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Superior Photodetectors Based on All-Inorganic Perovskite CsPbI Nanorods with Ultrafast Response and High Stability 3
Tao Yang, Yapeng Zheng, Zhentao Du, Wenna Liu, Zuobao Yang, Fengmei Gao, Lin Wang, Kuo-Chih Chou, Xinmei Hou, and Weiyou Yang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b08201 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018
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Superior Photodetectors Based on All-Inorganic Perovskite CsPbI3 Nanorods with Ultrafast Response and High Stability Tao Yang,†,‡ Yapeng Zheng,†,‡ Zhentao Du,†,§ Wenna Liu,†,‡ Zuobao Yang,† Fengmei Gao,† Lin Wang,† Kuo-Chih Chou,‡ Xinmei Hou,‡,* and Weiyou Yang,†,* †
Institute of Materials, Ningbo University of Technology, Ningbo, 315016, P.R.
China. ‡
State Key Laboratory of Advanced Metallurgy, University of Science and
Technology Beijing, Beijing 100083, P.R. China. §
College of Materials Science and Engineering, Hunan University, Changsha 410082,
P.R. China
Abstract:
Currently, one-dimensional (1D) all-inorganic CsPbX3 (X=Br, Cl and I) perovskites have attracted great attention, owning to their promising and exciting applications in optoelectronic devices. Herein, we reported the exploration of superior photodetectors (PDs) based on a single CsPbI3 nanorod. The as-constructed PDs had a totally 1 ACS Paragon Plus Environment
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excellent performance a responsivity of 2.92×103 A·W−1 and an ultrafast response time of 0.05 ms, respectively, which were both comparable to the best ones ever reported for all-inorganic perovskite PDs. Furthermore, the detectivity of the PDs approached up to 5.17×1013 Jones, which was more than 5 times to the best one ever reported. More importantly, the as-constructed PDs represented a high stability when maintained under ambient conditions.
KEYWORDS: photodetectors, perovskites, nanorods, stability, detectivity
Since the discovery of carbon nanotubes,1 one-dimensional (1D) nanostructurs, such as nanowires, nanobelts and nanotubes, have been attracting a great deal of interest over the past decades, due to their characteristics such as defect-free single crystal,2,
3
high crystalline quality,4 large specific surface area and Debye length
comparable to their sizes.5 More interestingly, the conductive channel of 1D nanostructures could confine the active area of charge carrier and shorten the carrier transit time,6, 7 making their advantage for exploration of highly efficient nanodevices.
Recently, the hybrid perovskites have attracted wide attention, due to their interesting applications in optoelectronic devices.8-10 However, the inevitable decomposition and volatilization of organic components within the hybrid perovskites render them with poor long-term stability.11, 12 In comparison to the organohalide perovskites, the all-inorganic counterparts are more stable and recognized as a rising star, which have attracted great interest due to their excellent charge transport 2 ACS Paragon Plus Environment
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properties13 and broad chemical tunability.14 It has been intensively reported that this class materials have triggered wide and exciting applications in high performance devices such as photovoltaic cells,15 light-emitting diodes,16 and lasers.17 Among them, the photodetectors (PDs) are considered as a representative one and being one of the hot topics currently.18, 19 For instances, most recently, Yang et al. reported the CsPbBr3 microcrystal-based PDs with a ultrahigh responsivity up to 6×104 A·W-1 and fast response time of ~1 ms;20 Shoaib et al. reported the CsPbBr3 nanowire-based PDs with an ultrahigh responsivity of 4.4×103 A·W-1 and a fast response speed of 0.252 ms,21 suggesting their very promising applications and bright prospect. However, how to bring the perovskite PDs with totally superior performances, especially with a satisfied stability to be serviced under air conditions, still encounters a grand challenge.
