and uniformity on curved surfaces of Ti wires. The modified Ti wire accompanied by
591
the transparent aligned CNT sheet was used as the two electrodes to produce a coaxial perovskite
592
solar cell fibre, A significantly high η of 7.1% was achieved with VOC ~ 0.85: the highest value
32
593
so far for a fibre-shaped device. Figure 11b shows a schematic of the fabrication and structure of
594
the FPSC device where TiO2 nanotube array engaged as an ETL was grown by an anodization
595
process on the Ti nanowires acting as a substrate.
596
Li et al., [133] presented CNT fibre-supported double-twisted PSCs (Figure 11c)
597
prepared using a homemade heat-assisted coating for setting up a dense perovskite layer. Lee et
598
al., [134] fabricated a fibre-shaped device with η= ~3.85% using silver nanowires as back
599
contact electrode instead of Au, and the schematic of the device is shown in Figure 11d. He et
600
al., [67] employed ZnO array as ETL in fibre-shaped devices, the step by step fabrication process
601
is depicted in Figure 11e. Similarly, Qui et al., [66] reported coaxial fibre-shaped PSCs for the
602
first time with η= ~3.3%. For the fabrication of a typical coaxial fibre-shaped device, a stainless
603
steel fibre with a compact blocking layer was used as an anode followed by deposition of
604
MAPbI3 as shown in Figure 11f. The spiro-OMeTAD layer as HTL was then deposited and
605
finally a CNT sheet was wound as a cathode. The collected data of PV parameters for flexible
606
and FPSCs are enlisted in Table 2.
33
607
608
609
610
611
612
613
614
615
Figure 11: (a) Schematic illustration of FPSC, (a) Fabrication steps and final device structure [131], (b) Schematic
illustration to the fabrication and structure of the perovskite solar cell fiber (PSCF) [132], (c) Structure (PMMA
layer not shown) of each layer in the double-twisted fibrous perovskite solar cell [133], (d) Schematic showing
structure of a fiber-shaped PSC using silver nanowires as the top electrode [134], (d) Schematic illustration showing
the fabrication process (e) Fiber-shaped PSC based on the aligned ZnO nano-obelisks like ETL[67], and (f) Co-axial
fiber-shaped PSCs [66].
616
617
Table 2: Available data in the literature for flexible and fibre PSCs, device architecture, PV parameters JSC (mA.cm2
), VOC (V), FF (%) and η(%) along with perovskite deposition method
Fibre shaped perovskite solar cells
PEN/ITO/c-TiO2/Perovskite/CNT
Deposition
Method
Dip-coating
JSC
(mA/cm2)
15.9
VOC
(V)
0.91
FF
(%)
65.6
η
(%)
9.49
[135]
Ti wire/c-TiO2/TiO2 NTs/perovskite/CNT sheet
Deposition
14.5
0.85
56
7.1
[132]
Ti wire/c&m-TiO2/MAPbI3-xClx/Au
Dip-coating
12.32
0.714
60.9
5.35
[51]
Stainless steel/c&m-TiO2/ MAPbI3/Spior/CNT sheet
dip-coating
10.2
66.4
48.7
3.3
[66]
Ti-polished /TiO2- MAPbI3/spiro-OMeTAD/Ag Nws
Dipping proc
11.87
73.2
37
3.21
[134]
Ti-dimple/TiO2-MAPbI3/spiro-OMeTAD/Ag Nws
Dipping proc
6.97
56.5
29
1.14
[134]
Regular device architecture/PSCs
Ref
618
34
619
PSCs have offered robust application for flexible and wearable electronics however high
620
temperature requirement for mesoscopic TiO2 limits its application as flexible polymers
621
substrates.[136] Efficient PSCs based on mesoporous TiO2, are however reported by adopting
622
low temperature process on a glass substrate that have their own limitations; one being rigid and
623
the other being their inability to twist in 3 D space as they appear in a planar structure making
624
then unsuitable to be integrated for wearable devices, thus demonstrating a lack in softness and
625
curvature.[62, 137-139] While at the same time flexible planar PSC can be made thin but still are
626
hefty and massive and hence cannot be incorporated into wearable electronics’ technology.
