1) The document discusses the basics of solar photovoltaic technology from the atomic level to full solar arrays. It covers topics like photo absorption and generation, energy levels, band structure, doping, p-n junction formation, and device fabrication for both crystalline silicon and thin film solar cell technologies.
2) The document then discusses generation and recombination processes in solar cells like radiative, non-radiative, Auger and Shockley-Read-Hall recombination and how they impact efficiency. It also covers the continuity equation and transport processes.
3) Finally, the basic one diode equivalent circuit model for solar cells is presented along with the Shockley diode equation and how it relates to the current-
What can Materials Science tell us about Solar Energy of the Future?glyndwruni
The document discusses materials science approaches to improving solar energy technologies. It describes current solar cell designs and their limitations. Thin film technologies offer lower costs but lower efficiencies. Research aims to apply materials science to develop new thin film methods and improve light absorption using nanostructures, down shifting layers, and other photon management techniques. The goal is integrating solar technologies into buildings in innovative ways while further increasing efficiencies and lowering costs to address climate change.
The document summarizes the key components and operation of dye-sensitized solar cells (DSSCs), also known as Grätzel cells. It describes how DSSCs work by using a sensitizing dye to absorb sunlight and generate excited electrons, which are then injected into a titanium dioxide semiconductor and collected via an electrolyte and cathode. The document also discusses research efforts to improve the efficiency of DSSCs beyond their current maximum of 12% by developing new dyes, electrolytes, and plastic hole conductors.
p-i-n Solar Cell Modeling with Graphene as ElectrodeWahiduzzaman Khan
Graphene is a 2-D atomic layer of carbon atoms with unique electronic properties like outstanding carrier mobility, high carrier saturation velocity, excellent thermal conductivity, high mechanical strength, transparency, thinness, and flexibility which make graphene an excellent choice of material for advanced applications in future solar cell design. We modeled a solar cell using graphene as the front electrode to study its performance and compare the performance with that of other possible contenders- indium tin oxide (ITO), widely used material at present and carbon nanotube (CNT), another promising material in this regard. Numerical solutions of the electrostatic and transport equations were obtained using the finite-element method. It was found that solar cell with graphene electrode can outperform the others. We also studied its performance as a function of various parameters. The developed model and obtained results are important for the design of solar cell with graphene as electrode.
Solar Cells: when will they become economically feasibleJeffrey Funk
Solar cell technologies are improving in several key ways:
1) Creating materials that better exploit the photovoltaic effect and having higher efficiencies.
2) Using multiple junctions with different bandgaps to capture more of the solar spectrum.
3) Decreasing costs through larger scale production and reductions in material thickness.
These improvements are driving down the costs of solar electricity and enabling new applications. Further advances could make solar a major electricity source.
The document discusses implementing carbon nanotubes in solar panel technology. It begins with an introduction to solar energy and the current materials used in solar panels. It then provides a history of carbon nanotubes, their advantages and disadvantages, and how they can be used to increase solar panel efficiency. Specifically, carbon nanotubes can increase efficiency up to 80% due to their thermal and electrical conductivity properties. The document concludes that carbon nanotubes are promising for miniaturizing electronics and creating more efficient solar panels.
The Role of Molecular Structure and Conformation in Polymer Opto-Electronicscdtpv
This document discusses the role of molecular structure and conformation in polymer optoelectronics. It summarizes that:
1) The molecular structure and ordering of conjugated polymers influences their optical, electrical, and ordering properties, which impact charge separation and transport in polymer solar cells.
2) Excitons in polymer:fullerene systems can dissociate at donor-acceptor interfaces, but the efficiency depends on factors like acceptor concentration, donor-acceptor distance, and molecular ordering.
3) "Low bandgap" copolymers with stronger electron acceptors can absorb more light and may improve charge separation, but performance also depends on polymer morphology and where polarons are localized within the material
Photosyntheis in a test tube-Dye sensitized solar cells (USPseminar)Atul Raturi
Dye-sensitized solar cells (DSSCs) mimic the process of photosynthesis by using a photosensitive dye to absorb sunlight and a nanocrystalline semiconductor to transport electrons, achieving efficiencies over 10%. DSSCs could provide low-cost solar power for the billions lacking electricity, especially in remote Pacific islands vulnerable to climate change. By emulating natural photosynthesis, DSSCs represent an example of biomimicry and have potential for widespread commercial production of affordable renewable energy.
Acceptor–donor–acceptor small molecules based on derivatives of 3,4-ethylened...Boniface Y. Antwi
Simple EDOT based photo-active molecules have been synthesised by fewer synthetic steps. The molecules separately acted as donor units in organic solar cells fabrications. Best device efficiency was 1.36%.
This document discusses the use of ionic liquids in dye-sensitized solar cells. It begins with an introduction to solar cells and a brief history before explaining the structure and operation of dye-sensitized solar cells. Common components like TiO2 films, dyes, and redox couples are described. The document then notes limitations of traditional liquid electrolytes and proposes ionic liquids as a solution. Several ionic liquids that have been used are listed, along with experimental results showing their effects on performance metrics. The summary concludes by identifying challenges for further developing the use of ionic liquids in dye-sensitized solar cells.
This document provides an overview of dye sensitized solar cells (DSSC). It discusses the principle and working of DSSCs, including the key components - a photosensitive dye, nanostructured semiconductor (typically TiO2), redox electrolyte, and two electrodes. Upon light absorption, electrons are injected from the dye into the semiconductor. The electrolyte regenerates the oxidized dye and transports electrons between the electrodes. The document outlines the preparation, applications, and commercial potential of DSSCs, noting their advantages over silicon solar cells.
The document summarizes research on understanding the physics of degradation in polymer solar cells. Key points include:
1) Polymer solar cells suffer from various degradation factors like oxygen, moisture, and light exposure that reduce efficiency over time.
2) Experiments showed degradation under light exposure in inert atmospheres, with blue photons found to be particularly detrimental. Increased sub-band gap states and reduced mobility were observed.
3) Post-degradation thermal annealing was able to partially recover performance by reducing sub-band gap states, suggesting the active layer was under-annealed initially.
The document discusses nantennas, which are nanoscopic antennas that can convert solar radiation into electricity. Nantennas address many limitations of traditional photovoltaic cells. They work by absorbing electromagnetic waves from solar radiation and thermal earth radiation. This induces an alternating current in the nantenna, which is then rectified into direct current using a diode. Nantennas show promise for applications like self-charging batteries and could be mass produced inexpensively using roll-to-roll manufacturing. Future research aims to improve rectifier efficiency and upscale the technology for widespread use.
This document summarizes a presentation on using carbon nanotubes in solar panel technology. It discusses how carbon nanotubes can improve the efficiency of solar cells compared to traditional organic solar cells. Carbon nanotubes are classified as single-walled or multi-walled nanotubes. Carbon nanotubes and a polymer called MEH-PPV-CN are used as materials in constructing a carbon solar cell. The cell works by generating electrons when exposed to light, which are transferred between energy bands and build up voltage. Adding carbon nanotubes can increase the cell's efficiency by improving light absorption and electron transport. Potential applications include using carbon nanotubes in the photoactive layer or as transparent electrodes.
1) The document describes a Monte Carlo model developed to simulate exciton diffusion in organic solar cells containing different porphyrin compounds.
