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Bulk heterojunction (BHJ) organic solar cells (OSCs) have attracted intensive research attention over the past two decades owing to their unique advantages including mechanical flexibility, light weight, large area, and low-cost fabrications. To date, OSC devices have achieved power conversion efficiencies (PCEs) exceeding 12%. Much of the progress was enabled by the development of high-performance donor polymers with favorable morphological, electronic, and optical properties. A key problem in morphology control of OSCs is the trade-off between achieving small domain size and high polymer crystallinity, which is especially important for the realization of efficient thick-film devices with high fill factors. For example, the thickness of OSC blends containing state-of-the-art PTB7 family donor polymers are restricted to ~100 nm due to their relatively low hole mobility and impure polymer domains. To further improve the device performance and promote commercialization of OSCs, there is a strong demand for the design of new donor polymers that can achieve an optimal blend morphology containing highly crystalline yet reasonably small domains. In this Account, we highlight recent progress on a new family of conjugated polymers with strong temperature-dependent aggregation (TDA) property. These polymers are mostly disaggregated and can be easily dissolved in solution at high temperatures, yet they can strongly aggregate when the solution is cooled to room temperature. This unique aggregation property allows us to control the disorder--order transition of the polymer during solution processing. By preheating the solution to high temperature (~100 °C), the polymer chains are mostly disaggregated before spin coating; as the temperature of the solution drops during the spin coating process, the polymer can strongly aggregate and form crystalline domains yet that are not excessivelylarge. The overall blend morphology can be optimized by various processing conditions (e.g., temperature, spin-rates, concentration, etc.). This well-controlled and near-optimal BHJ morphology produced over a dozen cases of efficient OSCs with an active layer nearly 300 nm thick that can still achieve high FFs (70--77%) and efficiencies (10--11.7%). By studying the structure--property relationships of the donor polymers, we show that the second position branched alkyl chains and the fluorination on the polymer backbone are two key structural features that enable the strong TDA property. Our comparative studies also show that the TDA polymer family can be used to match with non-fullerene acceptors yielding OSCs with low voltage losses. The key difference between the empirical matching rules for fullerene and non-fullerene OSCs is that TDA polymers with slightly reduced crystallinity appear to match better with small molecular acceptors and yield higher OSC performances. [ABSTRACT FROM AUTHOR]

2D conjugated side-chain engineering is an effective strategy that is widely utilized to construct benzodithiophene-based polymers. Herein, an unconjugated side-chain strategy to design fused-benzodithiophene-based non-fullerene small molecule acceptors (SMAs) via vertical aromatic side-chain engineering on the ladder-type core is employed. Three SMAs named BTTIC-Th, BTTIC-TT, and BTTIC-Ph with thiophene, thieno[3,2-b]thiophene, and benzene, respectively, as side chains, are designed and synthesized. Three SMAs exhibit similar absorption ranges but different lowest unoccupied molecular orbital (LUMO) energy levels due to the different strength of the δ-inductive effect between vertical aromatic side chains and their electron-rich core. Organic solar cells based on PBDB-T:BTTIC-TT achieve a power conversion efficiency (PCE) of 13.44%, which is higher than the PCE of devices based on PBDB-T:BTTIC-Th (12.91%) and PBDB-T:BTTIC-Ph (9.14%). The difference in device performance is investigated by electrical and morphological characterizations. A large domain size and different types of π- π stacking are found in the bulk heterojunction layer of PBDB-T:BTTIC-Ph blend film, which are detrimental to exciton dissociation and charge transport. Overall, it is demonstrated that when designing unconjugated side chains, thieno[3,2-b]thiophene is superior to thiophene and benzene through its dual roles of promoting the LUMO energy level and optimizing the morphology. These results shed light on the side-chain engineering of high-performance non-fullerene SMAs. [ABSTRACT FROM AUTHOR]

We report a novel small molecule acceptor (SMA) named FTTB-PDI4 obtained via ring-fusion between the thiophene and perylene diimide (PDI) units of a PDI-tetramer with a tetrathienylbezene (TTB) core. A small voltage loss of 0.53 V and a high power conversion efficiency of 10.58% were achieved, which is the highest value reported for PDI-based devices to date. By comparing the fused and nonfused SMAs, we show that the ring-fusion introduces several beneficial effects on the properties and performances of the acceptor material, including more favorable energy levels, enhanced light absorption and stronger intermolecular packing. Interestingly, morphology data reveal that the fused molecule yields higher domain purity and thus can better maintain its molecular packing and electron mobility in the blend. Theoretical calculations also demonstrate that FTTB-PDI4 exhibits a "double-decker" geometry with two pairs of mostly parallel PDI units, which is distinctively different from reported PDI-tetramers with highly twisted geometries and can explain the better performance of the material. This work highlights the promising design of PDI-based acceptors by the ring-fusion strategy. [ABSTRACT FROM AUTHOR]

To achieve efficient non-fullerene organic solar cells, it is important to reduce the voltage loss from the optical bandgap to the open-circuit voltage of the cell. Here we report a highly efficient non-fullerene organic solar cell with a high open-circuit voltage of 1.08 V and a small voltage loss of 0.55 V. The high performance was enabled by a novel wide-bandgap (2.05 eV) donor polymer paired with a narrow-bandgap (1.63 eV) small-molecular acceptor (SMA). Our morphology characterizations show that both the polymer and the SMA can maintain high crystallinity in the blend film, resulting in crystalline and small domains. As a result, our nonfullerene organic solar cells realize an efficiency of 11.6%, which is the best performance for a non-fullerene organic solar cell with such a small voltage loss. [ABSTRACT FROM AUTHOR]