Lecture 3Types of Solar Cells (experiment )-

3 generationsGeneration 1:

Single- and poly-Crystalline SiliconGrowth, impurity diffusion, contacts

Modules, interconnectionGeneration 2:

Polycrystalline thin films, crystal structure, deposition techniquesCdS/CdTe (II-VI) cells

CdS/Cu(In,Ga)Se2 cellsAmorphous Si:H cells

Generation 3:High-efficiency Multi-junction Concentrator Solar Cells based on III-V’s and III-V ternary analogs

Dye-sensitized cellsOrganic (excitonic) cells

Polymeric CellsNanostructured cells including Multi-carrier per photon cells, quantum dot and quantum

confined cells

Figure 3. The three generations of solar cells. First-generation cells are based on expensive silicon wafers and make up 85% of the current commercial market. Second-generation cells are based on thin films of

materials such as amorphous silicon, nanocrystalline silicon, cadmium telluride, or copper indium selenide. The materials are less expensive, but research is needed to raise the cells' efficiency to the

levels shown if the cost of delivered power is to be reduced. Third-generation cells are the research goal: a dramatic increase in efficiency that maintains the cost advantage of second-generation materials. Their

design may make use of carrier multiplication, hot electron extraction, multiple junctions, sunlight concentration, or new materials. The horizontal axis represents the cost of the solar module only; it must be approximately doubled to include the costs of packaging and mounting. Dotted lines indicate the cost

per watt of peak power (Wp). (Adapted from ref. 2,) Green.)

Generation I.

Single Crystal Ingot-based PVs

• Single crystal wafers made by Czochralski process, as in silicon electronics

• Comprise 31% of market

• Efficiency as high as 24.7%

• Expensive—batch process involving high temperatures, long times, and mechanical slicing Wafers are not the ideal geometry

• Benefits from improvements developed for electronics industry

http://hydre.auteuil.cnrs-dir.fr/dae/competences/cnrs/images/icmcb03.jpg

6.6.06 - 8.6.06Clemson Summer School

Dr. Karl Molter / FH Trier / molter@fh-trier.de

5

Production-Process

mono- or multi-crystalline Silicon

crystal growth process

6.6.06 - 8.6.06Clemson Summer School

Dr. Karl Molter / FH Trier / molter@fh-trier.de

6

Production process1. Silicon Wafer-technology (mono- or multi-crystalline)

Tile-production

Plate-production

cleaning

Quality-control

Wafer

Most purely silicon99.999999999%

Occurence:

Siliconoxide (SiO2)

= sand

melting / crystallization

SiO2 + 2C = Si + 2CO

Mechanical cutting:

Thickness about 300µm

Minimum Thickness:

about 100µm

typical Wafer-size:

10 x 10 cm2

Link to

Producers of Silicon Wafers

Energía Fotovoltaica

Celdas Solares

De Silicio monocristalino

Material: Silicio monocristalinoTemperatura de Celda: 25ºC Intensidad luminosa: 100%Área de la celda: 100 cm2

Voltaje a circuito abierto: Vca = 0.59 volts

Corriente a corto circuito: Icc = 3.2 A

Voltaje para máxima potencia: Vm = 0.49 volts

Corriente para máxima potencia: Im = 2.94 A

Potencia máxima: Pm = 1.44 Watts

Polycrystalline Ingot-based PVs

• Fastest-growing technology involves casting Si in disposable crucibles

• Grains mm or cm scale, forming columns in solidification direction

• Efficiencies as high as 20% in research

• Production efficiencies 13-15%

• Faster, better geometry, but still requires mechanical slicing

Polycrystalline Si Ribbon PVs• String method

– Two strings drawn through melt stabilize ribbon edge– Ribbon width: 8 cm

• Carbon foil method (edge-defined film-fed growth, EFG)– Si grows on surface of a carbon foil die– Die is currently an octagonal prism, with side length 12.5

cm

• Pros and Cons– Method can be continuous– Requires no mechanical slicing– Efficiencies similar to other polycrystalline PVs– Balancing growth rate, ribbon thickness and width

Generation II.

Flat-Plate Thin-Films

• Potential for cost advantages over crystalline silicon– Lower material use

– Fewer processing steps

– Simpler manufacturing technology

• Three Major Systems– Amorphous Silicon

– Cadmium Telluride

– Copper Indium Diselenide (CIS)

6.6.06 - 8.6.06Clemson Summer School

Dr. Karl Molter / FH Trier / molter@fh-trier.de

14

Production Process

semiconductor materials are evaporated on large areas

Thickness: about 1µm

Flexible devices possible

less energy-consumptive than c-Silicon-process

only few raw material needed

Typical production sizes:1 x 1 m2

Thin-Film-Process (CIS, CdTe, a:Si, ... )

