Graphene Multiheterojunction

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Perovskite/Poly(3-hexylthiophene)/Graphene Multiheterojunction Phototransistors with Ultrahigh Gain in Broadband Wavelength Region Chao Xie and Feng Yan* Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China S Supporting Information *

ABSTRACT: Organometal halide perovskite materials have attracted much attention recently for their excellent optoelectronic properties. Here, we report an ultrasensitive phototransist or based on t he mult ih eterojun ction o f CH3NH3PbI3−xClx perovskite/poly(3-hexylthiophene)/graphene for the first time. Since the photoexcited electrons and holes are effectively separated by the poly(3-hexylthiophene) layer, high-density electrons are trapped in the perovskite layer, leading to a strong photogating effect on the underlying graphene channel. The phototransistor demonstrates an unprecedented ultrahigh responsivity of ∼4.3 × 109 A/W and a gain approaching 1010 electrons per photon, respectively. More importantly, the device is sensitive in a broadband wavelength region from ultraviolet to near-infrared, which has not yet been achieved with other perovskite photodetectors. It is expected that the novel perovskite phototransistor will find promising applications as photodetection and imaging devices in the future. KEYWORDS: phototransistor, perovskite, graphene, responsivity, hole transport layer

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approaching 1014 Jones and fast photoresponse with 3-dB bandwidth up to 3 MHz.22 However, these perovskite photodiodes usually exhibit low responsivity and a gain less than 1.23 Perovskite photoconductors have the conductance that can be modulated significantly by light illumination.24 As a special type of photoconductor, phototransistors can provide high gain because of the inherent amplification function by the transistors.25,26 Recently, ambipolar phototransistors based on solution-processed pure CH3NH3PbI3 or CH3NH3PbI3−xClx perovskite films show the responsivity up to 320 A/W,27 which is lower than that of the photoconductors based on other semiconductors like PbS quantum dots (QDs).28 To further improve the responsivity, some groups reported hybrid perovskite phototransistors in combination with other highmobility materials, such as graphene, MoS2, and WS2, where perovskite only severs as light absorbing media while highmobility materials act as charge transport channels.29−33 These perovskite hybrid phototransistors show the responsivity up to ∼106 A/W,31,32 which is still much lower than that of their counterparts based on other materials (>108 A/W).34,35 On the other hand, perovskite devices have not been used as nearinfrared photodetectors until now because the absorption edge of the perovskite material CH3NH3PbI3 is around 800 nm.11 In this work, we demonstrate ultrasensitive perovskite phototransistors based on a CH3NH3PbI3−xClx/poly(3-hexylth-

INTRODUCTION Low-cost, high-performance photodetectors have broad potential applications, including remote sensing, imaging, optical communications, security, night-vision, process and environmental monitoring, or biomedical diagnostics, among others.1,2 So far, extensive attention has been devoted to exploit photodetectors with various types of semiconductor materials, such as silicon,3 GaN,4 ZnO,5 InGaAs,6 conjugated polymers,7 nanomaterials,8 graphene,9 and other two-dimensional materials,10 which demonstrated excellent optoelectronic performances. Recently, organometal halide perovskites especially CH3NH3PbX3 (X: I−, Br−, Cl−) have attracted much research interests for their broad applications in optoelectronic devices, including solar cells,11 photodetectors,12 light-emitting diodes,13 and lasers.14 The materials have shown appealing electronic and optoelectronic properties, such as appropriate direct bandgap, large light absorption coefficient, long-range balanced electron/hole transport lengths, and so on.15 Furthermore, these materials can be easily prepared by costeffective solution-based processes,16 making them promising candidates for low-cost and highly sensitive photodetectors. Currently, two types of perovskite photodetectors, namely, photodiodes and photoconductors, have been intensively investigated. The perovskite photodiodes usually have solarcell-like configurations,17,18 which exhibit low dark currents and fast response speeds, enabling them to operate at high frequency with low noise.19−21 Dou et al. first reported solution-processed photodiodes based on CH3NH3PbI3−xClx perovskite, which exhibited a high specific detectivity © XXXX American Chemical Society

