Formamidinium Lead Bromide (FAPbBr3) Perovskite Microcrystals for Sensitive and Fast Photodetectors

Because of the good thermal stability and superior carrier transport characteristics of formamidinium lead trihalide perovskite HC(NH2)2PbX3 (FAPbX3), it has been considered to be a better optoelectronic material than conventional CH3NH3PbX3 (MAPbX3). Herein, we fabricated a FAPbBr3 microcrystal-based photodetector that exhibited a good responsivity of 4000 A W−1 and external quantum efficiency up to 106% under one-photon excitation, corresponding to the detectivity greater than 1014 Jones. The responsivity is two orders of magnitude higher than that of previously reported formamidinium perovskite photodetectors. Furthermore, the FAPbBr3 photodetector’s responsivity to two-photon absorption with an 800-nm excitation source can reach 0.07 A W−1, which is four orders of magnitude higher than that of its MAPbBr3 counterparts. The response time of this photodetector is less than 1 ms. This study provides solid evidence that FAPbBr3 can be an excellent candidate for highly sensitive and fast photodetectors. Electronic supplementary material The online version of this article (10.1007/s40820-018-0196-2) contains supplementary material, which is available to authorized users.


Synthesis of CH(NH 2 ) 2 Br (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.

Synthesis of FAPbBr 3 MCs
FAPbBr 3 MCs were grown by a modified inverse temperature crystallization method using toluene as the antisolvent. In brief, 5 mmol PbBr 2 (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-lm 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 FAPbBr 3 with millimeter dimensions were formed after stirring on a hot plate of 80°C.

Device Preparation
Standard photolithography and hydrochloric acid etching were used to obtain conductive glass substrates with a channel length and width of 5 lm and 1 mm, respectively. After synthesizing FAPbBr 3 MCs using the above-described method, the substrates were deposited on these crystals and dried at 160°C for 10 min.

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 K a X-ray (k = 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 Quan-taMaster 400 system with an excitation of 470 nm. Timeresolved PL decay was obtained on the basis of the timecorrelated 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. Currentvoltage measurements were conducted using a Keithley 2400 source meter. For light characterization under onephoton excitation, a monochromatic source (LED, k = 495 nm) was used. Spectral responsivity (R) was calculated by the photocurrent (I ph ) and incident power (P inc ) according to the relation R = I ph /P inc . 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.

Results and Discussion
We synthesized FAPbBr 3 MCs using a modified antisolvent-assisted inverse temperature crystallization method [8,24,28]. To facilitate the removal of the solvent and form homogeneous crystalline films, 1 M precursor (equimolar FABr and PbBr 2 dissolved in a mixed solvent as shown in experimental section) was diluted three times in DMF. The antisolvent toluene was then added to obtain a saturated solution. This saturated solution was stirred at 80°C to accelerate the nucleation and increase the yield of the interconnected crystals. The mean size of the as-obtained FAPbBr 3 MCs was * 10 ± 5 lm, as shown in the top-view SEM image (Fig. 1a). These MCs were interconnected as a continuous film with a thickness of * 150 lm, as shown in Fig. 1b. XRD results shown in Fig. 1c confirmed the cubic phase of the FAPbBr 3 MCs [35,39]. The MCs exhibited an absorption band edge at 570 nm, corresponding to a bandgap of 2.18 eV, as obtained from the Tauc plot of the absorption spectrum (Fig. 1d). The emission peak of the MCs appeared at 567 nm (insets of Fig. 1d), which was consistent with previous results [33,37,40]. Additionally, PL decay was measured using the TCSPC technology (Fig. 1e), and a fast component (s 1 , 15 ± 1 ns) and slow decay (s 2 , 282 ± 5 ns) reflected the surface and bulk carrier lifetimes  Fig. 1f. Here, the trap-state density was calculated to be 6.98 9 10 11 cm -3 according to Eq.1: where V TFL is the voltage at which trap states are fully filled by injected carriers, q and L represent the elemental charge and film thickness, respectively, e 0 and e denote the vacuum permittivity and dielectric constant of FAPbBr 3 , with e = 43.6 [37]. The relatively fewer defects contributed to the formation of a high-quality film, thus promoting their application in photoelectronic devices [37,41].
In the next step, FAPbBr 3 MCs were directly deposited on interdigitated ITO substrates to form the prototype photodetector device. The length and width of the gaps between neighboring digits were 5 lm and 1 mm, respectively, as shown in Fig. S1. FAPbBr 3 MCs covered the entire active area, forming Schottky barriers due to contact with the ITO electrode. Once a voltage was applied to the detector device, ion migration and carrier trapping occurred in the active layer and at the electrode/perovskite interfaces, respectively. This indicated that an Ohmic contact between the FAPbBr 3 MCs and the ITO electrode was formed. The cathode collected abundant photogenerated holes, while holes from the anode were injected into the active layer, showing the photoconductivity of the device [42][43][44]. The compact morphology of the MCs minimized the grain boundary and consequently diminished interfacial charge recombination. As shown in Fig. 2a, the photocurrent drastically increased with increasing excitation light intensity. In addition, the small dark current of the device (seen in Fig. S2) indicated the low carrier concentration of FAPbBr 3 MCs [37]. The photoresponse was also reproducible under a periodic excitation of the light pulse, as demonstrated in Fig. S3.
As shown in Fig. 2b, the photocurrent increased linearly with the incident power and the corresponding responsivity (R, defined as photocurrent/the incident light power) linearly decreased. The responsivity was 4000 A W -1 at 5 V (k = 495 nm, probe intensity = 10 nW cm -2 ), which was two orders of magnitude higher than that of other FA-based perovskite photodetectors [7,38,45,46]. Furthermore, the external quantum efficiency (EQE) calculated by Eq. 2 was as high as 1.05 9 10 6 % ( Fig. 2c): In addition, the detectivity D* of the device was 3.87 9 10 14 Jones, as obtained from Eq. 3:

