Formamidinium Lead Bromide (FAPbBr3) Perovskite Microcrystals for Sensitive and Fast Photodetectors
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KeywordsFormamidinium lead trihalide (FAPbBr3) Perovskite microcrystals Photodetector External quantum efficiency
Formamidinium lead trihalide (FAPbBr3) microcrystal-based photodetectors facilitate efficient charge transfer.
The fabricated FAPbBr3 photodetector shows good responsivity, external quantum efficiency, and detectivity.
Two-photon performance of the photodetectors is better than that previously reported for MAPbBr3 photodetectors.
In recent years, photoelectric devices have been widely investigated [1, 2, 3], especially detectors with different morphologies and sizes, for exploring their mechanisms, simplifying their syntheses, and improving device efficiencies [4, 5, 6, 7, 8, 9, 10]. Photodetectors based on hybrid organolead trihalide perovskite CH3NH3PbX3 (MAPbX3, X = Cl, Br, I) have attracted significant attention in the optoelectronic field owing to their strong absorption coefficients (up to 105 cm−1) [11, 12, 13, 14], carrier mobilities (2.5–1000 cm2 v−1 s−1) [15, 16, 17, 18, 19], long carrier lifetimes (0.08–4.5 μs) [17, 19, 20, 21], and diffusion lengths (2–175 μm) [19, 20, 22, 23]. In particular, Saidaminov et al. produced a planar-integrated photodetector using MAPbBr3 single crystals, achieving high-performance light detection in the wide wavelength range . Bao et al. reported low-noise and large-linear-dynamic-range photodetectors based on MAPbBr3 and MAPbI3 . Yang et al. demonstrated the great potential of metal–semiconductor–metal structures for low-cost and high-performance optoelectronic devices . Moreover, Hu et al.  reported high-performance and low working-voltage perovskite thin-film photodetectors. Further, a UV-selective photodetector based on MAPbCl3 was also demonstrated [28, 29]. However, conventional MAPbX3 materials have poor stability [30, 31], which severely impedes device development.
HC(NH2)2PbX3 (FAPbX3), which replaces organic methylammonium cation (MA+) with the larger formamidinium cation (FA+) [32, 33], is much more stable in air because the high Goldschmidt tolerance factor of its lattice (≈ 1) [33, 34, 35]. Moreover, previous reports have revealed that FA-substituted compounds (e.g., FAPbX3) showed remarkably improved carrier transmission performance than MAPbX3 in both polycrystalline thin films and monocrystalline phases, including a longer carrier lifetime and lower dark carrier concentration [20, 22, 35, 36, 37]. As a result, FAPbBr3 is expected to perform better in photodetectors than MAPbBr3. However, a prototype device based on FAPbBr3 has not been reported yet, except for a two-dimensional (OA)2FAn−1Pb n Br3n+1 photodetector with low responsivity . Taking advantage of the high carrier mobility, stability of single crystals and the efficient charge transfer in the micron scale, we synthesized FAPbBr3 microcrystals (MCs) and deposited them as a photodetector. Detailed investigations under single- and two-photon excitation and front- and back-side excitation were performed to reveal the great potential of the fabricated photodetector as an optoelectronic device.
3.1 Chemicals and Reagents
Formamidine acetate, lead bromide (PbBr2, > 98%) and hydrobromic acid (48 wt% in water) were purchased from Alfa Aesar. Gamma-butyrolactone (GBL) and N,N-dimethylformamide (DMF) were purchased from Kermel. All compounds were used without any further purification.
3.2 Synthesis of CH(NH2)2Br (FABr)
FABr was synthesized by slowly dropping 10 mL hydrobromic acid into 50 mmol (5.205 g) formamidine acetate in a flask, accompanied by continuous stirring at 0 °C for 2 h under argon atmosphere. The product FABr was formed once the solvent was removed using a rotary evaporator at 70 °C. The crude white powder was dissolved in ethanol and subsequently reprecipitated in diethyl ether. Then, the filtered product was dried at 60 °C in a vacuum oven for 24 h for further use.