627
Hence the need to develop 3D light weight flexible PSC for its application in wearable electronic
628
devices with a highly efficient and low cost fabrication process is necessary. Unlike the
629
conventional DSSC and quantum dots (QD) based SCs, the PSCs layer is thinner and has high
630
surface area to fabricate flexible devices with high η and light absorbing efficiency. Electronic
631
textile (e-textile) have unique features of being light weight, flexible and wearable just like
632
textiles in addition to being warm and snug and hence have evolved in e-apparels leading to its
633
significant commercialization and so smart watches, bands and glasses have seen a tremendous
634
boost. [140]
635
A blend film such as poly(3-hexylthiophene) P3HT: PCBM as an alternative to dye molecules
636
used as the light harvesting layer for fabricating flexible fiber shaped PSC, is reported
637
demonstrating η = 1.23 % which was able to tolerate the strain upto 30%.[141] Further
638
improvement was carried out by using active layer of MAPbI3-xClx PSC layer to improve η. A
639
double twisted PSC warped over pristine CNT fiber with mesoporous and n-TiO2, was
640
sequentially prepared by solution process. [142] P3HT/SWCNT (single walled carbon nanotubes)
641
was used as the HTL on Ag nanowire to increase the contact area. η value of 3.03% with a
35
642
stability of 96 Hr was reported. A mild solution low temperature process to synthesize tunable
643
sized obelisk-like ZnO NPs arrays aligned perpendicularly on the substrate to substitute for TiO2
644
layer is reported for fabrication of highly efficient, light weighted and flexible PSC fiber and
645
fabrics acting as the ETL. [67] The voids in ETL scaffold comprising nano-obelisk ZnO array
646
was infiltrated with perovskite layer and was deposited over with HTL while the CNT was used
647
as back contact electrode. The size of CH3NH3PbI3 was compatible with ZnO scaffold which
648
otherwise may had resulted in poor performance and collapsing of scaffold by a high charge
649
recombination of charges with a small surface area. The ZnO scaffold provided an η=3.8 %
650
higher than PSC based on ZnO nanorods on polymer substrate with planar architecture and
651
offered 3D deformation of fiber shaped PSC with η maintained at 84% with a twisting angle of
652
30∘ and surviving 100 cycles of bending without being damaged. Additionally, cathodic
653
deposition method followed for perovskite layer to form a coaxial perovskite SC fiber with
654
η = 7.1 % was reported. [143] More recently η = 9.49% has been realized by using instead of
655
curved surface that limits fabrication of a dense perovskite layer, a 1D flat substrate to increase
656
the coverage and formation of micrometer sized crystal as the perovskite layer. [144]
657
The breakthrough in η for planar PSCs have led researchers to apply perovskite material as fiber
658
shaped PSCs, however limited by the difficulty to fabricate film on it and also to obtain high
659
purity and quality perovskite film on fiber surface, is a feat in itself. Hence, vapor assisted
660
deposition method for its preparation by applying novel changes to its fabrication process has
661
been employed and has demonstrated a η = 10.79% which is the highest so far reported.
662
However still certain issues including operational and environmental stability and washability
663
needs to be looked into.[145] Most PSCs have been fabricated employing glass or polyethylene
664
terephthalate (PET) substrates with an indium tin oxide (ITO). Recently a report shows for the
36
665
first time that paper has employed as substrate in F-PSCs which deliver η of 2.7%[14]. The
666
architecture of the device is paper/Au/ SnO2/ meso-TiO2/CH3NH3PbI3/Spiro- OMeTAD/MoOx/
667
Au/MoOx.
668
environmentally friendly recyclable substrate made from abundantly available cellulose
669
materials such as plants. Gao et al [145] used transparent nanocellulose paper (NCP) coated with
670
acrylic resin as a substrate in F-PSCs which yielded η 4.25%.
671
4.
As we know that paper is a lightweight, flexible, inexpensive, ubiquitous, and
Stability and durability of Flexible PSCs
672
Stability is one of the crucial aspects when considering the commercial prospects of
673
perovskite solar cells. Having said that, the PSCs are highly susceptible to degradation under the
674
influence of moisture, oxygen, temperature and ultra-violet UV radiation that serves as a
675
detriment for outdoor deployment.[29, 146-148] Though improvement and enhancement of
676
performance has been shown in numerous studies involving flexible PSCs, only few reports
677
discuss the stability of flexible PSCs.[149, 150] In this scenario, material selection is an important
678
criterion for ensuring the durability of flexible PSCs. Integration of hydrophobic materials in the
679
device structure or in the fabrication process significantly contributes to mitigation of moisture
680
degradation.
