2, University of California, Berkeley, Berkeley, California, United States
3, University of Illinois at Urbana-Champaign, Urbana-Champaign, Illinois, United States
4, National Renewable Energy Laboratory, Golden, Colorado, United States
We have fabricated a 100 cm2 prototype two-junction tandem photovoltaic consisting of a luminescent solar concentrator (LSC) top cell featuring quantum luminophor concentrators and InGaP microcells that is combined with a commercially-available Si passivated emitter rear contact (PERC) cell. Our LSC employs ultrahigh luminescence efficiency CdSe/CdS quantum dot (QD) luminophores, absorbing light in 300-500 nm wavelength range and re-emitting luminescence radiation at 635 nm, a wavelength which is nearly ideally matched to the bandgap of InGaP microcells formed by epitaxial growth, liftoff and patterning. We form an LSC planar optical waveguide by dispersing QDs throughout a poly(laurylmethacrylate) (PLMA) waveguide layer along with an array of InGaP micro-cells, tuned to the photoluminescence emission profile. This planar LSC architecture of InGaP micro-cells achieved concentrator factors of 40-100x. The microcell architecture allows for scaling of the LSC waveguide to conventional module size and enables greater optical collection efficiencies than for traditional waveguide edge-lining cell geometries, and InGaP cells are thus cost-effective since they comprise only 1-3% of the module aperture area. We surround the LSC structure with wavelength-selective, top and bottom notch filters in order to minimize photon escape cone loss, which enables high LSC concentration factors to be achieved. We have integrated this layered LSC with a Si PERC cell for use as a 4-terminal tandem prototype. With this optimized architecture, our module captures and utilizes short wavelength light in InGaP with higher achievable cell voltage and power conversion and long wavelength light, thus resulting in efficiencies far beyond that of a stand-alone Si cell. Using Monte Carlo ray-tracing simulations, we have explored the radiative limit of such a tandem LSC-on-Si module architecture, showing an overall power conversion efficiency increase from 20% for a stand-alone Si cell under direct normal incidence, to 30.8% for the LSC-on-Si under solar irradiance with a 40% direct/50% diffuse ratio, typical of average US continental solar irradiance. Results of outdoor on-sun testing of our module will be presented in the talk. In addition to our computational analysis of efficiency and tandem module fabrication, we have also developed a thorough techno-economic (TEA) for this LSC-on-Si architecture. With our current module design, we estimate fabrication costs as low at $80/m2, indicating that an LSC-on-Si tandem module is not only efficient under highly diffuse solar irradiance conditions, but also holds potential for mass market distribution.