Nearly all of the photovoltaic (PV) technologies that have emerged over the past two decades have been preceded by time-resolved optical spectroscopic investigations of the photoactive layers that comprise the devices. Transient absorption (TA) spectroscopy has revealed photoinduced electron transfer from the primary light harvesting materials to electron acceptors in systems such as organic photovoltaics (OPVs); while time-resolved photoluminescence (TRPL) has demonstrated long carrier diffusion lengths in the most efficient class of emerging PVs, the perovskites. Time-resolved spectroscopies have accelerated the search for new material systems to serve as PV active layers, while simultaneously enabling an understanding of the losses within PV devices.

In this talk, I will introduce some of the ways we have been using time-resolved spectroscopies to investigate novel organic and perovskite active layer systems and interfaces. I will then describe in detail our results on one particular system: interfaces between organic active layers and two-dimensional (2D) molybdenum disulfide (MoS₂) films. OPVs have reached efficiencies >19% thanks to the advancement of non-fullerene acceptor materials, such as the molecule BTP-4F (i.e., Y6). The unique molecular structure of Y6, having alternating sub-units of electron donating (D) and accepting (A) groups, in the form of A-DAD-A, gives rise to strong excitonic charge transfer characteristics, potentially enabling charge separated states even without an interpenetrating donor/acceptor interface typical for bulk heterojunction OPVs.

I will describe the charge generation, transfer, and separation processes that occur when Y6 thin films are interfaced with large-area, single crystal MoS₂ monolayers. Using femtosecond-to-microsecond TA and picosecond TRPL spectroscopies, we demonstrate that Y6/MoS₂ interfaces lead to long-lived charge separation in both layers. These results demonstrate the viability of using Y6/MoS₂ as bilayer OPVs, avoiding the need for multi-component bulk heterojunctions.