Temperature-Dependent Li+ Diffusion and Its Influence on Doping Profile and Stability of Perovskite Solar Cells

Abstract

The efficiency and stability of perovskite photovoltaic devices (PPVDs) are heavily influenced by defects and external dopants within the perovskite layer and by external environmental factors, such as oxygen, moisture, light, and heat. Still, the impact of dopant diffusion from the hole transport layer toward the perovskite absorbing layer as a function of the temperature is not fully understood. This study investigates the diffusion effect of lithium (Li) ions from Spiro-OMeTAD into the double-cation perovskite layer for PPVDs with regular architecture. For Li-containing devices, temperature-dependent capacitance–voltage (C–V) measurements and Mott–Schottky analysis, within the temperature range of 280–353 K, reveal an increased space charge density. This suggests a higher ionic mobility with increasing temperature. Consequently, a decrease in the depletion region width was observed as the temperature increased. Additionally, C–V profiles reveal two distinct peaks, M₁ and M₂, correlated with two different regions within the perovskite layer: the first peak is related with the embedded perovskite within the mesoporous TiO₂ scaffold, and, the second one with the perovskite capping layer. On another side, in aged Li-containing devices, the M₂ peak shifts to lower voltages (from 1.07 to 0.95 V) with aging indicating changes in its electronic structure and charge distribution in the embedded perovskite due to Li-ion diffusion. This accumulation of Li ions correlates with reduced device stability, highlighting that Li-ion migration adversely impacts long-term performance. The results emphasize differences in charge carrier distribution and interactions within the device, leading to variations in the local electronic properties. Stability tests revealed that after 280 days, Li-free devices retained approximately 75% of their initial efficiency, while Li devices maintained only 35%. These findings emphasize the role of Li in influencing degradation mechanisms and the importance of managing dopant migration for long-term device stability.