In the present work, single-crystalline all-inorganic CsPbI3 nanorods with high quality were synthesized in high yield via a solution process. Then, the PDs based on individual CsPbI3 nanorod were constructed. The obtained PDs exhibited a totally superior performance with a responsivity of 2.92×103 A·W−1 and an ultrafast response time of 0.05 ms, respectively, which were both comparable to the best ones ever reported for all-inorganic perovskite PDs. Furthermore, the detectivity of the PDs approached up to 5.17×1013 Jones, which was more than 5 times to the best one ever reported. More importantly, the PDs could be stable once maintained under ambient conditions, suggesting their very promising applications.
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RESULTS AND DISCUSSION
Figure 1(a) and Figure S1(a) in Supporting Information are the typical SEM images of the as-synthesized product under low magnifications, suggesting its high yield. Figure 1(b-c) and Figure S1(b-e) in Supporting Information are their closer observations, showing that the resultant nanorods are highly pure in morphology with uniform distribution in diameter and length, which are averagely sized in ~150 nm and ~2 µm, respectively. Meanwhile, the diameter of the rods is highly uniform along the axial direction. Figure 1(d) shows a typical TEM image of a single CsPbI3 nanorod (the observations under different magnifications are shown in Figure S2, Supporting Information). The down-left inset SAED pattern can be indexed to its orthorhombic phase, which is identical along the entire body, disclosing its single-crystalline nature. The measured d spacing of ∼0.478 nm between two neighbored lattice fringes (Figure S3, Supporting Information) responds to the distance of (100) crystal planes. Both TEM image and SAED pattern verify that the resultant nanorods grow along [100] direction, which is schematically shown as the crystal structures in Figure S4 in Supporting Information. The typical energy dispersive X-Ray spectroscopy (EDX) (Figure 1(e)) reveals that the as-synthesized nanorods are composed of Cs, Pb and I with a mole ratio of ∼1:1:3, in agreement with the stoichiometry of CsPbI3. The element mappings (Figure S5, Supporting Information) within a single rod present the uniform spatial distributions of Cs, Pb and I. The X-ray diffraction (XRD) pattern (Figure 1(f)) as well as the yellow color (the inset in Figure 1(f)) of the as-grown nanorods further confirms that they are 4 ACS Paragon Plus Environment
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orthorhombic perovskite CsPbI3 (JCPDS Card No. 18-0376). The detected signals of quartz in XRD pattern come from the used substrate. These experimental results represent that the as-synthesized products are of high-qualified single-crystalline perovskite CsPbI3 nanorods with a uniform diameter and length. The effect of reaction times, solvent amounts of OctAm and reactant amount of PbI2 on the growth of CsPbI3 nanorods has been investigated, which has been discussed in Supporting Information (Figure S6). It seems that the introduced reactant amounts play a fundamental role on the growth of high-quality CsPbI3 nanorods.
Figure 1. (a-c) Typical SEM image of as-synthesized CsPbI3 nanorods under different magnifications. (d) A typical TEM image of a single CsPbI3 nanorod. The down-left inset provides the SAED pattern. (e) A typical EDX spectrum of the nanorods. (f) The
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typical XRD pattern of the nanorods. The up-left inset is the photograph of the CsPbI3 nanorods dispersed in hexane.
Figure 2(a) is the typical UV–vis absorption spectrum of the obtained product, indicating that the CsPbI3 nanorods have a direct bandgap of ~2.7 eV, with an absorbance peak at 405 nm. Based on these nanorods, the photodetector (PD) nanodevice is constructed, which is schematically illustrated in Figure 2(b). The two ends of a single nanorod are covered by 30 nm Au film with a separation of 1 µm, as shown in Figure 2(c). The PD performances are then measured on a four-probe station in conjunction with a semiconductor characterization system, as shown in Figure 2(d).
Figure 2. (a) The representative UV–vis absorbance spectrum of CsPbI3 nanorods. The inset is the plots of (αhν)2 vs. hν. (b) Schematic diagram for the as-constructed
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PDs based on a single CsPbI3 nanorod. (c) A representative SEM image of the as-assembled PD with an individual nanorod. (d) The photograph showing the property measurements of the fabricated PDs conducted on a four-probe station.
