Enhancing the Phase Crystallinity of Buried Layer Perovskites through Organic Salt Molecule-Assisted Crystal Growth

The performance and stability of perovskite solar cells rely crucially on the purity of their active perovskite phase. While the two-step method has emerged as a well-known technique for fabricating high-performance cells, it suffers from significant PbI₂ phase impurities at the buried layer due to inefficient diffusion of cationic molecules into the preprepared PbI₂ layer. Herein, a simple yet highly effective method is presented to boost phase purity within the buried layer by introducing formamidinium iodide (FAI) seed molecules into the underlying PbI₂ layer. X-Ray diffraction analysis result reveals that this process significantly reduces the PbI₂ phase and enhances the purity of the perovskite's phase. It is also observed that this technique can produce perovskite layer with a remarkably smooth surface structure and large interconnected crystal grains, forming a continuous layer. These characteristics are subjected to further enhancement when hexamethylenetetramine molecules are concurrently introduced with FAI into the PbI₂ layer. Solar cells fabricated using this method, with an active area of 0.1 cm², achieve a remarkable power conversion efficiency of up to 24.52% with V_oc as high as 1.18 V, representing a substantial improvement over cells produced using the standard two-step method, which attains only 22.18% efficiency. With its simple yet impactful approach, the present method should find widespread adoption in the production of high-performance perovskite solar cells.