1. Improving Solar Cell Efficiencies through Periodicity
Peter Bermel*, Chiyan Luo, Marin Soljacic, John D. Joannopoulos – MIT
MIT CIPS Annual Meeting, May 5, 2006
* Invited Speaker

2. Solar Energy Solar spectrum spans a broad range of wavelengths
Silicon has near-optimal bandgap
Absorption of silicon limited in near IR
Solar power vs. wavelength
Absorption coefficient for Si vs. wavelength

3. Current Light Trapping Schemes
Geometrical techniques:
Texturing
Lambertian scattering
Effective path length limited to 4n2d [Yablonovitch & Cody, 1981]
Wave optics can do much better [Gee, 1999; Brendel, 2003]
Path length enhancement associated with texturing

4. Photonic Crystals
Periodic dielectric media; reflect certain wavelengths and transmit others
Can selectively trap light at desired frequencies via modification of the density of modes
1D, 2D and 3D periodic dielectric structures
Photonic band gap for a diamond structure

5. Proposed Solar Cell Design
Introduce two regions of photonic crystal:
Diffractive & refractive region
Reflective region (distributed Bragg reflector)
2D air holes in silicon + DBR
grating on top of DBR [Zheng et al., 2005]

6. Trapping mechanisms Diffraction: Refraction: Reflection:
Requires correct period a in perpendicular direction
Causes total internal reflection within critical wavelength range
( na > l > a )
Refraction:
Some modes strongly trapped in PhC region
Can complement diffraction-based absorption peaks
Reflection:
Prevents losses through bottom
Diffractive trapping with partial photonic crystal coverage in the middle of the solar cell

7. Diffraction Modes
Absorption selectively enhanced at wavelengths below diffraction threshold
Spacing set by thickness of material
Width set by absorption of material
Absorption of diffracted modes

8. Reflection at Normal Incidence
System simulated consists of weakly absorbing dielectric with or without PhC
Absorption enhanced above diffraction limit
Spectral reflection vs. frequency
Total reflection vs. frequency

9. Reflection at Oblique Incidence
Absorption changes at much lower frequencies
Effect can be negative sometimes
Strong trapping still observed above crossover
Spectral reflection vs. frequency
Total reflection vs. frequency

10. Enhancement of Light Trapping
Choose numbers appropriate for real systems: a=300 mm, etching depth=100 mm
Best enhancement observed for thinnest cells
May become increasingly important as wafer thicknesses decrease!
Light trapping improvement from grating vs. wavelength for several thicknesses

11. Enhancement of Light Trapping
Light trapping selectively enhanced in nm range vs. bare reflector
Photonic crystals offer similar performance as gratings at normal incidence
Absorption vs. wavelength –gratings vs. PhCs

12. Enhancement of Light Trapping
Periodicity in two directions enhances absorption
Greatest impact around 45° diffraction wavelength
Absorption vs. wavelength – 1D vs. 2D gratings

13. Enhancement of Light Trapping
Compared to existing light trapping techniques:
Texturing & PhCs give similar improvement (10% in this example)
But, they can be combined (15%)

14. Enhancement of Light Trapping
1D grating: 14% improvement
1D PhC: 13% improvement
Potential for system when refracted modes are included?
2D grating: 20% improvement
2D grating + texturing: projected 30% total enhancement

15. Conclusions
Photonic crystals have the potential to enhance light trapping of solar cells
Two different mechanisms can play a role
Diffraction
Coupling to PhC modes
Overall enhancement: 30% or more

16. Future Work Systematic investigation of case for oblique incidence
Maximize DOM for weakly absorbed light inside photonic crystal
Transparent conductive oxides with periodicity
Varying Si bandgap

17. Any Questions?

18. Solar Energy Market Solar cell energy sources in demand today
Silicon shortage  2-year backlog on solar cells!
Market at $7 billion in 2004; will grow to $30 billion by 2010 [Michael Rogol, CLSA]
Still more expensive than fossil fuels, but growth driven by 3 factors:
competes at grid prices not generator prices
subsidies can make it economical for consumers
innovations keep driving down costs
Only 0.1% total world energy market share  massive growth opportunities [solarbuzz.com]
Many companies became profitable for first time in 2004
cost = 4-8x generator price; 2x end-user price (without subsidies)
0.8% of global renewable energy (80% of renewables = non-commercial biomass; then hydropower, wind & solar)