Anatase, rutile, and brookite TiO2 electron transport layers for mesoporous carbon-based perovskite solar cells

This study investigates the influence of the TiO2 crystal structure and nanomaterial morphology on the effectiveness of the electron selective contact in triple-stack mesoporous, carbon-based perovskite solar cells (C-PSCs). We synthesized pure brookite and rutile TiO2 nanomaterials with different morphology compared to commercial anatase, formulated the corresponding screen-printing pastes, and fabricated solar cells; commercial anatase-based paste was used to fabricate reference devices. The mesoscopic triple-stack structure was infiltrated under two different humidity conditions (45% and 60% RH) to study the effect on solar cell performance. With infiltration at 45% RH, all device configurations demonstrated robust photovoltaic performance, with rutile-based cells achieving slightly higher power conversion efficiency and better reproducibility. Infiltration at 60% RH resulted in generally lower efficiencies, with anatase-based devices exhibiting slightly better performance than rutile-based cells. The performance of brookite-based devices decreased significantly, related to a change of the recombination mechanism. These effects can be explained by a combination of morphology aspects and the differences in hydrophilicity and surface hydroxylation, which directly affect perovskite crystallization. Impedance spectroscopy revealed similar electronic–ionic dynamics across the three TiO2 phases at 45% RH; however, at 60% RH, distinct low-frequency impedance signatures—including negative capacitance and inductive loops—emerged related to phase-dependent perovskite morphology variations. Collectively, these findings underscore the resilience and practical potential of triple-stack C-PSCs for scalable manufacturing in humid climates, highlighting homemade rutile paste as a promising alternative to widely used commercial anatase printing paste.