Revealing Intrinsic Free Charge Generation: Promoting the Construction of Over 19% Efficient Planar p-n Heterojunction Organic Solar Cells.

Due to high binding energy and extremely short diffusion distance of Frenkel excitons in common organic semiconductors at early stage, mechanism of interface charge transfer-mediated free carrier generation has dominated the development of bulk heterojunction (BHJ) organic solar cells (OSCs). However, considering the advancements in materials and device performance, it is necessary to reexamine the photoelectric conversion in current-stage efficient OSCs. Here, we propose that the conjugated materials with specific three-dimensional donor-acceptor conjugated packing potentially exhibit distinctive charge photogeneration mechanism, which spontaneously split Wannier-Mott excitons to free carriers in pure phases. Subsequently, the pure planar p-n heterojunction (PHJ) OSCs based on green orthogonal solvents were prepared and exhibited comparable even greater performance to that of BHJ OSCs. More interestingly, by introducing PVDF-TrFE as intrinsic region to regulate built-in electric field of the device, the planar p-i-n PHJ OSCs achieved much higher efficiency (>18%) and stability. Moreover, a prominent efficiency of over 19% has been obtained via ternary optimization, which is the new efficiency record for PHJ OSCs up to date. This study points towards the distinguishing intrinsic free charge generation mechanism, opens up a new avenue for OSCs to collectively realize high-efficiency, long-term duration, and simplified device engineering for future commercialization.