Solution-Processed Epitaxial Growth of MAPbI3 Single-Crystal Films for Highly Stable Photodetectors

Recent years, organic-inorganic hybrid perovskites (OIHPs) have been widely used in applications, such as solar cells, lasers, light-emission diodes, and photodetectors due to their outstanding optoelectronic properties. Nowadays photodetectors based on perovskite films (PFs) suffer from surface and interface traps, which result from low crystalline quality of perovskite films and lattice mismatch between perovskite films and substrates. Herein, we fabricate MAPbI₃ -(MA = CH₃NH₃) single-crystal films (SCFs) on MAPbBr₃ single crystal substrates in MAPbI₃ precursor solution during crystallization process via solution-processed epitaxy. Benefit from the good lattice matching, epitaxial MAPbI₃ SCFs with high crystallinity and smooth morphology are of comparable quality to MAPbI₃ PSCs and are of better quality than MAPbI₃ polycrystalline films. Here we report that epitaxial MAPbI₃ SCFs have a low trap density of 5.64×10¹¹ cm^–3 and a long carrier lifetime of 11.86 μs. In this work, photodetector based on epitaxial MAPbI₃ single-crystal film (SCF) exhibits an excellent stability of a long-term stable response after 120 days, a fast response time of 2.21 μs, a high responsivity of 1.2 A W^–1 and a high detectivity of 3.07 ×10¹² jones.

Introduction

Organic-inorganic hybrid perovskites (OIHPs) have attracted worldwide numerous attention in recent years due to their remarkable properties in optoelectronic applications, such as solar cells, lasers, light-emission diodes and photodetectors (Shao et al., 2014; Tan et al., 2014; Wang et al., 2014, 2019a; Cho et al., 2015; Stranks and Snaith, 2015; Ling et al., 2016). As a new generation of promising semiconductor materials, solution-processable OIHPs are low-cost and have high absorption coefficient, tunable bandgap and long carrier lifetime and diffusion length (Xing et al., 2014; Fang et al., 2015; Wei et al., 2017). Up to now, not only devices based on perovskite single crystals (PSCs) have been widely used but also perovskite films (PFs)-based devices have shown great potential for high-performance photodetectors for visible light, near-infrared light and high energy ray (Shao et al., 2014; Kim et al., 2017; Wang et al., 2018, 2020). For example, Kim et al. (2017) used MAPbI₃ films for large-area, low-dose X-ray imaging, Ji et al. (2018) reported OHIPs films-based photodetectors with high-performance and Hou et al. (2020) reported narrowband photodetectors based on mixed halide PFs. The critical factors of PFs-based devices are high crystallinity, smooth morphology of films and good lattice matching between layers to form high-quality epitaxial films on substrates, which result in low trap density, long carrier lifetimes, and long diffusion lengths (Shi et al., 2015; Ji et al., 2018; Tang et al., 2019; Chen et al., 2020). Although many epitaxy approaches, such as spin-coating method, vapor-phase epitaxy as well as van der Waals epitaxy have been used to grow PFs used for optoelectronic devices (Wang et al., 2015, 2021; Yang et al., 2015; Long et al., 2018; Kelso et al., 2019; Chen et al., 2020), it is also a challenge to overcome the lattice mismatching between epitaxial PFs and substrates and to obtain high-crystallinity PFs with smooth morphology and good stability. Furthermore, the high trap density, short carrier lifetimes and short diffusion length of epitaxial PFs have restricted the development of photodetectors based on PFs (Wang et al., 2014; Chen et al., 2015; Tong et al., 2017; Yang et al., 2017; Abdi-Jalebi et al., 2018).

