Vertical Miscibility of Bulk Heterojunction Films Contributes to High Photovoltaic Performance

The power conversion efficiencies (PCEs) of organic photovoltaic cells employing donor (polymer):acceptor (small molecule) bulk heterojunction (BHJ) are being rapidly improved. The phase segregation along vertical (film-depth) direction of the active layer, referring to vertical immiscibility between donor and acceptor, leads to vertically varied and nonlinearly distributed composition, optical and electronic properties. However, the correlation between vertical miscibility and photovoltaic performance is still confusing. Here, it is semi-empirically found that such vertical variations induced by vertical immiscibility deteriorate PCE. Subsequently, using PM6:Y6-based binary and ternary BHJ as examples, a combined statistical, experimental, and numerical investigation on the dependence of photovoltaic performance on vertical miscibility is reported. The vertical phase evolution of BHJ significantly depends on solvents and processing methods. As compared with other donor:acceptor systems, polymer:Y6 deposited from appropriate solvents could have the best vertical miscibility which is nearly independent on film-depth, leading to a higher PCE. PM6:Y6:fullerene ternary blends also have a good and uniform vertical miscibility, forming spatially well-mixed ternary BHJ. Consequently, under this design guideline, the optimized film-depth-dependent miscibility contributes to optimized vertical distribution of optical and electronic properties, leading to an optimized PCE 17.1% with a low sensitivity to fluctuation of film thickness.