Transient optical studies of carrier dynamics in perovskite solar cells

In this thesis, the charge carrier dynamics of trapping and transfer of electrons and holes are investigated for perovskite photovoltaics based on methylammonium lead iodide (MAPI3) model materials. To understand the recombination processes in perovskite photovoltaics, the photophysical properties of perovskite thin-films and perovskite with charge transport layers (CTL) are studied with various optical measurement tools such as photoluminescence (PL) spectroscopy, time-correlated single photon counting (TCSPC) and transient absorption spectroscopy (TAS). This thesis provides key new quantitative insights for the critical steps in the function of planar perovskite-based solar cells. One of the challenges of using spectroscopic tools to study perovskite is the reliability of the data. Strong intensity dependent behaviour is observed, which is important for interpretation data from different measurement techniques. Several studies have reported that trapping and recombination dynamics in MAPI3 films are highly, and non-linearly light intensity (and carrier density) dependent. However, studies on the impact of this non-linear, light intensity behaviour on interfacial charge transfer efficiency and the efficiency of photocurrent generation in devices under operation, have been very limited to date. Initially, perovskite structure and crystallization process, as well as the principles of photophysics and spectroscopy measurement techniques are introduced. A procedure for measuring charge carrier dynamics as a function of excitation power and the sample preparation is demonstrated. The first chapter of the results section describes the dependency between excitation conditions and charge carrier dynamics of MAPI3 with PEDOT:PSS and PC61BM as a CTL, to unveil the recombination behaviour in a perovskite device. The results show that different excitation density of the excitation source and repetition rate can cause significant difference on recombination processes. 1 Sun equivalent continuous-wave light emitting diode (LED) source allows collecting reliable PL quenching efficiency to estimate interlayers charge transfer rate. Steady-state and pulsed measurements indicate low transfer efficiencies at low excitation conditions (5E + 17 cm−3) due to fast bimolecular recombination. TCSPC and TAS were used to explore the dynamics. The second chapter describes the change of the perovskite crystal with oxygen when kept in the dark and explores the impact on photoluminescence behaviour. Spectroscopic tools such as PL, absorptance and TAS are used to analyse the evolution of PL properties which is corelated with X-Ray diffraction (XRD) and device performance. The third chapter focuses on the integrated perovskite device structure stacked with organic photovoltaics. In this structure, carrier transfer behaviour has been changed not only by the interface between CTL and perovskite-organics bulk heterojunction (BHJ) layer, but also by charge accumulation in whole device components such as electrode and Zinc-oxide (ZnO) hole blocking layer due to the electric field, which is the key to design new device structure. The last chapter provides conclusions from these studies and suggests further work.Open Acces