Dr. Terry Chien-Jen Yang received his PhD in Photovoltaic Engineering (2016) and BEng in Photovoltaic and Solar Energy Engineering with 1st Class Hons. (2011) from UNSW in Sydney, Australia. He joined StranksLab at the University of Cambridge as a new Marie Skłodowska-Curie postdoctoral fellow in July 2021 working on lightweight and flexible all-perovskite triple-junction solar cells. Previously, he worked on perovskite-silicon tandem solar cells as a postdoctoral research fellow at CSIRO Energy in Newcastle, Australia (2019-2021) and as a MSCA fellow / postdoctoral scientist at PV-Lab, EPFL in Switzerland (2017-2019). His research interests include: perovskite, crystalline silicon, and tandem/multijunction solar cells, as well as the materials science of other photovoltaic technologies. Supervisors: Prof. Sam Stranks Phone: +417743581716 Address: University of Cambridge, Philippa Fawcett Dr, Cambridge CB3 0AS
Fast diffusion of charge carriers is crucial for efficient charge collection in perovskite solar ... more Fast diffusion of charge carriers is crucial for efficient charge collection in perovskite solar cells. While lateral transient photoluminescence microscopies have been popularly used to characterize charge diffusion in perovskites, there exists a discrepancy between low diffusion coefficients measured and near-unity charge collection efficiencies achieved in practical solar cells. Here, we reveal hidden microscopic dynamics in halide perovskites through four-dimensional (directions x, y and z and time t) tracking of charge carriers by characterizing out-of-plane diffusion of charge carriers. By combining this approach with confocal microscopy, we discover a strong local heterogeneity of vertical charge diffusivities in a three-dimensional perovskite film, arising from the difference between intragrain and intergrain diffusion. We visualize that most charge carriers are efficiently transported through the direct intragrain pathways or via indirect detours through nearby areas with fast diffusion. The observed anisotropy and heterogeneity of charge carrier diffusion in perovskites rationalize their high performance as shown in real devices. Our work also foresees that further control of polycrystal growth will enable solar cells with micrometres-thick perovskites to achieve both long optical path length and efficient charge collection simultaneously.
accounts for only ≈3% of global electricity generation. [1] However, PV is experiencing an accele... more accounts for only ≈3% of global electricity generation. [1] However, PV is experiencing an accelerated growth globally with >130 GW installed in 2020, an acceleration that should continue in the future to provide 20-30% of the global electricity on the 2050 horizon. [2] The key to materializing this ambitious goal is to reduce the cost of PV-generated electricity to make solar energy significantly cheaper than that produced by fossil fuels, and to promote the implementation of storage technologies. [3] Currently, the major cost component of a PV system stems from the balance-ofsystems (BOS). [4] The BOS refers to all the components of a PV system other than the solar module, including wiring, inverters, land, installation, labor, etc. With cell costs typically accounting for less than 20% of the total module cost (and module costs typically account for around 40% at the system level), [4,5] increasing power conversion efficiency at the cell and module level is the most efficient way to reduce the levelized cost of electricity (LCOE), provided this efficiency gain comes at affordable manufacturing costs. [6] Increasing the solar module efficiency is even more important for residential rooftops, facades, or other applications where
The development of perovskite solar cells (PSCs) as an efficient and cost-effective alternative t... more The development of perovskite solar cells (PSCs) as an efficient and cost-effective alternative to traditional approaches to solar energy transduction has received much recent attention, and there has been considerable progress made with reported power conversion efficiencies now surpassing 25%. This development is encouraging and is a result of intensive research on device design, factors affecting long-term stability of PSCs, and systematic material development in which electron transport layer (ETL) materials play a crucial role to afford high-performance PSC devices. ETL materials, including (6-(1,10phenanthrolin-3-yl)naphthalen-2-yl)diphenylphosphine oxide (Phen-NaDPO), and n-type materials based on the naphthalene diimide (NDI) structure, appear to be amongst the most promising materials to date. This article provides an up-to-date review on organic n-type ETL materials, both polymeric and small molecules, based on NDI format, detailing reports of structures with key relevant parameters, such as the efficiency and stability of PSCs. The review is written from a perspective of organic chemistry and we believe this will provide fundamental advice on the future design of new ETL materials based on NDIs that will afford more efficient and stable PSCs.
Black phosphorus (BP) has emerged as an exciting 2D material for optics, photonics, and electroni... more Black phosphorus (BP) has emerged as an exciting 2D material for optics, photonics, and electronics. However, there are few studies on BP in terms of modulation and enhancement of their electronic and optical properties and their concurrent reactivity and hence reduction after exposure to the ambient environment. To resolve such challenges, creating inorganic–organic hybrid materials is a suitable approach that offers significant opportunities to enhance the utility of BP by combining them with an organic material which has a complementary set of properties. Herein, a hybrid-layered BP material coupled with a low bandgap donor–acceptor organic semiconducting polymer, selenophene-flanked diketopyrrolopyrrole with thienyl-vinylene-thienyl (PDPPSe–TVT) is reported, to broaden the optical absorption and tune the ambipolar field effect transistor characteristics. A highly sensitive, nongated broadband photodetection capability of the hybrid device with a detection range from UV–vis to near-IR (280–1050 nm) and responsivities of up to 4.22 × 103 A W−1 in ambient conditions. In addition to improved photodetection, simultaneous enhancement in both hole (71%) and electron (91%) mobilities is achieved while protecting the sensitive BP material from rapid environmental degradation. The findings therefore report a breakthrough in enhancing the utility of BP as a light-active material for versatile photonics and electronics applications without operating in an inert environment.
The power conversion efficiency (PCE) ofmetal halide perovskite solar
cells (PSCs) has improved d... more The power conversion efficiency (PCE) ofmetal halide perovskite solar cells (PSCs) has improved dramatically from 3.8% to 25.5% in only a decade. Gas quenching is a desirable method for fabricating high-efficiency cells as it does not consume antisolvents and is compatible with large-area depositionmethods such as doctor blading and slot-die coating. To further improve PCEs for gas-quenched PSCs, here, we develop complementary bulk and surface passivation strategies by incorporating potassium iodide (KI) in the perovskite precursor and applying n-hexylammonium bromide (HABr) to the perovskite surface.We show that (1) KI induces a spatial-compositional change, improving grain boundary properties; (2) KI and HABr reduce traps, especially at levels close to the mid-gap; and (3) HABr greatly improves the built-in potential of the device, thereby improving voltage output. The champion device achieves a steady-state PCE of 23.6% with a VOC of 1.23V, which is, to the best of our knowledge, the highest for PSC by gas quenching to date.
Halide perovskite/crystalline silicon (c-Si) tandem solar cells promise power conversion efficien... more Halide perovskite/crystalline silicon (c-Si) tandem solar cells promise power conversion efficiencies beyond the limits of single-junction cells. However, the local light-matter interactions of the perovskite material embedded in this pyramidal multijunction configuration, and the effect on device performance, are not well understood. Here, we characterize the microscale optoelectronic properties of the perovskite semiconductor deposited on different c-Si texturing schemes. We find a strong spatial and spectral dependence of the photoluminescence (PL) on the geometrical surface constructs, which dominates the underlying grain-to-grain PL variation found in halide perovskite films. The PL response is dependent upon the texturing design, with larger pyramids inducing distinct PL spectra for valleys and pyramids, an effect which is mitigated with small pyramids. Further, optimized quasi-Fermi level splittings and PL quantum efficiencies occur when the c-Si large pyramids have had a secondary smoothing etch. Our results suggest that a holistic optimization of the texturing is required to maximize light in-and out-coupling of both absorber layers and there is a fine balance between the optimal geometrical configuration and optoelectronic performance that will guide future device designs.
