Self-Powered All-Inorganic Perovskite Microcrystal Photodetectors

Apr 5, 2018 - Organic–inorganic lead halide perovskite microcrystal (MC) films are attractive candidates for fabricating high-performance large-area...
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Self-Powered All-Inorganic Perovskite MicroCrystal Photodetectors with High Detectivity Hai Zhou, Junpeng Zeng, Zhaoning Song, Corey R. Grice, Cong Chen, Zehao Song, Dewei Zhao, Hao Wang, and Yanfa Yan J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00700 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018

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The Journal of Physical Chemistry Letters

Self-powered All-Inorganic Perovskite Micro-Crystal Photodetectors with High Detectivity Hai Zhou, †,‡ Junpeng Zeng,‡ Zhaoning Song,† Corey R. Grice,† Cong Chen,† Zehao Song,† Dewei Zhao,† Hao Wang*,‡ and Yanfa Yan*, †



Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and

Commercialization, The University of Toledo, Toledo, Ohio, 43606, USA



Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei

Key Laboratory of Ferro & Piezoelectric Materials and Devices, Faculty of Physics & Electronic Science, Hubei University, Wuhan, 430062, P.R. China

Corresponding Author Yanfa Yan: [email protected]

Hao Wang: [email protected]

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ABSTRACT Organic-inorganic lead halide perovskite micro-crystal (MC) films are attractive candidates for fabricating high-performance large-area self-powered photodetectors (PDs), because of their lower trap-state density and higher carrier mobility than their polycrystalline counterparts and more suitability of synthesizing large lateral area films than their single-crystal counterparts. Here, we report on the fabrication of self-powered all-inorganic CsPbBr3 perovskite MC PDs with high detectivity, using a modified solution synthesis method. The MCs are up to about 10 µm in size and the MC layer is also about 11 µm in thickness. Under a 473 nm laser (100 mW) illumination, the CsPbBr3 MC PDs show responsivity values of up to 0.172 A W-1, detectivity values of up to 4.8×1012 Jones, on/off ratios of up to 1.3×105, and linear dynamic ranges of up to 113 dB. These performances are significantly better than that of PDs based on polycrystalline perovskite thin films and comparable with that of PDs based on perovskite single crystals.

TOC GRAPHICS

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Organic-inorganic metal halide perovskites have attracted tremendous attention in recent years due to their successful applications in high-performance photovoltaic solar cells,1-4 light-emitting diodes (LEDs),5 photodetectors (PDs),6,

7

and lasers,8 resulting from their

ultrafast charge generation, high absorption coefficient, defect tolerance9, 10 and relatively large carrier mobility. In recent years, the performance of organic-inorganic lead halide perovskite-based

PDs

has

been

significantly

improved.6,7,11,12

However,

the

commercialization of these PDs is still hindered by the material instability induced by oxygen and moisture, which is largely due to the volatility of the organic cations.13 As an alternative, all-inorganic CsPbX3 (X=Cl, Br, and I) perovskite materials have caught great interests because of their higher thermal stability.14-19 Li and co-authors introduced ZnO nanoparticles into CsPbBr3 precursor solution to fabricate self-powered perovskite polycrystalline (PC) thin-film PDs, showing responsivities (R) of 11.5 mA W-1 without bias.20 Sim and co-authors21 reported high-performance PDs based on α-CsPbI3 perovskite nanocrystal films, showing a detectivity (D*) of 1.8 × 1012 Jones and a linear dynamic range (LDR) of 86 dB. The optimized device also exhibited an excellent stability under high humidity conditions. Despite this exciting progress, the current PC perovskite thin film-based PDs suffer from limited R and LDR due to the relatively low carrier mobility (100 cm2 V-1 s-1). Huang’s group28 reported perovskite thin-single-crystal photodetectors with a vertical p-i-n structure showing a high specific detectivity of 1.5 × 1013 Jones and a linear dynamic range of 256 dB. For best device performance, the SCs should have a large lateral area but a low thickness, i.e., about 10 µm,28 so that the photogenerated carriers can be efficiently collected.29–32 However, synthesizing such SCs with a combination of large lateral area and narrow thickness presents a significant challenge. 3 ACS Paragon Plus Environment

