Manipulating the donor:acceptor (D:A) energetics, e.g. the highest occupied molecular orbital (HOMO) offset, is the key to balancing the charge separation and charge recombination for high-performance organic solar cells (OSCs)....

Enhancing the luminescence property without sacrificing the charge collection is one key to high-performance organic solar cells (OSCs), while limited by the severe non-radiative charge recombination. Here, we demonstrate efficient OSCs with high luminescence via the design and synthesis of an asymmetric non-fullerene acceptor, BO-5Cl. Blending BO-5Cl with the PM6 donor leads to a record-high electroluminescence external quantum efficiency of 0.1%, which results in a low non-radiative voltage loss of 0.178 eV and a power conversion efficiency (PCE) over 15%. Importantly, incorporating BO-5Cl as the third component into a widely-studied donor:acceptor (D:A) blend, PM6:BO-4Cl, allows device displaying a high certified PCE of 18.2%. Our joint experimental and theoretical studies unveil that more diverse D:A interfacial conformations formed by asymmetric acceptor induce optimized blend interfacial energetics, which contributes to the improved device performance via balancing charge generation and recombination.

Unveiling the correlations among molecular structures, morphological characteristics, macroscopic properties and device performances is crucial for developing better photovoltaic materials and achieving higher efficiencies. To achieve this goal, a comprehensive study is performed based on four state-of-the-art non-fullerene acceptors (NFAs), which allows to systematically examine the above-mentioned correlations from different scales. It’s found that extending conjugation of NFA shows positive effects on charge separation promotion and non-radiative loss reduction, while asymmetric terminals can maximize benefits from both terminals. Another molecular optimization is from alkyl chain tuning. The shortened alkyl side chain results in strengthened terminal packing and decreased π-π distance, which contribute high carrier mobility and finally the high charge collection efficiency. With the most-acquired benefits from molecular structure and macroscopic factors, PM6:BTP-S9-based organic photovoltaics (OPVs) exhibit the optimal efficiency of 17.56% (certified: 17.4%) with a high fill factor of 78.44%, representing the best among asymmetric acceptor based OPVs. This work provides insight into the structure-performance relationships, and paves the way toward high-performance OPVs via molecular design.

Photo-degradation of organic semiconductors remains as an obstacle preventing their durable practice in optoelectronics. Herein, we disclose that volume-conserving photoisomerization of a unique series of acceptor-donor-acceptor (A-D-A) non-fullerene acceptors (NFAs) acts as a surrogate towards their subsequent photochemical reaction. Among A-D-A NFAs with fused, semi-fused and non-fused backbones, fully non-fused PTIC, representing one of rare existing samples, exhibits not only excellent photochemical tolerance in aerobic condition, but also efficient performance in solar cells. Along with a series of in-depth investigations, we identify that the structural confinement to inhibit photoisomerization of these unique A-D-A NFAs from molecular level to macroscopic condensed solid helps enhancing the photochemical stabilities of molecules, as well as the corresponding OSCs. Although other reasons associating with the photostabilities of molecules and devices should not excluded, we believe this work provides helpful structure-property information toward new design of stable and efficient photovoltaic molecules and solar cells.

Conspectus Toward future commercial applications of organic solar cells (OSCs), organic photovoltaic materials that enable high efficiency, excellent stability, and low cost should be developed. Fused-ring electron acceptors (FREAs) have declared that OSCs are capable of showing efficiencies over 19%, whereas stability and cost are not solved yet. As the counterparts of FREAs, non-fused ring electron acceptors (NFREAs) are more flexible in molecular design. They have better stability because of the reduction of intramolecular tension via breaking fused backbone and have more advantages in cost with the reduction of synthetic complexity. However, the challenge for NFREAs is the relatively lower efficiencies (around 15% at current stage), which require better molecular designs for addressing the issues of conformational unicity and effective molecular packing. In this Account, we comprehensively summarize works about NFREAs carried out in our group from three main frameworks, including molecular design and efficiency optimization, material cost, and stability. First, in the part of molecular design and efficiency optimization, the existing rotatable single bond in NFREAs will bring the problem of conformational uncertainty, but it can be solved through proper molecular design, which also regulates the energy levels, light absorption range, and the packing mode of the molecule for obtaining higher performance. Thus, in this part, we discuss the evolution of NFREAs in three aspects, including molecular skeleton optimization, terminal modification, and side chain engineering. Many strategies are used in the design of a molecular skeleton, such as utilizing the quinoid effect, introducing functional groups with the electron push–pulling effect, and using multiple conformational lock. Furthermore, simplifying the skeleton is also the preferred development tendency. As for the terminal, the main modification strategy is adjusting the conjugation length and halogen atoms. What is more, by adjusting the side chain to induce appropriate steric hindrance, we can fix the orientation of molecules, thus regulating molecular packing modes. Second, regarding material cost, we compare the synthesis complexities between state-of-the-art FREAs and NFREAs. Because the synthesis processes of NFREAs reduce the complex cyclization reactions, the synthesis routes are greatly simplified, and the molecule can be obtained through three minimal steps. Third, regarding stability, we analyze the workable strategies used in NFREAs from the views of intrinsic material stability, photostability, and thermal stability. Finally, we conclude the challenges that should be conquered for NFREAs and propose perspectives that could be performed for NFREAs, with the hope of pushing the development of OSCs toward high performance, stability, and low cost.