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Self-Assembly of the Lateral In2Se3/CuInSe2 Heterojunction for Enhanced Photodetection Zhaoqiang Zheng, Jiandong Yao, and Guowei Yang* State Key Laboratory of Optoelectronic Materials and Technologies, Nanotechnology Research Center, School of Materials Science & Engineering, Sun Yat-sen University, Guangzhou, Guangdong 510275, P. R. China S Supporting Information *

ABSTRACT: Layered materials have been found to be promising candidates for next-generation microelectronic and optoelectronic devices due to their unique electrical and optical properties. The p−n junction is an elementary building block for microelectronics and optoelectronics devices. Herein, using the pulsed-laser deposition (PLD) method, we achieve pure In2Se3-based photodetectors and In2Se3/CuInSe2-based photodetectors with a lateral p−n heterojunction. In comparison to that of the pure In2Se3-based photodetector, the photodetectors based on the In2Se3/CuInSe2 heterojunction exhibit a tremendous promotion of photodetection performance and obvious rectifying behavior. The photoresponsivity and external quantum efficiency of the fabricated heterojunction-based device under 532 nm light irradiation are 20.1 A/W and 4698%, respectively. These values are about 7.5 times higher than those of our fabricated pure In2Se3-based devices. We attribute this promotion of photodetection to the suitable band structures of In2Se3 and CuInSe2, which greatly promote the separation of photoexcited electron−hole pairs. This work suggests an effective way to form lateral p−n junctions, opening up a new scenario for designing and constructing high-performance optoelectronic devices. KEYWORDS: layered materials, In2Se3, CuInSe2, lateral junction, photodetectors

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INTRODUCTION Over the past decade, layered materials (e.g., In2Se3) have attracted enormous attention due to their exotic electronic and optical attributes such as strong light−matter interaction,1 efficient light absorption,2 and large surface-to-volume ratio.3 These physical properties endow layered materials with the potential to trigger the revolution of a wide range of optoelectronic devices, including solar cells, light-emitting devices, and photodetectors.4−9 Moreover, the existence of direct bandgap in bulk form renders In2Se3 an attractive candidate in optoelectronic applications.2 In optoelectronics, photodetectors are fundamental devices which enable the conversion of optical signals to electronic signals. Photodetectors have been widely applied in various fields, including military applications and commercial products in daily life.10,11 The performance of photodetectors depends on various factors. These include light absorbency, photoexcitation, relaxation, free carrier generation, charge trapping/detrapping, recombination, etc.12 Under illumination, the active film absorbs photons with energy higher than that of the bandgap, which results in excited electrons with excess energy. Due to the small bandgaps of layered materials, energy dissipation will inevitably occur during the long relaxation process. The excess energy can be relaxed into thermal sinks via interactions between carriers with intrinsic acoustic or optical phonons.12 Thus, effective separation of photoexcited electron−hole pairs and conversion of excess photoexcited energy into electrical current are keys for superior photodetector devices.12−14 © 2017 American Chemical Society

Introduction of the p−n junction is a particularly powerful approach to improve the performance of photodetectors. The built-in potential in the depletion region can readily separate and drive the photogenerated electron−hole pairs that migrate toward the electrodes and eventually generate a superior photocurrent to enable efficient energy conversion.15−17 Recently, several efforts have been made toward realizing the p−n junction via, for example, the use of a local electrostatic gate, an appropriate ionic doping procedure, and so on.15,18 However, these methods require either complex device configurations or specialized fabrication procedures. These requirements are not compatible with current semiconductor fabrication technology. Hence, development of a facile and efficient approach to create a functional p−n junction in thin layered semiconductors and achieve a superior photodetector is desirable. In this contribution, we conducted a one-step approach to fabricate pure In2Se3-based photodetectors and In2Se3/CuInSe2based photodetectors with a lateral p−n heterojunction through a simple pulsed-laser deposition (PLD) procedure. Due to the suitable band structures of In2Se3 and CuInSe2, photoexcited electron−hole pairs can be separated readily and extracted by the electrodes. As a result, the photoresponsivity and external quantum efficiency (EQE) of the In2Se3/CuInSe2-based photodetector are about 7.5 times higher than that of our prepared Received: December 20, 2016 Accepted: February 9, 2017 Published: February 9, 2017 7288

DOI: 10.1021/acsami.6b16323 ACS Appl. Mater. Interfaces 2017, 9, 7288−7296

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a−d) Schematic diagrams illustrating the fabrication process of the lateral In2Se3/CuInSe2 heterojunction photodetector array. (e) Schematic device structure of a single In2Se3/CuInSe2 photodetector.

