γ-Ray Irradiation Enables Annealing- and Light-Soaking-Free Solution Processable SnO₂ Electron Transport Layer for Inverted Organic Solar Cells

The electrode buffer layer is crucial for high-performance and stable OSCs, optimizing charge transport and energy level alignment at the interface between the polymer active layer and electrode. Recently, SnO₂ has emerged as a promising material for the cathode buffer layer due to its desirable properties, such as high electron mobility, transparency, and stability. Typically, SnO₂ nanoparticle layers require a postannealing treatment above 150°C in an air environment to remove the surfactant ligands and obtain high-quality thin films. However, this poses challenges for flexible electronics as flexible substrates can't tolerate temperatures exceeding 100°C. This study presents solution-processable and annealing-free SnO₂ nanoparticles by employing y-ray irradiation to disrupt the bonding between surfactant ligands and SnO₂ nanoparticles. The SnO₂ layer treated with y-ray irradiation is used as an electron transport layer in OSCs based on PTB7-Th:IEICO-4F. Compared to the conventional SnO₂ nanoparticles that required high-temperature annealing, the y-SnO₂ nanoparticle-based devices exhibit an 11% comparable efficiency without postannealing at a high temperature. Additionally, y-ray treatment has been observed to eliminate the light-soaking effect of SnO₂. By eliminating the high-temperature postannealing and light-soaking effect, y-SnO₂ nanoparticles offer a promising, cost-effective solution for future flexible solar cells fabricated using roll-to-roll mass processing.