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

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

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

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

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]

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

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

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

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

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

Enhancement of Light Trapping Light trapping selectively enhanced in 700-1100 nm range vs. bare reflector Photonic crystals offer similar performance as gratings at normal incidence Absorption vs. wavelength –gratings vs. PhCs

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

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%)

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

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

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

Any Questions?

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)