Transition metal complexes offer cost-effective alternatives as hole-transport materials (HTMs) i... more Transition metal complexes offer cost-effective alternatives as hole-transport materials (HTMs) in perovskite solar cells. However, the devices suffer from low performance. We boost the power conversion efficiency of devices with transition metal complex HTMs from 2% to above 10% through energy level tuning. We further demonstrate the excellent photostability of the device based on the additive-free HTM. International excitement surrounding the prospects of perovskite solar cells shows no signs of abating. Many groups now achieve 420% light-to-electrical power conversion efficiency (PCE) for benchmark laboratory devices, 1 a level suitable for commercial application. A natural shift in research focus on improving the stability and cost of device components is occurring internationally to address these challenges. Indeed, many reports are showing that it is becoming clear that the incumbent hole transport material (HTM) 2,2 0 ,7,7 0-tetrakis[N,N-di-(4-methoxy-phenyl)amino]-9,9 0-spirobifluorene (spiro-OMeTAD) is not suitable for commercial application due to the high cost of production, even at volume, and chemical instability. In practice, the required additives for increased performance of spiro-OMeTAD and related HTMs comes at a cost to stability. Investigation of dopant-free HTMs are therefore an active field for synthetic chemists. 2 Indeed, techno-economic reviews suggest structural simplicity is required of all materials for commercial application to be produced at scale. 3,4 This motivates investigation to identify HTMs with high stability, low cost, and simplified synthetic processing. Originally used as redox mediators in liquid junction dye cells, 5 simple transition metal complexes have begun to receive attention for solid-state charge transport. Frietag et al. 6 seren-dipitously discovered excellent solid-state hole transport for bis Cu(I/II) complexes. Kashif et al. 7 employed a hexadentate Fe(II/III) coordination complex mediator with N-donors based on the heterocycle pyridine mediator to achieve a modest 2% PCE in a perovskite solar cell. Here, the novel wide-bandgap FAPbBr 3 was required for energy matching considerations owing to the deep HOMO level of the iron complex. We reasoned that higher PCEs would be readily attainable with state-of-the-art perovskite formulations, such as Cs 0.05 (FA 0.83 MA 0.17) 0.95 Pb(I 0.85 Br 0.15) 3 , 8 provided that we could adequately match the energy levels. We have previously reported tuning the redox potential of cobalt bipyridine complexes by as much as 420 mV through ligand derivatisation via electron withdrawing/donating groups. 5 We further employ this strategy to optimise the highest occupied molecular orbital (HOMO) level for compatibility with more conventional perovskite formulations. 8 We synthesised simple homoleptic iron(II/III) triflate complexes in Fig. 1a via a coordination reaction with 4,4 0-disubstituted-2,2 0-bipyridine ligands with electron-donating groups (-NH 2 ,-tBu,-OMe,-H). All ligands were commercially-sourced (Fig. S1, ESI †). We evaluated the HOMO level (Fig. S2, ESI †) and electronic transition (Fig. S3, ESI †) of the coordination complexes by photo-electron spectroscopy in air (PESA) and UV-visible spectroscopy, respectively. The energy level diagram constructed in Fig. S4 (ESI †) demonstrates a positive shift of the ionisation potential suitable for all complexes containing electron-donating substituents. A shift of B700 meV is recorded for the substituent with the highest donation strength. We have previously measured this perovskite valence band maximum to be À5.5 eV by PESA, 9 meaning that the hole injection is favourable for the electron-donating ligands. Furthermore, the magnitude of the shift in the
This work investigates halide segregation in methyl-ammonium-free wide bandgap perovskites by pho... more This work investigates halide segregation in methyl-ammonium-free wide bandgap perovskites by photoluminescence quantum yield (PLQY) and advanced electron microscopy techniques. Our study reveals how the formation of nano-emitting low-energy domains embedded in a wide bandgap matrix, located at surfaces and grain boundaries, enables a PLQY up to 25%. Intensity-dependent PLQY measurement and PL excitation spectroscopy revealed efficient charge funnelling and the failure of optical reciprocity between absorption and emission, limiting the use of PLQY data to determine the quasi-Fermi level splitting (QFLS) in these layers. Concomitantly, the small spectral overlap between emission and absorption reduces photon re-absorption. We demonstrate that phase segregation and charge funnelling, although harmful for the radiative efficiency of the mixed phase, are essential for achieving high PLQYs, selectively at low energies, otherwise not achievable in non-segregated perovskites with a similar bandgap. This promotes the applicability of this phenomenon in thermally stable high-efficiency emitting devices and color-conversion heterostructures. T he combination of exceptional optoelectronic properties makes metal halide perovskites highly promising for their application in optoelectronics, such as high-efficiency single junction 1 and multi-junction solar cells 2,3 and light emitting diodes. 4,5 One advantage of these materials is the ability of tuning the optical band gap (E G) through compositional engineering, e.g., by varying the type of the halogen anioncommonly I, Br, and Clon the X site of the perovskite crystal lattice ABX 3. Importantly, it has been noted that these anions intermix rather homogeneously within a certain composition range, 6 allowing for a continuous tuning of the bandgap energy. For example, in the archetypal MAPb-(I 1−x-Br x) 3 system, an E G range of ∼1.6−2.3 eV has been realized by changing the halogen ratio. 7 This phenomenon is of particular interest for multi-junction tandem devices, either in perovskite−Si or perovskite−perovskite device configurations, where an accurate selection of the E G is mandatory to ensure that the sub-cells are current-matched while producing the highest voltage. However, when a certain Br concentration is surpassed, the perovskite material exhibits intrinsic instability under illumination. A frequent problem widely reported in literature is the photo-induced segregation of the originally mixed perovskite phase into I-rich and Br-rich domains. 8−16 Various explanations have been put forward, often based on the different radii and mobilities of the two halide ions. For example, trapping of photogenerated electrons at the surface
In the past decade, the perovskite solar cell (PSC) has attracted tremendous attention thanks to ... more In the past decade, the perovskite solar cell (PSC) has attracted tremendous attention thanks to the substantial efforts in improving the power conversion efficiency from 3.8% to 25.5% for single-junction devices and even perovskite-silicon tandems have reached 29.15%. This is a result of improvement in composition, solvent, interface, and dimensionality engineering. Furthermore, the long-term stability of PSCs has also been significantly improved. Such rapid developments have made PSCs a competitive candidate for next-generation photovoltaics. The electron transport layer (ETL) is one of the most important functional layers in PSCs, due to its crucial role in contributing to the overall performance of devices. This review provides an up-to-date summary of the developments in inorganic electron transport materials (ETMs) for PSCs. The three most prevalent inorganic ETMs (TiO2, SnO2, and ZnO) are examined with a focus on the effects of synthesis and preparation methods, as well as an introduction to their application in tandem devices. The emerging trends in inorganic ETMs used for PSC research are also reviewed. Finally, strategies to optimize the performance of ETL in PSCs, effects the ETL has on J–V hysteresis phenomenon and longterm stability with an outlook on current challenges and further development are discussed.
Bernard Wenger, Henry J Snaith, Isabel H Sörensen, Johannes Ripperger, Samrana Kazim, Shahzada Ah... more Bernard Wenger, Henry J Snaith, Isabel H Sörensen, Johannes Ripperger, Samrana Kazim, Shahzada Ahmad, Edgar R Nandayapa, Christine Boeffel, Silvia Colodrero, Miguel Anaya, Samuel D Stranks, Iván Mora-Seró, Terry Chien-Jen Yang, Matthias Bräuninger, Thorsten Rissom, Tom Aernouts, Maria Hadjipanayi, Vasiliki Paraskeva, George E Georghiou, Alison B Walker, Arnaud Walter, Sylvain Nicolay
With the rapid progress of perovskite photovoltaics (PV), further challenges arise to meet meet the minimum standards required for commercial deployment. Along with the push towards higher efficiencies, we identify a need to improve the quality and uniformity of reported research data and to focus efforts upon understanding and overcoming failures during operation. In this perspective, as a large and representative consortium of researchers active in this field, we discuss which methods require special attention and issue a series of recommendations to improve research practices and reporting.
There has been considerable progress over the last decade in
development of the perovskite solar ... more There has been considerable progress over the last decade in development of the perovskite solar cells (PSCs), with reported performances now surpassing 25.2% power conversion efficiency. Both long-term stability and component costs of PSCs remain to be addressed by the research community, using hole transporting materials (HTMs) such as 2,2′,7,7′-tetrakis(N,N′-di-pmethoxyphenylamino)- 9,9′-spirbiuorene(Spiro-OMeTAD) and poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA). HTMs are essential for highperformance PSC devices. Although effective, these materials require a relatively high degree of doping with additives to improve charge mobility and interlayer/substrate compatibility, introducing doping-induced stability issues with these HTMs, and further, additional costs and experimental complexity associated with using these doped materials. This article reviews dopant-free organic HTMs for PSCs, outlining reports of structures with promising properties toward achieving low-cost, effective, and scalable materials for devices with long-term stability. It summarizes recent literature reports on non-doped, alternative, and more stable HTMs used in PSCs as essential components for high-efficiency cells, categorizing HTMs as reported for different PSC architectures in addition to use of dopant-free small molecular and polymeric HTMs. Finally, an outlook and critical assessment of dopant-free organic HTMs toward commercial application and insight into the development of stable PSC devices is provided.
Partial shading can trigger permanent damage in photovoltaic modules because the illuminated sola... more Partial shading can trigger permanent damage in photovoltaic modules because the illuminated solar cells drive the shaded cells into reverse bias. Under reverse bias conditions, perovskite solar cells have been shown to degrade quickly due to processes that have so far remained elusive. Here, we combine optical, microstructural, and electrical characterization to address the mechanisms governing perovskite solar cell degradation under reverse bias. Three main processes are shown to occur in cesium/formamidinium lead iodide/bromide cells depending on the reverse voltage applied, the duration of the reverse bias treatment and the cell structure. The first and most severe involves the formation of highly conductive shunts, preferentially in regions covered by the metal electrode (with apparently equal propensity for various metals) and, at higher reverse voltages, within the perovskite active area. Second, we find that iodide is reversibly driven into the organic electron transport layer, causing an S-shape in the current-voltage curve and lowering power conversion efficiency. Finally, under heavy reverse biasing the perovskite absorber is shown to degrade into iodide- and bromide-rich sub-layers, an irreversible process associated with a shift in the effective band gap and changes to the perovskite microstructure. Criteria that must be met to pass partial shading tests defined by the International Electrotechnical Commission are also discussed in relation to these issues, indicating the urgent need for device structures far more robust than those usually reported.
Advanced characterization methods avoiding transient effects in combination with solar cell perfo... more Advanced characterization methods avoiding transient effects in combination with solar cell performance monitoring reveal details of reversible light-induced perovskite degradation under vacuum. A clear signature of related deep defects in at least the 1 ppm range is observed by low absorptance photocurrent spectroscopy. An efficiency drop, together with deep defects, appears after minutes-long blue illumination and disappears after 1 h or more in the dark. Systematic comparison of perovskite materials prepared by different methods indicates that this behavior is caused by the lead halide residual phase inherently present in material prepared by the two-step method. X-ray photoelectron spectros-copy confirms that lead halide when illuminated decomposes into metallic lead and mobile iodine, which diffuses into the perovskite phase, likely producing interstitial defects. Single-step preparation, as well as preventing lead halide illumination, eliminates this effect.
