Hybrid Field-Effect Transistors and Photodetectors Based on Organic Semiconductor and CsPbI3 Perovskite Nanorods Bilayer Structure
KeywordsPerovskite Phototransistor Nanorod Organic semiconductor Photogating effect
A high-performance phototransistor with an organic semiconductor and CsPbI3 perovskite nanorod hybrid structure was fabricated and characterized.
The perovskite layer efficiently absorbs the input illumination, while the organic semiconductor layer acts as a transport channel for injected photogenerated carriers and provides gate tuning.
The hybrid phototransistor exhibits high performance with a photoresponsivity as high as 4300 A W−1, ultra-high photosensitivity of 2.2 × 106, and long-term stability of 1 month.
Recently, organometal halide perovskites have attracted a significant interest in the field of optoelectronics owing to their desirable electronic and optoelectronic properties, such as strong and broad light absorption, weakly bound excitons, long-range-balanced electron/hole transport lengths, and low-cost fabrication [1, 2]. These characteristics make them widely use in solar cells [3, 4], light-emitting diodes (LEDs) [5, 6], lasing , and photodetectors [8, 9, 10]. However, these perovskite materials usually have a low long-term stability owing to the environmental degradation of organic cations within the materials from moisture and heat [11, 12]. In contrast, all-inorganic CsPbX3 (X = I, Br, Cl) perovskites exhibit decent stabilities; among them, the CsPbI3 perovskite exhibited unique properties with a suitable bandgap, high quantum efficiency, and long radiative lifetime [13, 14]. However, this material has two phases: cubic phase and orthorhombic phase. The black cubic phase is stable above 300 °C; it quickly undergoes phase transformation into the wider-bandgap (3.01 eV) orthorhombic phase upon cooling to room temperature. This phase transformation largely limited its applications. In order to overcome this problem, low-dimensional CsPbI3 perovskites, including quantum dots, nanowires, nanorods, and nanoplates, have been prepared. Owing to both surface and nanoeffects, as the size of the perovskite material is reduced to the nanoscale, the phase stability can significantly differ from that in the bulk counterpart [15, 16, 17, 18].
Owing to the outstanding characteristics including few grain boundaries, large specific surface area, low charge carrier rate, long charge carrier lifetime, and high surface-to-volume ratio, one-dimensional (1D) CsPbI3 nanorods are promising materials for photodetector devices [19, 20, 21, 22, 23]. For example, Yang et al.  reported superior photodetectors based on a single CsPbI3 nanorod with an ultra-fast response and high stability. Tang et al.  demonstrated a simple method for the synthesis of CsPb(Br/I)3 nanorods; the photosensitivity of the nanorod-based photodetectors could reach up to 103. Owing to the charge-transport limitation in the perovskite layers, the CsPbI3 nanorod-based photodetectors exhibited a relatively low photoresponsivity (R) .
Organic semiconductors (OSCs) are promising materials with large potentials in next-generation flexible electronic and photoelectronic devices owing to their attractive advantages of easily tunable bandgap, simple synthesis, low cost, and resolvability [25, 26, 27, 28, 29, 30, 31]. 2,7-Dioctyl  benzothieno[3,2-b]  benzothiophene (C8BTBT) is an excellent OSC material, which does not respond to visible light, and has a good air stability and high hole-carrier mobility [32, 33, 34]. In this study, C8BTBT/CsPbI3 nanorod bilayer films were produced and used to fabricate high-performance phototransistors. The gate-tunable hybrid transistor devices illuminated by white light exhibited an outstanding photosensing performance with an R as high as 4.3 × 103 A W−1 and ultra-high Iphoto/Idark ratio of 2.2 × 106. Furthermore, the hybrid phototransistors possessed an excellent long-term stability when stored under ambient conditions for more than 1 month. Therefore, this study reveals a simple approach to fabricate hybrid phototransistors with comprehensive advantages including a simple fabrication, enhanced photosensitivity, and high stability.
3 Experimental Methods
C8BTBT (98%, Suna Tech Inc), Cs2CO3 (99.9%, Adamas), lead iodide (PbI2, 99.99%, Sigma-Aldrich), 1-octadecene (ODE, 90%, Energy Chemical), oleylamine (OA, 90%, Energy Chemical), oleylamine (OAm, 70%, Energy Chemical), and octane (99.9%, Sigma-Aldrich) were used without further purification.
