From Perylene Diimide Polymers to Fused‐Ring Electron Acceptors: A 15‐Year Exploration Journey of Nonfullerene Acceptors

Revealing the Role of Donor/Acceptor Interfaces in Nonfullerene-Acceptor Based Organic Solar Cells: Charge Separation versus Recombination

Organic solar cells (OSCs) based on nonfullerene-acceptors (NFAs) have achieved rapid development, while the role of donor/acceptor (D/A) interfaces in NFA based heterosystems has not been fully addressed. Here, we clarify that the photoinduced spontaneous charge separation efficiency in typical NFA heterosystems can reach up to 67%, and the charge separation efficiency contributed by the D/A interface is only 25%. The more important role of D/A interfaces is reducing the charge recombination rate, especially optimizing the competition between radiative and nonradiative charge recombination, thus reducing the nonradiative voltage loss. Systematical simulations demonstrate that there exists an optimal interfacial distance for a fixed energy offset, at which the D/A interface can reduce the nonradiative voltage loss by a maximum value of 0.12 V. Hence, we propose that optimizing the interfacial distance combined with the actual interfacial energy offset of a given heterosystem is important to develop its best photovoltaic performance.

Large-fused-ring-based D-A type electrochromic polymer with magenta/yellowish green/cyan three-color transitions

Large-fused-ring-based conjugated polymers possess wide application prospects in optoelectronic devices due to their high charge transport and wide optical absorption. In this paper, three low-bandgap donor-acceptor (D-A) type polymers PBIT-X (X = 1, 2, 3) based on alkylated benzodithiophene and tris(thienothiophene) as donors and thiadiazol-quinoxaline as an acceptor were synthesized via Stille coupling polymerization at different (donor/acceptor) D/A molar feed ratios. The band gaps of PBIT-1, PBIT-2, PBIT-3 were 1.10 eV, 1.04 eV and 1.02 eV, respectively. Spectroelectrochemistry studies showed that the three D-A type polymers have dual bands located in visible and near-infrared regions in the neutral state. The three D-A type polymers possess good electrochromic properties, such as an optical contrast of 56% and response time of 0.3 s. In particular, PBIT-3 could achieve three color changes from magenta to yellowish green to cyan during the oxidation process. The results indicate that these D-A type conjugated polymers based on large fused-ring units exhibit multiple color changes, endowing them with huge potential applications in visible and near-infrared electrochromic devices.

Development of non-fullerene electron acceptors for efficient organic photovoltaics

Compared to fullerene based electron acceptors, n-type organic semiconductors, so-called non-fullerene acceptors (NFAs), possess some distinct advantages, such as readily tuning of optical absorption and electronic energy levels, strong absorption in the visible region and good morphological stability for flexible electronic devices. The design and synthesis of new NFAs have enabled the power conversion efficiencies (PCEs) of organic photovoltaic (OPV) devices to increase to around 19%. This review summarises the important breakthroughs that have contributed to this progress, focusing on three classes of NFAs, i.e. perylene diimide (PDI), diketopyrrolopyrrole (DPP) and acceptor–donor–acceptor (A-D-A) based NFAs. Specifically, the PCEs of PDI, DPP, and A-D-A series based non-fullerene OPVs have been reported up to 11%, 13% and 19%, respectively. Structure–property relationships of representative NFAs and their impact on OPV performances are discussed. Finally, we consider the remaining challenges and promising directions for achieving high-performing NFAs.