One of the key properties of lead-halide perovskites employed in solar cells is the defect tolera... more One of the key properties of lead-halide perovskites employed in solar cells is the defect tolerance of the materials, in particular regarding intrinsic point defects, which mainly form shallow traps. Considering that high luminescence yields and photovoltaic performance are obtained by simple solution processing from commercial chemicals, it is commonly anticipated that the defect tolerance-at least to a considerable degree-extends to grain boundaries and extrinsic defects, i.e. impurities, as well. However, the effect of impurities has hardly been investigated. Here, we intentionally introduce small quantities of bismuth (10 ppm to 2%) in solution to be incorporated in the perovskite films based on mixed cation mixed anion compositions. We observe that Bi impurities in the %-regime reduce charge carrier collection efficiency and, more importantly, that the open-circuit voltage decreases systematically with impurity concentration even in the ppm regime. This strong defect intolerance against Bi impurities comes along with reduced electroluminescence yields and charge carrier lifetimes obtained from transient photoluminescence experiments. Calculations based on molecular dynamics and density functional theory predict delocalized (z0.16 eV) and localized deep (z0.51 eV) trap states dependent on the structural arrangement of the surrounding atoms. Structural characterization supports the idea of Bi being present as a homogeneously spread point defect, which substitutes the Pb 2+ by Bi 3+ as seen from XPS and a reduction of the lattice parameter in XRD. Sensitive measurements of the photocurrent (by FTPS) and surface photovoltage (SPV) confirm the presence of tail states. Photoelectron spectroscopy measurements show evidence of a deep state. These results are consistent with the common idea of shallow traps being responsible for the reduced charge collection efficiency and the decreased fill factor, and deeper traps causing a substantial reduction of the open-circuit voltage. As Bi is only one potential impurity in the precursor salts used in perovskite solar cell fabrication, our findings open-up a research direction focusing on identifying and eliminating impurities that act as recombination centers-a topic that has so far not been fully considered in device optimization studies.
High-bandgap (>1.7 eV) mixed halide perovskites for multijunction solar cells are usually affecte... more High-bandgap (>1.7 eV) mixed halide perovskites for multijunction solar cells are usually affected by photoinducedphase segregation, which triggers sub-bandgap defects that are detrimental to the open-circuit voltage. While this ef-fect may be reversed, e.g., when leaving the cells in the dark, new perovskite compositions that exhibit enhanced stability may be required. In this Perspec-tive, the compositional space beyond the conventional methylammonium- and formamidinium-based mixed halide compounds is reviewed in light of multijunc-tion applications. These alternative absorber compositions include: (1) layered or quasi-2D perovskites, where larger organic cations are incorporated into the structure; (2) inorganic perovskites (i.e., when the organic components are removed altogether); and (3) lead-free structures, where the toxic lead is substituted by one or more elements. The development perspectives of high-efficiency and stable perovskite materials based on these compositions are discussed in view of an integration in multijunction solar cells.
While it has recently been recognized that organic donor-acceptor charge transfer (CT) state ener... more While it has recently been recognized that organic donor-acceptor charge transfer (CT) state ener-gies can vary substantially under different interfacial morphologies, this behavior is under-appreciated in the context of organic singlet fission solar cells where a specific alignment between the triplet state of the fission material and the CT state of the donor-acceptor interface is necessary to the function of the device. In this work, we demonstrate that the CT state energy of a prototypical pentacene-C60 singlet fission system is around 1 eV in most systems, but can vary over 300 meV depending on the composition and morphology of the interface. Moreover, we show that the inclusion of a poly(3-hexylthiophene-2,5-diyl) underlayer, which commonly serves as a triplet blocker and hole collector in pentacene/C60solar cells, helps promote active layer morphologies with stabilized, low energy CT states. These trends in the interfacial energetics are correlated with structural characterization of the films by atomic force microscopy and x-ray diffraction.
Cesium−formamidinium-based mixed-halide perovskite materials with optical band gaps ranging from ... more Cesium−formamidinium-based mixed-halide perovskite materials with optical band gaps ranging from 1.5 to 1.8 eV are investigated by variable-angle spectroscopic ellipsometry. The deter-mined complex refractive indices are shown to depend on the fabrication procedure and environmental conditions during processing. This data is complemented by additional optical and structural characterization, as well as the demonstration of efficient perovskite solar cells. Finally, the data is used in optical simulations to provide guidelines for the optimization of perovskite/silicon tandem solar cells.
Effects of boron and phosphorus doping on the structural, electrical, and optical properties of s... more Effects of boron and phosphorus doping on the structural, electrical, and optical properties of silicon nanocrystals in superlattice thin films were investigated. Silicon nanocrystals were fabricated via magnetron sputtering of stoichiometric silicon rich oxide and silicon dioxide bilayers followed by high temperature annealing at 1100 degrees Celsius. The characterization techniques used include: high-resolution transmission electron microscopy with energy filtering, grazing incidence x-ray diffraction, Raman, photoluminescence, and photothermal deflection spectroscopy, as well as electrical measurements. Results showed that phosphorus doping causes the loss of the bilayer structure and an increase in the average size of the silicon nanocrystals due to softening of the silicon dioxide matrix during post-sputter annealing. The result was a decrease in quantum confinement and a redshift in photoluminescence spectrum with an absorption profile similar to crystalline silicon. The undoped (intrinsic) sample maintained its bilayer structure and displayed stronger quantum confinement with higher photoluminescence peak energy and higher absorption coefficient. In-between, the boron doped sample was more similar structurally to the intrinsic sample, although merging between bilayers resulted in an extensive silicon nanocrystalline network. Optically, it displayed different effects due to photoluminescence quenching and free carrier absorption. Finally, both doped samples exhibited a decrease in electrical resistivity.
Thin films consisting of silicon nanocrystals fabricated by high silicon content in silicon rich ... more Thin films consisting of silicon nanocrystals fabricated by high silicon content in silicon rich oxide show unique properties of decreasing resistivity and increasing light absorption while maintaining quantum confinement effects. With that said, the effect of the annealing temperature and doping element on the microscopic structure of silicon nanocrystals (Si NCs) and the film are still under research. In this study, individual intrinsic, boron-, and phosphorus-doped films are annealed at various temperatures, and their structural properties are analyzed via atom probe tomography together with glancing incidence x-ray diffraction, Raman spectroscopy (Raman), transmission electron microscopy (TEM), and energy filtered TEM. In addition, photoluminescence (PL) is performed and linked with their microstructural properties. The Si NC growth is confirmed at annealing temperatures of 1000 °C and 1100 °C. The microstructure of the Si NCs in the whole film is dramatically changed by increasing the annealing temperature from 1000 °C to 1100 °C. In addition, doping changes the arrangement of the Si NCs by assisting their penetration across the SiO2 barrier layers. This study helps to understand the relationship between the microscopic and macroscopic properties of the Si NC film, showing that the size and distribution of the Si NCs are correlated with the obtained PL profiles.
Fast diffusion of charge carriers is crucial for efficient charge collection in perovskite solar ... more Fast diffusion of charge carriers is crucial for efficient charge collection in perovskite solar cells. While lateral transient photoluminescence microscopies have been popularly used to characterize charge diffusion in perovskites, there exists a discrepancy between low diffusion coefficients measured and near-unity charge collection efficiencies achieved in practical solar cells. Here, we reveal hidden microscopic dynamics in halide perovskites through four-dimensional (directions x, y and z and time t) tracking of charge carriers by characterizing out-of-plane diffusion of charge carriers. By combining this approach with confocal microscopy, we discover a strong local heterogeneity of vertical charge diffusivities in a three-dimensional perovskite film, arising from the difference between intragrain and intergrain diffusion. We visualize that most charge carriers are efficiently transported through the direct intragrain pathways or via indirect detours through nearby areas with fast diffusion. The observed anisotropy and heterogeneity of charge carrier diffusion in perovskites rationalize their high performance as shown in real devices. Our work also foresees that further control of polycrystal growth will enable solar cells with micrometres-thick perovskites to achieve both long optical path length and efficient charge collection simultaneously.
accounts for only ≈3% of global electricity generation. [1] However, PV is experiencing an accele... more accounts for only ≈3% of global electricity generation. [1] However, PV is experiencing an accelerated growth globally with >130 GW installed in 2020, an acceleration that should continue in the future to provide 20-30% of the global electricity on the 2050 horizon. [2] The key to materializing this ambitious goal is to reduce the cost of PV-generated electricity to make solar energy significantly cheaper than that produced by fossil fuels, and to promote the implementation of storage technologies. [3] Currently, the major cost component of a PV system stems from the balance-ofsystems (BOS). [4] The BOS refers to all the components of a PV system other than the solar module, including wiring, inverters, land, installation, labor, etc. With cell costs typically accounting for less than 20% of the total module cost (and module costs typically account for around 40% at the system level), [4,5] increasing power conversion efficiency at the cell and module level is the most efficient way to reduce the levelized cost of electricity (LCOE), provided this efficiency gain comes at affordable manufacturing costs. [6] Increasing the solar module efficiency is even more important for residential rooftops, facades, or other applications where
The development of perovskite solar cells (PSCs) as an efficient and cost-effective alternative t... more The development of perovskite solar cells (PSCs) as an efficient and cost-effective alternative to traditional approaches to solar energy transduction has received much recent attention, and there has been considerable progress made with reported power conversion efficiencies now surpassing 25%. This development is encouraging and is a result of intensive research on device design, factors affecting long-term stability of PSCs, and systematic material development in which electron transport layer (ETL) materials play a crucial role to afford high-performance PSC devices. ETL materials, including (6-(1,10phenanthrolin-3-yl)naphthalen-2-yl)diphenylphosphine oxide (Phen-NaDPO), and n-type materials based on the naphthalene diimide (NDI) structure, appear to be amongst the most promising materials to date. This article provides an up-to-date review on organic n-type ETL materials, both polymeric and small molecules, based on NDI format, detailing reports of structures with key relevant parameters, such as the efficiency and stability of PSCs. The review is written from a perspective of organic chemistry and we believe this will provide fundamental advice on the future design of new ETL materials based on NDIs that will afford more efficient and stable PSCs.
