High-sensitive and fast response to 255 nm deep-UV light of CH3NH3PbX3 (X = Cl, Br, I) bulk crystals.

Deep-UV light detection has important application in surveillance and homeland security regions. CH3NH3PbX3 (X = Cl, Br, I) materials have outstanding optical absorption and electronic transport properties suitable for obtaining excellent deep-UV photoresponse. In this work, we have grown high-quality CH3NH3PbX3 (X = Cl, Br, I) bulk crystals and used them to fabricate photodetectors. We found that they all have high-sensitive and fast-speed response to 255 nm deep-UV light. Their responsivities are 10-103 times higher than MgZnO and Ga2O3 detectors, and their response speeds are 103 times faster than Ga2O3 and ZnO detectors. These results indicate a new promising route for deep-UV detection.

Introduction

The increasingly irreplaceable application of deep-ultraviolet (deep-UV: 200–280 nm) technology (imagery, warning and secure communication) in surveillance, homeland security and civil regions, makes the high-sensitive and fast-speed deep-UV detectors being urgently demanded [1-5]. Compared to cumbersome vacuum phototube detectors, semiconductor ones are lightweight, robust and have low operating voltage [6,7]. There are generally two detection strategies for semiconductor-based deep-UV detectors. One approach is to use wide bandgap semiconductors such as AlGaN, MgZnO, Ga2O3 or diamond [8-12]. However, the high-temperature and complex growth condition make it difficult to obtain high-quality materials, and thus the performance of the fabricated detectors are always far from expected [13]; another alternative approach is employing narrow band-gap Si diode detectors equipped with UV-pass filters [14,15]. However, the deep-UV detection of Si diode is still barely satisfactory, as the large absorption of deep-UV light of Si makes it difficult for the photo-generated carrier to reach the depletion layer. Therefore, it is still urgently needed to explore new semiconductor materials which have both facile growth method and excellent deep-UV response performance.

Recently, organic–inorganic perovskite CH3NH3PbX3 (X = Cl, Br, I) have attracted intensive attention in solar cells, luminescence, photodetection etc. [16-19]. High-crystalline quality CH3NH3PbX3 crystals can be easily grown using simple low-temperature (less than 100°C) solution method [20-22]. And they have large absorption coefficient of approximately 105 cm−1 in deep-UV spectral range [17,23,24], high carrier mobility even exceeds 100 cm2 V−1 s−1) [25] and long carrier transport length up to hundreds of micrometres [18,26], which make them promising for showing high-sensitive and fast-speed deep-UV response performance. Several studies have reported the photodetection properties of CH3NH3PbX3 [19,27,28], which generally concern visible light or radiation detection. Special comprehensive research on their deep-UV detection performance has not been reported. As mentioned above, narrow band-gap semiconductor also has application possibilities in deep-UV detection with the aid of UV-pass filters. Therefore, studies on deep-UV detection performance of CH3NH3PbX3 have practical significance.

In this work, we have grown high-quality bulk CH3NH3PbX3 crystals and used them to fabricate photodetectors. The deep-UV detection performance of CH3NH3PbCl3, CH3NH3PbBr3 and CH3NH3PbI3 were comprehensively studied. Under illumination of 1.5 mW cm−2 255 nm light and 5 V bias, CH3NH3PbCl3, CH3NH3PbBr3 and CH3NH3PbI3 respectively show responsivities of approximately 450, 300 and 120 mA W−1, and rise time of 15, 2.5 and 2 ms. These results manifest that CH3NH3PbX3 are promising candidates for deep-UV detection.

Material and methods

CH3NH3PbX3 powders were synthesized from halogen acid aqueous solution using the method illustrated in figure 1. Firstly, 5 g lead(II) acetate trihydrate (AR) was dissolved in 20 ml HCl/HBr/HI acid solution in a 50 ml flask under rigorous stirring. Then PbCl2/PbBr2/PbI2 powders are generated in the flask. Secondly, 10 ml methylamine (40% wt/wt aq. sol.) was added to the above blend solution and CH3NH3PbX3 powders precipitated. It should be noted that the addition of methylamine should be drop by drop under rigorous stirring because the reaction is violently exothermic. Once the CH3NH3PbCl3/Br3 powder was obtained, keeping the blend solution at 90°C and under rigorous stirring for 24 h until the powder was fully dissolved. Then stopping stirring and absorbing the supernatant liquid and placed them into a 20 ml serum bottle. Then slowly cooling this saturated precursor solution to room temperature, 1–3 mm3 CH3NH3PbCl3/Br3 crystals were obtained. Yellow needle-like CH3NH3PbI3 · nH2O crystals are formed when CH3NH3PbI3 encounters water below 50° [29]. Thus, CH3NH3PbI3 single crystals were grown from organic solvent γ-butyrolactone using inverse temperature crystallization (ITC) method due to its negative solubility temperature coefficient. By fully dissolving the CH3NH3PbI3 powder in γ-butyrolactone (approx. 0.3 g ml−1) and slowly heating the solution from room temperature to 80°C with a rate of approximately 5°C h−1, 5 mm3 CH3NH3PbI3 crystals can be obtained.

