2, Kyushu Institute of Technology, Kitakyushu, , Japan
3, University of Electro Communication, Tokyo, , Japan
4, National Institute of Advanced Industrial Science and Technology, Tokyo, , Japan
Lead (Pb) based perovskite solar cells(PSCs) are continuously getting improved in terms of stability, reproducibility and efficiency (22.1%). However, presence of Pb in PSCs has toxicity concern to environment and human health. Therefore, in recent time a lot of research has been done in looking for a Pb free solar cells based on tin (Sn), germanium (Ge) such as FASnI3, MAGeI3 etc . Although efforts are being done as described to solve the toxicity issue, but it costs the reduction in solar cell efficiency. So one of the alternative is to mix Pb with Sn making PSC relatively less toxic with the advantage of increased absorption spectra  and comparative photoconversion efficiency (PCE) to Pb PSCs. Our group is involved in fabricating the Sn/Pb alloyed solar cells and so far best PCE of 15.93%  has been achieved. It is well known that Sn/Pb possess low air stability due to rapid oxidation of Sn2+ to Sn4+, which results in relatively faster degradation of solar cell performance compared to purely Pb based solar cells. To elucidate this problem we tried to explore the role of multiple monovalent cations such as rubidium (Rb+), cesium(Cs+), formamidinium [(CH3(NH2)2+, FA+], methylammonium [(CH3NH3)+, MA+] etc. on the A position of ABX3 perovskite crystal structure , which is already an effective way to improve the stability and reproducibility of Pb based PSCs. Therefore, the present research work discusses about the role of multiple monovalent cations in improving the stability and performance of Sn/Pb PSCs as answered by X-ray diffraction (XRD) pattern, thermogravimetric analysis (TGA), scanning electron microscopy etc. We fabricated the solar cells with precursor solutions such as (FASnI3)0.6(MAPbI3)0.4 as (FAMA), (CsI)x[(FASnI3)0.6(MAPbI3)0.4]1-x as (Cs)x(FAMA)1-x. In result we found that with the addition of Cs(5%) showed increase in PCE from 10.26% (FAMA) to 11.66%. This improvement in efficiency was clearly observed in IPCE as well. Cs(5%) containing solar cell exhibits more than 60% absorption at 900 nm with a high photocurrent of 28 mA/cm2. We will finally report an optimized solar cell performance of ~16% with the further discussion on the role of interfacial engineering.
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