Black phosphorus (BP) has emerged as an exciting 2D material for optics, photonics, and electroni... more Black phosphorus (BP) has emerged as an exciting 2D material for optics, photonics, and electronics. However, there are few studies on BP in terms of modulation and enhancement of their electronic and optical properties and their concurrent reactivity and hence reduction after exposure to the ambient environment. To resolve such challenges, creating inorganic–organic hybrid materials is a suitable approach that offers significant opportunities to enhance the utility of BP by combining them with an organic material which has a complementary set of properties. Herein, a hybrid-layered BP material coupled with a low bandgap donor–acceptor organic semiconducting polymer, selenophene-flanked diketopyrrolopyrrole with thienyl-vinylene-thienyl (PDPPSe–TVT) is reported, to broaden the optical absorption and tune the ambipolar field effect transistor characteristics. A highly sensitive, nongated broadband photodetection capability of the hybrid device with a detection range from UV–vis to near-IR (280–1050 nm) and responsivities of up to 4.22 × 103 A W−1 in ambient conditions. In addition to improved photodetection, simultaneous enhancement in both hole (71%) and electron (91%) mobilities is achieved while protecting the sensitive BP material from rapid environmental degradation. The findings therefore report a breakthrough in enhancing the utility of BP as a light-active material for versatile photonics and electronics applications without operating in an inert environment.
The power conversion efficiency (PCE) ofmetal halide perovskite solar
cells (PSCs) has improved d... more The power conversion efficiency (PCE) ofmetal halide perovskite solar cells (PSCs) has improved dramatically from 3.8% to 25.5% in only a decade. Gas quenching is a desirable method for fabricating high-efficiency cells as it does not consume antisolvents and is compatible with large-area depositionmethods such as doctor blading and slot-die coating. To further improve PCEs for gas-quenched PSCs, here, we develop complementary bulk and surface passivation strategies by incorporating potassium iodide (KI) in the perovskite precursor and applying n-hexylammonium bromide (HABr) to the perovskite surface.We show that (1) KI induces a spatial-compositional change, improving grain boundary properties; (2) KI and HABr reduce traps, especially at levels close to the mid-gap; and (3) HABr greatly improves the built-in potential of the device, thereby improving voltage output. The champion device achieves a steady-state PCE of 23.6% with a VOC of 1.23V, which is, to the best of our knowledge, the highest for PSC by gas quenching to date.
Halide perovskite/crystalline silicon (c-Si) tandem solar cells promise power conversion efficien... more Halide perovskite/crystalline silicon (c-Si) tandem solar cells promise power conversion efficiencies beyond the limits of single-junction cells. However, the local light-matter interactions of the perovskite material embedded in this pyramidal multijunction configuration, and the effect on device performance, are not well understood. Here, we characterize the microscale optoelectronic properties of the perovskite semiconductor deposited on different c-Si texturing schemes. We find a strong spatial and spectral dependence of the photoluminescence (PL) on the geometrical surface constructs, which dominates the underlying grain-to-grain PL variation found in halide perovskite films. The PL response is dependent upon the texturing design, with larger pyramids inducing distinct PL spectra for valleys and pyramids, an effect which is mitigated with small pyramids. Further, optimized quasi-Fermi level splittings and PL quantum efficiencies occur when the c-Si large pyramids have had a secondary smoothing etch. Our results suggest that a holistic optimization of the texturing is required to maximize light in-and out-coupling of both absorber layers and there is a fine balance between the optimal geometrical configuration and optoelectronic performance that will guide future device designs.
Transition metal complexes offer cost-effective alternatives as hole-transport materials (HTMs) i... more Transition metal complexes offer cost-effective alternatives as hole-transport materials (HTMs) in perovskite solar cells. However, the devices suffer from low performance. We boost the power conversion efficiency of devices with transition metal complex HTMs from 2% to above 10% through energy level tuning. We further demonstrate the excellent photostability of the device based on the additive-free HTM. International excitement surrounding the prospects of perovskite solar cells shows no signs of abating. Many groups now achieve 420% light-to-electrical power conversion efficiency (PCE) for benchmark laboratory devices, 1 a level suitable for commercial application. A natural shift in research focus on improving the stability and cost of device components is occurring internationally to address these challenges. Indeed, many reports are showing that it is becoming clear that the incumbent hole transport material (HTM) 2,2 0 ,7,7 0-tetrakis[N,N-di-(4-methoxy-phenyl)amino]-9,9 0-spirobifluorene (spiro-OMeTAD) is not suitable for commercial application due to the high cost of production, even at volume, and chemical instability. In practice, the required additives for increased performance of spiro-OMeTAD and related HTMs comes at a cost to stability. Investigation of dopant-free HTMs are therefore an active field for synthetic chemists. 2 Indeed, techno-economic reviews suggest structural simplicity is required of all materials for commercial application to be produced at scale. 3,4 This motivates investigation to identify HTMs with high stability, low cost, and simplified synthetic processing. Originally used as redox mediators in liquid junction dye cells, 5 simple transition metal complexes have begun to receive attention for solid-state charge transport. Frietag et al. 6 seren-dipitously discovered excellent solid-state hole transport for bis Cu(I/II) complexes. Kashif et al. 7 employed a hexadentate Fe(II/III) coordination complex mediator with N-donors based on the heterocycle pyridine mediator to achieve a modest 2% PCE in a perovskite solar cell. Here, the novel wide-bandgap FAPbBr 3 was required for energy matching considerations owing to the deep HOMO level of the iron complex. We reasoned that higher PCEs would be readily attainable with state-of-the-art perovskite formulations, such as Cs 0.05 (FA 0.83 MA 0.17) 0.95 Pb(I 0.85 Br 0.15) 3 , 8 provided that we could adequately match the energy levels. We have previously reported tuning the redox potential of cobalt bipyridine complexes by as much as 420 mV through ligand derivatisation via electron withdrawing/donating groups. 5 We further employ this strategy to optimise the highest occupied molecular orbital (HOMO) level for compatibility with more conventional perovskite formulations. 8 We synthesised simple homoleptic iron(II/III) triflate complexes in Fig. 1a via a coordination reaction with 4,4 0-disubstituted-2,2 0-bipyridine ligands with electron-donating groups (-NH 2 ,-tBu,-OMe,-H). All ligands were commercially-sourced (Fig. S1, ESI †). We evaluated the HOMO level (Fig. S2, ESI †) and electronic transition (Fig. S3, ESI †) of the coordination complexes by photo-electron spectroscopy in air (PESA) and UV-visible spectroscopy, respectively. The energy level diagram constructed in Fig. S4 (ESI †) demonstrates a positive shift of the ionisation potential suitable for all complexes containing electron-donating substituents. A shift of B700 meV is recorded for the substituent with the highest donation strength. We have previously measured this perovskite valence band maximum to be À5.5 eV by PESA, 9 meaning that the hole injection is favourable for the electron-donating ligands. Furthermore, the magnitude of the shift in the
This work investigates halide segregation in methyl-ammonium-free wide bandgap perovskites by pho... more This work investigates halide segregation in methyl-ammonium-free wide bandgap perovskites by photoluminescence quantum yield (PLQY) and advanced electron microscopy techniques. Our study reveals how the formation of nano-emitting low-energy domains embedded in a wide bandgap matrix, located at surfaces and grain boundaries, enables a PLQY up to 25%. Intensity-dependent PLQY measurement and PL excitation spectroscopy revealed efficient charge funnelling and the failure of optical reciprocity between absorption and emission, limiting the use of PLQY data to determine the quasi-Fermi level splitting (QFLS) in these layers. Concomitantly, the small spectral overlap between emission and absorption reduces photon re-absorption. We demonstrate that phase segregation and charge funnelling, although harmful for the radiative efficiency of the mixed phase, are essential for achieving high PLQYs, selectively at low energies, otherwise not achievable in non-segregated perovskites with a similar bandgap. This promotes the applicability of this phenomenon in thermally stable high-efficiency emitting devices and color-conversion heterostructures. T he combination of exceptional optoelectronic properties makes metal halide perovskites highly promising for their application in optoelectronics, such as high-efficiency single junction 1 and multi-junction solar cells 2,3 and light emitting diodes. 4,5 One advantage of these materials is the ability of tuning the optical band gap (E G) through compositional engineering, e.g., by varying the type of the halogen anioncommonly I, Br, and Clon the X site of the perovskite crystal lattice ABX 3. Importantly, it has been noted that these anions intermix rather homogeneously within a certain composition range, 6 allowing for a continuous tuning of the bandgap energy. For example, in the archetypal MAPb-(I 1−x-Br x) 3 system, an E G range of ∼1.6−2.3 eV has been realized by changing the halogen ratio. 7 This phenomenon is of particular interest for multi-junction tandem devices, either in perovskite−Si or perovskite−perovskite device configurations, where an accurate selection of the E G is mandatory to ensure that the sub-cells are current-matched while producing the highest voltage. However, when a certain Br concentration is surpassed, the perovskite material exhibits intrinsic instability under illumination. A frequent problem widely reported in literature is the photo-induced segregation of the originally mixed perovskite phase into I-rich and Br-rich domains. 8−16 Various explanations have been put forward, often based on the different radii and mobilities of the two halide ions. For example, trapping of photogenerated electrons at the surface
In the past decade, the perovskite solar cell (PSC) has attracted tremendous attention thanks to ... more In the past decade, the perovskite solar cell (PSC) has attracted tremendous attention thanks to the substantial efforts in improving the power conversion efficiency from 3.8% to 25.5% for single-junction devices and even perovskite-silicon tandems have reached 29.15%. This is a result of improvement in composition, solvent, interface, and dimensionality engineering. Furthermore, the long-term stability of PSCs has also been significantly improved. Such rapid developments have made PSCs a competitive candidate for next-generation photovoltaics. The electron transport layer (ETL) is one of the most important functional layers in PSCs, due to its crucial role in contributing to the overall performance of devices. This review provides an up-to-date summary of the developments in inorganic electron transport materials (ETMs) for PSCs. The three most prevalent inorganic ETMs (TiO2, SnO2, and ZnO) are examined with a focus on the effects of synthesis and preparation methods, as well as an introduction to their application in tandem devices. The emerging trends in inorganic ETMs used for PSC research are also reviewed. Finally, strategies to optimize the performance of ETL in PSCs, effects the ETL has on J–V hysteresis phenomenon and longterm stability with an outlook on current challenges and further development are discussed.
