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Research Progress of Small Molecule Donors with High Crystallinity in All Small Molecule Organic Solar Cells

In the past few decades, organic solar cells (OSCs) have been extensively studied due to their advantages of sem-itransparency, light-weight and flexibility, which are considered to be an important renewable energy source in the future. Recently, bulk heterojunction all-small molecule organic solar cells (BHJ ASM-OSCs) composed of p-type small molecule donors and n-type non-fullerene acceptors as the active layers have achieved remarkable development and the power conver-sion efficiencies (PCEs) have exceeded 15%. The advantages of non-fullerene acceptors over fullerene competitors are their easily tunable energy levels, better absorption properties and proper molecular designs, which allow ASM-OSCs to achieve significantly higher open-circuit voltages, higher photocurrents and superior stability, thus extending the device lifetime of OSCs. Compared with p-type polymer donors, p-type small molecule donors have unique advantages, such as well-defined molecular structures, easy purification and low batch-to-batch variations. Although ASM-OSCs are expected to take advantages of non-fullerene acceptors and small molecule donors simultaneously, in consequence of inappropriate crystallinity and nano-scale bi-continuous interpenetrating networks in blended films resulting from similar acceptor-donor-acceptor (A-D-A) structures and physicochemical properties between small molecule donors and non-fullerene acceptors, the PCEs of current state-of-the-art ASM-OSCs are still much lower than that of polymer-based OSCs. In this review, based on the expan-sion of the central conjugated units of small molecular donors, we summarized the crystallinity of benzodithiophene (BDT), naphthodithiophene (NDT) and dithienobenzodithiophene (DTBDT) donors from the perspective of molecular design and provided suggestions for further molecular design and morphology improvement.