Towards understanding mechanisms of charge transfer in organic semiconductors

• Main Author: Macdonald, Josh
• Format: Dissertation
• Language: English
• Published: University of Bath 2021
• Subjects: charge transfer; organic semiconductor; photovoltaics

The broad potential for real-world applications of organic semiconductors is currently being limited by an incomplete understanding of their fundamental physics. The processes by which charges transfer both between and within molecules in organic semiconductors are crucial to device performance and are one such area where further understanding is required. This work seeks to contribute towards filling this lack of knowledge, firstly by using an ordered donor-acceptor crystal as a model system with which to study the optical signatures of charge transfer excitons without the presence of morphological disorder. In doing so, we discern that dynamic dipole disorder causes a multipart splitting of the charge transfer state due to dipole-dipole interactions of the charge transfer exciton with nearest-neighbour molecular dipoles. Secondly, we seek to better understand the processes of charge transfer that induce doping by comparing two separate doping mechanisms which utilise charge transfer in two distinct ways. The Lewis acid dopant BCF and the integer charge transfer dopant F4-TCNQ are both applied to multiple polymers. By measuring the resulting films' steady-state optical properties, we discern the spectral fingerprints of the two distinct doping mechanisms and confirm the protonation mechanism by which BCF dopes, as well as identifying the polaron absorptions induced by both dopants. Finally, we further our understanding of these doping processes by measuring their impact on ultrafast charge dynamics using pump-probe spectroscopy. In this study, we find that each dopant extends the lifetime of the exciton by enhancing the polymer mobility, albeit to a greater extent with BCF. Additionally, we identify a signature of doping-induced polaron formation in the ultrafast dynamics that changes the form of the transient decay. Finally, we investigate the effect of doping on the recombination of polarons, finding that the subpicosecond geminate recombination of polarons is suppressed by increased doping, while non-geminate recombination is relatively unaffected.