Tuning Porosity in Laminated Carbon Electrodes via Solvent Treatment Enables >20% Efficient Perovskite Solar Cells

Replacing metal top electrodes with carbon in perovskite solar cells (PSCs) is a promising strategy to address stability issues arising from metal‐induced degradation. However, conventional blade‐coated carbon deposition can damage underlying charge transport layers, such as Spiro‐OMeTAD, due to solvent incompatibility. In contrast, laminating predried carbon films mitigates this issue and significantly enhances open‐circuit voltage and photoluminescence intensity, reaching levels comparable to devices with gold electrodes. Still, laminated carbon (LMC) electrodes often exhibit increased series resistance, which reduces overall device efficiency. This study demonstrates that treating the predried carbon film with o‐xylene prior to lamination significantly reduces its porosity, especially at the interface between carbon and the underlying layer. This leads to a significant reduction of both the contact and the sheet resistance (and therefore the overall series resistance), improving the fill factor and boosting the power conversion efficiency (PCE) of carbon electrode‐based PSCs from 15.6% to 18.9%. By introducing an alternative electron transport layer formulation, the PCE of gold‐based reference devices and laminated carbon‐based PSCs was improved to 21.4% and 20.4%, respectively. These results demonstrate that solvent treatment of predried carbon films is a key enabler for high‐efficiency carbon‐based PSCs, offering an alternative to metal‐based electrodes.