Diffusion-Controlled Morphology Modification via Employing Host/Guest Acceptors to Improve the Stability of Organic Photovoltaics

Typical bulk-heterojunction organic photovoltaics (OPV) suffer from instability due to the slowly diffusing small-molecule acceptors (SMAs) under operational conditions. The originally well-phase-separated morphology will undergo SMA self-aggregation to form over-purified domains that will deteriorate the OPV performance. This issue can be partially alleviated in sequentially processed OPV cells with a planar-mixed heterojunction absorber, which is intrinsically more robust due to larger domain sizes and a lower degree of mixing. To further enhance the morphological stability, a strategy is developed to inhibit the disordered molecular diffusion that leads to the loss of crystallinity of SMA domains. Two SMAs, C9-4F and Y6-OBO, are incorporated as guest acceptors to form an alloy-like acceptor with the host acceptor BTP-eC9. These alloy-like acceptors improved the crystallinity of the acceptor phase by promoting more ordered molecular packing, effectively restraining the diffusion-caused loss of SMA crystallinity upon illumination. This approach helps maintain favorable gradients and ultimately ensures OPV morphological stability. Consequently, the derived devices exhibited considerably enhanced stability, with the T80 lifetime of C9-4F-based devices reaching approximate to 790 h and a remarkable approximate to 1010 h for Y6-OBO-based devices, significantly surpassing the approximate to 450 h observed from binary devices. The presence of alloy-like acceptor phases improves the ordering of acceptor stacking in ordered regions after illumination at 1 sun, as opposed to the neat acceptor, inhibiting the diffusion of small-molecule acceptors. This helps avoid the overmixing of donor and acceptor materials, prevents the development of unfavorable gradients, and consequently extends the lifetime of organic photovoltaics. image