2) The model simulated the diffusion and decay of excitons in a cube representing the solar cell material. Results showed less aggregation of PCBM molecules and longer exciton lifetimes for the compound TCO4PP compared to TCM4PP.
3) By varying the simulation parameters, the model determined TCO4PP had significantly longer exciton diffusion lengths than TCM4PP, indicating it could enable up to two times higher efficiencies in organic solar cells.
This document discusses the basic principles and measurements of dye solar cells. It begins by introducing dye solar cells and their advantages over other solar cell technologies. It then provides a simplified diagram of a dye solar cell's setup and components. The remainder of the document thoroughly explains the electrochemical processes and energy transfers that occur in a dye solar cell when exposed to light, including excitation of dye molecules, electron injection and transport, redox reactions, and regeneration of the electrolyte. It also defines and discusses important parameters that can be measured from dye solar cell I-V curves, such as short circuit current, open circuit potential, fill factor, series and shunt resistances, maximum power, and efficiency.
This document contains slides from a presentation given by Jarvist Moore Frost from the University of Bath on multiscale physics of photovoltaics. The presentation covers an introduction to solar cells and photovoltaics, the underlying physics from the atomic to device scale, and case studies applying different computational modeling techniques. Key topics discussed include semiconductor device physics concepts like the PN junction and drift-diffusion modeling, as well as thermodynamic limits on solar cell efficiency including the Shockley-Queisser limit. A variety of solar cell materials and technologies are also reviewed from silicon to organic and perovskite cells.
in this ppt it was explained that the importance of dssc and the working principles and the notes during the research work..
the concept was explained in the ppt was very clear......
This document discusses thin-film photovoltaics research and opportunities. It covers several topics:
- Thin-film solar cell technologies like CIGS, CdTe, and emerging materials like CZTS have higher efficiencies than earlier generations and lower production costs. Research aims to further improve efficiency and reduce costs.
- The Helmholtz-Zentrum in Berlin conducts R&D on thin-film photovoltaics including advanced materials, device concepts, and characterization techniques to develop more efficient and cost-effective solar cells.
- Issues like material scarcity for some thin-film technologies are being addressed through research into alternative materials and processes to produce solar cells on flexible substrates using less raw
This document describes a method for coating metallic surfaces with thin films of nano-dimensional carbon to reduce secondary electron emission and suppress multipactor phenomena. Carbon nano-particles 1-10 nm in size are produced using a multispark discharge in ethyl alcohol. Thin films are then deposited on copper plates via two methods: evaporation of a colloidal solution or electrophoresis. Secondary electron emission measurements found that samples coated with films deposited by evaporation or long-time electrophoresis had lower maximum emission and higher first crossover energy compared to uncoated samples, inhibiting multipactor excitation.
The document discusses the solar spectrum and how it relates to photovoltaic devices. It covers how the solar spectrum is affected by factors like the surface temperature of the sun and atmospheric absorption. It also summarizes key concepts in solar cells like black body radiation, the p-n junction, carrier generation and recombination, and how these concepts relate to metrics like open circuit voltage, short circuit current and fill factor. Finally, it discusses approaches to improving solar cell efficiency such as minimizing optical losses, reducing recombination losses, and lowering bulk resistivity.
The document discusses the solar spectrum and how it relates to photovoltaic devices. It covers how the solar spectrum is affected by factors like the surface temperature of the sun and atmospheric absorption. It also summarizes key concepts in solar cells like black body radiation, the photovoltaic effect, and how changing factors like temperature, shunt resistance and spectral response impact performance metrics like open circuit voltage and fill factor.
This document summarizes a research paper on dye sensitized solar cells (DSSCs). It provides background on the development of DSSCs since 1991 and their advantages over traditional silicon solar cells in terms of lower cost and simpler preparation. However, liquid electrolytes used in early DSSCs limited long-term performance. Recent research has focused on improving electrolytes, particularly developing quasi-solid state electrolytes, to enhance photoelectric performance and stability for practical applications of DSSCs. The document reviews progress on quasi-solid state electrolytes and their advantages over liquid electrolytes for DSSCs.
Maskless Nanopattering and Formation of Nanocorrals and Switches for Haloalka...ioneec
This document summarizes research on the self-assembly and stabilization of nanopatterns formed by haloalkane molecules on silicon surfaces. In 3 sentences:
1) The researchers show that propyl bromide molecules self-assemble into circular patterns on a Si(111)-7x7 surface at 50K, and these labile patterns can be stabilized through a "maskless imprinting" process using localized chemical reactions induced by photons or electrons, forming stable circular patterns of atomic bromine.
2) They also find that at room temperature, longer-chain octyl chloride and bromide molecules adsorb horizontally and spontaneously self-assemble into stable "nanocorrals" surrounding surface defects ("type II") or into
Solar energy originates from the sun and can be harnessed using various technologies. It has advantages over fossil fuels in that it is a renewable resource and produces no pollution. However, solar energy is diffuse and intermittent, requiring technologies to concentrate and store it. Current solar technologies include photovoltaics to directly convert sunlight to electricity using semiconductor materials like silicon. Ongoing research focuses on improving efficiency and lowering costs through thin-film cells, multi-junction cells, and new materials like organics and nanotechnology. As costs decrease, solar power is expanding globally in both off-grid and grid-connected applications.
This document provides an overview of solar energy, including its basic concepts, advantages and disadvantages, applications for heating spaces and water, photovoltaics, and the future of solar technology. It describes how solar energy originates from the sun's thermonuclear fusion reactions and can be harnessed through various collection, conversion, and storage methods. The core technologies discussed are solar thermal and photovoltaics, with explanations of how silicon solar cells work using intrinsic and extrinsic semiconductors to generate electricity from sunlight. The document also outlines the development of solar cell technologies from first to third generation and discusses various applications and the top producers globally.
This document discusses new generation silicon solar cells. It begins with an introduction to photovoltaics and the semiconductor properties relevant to solar cells. It then describes the functionality of solar cells including pn junctions, characteristics, and losses. Methods to optimize silicon solar cells are discussed, including surface passivation techniques to reduce recombination at surfaces and contacts. The document concludes that silicon still has potential for high efficiency due to its abundance and the existing manufacturing infrastructure, and that surface passivation is key to improving solar cell performance.
Organic solar cells the exciting interplay of excitons and nano-morphologyvvgk-thalluri
1) The document summarizes organic solar cells, which use a bulk heterojunction of a conjugated polymer donor and fullerene acceptor. When light is absorbed, excitons are formed that must dissociate at the donor-acceptor interface into free charges.
2) The bulk heterojunction morphology, consisting of an interpenetrating network of the donor and acceptor materials, allows more excitons to dissociate since the interface is throughout the volume. This leads to higher efficiencies than simple bilayer cells.
3) Efficiencies of over 6% have been achieved but further work is needed to improve stability and lower costs for organic solar cells to become commercially viable. Optimization of
This document discusses solar energy and its advantages as a renewable resource. It begins by explaining what solar energy is and how it has powered life on Earth for millions of years. The main advantages are that it is a clean energy source without polluting byproducts. Some disadvantages are that the sun does not shine consistently and solar energy must be concentrated to be useful. New technologies discussed include using solar panels to heat buildings and water, generating electricity via photovoltaics, and future technologies like organic and nanostructured solar cells.