Bernard Wenger, Henry J Snaith, Isabel H Sörensen, Johannes Ripperger, Samrana Kazim, Shahzada Ah... more Bernard Wenger, Henry J Snaith, Isabel H Sörensen, Johannes Ripperger, Samrana Kazim, Shahzada Ahmad, Edgar R Nandayapa, Christine Boeffel, Silvia Colodrero, Miguel Anaya, Samuel D Stranks, Iván Mora-Seró, Terry Chien-Jen Yang, Matthias Bräuninger, Thorsten Rissom, Tom Aernouts, Maria Hadjipanayi, Vasiliki Paraskeva, George E Georghiou, Alison B Walker, Arnaud Walter, Sylvain Nicolay
With the rapid progress of perovskite photovoltaics (PV), further challenges arise to meet meet the minimum standards required for commercial deployment. Along with the push towards higher efficiencies, we identify a need to improve the quality and uniformity of reported research data and to focus efforts upon understanding and overcoming failures during operation. In this perspective, as a large and representative consortium of researchers active in this field, we discuss which methods require special attention and issue a series of recommendations to improve research practices and reporting.
There has been considerable progress over the last decade in
development of the perovskite solar ... more There has been considerable progress over the last decade in development of the perovskite solar cells (PSCs), with reported performances now surpassing 25.2% power conversion efficiency. Both long-term stability and component costs of PSCs remain to be addressed by the research community, using hole transporting materials (HTMs) such as 2,2′,7,7′-tetrakis(N,N′-di-pmethoxyphenylamino)- 9,9′-spirbiuorene(Spiro-OMeTAD) and poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA). HTMs are essential for highperformance PSC devices. Although effective, these materials require a relatively high degree of doping with additives to improve charge mobility and interlayer/substrate compatibility, introducing doping-induced stability issues with these HTMs, and further, additional costs and experimental complexity associated with using these doped materials. This article reviews dopant-free organic HTMs for PSCs, outlining reports of structures with promising properties toward achieving low-cost, effective, and scalable materials for devices with long-term stability. It summarizes recent literature reports on non-doped, alternative, and more stable HTMs used in PSCs as essential components for high-efficiency cells, categorizing HTMs as reported for different PSC architectures in addition to use of dopant-free small molecular and polymeric HTMs. Finally, an outlook and critical assessment of dopant-free organic HTMs toward commercial application and insight into the development of stable PSC devices is provided.
Partial shading can trigger permanent damage in photovoltaic modules because the illuminated sola... more Partial shading can trigger permanent damage in photovoltaic modules because the illuminated solar cells drive the shaded cells into reverse bias. Under reverse bias conditions, perovskite solar cells have been shown to degrade quickly due to processes that have so far remained elusive. Here, we combine optical, microstructural, and electrical characterization to address the mechanisms governing perovskite solar cell degradation under reverse bias. Three main processes are shown to occur in cesium/formamidinium lead iodide/bromide cells depending on the reverse voltage applied, the duration of the reverse bias treatment and the cell structure. The first and most severe involves the formation of highly conductive shunts, preferentially in regions covered by the metal electrode (with apparently equal propensity for various metals) and, at higher reverse voltages, within the perovskite active area. Second, we find that iodide is reversibly driven into the organic electron transport layer, causing an S-shape in the current-voltage curve and lowering power conversion efficiency. Finally, under heavy reverse biasing the perovskite absorber is shown to degrade into iodide- and bromide-rich sub-layers, an irreversible process associated with a shift in the effective band gap and changes to the perovskite microstructure. Criteria that must be met to pass partial shading tests defined by the International Electrotechnical Commission are also discussed in relation to these issues, indicating the urgent need for device structures far more robust than those usually reported.
Advanced characterization methods avoiding transient effects in combination with solar cell perfo... more Advanced characterization methods avoiding transient effects in combination with solar cell performance monitoring reveal details of reversible light-induced perovskite degradation under vacuum. A clear signature of related deep defects in at least the 1 ppm range is observed by low absorptance photocurrent spectroscopy. An efficiency drop, together with deep defects, appears after minutes-long blue illumination and disappears after 1 h or more in the dark. Systematic comparison of perovskite materials prepared by different methods indicates that this behavior is caused by the lead halide residual phase inherently present in material prepared by the two-step method. X-ray photoelectron spectros-copy confirms that lead halide when illuminated decomposes into metallic lead and mobile iodine, which diffuses into the perovskite phase, likely producing interstitial defects. Single-step preparation, as well as preventing lead halide illumination, eliminates this effect.
One of the key properties of lead-halide perovskites employed in solar cells is the defect tolera... more One of the key properties of lead-halide perovskites employed in solar cells is the defect tolerance of the materials, in particular regarding intrinsic point defects, which mainly form shallow traps. Considering that high luminescence yields and photovoltaic performance are obtained by simple solution processing from commercial chemicals, it is commonly anticipated that the defect tolerance-at least to a considerable degree-extends to grain boundaries and extrinsic defects, i.e. impurities, as well. However, the effect of impurities has hardly been investigated. Here, we intentionally introduce small quantities of bismuth (10 ppm to 2%) in solution to be incorporated in the perovskite films based on mixed cation mixed anion compositions. We observe that Bi impurities in the %-regime reduce charge carrier collection efficiency and, more importantly, that the open-circuit voltage decreases systematically with impurity concentration even in the ppm regime. This strong defect intolerance against Bi impurities comes along with reduced electroluminescence yields and charge carrier lifetimes obtained from transient photoluminescence experiments. Calculations based on molecular dynamics and density functional theory predict delocalized (z0.16 eV) and localized deep (z0.51 eV) trap states dependent on the structural arrangement of the surrounding atoms. Structural characterization supports the idea of Bi being present as a homogeneously spread point defect, which substitutes the Pb 2+ by Bi 3+ as seen from XPS and a reduction of the lattice parameter in XRD. Sensitive measurements of the photocurrent (by FTPS) and surface photovoltage (SPV) confirm the presence of tail states. Photoelectron spectroscopy measurements show evidence of a deep state. These results are consistent with the common idea of shallow traps being responsible for the reduced charge collection efficiency and the decreased fill factor, and deeper traps causing a substantial reduction of the open-circuit voltage. As Bi is only one potential impurity in the precursor salts used in perovskite solar cell fabrication, our findings open-up a research direction focusing on identifying and eliminating impurities that act as recombination centers-a topic that has so far not been fully considered in device optimization studies.
High-bandgap (>1.7 eV) mixed halide perovskites for multijunction solar cells are usually affecte... more High-bandgap (>1.7 eV) mixed halide perovskites for multijunction solar cells are usually affected by photoinducedphase segregation, which triggers sub-bandgap defects that are detrimental to the open-circuit voltage. While this ef-fect may be reversed, e.g., when leaving the cells in the dark, new perovskite compositions that exhibit enhanced stability may be required. In this Perspec-tive, the compositional space beyond the conventional methylammonium- and formamidinium-based mixed halide compounds is reviewed in light of multijunc-tion applications. These alternative absorber compositions include: (1) layered or quasi-2D perovskites, where larger organic cations are incorporated into the structure; (2) inorganic perovskites (i.e., when the organic components are removed altogether); and (3) lead-free structures, where the toxic lead is substituted by one or more elements. The development perspectives of high-efficiency and stable perovskite materials based on these compositions are discussed in view of an integration in multijunction solar cells.
