Visit the Solar Institute for details of the 2011 Solar Symposium: solar.gwu.edu/Symposium.html

1. PV’s Leading Edge:
The Direction of Maximum Impact
Jim Sites
Physics Department
Colorado State University
(1) Where are we?
(2) What metrics indicate
leading edge?
(3) What next?
April 26, 2011 James Sites

2. The Typical Solar Cell
Most existing PV panels, silicon and thin-film, consist
of a semiconductor p-n junction, with top and bottom
contacts, sandwiched between two pieces of glass.
From NREL
April 26, 2011 James Sites

3. Large Photovoltaic Fields
April 26, 2011 James Sites

4. Annual World PV Production 1982-present
Production
[Peak Gigawatts]
100
Leading
Total PV Production (GW)
edge
10
Average increase
of 45%/yr for last
1
15 years!
0.1
0.01
0.001
1980 1985 1990 1995 2000 2005 2010 2015
Year Slide by Valerie Jacobson
April 26, 2011 James Sites

5. Technology Comparison
April 26, 2011 James Sites

6. Additional Types of Solar-Cells
Concentrated PV (CPV)
Small, high-efficiency cells, often in tandem
Organic PV (OPV)
Lots of materials choices, inexpensive
Dye Sensitized Solar Cell (DSSC)
Liquid electrolyte (can be solid
state) also inexpensive
Intermediate Band-gap PV
Utilizes intermediate band for lower energies
April 26, 2011 James Sites

7. Different Views of a Solar Cell?
GRAIN BOUNDARY
- potential barrier
- recombination CONTACT
- enhanced atomic diffusion
VOID
- gas transport
CdTe GRAIN SURFACE
- surface recombination
- depletion layer
- enhanced surface
diffusion
Electrical
Engineering
CdS
Materials
SnO x Science
GLASS
JUNCTION INTERFACE
GRAIN INTERIOR
- interface recombination
- bulk recombination
- S and Te interdiffusion
Physics
Manufacturing
April 26, 2011 James Sites

8. Comparison of J-V Curves
Voltage
Efficiency =
Electical Power
fill-
factor Light Power
Commercial High-Eff
CdTe (11%) Lab CdTe
(16.5%)
Current
High-Eff Crystalline (27%)
[Based on GaAs]
Two efficiency issues: (1) lab vs ideal, (2) commercial vs lab
April 26, 2011 James Sites

9. Organize and Quantify Efficiency Losses
Absolute efficiency loss and primary reason
Commercial – Lab Efficiency Lab – Ideal
Differences Efficiency Differences
1% 5%
Voltage Lower electron lifetime Low electron lifetime (GBs);
Low carrier density
2½% 2%
Current Thick CdS; TCO/glass abs; Various: Reflection; CdS abs;
Interconnect area loss TCO/glass abs; Collection
2% 3½%
Fill-Factor High series resistance; Low electron lifetime (GBs)→
Uniformity issues voltage-dependent current
Incremental progress expected Possible breakthough?
April 26, 2011 James Sites

10. Leading Edge Metrics
(1) Efficiency (electrical power/light power)
- defined at a specified temperature and illumination level, which
can be misleading.
- can vary significantly with temperature, illumination, and the PV
module parameters.
- however, the average efficiency (and from that the expected
annual power) at a specified location can be calculated with
reasonable accuracy.
April 26, 2011 James Sites

11. Variation in Effective Efficiency with Location
Calculated average c-Si
efficiency (relative to
STC) throughout Europe
T. Huld et al, Solar Energy 84, 324 (2010)
April 26, 2011 James Sites

12. Technology Comparison
Calculated differential in
relative power yield of
higher band-gap CdTe
compared to c-Si
T. Huld et al, Solar Energy 84, 324 (2010)
April 26, 2011 James Sites

13. Leading Edge Metrics
(1) Efficiency (electrical energy/light energy)
- defined at a specified temperature and illumination level, which
can be misleading.
- can vary significantly with temperature, illumination, and the PV
module parameters.
- however, the average efficiency (and from that the expected
annual power) at a specified location can be calculated with
reasonable accuracy.
(2) Cost
- here also a caveat is needed: total cost includes installation and
permitting, generally lower with higher efficiency.
(3) Reliability
(4) Additional practical considerations
- scalability, impact on land area, grid integration.
April 26, 2011 James Sites

14. Technology Pros and Cons
Arguments For Being
Technology Arguments Against
Leading Edge
Has maintained production lead
c-Si Good fundamental understanding Crystal growth and assembly relatively
Solid efficiency and reliability expensive
Credible roadmap to very large scale
Very high efficiency
Requires tracking system
CPV Cell area small
Only suitable for direct sunlight
Good fundamental understanding
Reasonable efficiency at modest cost
Thin Films Complex structures; fundamental
Impressive recent growth rate
(a-Si, CdTe, understanding light
Credible roadmap to very large scale
CIGS) a-Si limited by efficiency
CdTe and CIGS at low cost
Fundamental understanding very light
“Third Projections for very low cost OR very Difficulty producing high current and
Generation” high efficiency high voltage in same cell
No good roadmap to high efficiency
April 26, 2011 James Sites

15. Leading Edge Predictions
(1) Almost certainly a mix of technologies, and all will come
down in cost.
(2) c-Si, especially from China, will remain highly competitive
(3) Thin-film CdTe and CIGS will continue to increase their
market share
- CdTe straightforward to deposit; has momentum and potential for
higher efficiency
- CIGS is thin-film that suffers least from being polycrystalline; keep an
eye on higher band-gap CIGS
(4) Tandem CPV will have an impact in sunny areas; modest
overall
(5) Not clear that other approaches will gain traction
(6) Successful countries will be those with both healthy
manufacturing climate and intellectual infrastructure
April 26, 2011 James Sites