Perovskite solar cells (PSCs) have become one of the technologies with high potential for solar-harvesting devices for their capability in efficient energy conversion and suppressing cost in production. However, carbon-based PSCs (C-PSCs), which promote carbon as an electrode to replace the high cost of a noble-metal electrode, still suffer performance limitations owing to inferior interfacial contact and inefficient charge extraction at the interface between perovskite and carbon. In this study, the interfacial engineering of C-PSCs was optimized by using CuSCN as both a hole transport layer (HTL) and a carbon additive. Two forms of CuSCN were examined as carbon additives: powder and solution, with the solution prepared using dipropyl sulfide (DPS) and diethyl sulfide (DES) as solvents. Photoluminescence (PL) analysis showed enhanced charge collection and suppressed carrier recombination in the C-PSC using CuSCN in DPS as the carbon additive. Electrochemical impedance spectroscopy (EIS) further confirmed a decrease in series resistance and a boost in charge transport parameters. As a result, the CuSCN in the DPS-based device exhibited superior interfacial quality and achieved the highest efficiency of 11.09%, outperforming both the powder and DES-based counterparts. These findings demonstrate that optimizing CuSCN incorporation as a carbon additive, particularly its form and solvent medium, is essential for achieving better interfacial contact and higher C-PSC efficiency. Copyright © 2026. Published by Elsevier Ltd.