Figure 3(a) gives the typical current-voltage (I-V) characteristics of the CsPbI3 nanorod PD illuminated under a light of 405 nm (3.06 eV) with an average power of 10.69 mW•cm-2 and dark conditions, respectively. It discloses that the dark current of CsPbI3 nanorod PD is lower than 10 pA at a bias of 2 V. However, the photocurrent illuminated under 405 nm is three orders of magnitude higher than that under dark, which approaches to 31 nA at the same voltage of 2 V. As shown in Figure 3(b), the symmetric and linear I–V curves verify that the ohmic contact has been established between the CsPbI3 nanorod and the electrodes.
The sensitivity of the PDs is mainly characterized by the spectral responsivity (Rλ) and external quantum efficiency (EQE),22,
23
which can be calculated by
following equations, respectively:
=
=
=
(1)
(2)
Where ∆I is the difference between the photocurrent and the dark current, Llight is the incident light intensity, P is the light power, A is the effective area of the detector, h is Planck’s constant, c is the velocity of light, e is the electronic charge, and λ is the exciting wavelength, respectively. Since Rλ is proportional to the quantum yield of the 7 ACS Paragon Plus Environment
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PD, it is very important for a PD to have a high conversion rate from photons to electrons/holes, namely, with a high EQE. Figure 3(c and d) provide the ca. Rλ and EQE of the PDs illuminated under 405 nm light at an applied voltage of 2 V, which can be up to 2.92×103 A·W-1 and 0.9×106 %, respectively. Notably, the responsivity can be comparable to the best one ever reported for the perovskite PDs, implying its high sensitivity.20, 21 As shown in Figure 3(c), the spectral response of the PD has a discrimination ratio (i.e., among the wavelengths ranged in 250~405 nm UV light, 405~460 blue light and 460~600 nm visible light) up to 3 orders magnitude, suggesting that the as-built CsPbI3 nanorod nanodevice is a typical UV and blue light PD, in accordance with its band gap (Eg) of ~2.7 eV (~460 nm), as validated in Figure 2(a).
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Figure 3. (a) The typical logarithmic I–V and (b) I–V characteristics of the PDs under irradiation with 405 nm light (10.69 mW•cm−2) and in the dark. (c) The representative spectral response of the PDs with wavelength ranging from 250 to 600 nm at a bias of 2.0 V. (d) The ca. EQE and detectivity of the PDs at different wavelengths.
As for a PD device, its detectivity can be calculated by:
∗
=
( ) ! "
(3)
Where f is the electrical bandwidth, and In is the noise current, respectively. In this case, once the dark current is dominated by the shot noise, D* can be expressed as:
∗ =
#! " ($ % )
(4)
Where Ioff is the dark current, and e is the elementary charge, respectively. Evidently, the dark current of the PD should be depressed as low as possible to distinguish very weak optical signals. To obtain a small Ioff, the semiconductor should have low trap density, low thermal emission (recombination) rates with the desired good crystal quality to avoid the current leakage during the operation. In current case, the specific detectivity of the CsPbI3 nanorod PD illuminated at 405 nm light is calculated to be as high as 5.17×1013 Jones (cm•Hz1/2•W-1). From 250 to 450 nm, the ultrahigh detectivity of the PD device approaches 1013 Jones (at a bias of 2.0 V), which is more than one order of magnitude higher than that of Si PD in the identical spectral region (i.e., 4×1012 Jones),18, 24 and more than 5 times to the best one ever reported for the perovskite PDs (see Table 1).19-21,
25-35
The excellent performance of perovskite 9
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nanorod PDs is mainly attributed to following reasons. Firstly, the CsPbI3 nanorods have low recombination of charge carriers, low density of defects and high absorption coefficient, which could bring a very strong photoelectric effect;21 Secondly, the single-crystal nanorod can provide a smooth and short path for carriers transfer, which significantly enhance the response speed; Thirdly, the absorption coefficient of the perovskite can reaches the order of 104 cm-1.18 Thereby, a few hundred nanometer layers of the material are required for light absorption.18 In comparison to the PDs as reported in Table 1, CsPbI3 nanorods with a diameter of ~150 nm can absorb the light with an energy smaller than Eg completely, which benefits to the improvement on the detectivity.