While it has recently been recognized that organic donor-acceptor charge transfer (CT) state ener... more While it has recently been recognized that organic donor-acceptor charge transfer (CT) state ener-gies can vary substantially under different interfacial morphologies, this behavior is under-appreciated in the context of organic singlet fission solar cells where a specific alignment between the triplet state of the fission material and the CT state of the donor-acceptor interface is necessary to the function of the device. In this work, we demonstrate that the CT state energy of a prototypical pentacene-C60 singlet fission system is around 1 eV in most systems, but can vary over 300 meV depending on the composition and morphology of the interface. Moreover, we show that the inclusion of a poly(3-hexylthiophene-2,5-diyl) underlayer, which commonly serves as a triplet blocker and hole collector in pentacene/C60solar cells, helps promote active layer morphologies with stabilized, low energy CT states. These trends in the interfacial energetics are correlated with structural characterization of the films by atomic force microscopy and x-ray diffraction.
Cesium−formamidinium-based mixed-halide perovskite materials with optical band gaps ranging from ... more Cesium−formamidinium-based mixed-halide perovskite materials with optical band gaps ranging from 1.5 to 1.8 eV are investigated by variable-angle spectroscopic ellipsometry. The deter-mined complex refractive indices are shown to depend on the fabrication procedure and environmental conditions during processing. This data is complemented by additional optical and structural characterization, as well as the demonstration of efficient perovskite solar cells. Finally, the data is used in optical simulations to provide guidelines for the optimization of perovskite/silicon tandem solar cells.
Effects of boron and phosphorus doping on the structural, electrical, and optical properties of s... more Effects of boron and phosphorus doping on the structural, electrical, and optical properties of silicon nanocrystals in superlattice thin films were investigated. Silicon nanocrystals were fabricated via magnetron sputtering of stoichiometric silicon rich oxide and silicon dioxide bilayers followed by high temperature annealing at 1100 degrees Celsius. The characterization techniques used include: high-resolution transmission electron microscopy with energy filtering, grazing incidence x-ray diffraction, Raman, photoluminescence, and photothermal deflection spectroscopy, as well as electrical measurements. Results showed that phosphorus doping causes the loss of the bilayer structure and an increase in the average size of the silicon nanocrystals due to softening of the silicon dioxide matrix during post-sputter annealing. The result was a decrease in quantum confinement and a redshift in photoluminescence spectrum with an absorption profile similar to crystalline silicon. The undoped (intrinsic) sample maintained its bilayer structure and displayed stronger quantum confinement with higher photoluminescence peak energy and higher absorption coefficient. In-between, the boron doped sample was more similar structurally to the intrinsic sample, although merging between bilayers resulted in an extensive silicon nanocrystalline network. Optically, it displayed different effects due to photoluminescence quenching and free carrier absorption. Finally, both doped samples exhibited a decrease in electrical resistivity.
Thin films consisting of silicon nanocrystals fabricated by high silicon content in silicon rich ... more Thin films consisting of silicon nanocrystals fabricated by high silicon content in silicon rich oxide show unique properties of decreasing resistivity and increasing light absorption while maintaining quantum confinement effects. With that said, the effect of the annealing temperature and doping element on the microscopic structure of silicon nanocrystals (Si NCs) and the film are still under research. In this study, individual intrinsic, boron-, and phosphorus-doped films are annealed at various temperatures, and their structural properties are analyzed via atom probe tomography together with glancing incidence x-ray diffraction, Raman spectroscopy (Raman), transmission electron microscopy (TEM), and energy filtered TEM. In addition, photoluminescence (PL) is performed and linked with their microstructural properties. The Si NC growth is confirmed at annealing temperatures of 1000 °C and 1100 °C. The microstructure of the Si NCs in the whole film is dramatically changed by increasing the annealing temperature from 1000 °C to 1100 °C. In addition, doping changes the arrangement of the Si NCs by assisting their penetration across the SiO2 barrier layers. This study helps to understand the relationship between the microscopic and macroscopic properties of the Si NC film, showing that the size and distribution of the Si NCs are correlated with the obtained PL profiles.
Silicon quantum dots (QDs), a subset of Si nanocrystals (NCs), in dielectric matrices with bandga... more Silicon quantum dots (QDs), a subset of Si nanocrystals (NCs), in dielectric matrices with bandgap tunability are promising thin film materials for third generation photovoltaics, which aim to cost effectively exceed the Shockley-Queisser limit of efficiency. The Si QDs investigated in this thesis were fabricated by magnetron sputtered thin film superlattice bilayers of silicon rich oxide (SRO) and silicon dioxide (SiO2) followed by annealing at 1100 °C. The annealing causes solid-state nucleation and subsequent formation of Si NCs in the SRO layers. However, the main issue with this type of Si NC material is its poor carrier transport and material quality. Solar cell devices in the past have experienced heavy current crowding and high series resistance despite their reasonable open-circuit voltages up to 492 mV.
Aluminium doped zinc oxide (AZO) is a promising transparent conducting oxide (TCO) which is often used in thin-film solar cells as transparent contacts. From the literature surveyed, AZO has never been used to make Si NC solar cell devices before. The key advantage of AZO is its high melting point of 1975 °C, which is much higher than other common TCOs. This allows AZO to be annealed at 1100 °C together with the SRO/SiO2 bilayers, although the structural, electrical and optical properties of the AZO thin films change after annealing. The main issue is the heavy decrease in conductivity and cross-diffusion of elements.
For the first time, nucleation of Si NCs in SRO/SiO2 bilayers was observed in real-time via an aberration-corrected high resolution transmission electron microscope with in situ heating up to 600 °C. This particular experiment showed that nucleation of Si NCs begins at an unexpectedly low temperature (450 °C) which suggests that ex situ annealing at 1100 °C may not be necessary. However, through external furnace annealing temperature dependence experiments later it was shown that the higher the annealing temperature, the better the extent of crystallisation of the Si NCs. The advantages of high Si content SRO/SiO2 bilayer superlattices with boron and phosphorus doping were also investigated. It was also shown that AZO forms a good ohmic contact to both the high Si content B and P doped SRO/SiO2 bilayer superlattices. Finally, together with utilising AZO as a transparent contact, a prototype superstrate configured Si NC solar cell device is demonstrated.
With recent technological advances in tandem cell technology and given that the nature of the str... more With recent technological advances in tandem cell technology and given that the nature of the structure of these tandem cells are different to conventional single-junction solar cells, barriers are present in the characterisation of these tandem cells.
The goal of this thesis is to characterise Spectrolab/Boeing’sTM monolithic GaInP/Ga(In)As/Ge triple-junction solar cells with emphasis on spectral response measurements. The method uses the technique of light biasing involving the use of conventional edge-pass filtering of white light. In addition to light biasing another procedure called voltage biasing is required to attain accurate measurements.
Other measurements and investigations conducted on these triple junction tandem cells involve the angular reflectance measurements which are imperative in order to understand the nature of these tandem cells with respect to the angle of incidence of the light. This information is crucial for the optical optimisation of these tandem cells under concentration from reflector optics such as heliostats and Frensel lenses. This topic and spectrum splitting are discussed in more detail in other theses from the 46% efficient cell project.
Silicon quantum dots (QDs), a subset of Si nanocrystals (NCs), in dielectric matrices with bandga... more Silicon quantum dots (QDs), a subset of Si nanocrystals (NCs), in dielectric matrices with bandgap tunability are promising thin film materials for third generation photovoltaics, which aim to cost effectively exceed the Shockley-Queisser limit of efficiency. The Si QDs investigated in this thesis were fabricated by magnetron sputtered thin film superlattice bilayers of silicon rich oxide (SRO) and silicon dioxide (SiO2) followed by annealing at 1100 °C. The annealing causes solid-state nucleation and subsequent formation of Si NCs in the SRO layers. However, the main issue with this type of Si NC material is its poor carrier transport and material quality. Solar cell devices in the past have experienced heavy current crowding and high series resistance despite their reasonable open-circuit voltages up to 492 mV. Aluminium doped zinc oxide (AZO) is a promising transparent conducting oxide (TCO) which is often used in thin-film solar cells as transparent contacts. From the literature surveyed, AZO has never been used to make Si NC solar cell devices before. The key advantage of AZO is its high melting point of 1975 °C, which is much higher than other common TCOs. This allows AZO to be annealed at 1100 °C together with the SRO/SiO2 bilayers, although the structural, electrical and optical properties of the AZO thin films change after annealing. The main issue is the heavy decrease in conductivity and cross-diffusion of elements. For the first time, nucleation of Si NCs in SRO/SiO2 bilayers was observed in real-time via an aberration-corrected high resolution transmission electron microscope with in situ heating up to 600 °C. This particular experiment showed that nucleation of Si NCs begins at an unexpectedly low temperature (450 °C) which suggests that ex situ annealing at 1100 °C may not be necessary. However, through external furnace annealing temperature dependence experiments later it was shown that the higher the annealing temperature, the better the extent of crystallisation of the Si NCs. The advantages of high Si content SRO/SiO2 bilayer superlattices with boron and phosphorus doping were also investigated. It was also shown that AZO forms a good ohmic contact to both the high Si content B and P doped SRO/SiO2 bilayer superlattices. Finally, together with utilising AZO as a transparent contact, a prototype superstrate configured Si NC solar cell device is demonstrated.