Methylammonium lead halide perovskites (MAPbX₃, where X = Cl, Br, I) have been the most widely studied OIHPs (Wei et al., 2017; Jiang et al., 2019) and the lattice parameters of MAPbI₃ and MAPbBr₃ are calculated as 6.23 and 5.93 Å, respectively. The lattice mismatching rate is calculated to be only 4.81% at room temperature so that MAPbI₃ single-crystal films (SCFs) are able to be epitaxially grown on MAPbBr₃ single crystal substrate (Wang et al., 2020; Xu et al., 2020). Here in this work, Different from reported solution-processed epitaxy, such as spin-coating method, our work directly put MAPbBr3 substrates into MAPbI₃ precursor solution. Benefit from good lattice matching, MAPbI₃ layers can directly grow on the surface of MAPbBr3 substrates in MAPbI₃ precursor solution at optimal growth temperature during crystallization process. This method in our work can easily get high-quality epitaxial layers with high crystallinity, good morphology and thickness of micro level for stable photodetectors. The epitaxial MAPbI₃ SCFs are of comparable quality to MAPbI₃ PSCs and are of better quality than MAPbI₃ polycrystalline films. Furthermore, epitaxial MAPbI₃ SCFs with dense structure have much better stability than polycrystalline films. X-ray diffraction (XRD) is used to compare the crystallinity of epitaxial MAPbI₃ SCFs and scanning electron microscopy (SEM) is used to characterize the surface morphology and the thickness of epitaxial MAPbI₃ SCFs. Additionally, this work fabricate a high performance stable photodetector based on high-quality epitaxial MAPbI₃ SCFs on MAPbBr₃ single crystal substrate as well as a photodetector based on MAPbI₃ PSCs for comparison. Moreover, the trap density is characterized by space-charge-limited-current (SCLC) and the stability is exhibited by long-term dark currents and photocurrents. The high-crystallinity film has low trap density of 5.64 ×10¹¹ cm^–3 and long carrier lifetime of 11.86 μs. Briefly, this photodetector has outstanding optoelectronic performance with a good stability, a fast response speed of 2.21 μs, a responsivity of 1.2 A W^–1 and a high detectivity of 3.07 × 10¹² jones.

Materials and Methods

Preparation of CH₃NH₃I Powder and CH₃NH₃Br Powder

To get CH₃NH₃I powder, Hydroiodic Acid (40 wt% in water, Macklin) reacted with Methylamine (30–33 wt% in ethanol, Macklin) in a 50 ml bottle at 0°C for 2 h with stirring. After fully reaction, the mixed solution was heated to 100°C on a hot plate with magnetic stirring then the precipitate of CH₃NH₃I gradually appeared. The precipitate of CH₃NH₃I was washed in ether solution twice and was re-dissolved in absolute ethanol. The CH₃NH₃I powder was collected after dried at 75°C in a vacuum dryer (China) for 12 h. To get MAPbBr₃ powder, Hydrobromic Acid (55 wt% in water, Macklin) was mixed with Methylamine (30–33 wt% in ethanol, Macklin) and other steps are same as the synthesis of CH₃NH₃I powder.

Preparation of Precursor Solution

To get MAPbI₃ precursor solution, as-prepared CH₃NH₃I and Lead iodide (PbI2, 99%, Sigma Aldrich) with a ratio of 1:1 were dissolved in anhydrous γ-butyrolactone (GBL, 99%, Aladdin). The final concentrations were 0.1 M Lead iodide and 0.1 M CH3NH3I mixed in 100 ml GBL solution. And MAPbBr₃ precursor solution was a N, N-Dimethylformamide (DMF, 99.5%, SCR) solution which dissolves as-prepared CH₃NH₃Br and lead bromine (PbBr2, 98%, Sigma Aldrich) with a ratio of 1:1.

Device Fabrication

To grow high-quality MAPbBr₃ PSCs as substrate, the MAPbBr₃ precursor solution was heated in crystallizing dish in air from 55 to 80°C at a rate of 0.3°C h^–1. Then the MAPbBr₃ PSCs was put into MAPbI₃ precursor solution to grow epitaxial MAPbI₃ SCFs at 110°C. The absolute dichloromethane solution was use to wash off the remains of the precursor solution from the crystal surface. The programmable heating control system was realized on IKA-RET control-visc. The final device was cut by diamond wire (Φ0.35 mm) and polished by Glycerin and diamond powder (Φ0.5 μm) mixed solution. The cutting and polishing system (STX-202A, UNIPOL-802) was pursued from KEJING company (Shenyang, China). And the toluene solution was used to wash the mixed solution remaining on the surface of single crystal. The gold electrodes were deposited on the surface of MAPbI₃ single-crystal film by a metal shadow mask with 500 μm channel by thermal evaporation in vacuum.

Characterization

Scanning electron microscopy (SEM) images were taken with a Quanta 200 FEI (United States). Atomic force microscope (AFM) was obtained by Mulyimode-8-AM. X-ray diffraction (XRD) patterns were obtained by X’TRA (Switzerland). Photoluminescence (PL) spectra was measured by UV–vis spectroscopy (Lab Tech Bluestar, United States).

Device Performance Measurements

All measurements were performed on the probe station. Keithley 2410 was used as the voltage source to measure current-voltage curve (I-V) with 0.6 V/s voltage sweep rate. A 355 nm pulsed Nd:YAG laser with 6 ns pulse width at 20 Hz was used as the illumination source and response time was measured using an Agilent oscilloscope (DSO-X4054A) of KEYSIGHT. The 670 nm light source was obtained from L10762 of Hamamatsu. Agilent 81110A was use to power the light-emitting diode (LED). The working voltage and current of XRD copper target ray tube is 40 KeV and 40 mA, respectively. And the emission laser of 450 nm is obtained from NBET-LASER.

