From foe to friend: Taming the δ-phase of FAPbI3 for oriented growth and high-performance perovskite solar cells

The δ-phase of formamidinium lead iodide (FAPbI 3 ) is widely regarded as detrimental to achieving high efficiency and stability in perovskite solar cells (PSCs) due to its lack of optical activity. In fact, the δ-FAPbI 3 is crucial for the crystal growth of α-FAPbI 3 and establishing a Type-I band alignment with the α-phase FAPbI 3 in PSCs. In this work, we proposed that 1-(4-pyridyl) piperazine (PPDA) not only can successfully achieve controllable modulation of the α-phase/δ-phase ratio in FAPbI 3 , but also increase the transition temperature of δ-FAPbI 3 to α-FAPbI 3 through strong hydrogen bonding interaction. Moreover, we observed that with increasing annealing temperature, the breakage of hydrogen bonds between PPDA and FA + facilitates the complete phase transition from δ-FAPbI 3 to α-FAPbI 3 , while the liberated PPDA molecules preferentially adsorb onto the (110) facet of α-FAPbI 3 , promoting its oriented growth. Based on this strategy, the PPDA-treated device achieved a power conversion efficiency (PCE) of 23.31 %, which was further improved to 24.81 % after passivation treatment, which is one of the highest PCE among PSCs dominated by (110) preferred orientation and δ-phase modulation. Moreover, the devices exhibited remarkable operational stability, maintaining 87.4 % of its initial performance after 1150 h of storage without encapsulation. These findings underscore the importance of rational δ-phase regulation, presenting a synergistic design strategy that provides new theoretical insights and practical approaches for the development of high-performance PSCs.