Dr Junke Wang

Abstract:

This work investigates how excitation energy, internal electric fields and device architecture control ultrafast charge formation, separation and extraction in organic semiconductors. The thesis contrasts a vacuum-deposited single-component small-molecule system, DCV2-5T, with a solution-processed donor–acceptor blend, PM6:Y6. Transient absorption spectroscopy tracks excited-state populations, electromodulated differential absorption quantifies field-assisted separation and extraction, and optical-pump terahertz-probe spectroscopy evaluates photoconductivity and carrier localisation. Higher photon energy accelerates the initial formation of charges in both systems, yet long-lived charge populations emerge only when early extraction competes successfully with recombination. In DCV2-5T, the dominant bottleneck is extraction in the absence of a donor–acceptor interface rather than charge-generation kinetics. Introducing transport layers increases quantum efficiency but produces slower initial extraction that limits initial photoresponse, relevant for photodiode operation. External reverse bias improves extraction and exposes recombination and localisation as direct competitors to charge collection. In PM6:Y6, durable charge populations arise when excitation accesses delocalised Y6 aggregates at early times. Direct excitation of aggregated Y6 yields the most stable carriers, whereas populations originating from less connected regions show stronger field response. The thesis establishes extraction-limited behaviour in the single-component system and identifies aggregate connectivity with access to delocalised acceptor states as the decisive lever in the blended system