A paradigm study of polymer donor diluted bulk heterojunction films for application in semitransparent organic photovoltaics

Compared with inorganic photovoltaics, organic solar cells are more suitable for semitransparent applications in building-integrated photovoltaics (BIPV). The complementary absorption ranges of binary photoactive components enable enhancing the visible light transmittance by diluting the polymer donor content in bulk heterojunction (BHJ) films. However, this strategy brings morphology defects to disturb the photovoltaic performance, producing inevitable compromise between the optical and electronic properties to confine the achievable light utilization efficiency (LUE). To maximize the LUE, we conduct comparative studies on two polymer donors to find out ways to alleviate the performance loss in diluted BHJ films. Importantly, we find that the short-circuit current (J(SC)) loss differs based on polymer aggregation type and its microscopic blend morphology. For one thing, more J-type polymer aggregations preserve the hole transporting pathway connection, and for another, the prioritized reduction of the polymer from pure domains rather than mixed domains helps maintain sufficient heterojunction area for exciton separation. Followed by compensation for the fill factor (FF) loss via addition of an n-dopant, we obtain a high LUE of 3.83% in binary component semitransparent organic solar cells (ST-OSCs) without device optical engineering design. Further deposition of a MoO3 layer boosts the LUE to 4.37%.