A perovskite solar cell is a type of solar cell which includes a perovskite structured compound, most commonly a hybrid organic-inorganic lead or tin halide-based material, as the light-harvesting active layer.
Synthesis and characterisation of k doped zno 1Jeslin Mattam
Thin film technology has been developed for integrated circuits. Thin films are two-dimensional due to their small thickness and need a substrate. They are created through atomic/molecular processes like chemical vapor deposition or physical deposition. Structural properties are analyzed using electron diffraction and electrical properties depend on temperature. Optical measurements determine band structure and lattice vibrations. Applications include drug delivery, optical coatings, transistors, solar cells, and metallurgical coatings. Zinc oxide thin films were synthesized with potassium doping and analyzed. X-ray diffraction showed the films were phase pure wurtzite structure. Resistance decreased and particle size increased with higher doping concentration.
This document summarizes a lecture on thin film deposition techniques given by Dr. Toru Hara. It begins with definitions of thin films and their applications in electronic devices, optical coatings, optoelectronic devices, and quantum devices. It then provides brief introductions to specific applications like transistors, oxygen sensors, and LEDs. The main deposition techniques are also summarized, including chemical methods like plating, CSD, CVD, and ALD, as well as physical methods like thermal evaporation, sputtering, PLD, and MBE. Examples of equipment schematics are provided for many of the techniques.
The document provides an overview of a lecture on thin film deposition techniques given by Dr. Toru Hara. It discusses four main applications of thin films: 1) electronic semiconductor devices using band engineering, 2) optical coatings using refractive index engineering, 3) optoelectronic devices using both band and refractive index engineering, and 4) quantum devices using quantum dynamics design. It also describes common thin film deposition methods including chemical solution deposition, chemical vapor deposition, plating, and physical vapor deposition techniques and gives examples of their use in applications such as transistors, optical coatings, LEDs, and superlattices.
Implantation is a process used to dope semiconductors with impurities by accelerating ions into a solid target material. Ion implantation is advantageous over diffusion due to having no saturation limit. SRIM and TRIM software can be used to simulate ion implantation and predict values like ion range and damage. The thermal spike model describes how the energetic collisions from an ion create a brief high temperature region along its path, resulting in defect formation as the energy diffuses away. Observations from SRIM/TRIM include predicting the ion range, damage events within the target, and energy loss mechanisms during implantation.
This document summarizes the results of a simulation study investigating light absorption in organic solar cells with smooth and pyramidal textured surfaces. The study first optimized a flat organic solar cell structure by simulating the effects of varying each layer's thickness on short-circuit current and quantum efficiency. The optimized flat cell achieved 13.65 mA/cm2 short-circuit current and 84% quantum efficiency at 640 nm. It then proposed a pyramid textured surface to enhance light trapping and absorption compared to a smooth surface. The effects of period and height of the pyramids were analyzed to validate the light trapping model.
Band edge engineering of composite photoanodes for dye sensitized solar cellsvenkatamanthina
This document discusses engineering the band edges of composite photoanodes for dye-sensitized solar cells through doping. Specifically, it doped ZnO nanorods with cobalt to lower its conduction band minimum and doped TiO2 nanoparticles with zirconium to raise its conduction band minimum in order to overcome an energy barrier preventing electron transfer. Characterization with diffuse reflectance spectroscopy and open circuit voltage measurements under illumination confirmed the doping shifted the band edges as intended. However, dye-sensitized solar cells fabricated with the composite nanostructures did not show improved performance. The paper details a methodology for producing and measuring band edge shifts but notes limitations in applying it to improve device operation.
Zr doped TiO2 nanocomposites for dye sensitized solar cellsvenkatamanthina
This document discusses engineering the band edges of a composite photoanode for dye-sensitized solar cells through doping. ZnO nanorods were doped with cobalt to lower their conduction band minimum energy, and TiO2 nanoparticles were doped with zirconium to raise their conduction band minimum energy. This was done to overcome an energy barrier that previously prevented electron transfer from TiO2 to ZnO in the composite. Characterization showed the doping incorporated into the materials as desired without other changes. Open circuit photovoltage measurements indicated the doping shifted the band energies to enable electron transfer, but devices using the materials did not show improved performance. The methodology for producing and measuring band edge shifts through doping is detailed.
R.P.H. Chang presented research on using nanotechnology to develop future solar cells, supercapacitors, and sensors. Three examples from his lab were discussed: (1) hybrid solar cells using materials like SnIx-based perovskites as sensitizers, (2) integrating supercapacitors for energy storage, and (3) plasmonic sensors in the infrared region. Nanotechnology is urgently needed to address global problems through worldwide collaboration and education on technologies like more efficient solar cells and energy storage solutions.
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Lecture Conference Ourzazate ennaoui
1. Solar Photovoltaic Technology from atoms to arrays
Ahmed Ennaoui
Helmholtz-Zentrum Berlin für Materialien und Energie
Science Advisory Board Member of IRESEN - Morocco
E-mail: ennaoui@helmholtz-berlin.de
https://www.helmholtz-berlin.de
The International Renewable and Sustainable Energy Conference(IRSEC'13)
March 7-9 2013, Ouarzazate, Morocco
http://www.iresen.org/index.phpLecture 4 on Friday 09h30 10h15‐
2. Introduction: PV from atom to array
Arrays
Absorbed photon creates 1 electron-hole pair.
The electric field separates the electron-hole pair.
The electrons are collected in the external load.
Generation-Recombination.
Enery levels
Atom
Module
Solar cell
3. Prof. Ahmed Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
Photo absorption and photo generation
Direct and indirect band gap.
External and Internal Quantum Efficiency (EQE and IQE).
Absorption coefficient, absorption length, excess minority carrier.
Recombination: Non Radiative, Radiative, Auger.
Shockley-Read Hall Recombination.
Continuity equation and Transport process.
Basic J-V equation.
Equivalent Circuit model.
Silicon Technology versus. Thin Film technology.
Basic building block for PV: cells in series, cells in parallel.
Change in short circuit current and open-circuit with solar radiation.
Change in short circuit current and open-circuit with the temperature
Performance measurement standard conditions
What we have to learn
6. Task of Photovoltacis: Photo absorption and photo generation
Light = wave λ, and particle with energy E = hν
Albert Einstein
1879 - 1955
Max Planck
1858 - 1947
)(
1239
)(
hc
hE
nm
eVE
λλ
ν =⇒==
)rkexp()rk,()rk,( ⋅= iunnψ
Function with
the periodicity
of the crystal
lattice
Plane wave
)rkexp()rk,()rk,( ⋅= iunnψ
Function with
the periodicity
of the crystal
lattice
Plane wave
Use of Bloch functions
Band structure of Si E(k)
1000 nm 1.239 eV≅ 1.4 eV
Solving Schrödinger
equation
ψψψ ErV
m
=+∇− )(
2
2
0
2
Particle in a box: wave functions and energies
n ; the quantum number (n= 1, 2, 3,....)
L ; the length one dimensional) molecular box
m ; the mass of the particle (electron)
h ; Planck's constant
7. Device fabrication
1. Surface etch, Texturing
2. Doping: p-n junction formation
3. Edge etch: removes the junction at the edge
4. Oxide Etch: removes oxides formed during diffusion
5. Antireflection coating: Silicon nitride layer reduces reflection
Cells
Purifying the silicon:
STEP 1: Metallurgical Grade Silicon (MG-Silicon is produced from SiO2 melted
and taken through a complex series of reactions in a furnace at T = 1500 to
2000°C.
