Enhanced performance of Se-alloyed CdTe solar cells: The role of Se-segregation on the grain boundaries

Abstract

Recently, CdTe-based solar cells have achieved high power conversion efficiency by alloying with CdSe. Besides the increased photocurrent due to the reduced bandgap, it is also reported that the electron lifetime in the alloyed system is higher than that in the CdTe-based system. However, the origin of the improved lifetime is not clear. In this work, using first-principles calculations and the low energy Σ3 (112) grain boundary (GB) in polycrystalline CdTe as an example, we show that in the alloyed system, Se has the tendency to move toward the Σ3 (112) GB. Consequently, Se at the GBs in CdTe can effectively passivate the deep GB defect levels, thus reducing carrier recombination and improve solar cell performance. More specifically, we find that the Σ3 (112) GB with Te-core has the lowest formation energy among the electronically detrimental GB configurations in polycrystalline CdTe. The Σ3 (112) GB with Te-core introduces a deep defect state in the bandgap of CdTe, which can act as nonradiative recombination center and reduces the carrier lifetime of CdTe. When Se segregates to GB and substitutes the Te atom at the Te dimer site, due to the lower energy of Se 4p orbital and the weak coupling between the dimer elements in the GB core, the deep GB states will shift to shallower states toward the valance band maximum of CdTe. This can increase the carrier lifetime of the CdSeTe layer and thus provide a viable explanation to the improved lifetime and performance of Se-alloyed CdTe solar cells.