681
Numerous ways to mitigate the degradation has been explored so far. Recently, Hou et
682
al., developed a flexible PSC based on double hole transport layers of PEDOT:PSS/NiO
683
deposited on the surface of PET/ITO.[90] The resultant devices exhibited high η of 18%. The as-
684
fabricated devices also employed dual additives with 2 wt% DIO and 2 wt% H2O added to the
685
CH3NH3PbI3−xClx solution. The devices also exhibited high stability maintaining over 80%
686
efficiency of η even after 60 days. The H2O/DIO dual additives used in this work contributed to
37
687
the improvement of the interfacial energy of the perovskite film during deposition onto the
688
PEDOT:PSS surface. This resulted in a smooth perovskite film with less impurities, contributing
689
towards high stability and enhanced mechanical durability. The η decreased only by 8.3%
690
compared with the initial efficiency even after 1000 cycles.[90] Najafi et al., demonstrated
691
stability enhancement in flexible PSCs incorporating solution‐derived NiOx and ZnO
692
nanoparticle extraction layers.[151] The triple cation devices with ZnO interlayer retained >95%
693
of the initial performance after its storage for 210 days in the dark under nitrogen atmosphere
694
without encapsulation. The cells also exhibited high stability under continuous light soaking in
695
N2 atmosphere retaining more than 85% of the initial η after 1000 hr .[151]
696
Counter electrode and its material properties play a significant role in determining the
697
rate of degradation and the prospect of the devices’ stability in the long run. Metal electrodes
698
undergo oxidation in the presence of oxygen and result in the formation of halide compounds at
699
the interfaces. For example, in the case of gold (Au) electrode, the redox couple of the iodide
700
system from perovskite reacts readily with the Au resulting in the formation of AuI2- and
701
AuI3.[152] Lee et al., demonstrated the application of Silver nanowire (AgNW)‐based transparent
702
electrodes prepared via an all‐solution‐process as bottom electrodes for flexible PSCs.[153] In
703
their work, the authors deposited a layer of pinhole-free amorphous aluminium doped ZnO (a-
704
AZO) to enhance the chemical stability of the AgNW network. The a‐AZO/AgNW/AZO
705
composite electrode exhibits a transmittance of 88.6% at 550 nm and a sheet resistance of 11.86
706
Ω sq−1 comparable to that of commercial FTO. The PSCs fabricated using this composite
707
electrode a‐AZO/AgNW/AZO exhibited high stability and mechanical integrity retaining 94% of
708
its initial η even after 400 bending cycles with a bending radius of 12.5 mm.[153] Chu et al.,
709
developed a self-healing electrode based on liquid metal microcapsules and polymer composites
38
710
(LMC/polymer) for application in flexible PSCs as shown in Figure 12.[154] The devices
711
demonstrated perfect recovery of the photovoltaic parameters even after cutting the metal
712
contact, exhibiting a η retention of about 99% relative to the initial value (η = 15.07%). Such
713
self-repairing contact could overcome the mechanical durability issues arising out of bending
714
cycles and its negative influence on the device performance could be repressed. The self-healing
715
property of the contact was attributed to the filling of the damaged sites with out-flowing liquid
716
metal from the ruptured capsules. [154]
717
718
719
720
721
722
Figure 12. a) Schematic of a solar‐powered smart watch embedded with self‐healing conductors. The
interconnects are passivated with an electrically self‐healing film. b) Schematic illustrating the healing
mechanism of a metal conductor by a passivation film made of an LMC/polymer composite. Liquid metal
39
723
724
flows out from the ruptured microcapsules and is transported to the damaged site. [154] Copyright Wiley
Publications 2018.