Figure 2. Morphology and structure of the prepared heterojunction photodetectors. (a) Optical image of a photodetector chip containing seven detectors. (b) High magnification optical image of a single photodetector. (c) SEM image of the interface of the lateral heterojunction film. (d) Zoomedin SEM images of the In2Se3 region indicated by the red box in panel c. (e) Zoomed-in SEM images of the CuInSe2 region indicated by the orange box in panel c. (f) AFM height profile of the In2Se3 region of the heterojunction film. The inset shows the AFM image of the scan area. The thickness of the sample is deduced to be 14.1 nm. EDS mapping images of (g) In, (h) Se, and (i) Cu at the interface of the lateral heterojunction.

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pure In2Se3-based device. Moreover, under illumination, obvious short-circuit current (Isc) and open-circuit voltage (Voc) originating from the formation of the p−n junction are also observed from the heterojunction device. This study suggests a new route for designing and constructing superior optoelectronic devices.

EXPERIMENTAL SECTION

Self-Assembly Preparation of the In2Se3/CuInSe2 Photodetector Array and Pure In2Se3 Photodetector. As described in Figures 1a−d, PLD was adopted to self-assemble the In2Se3/CuInSe2 photodetector array. In this experiment, the PLD parameters were similar to those in our previous works.19−21 First, the Si/SiO2 substrate was washed in acetone, alcohol, and deionized water for 10 min to 7289


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Figure 3. Further characterizations of the lateral heterojunction film. (a) XRD patterns of the In2Se3/CuInSe2 heterojunction film. (b) Raman patterns of the In2Se3/CuInSe2 heterojunction film. (c) Raman mapping (CuInSe2 A1 peak at 178 cm−1) at the interface of the In2Se3/CuInSe2 heterojunction film. The wavelength of the laser used for Raman spectroscopy is 514 nm. remove contaminations (Figure 1a). Then, seven sets of Au−Cu electrodes were patterned onto the Si/SiO2 substrate using a standard photolithography process, followed by electron beam evaporation (Figure 1b). Subsequently, we covered the electrode pairs with a stainless steel shadow mask for patterning the In2Se3/CuInSe2 film (Figure 1c). Then, they were mounted in the deposition chamber at a distance of 7 cm away from and parallel to the In2Se3 target (99.99%). After the deposition system was evacuated to lower than 2 × 10−4 Pa, highly pure argon background gas with a flow rate of 50 sccm was guided into the system. Thereupon, the substrate was rapidly heated to 365 °C, and the system pressure was maintained at 20 Pa. Sequentially, a pulsed KrF excimer laser (λ = 248 nm, pulse duration of 20 ns, repetition rate of 4 Hz) was focused to ablate the target. The operating energy per pulse was set at 105 mJ, and the total pulse number is 2400. Finally, the system was cooled to room temperature over several hours. The In2Se3/ CuInSe2 photodetector array could be observed on the substrate when it was removed (Figure 1d). The device structure of a single In2Se3/ CuInSe2 photodetector is schematically shown in Figure 1e. PLD was also employed to fabricate the pure In2Se3 photodetector. The substrate was Si/SiO2 covered with Au−Au electrodes. The PLD parameters are consistent with those used for the deposition of the In2Se3/CuInSe2 films. Characterization of the In2Se3/CuInSe2 Heterojunction Film. The structure of the photodetector array was identified using an optical microscope. The surface morphologies of our In2Se3/CuInSe2 film were observed using a scanning electron microscope (SEM, FEI Quanta 400F). Atomic force microscopy (AFM, Bruker Dimension Fastscan) was used to perform the thickness profile measurement, and Kelvin probe force microscopy (KPFM) was used to perform measurements of the heterojunction film. Ultraviolet photoelectron spectroscopy (UPS, Escalab 250) with He I line (21.22 eV) was conducted to estimate the band structure of the samples. X-ray photoelectron spectroscopy (XPS, Escalab 250) with a monochromatic Al Kα source was used to measure the binding energies of CuInSe2. Transmission electron microscopy

(TEM) and selected area electron diffraction (SAED) were recorded by a transmission electron microscope system (FEI Tecnai G2 F30). X-ray diffraction (XRD, Rigaku D-MAX 2200 VPC) was recorded with Cu Kα radiation (λ = 0.15418 nm) at the speed of 7° min−1. Raman spectra were recorded using a Renishaw InVia Raman spectrometer with a 514 nm laser for excitation. UV−vis−NIR diffuse reflectance spectra (DRS) of the In2Se3 and CuInSe2 were obtained by a spectrophotometer (Lambda950, PerkinElmer). Optoelectronic Measurements of the Heterojunction Photodetector Array. We carried out the optoelectronic measurements on a Lakeshore probe station equipped with a Keithley 4200 semiconductor characterization system. The photocurrents were measured under light illumination. The illuminations with wavelengths ranging from ultraviolet to near-infrared were generated from monochromatic semiconductor lasers (Viasho), and the intensities of incident lights were measured by an optical power meter.

