2. Lecture 5. Experiment: Types of Solar Cells
•Generation I solar cells:
Single Crystal Si, Polycrystalline Si
Growth, impurity diffusion, contacts, anti-reflection coatings
•Generation II Solar cells:
Polycrystalline thin films, crystal structure, deposition techniques
CdS/CdTe (II-VI) cells
CdS/Cu(InGa)Se2 cells
Amorphous Si:H cells
•Generation III Solar Cells:
•High-Efficiency Multijunction Concentrator Solar cells based on
III-V’s and III-V ternary analogues
•Dye-sensitized solar cell
•Organic (excitonic) cells
•Polymeric cells
•Nanostructured Solar Cells including Multicarrier per photon cells,
quantum dot and quantum-confined cells
3. Background and Cost
• Photovoltaics convert
sunlight directly to
electric power
– Carbon-neutral
– Highly abundant—the
earth receives 120
quadrillion watts of power
from the sun, humans
currently use about 13 trillion watts
Lewis, et al. “Basic Research Needs for Solar
• Costs Energy Utilization.”
– Module cost
– Balance of system cost
– Power conditioning cost
– Currently about $0.30/kWh, a factor of 5-10 behind total cost of fossil
fuel generation
4. 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.)
6. 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
7. Production-
Process
mono- or multi-
crystalline Silicon
crystal growth process
Clemson Summer School
6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 7
trier.de
8. Production process
1. Silicon Wafer-technology (mono- or multi-crystalline)
Most purely silicon
99.999999999%
melting /
crystallization Occurence:
Siliconoxide (SiO2)
Tile-production
= sand
Mechanical cutting:
Plate-production Thickness about 300µm
typical Wafer-size:
Minimum Thickness:
cleaning
10 x 10 cm2
about 100µm
Quality-control
Wafer Link to
SiO2 + 2C = Si + 2CO
Producers of Silicon Wafers
Clemson Summer School
6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 8
trier.de
9. Energía Fotovoltaica
Celdas Solares
De Silicio monocristalino
Material: Silicio monocristalino
Temperatura 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
10.
11. 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
12.
13. 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
15. 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)
16.
17. Production Process
Thin-Film-Process (CIS, CdTe, a:Si, ... )
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
CIS Module
Clemson Summer School
6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 17
trier.de
22. 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
23.
24. Material Level of Level of efficiency in %
efficiency
in % Lab Production
Monocrystalline
Silicon Approx. 24 14 to 17
Polycrystalline
Silicon Approx. 18 13 to 15
Amorphous
Silicon Approx. 13 5 to 7
30. 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
33. Cadmium Telluride
• One of the most
promising approaches
• Made by a variety of http://www.nrel.gov/cdte/images/cdte_cell.gif
processes
– CSS
– HPVD
http://www.sandia.gov/pv/images/PVFSC29.jpg
34. Cadmium Telluride Solar Cells
D.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.
John A. Woollam, PV talk UNL 2007
35. CdTe and CIGS Review: 2006 World PV Conference
Noufi and Zweibel, NREL/CP -520-39894, 2006
John A. Woollam, PV talk UNL 2007 35
36. Nano-Structured CdS/CdTe Solar Cells
Graphite
CdTe
Nanocrystalline CdS
ITO
Glass
Nano CdS/ CdTe device Structure.
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.
37. 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
38.
39. Modulos Solares de CdTe
• Costo 60% de Si
• 20 años garantia
• Modulos de peliculas
delgadas
• Potencia 50 – 60 W
• Eficiencia 9%
40. 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
41.
42.
43. 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/PVFSC27.jpg http://www.sandia.gov/pv/images/PVFSC26.jpg
51. Tandem-
cell
Pattern of a multi-
spectral cell on the
basis of the
Chalkopyrite
Cu(In,Ga)(S,Se)2
Clemson Summer School
6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 51
trier.de
55. Multijunction Concentrators
• Similar in technique
• Exotic Materials
• More expensive processing (MBE)
http://www.nrel.gov/highperformancepv/entech.html
56. Spectrolab’s Triple-Junction Solar Cell
D.E.Carlson, BP Solar
Spectrolab: 40.7% conversion efficiency at ~ 250 suns.
John A. Woollam, PV talk UNL 2007
57. [edit] Gallium arsenide substrate
Twin junction cells with Indium gallium phosphide
and 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.
59. 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
60. 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.
61.
62.
63. 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
65. 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.
72. 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
73. 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.
74. Cu2S/CdS bulk and nano heterojunction solar cells
Bulk heterojunction Nano heterojunction
Cr
contacts Cu/Cr top contact
Thin layer of Cu ~
10 nm
Copper Sulfide
Cu2S
Inter-pore spacing
CdS
Nano-porous Alumina
Template
ITO
Cadmium Sulfide
Glass
ITO
81. 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 multiparticle
exciton 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.
82.
83. To study MEG processes in QDs, we detect
multiexcitons created via exciton multiplication
(EM) by
monitoring the signature of multiexciton decay in
the
transient absorption (TA) dynamics, while
maintaining a
pump photon fluence lower than that needed to
create
multiexcitions directly. The Auger recombination
rate is
proportional to the number of excitons per QD
with the
decay of a biexciton being faster than that of the
single
exciton. By monitoring the fast-decay component
of the
TA dynamics at low pump intensities we can
measure the
population of excitons created by MEG.
84.
85. The work reported here provides a confirmation of the
previous report of efficient MEG in PbSe. We observed a
previously unattained 300% QY exciting at 4Eg in PbSe QDs,
indicating that we generate an average of three excitons per
photon absorbed. In addition, we present the first known
report of multiple exciton generation in PbS QDs, at an
efficiency comparable to that in PbSe QDs. We have shown
that a single photon with energy larger than 2Eg can
generate
multiple excitons in PbSe nanocrystals, and we introduce a
new model for MEG based on the coherent superposition of
multiple excitonic states. Multiple exciton generation in
colloidal QDs represents a new and important mechanism
that may greatly increase the conversion efficiency of solar
cell devices.
86. For the 3.9 nm QD (Eg = 0.91 eV), the QY reaches a
surprising value of 3.0 at Ehn/Eg = 4. This means that on
average every QD in the sample produces three
excitons/photon.
