The most efficient single-junction polymer: fullerene organic solar cells (OSC) are now reaching power conversion efficiencies of 10%, but further device improvements are needed for this technology to become commercially viable. A major obstacle to achieve this is our poor understanding of the relationship between materials energetics, film morphology and charge photogeneration,   which does not allow us to control the charge photogeneration properties of the polymer: fullerene photoactive layer.

In this contribution, I will present our recent advances in understanding the factors controlling charge photogeneration and device short circuit current of low bandgap: PCBM bulk-heterojunction devices. I will give examples of how some of these factors can be exploited for the fabrication of more efficient OSC. In particular, the role of materials energetics and film morphology on charge photogeneration was evaluated for a series of devices consisting of low bandgap indacenodithiophene-containing (IDT) and diketopyrrolopyrrole-containing (DPP)copolymers and PCBM.

Using femtosecond to millisecond transient absorption spectroscopy, we compared the charge generation dynamics of IDT: PCBM blends with differing LUMO - LUMO offsets. For the blend with a low energy offset, we observed ultrafast population of bound-polaron-pair (BPP) states and their geminate recombination to the polymer triplet on the sub-nanosecond timescale. For the higher energy offset blends, charge generation was not limited by geminate recombination of BPP states, but instead by sub-optimal film morphologies. Our further analysis of charge photogeneration as a function of film composition of the high energy offset system revealed that the yield of polarons on the picosecond and microsecond timescales correlate well with the device photocurrent densities under different applied bias conditions.

In addition to charge generation from polymer excitons, we also investigated the kinetics of fullerene excitons in DPP: PC70BM blends. Our femtosecond transient absorption spectroscopy results demonstrated that PC70BM excitons can contribute strongly to device photocurrents but are often limited by exciton diffusion to the polymer-fullerene interface on a sub-nanosecond timescale. Using fractionated DPP-copolymers with differing molecular weights, we optimised the morphology of the active layer to reduce PC70BM exciton diffusion losses and thus achieved ~7 mA.cm^-2 photocurrent from PC70BM excitons.