Improving the Thermal Stability of Inverted Organic Solar Cells by Mitigating the Undesired MoO₃ Diffusion toward Cathodes with a High-Ionization Potential Interface Layer

Understanding the thermal degradation mechanism of organic solar cells (OSCs) and developing strategies to enhance their thermal stability are crucial before they can be commercialized. In this paper, we demonstrated that in a structure-inverted ITO/ZnO/PM6/L8-BO/TCTA/MoO₃/Ag solar cell, a thin 4,4′,4″-tris(carbazol-9-yl)-triphenylamine (TCTA) film between MoO₃ and PM6:L8-BO can significantly suppress the fast short circuit current (J_SC) loss and the slow but continuous open circuit voltage (V_OC) and fill factor (FF) decay upon 150 °C thermal annealing. XPS and TOF-SIMS results confirm that thermal annealing leads to the formation of (MoO₃)⁻ at the MoO₃/PM6/L8-BO interface and the diffusion of (MoO₃)⁻ through the photoactive layer. The diffused (MoO₃)⁻ act as acceptor-type impurities that leads to p-doping of the photoactive layer, increasing charge recombination within the photoactive layer and reducing J_SC. In addition, the accumulation of (MoO₃)⁻ at the cathode interface leads to p-doping at the cathode interface and consequently decreases V_OC and FF. The thermally induced interfacial degradation model is supported by detailed drift-diffusion simulations. The TCTA-interlayer minimizes the (MoO₃)⁻ diffusion, thereby stabilizing the cell performance against thermal annealing. The TCTA-incorporating cells showed a high PCE of over 16% after high-temperature hot-press encapsulation, and the resulting cells showed excellent thermal stability under 85 °C.