Manipulating Molecular Stacking for Semitransparent Organic Photovoltaics Achieving Light Utilization Efficiency >6%

The trade-off between average visible transmittance (AVT) and power conversion efficiency (PCE), governed by the molecular stacking of the donor and acceptor materials in semitransparent organic solar cells (ST-OSCs), significantly constrains improvements in light utilization efficiency (LUE). Here, simultaneous enhancement of AVT and PCE is achieved by meticulously designing host-guest active layers to fine-tune the molecular stacking. A systematic investigation of various host donor and guest material combinations reveals that the donor material (D18) with more electron-deficient hydrogen atoms tends to form C─H···O interactions with the guest material (BTO-BO) that features electron-rich oxygen atoms. Hydrogen bonding interactions between host donor D18 and guest BTO-BO facilitate the transition from mixed J-type and H-type molecular stacking modes of the donor to predominant J-type stacking during crystallization, significantly reducing visible absorption and enhancing hole transport. Additionally, BTO-BO can act as a nucleation agent for the host acceptor BTP-eC9 to increase the crystallinity and absorption coefficient of the active layer, thereby, enhancing near-infrared light absorption. The resultant toluene-processed ST-OSCs with optical modulation exhibit simultaneous improvement in PCE and AVT, delivering record LUEs of 6.02%. Notably, this host-guest active layer demonstrates exceptional compatibility with flexible devices and promising scalability for greenhouse photovoltaic applications.