High-Performance Photodiode-Type Photodetectors Based on Polycrystalline Formamidinium Lead Iodide Perovskite Thin Films

www.nature.com/scientificreports OPEN Received: 9 March 2018 Accepted: 22 June 2018 Published: xx xx xxxx High-Performance PhotodiodeType Photodetectors Based on Polycrystalline Formamidinium Lead Iodide Perovskite Thin Films Meng Zhang, Fan Zhang, Yue Wang, Lijie Zhu, Yufeng Hu, Zhidong Lou, Yanbing Hou & Feng Teng Photodetectors based on three dimensional organic–inorganic lead halide perovskites have recently received significant attention. As a new type of light-harvesting materials, formamidinium lead iodide (FAPbI3) is known to possess excellent optoelectronic properties even exceeding those of methylammonium lead iodide (MAPbI3). To date, only a few photoconductor-type photodetectors based on FAPbI3 single crystals and polycrystalline thin films in a lateral structure have been reported. Here, we demonstrate low-voltage, high-overall-performance photodiode-type photodetectors in a sandwiched geometry based on polycrystalline α-FAPbI3 thin films synthesized by a one-step solution processing method and post-annealing treatment. The photodetectors exhibit a broadband response from the near-ultraviolet to the near-infrared (330–800 nm), achieving a high on/off current ratio of 8.6 × 104 and fast response times of 7.2/19.5 μs. The devices yield a photoresponsivity of 0.95 AW−1 and a high specific detectivity of 2.8 × 1012 Jones with an external quantum efficiency (EQE) approaching 182% at −1.0 V under 650 nm illumination. The photodiode-type photodetectors based on polycrystalline α-FAPbI3 thin films with superior performance consequently show great promise for future optoelectronic device applications. Photodetectors have attracted significant attention that find extensive applications in the fields such as optical communications, video imaging, environmental monitoring, chemical/biological sensing, and space exploration1. As an optoelectronic device converting a light signal into a detectable electrical signal, it is highly desirable that a broadband photodetector operating at low voltages should possess high photoresponsivity and detectivity with a fast response and high ratio of photocurrent to dark current2–6. Since the first report on the solar cells based on methylammonium lead iodide (MAPbI3) perovskite thin films7, tremendous efforts have been devoted to improving the performance of perovskite solar cells due to the extraordinary physical properties of this new generation of light-harvesting materials, such as appropriate direct bandgap, high absorption coefficient, wide absorption spectrum, large carrier mobility, and long carrier diffusion length8. Meanwhile, three-dimensional organic–inorganic hybrid perovskites have been extensively studied and applied to fabricating other optoelectronic devices9, especially photodetectors10–17, which adopt a general formula ABX3, where A is an methylammonium (MA or CH3NH3+) or formamidinium (FA or NH2CH = NH2+) cation, B is a metal ion (Pb2+ or Sn2+), and X is a halide anion (Cl−, Br−or I−)18. Compared to MAPbI3, the larger FA cation in a FAPbI3 perovskite occupies the A site in the ABX3 perovskite structure, forming a more symmetric crystal structure and reducing the electronic band gap19,20. FAPbI3 perovskites also show wider absorption spectrum and better thermal stability21–23. They are consequently regarded as a more proper candidate material for light harvesting not only in solar cells, but also in photodetectors. Currently, most studies of FAPbI3 perovskites have been focused on FAPbI3-based solar cells20 and some on nanowire lasers of FAPbX324 and light emitting diodes based on FAPbBr325. Nevertheless, only a few photodetectors based on FAPbI3 single crystals and polycrystalline thin films26–28 have been reported. Additionally, a critical issue in synthesizing FAPbI3 perovskites is the phase purity since FAPbI3 materials have two phases. One is a desirable Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing, 100044, China. Correspondence and requests for materials should be addressed to Y. Hu (email: ) or Z.L. (email: ) ScIEntIfIc RePorTs | (2018) 8:11157 | DOI:10.1038/s41598-018-29147-6 1 www.nature.com/scientificreports/ Figure 1. (a) XRD patterns and (b–e) top-view SEM images of the FAPbI3 films annealed in a nitrogen-filled glove box at different temperatures. The photographs of the FAPbI3 films are illustrated in the inset of Fig. 1(a). semiconducting perovskite phase (α-phase), and the other is an insulating non-perovskite phase (δ-phase) that is preferentially formed at room temperature29–31. It is therefore particularly important to promote the formation of α-FAPbI3 for achieving high performance in FAPbI3-based optoelectronic devices. The performance parameters and operating mechanisms of perovskite photodetectors are determined by the device configuration as well as the optoelectronic properties of the photoactive perovskite material. Two major types of two-terminal photodetectors based on organic–inorganic hybrid perovskites: photoconductors in a lateral geometry10,14,32 and photodiodes in a sandwiched structure11–13, have been reported. Many MAPbI3 photoconductors have adopted a lateral structure and operated at relatively high voltages33, even as high as 10 V34, although some of them have demonstrated an external quantum efficiency (EQE) larger than 100%35. Meanwhile, they suffer from slow response that the rise time ranging from 0.02 ms to approximately 100 ms which limits their further applications in high-speed devices. The lateral photodetectors based on FAPbI3 single crystals28 and FAPbI3 polycryatalline thin films26 have also been demonstrated. The FAPbI3 crystal photodetector working at 0.1 V has response rise/fall times of 12.4/17.8 ms, even slower than those (5.4/10.9 ms) of the FAPbI3 film device at 10 V, while no EQE values have been reported in the two literatures. Photodiode-type photodetectors apparently operate at relatively low voltages and exhibit fast response owing to rather short lengths of the photoactive layers, but their EQE values are normally less than unity13,36,37, except that in some cases current amplification or photomultiplication has been realized by the interface-controlled charge injection and photoconductive gain38–40. The response times of the sandwiched photodetectors based on MAPbI3 are in the range from a few microseconds to tens of microseconds11,37,41. In contrast to MAPbI3, few reports on photodiode-type photodetectors based on FAPbI3 perovskites have been published so far. In this paper, we demonstrate photodiode-type photodetectors based on polycrystalline α-FAPbI3 thin films. Pure polycrystalline α-FAPbI3 thin films were prepared using a one-step solution processing method and post-annealing treatment. Nanocrystal titanium dioxide (TiO2) and 2,2′,7,7′- Tetrakis[N,N-di(4-Methoxyphenyl) aMino] −9,9′-spirobifluorene (Spiro-OMeTAD) were utilized r (...truncated)