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RESULTS AND DISCUSSION Morphology and Structure of the In2Se3/CuInSe2 Photodetector. An optical microscopy image of a typical pure In2Se3 photodetector is shown in Figure S1a. The greenish color is reflected by the In2Se3 active film. From the SEM image in Figure S1b, we can observe sheet-like morphology. Figure S1c presents the high-resolution TEM image of the In2Se3 active film. Obvious lattice fringes are visible, revealing the high quality of the In2Se3 film. The inset in Figure S1c shows the SAED pattern of the In2Se3 active film. The sharp diffraction rings indicate the polycrystalline nature of the film. Figure 2a shows the digital photograph of a heterojunction photodetector chip. The chip size is 1 × 1 cm, and there are seven photodetectors in a chip. A representative optical microscopy image of a single photodetector is presented in Figure 2b. As can be seen, the channel 7290


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Figure 4. (a) I−V curves of the In2Se3/CuInSe2 photoconductor measured in the dark and under 532 nm light illumination with various power densities. The inset shows enlarged I−V characteristics in the dark and under light illumination. (b) Photocurrent (blue) and responsivity (R, red) of the two photoconductors at Vds = 2 V. (c) Power intensity dependent EQE of the two photoconductors at Vds = 2 V. (d) Temporal response of the two photodetectors under 532 nm light illumination. The curves are normalized for clarity, and the time interval is 8.3 ms.

Figures S2c−e were observed at 932.6, 444.8, and 54.0 eV, respectively, which confirms the existence of Cu+, In3+, and Se2− states.25,26 According to the XPS results, the experimental stoichiometric ratio of Cu:In:Se was calculated to be 1:1.06:1.85, which is approximately consistent with the theoretical value (1:1:2). Therefore, we estimate the formation of CuInSe2. To further confirm the formation of the In2Se3/CuInSe2 heterojunction, XRD and Raman characterizations are performed. As shown the XRD patterns in Figure 3a, the pattern of CuInSe2 contains only one pronounced peak at 26.649°, which can be indexed as (112) plane (JCPDS 87-2265). For the pattern of In2Se3, all diffraction peaks are consistent with data of the βphase In2Se3 (JCPDS 35-1056).27,28 Raman patterns of the In2Se3/CuInSe2 heterojunction film are presented in Figure 3b. As can be seen, the Raman spectrum taken from the CuInSe2 region shows a strong peak at 178 cm−1 and two weak peaks at 211 and 230 cm−1. They can be regarded as the A1, B2, and E modes of CuInSe2, respectively.23 However, the positions of the vibration peaks taken from the In2Se3 region are mainly located at 110, 151, 205, and 240 cm−1, which can be assigned to A1, A1(TO), A1(LO + TO), and A1 symmetry mode of β-In2Se3, respectively.29,30 The weak peak at 175 cm−1 is attributed to the In−Se vibrations in the In2Se3/CuInSe2 film.30 Figure 3c shows the Raman mapping (CuInSe2 A1 peak of 178 cm−1) at the interface of the In2Se3/CuInSe2 heterojunction film. The CuInSe2 is homogeneously distributed on the bottom part, and the In2Se3/CuInSe2 interface is obviously visible. This Raman mapping has revealed the formation of the In2Se3/CuInSe2 heterojunction, and the boundary between the two materials is clearly visible.

spacing between two parallel electrodes was maintained at 30 μm. Moreover, the color of the active film near the copper electrode transformed into red, which may due to the reaction with the Cu electrode; layered In2Se3 in this region transformed into nonlayered CuInSe2.22,23 The SEM image of the middle of the active film shows that there is an obvious junction at the interface of the two materials (Figure 2c). High-magnification SEM images of the In2Se3 (labeled by the red box) and CuInSe2 regions (labeled by the orange box) are presented in Figures 2d and e, respectively. Sheet-like and particle-like continuous polycrystalline morphologies are observed. The difference in morphology suggests the creation of the In2Se3/CuInSe2 heterojunction. Figure 2f presents the AFM height profile across the edge of the heterojunction film. The thickness is calculated to be ca. 14.1 nm. To analyze the detailed chemical composition of the assynthesized heterojunction film, EDS measure at the interface of the lateral heterojunction is carried out. The EDS spectrum in Figure S2a manifests the coexistence of In, Se, and Cu elements. Their spatial distributions are depicted in the EDS mapping images in Figures 2e−i. Clearly, In and Se elements distribute uniformly throughout the whole heterostructure, while the element Cu locates mainly on one side. It suggests that In2Se3 in the bottom region converted to CuInSe2 successfully. XPS analysis was performed to verify the elemental composition of CuInSe2. As the wide scanning XPS spectrum in Figure S2b, the binding energies of Cu 2p, In 3d, and Se 3d core levels were consistent with the literature values.24 Figures S2c−e show the high resolution XPS spectra of 2p, In 3d, and Se 3d, respectively. The Cu 2P3/2, In 3P5/2, and Se 3d5/2 binding energy peaks in 7291


