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Despite the high-efficiency of these lead-based perovskite solar cells, the problem associated from the toxic nature of lead has open a new research direction which focuses on lead-free perovskite materials. As an alternative, tin has been proposed to replace lead. The highest efficiency obtained with Sn only perovskite was 9 % which was based on 2D and 3D mixture of FASnI3. However, Sn-based perovskites are known to have low stability in air. The use of germanium-based perovskite in solar cell was first realized by Krishnamoorthy et. al. The measured solar cell performance was notably low, 0.11 % for CsGeI3 and 0.20 % for MAGeI3. A theoretical study exploring hybrid tin and germanium perovskite showed that it is possible to prepare a stable Sn-Ge perovskite material that absorbs the sunlight spectrum. In this study, a new type of SnGe mixed metal perovskite solar cells are reported with enhanced efficiency and stability. In this report, FA0.75MA0.25Sn1-xGexI3 (abbreviated as SnGe(x)-PVK) were used for the mixed metal SnGe perovskite. XRD spectra showed that the structure is perovskite.
The structure of Ge-doped Sn perovskite was also discussed from the view point of band gap, conduction and valence band level, XPS analysis, and the urbach energy. It can be concluded that most of the Ge atoms passivate the surface of the Sn perovskite (graded structure).For SnGe(0)-PVK device, the averageJsc was 17.61 mA/cm2, VOC was 0.46 V, FF was 0.41 and PCE of 3.31 %. Upon doping with 5 wt% of Ge, the JSC increased up to 19.80 mA/cm2, FF improved up to 0.55 with an overall efficiency of 4.48 %. Upon increasing the Ge content more than 10wt%, all the photovoltaic parameters decreased significantly which resulted in an efficiency as low as 0.80 % for SnGe(0.2)-PVK device. After optimization, 7.75% of SnGe(5)-PVK device is reported. Significant effect on Ge doping was seen in the enhancement of the stability. The stability in air has been improved significantly with the Ge doping, retaining 80 % of its original performance, remarkable stability enhancement, compared with 10 % retention for non-doped sample. This work provides a platform for further research on lead-free Sn-Ge based perovskite solar cells.
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