Results and Discussion

As illustrated in Figure 1A, a solution-processed epitaxy is employed to fabricate MAPbI₃ SCF on MAPbBr₃ single crystal substrate. Firstly, a high-quality MAPbBr₃ single crystal is put into the precursor solution of MAPbI₃ heated to 110°C at which the MAPbI₃ SCFs can be densely and rapidly grown on the whole surface of the substrate. Then, after cutting and polishing, a MAPbI₃ SCF is deposited on MAPbBr₃ (200) single crystal substrate. SEM images of the epitaxial MAPbI₃ SCF surface and the cross section of the epitaxial MAPbI₃ SCF on MAPbBr₃ demonstrate the morphology and thicknesses of epitaxial MAPbI₃ SCFs. As shown in Figure 1B, epitaxial MAPbI₃ SCF with smooth surface are grown densely on the MAPbBr₃ (200) single crystal substrate. Moreover, the thickness of epitaxial films is about 37.1 μm which are two orders of magnitude thicker than films deposited by spin-coating method (Wang et al., 2014; Fei et al., 2017; Ji et al., 2018), as clearly shown in Figure 1C. And the small picture in Figure 1C shows the interface of the epitaxial layer and substrate. Epitaxial MAPbI₃ SCF with smooth morphology and controllable thickness can be directly grown on MAPbBr₃ single crystal substrate due to good lattice matching (Li et al., 2020; Pan et al., 2020; Xu et al., 2020). As shown in Figures 1D,E, Atomic force microscope (AFM, Mulyimode-8-AM) was performed to further characterize the surface of MAPbI₃ film and the roughness is approximately 14.9 nm.

FIGURE 1

Figure 1. (A) Processing of epitaxial growth of MAPbI₃ SCF via solution-process epitaxy. Step 1: put substrate into precursor solution of MAPbI₃. Step 2: Cutting and polishing. (B) SEM image of the epitaxial MAPbI₃ SCF surface. (C) SEM image of the cross section of the epitaxial MAPbI₃ SCF on MAPbBr₃ single crystal substrate. The small figure is SEM image of the interface of the epitaxial layer and substrate with 1 μm scale. (D) Atomic force microscope (AFM) image of the surface of MAPbI₃ film. (E) 3D image of the surface of MAPbI₃ film.

XRD and PL characterization are performed to determine the quality and crystallinity of the epitaxial MAPbI₃ SCFs. For comparison, the XRD spectra of the epitaxial MAPbI₃ on MAPbBr₃ (orange line), MAPbBr₃ PSC (green line) and MAPbI₃ PSC (blue line) are shown, respectively, in Figure 2A. Different from MAPbI₃ single crystal of hexagonal structure, the epitaxial MAPbI₃ film changes to cubic structure because of the cubic MAPbBr3 substrate (Wang et al., 2019b). Additionally, Figure 2B shows a clear MAPbI₃ (200) peak at a Bragg angle of 2θ = 28.22° and a MAPbBr₃ (200) peak at 2θ = 28.47°. Furthermore, MAPbI₃ (200) peak with a full width at half-maximum (FWHM) of 0.0925 in Figure 2B, which is comparable to MAPbI₃ single crystal (200) peak with a FWHM of 0.1121, indicates the high crystallinity of epitaxial MAPbI₃ SCFs (Liu et al., 1991, 2016). Figure 2C demonstrates the steady-state PL spectra at room temperature with a strong emission peak at 791 nm under the 450 nm excitation with a FWHM of 32.3 nm, which is smaller than 60 nm of MAPbI₃ single crystal and 55 nm of MAPbI₃ polycrystalline film (Han et al., 2018), confirming the high quality of epitaxial film.

FIGURE 2

Figure 2. (A) XRD spectra of the epitaxial MAPbI₃ SCF on MAPbBr₃, MAPbBr₃ PSC, and MAPbI₃ PSC. (B) XRD spectra of the epitaxial MAPbI₃ SCF on MAPbBr₃ at Bragg angle from 27.8° to 29.0°. (C) Photoluminescence (PL) spectra of the epitaxial MAPbI₃ SCF on MAPbBr₃ at room temperature. The PL excitation wavelength is 450 nm. (D) Absorption spectra of the epitaxial MAPbI₃ SCF. (E) Time-resolved PL spectra of MAPbI₃ SCF on MAPbBr₃ at the peak emission wavelength of 791 nm. (F) Current-voltage curve for the epitaxial MAPbI₃ based structure.

