Methylammonium lead iodide perovskite-graphene hybrid channels in flexible broadband phototransistors
Introduction
Recently, photodetectors capable of sensing light from the ultraviolet (UV) and visible to the infrared (IR) spectrum have received considerable attention due to their broad applications in the fields of medicine, displays, military, imaging, environmental monitoring, security checks, optical communication, scientific research, and industrial process control. Currently, photodetectors have been studied on both rigid (glass, SiO2/Si) and flexible substrates (polyimide, polyethylene, terephthalate, and PET). By comparing the photoresponse properties between rigid and flexible photodetectors, X. Wang et al. concluded that the photocurrent-UV irradiance of photodetectors on flexible PET substrates were as high as those built on rigid glass substrates [1]. However, almost all flexible optoelectronic devices must be fabricated at low temperatures. Flexible optoelectronic devices also exhibit particular advantages including: simple processing methods, low cost, lightweight, shock resistance, durability, portability, and flexibility [2], [3]. Therefore, flexible photodetectors can enable new types of applications. For instance, they can be wrapped on curvilinear surfaces or attached onto human skin for human–machine interfaces or incorporated into clothing to detect harmful light while being worn.
In classifications based on the detection band, there are two main types of photodetectors: special wavelength and broadband detectors [1], [4], [5], [6], [7], [8]. Special wavelength photodetectors detect at certain single wavelengths or narrow wavelength regions. For example, ZnO nanowire UV photodetectors exhibit high photoconductive gains under 390 nm wavelength light [7], and ZnO nanorods/graphene hybrid channel field-effect phototransistors exhibit excellent responsivities under 365 nm wavelength light [9], [10]. Ultrasensitive monolayer MoS2 phototransistors with improved device mobilities exhibit photoresponse within the visible light region with a maximum external photoresponsivity of 880 A W−1 at a wavelength of 561 nm [11]. On the contrary, photodetectors that can detect light from UV to visible [4], [12], visible to IR [5], [6], [13] or UV to IR [14] spectrum are referred to as broadband detectors. For instance, the PbS:P3HT:PCBM:ZnO photodetector exhibits high response under UV, visible, and near IR wavelengths, with responsivities of 4.58 A W−1, 5.6 A W−1, and 1.24 A W−1 at 350 nm, 500 nm, and 930 nm, respectively [14]. A UV/visible photodetector of Cu2O/ZnO hybrid nanofilms on single-walled carbon nanotube-based flexible conducting substrates exhibited high performance within the broadband photodetection range of 365–625 nm [12], that could be applied in our daily lives such as in the detection of sunlight or some other illuminated light [4]. Other photodetectors based on Gr-Bi2Te3 heterostructures not only exhibited high responsivities and excellent photoconductive gains (up to 83%), but also exhibited broadband photodetection capabilities between the visible to IR spectrum [5] which could be used in high-power infrastructure applications for future indoor/outdoor public ubiquitous data communication technologies [4]. Therefore, broadband photodetectors have currently become the subject of intense research due to their wide application potential. However, finding active materials that possess the ability to be sensitized by incident radiation over a broad wavelength range is still a big challenge with regard to broadband photodetectors due to the limited absorption of single phase materials. Therefore, a plausible approach has been to utilize hybrid materials for multi-spectral light absorption over a broad range.
Bearing this in mind, one material candidate with a broad absorption range is the organometaltrihalide perovskite [4], [15], [16], [17], an organic-inorganic hybrid perovskite. The crystalline structure is ABX3 [17], where A is an organic cation (CH3NH3+ or HNCHNH3+), B is a metal cation (Pb2+ or Sn2+), and X is a halide anion (Cl−, Br−, or I−). Among them, methylammonium lead iodide perovskite (CH3NH3PbI3, henceforth MAPbI3) has been extensively used in optoelectronic device applications including solar cells [18], [19], [20], [21], lasers [22], and photodetectors [4], [16], [23], [24] due to the possession of a direct band gap, long carrier lifetimes, extra-long carrier transport distances, broad absorption spectra, high carrier mobilities, ambipolar diffusion, and long carrier diffusion lengths [16], [17], [18], [25], [26]. For instance, MAPbI3 films deposited onto ITO coated substrates, yielded external quantum efficiency (EQE) values of 1.19 × 103% and 5.84% at excitation wavelengths of 365 nm (UV) and 780 nm (visible), respectively, using a voltage bias of 3 V [4]. High sensitivity and fast photoresponse speeds with 20 μs response times and 17 μs fall times were observed in a perovskite-optocoupler based on a white tandem organic light-emitting diode (OLED) light source and low-voltage organic-inorganic hybrid photodetector using MAPbI3 as a light harvester [16] due to the small travel distance required for the photogenerated charge carriers [27]. A large detectivity approaching 1014 Jones, a linear dynamic range over 100 dB, and a fast photoresponse with 3 dB bandwidth up to 3 MHz were also achieved with a photodetector based on an organic-inorganic hybrid perovskite material [27]. Interestingly, MAPbI3 could be fabricated using low-temperature solution processing that provides high compatibility with unconventional substrates such as flexible polymer [28]. Indeed, external bending stress was uninfluential to the photocurrent of MAPbI3 flexible photodetectors under both 365 nm and 780 nm light exposure. The conductance of the MAPbI3 film remained nearly unchanged even after 120 bending cycles [4]. Although the perovskite photodetector displayed high EQEs, fast response, and high detectivity within the UV to visible region, responsivity was still low (a few AW−1) [4], [16], [24].