STEP 2: Trichlorosilane (TCS) is created by heating powdered MG-Si at around
300°C in the reactor, Impurities such as Fe, Al and B are removed.
Si + 3HCl SiHCl3 + H2
STEP 3: TCS is distilled to obtain hyper-pure TCS (<1ppba) and then vaporized,
diluted with high-purity hydrogen, and introduced into a deposition reactor to form
polysilicon: SiHCl3 + H2→Si + 3HCl Electronic grade (EG-Si), 1 ppb Impurities
STEP 1
STEPE 2 and 3
Electronic
Grade Chunks
Source: Wacker Chemie AG, Energieverbrauch: etwa 250kWh/kg im TCS-Process, Herstellungspreis von etwa 40-60 €/kg Reinstsilizium
Ingot sliced
to create wafers
Making single
crystal silicon
Czochralski (CZ) process
crucible
Seed crystal slowly grows
Microelectronic
1G: Crystalline Si PV technology
9. External Load +-
Emitter Base Rear Contact
Front Contact
Antireflection coating
Absorption of photon creates an
electron hole pair. If they are
within a diffusion length of the
depletion region the electric field
separates them.
The electron after passing
through the load recombines
with the hole completing the
circuit
n pFront contact
Task of Photovoltacis: Photo absorption and photo generation
1. Light absorption: Generation of free excess
2. Charge separation:
a) Photocurrent, I [A] (Ampere)
b) Photovoltage, V [V] (Volt)
3. Recombintion (defect recombination centers)
V[A] x I[V] = Power [Watt]
Light flux
Valence band
Conduction band
10. ZnO
,2500
Å
CdS700
Å
Mo
0.5-1
µm
Glass, Metal Foil, Plastics
Glass
Cd2
SnO 4
SnO 2
0.2-0.5 µm
CdS
600-2000
Å
CdTe
2-8
µm
CIGS1-2.5µm
C-Paste
with
Cu,
CdTe based device
Quelle: Noufi, NREL, Colorado, USA,
*CIGS based device
CdTe and CIGS Thin Film Solar cells (2G)
11. Glass
Moly rear
contact
CIGS
Buffer
ZnO Front
contact
Technology: monolithic" interconnect from three scribes P1 to P3
P1
Step 1: Deposition of Cu, In,Ga (Se)
(sputtering, codeposition, Electrodeposition)
Step 2: Rapid Thermal Processing (RTP)
Pulsed Picosecond Laser
Front ZnO of one cell is connected to back Mo contatc of the next.
dead-zone width can be up to 500 μm for mechanical scribing.
Se Cu
Ga In
Cu(In,Ga)Se2
P3
P2 P1
P1 periodic scribes to defines the width of the cells.
P2 scribe removes the CIGS
down to the Moly back contact.
P3 scribe can also remove the whole layer stack down
to the Moly
Si
Module
Vmodule= Vcell x Ncell 24 V for battery charging
Quelle: HZB / M. Lux-Steiner
12. Radiative
recombination
EV
EC Augerrecombination
Excessenergygiven
toanothercarrierin
thesameband
EC
EV
Electron thermalizes
to band edge
Ahmed Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
Recombination (r) is the opposite of generation, leading to voltage and current loss.
Non-radiative recombination phonons, lattice vibrations.
Radiative recombination photons (dominating in a direct bandgap materials )
Auger recombination charge carrier may give its energy to the other carrier.
E(eV)
Non-radiative
recombination
EC
EV
Phonon
Recombination processes are characterized by the minority carrier lifetime τ.
Equilibrium: charge distributions np = ni
2
Out of equilibrium: The system tries to restore itself towards equilibrium through R-G
Steady-state rates: deviation from equilibrium
( )npnBgrR
BnnB.pg
.pnBr 2
i2
i00
−=−=
==
=
/scm102B(Si) 315−
×=
Generation vs. recombination processes
13. Summary: Generation & Recombination
Ahmed Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
Shockley-Read Hall
recombination
Direct
recombination
direct band
Auger recombination
(dominant effect at high carrier concentration)
EV
EC
Ekin
Ekin= -qELsc
Generation
Impact ionization is a
generation mechanism.
When the electron hits an
atom, it may break a
covalent bond to generate
an electron-hole pair.
The process continues with the newly
generated electrons, leading to avalanche
generation of electrons and holes.
τ : average time it takes an excess minority carrier to recombine
(1 ns to 1 ms) in Si
τ : depends on the density of metallic impurities and the density
of crystalline defects.
t/teff
τ
( )2
DAugern,DTn .NcBNNcΔn ++=
++∆=++=
AugerDirectSRH
111
n
τττ
RRRR AugerDirectSRH
( ) 1
eff
−
++=⇒ 2
DAugern,DTn .NcBNNcτ
Loss to thermal
vibrations
14. Ahmed Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
Sun
Task of Photovoltacis
• 100 W light bulb is turning on for one hour
• Energy consumed is 100 W·h = 0.1 kW.h.
Production vs. consommation
100 W
light bulb
15. Controller, (charge regulator) regulates the voltage and current
coming from the solar panels Determines whether this power is
needed for home use or whether it will charge a deep-cycle solar
battery to be drawn upon later on.
All other current must pass
through a DC to AC inverter,
transforming it into electricity
usable by general household
appliances.
DC-current from the
controller can be used to run
electronic devices that don't
require an AC-current.
all surplus electricity not being
drawn by your home can be
sent to your utility company's
power grid.
Photovoltaic
P > C
Traditional System
Photovoltaic
P < C
Copyrighted Material, from internet
Task of Photovoltacis
16. Efficiencies beyond the Shockley-Queisser limit
(1) Lattice thermalization loss (> 50%)
(2) Transparency to hν < Band gap
(3) Recombination Loss
(4) Current flow
(5) Contact voltage loss
Not all the energy of absorbed photon can be captured
for productive use (Th. Maxi efficiency ~32% ).
R.R. King; Spectrolab Inc., AVS 54th International Symposium, Seattle 2007
Reflection loss
Recombination
loss
Resistive loss Top contact
loss
Back contact
„Loss“
Good surface passivation.
Antireflection coatings.
Low metal coverage of the top surface.
Light trapping or thick material
(but not thicker than diffusion length).
High diffusion length in the material.
Junction depth optimized for absorption
in emitter and base.
