In recent years, zinc oxide (ZnO)-based methyl ammonium lead halide perovskite photovoltaic (PPv) technologies have achieved high power conversion efficiency. However, several key factors have hindered the commercialization of PPv devices. We report a ZnO electron transport layer surface decorated with solution-processable carbon quantum dots (CQDs), which significantly enhances interfacial charge transport between the perovskite and electron transport layer (ETL) interface. With a simple synthesis method, the ETL surface was coated with CQDs. Waste coffee grounds mixed with water, ethanol, and n-hexane solvents were used to extract carbon quantum dots of various sizes, whose optical, structural, and morphological properties were confirmed by sophisticated characterizations. The PPv device exhibited an increase in power conversion efficiency (PCE) from 9.09% to 11.71% due to enhanced charge extraction. The planar perovskite photovoltaic device with an 8 nm-sized CQDs-ZnO hybrid layer attained degradation percentages of 16.39%, 8.66%, 5.87%, and 1.96% for ZnO, E-CQDs, nH-CQDs, and W-CQDs, respectively. This work provides a cost-effective and eco-friendly ZnO-based material as a promising candidate for planar PPv structures. © 2025 Elsevier B.V., All rights reserved.