Figure 1.

Schematic representation of powder synthesis and single crystals growth of CH3NH3PbX3 (X = Cl, Br, I). CH3NH3PbX3 powders are synthesized through reaction between Pb(Ac)2, CH3NH2 and HX aqueous solution. CH3NH3PbCl3/Br3 crystals are grown by cooling the saturated precursor solution from 90°C to 25°C. CH3NH3PbI3 crystals are grown from the saturated solution of CH3NH3PbI3 powder in γ-butyrolactone (GBL) by gradually heating (from 25°C to 80°C) due to its negative solubility temperature coefficient.

Au films were deposited on the crystal surface as electrodes by thermal evaporation. The photo-response performance was measured using self-built system with 255 nm LED as light source.

Results and discussion

The powder XRD patterns of CH3NH3PbX3 crystals are shown in figure 2a, which agree well with previously reported results [18,20,21]. The residual weak peaks denoted by stars in the pattern of CH3NH3PbCl3 come from PbCl2. XRD patterns of CH3NH3PbCl3 and CH3NH3PbBr3 are very close because they both belong to cubic system (space group of Pm-3m) with different lattice constants of 5.67 Å for CH3NH3PbCl3 and 5.92 Å for CH3NH3PbBr3. CH3NH3PbI3 belongs to tetragonal phase (space group I4/m) with lattice constants of a = b = 8.83 Å and c = 12.69 Å. Their different crystal structures resulted from the different ion radius of Cl (1.81 Å), Br (1.96 Å) and I (2.2 Å). The large ion radius of I makes CH3NH3PbI3 distort from cubic to tetragonal phase.

Figure 2.

(a) Powder X-ray diffraction (XRD) patterns of the three crystals. CH3NH3PbCl3 and CH3NH3PbBr3 both belong to cubic phases, and CH3NH3PbI3 belongs to tetragonal phase. (b) The dependence of absorption of CH3NH3PbX3 on the photon energy. (c) Photoluminescence spectra of CH3NH3PbX3 crystals excited by 325 nm laser. For clarity, the photoluminescence intensity of CH3NH3PbCl3 was multiplied by 10 times.

Furthermore, detailed optical properties of CH3NH3PbX3 crystals were also studied comprehensively. Steady state UV-Vis diffuse reflection spectra of CH3NH3PbX3 powder were collected. According to Kubelka–Munk function, the dependence of (F(R∞)hv)2 on photon energy is given in figure 2b. As can be seen, sharp band edges are clearly observed, indicating the direct bandgaps of CH3NH3PbX3. Relying on estimation from Tauc/Davis–Mott model [30,31], through extrapolating the linear range of (F(R∞)hv)2 to photon energy (hv) intercept, the bandgaps of CH3NH3PbCl3, CH3NH3PbBr3 and CH3NH3PbI3 are estimated to be 2.95, 2.28 and 1.52 eV, respectively. The gradual lowering of CH3NH3PbX3 bandgap with halogen changing from Cl to Br to I is mainly ascribed to the lowering valence band maximum formed by halogen orbitals from 3p to 4p to 5p [32]. Room temperature photoluminescence spectra of CH3NH3PbX3 single crystals are displayed in figure 2c, and the strong band–band emission indicates the high crystalline quality of CH3NH3PbX3 single crystals. Luminescence peak positions show a gradual red-shift from 405 nm for CH3NH3PbCl3 to 545 nm for CH3NH3PbBr3 to 787 nm for CH3NH3PbI3, which are ascribed to their different bandgaps. Compared to previously reported CH3NH3PbCl3 which has no emission at room temperature [24], the observed emission in our CH3NH3PbCl3 crystal indicates its high crystalline quality. The luminescence intensity of CH3NH3PbBr3 and CH3NH3PbI3 are even 102 times higher than that of CH3NH3PbCl3 under the same measurement condition, which are attributed to their different band structures, exciton energies and carrier lifetimes.

