Non-fullerene acceptors (NFA) have revolutionized organic photovoltaic (OPV) technology, yielding devices with power conversion efficiencies (PCE) exceeding 19%. Despite this progress, fundamental questions remain unclear regarding the photophysical properties and excited state dynamics governing their performance. This dissertation investigates the photophysical origins of the superior performance of the state-of-the-art NFA, Y6, using steady-state and advanced ultrafast spectroscopic techniques along with device fabrication and characterizations.  

The first part of this dissertation addresses the debated question of whether the state-of-the-art NFA Y6 intrinsically photogenerate free charges. Our results demonstrate that Y6 does not intrinsically generate free charges. Instead, photoexcitation produces a charge-transfer (CT)-like exciton that evolves from a short-lived hot exciton. Using cw- photoinduced absorption, doping-induced absorption, spectroelectrochemistry, and transient absorption spectroscopy, we identify the short-lived species to hot excitons and the subsequent longer-lived species to CT-like exciton, where electrons and holes located on adjacent molecules. This CT-like exciton forms directly upon bandgap excitation and decays on a ~1 ns timescale. Excitation-dependent photoluminescence (PL) and PL quantum yield (PLQY) confirm the excitonic nature of this emissive state: the emitted photon number scales linearly with excitation density at low fluence then deviate to sublinear at high fluence due to singlet-singlet annihilation. Delayed fluorescence exhibits superlinear dependence and a power-law decay, consistent with thermally activated recombination of CT states formed after triplet generation. Temperature-dependent measurements yield an activation energy of ~50 meV, while transient absorption shows triplet formation within ~100 ps followed by decay within tens of nanoseconds. 

The second part examines charge generation and recombination processes in high-performance OPV blends, namely D18:Y6 and D18:BTP-eC9. Although D18:BTP-eC9 exhibits higher open-circuit voltage and fill factor, time-delayed collection field (TDCF) measurements reveal an ~11% lower external charge-generation efficiency compared to D18:Y6. Ultrafast spectroscopy shows efficient exciton dissociation in both blends, but long-delay transient absorption and TDCF measurements indicate bimolecular recombination that is approximately an order of magnitude faster in D18:BTP-eC9. Gated TRPL further reveals delayed fluorescence arising from recombination pathways that compete with charge extraction, attributed to faster exciton decay and higher triplet yield. 

Overall, this work identifies exciton lifetime, triplet formation, and recombination dynamics as key factors limiting photocurrent generation, providing photophysical design guidelines for next-generation OPVs targeting efficiencies beyond 20%.