Hsu, Julia W. PO, Kenneth K.2023-08-242023-08-242021-082021-08-01August 202https://hdl.handle.net/10735.1/9784Organic photovoltaics (OPV) has been one of the consistently researched photovoltaic (PV) technology for the past three decades. Efficient charge generation, charge transport, and collection process account for higher performance in OPV devices. Several donor and nonfullerene (NFA) acceptors are developed to enhance charge generation. However, charge transport and collection still need critical understanding to further boost the device performance. In this dissertation, we focus on characterizing the defect states for efficient charge collection and charge transport mechanism in fullerene-based OPV devices. Surface photovoltage spectroscopy (SPS) was applied to probe the defect states without fabricating the complete devices. The physical location and the energetics of defect states are determined by comparing two types of SPS and top layer deposition. Understanding the charge transport mechanism is highly important in fullerene-based OPVs, where donor concentration is too low to form a percolation path to the anode. The effect of device architecture was studied to gain insights into the charge transport in fullerene-based OPV devices. From experimental results combined with drift-diffusion simulations, we found the imbalance in carrier mobility between electrons and holes results in inferior performance in inverted devices. Thienothiophene (TT)-based small molecule donors was designed and synthesized to study the photocurrent generation in fullerenebased OPVs. The donor and acceptor must form type-II energy level alignment at its interface for efficient exciton dissociation. We showed that the hole back transfers from donor to acceptor and transports to the anode via fullerene matrix. The photocurrent generation results from the hole back transfer mechanism in fullerene-based OPVs.application/pdfenEngineering, Materials ScienceEnergyChemistry, OrganicThesis2023-08-24