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Figure 5. Broadband photodetection of the In2Se3/CuInSe2 photoconductor. (a) Temporal photoswitching curves under different illumination wavelengths. The time interval is 8.3 ms. (b) Photocurrent and the corresponding (c) responsivity of the In2Se3/CuInSe2 photoconductor under various illumination wavelengths at Vds = 2 V.

effective area of the photosensitive region.32 We calculated the R values under different incident light intensities in Figure 4b (red). At the light intensity of 7 μW/cm2, our pure In2Se3-based device shows a responsivity of 2.69 A/W. Remarkably, our heterojunction photodetector exhibits a high responsivity of 20.1 A/W, which is ca. 7.5 times higher than that of our prepared pure In2Se3-based device. It is notable that the R decreases with the increasing irradiation power, which may be explained by the trap states existing at In2Se3/CuInSe2 or the dielectric substrate.23,33 With the measured R here, as shown in Figure 4c, the EQEs of our fabricated pure In2Se3-based and In2Se3/CuInSe2-based devices were calculated to be ∼629 and ∼4698%, respectively, following the relation EQE = hcR/λe, where h is Planck’s constant, c is the light velocity, λ is the excitation wavelength, and e is the electronic charge.34 Figure S4 presents the statistic responsivity of 7 In2Se3/CuInSe2 devices’ response to 7 μW/cm2 light illumination. Clearly, the heterojunction detector arrays have high responsivity with a superior uniformity. Reliable and fast responses to light illumination are crucial for high-performance photodetectors. The cycling behaviors of the two photodetectors were checked under the pulse illumination. As depicted in Figure S5, definite photoswitching behaviors were observed and maintain reproducibility for multiple cycles. In Figure 4d, temporal responses of the two photodetectors in a complete on/off cycle were recorded with a high temporal resolution. For the In2Se3/CuInSe2 detector, after the illumination was turned on, the photocurrent rapidly rises by more than 95% in the initial 8.3 ms. While for the pure In2Se3 device, the photocurrent rises by only 75.5%, followed by a relatively slow

Optoelectronic Performance of the In2Se3/CuInSe2 Photodetector. Photodetection performances of the In2Se3/ CuInSe2 photodetectors were evaluated upon various light irradiations. We first measured the current versus bias voltage (I−V) characteristics of the In2Se3/CuInSe2 devices in dark and under light illumination (532 nm) with various power intensities ranging from 0.007 to 27 mW/cm2. The corresponding results were logarithmic presented in Figures 4a. Interestingly, these I− V curves exhibit an obvious rectifying behavior. The rectification ratio, defined as the ratio of the forward/reverse current,31 is about 10 at Vds = ± 2 V under 27 mW/cm2 light irradiation. Considering that the I−V curves of the pure In2Se3 photodetector are symmetrical (Figure S3), the rectifying characteristic of the In2Se3/CuInSe2 heterojunction can thus be attributed to the interaction between In2Se3 and CuInSe2. For further comparison, the insets in Figure 4a and Figure S3 present the enlarged I−V characteristics of the two devices under dark and light illumination. Compared to the pure In2Se3 device, more obvious photovoltaic-like behaviors such as Voc and Isc are exhibited in the In2Se3/CuInSe2 photodetector. The light intensity dependent photocurrent (Iph = Ilight − Idark) is investigated in Figure 4b (blue). We observed that the photocurrent increases continuously with the irradiation power, and the photocurrent of the heterojunction device is much higher than that of the pure In2Se3-based one. Responsivity (R), one of the critical parameters for a photodetector, is defined as the generated photocurrent per unit effective incident power. R can be calculated by the equation R = Iph/PS, where Iph is the generated photocurrent, P is the light power density, and S is the 7292


