Synergistic Effect of Guanidinium Iodide and Cesium Iodide for Enhanced Perovskite Crystal Quality and Device Performance in Hybrid Perovskite Solar Cells.

Guanidinium iodide (GAI) and cesium iodide (CsI) have been demonstrated as effective functional additives to formamidinium iodide (FAI) and lead iodide (PbI2), respectively, significantly enhancing power conversion efficiency and stability of devices. Herein, GAI was incorporated into the PbI2 solution to establish hydrogen-bonding interactions with the Pb-I framework, enhancing the stability of the [PbI6]4- octahedral structure. However, the relatively large ionic size of the guanidinium cation (GA+) requires further lattice regulation to achieve structural balance. To address this, the current study introduces Cs+ ions, which have smaller atomic radii, to synergistically regulate crystal growth kinetics and successfully achieve lattice stress balance. Experimental results show that the synergistic effect of GAI and CsI significantly enlarges the perovskite grain size (from 433 ± 141 to 586 ± 243 nm) with fewer pinholes in the perovskite films, in favor of stability improvement. An n-i-p structured device based on this approach achieves an efficiency of 24.29% (compared to 22.66% for the control) and exhibits excellent storage stability under ambient conditions with 80 ± 5% relative humidity at room temperature-retaining 86% of its initial efficiency after 1000 hours of storage. This study provides a promising technological pathway for improving perovskite crystal quality and device performance through cation-size-engineering strategies.