Enhancing Perovskites Cell Performance through Alkali Metal Modification of TiO₂: Exploring Electronic Nature and Interface Properties

Metal halide perovskite solar cells (PSC) have achieved a remarkable efficiency above 26%, which is comparable to the conventional Si photovoltaics. In the evolution of PSC technology from dye-sensitized solar cells, TiO₂ played a pivotal role in achieving remarkable power conversion efficiency. TiO₂, as a charge extracting layer, efficiently extracts photo-generated electrons and acts as a barrier for holes. Being widely used as a blocking layer, the TiO2 suffers from many fundamental challenges like surface defects, which work as trap states, low electron mobility, and high catalytic activity, collectively contributing to hysteresis and instability in perovskite solar cells. Therefore, to improve the electron transporting properties of TiO_2, various surface modification and doping strategies have been tried. Among various approaches, the alkali metal doping of TiO₂ has proven to be beneficial to achieving overall improvement of the PSCs' performance.<br/>Our research explores how adding lithium (Li), sodium (Na), and potassium (K) to mesoporous TiO₂ affects its electronic properties. We also investigate the chemical interaction between the perovskite layer and TiO₂, as well as the doped TiO₂ with alkali metals. We use soft (XPS) and hard X-ray photoelectron spectroscopy (HAXPES), along with resonance photoemission spectroscopy and X-ray absorption spectroscopy (XAS). Ultimately, we evaluate how alkali metal doping influences the efficiency and stability of the device.<br/>XPS and HAXPES analyses reveal that upon introducing Li, Na, and K to TiO₂, surface Ti⁴⁺ undergoes reduction to Ti³⁺, while bulk Ti⁴⁺ states remain unchanged. The reduction is more pronounced with Li, decreasing from Na to K. This implies Li acts as a dopant, whereas Na and K modify the TiO₂ surface. XAS studies support this conclusion, showing changes in the TiO₂layers. Resonance photoemission spectroscopy indicates that Li induces more defect states that result from Ti⁴⁺ reduction. Upon perovskite deposition, these defect states diminish, indicating electron transfer from perovskite to TiO₂, oxidizing it. The reduction of defect states is highest in Li-doped TiO₂. Additionally, Ti2p peak shifts to lower binding energy, indicating upward band bending at the TiO₂/perovskite interface. This band bending is consistent across all TiO₂ types and their perovskite interfaces studied here. Notably, the TiO₂/perovskite interface with Na and K modifications exhibits a higher presence of Pb0 states, suggesting a defective interface. The Li doped TiO₂ based PSCs have higher power conversion efficiency compared to the others. The enhanced power conversion efficiency observed in Li-doped TiO₂-based PSCs can be attributed to the improved conductivity resulting from lithium incorporation. This augmentation facilitates the efficient transportation of photo-generated electrons. However, for sustained and stable PSC performance, Na and K-modified TiO₂ devices exhibit superior characteristics.<br/>The spectroscopic investigation, in conjunction with device performance, shows that there is a favorable upward band bending from TiO₂ to the perovskite, and the defects of TiO₂ are passivated by the perovskite itself. Among used alkalis modifiers, the Li dopes the TiO₂ to increase the electron conductivity and with fewer interface defects, amounting to superior power conversion efficiency. On the other hand, Na and K act as modifiers for the TiO₂ layer, contributing to a more stable device performance. This stability can be attributed to their relatively larger ionic radii compared to Li.