Ultrafast electronic processes in doped metal-halide perovskite semiconductors

Electronic doping of semiconductors is an important topic in optoelectronics. Introducing small amounts of electron-rich or -poor dopants into the crystal structure of the semiconductor increases its conductivity and shifts the Fermi level, enabling efficient transistor, diode and photovoltaic device architectures. This thesis discusses the effects of doping in metal-halide perovskites, a novel group of semiconducting materials highly relevant for optoelectronic applications.
The properties of the doped materials are studied in this work with a variety of spectroscopic techniques, notably including terahertz (THz) spectroscopy. The analysis of terahertz transmission through thin films has proven to be an excellent tool for investigating optoelectronic properties of novel semiconductors. As the relation between the conductivity of the material and the THz transmission function is generally complicated, simple analytical expressions have been developed to enable straightforward calculations of frequency-dependent conductivity from experimental data in the regime of optically thin samples. However, significant deviations of the calculated photoconductivity from its actual value are observed in this thesis in semiconductors with high background conductivity when using these expressions. An alternative analytical formula is therefore developed, which greatly improves the accuracy of the estimated value of the photoconductivity while remaining simple to implement experimentally.
This improvement of data analysis methodology is highly relevant for studies of photoexcited charge-carrier dynamics in electrically-doped semiconductors, such as tin-iodide perovskites, where the commonly used expression for photoconductivity would result in an underestimate of charge-carrier mobility by over 50%. Tin-iodide perovskites are an important group of semiconductors for photovoltaic applications, as they exhibit higher intrinsic charge-carrier mobilities and lower toxicity than their lead-based counterparts. The spontaneous but controllable p-type doping in these materials provides an interesting opportunity to obtain novel insight into the effect of electronic doping on the dynamic processes, particularly on the intraband transitions of hot charge carriers in metal-halide perovskites. Using a new experimental technique, this thesis reveals that newly photogenerated charge carriers relax within the bands of FA0.83Cs0.17SnI3 on a sub-picosecond timescale when a large, already fully thermalized (cold) population of charge carriers is present. Such rapid dissipation of the initial charge-carrier energy suggests that the propensity of tin-halide perovskites towards unintentional self-doping resulting from tin vacancy formation makes these materials less suited to implementation in hot-carrier solar cells than their lead-based counterparts.
Finally, an investigation of charge-carrier trapping and conduction in films of MAPbBr3 perovskite chemically doped with bismuth is presented in this thesis. Successful chemical doping of metal halide perovskites with small amounts of heterovalent metals has attracted research attention because of its potential to improve long-term material stability and tune absorption spectra. However, some additives have been observed to impact negatively on optoelectronic properties, highlighting the importance of understanding charge-carrier behaviour in doped metal-halide perovskites. It is here found that the addition of bismuth has no effect on either the bandgap or exciton binding energy of the MAPbBr3 host. However, a substantial enhancement of electron-trapping defects upon bismuth doping is observed, which results in an ultrafast charge-carrier decay component, enhanced infrared emission and a notable decrease in charge-carrier mobility. Such defects arise from the current approach to bismuth-doping through the addition of BiBr3 salt, which may enhance the presence of bromide interstitials.

Advisor: Herz, L
Johnston, M