Low reflection by texturing
17. Route to high efficiency solar cells
Traditional cell design PERLPERCIBCPESCMINP
(1) (2) (3)
(1) PERL developed at UNSW (EFF. 25%) Passivated Emitter and Rear Locally diffused1
(2) Localized Emitter Cell Using Semiconducting Fingers. (EFF. 18.6%, CZ n-type)
(3) Laser-grooved, buried front contact (LGBC; EFF. 21.1%)
1
Martin Green, PIP 2009; 17:183–189, University of New South Wales, Australia
Copyrighted Material, from internet
18. Ahmed Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
We need to use most of the solar spectrum: Tandem solar cells
Power [Watt/cm2
] = Voltage [Volt ] x Current density [A/cm2
]
Materials with small Band gap
But low voltage
Excess energy lost to heat
Generating a large current (JSC)
Materials with large band gap
But low current
Sub-band gap light is lost
Generating a large voltage (VOC)
Solar cell
versus
Solar spectrum
19. = (in flow – out flow) + Rain -
Evaporation
rain
In flow
Out flow
Evaporation
Rate of
increase of
water level
in lake r-g.J
q
1
nnn +∇=
dt
dn
nnnn
nnn
qDEqnμJ
r-g.J
q
1
t
n
∇+=
+∇=
∂
∂
Ahmed Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
A little bit of Math: Continuity equation and Transport process
t
n
0=
∂
∂
20. Voc
0 La= 1/αLp
W
Rec
ΕF,n=µe
ΕF,p=µh
WLn
La= 1/α
Rec
ΕF,p=µh
ΕF,n=µe
Generated closer to the junction
Generated within a diffusion length of the junction
Key issues:
Minority carrier diffusion
Surface recombination
Collection near front surface and also rear
conditionsBondaryGτ
L
x
Bexp
L
x
AexpΔn(x) n
nn
←+
+
+
−
=
t
n
0=
∂
∂
Differential equation is simple only when G = constant.
n
2
n
2
2
D
x)G(λ(
L
Δn
dx
Δnd
−=
p
2
p
2
2
D
x)G(λ(
L
Δp
dx
Δpd
−=
Ahmed Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
Basic: Continuity equation and Transport process
21. Basic Diode J-V equation
NL
nD
NL
nD
qJ
Dp
2
ip
An
2
in
0
+=
+ JD
)( 1
Tk
qV
expp
L
D
qn
L
D
qJ
B
n,0
p
p
p,0
n
n
−
+=
0J
L
Jcurrent,Dark
Tn.k
qV
0 J1expJJ
D
B
−
−=
- JL
W)LqG(L pn ++−
LJntPhotocurre
Applying boundary conditions (ideal diode case)
Differentiating to find the current
Equating the currents on the n-type and p-type sides
J0 : saturation current
kB : Boltzmann`s constant, 1.381 10-23
J/Kelvin
n : ideality factor
ni: carrier concentration
NA,ND. Doping concentration
dx
pd
qDJ n
pp
∆
=
dx
nd
qDJ
p
nn
∆
=
Ahmed Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
22. One diode model / Equivalent Circuit
RLoad
J
VD
JD
Ideal diode (dark current , ID)
(Shockley diode equation)
−= 1exp0
nkT
qV
JJ D
D
SD RJVV .+=
add a serie resistance RS
jsh . Rsh
Current
loss
R
J.RV
J-
Sh
S
L
+
+
−
−
= 1
nkT
)R.JV(q
expJJ S
0
Add a shunt resistance
Sshsh RJVRi .. +=
JL
L
S
J
nkT
RJVq
JJ −
−
−
= 1
).(
exp0
Under illumination
VOC
JSC
- JL
4TH
Quadrant
J = I/A
VReverse
Forward
0
Solar cell in the dark
−
−
= 1
).(
exp0
nkT
RJVq
JJ S
D
+=
Dp
ip
An
in
NL
nD
NL
nD
qJ
22
0
J. RS
(Voltage drop)
V
Dark characteristics being shifted down by
photocurrent which depend on light
intensity.
P
N
Slope -1/RLoad
Photogenerated carriers can also flow through the crystal
surfaces or grain boundaries in polycrystalline devices
23. Two diodes model / Equivalent Circuit
R
J.RV
Sh
S+
+
−
−
+
−
−
= 1
).(
exp1
).(
exp
2
02
1
01
kTn
RJVq
J
kTn
RJVq
JJ SS
RLoad
J
+
-
RS
V
J01
,n1
J02
,n2
Rsh
JL
LJ-
R
J.RV
Sh
S+
−
−
−
−
−
−
−= 1
).(
exp1
).(
exp
2
02
1
01
kTn
RJVq
J
kTn
RJVq
JJJ SS
L
1st
Quadrant
4th
Quadrant
1st
Quadrant
4th
Quadrant
J
V
A. Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
24. Photocurrent analysis: Quantum efficiency measurments
Acceptor
Voc
x = 0 La= 1/α
x = Ln
x = W
EJ σ=
dx
dp
Dp
Donor
Rec
µh
µeE
→
p∇
→
Load
• How much light converted?
• Limited information on the electronic properties
• Information on the optical properties of the device
)(R λ−
=
1
EQE
IQE
λ
hc
e
J
Φ
EQE
)(
)(
1 λ
λ
=
This ratio can be measured
Ahmed Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
Φ 0
x R λ
Φ
0
[ ]
[ ]Joulehν
Watt/cmΦ
N
2
photons
in =
[ ]
[ ]Coulombe
A/cmJ
N
2
electrons
out =
Electrons
collected
Photons
absorbed
Φ 0
x Rλ
h(c/λ) < EG
x
0 ).eR.(1ΦΦ α
λ
−
−=
Φ
0
25. EQE and and absorption coefficient
Photon absorption
direct band-gap
( ) GG
2
1
E)E(hνvs..hν →−α
2
G )E(h
hν
B
−= να
Direct Bandgap Eg
EC
EV
Photon
Conduction
Band
Valence
Band
E(k)
GaAse.g.
+k-k
Photon absorption
indirect band-gap
( ) GG
2
E)E(hvs..h →−ννα
2
1
G )E(h
h
A
−= ν
ν
α
Photon
+k-k
Eg
EC
EV
Conduction
Band
Valence
Band
Phonon
EG+Ep
Ep
E(k)
Sie.g.
Ahmed Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie and IRESEN
Cut-off λ vs. EG
[eV]E
1.24
m][μλ
G
G =
∫Φ=
λ
λλλ dEQEqJsc )()(
λ
hc
e
J
Φ
EQE
)(
)(
1 λ
λ
=
26. hν
Band Gap - absorption coefficient - absorption length
Temperature changes:
EG ↑ as T ↓
Changing the absorption edge
Ahmed Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
A(T)(0)E(T)E gG −=
Si Ge GaAs
EG (eV) 1.12 0.66 1.42
λλλ ++= TAR100%
)R.(1Φ
Φ
ln.
d
1
α
λ0 −
−=λ
αx-
o R).α).-(E).(1Φ
dx
dΦ
x)G(E, =−=
Absorption
x
0 ).eR.(1ΦΦ α
λ
−
−=
Generation
Φ
ΦΦΦΦ TAR0 ++=
27. Quantum efficiency measurements
2 – Cell Measurement
2CELL
CELL
sc .Φq.EQEJ =
2MON
MON,2
sc .aΦq.EQEJ =
.a.EQE
J
J
.aEQE MONMON,2
sc
CELL
sc
CELL =
3 – Final Result
REFREF
sc
MON,1
sc
MON,2
sc
CELL
sc
CELL EQE
J
J
.
J
J
EQE =
Monochromator
equipped with more gratings*Chopper
Beam splitter
*Gratings should have line density as high as possible for achieving high resolution and high
power throughput. (600 – 3000 lines/mm).
