Phase-stable FAPbI3-based single crystals with 600-μm electron diffusion length

Long carrier diffusion length is important for optoelectronic devices that require large electrode spacing or thick active layers. Formamidinium lead triiodide (FAPbI3) perovskite is a promising candidate with ultra-long carrier diffusion length, but the existence of internal tensile strain and formation of ion vacancies make its photoactive black phase thermodynamically unstable. Herein, phase-stable FAPbI3-based single crystals with negligible absorption loss are achieved by synergy of iodide immobilization and compressive strain. Incorporation of guanidinium (GA+) cations strengthens the interaction with the inorganic framework and suppresses the formation of iodide ion vacancies. Besides, the interface compressive strain introduced from the indium tin oxide (ITO) substrate counteracts the lattice tensile strain. The as-grown GA0.015FA0.985PbI3 single crystals exhibit no phase transition after storage in air for 2,000 h. Moreover, the GA0.015FA0.985PbI3 single crystals exhibit ultra-long electron carrier diffusion length of about 600 μm, which is a record value for perovskite materials and only half of silicon single crystals.