1. A brief overview of Plasmonic Nanostructure Design for Efficient Light Coupling into Solar Cells V.E. Ferry, L.A. Sweatlock, D. Pacifici, and H.A. Atwater, Nano Letters, 8 4391 Douglas Detert EE235 Prof. Connie Chang March 2, 2009

2. Douglas Detert — EE235 — March 2, 2009 Solar Cell Design/Material Considerations Conventional solar cells (e.g. Silicon) require thick absorption layers for complete absorption Thin film solar cells (e.g. CdTe, CIGS) decrease bulk recombination effects and allow for higher quality absorber materials Problem: Thin film cells are limited by decreased absorption, carrier excitation, and photocurrent Solution: Texture top/bottom surfaces to enhance light absorption

3. Douglas Detert — EE235 — March 2, 2009 Surface Plasmon Polariton Enhanced Solar Cells Surface Plasmon Polaritons (SPPs) are collective oscillations of free electrons at metal/dielectric boundaries SPPs are highly localized to interfaces and propagate easily for microns. Energy in SPP modes enhances absorption Momentum mismatch between incident light and SPPs does not allow for direct excitation of SPPs Goal: Design a nanostructure back contact that scatters light into SPP mode Barnes. J Opt A-Pure Appl Op 8 S87-S93 (2006)

4. Douglas Detert — EE235 — March 2, 2009 Scattering From a Single Groove Light energy is scattered into two key modes Photonic (~semiconductor) SPP (~interface) Both enhance photoabsorption, but photonic modes are not supported in extremely thin structures HyHy

5. Douglas Detert — EE235 — March 2, 2009 Results: Scattering From a Single Groove Finite-difference time-domain (FDTD) simulations paired with modal decomposition analysis Three physical effects involved in incoupling efficiencies: Fabry-Pérot resonance of thin film Photonic mode excitation at SPP resonance wavelength Polarization resonance of scatterer Film thickness and scatterer geometry affect above properties

6. Douglas Detert — EE235 — March 2, 2009 Effect of Groove Dimensions Groove width: SPP modes break down at large groove sizes, photonic mode flattens out Groove depth has little effect on incoupling efficiency Ridge-like structure: enhances photonic mode

7. Douglas Detert — EE235 — March 2, 2009 Conclusion & Outlook Groove-like nanostructures improve photoabsorption in thin film solar cells by coupling light to various modes, including interfacial SPP modes. Incoupling to SPP modes allows for enhancement in thin film solar cells To date, solar cells enhanced by SPPs have been fabricated with only top-layer patterning. Pillai et al. JAP 101 093105 (2007)