3.2 Preparation of CsPbI3 Nanorod
CsPbI3 nanorods were synthesized using the ligand-assisted method [23, 35]. Cs2CO3 (0.40 g, 1.23 mmol) was added to a 50-mL round-bottom flask along with OA (1.5 mL) and ODE (25 mL) and stirred under vacuum for 1 h at 120 °C. The flask was filled with Ar. Subsequently, all Cs2CO3 reacted with the OA when the solution was clear. PbI2 (1.00 g, 2.17 mmol) and ODE (50 mL) were loaded into a 150-mL round-bottom flask and dried under vacuum at 120 °C for 1 h. Ar was then flowed into the flask. OA and OAm (5 mL each) were injected at 120 °C. The PbI2 completely dissolved, leading to the formation of a clear solution. The Cs-oleate solution (~ 0.046 M, 5.9 mL), prepared as described above, was injected and the reaction mixture was quenched by immersion of the flask into an ice bath after a 5-min reaction. The crude nanorod solution was added into 200 mL methyl acetate and centrifuged at 8000 rpm for 5 min. The precipitates were dispersed in 6 mL hexane and 20 mL methyl acetate and then centrifuged at 8000 rpm for 2 min. The nanorods were dispersed in 10 mL of hexane and centrifuged again at 4000 rpm for 5 min. Finally, the as-prepared nanorods were dispersed in octane for further use.
3.3 Device Fabrication
Heavily n-doped Si wafers with a thermally grown 300-nm-thick SiO2 layer were used as substrates. Before device fabrication, the substrates were cleaned using ultra-sonication consecutively with acetone, isopropanol, and deionized water for 30 min and finally dried by N2. The pre-cleaned substrates were immersed into a CsPbI3 nanorod solution (~ 20 mg mL−1 in octane) and pulled out at an optimized speed of 5 μm s−1 (SYDC-100H DIP COATER). The samples were then placed under vacuum for 2 h. A 30-nm-thick C8BTBT film was then deposited by vacuum evaporation, and 50-nm-thick Au source/drain electrodes on top of the hybrid films were thermally evaporated by shadow masks with a channel length of 5 μm and channel width of 1 mm. For the pure-CsPbI3-nanorod-based photodetectors, the nanorod solution was drop-cast directly on Si/SiO2 substrates with pre-patterned gold electrodes, followed by drying under vacuum for 1 h.
3.4 Characterizations and Measurements
The surface morphologies of the CsPbI3 nanorod films were investigated by an atomic force microscope (AFM, SEIKO SPA-300HV) operated in the tapping mode. Ultraviolet–visible (UV–Vis) absorption spectra of thin films on quartz substrates were acquired using a Cary 60 UV–Vis spectrometer (Agilent Technologies). Powder X-ray diffraction (XRD) was performed using a DX-2700 X-ray diffractometer (Dandong Haoyunan Instrument Co. Ltd) with Cu Kα radiation (1.54 Å) at 40 kV and 50 mA. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were recorded with a JEOL JEM-2100F TEM operated at 200 kV. The performances of the photodetectors were studied using a Keithley 4200 SCS. Monochromatic lights with different wavelengths were provided by a 300-W xenon lamp filtered with a double-grating monochromator (Omno 330150, Beijing NBeT, China). All of the measurements were taken in air at room temperature.
4 Results and Discussion
An additional benefit of the use of C8BTBT as the OSC is that C8BTBT does not strongly adsorb visible light and hence has no responsivity to visible light, while the CsPbI3 nanorods have a broad absorption in both UV and visible regions. When the device was illuminated by visible light, only the CsPbI3 nanorods could act as the light adsorption materials, while C8BTBT acted only as a charge-transport material. It is beneficial to understand the underlying sensing mechanism of our devices. Figure 1e shows the UV–Vis absorbance spectra of a CsPbI3 nanorod film, C8BTBT film, and C8BTBT/CsPbI3 nanorod hybrid film. Although there was an intense absorption peak observed around 365 nm, the pure C8BTBT film did not exhibit an obvious absorption in the range of 400–800 nm. In contrast, the CsPbI3 nanorod film exhibited a strong and broad absorption in both UV and visible regions. The spectrum of the hybrid film included the absorptions of the C8BTBT and CsPbI3 nanorod films; the superimposed effect of the hybrid film obviously enhanced its light-absorption range.