EG
EQE vs. λ
1REF
REF
sc .Φq.EQEJ =
1MON
MON,1
sc .aΦq.EQEJ =
1
MON,1
sc
MON
qΦ
J
.aEQE =
1 - Reference measurement
photon1ofrgyphoton/eneofpowerTotal
electron1ofargecurrent/ch
EQE""EfficiencyQuantumExternal =
Ahmed Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
28. Design to high efficiency solar cells
Light trapping
Reflection Loss: ARC
Material Parameter absorption
Important cost factor €/kg
+
−−= −αW
p
e
αL1
1
1R)(1η
λ
hc
e
)J(
Φ(λ)
1 λ
η =
Decisive Material Parameter
The band gap
0.3 0.5 0.7 0.9 1.1
20
0
40
60
80
100
0
1
2
3
4
5
NumberofSunlightPhotons(m-2
s-1
micron-1
)E+19
RExternalQuantumEfficiency,%
µc-Si:H junctiona-Si:H junction
AM 1.5 global spectrum
Wavelength, microns
a-Si:H/µc-Si:H Cell Spectral Response
Textured TCO
a-Si
Top cell
Back Reflector
Glass substrate
Thin film mc-Si
Bottom cell
[ ]∫ λ−=
GE
λ0λsc dλ.dα-exp.)().ΦR(1.η(λ).qJ
Light from the sun
Ahmed Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie and IRESEN
29. Power output characteristics
Jsc VOC
Pmax
Sun
OCSC
P
.FF.VJ
EFF. =
Vmpp
Pmpp= Impp x Vmpp
OCSC
mppmpp
.VJ
.VJ
Inverse of slope Vmpp/Impp
is characteristic resistance
Jmpp mmp
Rmpp
V
J
mpp = Maximum Power Point
P=I.V
Fill Factor
OCSC
mppmpp
.VJ
VxJ
Sun
mppmpp
P
V.J
EFF =
A. Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
30. Solar cell efficiency under simulated sun light
Earth´ s Surface
AM1
AM0
AM1.5
d=1.5 atmos d=1 atmos
Challenges
To simulate a spectrum as similar as possible to the sun spectrum with excellent
homogeneity over relatively large areas
A. Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
31. Principle of a sun simulator
The unit of the photon flux
A. Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
Reference cell
solar cell
Sources: Thomas FU-Berlin
32. Contact
grid
Total
Area
Including
grid
Iluminated
Area (2)
JSC is rather accurately determined by EQE measurements
0.5 cm
1 cm
Iluminated
Area (1)
0.5 cm
1 cm
∫Φ=
λ
λλλ dEQEq )()(J 0sc
From
monochromator
Performance measurement standard conditions
A. Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
(1) effective area or
(2) total area
33. A. Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
Photons to electrons, solar power to electrical power
You need a computer for this exercise
Physical constants: elementary charge, e = 1.60 x 10-19
C
Planck’s constant, = 6.63 × 10-34
J s
speed of light, c = 3.00 x 108
m s-1
Exercice: An ideal solar cell has a band gap energy . The solar cell absorbs 100% of photons with energy
and 0% of photons with energy . All absorbed photons are converted to current with 100% quantum efficiency.
The solar cell has a fill factor of 70% and an active area of 1 cm2. The external quantum efficiency (EQE)
spectrum and current-voltage (I-V) curves are sketched below:
0
20
40
60
80
100
120
Eg
Current
Voltage V
OC
I
SC
V
mpp
EQE(%)
a) The international standard AM1.5 solar spectrum is provided in the text file “Solar spectral irradiance.txt”
(from NREL.gov). Use it to calculate the short circuit current, ISC
, for the ideal solar cell made from:
Crystalline silicon Si, EG = 1.1 eV; Germanium Ge, EG = 0.67 eV ; Gallium arsenide GaAs EG = 1.42 eV
Amorphous Si, EG = 1.75 eV.
b) If the open-circuit voltage is given by Voc = EG/e, what is the maximum power conversion efficiency of
each of the four cells? (The total terrestrial irradiance is 1000 W m-2
.).
c) What is the optimum band gap for an ideal solar cell?
0
0.5
1
1.5
2
2.5
0 500 1000 1500 2000
Spectralirradiance(Wm-2nm-1)
Solar spectral irradiance
Extraterrestria
lTerrestrial
34. From Cells to a Module
The basic building block for PV applications is a module consisting of a number
of pre-wired cells in series.
Typical module Silicon technolog/ 36 cells in series referred to as 12V.
Large 72-cell modules are now quite common.
Multiple modules can be wired in series to increase voltage
and in parallel to increase current.
Such combinations of modules are referred to as an array
Cells wired in series
35. From Cells to a Module
0.6 V each cell
N°1
N° 36
4 cells
4 x 0.6V
36 x
36 x 0.6V = 21.6 V
Adding cells in series
Vmodule = n (Vd – I.RS)
Series resistance RS
Cell 1 Cell 2 Cell 36
. . . . .
+ - + - + -
36. From Cells to a Module
A parallel association of n cells is possible and enhances the output current of the
generator created.
In a group of identical cells connected in parallel, the cells are subjected to the same
voltage and the the resulting group is obtained by adding currents
VSC,n
Cell n
n Cells
Cell 1
n Cells
in parallele
n x ISC
ISC,n
37. From Module to array
For modules in series, the I –V curves are simply added along the voltage axis at any given
current which flows through each of the modules), the total voltage is just the sum of the
individual module voltages.
38. For modules in parallel, the same voltage is across each module and the total
current is the sum of the currents at any given voltage, the I –V curve of the
parallel combination is just the sum of the individual module currents at that
voltage.
From Module to array
39. Two ways to wire an array with three modules in series and two modules in parallel.
The series modules may be wired as
strings, and the strings wired in parallel.
The parallel modules may be wired together
first and those units combined in series
V V
If an entire string is removed from service
for some reason, the array can still
deliver whatever voltage is needed by the
load, though the current is diminished,
which is not the case when a parallel
group of modules is removed.
From Module to array
40. Two ways to wire an array with three modules in series and two modules in parallel.
The series modules may be wired as
strings, and the strings wired in parallel.
The parallel modules may be wired together
first and those units combined in series
V V
If an entire string is removed from service
for some reason, the array can still
deliver whatever voltage is needed by the
load, though the current is diminished,
which is not the case when a parallel
group of modules is removed.
From Module to array
41. Standard conditions of your PV module
Standard Test Conditions:
• 1 kW/m2
, AM 1.5, 25°C Cell Temperature
• Solar irradiance of 1 kW/m2
(1 sun)
• Air mass ratio of 1.5 (AM 1.5).
• Key parameter: rated power PDC,STC
• I –V curves at different insolation and cell temperature
• NOCT: Nominal Operating Cell Temperature
(T = 20°C,Solar Irradiation= 0.8 kW/m2
, winds speed 1 m/s.)
.S
0.8
C20NOCT
TT ambCell
°−
+=
cell temperature (°C)
ambient temperature (°C)
Insolation
(1 kW/m2
)
VMPP
MPP
VMPP
V
MPP
43. A. Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
Impact of Cell Temperature on Power for a PV Module.
Estimate cell temperature, open-circuit voltage, and maximum power output for the
150-W BP2150S module under conditions of 1-sun insolation and ambient
temperature 30°C. The module has a NOCT of 47°C.
C64.1
0.8
C204
3.S
0.8
C20NOCT
TT ambCell °=
°−
+=
°−
+=
7
0
From The table for this module at the standard T = 25°C, VOC = 42.8V
VOC drops by about 0.37% per °C , the new VOC = 42.8[1 − 0.0037(64 − 25)] = 36.7 V
with decrease in maximum power available of about 0.5%/°C.
