Perovskite solar cells (PSCs) have made significant progress in recent years. However, the presence of defects in the photoactive absorption layer has become a limiting factor in the efficiency of PSCs. In this research, methylammonium lead iodide perovskite films with improved quality and reduced defect density were obtained with the addition of crystalline nanocellulose (CNC) biopolymer. CNC produced from biomass contains many hydroxyl groups that were able to passivate the defects in perovskite, increase grain size, and reduce recombination. The PSC made with CNC modification achieved an efficiency of 9.22 %, higher than similar PSC without the addition of CNC that produced an efficiency of 7.28 %. This work highlights the passivating impact of low-cost green cellulose for enhancing the electrical performance of PSC.