Received: September 13, 2016 Accepted: December 19, 2016

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DOI: 10.1021/acsami.6b11631 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic diagram of the perovskite/graphene vertical heterojunction phototransistor with P3HT hole transport layer (left panel). Side view of the hybrid phototransistor (right panel). Incident photons generate electron−hole pairs in the perovskite film. Holes are then transferred to the graphene channel and drift toward the source, while electrons remain trapped in the perovskite film, due to selective charge transfer by the P3HT. (b) A typical plane view SEM image of the CH3NH3PbI3−xClx perovskite on the P3HT/graphene. (c) XRD spectrum of CH3NH3PbI3−xClx perovskite on the P3HT/graphene. (d) Absorption spectra of the CH3NH3PbI3−xClx perovskite film and the CH3NH3PbI3−xClx perovskite/P3HT/graphene films on glass. CH3NH3PbI3−xClx perovskite/P3HT/graphene phototransistor, the precursor solution was spin-coated on the P3HT film treated with O2 plasma for 90 s at 2500 rpm for 1 min and annealed at 100 °C for 45 min to form a CH3NH3PbI3−xClx perovskite film with thickness of ∼300 nm. The phototransistor with thinner perovskite layer was obtained by adjusting concentration of the precursor and speed of the spin-coating. The CH3NH3PbI3 perovskite/P3HT/graphene control phototransistor was fabricated by spin-coating a triiodide perovskite precursor solution (CH3NH3I and PbI2 dissolved in a mixed solvent of DMF and dimethyl sulfoxide (DMSO) (7:3 volume ratio) at molar ratio of 1:1) onto the P3HT film at 4000 rpm for 1 min and followed by annealing at 90 °C for 45 min. The CH3NH3PbI3−xClx perovskite/ graphene control phototransistor was fabricated by spin-coating the mixed halide perovskite precursor solution directly onto a cleaned SiO2/Si substrate with predefined Cr (10 nm)/Au (100 nm) electrodes covered with monolayer graphene film and annealed at 100 °C for 45 min. Material and Device Characterization. The surface and crosssectional morphologies of the perovskite/P3HT/graphene multiheterojunction were characterized by SEM (FEI Nova 450 Nano). XRD measurement was performed using a Rigaku SmartLab X-ray Diffractometer operating at room temperature. The UV−vis absorption spectra were recorded on a Shimadzu UV-2550 UV−vis spectrometer. Raman measurement was performed using a Jobin Yvon/Labram HR800 Raman spectroscopy with a 633 nm laser at a beam size of 2 μm2. Electrical and optoelectrical measurements were performed by using a semiconductor parameter analyzer (Agilent 4156 C) under light illumination with different intensities in a glovebox. The light sources are LEDs with wavelengths of 598, 895, 1050, 1200, and 1300 nm, respectively.

iophene)/graphene vertical multiheterojunction. In the devices, the CH3NH3PbI3−xClx layer can absorb light and generate electron and hole pairs, poly(3-hexylthiophene) (P3HT) acts as a hole transport layer to efficiently separate electrons and holes and prohibit charge recombination, while the graphene layer provides a fast charge transfer channel.36 Consequently, the phototransistors exhibit unprecedented responsivities as high as ∼4 × 109 A/W and ultrahigh gains close to 1010 at the operational voltage of only 0.1 V. Moreover, the perovskite phototransistors can detect infrared light with the wavelength beyond 1300 nm due to the trap states in the bandgap. Thanks to the convenient fabrication processes, the devices are compatible with current electronic technologies and promising for applications with mass production.

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EXPERIMENTAL METHODS

Device Fabrication. SiO2 (300 nm)/Si substrates were ultrasonically cleaned in alcohol, acetone, and deionized water in sequence, and they were dried under a stream of nitrogen gas. Cr (10 nm)/Au (100 nm) electrodes with a channel width and length (W/L) of 800 and 5 μm, respectively, were patterned by photolithography and magnetron sputtering. Single-layer graphene was prepared on copper foils by the chemical vapor deposition (CVD) method and transferred onto Si/ SiO2 substrates. Then a P3HT film was prepared on graphene by spincoating a solution of P3HT (2.5 mg/mL in chlorobenzene) at the spin-coating speed of 2000 rpm for 1 min and annealed at 120 °C for 1 h in a glovebox filled with nitrogen gas. The thickness of the P3HT film is ∼20 nm. Methylammonium iodide (CH3NH3I) and lead chloride (PbCl2) (99.5 wt %) were dissolved in anhydrous N,N-dimethylformamide (DMF) (99.8 wt %) at a 3:1 molar ratio of CH3NH3I to PbCl2, and stirred at 45 °C for 12 h inside a nitrogen-filled glovebox to produce a mixed halide perovskite precursor solution. In order to fabricate the B


