Bandgap gradient strategy with ultra-thin passivation layer (1 nm) enabling lower SRH voltage loss in perovskite/CdTe Tandem Solar Cells

An experimentally demonstrated open-circuit voltage of a CdTe-based solar cell is only 904.8 mV, which is ∼235.2 mV lower than the Shockley–Queisser voltage limit. This voltage loss can be attributed to the factors such as radiative, nonradiative, and thermodynamic recombination losses. To circumvent the voltage loss issue, this study proposes a strategy of implementation a CdSe_0.2Te_0.8 passivation layer in conjunction with graded bandgap absorber layers. Initially, we examined a device with a configuration of V₂O₅/CdTe/ZnSe in absence of passivation layer. This device resulted in a larger Shockley-Read-Hall (SRH) recombination voltage loss of 281 mV and a total voltage loss of 716 mV. We modified this device configuration by utilizing an ultrathin layer (1 nm) of CdSe_0.2Te_0.8, i.e., V₂O₅/CdTe/CdSe_0.2Te_0.8/ZnSe. An ultrathin layer of CdSe_0.2Te_0.8 works as an effective passivation layer, substantially reducing the SRH recombination voltage loss to 46 mV from 281 mV. Interestingly, when a thicker layer of CdSe_0.2Te_0.8 is utilized, it not only acts as a passivation layer but also functions as an absorber layer, creating a bandgap gradient. However, improving the grain boundary between CdSe_0.2Te_0.8 and ZnSe is necessary to further cuts down to SRH recombination voltage loss below 46 mV. To overcome this issue, a thin window layer of CdS_0.102Se_0.336Te_0.562 has been incorporated in between CdSe_0.2Te_0.8 and ZnSe, close to the front contact, which reduces the SRH recombination voltage loss to 20 mV. This SRH recombination voltage loss can be further minimized to zero from 20 mV when the back interface has been optimized. All simulation data have been justified by previous reported experimental finding to validate the proposed simulation models. Additionally, two-terminal and four-terminal perovskite/CdTe tandem configurations have also been proposed and simulated, yielding power conversion efficiency of 28.64 % and 29.80 %, respectively. These findings demonstrate the efficacy of passivation layer, double absorber layer, bandgap grading, window layer, and interface engineering in mitigating SRH recombination voltage loss, offering a roadmap for future perovskite/CdTe tandem solar cells.