CIS Module

Photon Energy

Amorphous Silicon

• a-Si:H Discovered in 1970’s

• Made by CVD from SiH4

http://www.solarnavigator.net/images/uni_solar_triple_junction_flexible_cell.jpg

Material Level of

efficiency

in % Lab

Level of efficiency in %

Production

Monocrystalline

Silicon

Approx. 24

14 to 17

Polycrystalline

Silicon

Approx. 18

13 to 15

Amorphous

Silicon

Approx. 13

5 to 7

Amorphous Silicon Growth by Thermal CVD

Basic Cell Structure

• p-i-n structure

– Intrinsic a-Si:H between very thin p-n junction

– Lower cells can be a-Si:H, a-SiGe:H, or microcrystalline Si

• Produces electric field throughout the cell

http://www.sandia.gov/pv/images/PVFSC36.jpg

CdTe

CdTe/CdS Solar Cell

• CdTe : Bandgap 1.5 eV; Absorption coefficient 10 times that of Si

• CdS : Bandgap 2.5 eV; Acts as window layer

Limitation :

Poor contact quality with p-CdTe (~ 0.1 Wcm2)

Cadmium Telluride

• One of the most promising approaches

• Made by a variety of processes

– CSS

– HPVD

http://www.nrel.gov/cdte/images/cdte_cell.gif

http://www.sandia.gov/pv/images/PVFSC29.jpg

John A. Woollam, PV talk UNL 2007 31

CdTe and CIGS Review: 2006 World PV ConferenceNoufi and Zweibel, NREL/CP -520-39894, 2006

John A. Woollam, PV talk UNL 2007

Cadmium Telluride Solar CellsD.E.Carlson, BP Solar

CdS/CdTe heterojunction: typically chemical bath CdS deposition, and CdTe sublimation.

Cd Toxicity is an issue.

Best lab efficiency = 16.5%

First Solar plans 570 MWp production capacity by end of 2009.

Nano-Structured CdS/CdTe Solar Cells

Nanocrystalline CdS

CdTe

ITO

Glass

Graphite

Band gap of CdS can be tuned in the range 2.4 - 4.0 eV.

Nano-structured CdS can be a better window material and may

result in high performance, especially in short circuit currents.

Nano CdS/ CdTe device Structure.

Pros and Cons

• Pros– A material of choice for thin-flim PV modules

• Nearly perfect band-gap for solar energy conversion• Made by a variety of low-cost methods• Future efficiencies of 19%• "CdTe PV has the proper mix of excellent efficiency and manufacturing cost to make

it a potential leader in economical solar electricity." Ken Zweibel, National Renewable Energy Laboratory

• Pros– Health Risks– Environmental Risks– Safety Risks– Disposal Fees

Modulos Solares de CdTe

• Costo 60% de Si

• 20 años garantia

• Modulos de peliculas

delgadas

• Potencia 50 – 60 W

• Eficiencia 9%

Modulos Solares de CdTe

• Costo 60% de Si

• 20 años garantia

• Modulos de peliculas

delgadas

• Potencia 50 – 60 W

• Eficiencia 9%

100 kW – 1 MW

Copper Indium Diselenide

• Also seen as CIGS

• Several methods of production

http://www.sandia.gov/pv/images/PVFSC25.jpg

http://www.sandia.gov/pv/images/PVFSC26.jpghttp://www.sandia.gov/pv/images/PVFSC27.jpg

Tandem Cells

• Current output matched for individual cells

• Ideal efficiency for infinite stack is 86.8%

• GaInP/GaAs/Ge tandem cells (efficiency 40%)

6.6.06 - 8.6.06Clemson Summer School

Dr. Karl Molter / FH Trier / molter@fh-trier.de

49

Tandem-cell

Pattern of a multi-spectral cell on the basis of the Chalkopyrite Cu(In,Ga)(S,Se)2

Generation III.

High Efficiency ConcentratorSolar Cells

Multijunction Concentrators

• Similar in technique

• Exotic Materials

• More expensive processing (MBE)

http://www.nrel.gov/highperformancepv/entech.html

John A. Woollam, PV talk UNL 2007

Spectrolab’s Triple-Junction Solar CellD.E.Carlson, BP Solar

Spectrolab: 40.7% conversion efficiency at ~ 250 suns.

[edit] Gallium arsenide substrateTwin junction cells with Indium gallium phosphideand gallium arsenide can be made on gallium arsenide wafers. Alloys of In.5Ga.5P through In.53Ga.47P may be used as the high band gap alloy. This alloy range provides for the ability to have band gaps in the range of 1.92eV to 1.87eV. The lower GaAs junction has a band gap of 1.42eV.The considerable quantity of photons in the solar spectrum with energies below the band gap of GaAs results in a considerable limitation on the achievable efficiency of GaAs substrate cells.

Dye-Sensitized solar cells

Dye-sensitized Solar Cells

• O’Regan and Grätzel 1991

• Organic dye molecules + nanocrystalline titanium dioxide (TiO2)

• 11% have been demonstrated

• Benefits: low cost and simplicity of manufacturing

• Problems: Stability of the devices

Operation

Sunlight enters the cell through the transparent SnO2:F top

contact, striking the dye on the surface of the TiO2. Photons

striking the dye with enough energy to be absorbed will create an

excited state of the dye, from which an electron can be "injected"

directly into the conduction band of the TiO2, and from there it

moves by diffusion (as a result of an electron concentration

gradient) to the clear anode on top.

