The U.S. Department of Energy (DOE)’s SunShot Initiative is a collaborative national effort to reduce the price of solar energy to 6¢/kWh without subsidy for utility scale installations by 2020. Thin-film photovoltaics offer a promising path to reach this goal. Analysis of the levelized cost of energy (LCOE) from photovoltaics (PV) highlights the dependence on the module and system efficiency and lifetime in addition to module price. Here we summarize challenges and opportunities for CdTe and CIGS PV research and show that a substantial effort is still needed in areas such as device design and material improvement to reach higher efficiency and reliability connected with low-cost and robust module-scale implementation. We also discuss how SunShot Initiative funding is addressing key research areas in CdTe and CIGS PV and show how recent progress in SunShot projects is guiding funding priorities in thin-film PV research.

Current photovoltaic technologies harvest only a fraction of incoming solar energy since they are unable to utilize photons with energies below the cell band gap. Placed behind a solar cell, the upconverter converts transmitted low-energy photons to photons with energies higher than the cell band gap. The higher energy photons are absorbed by the solar cell and contribute to the photocurrent. We developed optical models of several state-of-the-art commercial and research thin-film solar cells incorporating the upconversion layer. We present both analytical models based on published EQE data as well as detailed finite difference time domain (FDTD) models that incorporate absorption in all cell layers. We model the improvement in absorption and overall cell performance of amorphous Si, CIGS, GaAs, CdTe, and Cu2O cells with upconverting layers. We incorporate and discuss the effect of interface texture and different cell layers on the absorption of upconverted photons and make suggestions for improving the overall cell design to get the maximum benefit from upconversion. We estimate that the cell efficiency enhancement can range from 0.5% to up to 5% absolute depending on the cell type and upconversion efficiency. This work connects to the fundamental efficiency limit analysis of narrow-bandwidth solar upconversion by our collaborators [1], but presents concrete optical models of current solar cells and discusses the promise of upconversion for particular applications.