Organic photovoltaics (OPVs) have recently attracted intensive research interest for their unique properties and rapid development. In the past two decades, fullerene derivatives have been the dominant electron acceptor in the bulk heterojunction OPV. Recently, the emergence of novel non-fullerene acceptors (NFA) has changed the situation and has made remarkable progress in power conversion efficiency. The NFAs benefit from the tunable bandgap and energy levels and enable efficiency charge separation at very small or even negative energetic offset. However, the charge separation mechanism in these low offset systems remains unclear. Understanding the physics behind charge generation under unfavorable offset is critical in OPV.

This thesis focuses on the photophysics study of the OPV sys...[ Read more ]

Organic photovoltaics (OPVs) have recently attracted intensive research interest for their unique properties and rapid development. In the past two decades, fullerene derivatives have been the dominant electron acceptor in the bulk heterojunction OPV. Recently, the emergence of novel non-fullerene acceptors (NFA) has changed the situation and has made remarkable progress in power conversion efficiency. The NFAs benefit from the tunable bandgap and energy levels and enable efficiency charge separation at very small or even negative energetic offset. However, the charge separation mechanism in these low offset systems remains unclear. Understanding the physics behind charge generation under unfavorable offset is critical in OPV.

This thesis focuses on the photophysics study of the OPV systems with unfavorable offset. We investigate a series of systems based on novel NFA by optical approaches. We first study a PDI-based OPV system with small offset, and establish an endothermic charge separation model, in which the charge-transfers (CT) states require thermal activation to separate. We further quantify the charge separation at various temperatures by numerically solving the transient absorption spectra. Then, we systematically investigate the field-assisted charge separation behavior of A-D-A systems with various energetic offsets, and find that the bound CT states can be separated by external electrical field. Then the high blend PL and slow hole transfer kinetics of model unfused A–D–A′–D–A systems are studied. We find a well-established equilibrium between singlet excitons, CT states and free charge carriers that can achieve energetically unfavorable charge separation under short-circuit. In contrast, small nonradiative loss can be achieved under open-circuit. This thesis provides a fundamental understanding of different mechanisms in different NFA systems. Our work sheds light on the design of next-generation OPVs that can break the tradeoff between the photocurrent and energy loss in OPVs that limits further efficiency improvement.