Table 1. Key performances of the typical perovskite PDs ever reported. Bias
Rise/decay
Responsivity
Detectivity
(V)
time (ms)
(A·W−1)
(Jones)
Photodetectors
Ref.
CsPbBr3 microparticles
10
1.8/1.0
0.18
6.1 × 1010
27
CsPbI3 nanocrystals films
1
24/29
-
-
31
CsPbBr3 nanoparticles/Au 2 anocrystals
0.2/1.2
0.01
1.68 × 109
25
CsPb(Br/I)3 networks
680/660
-
-
19
nanorod 8
CsPbBr3 nanosheets
5
0.019/0.025
0.25
-
34
CsPbBr3 thin films
6
0.43/0.318
55
9 × 1012
29
CsPbBr3 nanoarrays
5
0.0215/0.02 34
1 ×103
-
26
CsPbI3 nanoarrays
1
292/234
0.0067
1.57 × 1012
35
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CsPbBr3 nanosheets/carbon 10 nanotubes
0.016/0.38
31.1
-
28
CsPbBr3 nanoplatelets
0.6/0.9
34
7.5 × 1012
30
CsPbBr3 bulk single crystals 0
0.23/0.06
0.028
1.7 × 1011
32
CsPbBr3 bulk single crystals 5
0.069/0.261
2
-
33
CsPbBr3 microcrystals
3
0.5/1.6
6 × 104
1 ×1013
20
CsPbBr3 nanowires
3
0.252/0.3
4.4 × 103
-
21
CsPbI3 nanorods
2
0.05/0.15
2.92 × 103
5.17 × 1013
1.5
This work
Figure 4(a) gives the time response of the PD devices, which is measured by periodically turning on and off 405 nm light under air conditions at a bias of 2.0 V. The current clearly exhibits two distinct states when the light irradiation is on and off, respectively. It seems that the dark current is only 10 pA, nevertheless the photocurrent can be significantly enhanced to a stable value of 31.27 nA. It is worthy pointing out that the as-built PD behaves an excellent stability and reproducibility, which is evidenced by switching the light on/off for more than 200 cycles prior to test under illumination. As a result, the switching ratio (SR) is calculated as:
SR =
(
)*+,
=
(5)
Where Ion is the current measured by turning on 405 nm light, and Ioff is the one measured by turning off the light. Accordingly, the SR can be ca. 3.13 × 103. To obtain the response time, the response speed is measured using a 405 nm continuous laser triggered with a pulse width of 100 Hz, as shown in Figure 4(b). The currents 11 ACS Paragon Plus Environment
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increase very sharply from a state to another one, indicating its fast response to the light illumination. The response and recovery times, which are defined as the values needed for the dark current to reach 90% of the maximum photocurrent and vice versa down to 10%, are measured as ~0.05 and 0.15 ms, respectively. Notably, the response time of 0.05 ms in current case is around 10 and 5 times faster to those based on CsPbBr3 microcrystals (i.e., 0.5 ms)20 and CsPbBr3 single-crystalline nanowires (i.e., 0.252 ms)21 (see Table 1), evidencing the ultrafast response of the as-constructed perovskite PD device.
Figure 4(c) shows the time-response curves of CsPbI3 nanorod PD irradiated at 405 nm light in respect to the light intensities, in which the photocurrents increase from 3.12 to 16.07 mW•cm-2 with the raise of the light intensities at a bias of 2 V. The stable photocurrent without evident fluctuation can increase drastically from 0.010 to 33.83 nA. Notably, the CsPbI3 nanorod PD exhibits a good repeatability and fast response, even after being subjected to the largest photocurrent of 33.83 nA for a long time. The dependence of the photocurrent on the light intensity can be expressed by a power law: I = αPθ,36, 37 where α is a constant for a given wavelength, P is the light intensity, and θ is the exponent (0