To estimate the trap densities of the three CH3NH3PbX3 crystals, we fabricated sandwich-type devices by depositing two Au electrodes on the top and bottom faces of the crystals. Their I–V plots of them under dark condition are shown in figure 3a–c. As seen from figure 3a, the dark current of CH3NH3PbCl3 shows linear dependence on voltage (I ∝ V) under low voltage, which belongs to ohmic region. When voltage is larger than 10.3 V, charge carriers start to occupy the trap states and the current rises sharply with increasing voltage (I ∝ V, n > 3), which is trap-filled region. Similarly, CH3NH3PbBr3 and CH3NH3PbI3 both have such transition at about 3.9 V and 5.5 V, respectively. According to the space-charge-limited current (SCLC) model [20,21], the transition voltage (VTFL) from ohmic to trap-filled region is proportional to traps density (ntraps) and the square of electrode gap (L) as described by the relation:dielectric constant ɛ(CH3NH3PbCl3) = 23.9, ɛ(CH3NH3PbBr3) = 25.5, ɛ(CH3NH3PbI3) = 28.8 [33], 0 denotes vacuum permittivity dielectric constant 8.85 × 1012 C V−1 m−1), L is electrode gap (equal to the crystal thickness, 0.8 mm for CH3NH3PbCl3, 1 mm for CH3NH3PbBr3 and 3 mm for CH3NH3PbI3) and e is the elementary charge e = 1.6 × 10−19 C. According to the formula (3.1), we find that ntraps (CH3NH3PbCl3) is estimated to be approximately 8.4 × 1010 cm−3, ntraps (CH3NH3PbBr3) is around approximately 2.1 × 109 cm−3, and ntraps (CH3NH3PbI3) is about approximately 3.2 × 109 cm−3.

Figure 3.

Current versus voltage in logarithmic coordinates (logI–logV) under dark condition of sandwich structure (a) CH3NH3PbCl3, (b) CH3NH3PbBr3, and (c) CH3NH3PbI3 devices, which show different regions marked as ohmic (I ∝ V), trap filling (I ∝ V3), and Child (I ∝ V2). The insets show the diagram of device structure. (d) Schematic diagram of planar MSM detectors and photoresponse measurement system. (e) Dark currents (dotted lines) and photocurrents (solid lines) under illumination of 5.3 mW cm−2 255 nm light versus voltage of three planar MSM detectors. (f) Dark current versus voltage (I–V) of three planar MSM detectors. (g) Photo/dark current ratio and (h) responsivity of three CH3NH3PbX3 detectors with varying voltage. (i) The responsivity of three CH3NH3PbX3 detectors with increasing power intensity.

To obtain high performance detector, carrier recombination should be decreased to the largest extent. CH3NH3PbX3 have been demonstrated to have large absorption coefficient of approximately 105 cm−1 for deep-UV light [23], it is estimated that penetration depth of incident deep-UV photons is only about hundreds of nanometres. Thus, the materials for fabricating detectors should be as thin as possible on the premise of completely absorbing the incident light. The schematic diagram of the detectors is shown in figure 3d. The Au electrodes are connected to the outer circuit (source meter) using a four-probe station equipped with a microscope.

