Publication date: 25th September 2020
The perovskite solar cells have been extensively developed since 2009 but still the ultrafast and fast processes occurring at the interfaces in this system are not fully understood. We focused on the investigation of electron transport paths: after light absorption, an electron is promoted from the valence to the conduction band, and then several processes can occur. First few hundreds of femtosecond after light absorption are governed by cooling of the hot carriers. When the process is finished, sharp bleach due to the band filling phenomena occurs at absorption edge in the transient absorption measurements. Its decay correlates with photoluminescence kinetics and represents the excited carrier lifetime [1,2]. That decay proceeds by several paths such as the recombination (first-, second- and third-order) and the charge injection to an electron transporting material (ETM) or a hole transporting material (HTM).
In this work we focused on a triple cation perovskite FA0.76MA0.19Cs0.05(I0.81Br0.19)3 sandwiched between a spiro-OMeTAD (HTM) and mesoporous TiO2 (ETM) layers prepared under ambient (in the presence of oxygen and ambient room humidity) conditions. The studies were based on femtosecond to nanosecond transient absorption, picosecond to nanosecond time-resolved emission, electrochemical impedance spectroscopy and x-ray diffraction measurements of the prepared cells. By properly tuning the excitation wavelength, changing the excitation side and grazing angles we were able to selectively probe the titania/perovskite or perovskite/spiro-OMeTAD interfaces of the cells.
Difference in PbI2 content as well as difference in charge dynamics on the ETM and HTM interface were detected. The transient bleach and stationary emission band maximum was also shifted when HTM or ETM side was investigated. We also checked influence of DMSO content in the precursor solution on the cell parameters. We found that higher DMSO concentration causes an increase of the ideality factor in electrochemical impedance studies [3].
The study was supported by Polish Ministry of Science and Higher Education (Ministerstwo Nauki i Szkolnictwa Wyższego) under project “Diamentowy Grant” 0019/DIA/2017/46.
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