725
726
Ryu et al., synthesized nanocrytsalline Ti-based metal–organic framework (nTi-MOF)
727
particles of about 6 nm in diameter.[155] The nanoparticle layer served as an intermediate layer
728
between the perovskite and the substrate, thereby protecting it from chemical etching as well as
729
facilitating electron transport. The flexible PSCs thus fabricated exhibited η of about 17% and
730
demonstrated high mechanical durability even after 700 bending cycles retaining η at
731
~15.5%.[155] Ligand capped ultrafine SnO2 quantum-dots (QDs) have also demonstrated to
732
serve as a multifunctional interlayer improving the charge transport across the interface as well
733
as enhancing the stability of the flexible PSCs.[156] The device exhibited an η value of 17.7%
734
and retained 92% of the initial η even after 1000 bending cycles for a bending radius of 18
735
mm.[156] It was also reported that the use of Cr/Cr2O3 interlayer as buffer layer between the
736
perovskite and counter electrode reduces the chemical etching of the metal caused by the iodide
737
component disintegrated from the perovskite.[152] Bu et al., demonstrated the efficacy of
738
SnO2 nanocrystals (NCs) as ETLs for flexible PSCs in small devices and modules.[157] Power
739
conversion efficiency (η) of 16.5% was achieved using 120 oC sintered and UVO treated SnO2
740
ETLs in small flexible devices. In flexible modules of size 5 x 5 cm2, η of 12.4% was achieved
741
which retained 80% of its initial performance even after 1000 bending cycles with a bending
742
radius of 25 mm.[157] Zhou et al., reported the synergetic effect of anatase TiO2 NPs and
743
pristine C60 (TiO2/C60) interlayer in enhancing the stability and mechanical durability of flexible
744
PSCs.[158] The TiO2/C60 based flexible devices exhibited η= ~16% while retaining 80% of its
745
initial value even after 1500 cycles. In addition, the devices also demonstrated longer device
40
746
lifetime under light soaking conditions retaining 100% of its original η even after 100 minutes of
747
continuous illumination. The reason behind such an enhancement is attributed to superior charge
748
extraction at the TiO2/C60 interface, reduction of interfacial trap states by fullerene acting as a
749
passivation layer and immobilization of iodine ions (I-). It is also shown to reduce the formation
750
of PbI2 originating from the reaction between the iodine ions of the perovskite and oxygen
751
vacancies of TiO2, preventing thus material degradation. [158]
752
Similarly, Carbon-based counter electrodes exhibit non-oxidative and hydrophobic
753
properties which makes it non-reactive with the perovskite components, and a suitable candidate
754
for developing highly stable devices.[159] Luo et al., reported an all-carbon-electrode based
755
flexible PSCs employing graphene as transparent anode and carbon nanotubes (CNT) as
756
cathode.[159] The devices exhibited maximum η of 11.9% incorporating Spiro-OMeTAD as
757
HTL in conjunction with CNT. The carbon-based flexible PSCs retained 86% of their initial
758
η even after 2000 bending cycles at a curvature radius of 4mm which is exceptionally high
759
compared to conventional flexible PSCs fabricated with transparent ITO/PEN electrode and
760
metal based counter electrode. The all-carbon based electrodes also demonstrated enhanced
761
device lifetime of the flexible PSCs retaining over 90% of the initial η after 1000 hr of
762
continuous illumination in humid air as shown in Figure 13. The authors attributed the enhanced
763
device stability to the hydrophobic surface of CNTs and also to the thickness of CNT-Spiro-
764
OMeTAD layer. The combined effect together lowers the moisture ingression and also improves
765
thermal stress stability of the flexible PSCs.[159]
766
41
767
768
769
770
771
772
773
Figure 13. a) η of the flexible devices upon increasing bending curvature radius after 200 bending cycles.
b) Bending durability of the ITO/PEN‐based and all‐carbon‐electrode‐based flexible solar cells as a
function of bending cycles. c) Efficiency stability of the standard and all‐carbon‐electrode‐based flexible
PSCs as a function of soaking time in different conditions c) in ambient atmosphere under AM 1.5G
illumination in air without UV filter, and d) in ambient atmosphere with constant heating temperature of
60 °C. [159] Copyright Wiley Publications 2018.
774
775
Encapsulation is one of the prominent ways to improve the device lifetime which could facilitate
776
the successful commercialization of flexible PSCs in future. Weerasinghe et. al., demonstrated the
777
encapsulation effects on flexible PSCs.[117] In their work, flexible PSCs based on the
778
configuration (PET/ITO/TiO2/MAPbI3/SM-Au) was studied in three categories namely, non-
779
encapsulated, partially-encapsulated and completely encapsulated PSCs. The plastic encapsulant
780
employed is about 240 µm in thickness with a water vapour transmission rate (WVTR) of 10−3 g
781
m−2day−1 and 89% transparency in the visible light spectrum. The device was sealed with the
42
782
encapsulant on the metal electrode side using acrylic adhesive. The moisture content in the
783
devices are removed by drying in vacuum for 12hr before employing for the stability tests. The η
784
of the fully encapsulated devices retained over 80% of the initial efficiency even after 500 hr
785
while the partially encapsulated devices exhibited significant drop in performance after 400 hr.
