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High-performance photodetectors based on single all-inorganic CsPbBr3 perovskite microwire Pengbin Gui, Zhao Chen, Borui Li, Fang Yao, Xiaolu Zheng, Qianqian Lin, and Guojia Fang ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01567 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 1, 2018
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ACS Photonics
High-performance
photodetectors
based
on
single
all-inorganic CsPbBr3 perovskite microwire Pengbin Gui, Zhao Chen, Borui Li, Fang Yao, Xiaolu Zheng, Qianqian Lin*, Guojia Fang*
Key Lab of Artificial Micro- and Nano-Structures of Ministry of Education of China, School of Physics and Technology, Wuhan University, Wuhan 430072, P. R. China
Corresponding Authors： ： *Email:
[email protected];
[email protected] Keywords: all-inorganic perovskites, weak light detectors, polycrystalline single microwire, Schottky barrier, dark current
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Abstract In recent years, hybrid organic-inorganic perovskites have emerged as promising photo-sensing materials for next generation solution-processed photodetectors, achieving high responsivity, fast speed and large linear dynamic range. In particular, perovskite photoresistors possess low-cost fabrication, easy integration with low dimensional structures. However, relatively large dark current is still limiting the further development of perovskite photoresistors. Herein, we introduce full-inorganic perovskite polycrystalline microwires for high-performance photodetection, in order to enhance the device stability. Furthermore, dark current and noise can be effectively suppressed by tuning the contacts. All-inorganic CsPbBr3 microwires with a number of nano-crystals on wire surface are prepared by a simple, low cost two-step solution-processed method at room temperature. Photodetectors based on this CsPbBr3 polycrystalline single microwire are assembled on indium tin oxide (ITO) electrodes and demonstrate a decent responsivity up to 118 A/W and a fast response within 40 ms. In addition, such optimized photoresistors possess a fairly tiny dark current and noise, which result an improved detectivity of >1012 Jones, and demonstrate excellent characteristics to detect weak light.
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Introduction Hybrid organic-inorganic perovskites (HOIPs) with a typical formula of ABX3 (A=CH3NH3, CH2(NH2)2 or Cs; B=Pb or Sn, X=Cl, Br or I) have attracted great attention in the field of opto-electronics thanks to their superior opto-electronic properties, including high charge carrier mobility, long diffusion length, tunable direct band gap, ease of processing, and facile integration with both organic and inorganic semiconductor materials.1-6 Moreover, low-dimensional perovskite materials, such as zero-dimensional (0D) quantum dots,7,
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two-dimensional (2D) nanosheets,11,
provide another avenue to modulate the
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one-dimensional (1D) nanorods9,
10
and
materials properties and have been investigated extensively in the last few years, achieving enhanced stability, fewer defects and higher photoluminescence quantum yield. In particular, recent advances in 1D perovskite nanorods demonstrate an improved crystal quality and enhanced light absorption ability-effectively controlled by the morphology of nanorods.13-15 For instance, the perovskite crystal phase of all-inorganic CsPbI3 is instable at room temperature, which can be mitigated in nanoscale.16 Compared with their bulk counterparts, low dimensional perovskites also have emerged as promising candidates for miniaturized and integrated devices. Benefiting from the unprecedented progress of perovskite solar cells along with the fundamental understanding, HOIPs have been successfully implemented in various opto-electronic platforms, such as photovoltaics,17, 18 photodetection19-21 and light-emitting devices.22 Particularly, photodetectors as one of the most important opto-electronic devices can directly transform light radiation into electrical signal, which underpin modern science and technology, such as imaging, surveillance and 3
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machine vision.23-25 As excellent photoactive materials, HOIPs were first introduced to detect light in 2014 with a photoresistor architecture.26 Later on, perovskite photodiodes were achieved based on a p-i-n linear junction structure and perovskite thin films, with enhanced frequency response and reduced dark current and noise.19, 27 More recently, high-speed perovskite photodetectors with nanosecond photoresponse have been demonstrated with minimized device area and resistor-capacitance (RC) constant.28 These achieved metric of performance is on a par with the commercial silicon technologies. Moreover, perovskite photodetectors also demonstrated decent sensitivity to high-energy radiation29 and infrared rays,30-32 which have the potential to be used in biological sensing and communications, respectively. Despite such rapid progress, most of the perovskite photodetectors are built on photodiode structure with perovskite thin-films as light harvesting layer,20, 33, 34 which require electron transport layers (ETLs) and/or hole transport layers (HTLs). The multi-layer thin-film junctions also have stringent requirements on film morphology and surface roughness, which dominate the current leakage, noise and eventually the detectivity. Besides, as we all know HOIPs are instable and prone to degrade once they are exposed to moisture or heat.35-39 All-inorganic low-dimensional CsPbX3 perovskite materials possess superior stability with the absence of organic cations.40-42 However，it remains challenges facing perovskite photodetectors in terms of the size and the complexity of device fabrication. To precisely control and construct junctions based on individual low dimensional perovskite particles or nanorods is even more challenging. In order to overcome these limitations, Zeng et al.43 attained a stable 4
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perovskite photodetector based on 2D CsPbBr3 nanosheets with a planar photoresistor architecture, which exhibited a high on/off ratio of >104. In a similar manner, photodetectors based on CsPbBr3 nanowire arrays with a polarization-sensitive character are also reported by Wu and co-workers.44 However, the dark current of photoresistors cannot compete with heterojunction photodiodes, which preclude photoresistors to achieve high detectivity and viable devices. More recently, Schottky contacts between perovskite and ITO or metal electrodes were reported as an effective approach to enhance the device performance of perovskite photodetectors.45-48 However, the modulation of Schottky contacts between perovskite and various modified electrodes has not been systematically investigated. Motivated by these factors, we deposit CsPbBr3 perovskite films on various patterned electrodes and investigate the impact of contacts on the device performance. Then, we assemble high-performance photodetectors with the optimized contacts based on 1D CsPbBr3 single microwire, which are synthesized via a simple, low cost two-step solution process method. Here, the obtained microwires possess a porous structure with many tiny nanocrystals distributed on microwire surface. This typical structure possesses larger specific surface for light absorption and scattering. The dark current of the photoresistors is expected to be simply modulated with the metal-semiconductor contacts, which do not introduce complicate fabrication process. This new strategy can also effectively break the dark current-photoconductive gain compromise, leading to sufficient detectivity for state-of-the-art light detection.
