Modulation of Excited State Dynamics in Semiconductor Electrodes with Electrical Bias

Abstract

Semiconducting materials are essential building blocks of the majority of devices in the 21st century. The ability of semiconductors to generate charge carriers under band gap irradiation makes them suitable for a myriad of applications (e.g., solar cells, photocatalysts, photodetectors). In recent years the emergence of organic-inorganic lead halide perovskites revitalized these research fields. As a result, the efficiency of derived solar cells reached 22.3% in less than 10 years. To unravel the reasons behind their outstanding performance, it is vital to understand their optoelectronic properties. Spectroelectrochemical methods are suitable to determine fundamental optoelectronic properties (band edge and trap state energies) and electrochemical bias induced chemical changes in these materials. However, to probe charge carrier dynamics in these materials, it is essential to utilize ultrashort laser techniques, as these fast processes fall into the femto- or picosecond timescale. In my presentation the effect of applied bias on the charge carrier dynamics of semiconductor electrodes will be discussed. Ultrafast spectroelectrochemical experiments were carried out on FTO/TiO2/CsPbBr3 and ITO/CuI systems by coupling ultrafast transient absorption spectroscopy with electrochemical techniques.1 These two systems have practical relevance in perovskite based solar cells, and their excited state behavior under operating conditions was scrutinized. It was found that the excitonic features of both CsPbBr3 and CuI are responsive to the applied external bias, within their electrochemical stability window. The accumulation of electrons on the TiO2/CsPbBr3 or ITO/CuI interface have a pronounced effect on charge carrier lifetimes in these studied materials. The change in charge carrier lifetimes in both CsPbBr3 and CuI was completely reversible showing the dependence of excited state dynamics on the externally controlled charge carrier density. This validates the in situ electrochemical transient absorption measurement as a useful tool to probe the charge carrier injection process in different semiconductor systems. Scheidt, R. A., Samu, G. F., Janáky, C. & Kamat, P. V. Modulation of Charge Recombination in CsPbBr3 Perovskite Films with Electrochemical Bias. J. Am. Chem. Soc. 140, 86–89 (2018).