The I–V plots of CH3NH3PbX3 detectors under illumination of 5.3 mW cm−2 255 nm light and dark condition are given in figure 3e, and the enlarged plots of I–V curves under dark condition are shown in figure 3f. As seen from figure 3f, under 5 V bias, the dark currents are approximately 3 nA for CH3NH3PbCl3, approximately 0.8 nA CH3NH3PbBr3 and approximately 0.6 nA for CH3NH3PbI3. When the voltage is smaller than 2.5 V, the dark current of CH3NH3PbBr3 is smaller than that of CH3NH3PbI3; when the bias is larger than 2.5 V, the result reverses. As seen from figure 3e, the photocurrents show gradual decrease with halogen varying from Cl to Br to I. For CH3NH3PbCl3 and CH3NH3PbBr3, photocurrents approach saturation with increasing voltage, which is attributed to phonon scattering on photo-generated carriers. Under high voltage, carriers are scattered heavily by phonons, thus the dependence of carrier drift velocity on voltage deviates from linear relation and approaches saturation, which leads to current saturation. While the photocurrent of CH3NH3PbI3 does not show obvious saturation within the measured voltage range, indicating that CH3NH3PbI3 has a higher saturation voltage than CH3NH3PbCl3 and CH3NH3PbBr3, which can be ascribed to their different intrinsic carrier concentration and phonon energy. Figure 3g displays the photo/dark current ratios of CH3NH3PbX3 detectors under increasing voltage, the maximum photo/dark current ratios are nearly 900 for CH3NH3PbCl3, 320 for CH3NH3PbBr3 and 190 for CH3NH3PbI3. The responsivity is defined as R = I/AP, where I represents the photocurrent, P is the incident light power, and A is the absorption area of device [34-37]. Illuminated under 255 nm light with power intensity of 5.3 mW cm−2, the responsivity versus voltage is shown in figure 3h. At 5 V voltage, the responsivities are 210 mA W−1 for CH3NH3PbCl3, 190 mA W−1 for CH3NH3PbBr3 and 40 mA W−1 for CH3NH3PbI3.

Responsivities under illumination with increasing powder intensity given in figure 3i show a gradual decreasing trend, indicating that the detectors operate on photoconductive effect of CH3NH3PbX3 elaborated as follows. Once the incident photons are absorbed, excitons are generated inside the perovskite crystals. Under the applied voltage, these excitons were dissociated to be free electrons and holes and transported to the external circuit; finally the photocurrent is measured. Under higher power intensity light illumination, the effective traps are filled, leading to the decrease of photoconductive gain and thus responsivity also decreases [38]. As seen from figure 3i, under illumination of 1.5 mW cm−2 255 nm light, the responsivities are 450 mA W−1 for CH3NH3PbCl3, 300 mA W−1 for CH3NH3PbBr3, and 120 mA W−1 for CH3NH3PbI3, respectively. As summarized in table 1, these results are 101–103 times larger than previously reported wide bandgap semiconductors based deep-UV detectors such as AlGa1−N (34 mA W−1) [8], MgZn1−O (0.1 mA W−1) [9], LaAlO3 (72 mA W−1) [43], Ga2O3 (0.32 mA W−1) [39], SrRuO3/BaTiO3/ZnO [40], ZnO-Ga2O3 (9.7 mA W−1) [41] and MgZnO (0.16 mA W−1) [42]. As another determinant of detector performance, external quantum efficiency (EQE) is defined as the number of generated electrons per incident photon. EQE equals to Rhc/eλ, where h is the Planck's constant, c is the velocity of light, and λ is the wavelength of incident light [34,44]. Illuminated under 255 nm light with power intensity of 5.23 mW cm−2, EQE is 219% for CH3NH3PbCl3, 146% for CH3NH3PbBr3 and 58% for CH3NH3PbI3, respectively. When illuminated under 1.5 mW cm−2 light, the EQE is 102%, 93% and 19%, respectively.

Table 1

Comparison of the responsivity of different semiconductor materials to deep-UV light.

material	light (nm)	bias (V)	R (mA W−1)	EQE (%)
CH3NH3PbCl3	255	5	450	219
CH3NH3PbBr3	255	5	300	146
CH3NH3PbI3	255	5	120	58
AlxGa1−xN [8]	267	20	34	16
MgxZn1−xO [9]	250	10	0.1	0.05
Ga2O3 [39]	185	10	0.3	0.2
SrRuO3/BaTiO3/ZnO [40]	260	6	71.2	34
ZnO-Ga2O3 [41]	251	0	9.7	—
MgZnO [42]	250	0	0.16	—

Compared to CH3NH3PbBr3 and CH3NH3PbI3, CH3NH3PbCl3 shows higher responsivity and EQE. For photoconductive detector, trap states inside the photosensitive materials will capture the photo-generated electrons (holes) and carrier lifetime of holes (electrons) will be elongated and hence responsivity is improved [38]. Simultaneously, the captured carriers also slow the response speed, meaning that responsivity increase is always accompanied by response speed decrease.

