Organic solar cell (OSC) technology has attracted much attention due to its promise as low-cost conversion of solar energy. Despite recent progress, several limitations are holding back OSC development. For instance, best-efficiency OSCs are mostly based on relatively thin (100 nm) active layers. Thick-film OSCs generally exhibit lower fill factors and efficiencies compared to the best thin-film OSCs. Here we report multiple cases of high-performance thick-film (300 nm) OSCs (efficiencies up to 11.7%, fill factors up to 77%). Our simple temperature dependent aggregation control and materials design rules allowed us to develop, within a short time, over twenty polymer:fullerene combinations, all of which yielded higher efficiency than previous state of art devices (~10%). The common structural feature of the three new donor polymers, the 2-octyldodecyl (2OD) alkyl chains sitting on quaterthiophene, causes a temperature-dependent aggregation behavior that allows for the processing of the polymer solutions at moderately elevated temperature, and more importantly, controlled aggregation and strong crystallization of the polymer during the film cooling and drying process. This results in a well-controlled and near-ideal polymer:fullerene morphology (containing highly crystalline, preferentially orientated, yet small polymer domains) that is controlled by polymer aggregation during warm casting and thus insensitive to the choice of fullerenes.

This approach can be applied to non-fullerene OSCs. The energy loss from the optical bandgap (Egap) to the open-circuit voltage (Voc) of a solar cell is a simple measure of its effectiveness in generating voltage. For best-efficiency (>10%) organic solar cells (OSCs), the energy loss is typically in the range of 0.85-0.9 eV, while the loss is only 0.4-0.55 eV for other more efficient solar cell systems. High energy loss is one key factor that limits the performance of OSCs. In this paper, we report efficient (9.5%) OSCs with a high Voc of 1.11 V, despite a relatively narrow optical bandgap of 1.66 eV for the absorber. Importantly, the high efficiency and low energy loss were achieved without using the conventional fullerene acceptors, which have dominated OSCs for nearly two decades. The origin of the small energy loss of our non-fullerene OSCs can be attributed to two factors. First, our OSC exhibits a high electroluminescence (EL) quantum efficiency that is comparable to those of inorganic solar cells and that helps to reduce non-radiative recombination loss.