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Results and discussion It is well recognized that introducing barriers, either low-conductive blocking layers or energetic barriers, is a straightforward way to reduce the dark current of photodetectors. As we can see from Figure 1a, both of the current in dark (Id) or under light (Iph) are proportional to the bias voltage according to Ohm’s law. Furthermore, responsivity can be derived from irradiance and the difference of Iph and Id, which can be enhanced by increasing the electric field, i.e., improved photoconductive gain. However, dark current can be simultaneously amplified to a great extend under high bias voltage, which deteriorate detectivity a lot. In this regard, introducing energetic barriers, e.g., Schottky contacts, provides a subtle and effective approach to overcome the trade-off between responsivity and dark current as shown in Figure 1b. Although the injected dark current tends to increase exponentially and behaves rectification characteristics when the bias voltage overtakes the Schottky barrier, small dark current can be ensured within the operational region at low bias voltage. The reduced dark current is a key feature for the high-performance photodetector.
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Figure 1. Schematic illustration and comparison of I-V curves obtained from photoresistors contacts (a) without and (b) with energetic barriers. To prove the concept, we deposited CsPbBr3 thin films on various electrodes, including metal, metal/oxide and conductive ITO with the same device architecture, i.e., identical channel width of 10 µm and the width and length of the electrode are 50
µm. Figure S1 displays the surface and cross-sectional field emission scanning electron microscope (FE-SEM) images of prepared CsPbBr3 films, indicating a decent coverage of CsPbBr3 films with a thickness of ~300 nm on various electrodes. Intriguingly, none of the prepared devices presented ohmic contacts between above CsPbBr3 films and the electrodes, and the dark current varies over a few magnitudes. We depict the obtained dark current in the left panel of Figure 2 and sketched the energy band level diagrams of the corresponding contacts in the right. When the CsPbBr3 films attach to the electrodes, electrons will be drifted until the Fermi level balanced, which causes the band bending and form an energetic barrier. As shown in Figure 2, the CsPbBr3/Al junction presented the largest dark current and less rectified current-voltage (I-V) curve - more like an ohmic contact. Incongruously, the dark current measured at 0 V is not 0 nA. We also find this phenomenon is common and exists in other perovskite photodetectors, and sometimes the “none-zero” current can reach up to 10 nA.12, 45, 48, 49 Hence, the dark current at 0 V based on perovskite/Al contact is pretty low and can be negligible. Similar to hysteresis, it can be attributed to ion migration, interfacial polarization, light soaking, test history and degradation etc. When the Al electrodes were decorated with a thin layer of SnO2, the I-V curve 7
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appeared as a rectifying feature due to the distinct mismatch of energy level alignment. Not surprisingly, the dark current of this structure dropped evidently at low bias region, indicating the formation of depletion region between the SnO2 modified Al electrodes and CsPbBr3 perovskite. Optimum dark current of ~10 pA arrived from the ITO electrodes with the highest work function, which provided the largest energy barrier and thicker depletion region for the injection of carriers and offered a wider operational bias region. We observed that the dark current of this type of devices is 1-3 orders of magnitude lower than the CsPbBr3/Al device, with a pronounced dark current suppression at higher bias. This property allows the photoresistors to operate under a relatively high bias and obtain a compelling responsivity. Figure S2 also presents a collection of dark current of such perovskite photoresistors based on various contacts, indicating a good reproducibility and precise control over the dark current.