Moreover, Figure 2D illustrates that the absorption onset of epitaxial MAPbI₃ SCF is 840 nm, which are comparable to the 850 nm absorption onset of MAPbI₃ single crystal (Dong et al., 2015). And the redshift is obvious compared to the 780 nm absorption onset of polycrystalline MAPbI₃ film (Zhang et al., 2016). As shown in Figure 2E, double exponential fitting is performed to evaluate the carrier lifetime (τ) of epitaxial MAPbI₃ SCF. The short lifetime of 0.874 ±0.002 μs indicates surface recombination while long lifetime of 11.86 ±0.14 μs suggests bulk recombination (Shi et al., 2015). The carrier lifetime of epitaxial MAPbI₃ SCF is much longer than the carrier lifetime of polycrystalline film of nanosecond scale (Tong et al., 2017). In addition, space-charge-limited current (SCLC) technique is used to estimate the trap density of epitaxial MAPbI₃ SCF (Rose, 1955). The formula is ${\mathit{\text{n}}}_{\mathit{traps}}=\frac{\mathbf{2}\mathrm{\epsilon }{\mathit{V}}_{\mathit{TFL}}}{{\mathit{qL}}^{\mathbf{2}}}$, where ε is relative dielectric constant, V_TFL is the trap-filled limit voltage, q is the electronic charge. As to the horizontal device architecture in this work, L is the channel length between two gold electrodes, which were deposited on the surface of MAPbI₃ films by a shadow mask. As shown in Figure 2F, there is a linear current-voltage curve (n = 1) obeying Ohm’s law since the injected charge carriers at low applied voltage is too weak to completely fill trap center. As the applied voltage gradually increases, the injected carrier concentration is greater than the thermally excited carrier concentration and the conduction law changes from the ohmic mode to the SCLC mode. When the applied voltage further increases, all injected charge carriers are used to fill deep traps and all traps will be filled at V_TFL (Quah and Cheong, 2013a,b). Figure 2F exhibits the current-voltage curve for the epitaxial MAPbI₃ based structure. The L is 500 μm with 1% error and V_TFL is 50 V so that the low trap density is calculated as 5.64(±0.12) × 10¹¹ cm^–3, which is much lower than polycrystalline films (Dong et al., 2015; Li et al., 2018). The high quality and crystallinity of epitaxial MAPbI₃ SCF benefit the wider absorption range, longer carrier lifetime and lower trap density than polycrystalline films.

To further validate the quality of the epitaxial MAPbI₃ SCF, a horizontal architecture photodetector with two golden electrodes deposited on the surface of epitaxial MAPbI₃ SCF is fabricated as shown in Figure 3A. A light-emitting diode at 670 nm is used as light source for characterization of optoelectronic performance. Figures 3B,C plot the typical I-V curve under various light illumination intensities and the temporal response at different voltage bias of our photodetector, respectively. The dark current is very low of 7.1 nA at 0.2 V, which benefits the high photocurrent to dark current ratio (I_ON/I_OFF) of 331.7 with 2.5 mW cm^–2 light illumination. Additionally, responsivity ($\mathbf{\text{R}}=\frac{({\mathbf{J}}_{\mathbf{l}ight}-{\mathbf{J}}_{\mathbf{d}ark})}{{\mathbf{P}}_{\mathbf{l}ight}}$) and detectivity ($\mathbf{\text{D}}=\frac{{\mathbf{J}}_{\mathbf{l}ight}}{({\mathbf{P}}_{\mathbf{l}ight}\sqrt[\mathbf{2}]{(\mathbf{2}q{\mathbf{J}}_{\mathbf{d}ark})})}$) are calculated to evaluate the performance of this photodetector (Gong et al., 2009; Zhang et al., 2018). As shown in Figure 3D, R is as high as 1.38 A W^–1 and the maximum D is 3.07 × 10¹² jones at 20 V bias with 90 μW cm^–2 illumination at 670 nm, which are higher than polycrystalline films-based devices, reflecting the efficiency and ability of this photodetector responds to weak optical signal (Jiao et al., 2017; Wang et al., 2019c). Additionally, responsivities as a function of wavelength from 400 to 900 nm are illustrated in Figure 3E. And the normalized EQE of MAPbBr₃ single crystal in Figure 3E indicates that photodetectors based on epitaxial films can widen the absorption range of substrate. As shown in Figure 3F, a 6 ns pulsed laser is used to measure the response speed and it takes only 2.21 μs for current decay to 63% at 20 V bias, which is much faster than devices based on polycrystalline films, owing to the high quality of epitaxial MAPbI₃ SCF (Horvath et al., 2014; Mi et al., 2017; Wu et al., 2020).