786
Non-encapsulated devices deteriorated rapidly even before 100 hr. Investigation of the moisture
787
ingression pathways was carried out by employing metallic calcium (Ca) sensor. It was found
788
that the moisture enters the partially encapsulated devices through the sides or edges while the
789
moisture ingression happens in the fully encapsulated devices through the copper wire contacts
790
provided for electrical connections in the modules.[117]
791
Recent advances in encapsulation technology applied for rigid PSCs could also be
792
extended for flexible devices with appropriate modifications. Cheacharoen et al., reported a
793
comparative study of PSCs packaged between glass and two commonly used encapsulants with
794
different elastic moduli (i.e., Surlyn and EVA).[160] It was found that the devices encapsulated
795
with Surlyn, which has a stiffer ionomer, degraded rapidly with temperature cycling and
796
delaminated. On the other hand, PSCs encapsulated with softer ethylene vinyl acetate (EVA)
797
endured temperature cycling and retained over 90% of their initial performance even after 200
798
temperature cycles. The results indicate that the encapsulant with a low elastic modulus is much
799
suitable for device packaging to ensure mechanical stability and long operating lifetime under
800
high thermal stresses.[160] . Park et al[159] uses Noland Optical Adhesive 63, a shape
801
recoverable polymer as a substrate in PSC. The employed stretchable substrate was quite
802
impressive and prevented mechanical damage of the perovskite layer. Before bending at a
803
radius of 1 mm, an efficiency (η
η) ~ 10.75% and after bending at a similar radius, η ~10.4%
43
804
have been reported. In addition, the shape recoverable device exhibits η= ~6.07% even after
805
crumpling.
806
Hu et al., [88] employed single piezoelectric ZnO microwire as an ETL on a flexible substrate in
807
PSCs. The incorporation of piezo-phototronics microwire can significantly enhance the
808
performance of ZnO/PSCs with η= ~0.0216%. The improved performance is due to the fact that
809
at the vicinity of ZnO/perovskite interface the strain-induced piezoelectric polarization charges
810
can modulate the transport and separation processes of photo-generated carriers within the
811
photovoltaic device. It has been reported that there is a profound effect of the c-axis of the ZnO
812
NW when pointed towards perovskite. The VOC and JSC of the device improved by 25.42% and
813
629.47%, respectively resultantly the η enhanced by 1280% (from 0.0216% to 0.298%) under a
814
compressive strain of 0.8%.
815
816
Progress in flexible module
817
Organic SCs had witnessed a boom in production due to being cheap, having a low energy
818
budget, adopting a solution process and being flexible SCs. This originated in utilizing
819
economical techniques such as spin coating and thermal evaporation for their fabrication and
820
constitute a small area cell[161]. With progress of time, the focus shifted to improving
821
η efficiencies compatible to the supply of global energy. However, the recent focus has reverted
822
back to high throughput production of SCs. The methods used previously were not economical
823
for R2R processing and hence advancements in large area SCs was limited due to these factors
824
where η for large area cell stands at < 3.5%. [162, 163] The focus has been on large area
825
production with emphasis on R2R processing to attain high throughput production of SCs. [164]
44
826
PSC have amassed a lot of interest due to their ease of processing as well as quest for improving
827
low cost technology by favourable combination of highly efficient small lab devices with
828
η above 20% [165, 166] and η (11-16%) for small area modules.[112, 167, 168]
829
Flexible PSCs have theoretically a low cost of fabrication than their counterparts made up of
830
glass substrates, however exhibit low η values for laboratory scale PSCs. [169] Yet they are
831
better than the existing DSSCs and organic photovoltaics and are fast emerging as the state-of-art
832
flexible PSC technology. [170, 171] In future the gap between the glass and plastic PSCs has to
833
be closed.
