Subtle Alignment of Organic Semiconductors at the Donor/Acceptor Heterojunction Facilitates the Photoelectric Conversion Process

The aligned molecular packing structure is vital to the anisotropic charge transport in conjugated polymer and small molecule thin films. However, how this molecular packing motif influences the photoelectric conversion process at the donor/acceptor heterojunction is still mysterious. Herein, we employed a PM6/Y6 bilayer model to investigate the long-range alignment of molecular packing induced photoelectric conversion process. Both PM6 and Y6 layers were properly controlled to exhibit the uniaxially oriented molecular packing compared to their as-cast counterparts, as revealed by the polarized absorption spectra and transmission electron microscopy. After analyzing the photovoltaic performance of bilayer devices, the smaller energy loss, lower energetic disorder, and longer charge carrier lifetime can be observed in the bilayer devices with aligned Y6 molecules, which contribute to a higher power conversion efficiency (PCE) than the as-cast devices. While the molecular packing structure of PM6 layer exhibited negligible influence on the device performance, probably resulting from the intrinsic semicrystalline nature of PM6 molecules. Our results indicate that the alignment of small molecular acceptor at the donor/acceptor interfaces should be a powerful strategy to facilitate the photoelectric conversion process, which will definitely pave the way to highly efficient bulk heterojunction photovoltaic device.