The ever-growing demand for carbon-neutral energy source has led photovoltaic (PV) technologies to become fastest-growing form of renewable energy. Among many different PV technologies, organic photovoltaic (OPV), being based on earth-abundant materials and showing short energy payback times, has been exclusively attractive. Donor:acceptor bulk-heterojunction structure has enabled efficient photon – electron conversion mechanism and is by far the most widespread structure for OPV. For almost two decades highest power conversion efficiencies (PCE) were obtained with fullerene type acceptors. Fullerene owning to its ball-like fully conjugated structure, which enables high electron accepting and isotropic electron transport capabilities was believed to be a crucial part of efficient OPV technology. Other acceptor molecules, generally named nonfullerene (NF) molecules, usually shows low PCE’s, mostly due to difficult morphological control. However, NF acceptor resurgence was witnessed in the last three years, with power conversion efficiency skyrocketing to record-breaking >13.5% values, exceeding those reached by fullerene-based systems.

Today there are >100 different acceptor molecules displaying high efficiencies. Most of them are so-called A-D-A (or D-A-D) type molecules, distinct by its two symmetrical electron accepting (donating) complexes connected via conjugated electron donating (accepting) backbone, which itself is surrounded by non-conjugated tails. Most striking feature of such molecules is efficient exciton splitting at low, or even negligible, driving forces made by HOMO/LUMO level mismatch at D/A interface. It enables energy loss minimisation and results in high V_OC values. Nevertheless, fundamental question on how exciton split into charge carrier at low driving forces and their initial kinetics afterwards remains open.

To address this problem, we have performed both conventional transient absorption and time resolved electroabsorption measurements with 4 different donor:acceptor material combinations (PBDB-T:Y1; PBDB-T-2Cl:ITIC; PDCBT-2F:ITIC; PBDB-T: PCBM) each with few different D/A mass ratios. Ten samples in total were investigated with PCE ranging from 3.2% to 11.1%. Excitation wavelength was chosen to excite either donor or acceptor specifically, thus electron transfer both from donor to acceptor and from acceptor to donor could be observed.

As expected, after exciting acceptor extremely fast (<10ps) hole transfer from acceptors HOMO to donor HOMO orbital was observed, for all the samples with A-D-A (D-A-D) type acceptor, which is one of already known distinct feature of NF molecules. However, less expected result manifested after donor’s excitation took place. In this case, ground state bleaching of an acceptor was extremely low, especially in case of best performing PBDB-T:Y1 sample. Acceptor's absorption spectra indifference to received electron was later confirmed by frontier molecular orbital calculations of neutral and ionized molecular states. Bleaching decay rate of an ionized donor was more rapid in BHJ with non-fullerene acceptors compared to fullerene based devices, which indicates either faster recombination rate and/or carrier return to a geminate molecule enabled by minor energy level offset. Both processes detrimental to photovoltaic performance.