Transition metal complexes offer cost-effective alternatives as hole-transport materials (HTMs) i... more Transition metal complexes offer cost-effective alternatives as hole-transport materials (HTMs) in perovskite solar cells. However, the devices suffer from low performance. We boost the power conversion efficiency of devices with transition metal complex HTMs from 2% to above 10% through energy level tuning. We further demonstrate the excellent photostability of the device based on the additive-free HTM.
Solid-state nucleation of Si nanocrystals in a SiO2 bilayered matrix was observed at temperatures... more Solid-state nucleation of Si nanocrystals in a SiO2 bilayered matrix was observed at temperatures as low as 450 °C. This was achieved by aberration corrected high-resolution transmission electron microscopy (HRTEM) with real-time in-situ heating up to 600 °C. This technique is a valuable characterization tool especially with the recent interest in Si nanostructures for light emitting devices, non-volatile memories, and third-generation photovoltaics which all typically require a heating step in their fabrication. The control of size, shape, and distribution of the Si nanocrystals are critical for these applications. This experimental study involves in-situ observation of the nucleation of Si nanocrystals in a SiO2 bilayered matrix fabricated through radio frequency co-sputtering. The results show that the shapes of Si nanocrystals in amorphous SiO2 bilayered matrices are irregular and not spherical, in contrast to many claims in the literature. Furthermore, the Si nanocrystals are w...
With the rapid progress of perovskite photovoltaics (PV), further challenges arise to meet meet t... more With the rapid progress of perovskite photovoltaics (PV), further challenges arise to meet meet the minimum standards required for commercial deployment. Along with the push towards higher efficiencies, we identify a need to improve the quality and uniformity of reported research data and to focus efforts upon understanding and overcoming failures during operation. In this perspective, as a large and representative consortium of researchers active in this field, we discuss which methods require special attention and issue a series of recommendations to improve research practices and reporting.
Black phosphorus (BP) has emerged as an exciting 2D material for optics, photonics, and electroni... more Black phosphorus (BP) has emerged as an exciting 2D material for optics, photonics, and electronics. However, there are few studies on BP in terms of modulation and enhancement of their electronic and optical properties and their concurrent reactivity and hence reduction after exposure to the ambient environment. To resolve such challenges, creating inorganic–organic hybrid materials is a suitable approach that offers significant opportunities to enhance the utility of BP by combining them with an organic material which has a complementary set of properties. Herein, a hybrid-layered BP material coupled with a low bandgap donor–acceptor organic semiconducting polymer, selenophene-flanked diketopyrrolopyrrole with thienyl-vinylene-thienyl (PDPPSe–TVT) is reported, to broaden the optical absorption and tune the ambipolar field effect transistor characteristics. A highly sensitive, nongated broadband photodetection capability of the hybrid device with a detection range from UV–vis to near-IR (280–1050 nm) and responsivities of up to 4.22 × 103 A W−1 in ambient conditions. In addition to improved photodetection, simultaneous enhancement in both hole (71%) and electron (91%) mobilities is achieved while protecting the sensitive BP material from rapid environmental degradation. The findings therefore report a breakthrough in enhancing the utility of BP as a light-active material for versatile photonics and electronics applications without operating in an inert environment.
Summary The power conversion efficiency (PCE) of metal halide perovskite solar cells (PSCs) has i... more Summary The power conversion efficiency (PCE) of metal halide perovskite solar cells (PSCs) has improved dramatically from 3.8% to 25.5% in only a decade. Gas quenching is a desirable method for fabricating high-efficiency cells as it does not consume antisolvents and is compatible with large-area deposition methods such as doctor blading and slot-die coating. To further improve PCEs for gas-quenched PSCs, here, we develop complementary bulk and surface passivation strategies by incorporating potassium iodide (KI) in the perovskite precursor and applying n-hexylammonium bromide (HABr) to the perovskite surface. We show that (1) KI induces a spatial-compositional change, improving grain boundary properties; (2) KI and HABr reduce traps, especially at levels close to the mid-gap; and (3) HABr greatly improves the built-in potential of the device, thereby improving voltage output. The champion device achieves a steady-state PCE of 23.6% with a VOC of 1.23V, which is, to the best of our knowledge, the highest for PSC by gas quenching to date.
In the past decade, the perovskite solar cell (PSC) has attracted tremendous attention thanks to ... more In the past decade, the perovskite solar cell (PSC) has attracted tremendous attention thanks to the substantial efforts in improving the power conversion efficiency from 3.8% to 25.5% for single-junction devices and even perovskite-silicon tandems have reached 29.15%. This is a result of improvement in composition, solvent, interface, and dimensionality engineering. Furthermore, the long-term stability of PSCs has also been significantly improved. Such rapid developments have made PSCs a competitive candidate for next-generation photovoltaics. The electron transport layer (ETL) is one of the most important functional layers in PSCs, due to its crucial role in contributing to the overall performance of devices. This review provides an up-to-date summary of the developments in inorganic electron transport materials (ETMs) for PSCs. The three most prevalent inorganic ETMs (TiO2, SnO2, and ZnO) are examined with a focus on the effects of synthesis and preparation methods, as well as an introduction to their application in tandem devices. The emerging trends in inorganic ETMs used for PSC research are also reviewed. Finally, strategies to optimize the performance of ETL in PSCs, effects the ETL has on J–V hysteresis phenomenon and longterm stability with an outlook on current challenges and further development are discussed.
This work investigates halide segregation in methylammonium-free wide bandgap perovskites by phot... more This work investigates halide segregation in methylammonium-free wide bandgap perovskites by photoluminescence quantum yield (PLQY) and advanced electron microscopy techniques. Our study reveals ho...
The mechanisms controlling the degradation of p–i–n perovskite solar cells in reverse bias are id... more The mechanisms controlling the degradation of p–i–n perovskite solar cells in reverse bias are identified using various optoelectronic and microstructural characterization techniques.
Advanced characterization methods avoiding transient effects in combination with solar cell perfo... more Advanced characterization methods avoiding transient effects in combination with solar cell performance monitoring reveal details of reversible light-induced perovskite degradation under vacuum. A clear signature of related deep defects in at least the 1 ppm range is observed by low absorptance photocurrent spectroscopy. An efficiency drop, together with deep defects, appears after minutes-long blue illumination and disappears after 1 h or more in the dark. Systematic comparison of perovskite materials prepared by different methods indicates that this behavior is caused by the lead halide residual phase inherently present in material prepared by the two-step method. X-ray photoelectron spectroscopy confirms that lead halide when illuminated decomposes into metallic lead and mobile iodine, which diffuses into the perovskite phase, likely producing interstitial defects. Single-step preparation, as well as preventing lead halide illumination, eliminates this effect.
Cesium–formamidinium-based mixed-halide perovskite materials with optical band gaps ranging from ... more Cesium–formamidinium-based mixed-halide perovskite materials with optical band gaps ranging from 1.5 to 1.8 eV are investigated by variable-angle spectroscopic ellipsometry. The determined complex refractive indices are shown to depend on the fabrication procedure and environmental conditions during processing. This data is complemented by additional optical and structural characterization, as well as the demonstration of efficient perovskite solar cells. Finally, the data is used in optical simulations to provide guidelines for the optimization of perovskite/silicon tandem solar cells.
This paper investigated the property evolutions of Mo thin films that were subjected to post-sput... more This paper investigated the property evolutions of Mo thin films that were subjected to post-sputtering heat treatments from 700 °C to 1100 °C. It was found that, after annealing, the use of Si wafers eliminated crack formations found in previously reported Mo thin films sputtered on fused silica substrates. The recrystallization of the Mo thin film was found to start at 900 °C, which led to rearrangements of the preferred crystalline orientation and enhancement of grain size when the annealing temperature was further increased. The electrical conductivity of the Mo thin films was majorly affected by the increase of Mo crystallite size as the annealing temperature was increased. Overall, the improvement of material sustainability and compatibility in the high temperature annealing process has made it positive to implement a Mo-Si contact-substrate scheme for vertical structured Si QDs solar cells.
While it has recently been recognized that organic donor-acceptor charge transfer (CT) state ener... more While it has recently been recognized that organic donor-acceptor charge transfer (CT) state energies can vary substantially under different interfacial morphologies, this behavior is under-appreciated in the context of organic singlet fission solar cells where a specific alignment between the triplet state of the fission material and the CT state of the donor-acceptor interface is necessary to the function of the device. In this work, we demonstrate that the CT state energy of a prototypical pentacene-C60 singlet fission system is around 1 eV in most systems, but can vary over 300 meV depending on the composition and morphology of the interface. Moreover, we show that the inclusion of a poly(3-hexylthiophene-2,5-diyl) underlayer, which commonly serves as a triplet blocker and hole collector in pentacene/C60 solar cells, helps promote active layer morphologies with stabilized, low energy CT states. These trends in the interfacial energetics are correlated with structural characterization of the films by atomic force microscopy and x-ray diffraction.While it has recently been recognized that organic donor-acceptor charge transfer (CT) state energies can vary substantially under different interfacial morphologies, this behavior is under-appreciated in the context of organic singlet fission solar cells where a specific alignment between the triplet state of the fission material and the CT state of the donor-acceptor interface is necessary to the function of the device. In this work, we demonstrate that the CT state energy of a prototypical pentacene-C60 singlet fission system is around 1 eV in most systems, but can vary over 300 meV depending on the composition and morphology of the interface. Moreover, we show that the inclusion of a poly(3-hexylthiophene-2,5-diyl) underlayer, which commonly serves as a triplet blocker and hole collector in pentacene/C60 solar cells, helps promote active layer morphologies with stabilized, low energy CT states. These trends in the interfacial energetics are correlated with structural characterization of the films by a...
