optical properties. However, such materials are either expensive or require vacuum equipment, e.g., metal-organic chemical vapor deposition, to fabricate, [7][8][9] which places a restriction on a wide deployment. In recent years, organometal trihalide perovskites (OTPs) (with a structure of ABX 3 , where A is an organic cation CH 3 NH 3 + (MA), B is Pb 2+ , X is a halide anion or mixed halide) have drawn great attention and been a very promising candidate for opto-electronic applications due to low cost and high throughput solution process. Since the discovery of perovskitebased solar cells (PSCs) by Miyasaka and co-workers [10] power conversion efficiencies have exceeded 22% in less than seven years, [11] thanks to the outstanding physics properties, including the low exciton binding energy, strong light absorption, long carrier lifetime, large carrier diffusion coefficient, and low charge recombination rate. [12][13][14][15][16] These features also make the emerging perovskite materials a promising alternative to conventional semiconductors used in PDs. Indeed, solution-processed OTPs have yielded PDs with excellent device performance. [16][17][18][19][20][21][22][23][24] For instance, both polycrystalline films and single crystals of OTPs have been successfully used to fabricate the narrowband and broadband photodetectors. [25][26][27] As one of the earliest discovered and extensively researched perovskite materials, MAPbI 3 has been regarded as one of the most potential materials for PDs due to its broadband absorption and superb light sensitivity. Dong et al. reported a MAPbI 3 -based photodetector with excellent photoconductive properties. [20] Su et al. reported a self-powered photodetector based on MAPbI 3, which exhibited excellent responsivity and rapid response time for wavelength ranging from ultraviolet to visible light. [28] Chen et al. fabricated a flexible UV-vis-NIR photodetector based on MAPbI 3 with excellent mechanical flexibility and durability. [18] However, some issues about this material still exist. MAPbI 3 tends to degrade and dissociate into MAI and PbI 2 in air. [29][30][31] Recent work on FAPbX 3 (FA: CH 3 (NH 2 ) 2 + , X = I, Br, Cl) PSCs demonstrates better thermal durability than methylammonium perovskites. [31,32] However, FAPbI 3 has two different phases at room temperature: α-phase (desired perovskite phase) and δ-phase (photo-inactive phase). Also, the α-phase perovskite of FAPbI 3 , which is sensitive to solvents and moisture, would turn into the undesired δ-phase in an air atmosphere. [33] Photodetectors, which can convert light signals into electrical signals, are important opto-electronic devices in imaging, optical communication, biomedical/biological sensing, and so on. Here a solution-processed photodetector based on the triple cation perovskite is demonstrated. The perovskite photodetectors show a high detectivity, high speed, as well as excellent environmental stability. Operating at a low voltage bias of 2 V, the photodetectors exhibit a large on/off ratio of 10 5 , ...
