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Organic solar cells (OSCs) are one of the most promising cost‐effective options for utilizing solar energy, and, while the field of OSCs has progressed rapidly in device performance in the past few years, the stability of nonfullerene OSCs has received less attention. Developing devices with both high performance and long‐term stability remains challenging, particularly if the material choice is restricted by roll‐to‐roll and benign solvent processing requirements and desirable mechanical durability. Building upon the ink (toluene:FTAZ:IT‐M) that broke the 10% benchmark when blade‐coated in air, a second donor material (PBDB‐T) is introduced to stabilize and enhance performance with power conversion efficiency over 13% while keeping toluene as the solvent. More importantly, the ternary OSCs exhibit excellent thermal stability and storage stability while retaining high ductility. The excellent performance and stability are mainly attributed to the inhibition of the crystallization of nonfullerene small‐molecular acceptors (SMAs) by introducing a stiff donor that also shows low miscibility with the nonfullerene SMA and a slightly higher highest occupied molecular orbital (HOMO) than the host polymer. The study indicates that improved stability and performance can be achieved in a synergistic way without significant embrittlement, which will accelerate the future development and application of nonfullerene OSCs.

Intra‐ and intermolecular ordering greatly impacts the electronic and optoelectronic properties of semiconducting polymers. The interrelationship between ordering of alkyl sidechains and conjugated backbones has yet to be fully detailed, despite much prior effort. Here, the discovery of a highly ordered alkyl sidechain phase in six representative semiconducting polymers, determined from distinct spectroscopic and diffraction signatures, is reported. The sidechain ordering exhibits unusually large coherence lengths (≥70 nm), induces torsional/twisting backbone disorder, and results in a vertically multilayered nanostructure with ordered sidechain layers alternating with disordered backbone layers. Calorimetry and in situ variable temperature scattering measurements in a model system poly{4‐(5‐(4,8‐bis(3‐butylnonyl)‐6‐methylbenzo[1,2‐b:4,5‐b′]dithiophen‐2‐yl)thiophen‐2‐yl)‐2‐(2‐butyloctyl)‐5,6‐difluoro‐7‐(5‐methylthiophen‐2‐yl)‐2H‐benzo[d][1,2,3]triazole} (PBnDT‐FTAZ) clearly delineate this competition of ordering that prevents simultaneous long‐range order of both moieties. The long‐range sidechain ordering can be exploited as a transient state to fabricate PBnDT‐FTAZ films with an atypical edge‐on texture and 2.5× improved field‐effect transistor mobility. The observed influence of ordering between the moieties implies that improved molecular design can produce synergistic rather than destructive ordering effects. Given the large sidechain coherence lengths observed, such synergistic ordering should greatly improve the coherence length of backbone ordering and thereby improve electronic and optoelectronic properties such as charge transport and exciton diffusion lengths.

Solution‐processed Cu(In,Ga)(S,Se)₂ (CIGS) has a great potential for the production of large‐area photovoltaic devices at low cost. However, CIGS solar cells processed from solution exhibit relatively lower performance compared to vacuum‐processed devices because of a lack of proper composition distribution, which is mainly instigated by the limited Se uptake during chalcogenization. In this work, a unique potassium treatment method is utilized to improve the selenium uptake judiciously, enhancing grain sizes and forming a wider bandgap minimum region. Careful engineering of the bandgap grading structure also results in an enlarged space charge region, which is favorable for electron–hole separation and efficient charge carrier collection. Besides, this device processing approach has led to a linearly increasing electron diffusion length and carrier lifetime with increasing the grain size of the CIGS film, which is a critical achievement for enhancing photocurrent yield. Overall, 15% of power conversion efficiency is achieved in solar cells processed from environmentally benign solutions. This approach offers critical insights for precise device design and processing rules for solution‐processed CIGS solar cells.