Figure 2. Measured dark current of perovskite thin-film photoresistors with various contacts and the corresponding energy band level diagrams. These findings based on thin-film devices verified the concept of reducing dark 8
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current by manipulating the contacts and energetic barriers. We now turn to further validate the idea with the photodetectors based on CsPbBr3 microwires, which attract more attention due to its outstanding stability and the potential for image sensor arrays. CsPbBr3 microwires were prepared via a two-step method (Figure S3) and the details are described in supporting information. Morphology as one of the most important aspects we concerned can strongly affect the light absorption and carrier transport. Figure S4 displays typical optical photographs of PbI2 microwires and CsPbBr3 microwires. It is clear that the PbI2 microwires had a length of around 50 µm, and the CsPbBr3 microwires maintained the original linear shape but the surface roughness increased after the ionic exchange reaction, reflecting the successfully conversion from PbI2 microwires to CsPbBr3 microwires. To further probe the surface morphology, FE-SEM images provided greater resolution. As we can see from Figure 3a, the surface of PbI2 microwires with a diameter of ~2 µm is relatively smooth and clean. Figure 3b-3d monitored the surface morphology evolution of CsPbBr3 perovskite microwires with various reaction times. I- ions were exchanged by Br- ions and released into solution. Then, numerous pores formed, which were beneficial to the further conversion. A prolonged reaction time eventually led to the generation of porous structure perovskite microwires with a large number of nanocrystals piled up on the surface. This structure possesses greater specific surface area and could be beneficial for attenuating the reflection due to the scattering affect. Figure 3e presents the X-ray diffraction (XRD) pattern of the CsPbBr3 microwires, with sharp diffraction peaks emerged at 15.3°, 21.5°, 30.4° and 30.7°, which are indexed to the crystal plane 9
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of (110), (020), (004), (220) of orthorhombic CsPbBr3 crystals at room temperature, respectively.50 The absence of diffraction peak of PbI2 crystals at 12.7° confirms that the PbI2 microwires have transformed into CsPbBr3 microwires completely. Figure 3f displays the steady-state photoluminescence (PL) spectrum of this as-grown CsPbBr3 microwires peaked at 520 nm with a full width at half maximum (FWHM) of 28 nm, which is consistent with its band gap of 2.3 eV, and this is also in line with the references.51, 52 These results indicate that the CsPbBr3 microwires were successfully synthetized by a low-cost solution processed method at room temperature.
Figure 3. Morphology evolution of perovskite microwires monitored by SEM images 10
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with a reaction time of (a) 0 min, (b) 10 min, (c) 30 min and (d) 60 min, (e) X-ray diffraction pattern and (f) photoluminescence spectrum of as-grown CsPbBr3 microwires. Having established a facile preparation method, we then fabricated photodetectors based on this 1D CsPbBr3 microwires. The photograph of the single microwire perovskite photoresistors (MWPPDs) is shown in Figure 4a. The substrates we used here are Si/SiO2 and the gap of pre-patterned ITO electrodes was designed to be 10 µm. Figure S5 compared the dark I-V characteristics obtained with a scanning rate of 0.2 V/s of photodetectors based on CsPbBr3 thin-film and single microwire. The nonlinear curves again confirmed that our devices do not exhibit ohmic contacts, which is consistent with aforementioned discussion. Compared with photodetectors based on perovskite films, CsPbBr3 single microwire based devices present slightly smaller dark current due to the reduced active area of 3×10-7 cm2 (defined as shown in Figure S6). From these curves, we can also see that our device has a spectacularly low dark current at a relatively larger bias, which further indicated that this device can be allowed to operate under a relatively high bias and obtain a compelling responsivity. The temporal photo-response under periodic illumination is displayed in Figure S7, implying that the photocurrent and dark current remain at the initial level under the repetition of light on and off. Inspiringly, the on/off ratio at 520 nm with a luminous power of 26.5 µW is more than two orders of magnitude offering great potential for detecting visible lights. Additionally, the photocurrent clearly ascended as the bias voltage increased from 1 V to 5 V (Figure S8 in SI). 11
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These results suggest the robustness and repeatability of the device performance. Spectral response is another critical evaluation factor of photodetectors, which can be calculated as
=
(1)
where R is the responsivity and P denotes illumination power. Figure 4b presents the responsivity as a function of wavelength. The devices exhibited a similar trend with a photoresponse window of 350 nm~520 nm with different bias voltage, i.e., 1 V, 3 V, 5 V. The sharp decrease of responsivity at around 520 nm is in line with the band gap of CsPbBr3 microwires. The optimized MWPPDs demonstrated a maximum responsivity of 118 A/W at 520 nm with a bias of 5 V. Even at a low bias of 1 V, the responsivity achieved 3.96 A/W at 520 nm. To gain an accurate view of the device noise, Figure 4c displays the noise spectral power as a function of bias voltage obtained from Fast Fourier Transform of the dark current as a function of time for the MWPPDs. The noise current (In) in these MWPPDs was found to be low and shows little bias dependent around 1~10 fA/√Hz, mainly dominated by the shot noise calculated from dark current. We then examined the noise equivalent power (NEP) as follows,
NEP=
(2)
Based on the measured noise current and responsivity, the lowest NEP was estimated to be 1012 Jones with a bias voltage of 5 V, without the compromise of low dark current, which is non-trivial from a photoresistor structure. Moreover, these detectors also possess extremely low noise current of 1~10 fA/√Hz, excellent weak-light detection ability, and fast response speed 1012 Jones, and demonstrate excellent characteristics to detect weak light.
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