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An alternative approach is to use ~10 µm thick absorber layers consisting of perovskite micro-crystals (MCs) with sizes of ~10 µm. A recent study has shown that such perovskite MCs can exhibit a low trap-state density (1012 cm−3) and a high carrier mobility (over 100 cm2 V−1 S−1). Non self-powered perovskite MC-based PDs with the metal-semiconductor-metal structure showed R values of up to 6 × 104 and 6 A W-1 at a bias of 3 V, using one-photon and two-photon excitations, respectively.33 The results suggest that self-powered PDs using such perovskite MC absorber layers should lead to performances much higher than the PDs based on PC perovskite thin films. Herein, we report on the fabrication of self-powered all-inorganic CsPbBr3 perovskite MC thin-film PDs with high detectivity using a simple and scalable solution process method. The PDs use plasma-enhanced atomic-layer deposited (PEALD) SnO2 as the electron selective layer

(ESL)

and

spin-coated

2,2’,7,7’-tetrakis(N,N’-di-p-methoxyphenylamine)-9,9’-spirobifluorene (Spiro-OMeTAD) as the hole selective layer (HSL). The device configuration consists of glass/indium tin oxide (ITO)/PEALD-SnO2/perovskite

MCs/Spiro-OMeTAD/Au,

where

the

~11

µm-thick

perovskite MC layer consists of CsPbBr3 MCs with grain sizes of up to 10 µm. Under a 473 nm laser (100 mW) illumination, the CsPbBr3 MC PDs show R values of up to 0.172 A W-1, detectivity values of up to 4.8×1012 Jones, on/off ratios of up to 1.3×105, and LDR of up to 113 dB without bias. These performances are significantly better than that of PDs based on PC perovskite thin films and comparable with that of perovskite single crystal-based PDs reported in the literature.

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Figure 1 (a) Top-view SEM image of a CsPbBr3 MC perovskite film. The scale bar is 50 µm. The inset is a digital photograph of the perovskite film under 365 nm purple flashlight. (b) Cross-sectional SEM image of a CsPbBr3 MC perovskite-based PD.

The scale bar is 10 µm.

(c) Absorption spectrum of a CsPbBr3 MC perovskite film. The inset is the corresponding Tauc plot. (d) Dark I–V trace measured from a Au/CsPbBr3 MC/Au device.

Figure 1a shows the top-view scanning electron microscopy (SEM) image of a CsPbBr3 MC thin film grown on an ALD-SnO2-coated substrate. The CsPbBr3 MCs cubes with dimensions of ~ 10 µm are interconnected. The prism shape is consistent with the orthorhombic structure (space group, Pnma) of CsPbBr3. The average size of the MCs should be similar to the film thickness to ensure high quality MC films and low trap-state densities. If the size of MCs is larger than the thickness of the film, voids and excessive surface roughness will result. On the other hand, if the MC size is smaller than the film thickness, grain boundaries parallel to the film surface will form and will reduce the efficiency of carrier 5 ACS Paragon Plus Environment

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transfer between the front and back contacts. The inset is the digital photograph of the perovskite under 365 nm purple flashlight, showing crystal quality of our CsPbBr3 MCs. The cross-sectional SEM image (Figure 1b) confirms the thickness of the CsPbBr3 MC layer (11 µm) and the interconnection between perovskite CsPbBr3 MCs.

The phase purity of the

CsPbBr3 MCs and orthorhombic crystalline structure were confirmed by X-ray diffraction (XRD) (Figure S2), in good agreement with the previous reports.29-33 The CsPbBr3 MC thin films exhibit excellent optical absorption properties. As shown in Figure 1c, the measured UV-Vis absorption spectrum shows a very sharp absorption edge at 554 nm, and the Tauc plot (inset of Figure 1c) indicates a direct bandgap of 2.24 eV. To evaluate the quality of the CsPbBr3 MC film, the dark current-voltage (I-V) characteristic of the MC film is measured by using the space charge limited current method with a device structure of Au/50 µm CsPbBr3 MC/Au.30 The result (Figure 1d) shows an Ohmic region followed by a steep increase of slope, indicating the presence of a trap-filling region, starting at VTFL = 2.5 V.

