Molecular Asymmetry Boosts Organic Solar Module Stability

Abstract

The symmetry-breaking design strategy of nonfullerene acceptor can improve the performance of semitransparent organic solar cells (ST-OSCs). However, no report exists on the "asymmetric molecular interaction" induced by symmetric molecular structure in nonfullerene acceptors. Herein, we showcase that 2D fluorophenyl outer groups in symmetric 4FY promote dipole-driven self-assembly through asymmetric molecular interactions, resulting in a tighter packed structure than Y6 with the same symmetric geometry. Such unique properties lead to high-performance layer-by-layer OSCs, accompanied by simultaneously reduced energy and recombination losses and improved charge-related characteristics. ST-OSCs based on PCE10-2F/4FY achieve notable power conversion efficiency (PCE) of 10.81%, average visible transmittance of 45.43%, and light utilization efficiency (LUE) of 4.91%. Moreover, exceptional diurnal cycling stability is observed in the ST-OSCs based on PCE10-2F/4FY with much prolonged T80 up to 134 h, which is about 17 times greater than the reference PCE10-2F/Y6. Lastly, we fabricate highly efficient semitransparent organic solar modules based on PCE10-2F/4FY (active area of 18 cm2), which shows PCE of 6.78% and the highest LUE of 3.10% to date for all-narrow bandgap semitransparent organic solar modules. This work demonstrates that asymmetry-driven molecular interactions can be leveraged to fabricate large-area ST-OSCs that are efficient and stable under realistic operating conditions.

A new type of semitransparent organic solar cell (ST-OSC) with an efficiency of over 10% has been developed, bringing us closer to a future where windows and mobile display screens could function as invisible power sources.

Professor Changduk Yang and his research team from the School of Energy and Chemical Engineering at UNIST announced the development of ST-OSCs, featuring a power conversion efficiency (PEC) of 10.81% and a visible light transmittance of 45.43%.

While traditional solar panels installed on rooftops or along roadsides appear dark because they absorb sunlight to generate electricity, transparent solar cells must allow most light to pass through without absorption. Developing highly efficient transparent devices has therefore been a significant challenge.

The key to this innovation lies in a specially designed photoactive layer that selectively absorbs only infrared wavelengths. This layer transmits nearly half of the visible light spectrum while harvesting energy from the invisible infrared portion of sunlight to produce electricity.

Typically, capturing infrared light results in lower efficiency compared to absorbing high-energy visible photons. However, the team overcame this obstacle through innovative molecular design of the active layer's receptor molecules. The active layer in OSCs consists of donor and acceptor molecules that facilitate charge transfer.

The newly synthesized acceptor molecule, called 4FY, is generally symmetrical and features an A-D-A structure. Nonetheless, it was intentionally designed to induce localized asymmetry-specifically between fluorine and hydrogen atoms, and fluorine and sulfur atoms-that enhances molecular alignment and improves charge transport, leading to higher device efficiency.

First author Sangjin Yang explained, "While asymmetry can boost efficiency, it often reduces device lifespan and complicates synthesis." He added, "Our molecular design introduces localized asymmetry within an overall symmetrical structure, leveraging the advantages of both."

The device demonstrated impressive durability, maintaining most of its initial performance during a 134-hour outdoor cycling stability test that simulated day-night conditions. This durability is approximately 17 times greater than that of previous semi-transparent OSCs based on Y6 acceptor molecules.

Professor Yang commented, "We introduced a new approach to generating electricity from invisible light," adding, "This technology has potential applications in smartphone protective films, building windows, and transparent displays-effectively turning everyday surfaces into invisible power plants."

Their findings were published in Angewandte Chemie International Edition on June 10, 2025. This study was supported by the Ministry of Science and ICT (MSIT), the National Research Foundation (NRF) of Korea, the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and other agencies.

Journal Reference

Sangjin Yang, Xuexiang Huang, Yongjoon Cho, et al., "Efficient Semitransparent Organic Solar Modules with Exceptional Diurnal Stability Through Asymmetric Interaction Induced by Symmetric Molecular Structure," Angew. Chem. Int. Ed., (2025).