Light intensity (mW cm −2 )
Incident power (mW cm −2 ) This value of D* was higher than that of state-of-art MAPbI 3 photodetectors (* 10 13 Jones) [46]. Moreover, the FAPbBr 3 MC photodetector exhibited a rapid response with a rise time (s rise ) of 0.67 ms and fall time (s fall ) of 0.75 ms (Figs. 2d and S4), where s rise and s fall 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.
Previous research has revealed that the photoresponse to the excitation wavelength is different for front-side excitation and back-side excitation [46]. In back-side excitation, charge carriers are efficiently collected in the vicinity of the electrodes. However, in front-side excitation, it is more difficult for charge carriers generated by shortwavelength photons to penetrate the thick film to electrodes than that of long-wavelength photons with energy comparable to the bandgap [46]. In our case, the wavelengthdependent light response using front-side excitation resembled that using back-side excitation, indicating that our device was a broadband photodetector (Figs. 3a and S5). It was found that the thickness of the FAPbBr 3 microcrystalline film of about 150 lm allowed most photons to be transmitted through the film to generate corresponding photocurrents. Once the film became sufficiently thick ([ 200 lm), it blocked the short-wavelength photons to the microcrystalline film to form a narrowband photodetector, which was also confirmed by Saidaminov and coworkers [46]. In addition, the generally lower photocurrent for front-side excitation could be attributed to the insufficient diffusion length of the photogenerated charges for the given film thickness, which hindered charge transportation to the bottom electrode [46]. Figure 3b shows the photocurrent variation for different bias voltages. As shown in Fig. 3c, d, when the bias voltage was decreased, the photocurrents of the photons excited far above the absorption band edge degraded more.
Perovskites have also attracted considerable attention as nonlinear semiconductor absorbers for optical limiting [47], ultrafast optical signal characterization [48], microscopy [49], and lithography [50]. Therefore, we characterized our device under two-photon excitation, where the photocurrent was generated by an 800-nm pulse laser with a photon energy much smaller than the bandgap of FAPbBr 3 (2.18 eV). Figure 4a shows the PL mechanism of FAPbBr 3 at 567 nm obtained under 800-nm two-photon absorption. Under 800-nm back-side illumination at a fixed bias of 5 V, the tendency of light current variation with  Fig. 4b. Under ideal conditions, the photocurrent generated by two-photon absorption showed a square (n = 2) dependence on the input intensity [51]. However, our tested photocurrents (with 1 \ n \ 2) exhibited highly dependent on incident power, which was mainly attributed to the effect of trap-state's sub-gap absorption [8,52]. This tendency was consistent with that reported for near-infrared CsPbBr 3 and MAPbBr 3 photodetectors [8,51]. Consequently, the responsivity of the two-photon pumped FAPbBr 3 MC detector increased with increasing input intensity in the linear excitation region (Fig. 4c). The largest responsivity under 800-nm excitation (0.07 A W -1 ) was much higher than that previously reported for MAPbBr 3 single crystals [51]. The fast response of our detector under two-photon excitation with a fast fall time (0.72 ms) is also shown in Fig. 4d. The result is similar to that observed under one-photon excitation (0.75 ms). The rapid pulse light with periodic changes was responsible for the missing rising stage. Moreover, the amplified spontaneous emission (ASE) behavior of the FAPbBr 3 microcrystalline film was measured using 800-nm laser pulses with tunable intensities. The emission spectra (Fig. S6)

Conclusion
In summary, organolead trihalide perovskite FAPbBr 3 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 9 10 6 % and 3.87 9 10 14 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 MAPbBr 3 single crystals (10 -6 A W -1 ). This deposited FAPbBr 3 microcrystalline photodetector showed great potential for developing fast and sensitive photodetectors. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://crea tivecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.