Meanwhile, the dye molecule has lost an electron and the

molecule will decompose if another electron is not provided. The

dye strips one from iodide in electrolyte below the TiO2, oxidizing

it into triiodide. This reaction occurs quite quickly compared to the

time that it takes for the injected electron to recombine with the

oxidized dye molecule, preventing this recombination reaction

that would effectively short-circuit the solar cell.

The triiodide then recovers its missing electron by mechanically

diffusing to the bottom of the cell, where the counter electrode re-

introduces the electrons after flowing through the external circuit.

Organic Solar Cells

Organic and Nanotech Solar Cells

Benefits:

• 10 times thinner than thin-film solar cells

• Optical tuning

• Low cost for constituent elements

• High volume production

Problems:

• Current efficiencies < 3-5%

• Long term stability

Fig. 1. The scheme of plastic solar cells. PET -Polyethylene terephthalate, ITO - Indium Tin Oxide, PEDOT:PSS - [[Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), Active Layer (usually a polymer:fullerene blend), Al - Aluminium.

Nanostructured Solar cells

Nanostructured Solar Cells

• Nanomaterials as light harvesters leading to direct conversion or chemical production alone or imbedded in a matrix.

Questions: art_nozik@nrel.gov

Fig.2 (a) Nanostructure of anodically formed Al2O3 template. (b) its cross-section, (c) catalyst deposited at the bottom of the pores, (e) vertically aligned nanotubes, and (f) TEM

image of a nanotube.

Cu2S/CdS bulk and nano heterojunction solar cells

Glass

ITO

CdS

Cu2S

Thin layer of Cu ~

10 nm

Cr

contacts

Nano-porous Alumina

Template

Inter-pore spacing

ITO

Cadmium Sulfide

Copper Sulfide

Cu/Cr top contact

Bulk heterojunction Nano heterojunction

zz

n-CdS

Alumina

p-CIS

Mo/Glass

ITO

PTCBI

Porous Al2O3

CuPc

ITO

ITO

Al or Ag

CuPc

PTCBI

John A. Woollam, PV talk UNL 2007

PV Module Conversion EfficienciesD.E.Carlson, BP Solar

Modules Lab

Dye-sensitized solar cells 3 – 5% 11%

Amorphous silicon (multijunction) 6 - 8% 13.2%

Cadmium Telluride (CdTe) thin film 8 - 10% 16.5%

Copper-Indium-Gallium-Selenium (CIGS) 9 - 11% 19.5%

Multicrystalline or polycrystalline silicon 12 - 15%20.3%

Monocrystalline silicon 14 - 16%23%

High performance monocrystalline silicon 16 - 19%24.7%

Triple-junction (GaInP/GaAs/Ge) cell (~ 250 suns) - 40.7%

Generation III Solar Cells not yet realized experimentally

Multiband Cells

• Intermediate band formed by impurity levels.

• Process 3 also assisted by phonons

• Limiting efficiency is 86.8%

Quantum Dots

Multiple Quantum Well

• Principle of operation similar to multiband cells

Multiple E-H pairs

• Many E-H pairs created by incident photon through impact ionization of hot carriers

• Theoretical efficiency is 85.9%

Figure 3. Photoexcitation at 3Eg creates a 2Pe-2Ph exciton state.This state is coupled to multiparticle states with matrix element V

and forms a coherent superposition of single and multiparticleexciton states within 250 fs. The coherent superposition dephases

due to interactions with phonons; asymmetric states (such as a 2Pe-1Sh) couple strongly to LO phonons and dephase at a rate of ô-1.

To study MEG processes in QDs, we detectmultiexcitons created via exciton multiplication (EM) bymonitoring the signature of multiexciton decay in thetransient absorption (TA) dynamics, while maintaining apump photon fluence lower than that needed to createmultiexcitions directly. The Auger recombination rate isproportional to the number of excitons per QD with thedecay of a biexciton being faster than that of the singleexciton. By monitoring the fast-decay component of theTA dynamics at low pump intensities we can measure thepopulation of excitons created by MEG.

The work reported here provides a confirmation of theprevious report of efficient MEG in PbSe. We observed apreviously unattained 300% QY exciting at 4Eg in PbSe QDs,indicating that we generate an average of three excitons perphoton absorbed. In addition, we present the first knownreport of multiple exciton generation in PbS QDs, at anefficiency comparable to that in PbSe QDs. We have shownthat a single photon with energy larger than 2Eg can generatemultiple excitons in PbSe nanocrystals, and we introduce anew model for MEG based on the coherent superposition ofmultiple excitonic states. Multiple exciton generation incolloidal QDs represents a new and important mechanismthat may greatly increase the conversion efficiency of solarcell devices.

For the 3.9 nm QD (Eg = 0.91 eV), the QY reaches asurprising value of 3.0 at Ehn/Eg = 4. This means that onaverage every QD in the sample produces threeexcitons/photon.

Fig. 2. Calculated efficiencies for different QYII models.