A research team from Wuhan University of Technology led by Professor Tao Wang has developed a vacuum-assisted thermal annealing (VTA) process that addresses the associated challenges of efficiency and stability in organic solar cells. The method designs the internal structure of the photovoltaic active layer so that devices maintain strong performance over long operating times.
The work focuses on controlling the arrangement of nanoscale donor and acceptor materials in the active film. The research team stated that “the VTA process accelerates solvent evaporation while promoting favorable molecular organization.” Under VTA, donor components accumulate toward the bottom of the film and acceptor components toward the top, creating a stratified but connected pseudo-bulk heterojunction with a pin-type profile that supports charge transport and extraction.
Using characterization tools including X-ray photoelectron spectroscopy, depth profiling and wide-angle X-ray scattering with grazing incidence, the team found that VTA-treated films exhibit higher molecular order and tighter pp stacking than films produced via standard processing routes. This improved packaging immediately led to higher device performance, with Champion cells achieving an energy conversion efficiency of 20.5 percent.
Stability tests under continuous illumination showed that conventional devices maintained 80 percent of their initial output (T80) for more than 3,900 hours. In device architectures optimized for stability, the extrapolated lifetime of the T80 has increased to approximately 54,000 hours, which is one of the longest lifetimes reported to date for organic solar cells that combine high efficiency with extended operational stability.
The researchers noted that “the enhanced crystallinity and suppressed phase separation created by the VTA treatment create a robust morphological structure that resists degradation.” Additional measurements indicated that VTA-treated films undergo less structural relaxation and domain coarsening when exposed to thermal stress, explaining the improved resistance to performance degradation.
The VTA approach was validated for several material systems, including PM6/L8-BO, D18/L8-BO and the all-polymer combination PM6/PY-IT, indicating that the process can be widely applied in the manufacture of organic solar cells. The study shows that relatively simple post-processing can address the trade-off between high efficiency and long-term stability, supporting the future development of flexible, lightweight organic photovoltaic modules for practical use.
Research report:Vacuum-assisted vertical component distribution in pseudo-bulk heterojunctions: a path to powerful and stable organic solar cells