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Figure 2. (a) Channel current of the CH3NH3PbI3−xClx perovskite/P3HT/graphene hybrid phototransistor as a function of back-gate voltage VG under different illumination levels. VDS = 0.1 V. For any intensity, the device was illuminated for 2 min before the measurement. Wavelength: 598 nm. (b) Shift of Dirac point VD as a function of the light illuminating on the phototransistor. Inset shows the energy band and charge transfer diagram of the phototransistor under illumination. (c) Photocurrent and (d) responsivity (R) of the device as a function of the back-gate voltage VG under different illumination levels.

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enhanced absorption for the multilayer film in the short wavelength region (350−550 nm) can be attributed to the P3HT and graphene layers. Photoresponse Characterization. To shed light on the influence of the modified layers on device performance, we characterized a graphene transistor before and after the coating of the CH3NH3PbI3−xClx/P3HT film. Figure S2 shows the channel current of the transistor as a function of gate voltage (VG) measured in dark. The Dirac point VD (i.e., the charge neutrality point) is found to shift from ∼5 V for the pristine graphene to ∼ −23 V for the hybrid, indicating that some electrons are transferred from the CH3NH3PbI3−xClx/P3HT layers to the graphene (n-type doping of graphene) upon the film coating due to the mismatch of their Fermi levels. Meanwhile, the field-effect mobilities for electrons and holes decrease slightly from ∼2290 cm2 V−1 s−1 and ∼1945 cm2 V−1 s−1, to ∼2120 cm2 V−1 s−1 and ∼1840 cm2 V−1 s−1, respectively, which are comparable to those of CVD-grown graphene reported before38 and can meet the demand for fast charge transfer in highly sensitive phototransistors. Then the photoresponse of the device was characterized under light illumination. Figure 2a shows the channel currents of the perovskite phototransistor as a function of VG under light illumination (wavelength: 598 nm) with different intensities. We can find that the transfer curve shifts dramatically toward positive VG with increasing illumination intensity and the Dirac point VD shifts for over 40 V, which can be regarded as a photogating effect due to the transfer of photoexcited holes from the perovskite to graphene. The shift of the Dirac point VD (ΔVD) of the device as a function of light irradiance (Ee) is shown in Figure 2b, which can be fitted well with a widely

RESULTS AND DISCUSSION Device Fabrication. Figure 1a shows the schematic diagrams of our perovskite phototransistor, which has a graphene channel modified with a thin layer of P3HT and a CH3NH3PbI3−xClx perovskite layer. The thickness of the perovskite layer is ∼300 nm unless otherwise specified. The graphene film was prepared by a chemical vapor deposition (CVD) method on a copper foil and transferred onto a SiO2/Si substrate with the conventional method (see Experimental Methods).37 The Raman spectrum of the graphene film demonstrates excellent monolayer characteristics (Figure S1a). Then P3HT in chlorobenzene solution was coated on the graphene layer and formed a uniform P3HT film on the surface, followed by the coating of CH3NH3PbI3−xClx perovskite film. The surface morphology of the CH3NH3PbI3−xClx perovskite fabricated on the P3HT/graphene was characterized by scanning electron microscopy (SEM), as shown in Figure 1b, which indicates a relatively full-coverage of the perovskite film on the P3HT layer. The corresponding cross-sectional SEM image is depicted in Figure S1b, from which a layerstacked vertical multiheterojunction can be clearly observed. The X-ray diffraction (XRD) pattern shown in Figure 1c displays strong peaks at 14.09°, 28.43°, 43.23°, and 58.90°, which can be assigned to (110), (220), (330), and (440) diffraction peaks of CH3NH3PbI3−xClx perovskite, respectively, indicating the good crystallinity of the perovskite film on P3HT. Figure 1d shows the absorption spectra of a CH3NH3PbI3−xClx/P3HT/graphene multilayer film and a CH3NH3PbI3−xClx film on glass for comparison. Strong and broadband absorption in UV−visible range with sharp absorption edges located at ∼800 nm can be observed. The C