From this dark I-V, we extract a trap state density of ntrap = 2.8 × 1012 cm−3,

by using the formula of ntrap = (2εε0VTFL)/(qL2), where VTFL is the trap-filled limit voltage, L is the thickness of the film, q is the electron charge, ε0 is the vacuum permittivity, ε is the relative dielectric constant of CsPbBr3 (≈22

30

).

The value of ntrap is four orders of

magnitude lower than that of the spin-coated PC thin films (1016 cm−3).6 A low ntrap is critical for achieving low 1/f noise of PDs.28

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Figure 2 (a) Schematic illustration of the MC perovskite PD. (b) Energy band diagram of the MC perovskite PD. (c) I-V curves of the CsPbBr3 MC PD at dark and under illumination. Inset: enlarged I-V curves showing self-powered performance. (d)

Photocurrent response of

the CsPbBr3 MCs PD at zero bias.

The low trap-sate density encourages us to fabricate self-powered CsPbBr3 MC thin-film PDs. The structure of our CsPbBr3 MC thin-film PDs (glass/ITO/PEALD-SnO2/perovskite MCs/Spiro-OMeTAD/Au) is shown in Figure 2a and the band diagram of the MC perovskite PD is shown in Figure 2b. In this structure, PEALD-SnO2 and spiro-OMeTAD are used as ESL and HSL, respectively. The PDs show good diode characteristics in dark with rectification ratios as high as 6792 at ±1V (Figure 2c).

Under illumination of a 473 nm laser

with the intensity of 0.64 W cm-2, the current is increased greatly due to the large contribution from photogenerated carriers (Figure 2c).

The device also exhibits photovoltaic properties

with an open-circuit voltage (Voc) of 0.45 V and a short-circuit current (Isc) of 0.6 mA (inset of 7 ACS Paragon Plus Environment

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Figure 2c).

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When the laser is switching on and off, the PD shows a rapid step-responses in

the cirrent-time (I-T) curves at zero bias, as illustrated in Figure 2d. shows a very low noise current within a few nA.

In dark, the device

The calculated on/off ratio is 1.3×105,

which is significantly higher than the values reported to date for PC perovskite thin-film PDs (~ 103),11, 12, 15 and comparable with the values of perovskite single crystal PDs (~ 105).28, 30, 32, 34

The detailed comparison is shown in Table S1. In addition, the PDs shows good switching

characteristic and illumination stability, the photocurrent degradation is less than 5% after 30 min illumination (Figure S3).

Figure 3 Photoresponse characteristics of a CsPbBr3 MC PD to a 473 nm laser irradiation at frequencies of (a) 500 and (b)1000 Hz at zero bias. estimating rise and fall time at 500 Hz.

(c) Rising and falling edges for

(d) Transient photovoltage decay of the PD.

Ln (△V) vs time curve.

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Inset:

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To evaluate the frequency response characteristic of the PDs, a rotary optical chopper is applied to the 473 nm laser.

Figure 3a and 3b depict the photoresponse characteristics of a

PD to the laser irradiation at chopping frequencies of 500 and 1000 Hz at 0 V, respectively. The device responses to the varying light signal accurately and quickly. Moreover, the light and dark currents of the PD show almost no change during the course of the measurement, implying that it can detect fast varying light signals. The rise (trise) and the fall time (tfall) is 0.14 ms and 0.12 ms, respectively, at 500 Hz (Figure 3c). Transient photovoltage (TPV)35 measurement is conducted to understand photocarrier dynamics during device operation.

