Improved Performance of Hole-Transporting Material-Free Perovskite Solar Cells Using a Low-Temperature Printed Carbon Paste

The combination of chemically and structurally unstable hole transport materials (HTMs) and the metal ion diffusion from counter electrodes (CEs) toward the perovskite layer are reported as primary causes of the insufficient stability of perovskite solar cells (PSCs) and modules. Carbon-based CEs (C-CEs) directly deposited atop the perovskite layer without interposing any HTM represent a promising path to improving PSC stability while lowering the environmental impact and the manufacturing cost. In this work, we present a cost-effective approach to fabricating C-CEs using two different carbon pastes with distinct formulations, successfully replacing expensive metal-based electrodes. We engineered HTM-free PSCs based on a mesoscopic n-i-p structure and printable C-CEs (C-PSCs), with a 2D perovskite passivation layer as an electron-blocking layer between the perovskite and the C-CE. The devices using a low-temperature processed carbon counter electrode (LTPC-CE) improved the performance of the devices compared to the cells produced with a medium-temperature processed carbon counter electrode (MTPC-CE). This behavior is associated with enhanced charge carrier lifetime, charge transfer, and charge extraction processes enabled by effective solvent removal during the C-CE deposition as well as the highly electrically conductive pathways offered by graphene flakes. In particular, in small-area devices, the power conversion efficiencies (PCE) of champion devices using the LTPC-CE were increased from 14.99% for the MTPC-CE cell to 17.68%. In large-area devices, PCE improved from 12.24 to 15.01%. Transient photovoltage and photocurrent measurements confirmed the enhanced performance of the devices incorporating the LTP graphene-based carbon paste as the CE. Our findings highlight the high potential of low-temperature processed carbon electrodes for stable and efficient PSCs, offering a promising approach for the massive and affordable production of perovskite-based photovoltaics.