Microstructure-controlled charge transport in disordered carbon electrodes for HTM-free perovskite solar cells

Carbon-based perovskite solar cells (C–PSCs) are attractive for low-cost and stable photovoltaics, yet their performance is often limited by the poorly understood role of carbon electrodes. In particular, graphite–carbon black (G–CB) mixed electrodes are widely used, despite the lack of clear design principles linking electrode composition, microstructure, and charge transport. Here, we uncover a counterintuitive structure–transport relationship in hole-transport-material-free mesoporous layered perovskite solar cells (MPLE–PSCs): increasing the carbon black content reduces the electrical conductivity of the standalone electrode, while simultaneously enhancing the fill factor and overall device performance. By systematically varying the graphite-to–carbon black ratio, combined with cross-sectional electron microscopy and electrochemical impedance spectroscopy, we show that carbon black plays a critical structural role rather than a conductive one. Carbon black fills interparticle voids between graphite flakes and improves physical contact at the ZrO<inf>2</inf>/carbon interface, leading to a denser electrode microstructure and more continuous pathways for hole transport. This structural optimization effectively suppresses transport losses, whereas changes in interfacial charge-extraction kinetics have a secondary impact on device performance. An optimal balance between electrode microstructure and transport is achieved at intermediate carbon black contents, yielding fill factors above 70% and markedly improved photovoltaic performance. These results demonstrate that electrode microstructure, rather than conductivity alone, governs charge transport in carbon-based perovskite solar cells, providing a general design principle for scalable and stable carbon electrodes.