For the TPV measurement, the device is operated at

open-circuit voltage condition and a laser pulse is generated to perturb the device, afterwards the decay of photovoltage was measured. The carrier lifetime is estimated to be the decay time at which the intensity becomes 1/e of the initial intensity. As shown in Figure 3d, the decay of transient photovoltage exhibits almost a linear relationship between ln(△V) and log10(t) (inset of Figure 3d), and the carrier lifetime is determined to be 10.78 ms, which is much longer than that of the PC perovskite thin-film PDs (~7.6 µs).35

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Figure 4 (a) I–V curves under different light intensity. (b) I-T curves under various light intensity at 0 V. (c) Power-dependent photocurrent under 0 V for CsPbBr3 MC based PDs. (d) Power-dependent R and D* under 0 V bias.

We further evaluate the I–V curve dependence on the incident light intensity. As shown in Figure 4a, the photocurrent depends more sensitively on the incident light intensity (varying from 6.4 ×10-6 to 6.4 ×10-1 W cm-2) than the photovoltage, which is reasonable since photocurrent is nearly linear dependent on the number of photogenerated electron–hole pairs, while photovoltage in the device is mainly determined by the band bending (or the Fermi level pinning) at the selective contacts due to the injection of charges.36 The results suggest an efficient carrier collection within the ~11 µm thick CsPbBr3 MC absorber layer. The photocurrent also strongly depends on the bias voltage. Figure 4b plots the photocurrent at various light intensity at zero bias, from which the photocurrent is found to increase gradually with the increase of light intensity. 10 ACS Paragon Plus Environment

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For PD applications, such as image sensors and illumination meters, the detection of light over a wide light-intensity range is required. Having a constant responsivity over a wide range of light intensity presents an important performance parameter of a PD.

The LDR is a

figure-of-merit for PDs to characterize the light intensity range in which the PDs have a constant responsivity, which is calculated by the following equation: (1)

LDR = 20 log( I p / I d )

where Ip and Id are the photocurrent and dark current, respectively.

The LDR of one of our

CsPbBr3 MC PDs is measured by recording the photocurrent of the devices under a 473 nm laser illumination with various light intensities (ranging from 6.4 ×10-6 to 6.4 ×10-1 W cm-2). The results are shown in Figure 4c. By linearly fitting the data, we find that the R-squared value (coefficient of determination) of the linear fitting for the CsPbBr3 MC PD is 0.94, which is close to 1, indicating the good linearity within this measurement region. The current has a linear change from 1.5 nA to 0.69 mA, corresponding to a LDR of at least 113 dB, which is higher than that of perovskite PC thin-film PDs and comparable with perovskite SC PDs (Table S1). The spectral responsivity, R, and the previously described detectivity value, D*, are two important parameters of a PD. When the dark current is dominated by shot noise, D* is given by the following equation,37 D* =

J ph

R 1 1 = 1 Llight (2 qJ d ) 2 (2 qJ d ) 2

(2)

where, Jd is the dark current density, Jph is the photocurrent density, and Llight is the light intensity.

The calculated R and D* are plotted in Figure 4d, showing linear changes for both

R and D* and their values decrease with the increase of light intensity.

Under strong

illumination (0.64 W cm-2), the PD shows good performance with R and D* values of 0.075 A

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W-1 and 2.1×1012 Jones (1 Jones = 1 cm Hz1/2 W-1), respectively.

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Under weak illumination

(6.4×10-6 W cm-2), it shows R and D* values of 0.172 A -W and 4.8×1012 Jones, respectively. For comparison, we also fabricate PDs using thinner (~5 µm) and thicker (~50 µm) CsPbBr3 MC absorber layers. The results are listed in Table S2. The performances of these PDs are much worse than that of the PDs using ~11 µm CsPbBr3 MC absorber layer. As shown in Figure S4, the ~5 µm thick CsPbBr3 MC absorber layer is not continuous, leading to direct contact between HSL and ESL, with the resulting the PD showing no useful performance.

The ~50 µm thick CsPbBr3 MC absorber layer (Figure S5) shows a high

density of voids, inhibiting the transport of carriers. As a result, poor performance is observed (Figure S6-S10).