FIGURE 3

Figure 3. (A) Schematic illustration of our photodetector and the incident light. The size of the photodetector is 0.025 cm². (B) I-V curves of our photodetector under dark and various light illumination intensities from 90 μW cm^–2 to 2.5 mW cm^–2. The wavelength of incident light is 670 nm. (C) Temporal response of the photodetector at different voltage bias at 670 nm under 90 μW cm^–2 light illumination intensity. (D) Plots of responsivity and detectivity of our photodetector at 20 V bias as a function of light intensity at 670 nm. (E) Responsivity at various incident light wavelength from 400 to 900 nm under different voltage bias and the normalized External Quantum Efficiency (EQE) of MAPbBr₃ single crystal. (F) Response time at 20 V bias under a 6 ns pulsed laser.

To further indicate the high quality of epitaxial MAPbI₃ SCF, variable temperature crystallization (VTC) method (Wang et al., 2017) is employed for synthesis of MAPbI₃ PSC and a photodetector based on MAPbI₃ PSC is fabricated and tested under same test environment for comparison in this work. Figures 4A,B illustrate the dark current and the photocurrent with 90 μW cm^–2 illumination at 670 nm of epitaxial film-based device and PSC-based photodetector. Epitaxial film-based device has lower dark currents at voltages from 0.2 to 10.4 V bias and higher photocurrents than PSC-based photodetector. As shown in Figure 4C, the I_ON/I_OFF ratios of epitaxial film-based device are about 10 times higher than those of PSC-based photodetector. Figure 4D shows the dark current densities at 20 V bias after various days, which of 25.3 μA cm^–2 on day 0 and of 33.11 μA cm^–2 on day 120. In addition, Figure 4E shows the long-term stable response of our device on day 120 at 20 V bias. The increasing rate formula is $\text{Rate}=(\frac{({\text{J}}_m-{\text{J}}_n)/(m-n)}{{\text{J}}_n})\times 100\%$, where J_m and J_n are the dark current densities on day m and day n. The calculated rate of change in dark current density is about 0.255% per day. The good morphology and crystallinity of epitaxial films benefit better stability than polycrystalline films (Salim et al., 2015; Tailor et al., 2020). The comparison of photoelectric performance between different MAPbI₃-based devices is shown in Table 1. The excellent performance actually validates the high-quality of the epitaxial MAPbI₃ SCFs.

FIGURE 4

Figure 4. (A) Dark current of our photodetector and MAPbI₃ PSC-based photodetector under different voltage bias. (B) Photocurrent of our photodetector and MAPbI₃ PSC-based photodetector at 670 nm under 90 μW cm^–2 light illumination intensity. (C) I_ON/I_OFF current ratios of our photodetector and MAPbI₃ PSC-based photodetector at 20 V bias under various light illumination intensity from 90 μW cm^–2 to 3.2 mW cm^–2. (D) Long-term dark current density test at 20 V bias. (E) Long-term response of devices after 120 days with a 0.5 Hz LED at 670 nm of 90 μW cm^–2 as light source.

TABLE 1

Table 1. Device performance comparison between MAPbI₃-based photodetectors.

Conclusion

In summary, this work demonstrates the epitaxial growth of high-quality MAPbI₃ single-crystal film on MAPbBr₃ substrate with good lattice matching. XRD, AFM, and SEM characterizations are performed to confirm the high crystalline and smooth morphology, which result in the low trap density of 5.64 ×10¹¹ cm^–3 and long carrier lifetime of 11.86 μs. Additionally, we fabricate a high-performance photodetector based on epitaxial MAPbI₃ SCF with responsivity of 1.2 A W^–1, detectivity of 3.07 ×10¹² jones, response time of 2.21 μs and excellent stability for 120 days. Therefore, this epitaxial growth method provides a new path for high-stability perovskite film-based devices.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation, to any qualified researcher.

Author Contributions

YX, JZ, and YL grew the perovskite single crystals. YX and JZ did the epitaxial experiments. YX, YP, and JZ did the measurements. XW and YX analyzed these results. YX wrote the manuscript. All authors made comments on this manuscript.

Funding

This work was financially supported by the National Key Research and Development Program of China (2018YFE0125500, 2016YFB0401600), the Program 111_2.0 in China (BP0719013), the National Natural Science Foundation Project of China (61775034, 51879042, 61674029, 12005038), the Research Fund for International Young Scientists (62050410350), the International Cooperative Research Project of Jiangsu Province (BZ2018056), and the Leading Technology of Jiangsu Basic Research Plan (BK20192003).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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