Experimental and theoretical study on the effect of shallow and deep defects on photovoltaic perf... more Experimental and theoretical study on the effect of shallow and deep defects on photovoltaic performance, luminescence, surface photovoltage, and density of states.
Effects of boron and phosphorus doping on the structural, electrical, and optical properties of s... more Effects of boron and phosphorus doping on the structural, electrical, and optical properties of silicon nanocrystals in superlattice thin films were investigated. Silicon nanocrystals were fabricated via magnetron sputtering of stoichiometric silicon rich oxide and silicon dioxide bilayers followed by high temperature annealing at 1100 degrees Celsius. The characterization techniques used include: high-resolution transmission electron microscopy with energy filtering, grazing incidence x-ray diffraction, Raman, photoluminescence, and photothermal deflection spectroscopy, as well as electrical measurements. Results showed that phosphorus doping causes the loss of the bilayer structure and an increase in the average size of the silicon nanocrystals due to softening of the silicon dioxide matrix during post-sputter annealing. The result was a decrease in quantum confinement and a redshift in photoluminescence spectrum with an absorption profile similar to crystalline silicon. The undoped (intrinsic) sample maintained its bilayer structure and displayed stronger quantum confinement with higher photoluminescence peak energy and higher absorption coefficient. In-between, the boron doped sample was more similar structurally to the intrinsic sample, although merging between bilayers resulted in an extensive silicon nanocrystalline network. Optically, it displayed different effects due to photoluminescence quenching and free carrier absorption. Finally, both doped samples exhibited a decrease in electrical resistivity.
Uploads
cells (PSCs) has improved dramatically from 3.8% to 25.5% in
only a decade. Gas quenching is a desirable method for fabricating
high-efficiency cells as it does not consume antisolvents and is
compatible with large-area depositionmethods such as doctor blading
and slot-die coating. To further improve PCEs for gas-quenched
PSCs, here, we develop complementary bulk and surface passivation
strategies by incorporating potassium iodide (KI) in the perovskite
precursor and applying n-hexylammonium bromide (HABr) to
the perovskite surface.We show that (1) KI induces a spatial-compositional
change, improving grain boundary properties; (2) KI and
HABr reduce traps, especially at levels close to the mid-gap; and
(3) HABr greatly improves the built-in potential of the device,
thereby improving voltage output. The champion device achieves
a steady-state PCE of 23.6% with a VOC of 1.23V, which is, to the
best of our knowledge, the highest for PSC by gas quenching to
date.
With the rapid progress of perovskite photovoltaics (PV), further challenges arise to meet meet the minimum standards required for commercial deployment. Along with the push towards higher efficiencies, we identify a need to improve the quality and uniformity of reported research data and to focus efforts upon understanding and overcoming failures during operation. In this perspective, as a large and representative consortium of researchers active in this field, we discuss which methods require special attention and issue a series of recommendations to improve research practices and reporting.
development of the perovskite solar cells (PSCs), with reported
performances now surpassing 25.2% power conversion efficiency.
Both long-term stability and component costs of PSCs remain to be
addressed by the research community, using hole transporting materials
(HTMs) such as 2,2′,7,7′-tetrakis(N,N′-di-pmethoxyphenylamino)-
9,9′-spirbiuorene(Spiro-OMeTAD) and poly[bis(4-phenyl)
(2,4,6-trimethylphenyl)amine] (PTAA). HTMs are essential for highperformance
PSC devices. Although effective, these materials require a
relatively high degree of doping with additives to improve charge mobility
and interlayer/substrate compatibility, introducing doping-induced
stability issues with these HTMs, and further, additional costs and
experimental complexity associated with using these doped materials.
This article reviews dopant-free organic HTMs for PSCs, outlining reports
of structures with promising properties toward achieving low-cost,
effective, and scalable materials for devices with long-term stability. It
summarizes recent literature reports on non-doped, alternative, and more
stable HTMs used in PSCs as essential components for high-efficiency
cells, categorizing HTMs as reported for different PSC architectures in
addition to use of dopant-free small molecular and polymeric HTMs.
Finally, an outlook and critical assessment of dopant-free organic HTMs
toward commercial application and insight into the development of
stable PSC devices is provided.
cells (PSCs) has improved dramatically from 3.8% to 25.5% in
only a decade. Gas quenching is a desirable method for fabricating
high-efficiency cells as it does not consume antisolvents and is
compatible with large-area depositionmethods such as doctor blading
and slot-die coating. To further improve PCEs for gas-quenched
PSCs, here, we develop complementary bulk and surface passivation
strategies by incorporating potassium iodide (KI) in the perovskite
precursor and applying n-hexylammonium bromide (HABr) to
the perovskite surface.We show that (1) KI induces a spatial-compositional
change, improving grain boundary properties; (2) KI and
HABr reduce traps, especially at levels close to the mid-gap; and
(3) HABr greatly improves the built-in potential of the device,
thereby improving voltage output. The champion device achieves
a steady-state PCE of 23.6% with a VOC of 1.23V, which is, to the
best of our knowledge, the highest for PSC by gas quenching to
date.
With the rapid progress of perovskite photovoltaics (PV), further challenges arise to meet meet the minimum standards required for commercial deployment. Along with the push towards higher efficiencies, we identify a need to improve the quality and uniformity of reported research data and to focus efforts upon understanding and overcoming failures during operation. In this perspective, as a large and representative consortium of researchers active in this field, we discuss which methods require special attention and issue a series of recommendations to improve research practices and reporting.
development of the perovskite solar cells (PSCs), with reported
performances now surpassing 25.2% power conversion efficiency.
Both long-term stability and component costs of PSCs remain to be
addressed by the research community, using hole transporting materials
(HTMs) such as 2,2′,7,7′-tetrakis(N,N′-di-pmethoxyphenylamino)-
9,9′-spirbiuorene(Spiro-OMeTAD) and poly[bis(4-phenyl)
(2,4,6-trimethylphenyl)amine] (PTAA). HTMs are essential for highperformance
PSC devices. Although effective, these materials require a
relatively high degree of doping with additives to improve charge mobility
and interlayer/substrate compatibility, introducing doping-induced
stability issues with these HTMs, and further, additional costs and
experimental complexity associated with using these doped materials.
This article reviews dopant-free organic HTMs for PSCs, outlining reports
of structures with promising properties toward achieving low-cost,
effective, and scalable materials for devices with long-term stability. It
summarizes recent literature reports on non-doped, alternative, and more
stable HTMs used in PSCs as essential components for high-efficiency
cells, categorizing HTMs as reported for different PSC architectures in
addition to use of dopant-free small molecular and polymeric HTMs.
Finally, an outlook and critical assessment of dopant-free organic HTMs
toward commercial application and insight into the development of
stable PSC devices is provided.
Aluminium doped zinc oxide (AZO) is a promising transparent conducting oxide (TCO) which is often used in thin-film solar cells as transparent contacts. From the literature surveyed, AZO has never been used to make Si NC solar cell devices before. The key advantage of AZO is its high melting point of 1975 °C, which is much higher than other common TCOs. This allows AZO to be annealed at 1100 °C together with the SRO/SiO2 bilayers, although the structural, electrical and optical properties of the AZO thin films change after annealing. The main issue is the heavy decrease in conductivity and cross-diffusion of elements.
For the first time, nucleation of Si NCs in SRO/SiO2 bilayers was observed in real-time via an aberration-corrected high resolution transmission electron microscope with in situ heating up to 600 °C. This particular experiment showed that nucleation of Si NCs begins at an unexpectedly low temperature (450 °C) which suggests that ex situ annealing at 1100 °C may not be necessary. However, through external furnace annealing temperature dependence experiments later it was shown that the higher the annealing temperature, the better the extent of crystallisation of the Si NCs. The advantages of high Si content SRO/SiO2 bilayer superlattices with boron and phosphorus doping were also investigated. It was also shown that AZO forms a good ohmic contact to both the high Si content B and P doped SRO/SiO2 bilayer superlattices. Finally, together with utilising AZO as a transparent contact, a prototype superstrate configured Si NC solar cell device is demonstrated.
The goal of this thesis is to characterise Spectrolab/Boeing’sTM monolithic GaInP/Ga(In)As/Ge triple-junction solar cells with emphasis on spectral response measurements. The method uses the technique of light biasing involving the use of conventional edge-pass filtering of white light. In addition to light biasing another procedure called voltage biasing is required to attain accurate measurements.
Other measurements and investigations conducted on these triple junction tandem cells involve the angular reflectance measurements which are imperative in order to understand the nature of these tandem cells with respect to the angle of incidence of the light. This information is crucial for the optical optimisation of these tandem cells under concentration from reflector optics such as heliostats and Frensel lenses. This topic and spectrum splitting are discussed in more detail in other theses from the 46% efficient cell project.