The R-squared value of the linear fitting of the ~50 µm thick CsPbBr3 MC

PD is small (Figure S10a). The R and D* curves display obvious increase with the decreasing light power at the linear response range (Figure S10b), suggesting that photogenerated carriers are more difficult to be collected in the thicker CsPbBr3 MC layers. In summary, we reported the fabrication of self-powered all-inorganic CsPbBr3 MC PDs with high detectivity. The MCs are up to 10 µm in size and the MC single layer is about 11 µm thick. Under a 473 nm laser (100 mW) illumination, the CsPbBr3 MC devices showed responsivity values up to of 0.172 A W-1, detectivity values as large as 4.8×1012 Jones, on/off ratio up to 1.3×105, and linear dynamic range up to 113 dB. These performances are significantly better than the perovskite film PDs based on PC perovskite thin films and comparable with the perovskite single crystal-based PDs. Our results demonstrate that high-performance perovskite PDs can be fabricated by scalable solution process methods, benefiting the comercialization of perovskite PD technology.

Experimental Methods

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SnO2 film preparation: Glass substrates with patterned ITO were washed by ionized water, acetone and anhydrous ethanol for 30 min, respectively.

Then, the SnO2 ESLs were

deposited onto cleaned ITO substrates by PEALD (Ensure Scientific Group Auto ALD-PE V2.0) following the process and treatment described in our earlier work.38 CsPbBr3 MC preparation: The CsPbBr3 MCs were prepared directly on SnO2-coated substrates by a modified inverse temperature crystallization.30, 33 As illustrated in Figure S1, 0.1 M CsBr and PbBr2 (mole ratio 1:2) were first dissolved in 10 µL dimethyl sulfoxide (DMSO) solution which was stirred on a hot plate. Then the antisolvent solution (toluene) was added with the same volume of DMSO. Then SnO2-coated substrates were immersed in the solution with a magnetic stirrer nearby, and the solution was heated up to 120 °C under stirring to increase the nucleation site within the solution.

After 5 min, another 5 µL toluene

was added dropwise under stirring and large amount of micrometer-size single crystals were obtained on the substrate.

20 min later, the remaining solution was removed and the samples

were annealed at 140 °C for 30 min to evaporate the solvent quickly and obtain better crystal quality. Highly crystalline CsPbBr3 MC layers with ~11 µm thickness were obtained. Device fabrication: After the preparation of the CsPbBr3 MCs, poly(methyl methacrylate) ( PMMA ) was spin-coated for filling the voids in CsPbBr3 MC layer at a speed of 1000 rpm and then spiro-OMeTAD was deposited following a previously reported method.38

Finally,

50 nm Au was thermally evaporated through a patterned mask onto the HSL. The active area of 0.12 cm2 for each device was defined by a mask. Characterization: High-resolution SEM images of CsPbBr3 perovskite films were taken with a field emission SEM (Hitachi S-4800). The crystalline structure of CsPbBr3 perovskite films was examined by XRD (RigakuUltima III) with Cu Kα radiation under operation conditions of 40 kV and 44 mA excitation. The optical absorption spectra were measured by UV-Vis spectrophotometer (PerkinElmer Lambda 1050).

The I-V characteristic was measured by a

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Keithley 2401 sourcemeter.

The I-T and TPV measurements were conducted by a Solarton

Modulab potentiostat/galvanostat.

The photosensitivity was performed by using a 473 nm

laser source (100 mW), the optical power of which was calibrated by a standard Si diode and modulated by neutral density optical filters. All characterizations and measurements were performed in the ambient.

ACKNOWLEDGMENT This work is financially supported by National Science Foundation under contract no. CHE-1230246, DMR−1534686, and ECCS1665028, and the Ohio Research Scholar Program.

ASSOCIATED CONTENT The Supplementary Information provides information about the characterization and the detailed performance of PDs.

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(13) Tang, X.; Zu, Z.; Shao, H.; Hu, W.; Zhou, M.; Deng, M.; Chen, W.; Zang, Z.; Zhu, T.; Xue,

J.

All-Inorganic

Perovskite

CsPb

(Br/I)3

Nanorods

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Optoelectronic

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