Nonlinear Optical Probes for Organic Field Effect Transistors and Halide Perovskites

Abstract

As the field of semiconductors is booming at an exponential rate, surpassing popular industry predictions like Moore’s law, small molecule and polymer based semiconductors such as organic and hybrid (organic-inorganic) materials are gaining popularity. Field effect transistors (FETs) form the cornerstone of such electronic technology. Unlike MOSFETs, the organic FETs operate in accumulation regime which is influenced by the metal-semiconductor and semiconductor-dielectric interfaces. Traditional current-voltage characterization is often ineffective in accurately probing these devices due to inherent contact resistance issues and high density of trapped states. Thus, a need for alternate techniques for visualizing and measuring transport in organic FETs is much sought. Transient nonlinear optical techniques have emerged as powerful tools in characterizing such devices, not only for estimating intrinsic transport parameters free from contact issues, but also by aiding in visualization of the interface conditions during formation of accumulation layer while the devices are operating.

In this thesis, we setup a microimaging and spectroscopic probing system based on an ultrafast laser source equipped with a prism compressor setup for compensating group velocity dispersion effects from a broadband source for studying an array of optoelectronic systems with a focus on organic thin film devices. We demonstrate contact-resistance free transport parameters from p-type organic FETs showing upto 20 times improvement in mobility estimation utilizing time-resolved electric field induced second harmonic generation (TR-EFISHG) techniques. An order of magnitude improvement in mobility estimation is observed in case of n-type FETs. The majority carriers in these devices are easily differentiable using this technique. Further, this technique enables us to map the electric field distribution in the device channel as well as help visualizing the formation of accumulation layer in each of the cases. Comparison of a small molecule semiconductor with a polymer while using different polymer dielectrics and oxide layers help discern the effect of the semiconductor dielectric interface on the device performance thus providing an effective way of improving such performance.

Furthermore, we use this nonlinear optical probing system to explore lead halide perovskites. Lead halide perovskites offer an opportunity to investigate nonlinear optical properties with third and higher-order harmonic generation being possible despite the centrosymmetric crystal structure of 3D lead halide perovskites, which prohibits second harmonic generation. This research delves into the third harmonic generation (THG) from CsPbBr₃ nanocrystals (NCs) and compares it to the THG from CsPbBr₃ NCs with Ruddlesden-Popper planar faults (RP-CsPbBr₃). The THG from CsPbBr₃ NCs is negligible in comparison to RP-CsPbBr₃ NCs for a wide range of femtosecond excitation wavelengths. Furthermore, the THG of a thin film of RP-CsPbBr₃ is compared to that of a single crystal of methylammonium lead bromide (MAPbBr₃), revealing that the THG efficiency of RP-CsPbBr₃ is three times greater than that of MAPbBr₃. An effective third-order susceptibility of approximately 10⁻¹⁸ m² V⁻² is obtained for a RP-CsPbBr₃ film, indicating the potential of inorganic halide perovskite NCs with planar defects for various nonlinear optical applications.