From analysis on the dark current of three sandwich-type CH3NH3PbX3 devices using SCLC model, it is suggested that CH3NH3PbCl3 has higher density of trap states than that of CH3NH3PbBr3 and CH3NH3PbI3. If the higher density of trap states of CH3NH3PbCl3 leads to the higher responsivity, it will also induce the slower response speed of CH3NH3PbCl3 detectors. To verify this point, analysis on response speed will be given as follows.

To measure response speed of the photodetectors, time-dependent response of CH3NH3PbX3 photodetectors under modulated illumination were measured as shown in figure 4a–c, showing good repeatability of our detectors. Photo-switching response under different voltage is given in figure 4d–f. The estimated rise/decay time versus voltage is displayed in figure 4g,h, respectively. As voltage increases, response time decreases originally and saturates finally. As we known, the response time t is decided by the carrier lifetime. As we mentioned above, the three detectors operate on photoconductivity mechanism, in which the trap states elongate the carrier lifetime. Thus, we speculate that the increased voltage weakens the trapping time of trap states on holes (electrons), which means that the carrier lifetimes are relatively decreased and then the response speed is decreased. As seen from table 2, the response times of CH3NH3PbX3 detectors are 101–103 times shorter than previously reported SrRuO3/BaTiO3/ZnO (7.1 s, 2.3 s) [40], β-Ga2O3 (3.33 s, 0.4 s) [45] and NaTaO3 (50 ms) [46].

Figure 4.

Photo-switching characteristics of (a,d) CH3NH3PbCl3 (b,e) CH3NH3PbBr3 and (c,f) CH3NH3PbI3 photodetectors illuminated under modulated 255 nm light. The rise time (g) and decay time (h) of CH3NH3PbX3 detectors with varying voltage.

Table 2

Comparison of response speed to deep-UV from several different semiconductors.

materials	light (nm)	bias (V)	rise time	decay time
CH3NH3PbCl3	255	10	15 ms	31 ms
CH3NH3PbBr3	255	10	2.5 ms	2.5 ms
CH3NH3PbI3	255	10	2 ms	2 ms
SrRuO3/BaTiO3/ZnO [40]	260	6	7.1 s	2.3 s
β-Ga2O3 [45]	236	20	3.33 s	0.4 s
NaTaO3 [46]	280	5	50 ms	50 ms

Among the three perovskite detectors, CH3NH3PbCl3 detector has the slowest response speed with rise time and rise time of 31 ms and 15 ms, respectively, which are 10 times longer than that of CH3NH3PbBr3 and CH3NH3PbI3 detectors, which have rise/decay time of about 2 ms. The slower response speed of CH3NH3PbCl3 detector can be attributed to the higher density of trap states in CH3NH3PbCl3 single crystals. This point is consistent with previous analysis on the responsivities.

Present research on CH3NH3PbX3 mainly focuses on polycrystalline-film-based solar cells, while their potential for deep-UV detection is not developed although they have outstanding optoelectronic properties suitable for deep-UV detection. Herein, we firstly give comprehensive studies on deep-UV detection performance of CH3NH3PbX3 (X = Cl, Br, I) single crystals.

To reveal the decisive role of intrinsic optoelectronic properties of perovskite on detector performance, high quality bulk crystals are used to fabricate planar-type MSM detectors, which operate on photoconductivity of CH3NH3PbX3. For such photoconductive detectors, there generally exists persistent photoconductivity mechanism. Trap states capture photo-generated carriers, and persistent photoconductivity (PPC) is formed, leading to an increased responsivity, simultaneously; the response speed is slowed.

From previous analysis on the dark current of the detectors shown in figure 3a–c, it is concluded that CH3NH3PbCl3 has highest density of traps among the three crystals. Therefore, according to the PPC mechanism, CH3NH3PbCl3 detector theoretically has the largest responsivity and slowest response speed, which is consistent with the measured results summarized in tables 1 and 2. CH3NH3PbBr3 and CH3NH3PbI3 crystals with low density of trap states have high responsivities and ultra-fast response speed, which seems more suitable for application in fast speed deep-UV detection.

Conclusion

In summary, we have grown millimetre-sized CH3NH3PbX3 (X = Cl, Br and I) bulk single crystals and used them to fabricate photodetectors. Benefiting from the excellent optoelectronic properties and high crystalline quality of CH3NH3PbX3 crystals, the detectors have low dark current, high photo/dark current ratio, sensitive and fast response speed to 255 nm deep-UV light. These excellent response performances make CH3NH3PbX3 materials promising candidates for deep-UV detection.