Novel upconversion nanostructure for high efficiency photovoltaics: theoretical model and material study
Date
2018
Authors
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Publisher
University of Delaware
Abstract
Upconversion is a process that converts two low energy photons to one high energy photon. It could be applied to solar cell to increase the solar power conversion efficiency. A novel upconversion nanostructure consisting of InAs quantum dots (QDs) and Al-containing dilute Bismuthides was proposed previously. Here I will discuss the kinetic rate equation model that was developed to describe the kinetic processes and predict the upconversion nanostructure performance. Combined with detailed balance approach for the solar cell, the calculation showed that the structure could potentially improve the solar cell performance significantly.
We also analyze the robustness of the model by considering how the performance of both the upconverter and an upconverter-backed solar cell are affected by variations in the assumptions made in the model, including QD absorption cross section, solar spectrum splitting and carrier lifetimes. We further analyze the performance of the nanostructure as a function of solar concentration. The theoretical performance of this upconversion paradigm under concentrated sunlight exceeds 60% solar energy conversion efficiency. Moreover, the results show that the predicted performance is relatively insensitive to the assumptions made in the model, suggesting that practical realization of such a semiconductor upconverter heterostructure paradigm is possible. ☐ Incorporating a small amount of bismuth could cause a large band gap reduction. Previous studies on GaBiAs and InGaBiAs have shown that the band gap reduction occurs mostly in the valence band. This property is helpful for application in the aformentioned upconversion nanostructure. MBE growth of Al-containing dilute bismuthides (In)AlBiAs at low temperature was demonstrated. Good quality InAlBiAs thin films was achieved. However, AlBiAs growth condition needs to be further explored to improve the material quality. We estimate band gap reduction from Bi incorporation of 47 meV/%Bi and 63 meV/%Bi for InAlBiAs and AlBiAs respectively (linear band gap reduction value is often used when discussing the Bi-induced band gap reduction). The experimental determined InAlBiAs band gap agree with valence band anticrossing (VBAC) theory fairly well when Bi% is less than 3%. Further investigation on the VBAC model and optimization of the (In)AlBiAs quality are needed.