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Figure 3. (a) Photocurrent and (b) responsivity (R) of the phototransistor as a function of drain-source voltage VDS under different illumination levels. (c) Time-dependent photoresponse of the phototransistors with perovskite layer thickness of ∼300 nm (top panel) and ∼40 nm (bottom panel) to periodical on/off illumination (intensity: 14.15 nW/cm2) at VG = −25 V, respectively. VDS = 0.1 V. (d) Comparison of the gain (G) as a function of illumination time at VG = −25 V for the CH3NH3PbI3−xClx perovskite/P3HT/graphene, CH3NH3PbI3 perovskite/P3HT/graphene, and the CH3NH3PbI3−xClx perovskite/graphene phototransistors. Wavelength: 598 nm.

applicable power law: ΔVD = αEβe , where α and β are constant and β ≈ 0.35.23,25 The band structure and charge transfer diagram are shown in the inset of Figure 2b. Photons absorbed by the perovskite layer create excitons that quickly dissociate into electrons and holes in the perovskite due to the low exciton binding energy (35−75 meV).39 Holes are transferred to graphene through the thin P3HT film due to the decreased energy, while electrons accumulate in the perovskite layer because the conduction band level of the perovskite material is much lower than the lowest unoccupied molecular orbital (LUMO) level of P3HT. So the P3HT layer plays an important role in separating electrons and holes in the device, leading to high-density electrons accumulated in the perovskite. The accumulated electrons can lead to an effective negative gate voltage (VG) and induce positive carriers in the graphene channel through capacitive coupling, which can be regarded as the photogating effect and explains the observed shift of Dirac point toward positive VG under illumination. The net photocurrent (Iph) can be extracted by deducting the dark current from the channel current under illumination (Ilight − Idark), as shown in Figure 2c. It is notable that the photocurrent increases with increasing light radiance when VG < VD (i.e., p-channel) while decreases to be more negative with increasing light radiance when VG > VD (i.e., nchannel). To elucidate the effect of P3HT hole transport layer, we prepared another phototransistor with only CH3NH3PbI3−xClx layer modified on the graphene channel. We found that the photocurrent of the device under 598 nm light is much lower than that of the device with P3HT layer under the same measurement condition (Figure S3). For the device without the

P3HT interlayer, both photoexcited electrons and holes generated in the perovskite can be injected into the graphene, and thus, electrons will accumulate in the perovskite with lower density, which leads to a reduced photogating effect. Meanwhile, direct deposition of the CH3NH3PbI3−xClx layer decreases the hole mobility of the graphene channel dramatically to ∼250 cm2 V−1 s−1, presumably due to the high-density trap states at the interface and the perovskite layer.26,40 So the P3HT interlayer can separate the graphene layer from the CH3NH3PbI3−xClx layer and keep high carrier mobility of the transistor. To better understand the device physics, a control phototransistor with CH3NH3PbI3 perovskite as a light harvesting material was prepared and examined under the same condition. As shown in Figure S4a, the control device displays a photoresponse behavior similar to that of the device employing CH3NH3PbI3−xClx perovskite. The transfer curve shifts toward positive VG with a Dirac point shift of ∼25 V at illuminating intensity of 76 μW/cm2. The photocurrent as a function of VG under different illumination levels is also depicted in Figure S4b. Apparently, both the shift of the Dirac point and the photocurrent are relatively lower than those observed in the CH3NH3PbI3−xClx perovskite phototransistor, indicating a weaker photogating effect in the former case. The weaker photogating effect can be attributed to less photogenerated holes transferred to the graphene due to much shorter carrier diffusion lengths in the CH3NH3PbI3 perovskite.41 Therefore, the carrier diffusion length in the perovskite layer is critical to the device performance. Responsivity (R) is an important figure-of-merit for photodetectors and is given by23 D


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Table 1. Comparison in Device Performance of the Perovskite Phototransistors in This Work with Previous Reports device structure

operation voltage (V)

responsivity (A/W)

gain

reference

CH3NH3PbI3 film CH3NH3PbI3−xClx film CH3NH3PbI3 film/graphene CH3NH3PbBr2I island/graphene CH3NH3PbI3 nanowire/graphene CH3NH3PbI3 film/APTES-doped MoS2 CH3NH3PbI3 film/WS2 CH3NH3PbI3−xClx film/P3HT/graphene CH3NH3PbI3−xClx film/graphene CH3NH3PbI3 film/P3HT/graphene

