Publication date: 17th February 2025
SnO2 is the most popular electron transport layer (ETL) for n-i-p perovskite solar cells. The good performance of SnO2 ETLs is surprising, as the conduction band minimum is expected to be well below that of lead halide perovskite, which should lead to considerable voltage losses in the solar cell.
In order to investigate processes at the SnO2 / perovskite interface we performed simple electrochemical measurements such as cyclic voltammetry and chronoamperometry on complete perovskite solar cell devices in the dark. The device structure used was ITO/SnO2/FAPbI3/spiro-OMeTAD/Au.
Time-resolved potential steps in forward direction (cathodic) displayed a biphasic current response, where the faster part can be attributed to ionic displacement (with charge: mC cm-2 regime) and the slower part (seconds) to an electrochemical reaction (charge: 100s mC cm-2). Cyclic voltammetry displays a quasi-reversible electrochemical reduction reaction occurring at about -0.8 V (vs the spiro-OMeTAD/Au contact, which serves as both counter and reference electrode), see Figure 1. We attribute the reduction to proton insertion into the SnO2, where the protons are provided by formamidinium cations (H2NCHNH2+):
SnO2 + H2NCHNH2+ + e- → SnO2-H + H2NCHNH
The reversibility of this electrochemical reaction is poor, as electrons in SnO2 can react with holes in the perovskite:
SnO2-H + H2NCHNH + h+ → SnO2 + H2NCHNH2+
The reactions observed here explain the significant charging effect that is required before n-i-p perovskite solar cells attain their maximum photovoltage upon illumination, a process that can require several seconds.
This research was partially funded by the European Union's Horizon Europe programme, through a FET Proactive research and innovation action under grant agreement No. 101084124 (DIAMOND).
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