834
There is ample scope of developing large area modules. [60] Flexible PSCs have been reported
835
as small area devices while large area modules have also been actively followed for their
836
characteristics such as being flexible, light weight and thin leading to upscaling of small area
837
devices to large area modules which is supported by rather a few reports. The following must be
838
considered for upscaling the facility:
839
1. Coating techniques applied to large area PSC modules
840
2. Patterning
841
3. Optimization of the cell
842
4. Interconnection and integration of design of cell [60]
843
Giacomo et al., first showed large area flexible PSCs module by using porous ITO/PET precisely
844
etched through laser patterning over a compact TiO2 layer deposited through atomic layer
845
deposition and a mesoporous n-i-p type TiO2 layer by UV screen printing. [111]The PSC module
846
requires as other thin film technologies, optimization of the cell architecture through laser
847
scribing which has resulted in η=3.1% on a plastic substrate of area 5.6 x 5.6 cm2.[172] Small
45
848
device fabrication is closely mirrored. At the same time, Hwang et al., have reported slot die
849
coating and introduced R2R processing for flexible PSC module fabrication for large area
850
manufacturing, however the module has very low efficiency. [173] Hu et al., have manufactured
851
a PSC module with 6 small scale devices with a η= 12.32% and an active area of 1.01 cm2.
852
However, still the performance of large PSC modules posits a great deal of challenge that needs
853
to be overcome to successfully implement it for large scale production. This is an active area of
854
research which is yet to be fully understood which is far inferior to same design as ITO and FTO
855
based PSC but rigid substrate with an η= 18.84%. [174] Blade coating technique as well as R2R
856
with inverted structure with η < 5% has also been investigated for exploring large area
857
perovskite solar cells.[175, 176] Currently the efficiency for flexible PSCs with single junctions
858
for large area module stands at 11.7% [177] while world’s highest performing PSC large area
859
module is reported in [178] employing 2 step process by adopting bar coating methodology to
860
fabricate the same PSC module.. This shows that small devices are highly efficient and robust
861
but do not give satisfying results when it comes to large modules. PSCs with rigid substrates
862
have been developed into large area module and have displayed superb performance and
863
efficiency with relation to their flexible counterparts. [179, 180] Additionally tandem devices of
864
PSC with Si sub cell or CIGS have demonstrated η =20% [181, 182].
865
866
5.
Conclusion and outlook
867
There has been a soaring interest in perovskites solar cells (PSCs) due to their unique
868
features especially such as wearabilty because of being light weight, portable and deployment for
869
near space and very light weight space applications. PSCs with different architectures have been
46
870
explored and investigated in detail which include, mesoporous, planar (regular and inverted),
871
HTL-free and ETL-free PSCs constructs. Among these, the inverted class of PSCs possessing a
872
p-i-n structure is an excellent choice for flexible devices than the regular planar devices owing to
873
its lower hysteresis and fabrication at a low temperature through an in-situ solution processable
874
route which presents itself as a great prospect to realize its importance for its
875
commercialization. Similarly, modifying the chemistry of perovskites via suitable substitutions
876
or exploring different configurations as well as tailoring various interfacial layers and
877
microstructures have notable effects on the performance and stability of the PSCs devices and
878
have shown to have a positive impact on its efficiencies. Various device designs for PSC by
879
changing the orientation or tailoring the material’s thickness which includes TiO2 with different
880
architectures have been employed. It is very crucial to carefully select materials for PSC from the
881
substrate to the top counter electrode that can guarantee good resistance to deformation of
882
various kinds. Research has been directed in the recent years to investigating ITO free TCE by
883
employing graphene on top of polymer film to overcome challenges associated with using ITO
884
and FTO based transparent conductive electrode. Other studies have shown that ZnO as ETL
885
could be used for flexible PSCs in comparison to its TiO2 ETL counterpart. The ss-IL has also
886
been known for its unique characteristics and used as ETL exhibiting no hysteresis with respect
887
to scan direction. Fibre-shaped solar cells due to their symmetrical structure and adjustable
888
aspect ratio, can be the potential flexible power sources. The importance of flexible fibre-shaped
889
devices had been realized for DSSC and organic solar cells with η ≈8%. FPSCs with η= ~4.81 –
890
5.22% have also been reported.
891
Counter electrode and its material properties play a significant role in determining the
892
rate of degradation and the prospect of the devices’ stability in the long run. Thus protecting
47
893
bottom electrode for flexibles PSCs by employing nanoparticles or ligand capped ultrafine SnO2
894
quantum-dots from chemical etching as well as facilitating the electron transport is incumbent.
895
Similarly, Carbon-based counter electrodes exhibit non-oxidative and hydrophobic properties
896
which makes them non-reactive with the perovskite components, and a suitable candidate for
897
developing highly stable devices. Encapsulation is one of the prominent ways to improve the
898
device lifetime which could facilitate successful commercialization of flexible PSCs in future.