30 30 0.1 3 0.01 5 5 0.1 0.1 0.1

320 47 180 6.0 × 105 2.6 × 106 1.9 × 106 ∼17 4.3 × 109 1.8 × 108 1.4 × 109

10−102 10−102 / 2.01 × 109 4 × 106 / / 8.9 × 109 3.7 × 108 2.9 × 109

27 27 29 30 31 32 33 this work this work this work

R = |Iph| /Popt

for the slow photoresponse. To verify this hypothesis, another phototransistor with identical device architecture except for reduced perovskite thickness (∼40 nm) was assembled. As expected, this device shows a relatively poor photoresponse (Figure S6a,b) due to weakened photogating effect as a result of limited light absorption (Figure S6d). Even so, a responsivity as high as ∼4.2 × 108 A/W at low light intensity can still be attained in the phototransistor thanks to the unique device architecture (Figure S6c). Interestingly, the device exhibits a much enhanced response speed (rising time 800 nm), the absolute value is still very high due to the ultrahigh gain. To better understand the NIR response, we further recorded the channel current of another phototransistor with similar device performance as a function of VG under 895 nm light illumination with different intensities and calculated the responsivity. Similarly, a dramatic shift of the Dirac point VD up to ∼60 V toward positive VG with increasing illumination

intensity was observed (Figure 4b). As depicted in Figure 4c, the responsivity can be as high as 1.1 × 109 A/W at low excitation intensity, which corresponds to a gain of 1.5 × 109. Time-dependent photoresponses were further measured under on/off NIR light modulation with different excitation wavelengths at VG = −15 V (Figure 4d). Obviously, the channel current increases/decreases when switching on/off the illumination for all the wavelengths. Under 1300 nm light excitation, the device still shows clear photoresponse with photocurrent of ∼25 μA, corresponding to a responsivity of ∼876 A/W. The strong photoresponse in the NIR region beyond the absorption edge of the CH3NH3PbI3−xClx perovskite is attributed to the excitation of carriers from the valence band to the traps states within the perovskite bandgap (inset in Figure 2b), similar to the extrinsic photoconductors based on normal semiconductors.46 It is expected that the NIR photoresponse could be further improved by means of employing perovskite materials with narrower bandgap as photoactive materials,47 doping perovskites by introducing foreign atoms to form appropriate defect/trap centers within the bandgap of perovskites,48 as well as integration with plasmonic metallic nanostructures resonated at NIR region.49,50

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CONCLUSIONS In summary, we report for the first time ultrasensitive phototransistors based on CH3NH3PbI3−xClx/P3HT/graphene vertical multiheterojunctions. The device exhibits record responsivity of ∼4.3 × 109 A/W and a gain close to 1010 electrons per photon, which are several orders of magnitude higher than those have been achieved previously with perovskite-based phototransistors. The superior performance can be attributed to the pronounced photogating effect on the F


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graphene channel induced by high-density accumulated electrons in the perovskite, due to the strong light absorption, long carrier diffusion lengths, and effective photoexcited carrier separation. This work paves a way for the application of organometal halide perovskites as cost-effective and highly sensitive photodetectors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11631. Raman spectrum of the graphene; cross-sectional SEM image of the CH3NH3PbI3−xClx perovskite/P3HT/ graphene multiheterojunction on a SiO2/Si substrate; transfer curves of pristine graphene and CH3NH3PbI3−xClx perovskite/P3HT/graphene in dark; photoresponse characteristics of CH3NH3PbI3−xClx perovskite/graphene, CH3NH3PbI3 perovskite/P3HT/graphene, and CH3NH3PbI3−xClx perovskite (∼40 nm)/ P3HT/graphene phototransistors; time-dependent photoresponse of the CH3NH3PbI3−xClx perovskite/P3HT/ graphene phototransistor at VG= 30 V and the stability measurement result (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chao Xie: 0000-0003-4451-767X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Research Grants Council (RGC) of Hong Kong, China (Project No. T23-713/ 11) and the Hong Kong Polytechnic University (Project No. G-YBB7).

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REFERENCES

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