Publication date: 16th July 2025
Optimizing open circuit voltage (VOC) in narrow bandgap (NBG) perovskite solar cells is essential for achieving highly efficient all-perovskite tandem solar cells (TSCs). NBG perovskite solar cells, particularly those utilizing tin-lead mixed perovskite absorbers, experience significant VOC losses primarily due to high defect density and charge carrier recombination at the device interfaces. Addressing these VOC losses is a critical challenge in enhancing the performance of these solar cells. This study aims to investigate and mitigate the factors contributing to VOC losses in NBG perovskites, thereby improving the overall efficiency of both single junction and all-perovskite TSCs. We explored the application of advanced materials, specifically a poly(triarylamine) (PTAA) interlayer between the hole transport layer (HTL) of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and the active layer, which consists of (FASnI3)0.6(MAPbI3)0.4 perovskite (referred to as “NBG perovskite”). We compared three different HTL strategies: PEDOT:PSS-based PSCs, PTAA-based PSCs, and cells employing a PEDOT:PSS/PTAA bilayer HTL. Comprehensive photophysical characterization was performed to understand the loss mechanisms and implement strategies to reduce recombination losses. The devices utilizing the bilayer HTL achieved a power conversion efficiency (PCE) of 20.3%, an increase from 17.8% for PEDOT:PSS-based PSCs and 16.0% for PTAA-based single-junction PSCs. The improved PCE and VOC of the bilayer-based PSCs are attributed to enhanced charge extraction and reduced interfacial recombination. We further demonstrated that the optimized NBG device employing the highly efficient bilayer as HTL can serve as the bottom subcell in an all-perovskite tandem solar cell (TSC). By combining the optimized NBG perovskite subcell with a wide bandgap (WBG) composition, specifically Cs0.3FA0.6MA0.1Pb(I0.7Br0.3)3 (referred to as “WBG perovskite”), we achieved a remarkable champion VOC of 2.00 V and a high PCE of 25.1%. This study presents effective strategies for overcoming VOC losses in narrow bandgap perovskites, leading to improved efficiency in all-perovskite tandem solar cells. The study presents effective strategies to overcome VOC losses in narrow bandgap perovskites, leading to higher efficiency in all-perovskite tandem solar cells. It underscores the effectiveness of interface engineering and material optimization in overcoming VOC losses in a device.
Keywords: Narrow bandgap perovskites, open circuit voltage, power conversion efficiency, tandem solar cells, defect passivation, charge carrier recombination
Y.A.T. acknowledges funding by the International Science Program (ISP, IPPS ETH: 03, 2021-2026) and the German Academic Exchange Service (DAAD) binationally supervised doctoral degrees/cotutelle (funding ID 57588368 and ref no. 91834339). M.I.H. and D.T.C. gratefully acknowledge funding from the German Federal Ministry for Economic Affairs and Climate Action (BMWK) through the APERO project (ref number 03EE1113C). A.F. acknowledges financial support from the European Commission in the framework of Marie Skłodowska-Curie Individual Fellowships (grant number 101030985 - RADICEL). A.F., L.V.M., and P.D.V. acknowledge funding from the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 101006715 (VIPERLAB).
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