Impact of solar upconversion on photovoltaic cell efficiency: optical models of state-of-the-art solar cells with upconv
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Impact of solar upconversion on photovoltaic cell efficiency: optical models of state-of-the-art solar cells with upconverters Inna Kozinsky1 , Yi Xiang Yeng1,2, and Yao Huang1 1 Research and Technology Center, Robert Bosch LLC, Palo Alto, CA 94304, U.S.A. 2 Research Laboratory of Electronics, MIT, Cambridge, MA 02139, U.S.A. ABSTRACT 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. INTRODUCTION Most commercial photovoltaic cells have a single p-n semiconductor junction with one discrete band gap and can only absorb photons with energies exceeding this band gap. This limitation gives rise to the Shockley-Quiesser limit for overall cell efficiency, which does not reach beyond 33% for any single-junction cell [2]. Upconversion can help overcome this limit by capturing the lost sub-band-gap photons and converting them to visible photons, which are then re-absorbed by the solar cell. This approach is, in principle, more efficient than adding a lowerband-gap cell to capture the transmitted photons because the cell operating voltage remains high due to the higher band gap and no current-matching is required. The radiative limit of the energy conversion efficiency of a single-junction cell with upconversion was calculated using a detailed balanced model to be 47.6% for nonconcentrated light [3]. This limit was later revised by including non-idealities, such as free carrier absorption and, additionally, narrow absorption bandwidth, to be around 40% [4] and 34% [1] respectively. However, the predicted substantial improvement from upconversion has not been seen experimentally. Goldschmidt et al. [5] report a rel
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