Studies on spatial dependence of photocarrier transport in hybrid perovskite based devices and applications

Abstract

Hybrid organic inorganic perovskites (HOIP) have emerged as a promising semiconducting materials for optoelectronic applications. The excellent performance of these materials have been attributed to properties such as long carrier lifetime and diffusion lengths, high absorption coefficient and ease of solution processability. In a device architecture, the carrier transport properties is influenced by the spatial boundary conditions and the potential landscape. This thesis focuses on study of spatial signatures of carrier transport in hybrid perovskite based devices, and its applications. The first part of the work deals with the impact of trap filling on carrier diffusion in Methyl Ammonium lead bromide (MAPbBr3) single crystals. The use of millimeter sized single crystals overcomes the limitation of grain boundary recombination. Using the technique of spatial photocurrent scanning microscopy (SPCM), the effective carrier diffusion length, Ld was estimated, and this parameter was found to reduce upon the introduction of low intensity light bias, suggesting that the recombination dynamics are not monomolecular. This observation were then correlated with intensity dependent transient PL studies that reveal distinct dynamics corresponding to band recombination and trap emission. Intensity dependent analysis reveals that the sub-band-gap trap recombination influences carrier transport in the low-intensity excitation regime, while bimolecular recombination and transport dominate at high intensity. The next part of the work, i.e. Chapter 3 presents the different regimes of carrier transport in a hybrid perovskite based lateral metal-semiconductor-metal device structures, with asymmetric electrodes. The device characteristics exhibit a cross-over from ohmic behavior to SCLC regime as function of inter-electrode distance and applied bias. In the observed device characteristics, the influence of carrier energetics at the metal-perovskite interface was studied using Kelvin Probe Force Microscopy (KPFM). KPFM on lateral MSM structures indicates the presence of a transport-barrier at Al-perovskite and an ohmic contact at the Au-perovskite interface. Additionally, the potential map also points out to ineffective screening due to mobile ions, confirming the reliability of the observed SCLC behavior. The spatial behavior of photogenerated carriers were understood in response to the already present potential profile, using the technique of scanning photocurrent microscopy (SPCM). In the presence of an applied bias, the potential distribution profiles indicate constant electric field in the device, and the light response were understood in the context of drift-diffusion formalism.