3.3 Synthesis of FAPbBr3 MCs
FAPbBr3 MCs were grown by a modified inverse temperature crystallization method using toluene as the antisolvent. In brief, 5 mmol PbBr2 (1.835 g) and 5 mmol FABr (0.625 g) were dissolved in 10 mL mixed DMF:GBL (1:1 v/v) solvent at 25 °C, and the solution was filtered using nylon filters with a 0.22-μm pore size. Then, 1 mL precursor was diluted in 2 mL DMF and antisolvent toluene was added to obtain a saturated solution. Single crystals of FAPbBr3 with millimeter dimensions were formed after stirring on a hot plate of 80 °C.
3.4 Device Preparation
Standard photolithography and hydrochloric acid etching were used to obtain conductive glass substrates with a channel length and width of 5 µm and 1 mm, respectively. After synthesizing FAPbBr3 MCs using the above-described method, the substrates were deposited on these crystals and dried at 160 °C for 10 min.
3.5 Measurements and Characterizations
Scanning electron microscopy (SEM) was performed using a field-emission scanning electron microscope (JEOL, JSM-7800F, 3 kV). X-ray diffraction (XRD) was performed using an X’pert PRO diffractometer equipped with Cu Kα X-ray (λ = 1.54186 Å) tubes. UV–Vis diffuse reflectance spectra were recorded at room temperature on a JASCO V-550 UV–Vis absorption spectrometer with an integrating sphere operating in 200–900 nm. Photoluminescence (PL) was recorded using a Horiba PTI QuantaMaster 400 system with an excitation of 470 nm. Time-resolved PL decay was obtained on the basis of the time-correlated single-photon counter (TCSPC) technology using a light-emitting diode (LED) to provide a 376-nm excitation beam. The decay data were analyzed using commercial software provided by Horiba. For trap-state density evaluation, one layer of conductive glass (ITO) undertaken the crystal film was used as one electrode. An 800-nm-thick gold (Au) layer deposited on top of the film by thermal evaporation was used as the other electrode. This structure had a rather simple geometry with the sample deposited on ITO and evaporated Au on opposite sides, and the structure should be kept in the dark. Current–voltage measurements were conducted using a Keithley 2400 source meter. For light characterization under one-photon excitation, a monochromatic source (LED, λ = 495 nm) was used. Spectral responsivity (R) was calculated by the photocurrent (Iph) and incident power (Pinc) according to the relation R = Iph/Pinc. For two-photon irradiation, the photocurrent was generated using a femtosecond laser system (Spitfire Pro, SpectraPhysic) with an output wavelength of 800 nm and a repetition rate of 1000 Hz as the light source.
4 Results and Discussion
This value of D* was higher than that of state-of-art MAPbI3 photodetectors (~ 1013 Jones) . Moreover, the FAPbBr3 MC photodetector exhibited a rapid response with a rise time (τrise) of 0.67 ms and fall time (τfall) of 0.75 ms (Figs. 2d and S4), where τrise and τfall are defined as the time required for light response from 10 to 90% in the rising stage and from 90 to 10% in the falling stage, respectively.
Moreover, the amplified spontaneous emission (ASE) behavior of the FAPbBr3 microcrystalline film was measured using 800-nm laser pulses with tunable intensities. The emission spectra (Fig. S6) with increasing light intensity exhibited an ASE threshold of 1.75 mJ cm−2, which was similar to the reported two-photon ASE threshold of MAPbBr3 photodetectors (2.2 mJ cm−2) .
In summary, organolead trihalide perovskite FAPbBr3 MCs were synthesized and then deposited as a photodetector. The photodetector exhibited a good responsivity of up to 4000 A W−1 under back-side one-photon excitation with an EQE and detectivity of up to 1.05 × 106% and 3.87 × 1014 Jones, respectively. Besides, the two-photon responsivity under 800-nm excitation was 0.07 A W−1, which was four orders of magnitude higher than that reported for MAPbBr3 single crystals (10−6 A W−1). This deposited FAPbBr3 microcrystalline photodetector showed great potential for developing fast and sensitive photodetectors.
We would like to acknowledge all funding sources, including grant numbers and funding agencies. We are grateful to the National Key R@D Program of China (Grant 2017YFA0204800) and the National Natural Science Foundation of China (Grant Nos: 21533010, 21321091, 21525315, 91333116 and 21173169) for their financial supports.
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