899
Recent advances in encapsulation technology applied for rigid PSCs could also be extended for
900
flexible devices with appropriate modifications. The results indicate that the encapsulant with a
901
low elastic modulus is much suitable for device packaging to ensure mechanical stability and
902
long operating lifetime under high thermal stresses. In the future novel, implementable and
903
rational innovations in PSCs by using R2R are projected.
904
It goes without saying that so far the conductive substrates such as ETL and HTL incur the most
905
cost for fabrication of the flexible PSCs. Hence striving to search for new materials as HTLs,
906
ETL as well perovskites and their combinations seems a plausible solution to advance in the
907
realm of renewable energy. Substrate selection also poses challenges as well, which needs to be
908
investigated especially with relation to flexible substrates. The substrate stability comes to fore
909
when especially polymer substrates are employed which allows ingression of moisture and O2.
910
Additionally lead based perovskite offer promising efficiencies yet is rapidly degraded in
911
presence of moisture and UV light.[181, 183] Lead plays a crucial role in PSC with highest
912
efficiency so far. Lead is however toxic yet the environmental impact seems minimal, [184]
913
however, research is ongoing to substitute lead with suitable nontoxic metals. It is argued that
914
non-lead based perovskites have demonstrated significantly low value of efficiencies as
915
compared to their lead based counterparts and hence challenges associated with lead replacement
48
916
and sustaining a high efficiency are still unresolved. It is noted that the η value for flexible PSCs
917
are far lower as compared to its rigid substrate counterpart due to a high transmittance value
918
which stands at 22% efficiency. While it is also true that studies on optimisation of devices in
919
terms of efficiencies has been more a subject of research rather the stability and degradation
920
studies of PSCs. Hence focus should be on arenas which will fetch the best results in terms of
921
effectively harvesting the solar energy to our best use.
922
Temperature consideration is an important parameter to ponder upon especially for low
923
temperature solution processing of conducing layers that will direct the research towards
924
achieving high performance PSCs on flexible substrates. This along with proper selection of
925
substrate will lead to large scale production of flexible PSCs by employing different solution
926
processing methods that include but not limited to slot dies coating, spray coating, etc., and 3D
927
printing in future. Integrating these flexible PSCs into wearable ingenious gadgetry with n-i-p or
928
inverted p-i-n configuration thus requires an adequate understanding, discovery and selection of
929
materials, techniques for depositing different layers, novel engineering processing under ambient
930
conditions, device architecture, cost analysis and an emphasis on durability and stability. This
931
may lead to replacing the existing materials with further research without compromising the
932
efficiency of the device.[149] This is important if large scale production of PSCs has to be
933
realized that promises outstanding resistance to bending and hence stability. It is however
934
contended that the road leading to manufacturing large scale PSCs devices is paved with
935
challenges that need to be overcome in order to attain high efficiency ,stability and low cost for
936
flexible PSCs modules by adopting large area techniques applicable to all layers just as has been
937
demonstrated for rigid substrate based PSCs module.[180, 185, 186] Thus results achieved for
938
lab scales PSC devices have not shown satisfying results when up-scaled to large modules.
49
939
Despite the achievements with small devices, large modules do not yet show satisfying results. In
940
addition, due to the absence of a large-scale manufacturing process, it is currently difficult to
941
provide a detailed cost analysis for flexible PSCs. It is more appropriate to suggest a more
942
detailed cost analysis of the R2R processing for large scale manufacturing as well as
943
performance and compatibility studies of the different techniques employed to fabricate flexible
944
PSCs in order to present an in-depth analysis of the cost.
945
946
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1. Various sources of instability in flexible PSCs are reviewed.
2. Different substrates employed in F-PSCs were discussed.
3. Roll-to- roll technology have been discussed.
4. Cost of module and small area F-PSCs are compared.
Declaration
The authors Qamar Walia*, Faiza Jan Iftikhara, Naveen Kumar Elumalaib, Yaseen Iqbalc Sidra Yousafc, Shahid
Iqbala, and Rajan Josed declared no conflict of interest.
a
School of Applied Sciences and Humanities, National University of Technology, Islamabad, 44000, Pakistan,
Faculty of Engineering and Science, Curtin University, Sarawak Malaysia, 98009 Miri, Sarawak, Malaysia,
c
Materials Research Laboratory, Department of Physics, University of Peshawar, 25120, Pakistan, dNanostructured
Renewable Energy Materials Laboratory, Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang
26300, Malaysia
b