In order to enhance responsivity, the hybrid perovskite was combined with 2-dimensional (2-D) materials within the photodetector active channel. Indeed, for single perovskite materials, electron–hole pairs generated via photo-induction would recombine within a few picoseconds, nevertheless, the charge carriers transferred from perovskite to 2-D materials within the hybrid channel filled the empty states in the perovskite valence band under light exposure [23], and hence, could reduce recombination of the photogenerated electron–hole pairs. With the highest mobilities (up to 106 cm2V−1s−1 at room temperature) [29], [30], [31], [32], [33], excellent electrical conductivity [34], high optical transparency (97.7% throughout the visible light regime) [33] and great mechanical flexibility [35], graphene (Gr) has been not only applied as the transparent electrodes in organic photovoltaic devices and light emitting diodes [36] but also become the most promising 2-D candidate material for the transport layer in nanohybrid electronic devices. Recently, Gr has been presented as a template for the formation of 0-D materials (nanoparticles, quantum dots) [37], [38], [39], 1-D materials (nanorods, nanowires) [9], [10], [40], [41], [42], and 2-D materials (thin films) [23], [43], [44], [45]. Gr hybrids with MAPbI3 perovskite could be used as a photodetector channel to not only overcome the drawbacks of Gr photodetectors (low responsivity due to the low photoabsorption of Gr) but also enhance the photocurrent in perovskite devices by reducing the charge recombination rate. In the Gr photodetectors, Gr often worked as both sensing and transport layers [46], [47], which made the recombination of electrons and holes inside the layer increase, resulting in low responsivity. In contrast, hybrid photodetectors consisted of Gr channel and ‘antenna’ materials such as perovskite and ZnO nanorods that played a role of photocurrent transport layer from source to drain electrode and a role of sensing layer to photo stimuli, respectively [9], [10], [23], [48]. For example, Cho and his co-workers demonstrated the perovskite-Gr hybrid photodetector on rigid substrates with high photoresponsivities of 180 A W−1 and high photodetectivity of 109 Jones at a fixed wavelength of 520 nm [23]. With a transistor structure, this study reported on the sensing mechanism related to charge transfer from perovskite to Gr, however, the far positive position of the Dirac point (VDirac) (∼80 V) and the unclear right-shifting of VDirac made it difficult to detect the photovoltage under light illumination. Furthermore, the perovskite-Gr photodetector was fabricated on a rigid substrate (Si substrate), limiting their usage in applications requiring flexibility, such as wearable electronics [23].
Herein, we report a high performance MAPbI3 perovskite-Gr hybrid phototransistor on a flexible substrate. Using polyethyleneimine (PEI) treatment of Gr, VDirac was shifted to a low voltage (<10 V). The clear positive shift of VDirac under light exposure allowed for a deep understanding of the sensing mechanism and detection of both the photocurrent and photovoltage. The sensing mechanism was attributed to the transfer of holes from perovskite to Gr after the photogeneration of electron–hole pairs under incident light radiation. The shift of VDirac calculated from the fit of the dependence of VDirac on the light intensity curve was 3200 V per mW. Our results not only demonstrated the high responsivity (up to 115 A W−1) at 515 nm, but also allowed for an investigation of the bandwidth by measuring the photoresponse at various frequencies. Another significant advancement of our hybrid device was that the flexible photodetector exhibited high stabilities after cyclic bending for 3000 cycles at a strain of 0.5% under a 12 mm bending radius with near-invariance of the photocurrent under flexure. More interestingly, besides the high responsivity within the visible region, our device also exhibited comparable performance within the near UV region, attesting to the capability for broadband detection.
Section snippets
Fabrication of the flexible Gr field-effect transistors (GFET)
Cross-sectional schematics of the photodetectors are described in Fig. 1a. The GFET was fabricated on flexible substrates following methods provided in our previous report [9]. First, the gate electrode (60 nm Ni) was deposited on a polyimide (PI) substrate via e-beam evaporation using a shadow mask. Next, the flexible gate dielectric of sandwiched Al2O3 (20 nm)/polyvinyl phenol (400 nm)/Al2O3 (20 nm) structure was deposited onto the gate electrode and then chemical-vapor deposited (CVD) Gr was
Device structure and the quality of perovskite on Gr layer
The device consisted of a MAPbI3 perovskite-Gr hybrid material that was a channel within the phototransistor. Gr played a role within the carrier transport layer, and MAPbI3 was used as the photon absorbing material. The hybrid channel was placed on top of the Al2O3/polyvinylphenol (PVP)/Al2O3 gate dielectric layer formed on the Ni gate electrode over the polyimide (PI) substrates shown in the cross-sectional schematic of the device (see Fig. 1a and the Experimental section for details related
Conclusion
In summary, a flexible phototransistor with a broad photoresponse range was fabricated by using a CH3NH3PbI3 perovskite-Gr hybrid channel field-effect transistor on a flexible substrate. The positive shift of VDirac in transfer characteristics under incident light radiation allowed for a clear explanation of the sensing mechanism that was attributed to the transfer of holes from perovskite to Gr after the photogeneration of electron–hole pairs. The responsivity of the device increased with a
Acknowledgments
This research was supported by the Basic Science Research Program (2013R1A2A1A01015232) through the National Research Foundation (NRF), funded by the Ministry of Science, ICT and Future Planning.
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