A schematic band diagram of the CsPbI3 nanorods and C8BTBT is shown in Fig. 1f. The values of the conduction band (CB) and valence band (VB) of perovskite are at − 3.45 and − 5.44 eV, respectively . The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of C8BTBT are − 5.39 and − 1.55 eV, respectively . Therefore, the CsPbI3 perovskite and C8BTBT form a type-II heterojunction . The VB of CsPbI3 aligns well with the HOMO of C8BTBT, facilitating the injection of photogenerated holes from the perovskite into the OSC. Furthermore, the high LUMO energy level of C8BTBT effectively blocks electrons and prevents them diffusing into the OSC layer, thus efficiently reducing the photogenerated charge recombination.
Figure 2d shows the transfer curves (ID–VG) of the hybrid phototransistors with a gate voltage sweeping from − 60 to 20 V and fixed drain voltage of − 30 V under illuminations of 0.05, 0.5, 1.5, 5, and 10 mW cm−2 and in the dark state, suggesting a typical p-type semiconductor behavior. The photocurrent (Iphoto= Ilight − Idark) significantly enhanced with the increase in the illumination power density (Fig. 2e). The results show that a larger applied gate voltage leads to a larger increase in the photocurrent for the p-type phototransistors. Therefore, the photosensitive performances of the devices can be modulated by applying different gate voltages. Devices based on pure CsPbI3 nanorods were also fabricated by drop casting directly on Si/SiO2 substrates with pre-patterned gold electrodes; the I–V curves of the devices in the dark state and under an illumination of 5 mW cm−2 are shown in Fig. S3. The hybrid phototransistors exhibited not only a higher photocurrent, but also a significantly larger photocurrent to dark current ratio than those of the pure CsPbI3 nanorod devices, although the pure CsPbI3 nanorod devices exhibited a relatively high response speed .
The long-term stabilities of the hybrid phototransistors were also studied. As shown in Fig. 4f, the transfer curves of the hybrid phototransistors after they were stored in an atmospheric environment (average temperature of 20 °C and average relative humidity of 50%) for 1 month were almost perfectly overlapped with those of the fresh samples, indicating their excellent stabilities with respect to oxygen and moisture. Furthermore, Table S1 presents key parameters of the hybrid devices, compared with those of recently reported low-dimensional all-inorganic perovskite-based photodetectors; the photosensitivity, responsivity, and stability values of our devices are among the highest reported values. The excellent stabilities of the devices can be mainly attributed to the following factors. First, C8BTBT is an excellent OSC material possessing a good air stability. Second, 1D all-inorganic perovskites with a better crystallization tend to have higher stabilities than those of the organic–inorganic hybrid counterparts. Third, the OSC layer fabricated in the upper part of the hybrid layers protects the perovskite layer from atmospheric moisture.
We demonstrated high-performance phototransistors based on the C8BTBT/CsPbI3 nanorod hybrid layers with high photosensitive performances and long-term stabilities in ambient conditions. The high-quality perovskite films were necessary to provide the desirable device performance. The perovskite layer could efficiently absorb the input illumination and generate a large amount of charge carriers, while a transport channel for the injection of photogenerated carriers and gate-tuning property was provided by the OSC layer. Owing to the photogating effect, the gate-tunable hybrid devices illuminated by white light exhibited outstanding photosensitivity performances with an R as high as 4300 A W−1 and ultra-high Iphoto/Idark ratio of 2.2 × 106, significantly higher than those of most of the previously reported perovskite-based photodetectors. Furthermore, the hybrid phototransistors possessed long-term stabilities owing to the high stabilities of the two materials and device structure under ambient conditions. Therefore, this study provides a strategy to combine the advantages of perovskite semiconductors and OSCs to obtain high-performance photodetectors.
This research was supported by the National Key Research and Development Program of China (2017YFA0103904), the National Nature Science Foundation of China (51741302 and 51603151), Science & Technology Foundation of Shanghai (17JC1404600), the Fundamental Research Funds for the Central Universities, and the support of College of Transportation Engineering, Tongji University’s Shanghai “Gaofeng” subject.
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