With maximum power expected to drop about 0.5%/°C, this 150-W module at
its maximum power point will deliver:
Pmax = 150 W· [1 − 0.005(64 − 25)] = 121 W
This is a significant drop of 19% from its rated power.
Standard conditions of your PV module
44. • Module with Power of 240 WC
• 240 Wc and efficiency 14.8%
• 1.64×0.99=1.6236 m².
• ηSTC=240/(1000×1.6236) = 14.78 %≈ 14.8 %
Standard conditions of your PV module
45. • Module with Power of 240 WC
• 240 Wc and efficiency 14.8%
• 1.64×0.99=1.6236 m².
• ηSTC=240/(1000×1.6236) = 14.78 %≈ 14.8 %
Siliken modules were awarded the
Number one test modules 2010 and
Number two test modules 2011.
Standard conditions of your PV module
46. • Module with Power of 240 WC
• 240 Wc and efficiency 14.8%
• 1.64×0.99=1.6236 m².
• ηSTC=240/(1000×1.6236) = 14.78 %≈ 14.8 %
• Module with Power of 240 WC
• 240 Wc and efficiency 14.8%
• 1.64×0.99=1.6236 m².
• ηSTC=240/(1000×1.6236) = 14.78 %≈ 14.8 %
• KT(P) = -0.41 %/°C Power decreases by (0.41% × 240W)
= 0.984 W /°C
• KT(Uco) = -0.356 %/°C Load voltage decreases by
(0.356 × 37V) = 0.13 V / °C.
• KT(Icc) = 0.062 %/°C Isc enhanced by
(0.062% × 8.61 = 0.0053 A / °C
• NOCT = 49°C (±2°C).
).S(kW/m
0.8
C20C249
C)(TC)(T 2
ambCell
°−°±
+°=°
NOCT terms:
Level of illumination: 800 W / m²
Outdoor temperature: 20 ° C
Wind speed: 1 m / s
Air mass AM = 1.5
Siliken modules were awarded the
Number one test modules 2010 and
Number two test modules 2011.
Standard conditions of your PV module
47. Exercice: Electronic Structure of Semiconductors and Doping
Physical constants: Planck’s constant, h = 6.63 × 10-34
J s
Boltzmann’s constant, k = 1.38 × 10-23
J K-1
= 8.62 x 10-5
eV K-1
speed of light, c = 3.00 x 108
m s-1
Rest mass of an electron, m0 = 9.11 x 10−31
kg
Elementary charge, e = 1.60 x 10-19
C
1) Germanium has an effective density of states (DOS) NC = 1019
cm-3
for the conduction band and a band gap
EG = 0.66 eV. The intrinsic carrier density at 300 K is 1.8 x 1013
cm-3
.
i) What is the effective DOS for the valence band, NV ?
ii) If the material is n-doped to give an electron density of ne = 1018
cm-3
, what is the hole density?
iii) What is the intrinsic carrier density at 100 K? You can assume that the effective DOS do not change with temperature.
2) (Only attempt this question if you like calculus or use a program like Mathematica)
The conduction-band DOS in a direct band gap semiconductor is given by
where is the conduction band minimum and is the electron effective mass. Show that the conduction band electron
density can be approximated by:
where EF is the Fermi level and is the effective DOS.
Ahmed Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
48. Exercice: Electronic Structure of Semiconductors and Doping
3) The effective electron mass in crystalline GaAs is .
The effective hole mass is = 0.47 m0 , and the band gap is EG = 1.42 eV.
i) Sketch the band structure (energy versus momentum) for GaAs.
ii) Using the expression for the effective DOS given in question 2, determine the intrinsic carrier density at 300K.
iii) A GaAs crystal is doped with 1016
cm-3
Si atoms, acting as electron donors by replacing Ga atoms in the lattice.
What is the electron and hole density, assuming that all dopants are ionised?
iv) What is the position of the Fermi-level relative to the conduction band onset? (Give your answer in electron volts.)
4) Crystalline silicon has an effective DOS of NC = 3 x 1019
cm-3
for the conduction band and NV = 2 x 1019
cm-3
for valence band, and a
band gap EG = 1.1 eV. A silicon crystal is doped with 1017
cm-3
boron (B) atoms. (Boron is a group III element.)
i) What is the position of the Fermi-level relative to the valence band maximum, EV, and conduction band maximum, EC, at 300 K?
ii) If the acceptor state energy, ED, is 0.05 eV above the valence band maximum (see diagram), use the Fermi-Dirac
distribution and the Fermi level calculated in (i) to calculate the fraction of dopant atoms that are ionized.
iii) Using the same approximations, calculate the Fermi-level and fraction of ionized dopants at 77 K. Is the assumption of
complete ionization still valid?
iVI Roughly sketch the variation of hole density with temperature over a wide temperature range.
energy
EV
ED
EC
0.05 eV
Ahmed Ennaoui / Helmholtz-Zentrum Berlin für Materialien und Energie
49. At wavelength λ = 1050 nm, the refractive index of silicon is n = 3.6 and the absorption
coefficient is α = 15 cm-1
.
i)A Si wafer of thickness d = 0.25 mm is illumined by light at wavelength λ = 1050 nm at normal
incidence. What fraction of light is reflected and what fraction of light is absorbed?
ii)A perfect reflective surface is added to the back side of the wafer. What is the absorptivity
now?
iii)An estimate for the absorptivity of a wafer with a light-trapping surface is given by Würfel as:
(P. Würfel, Physics of Solar Cells, p144) where R is the reflectivity of the surface. By what
factor would you expect the external quantum efficiency at 1050 nm of the corresponding solar
cell to be improved by a light-trapping surface?
Use the absorption coefficient and refractive index for silicon as a function of wavelength and
the solar irradiance spectrum to calculate the reflectivity and absorptivity spectra for a 0.25 mm-
thick Si solar cell with a reflective back side and no light trapping (assume normally incident
light). Plot these spectra as functions of wavelength. Assuming that each absorbed photon
generates one electron in the external circuit (external quantum efficiency = absorptivity),
calculate the short-circuit current for the cell under AM1.5 illumination.
50. Exercice: Charge transport and p-n diodes
Physical constants: Reduced Planck’s constant = 1.05 × 10-34
J s = 6.58 × 10-16
eV s
Boltzmann’s constant, k = 1.38 × 10-23
J K-1
= 8.62 x 10-5
eV K-1
speed of light, c = 3.00 x 108
m s-1
elementary charge, e = 1.60 x 10-19
C
1. A crystalline silicon wafer, has a band gap EG = 1.1 eV and an intrinsic carrier density of ni = 1.3 x 1010
cm-3
at 300 K.
The wafer is 200 µm thick and has an area of 1 cm-2
. The electron mobility is µe = 1000 cm2
V-1
s-1
, and the hole
mobility is µh = 100 cm2
V-1
s-1
.
i) What is the conductivity of the undoped wafer?
ii) The wafer is doped with a donor density ND = 1018
cm-3
. Is the doped wafer n-type or ptype? Which
carrier types (electron or hole) are the minority and majority carriers?
iii) A voltage of 1.0 V is applied across the wafer. Sketch the energy band diagram (energy vs depth),
indicating the direction of travel of holes and electrons. How large is the drift current?
iv) The minority carrier lifetime is 1 µs. On average how far does a minority carrier travel (under 1.0 V
applied bias) before recombining? How does this affect the photocurrent?
v) Why is the n-type region made thin relative to the p-type region in typical crystalline silicon solar cells?
