image: Schematic of the VTA process for fabricating pseudo-bulk heterojunction films.
Credit: ©Science China Press
Research team from Wuhan University of Technology, led by Professor Tao Wang, has developed an innovative processing technique that simultaneously addresses two key challenges in organic solar cell development: efficiency and stability. Their vacuum-assisted thermal annealing (VTA) method creates an optimal internal architecture within the photovoltaic material, resulting in devices that not only perform well but maintain their performance over extended periods.
The breakthrough concentrates on controlling the nanoscale organization of materials within the active layer film. "The VTA process accelerates solvent evaporation while promoting favorable molecular organization" explained the research team. This creates a stratified yet interconnected structure where donor materials concentrate toward the bottom and acceptor materials toward the top, forming an optimal p-i-n configuration for charge transport and extraction.
Through detailed characterization using techniques including X-ray photoelectron spectroscopy depth profiling and grazing-incidence wide-angle X-ray scattering, the researchers confirmed that VTA-treated films exhibit enhanced molecular ordering and tighter π-π stacking compared to conventionally processed counterparts. This improved structural organization directly translated to better device performance, with champion cells reaching 20.5% power conversion efficiency.
The stability results proved particularly noteworthy. Under continuous illumination testing, conventional device structures maintained 80% of initial performance (T80) for over 3,900 hours. In specially engineered stable configurations, the extrapolated T80 lifetime reached approximately 54,000 hours, which is among the longest reported for high-efficiency organic photovoltaics.
"The enhanced crystallinity and suppressed phase separation created by VTA treatment establish a robust morphological structure that resists degradation," the researchers noted. Additional investigations revealed that the VTA-treated films showed significantly reduced structural relaxation and domain coarsening under thermal stress, explaining the improved stability.
The method also proved effective in multiple material systems, including PM6/L8-BO, D18/L8-BO, and the all-polymer PM6/PY-IT, validating its versatility for OSCs. This research provides a practical approach to overcoming the trade-off between performance and stability, which is one of the most significant barriers to organic solar cell commercialization. This ability through simple post-processing modifications represents meaningful progress toward flexible, lightweight photovoltaic applications.