Owing to the increased global need for renewable energy, cost effect solar energy generation has become key to energy sustainable. Among the new thin film solar cells perovskite solar cells are considered as the suitable alternatives to the market giant c-Si based technology. Among the different perovskites FAPbI₃ is the most popular one for their better efficiency, cost-effectiveness, thermal stability, direct bandgap, and improved light absorption compared to other perovskites. However, the commercialization of FAPbI₃ is hindered by its phase shift from the photoactive cubic to the photo-inactive hexagonal phase as well as stability issues in humid conditions. This study aims to overcome the limitations of humidity effect on this perovskite layer using a multifunctional additive engineering strategy. 2-Aminoethylthiol was added to the FAPbI₃ precursor solution and active layer was spin coated on the precleaned FTO and FTO/SnO₂ glass substrates. Through various structural, opto-electronic, and morphological analyses, we found that the additive-modified films showed increased light absorption, photoluminescence emission, and crystallinity by 20%, 400%, and >300%, respectively, promising a higher solar cell current density. The percentage improvements were measured by comparing the peak intensities of the data obtained from the modified film with those obtained from the control film. When these films aged for 200 h at 85% relative humidity and 85 °C temperature, the target film retained photophysical integrity >600% better than the control film, proving their robustness in harsh environments.