51. Exercise : Crystalline silicon solar cells
2. A 250 micrometer-thick crystalline silicon wafer is doped with 5×1016
acceptors per cubic centimetre. A 1 micrometer-
thick emitter layer is formed at the surface of this wafer with a uniform concentration of 3×1019
cm-3
donors. Assume that
all doping atoms are ionized. The intrinsic carrier concentration in silicon at 300 K is ni = 1.3 x 1010
cm-3
. How large is (at
300 K and thermal equilibrium):
i)The electron and hole concentration in the p-type region and n-type region? Which charge carriers are the majority
carriers in the p-type region and what is their concentration?
ii)What is the position of the Fermi level (in eV) in respect to the conduction band in the ptype and n-type region,
respectively?
iii)The built-in voltage of the p-n junction? iv) Draw the corresponding band diagram of the p-n junction.
iv)The width of the depletion region of the p-n junction. Compare it with the total thickness of the Si wafer.
3. A 200 micrometer-thick multicrystalline silicon cell is doped with 5×1017
acceptors per cubic centimetre. A 1 micrometer-
thick n-type emitter layer is formed at the surface of this cell with a uniform concentration of 3×1019
cm-3
donors. Assume
that all doping atoms are ionized. The intrinsic carrier concentration in silicon at 300 K is ni = 1.3 x 1010
cm-3
, and the
dielectric constant is ε = 11.7. At 300 K and thermal equilibrium:
i) The electron mobility is µe
= 500 cm2
V-1
s-1
, and the hole mobility is µh
= 50 cm2
V-1
s-1
. The minority
carrier lifetime for electrons is τe
= 400 ns and τh
= 100 ns for holes. The diffusion constant is given
by the Einstein relation, D
52. Exercice: Charge transport and p-n diodes
iVI) Estimate the saturation current density for the cell, neglecting recombination in the depletion zone. How does the
saturation current affect the open-circuit voltage of the cell?
iV) Minority carriers generated within one diffusion length of the depletion zone will be collected and will contribute to the
measured photocurrent. Those generated outside of this region will recombine and will not contribute to the current. The
absorption coefficient for silicon at 950 nm is α(950nm) = 104
m-1
. Using the Beer-Lambert law for absorption, estimate the
quantum efficiency for this cell at 950 nm. (The light has normal incidence and shines on the n-type side of the cell.)
Vi) Sketch the energy band diagram for the cell, labelling all relevant distances. Explain why
reducing the doping in the p-type region might increase the short-circuit current of the cell. How
might this affect the open-circuit voltage?
53. PV module made up of 36 identical cells, all wired in series. With 1-sun insolation
(1 kW/m2
), each cell has short-circuit current ISC = 3.4 A and at 25°C its reverse saturation
current is I0 = 6 × 10−10
A. Parallel resistance RP = 6.6 Ω and series resistance RS = 0.005Ω .
a) Find the voltage, current, and power delivered when the junction voltage of each cell is
0.50 V.
b) Set up a spreadsheet for I and V and present a few lines of output to show how it works.
Using Vd = 0.50 V along with the other data
The voltage produced by the 36-cell module:
Vmodule = n(Vd − I x RS ) = 36(0.50 − 3.16 x 0.005) = 17.43 V
Power dilevred:
P(watts) = Vmodule x I = 17.43 × 3.16 = 55.0 W
R
I.RV
1
n.k.T
)I . Rq(V
exp.-III
p
SS
0ph
+
−
−
−
=
[ ]
p
dV9.38
0ph
R
V
1e.-III d
−−=
[ ] A6.3
6.6
5.0
1e.10x6-4.3I 5.0x9.3810
=−−= −
Voltage and Current from a PV Module
54. A spreadsheet might look something like the following:
From Cells to a Module
55. Gonçalves et al., Dye-sensitized solar cells, Energy Environ. Sci. 1, 655 (2008), is a very nice
summary of the current state of DSSCs. Use it as a reference to answer the following questions:
(only brief answers required)
What is the main reason for the lower efficiency of DSSCs compared to crystalline silicon
cells?
What is the main difference in the physical process of charge generation and transport
compared to silicon cells?
After excitation, what prevents the dye from returning to its ground state via
fluorescence?
What are the main requirements when choosing a dye?
What are the main requirements that the semiconductor (TiO2) layer must satisfy to in
order to make an efficient cell?
What causes the lack of stability of DSSCs? How can this potentially be solved?
56. Exercise : Tandem Solar Cells
A tandem cell is made from two sub-cells, A and B. The individual sub cells are ideal diodes, with current-voltage (J-V) characteristics
given by:
Where J0 is the reverse saturation current density, and Jph is the photocurrent density.
These have values of J0,A = 10-10
mA/cm2
, Jph,A= 25 mA/cm2
and J0,B = 10-18
mA/cm2
, Jph,B= 20 mA/cm2
for sub-cells A and B
respectively at temperature T = 300 K.
Calculate the open-circuit voltage for each sub-cell. Which sub cell do you suppose has the highest band gap?
The two sub-cells can be connected together in series or in parallel to make a tandem cell. Sketch the J-V characteristics of the
individual sub-cells as well as the two possible configurations of tandem cell.
Write an expression for the J-V characteristic of the parallel-connected tandem cell. Calculate the short-circuit current and the open-
circuit voltage.
Calculate the short-circuit current and open-circuit voltage for the series-connected tandem cell.
(optional) Using a computer or otherwise, calculate the fill-factor for each sub-cell and the two possible tandem-cell configurations. (Hint:
it is simpler to calculate power as a function of current for the series-connected cell.) Assuming the J-V curves were generated with
AM1.5 radiation (100 mW/cm2
), what are the corresponding power conversion efficiencies?
The series configuration is more efficient than the parallel configuration. Why?
Light passes through sub-cell B before reaching sub-cell A. The band gaps of each sub-cell can be adjusted to optimise the overall
efficiency. How are the J-V curves of sub-cells A and B affected by changing the band gaps of the two materials? What is an important
criterion for optimising the efficiency of a series-connected stacked tandem cell?
Editor's Notes
Just hit return once to drive the whole graphic
There are three most used methods for producing thin films of CIGS. The first one is the coevaporation process. This method is named coevaporation because in the beginning all needed elements were evaporated in vacuum at the same time. The thin film is produced by evaporating Cu, In, Ga, Se from elemental sources. In order to achieve the favored film composition, a precise control of the particular evaporation rates is necessary. There an electron impact emission spectrometer and an atom absorption spectrometer or a mass spectrometer is used. But the process also requires a substrate temperature between 300 and 550°C for a certain time during film growth. There are several processes of coevaporation, but one of the most favored ones is the inverted three-stage process, which you can see on the right. At first In, Ga, and selenium are evaporated with different rates and deposited as (In,Ga) 2 Se 3 at 300°C on the substrate. Afterwards Cu and selenium are evaporated and deposited on the substrate at elevated temperatures. At last In, Ga, and selenium are evaporated again. The inverted three-stage process leads to smoother film morphology and to high efficiency solar cells.