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Figure 6. (a) UPS spectra near the onset part of CuInSe2 and In2Se3. UPS spectra near the offset parts of (b) CuInSe2 and (c) In2Se3. Helium I with photon energy of 21.22 eV is used here. (d) UV−vis−NIR absorption spectra of CuInSe2 and In2Se3 and corresponding Tauc plots of (e) CuInSe2 and (f) In2Se3.

responsivity of the pure In2Se3 photoconductor under various illumination wavelengths in Figure S7. Clearly, our In2Se3/ CuInSe2 photoconductor exhibits performance much better than that of the pure In2Se3 device over the UV−vis−NIR range. Remarkably, for the In2Se3 photodetector in 1550 nm (Figure S8a), the photocurrent is still discernible but very weak. It may arise from the intrinsic defects or native oxides that grow at the surface of the In2Se3 film that could act as efficient energy converters of incident light.26 While for the In2Se3/CuInSe2 device, as shown in Figure S8b, we can observe a clear photoresponse in the wavelength of 1550 nm. This property means that the synergistic effect between In2Se3 and CuInSe2 film may broaden the response range of the In2Se3 detector. Heterojunction Enhanced Photodetection Mechanism. We can conclude from the above analyses that the In2Se3/CuInSe2 heterojunction can boost the photodetection of pure In2Se3. In this section, we attempt to unveil the plausible photodetection mechanism, especially the synergistic effect between In2Se3 and CuInSe2. At first, the band structures of the In2Se3 and CuInSe2 were estimated. UPS is a useful technique to estimate the band structure of samples.36 Here, UPS measurements at the CuInSe2 and In2Se3 regions were carried out, and the corresponding UPS spectra at low kinetic regions are shown in Figure 6a. By linear extrapolation of their leading edges to the baseline, the work function of our In2Se3 and CuInSe2 can be measured at 4.51 and 4.9 eV, respectively. Meanwhile, UPS spectra near the offset part of CuInSe2 and In2Se3 are presented in Figures 6b and c, respectively. The difference between the valence band (VB) energy and the Fermi energy can be calculated to be 0.29 eV (21.22 − 20.93 eV) for the CuInSe2 and 1.28 eV (21.22 − 19.94 eV) for In2Se3. Thereupon, the bandgaps of the CuInSe2 and

tail. The faster photoresponse of the heterostructured In2Se3/ CuInSe2 photoconductor could be attributed to the efficient charge separation at the interface. For clear comparison, Table S1 summarizes the relevant performance parameters of other recently developed heterojunction-based photodetectors.14,21,28−31 In general, our device stands out considering the overall performances, which reveals the superiority of the In2Se3/ CuInSe2 heterojunction in the photodetection applications. Then, broadband special responses of the constructed photodetectors were also investigated. Figure 5a presents the temporal photoswitching curves of our In2Se3/CuInSe2 photodetector under 370, 447, 808, and 1064 nm illuminations. Defining the response time as the photocurrent increased from 0 to 80% of the stable photocurrent, the response times of our heterojunction device over the 370−808 nm range are shorter than 8.3 ms, which is the limited sampling interval of our measurement system. Even for 1064 nm light, the response time was less than 16.6 ms. Figure S6 presents the I−V curves of our In2Se3/CuInSe2 photoconductor measured in the dark and under various illumination wavelengths. In all cases, obvious rectifying behavior and significant photocurrents were observed. Figure 5b summarizes the power dependent photocurrent upon illumination with different wavelengths at Vds = 2 V. The photocurrents also exhibit positive dependence on the incident light density, which can be attributed to the increase in the number of photogenerated carriers. The corresponding power dependent responsivities are presented in Figure 5c. In all cases, the responsivity also exhibits a negative dependence on the incident light density. Meanwhile, these responsivities are comparable to those of the of the state-of-the-art commercial Si and Ge photodetectors (R ≈ 0.05 to 0.85 A/W).35 Sequentially, we recorded the photocurrent and corresponding 7293


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CuInSe2 can readily separate photoexcited electron−hole pairs, bringing about the enhanced photodetection.

In2Se3 are determined to be 0.93 and 1.35 eV according to their UV−vis−NIR diffuse reflection spectra (Figure 6d) and their corresponding Tauc plots (Figures 6e and f). Then, we sketched simple bandgap models of CuInSe2 and In2Se3 in Figure 7a. They

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CONCLUSION In summary, we demonstrated the fabrication of the lateral In2Se3/CuInSe2 heterojunction film via a facile PLD technique. The suitable band structures of In2Se3 and CuInSe2 lead to the formation of the type-II heterostructures, which can separate photoexcited electron−hole pairs timely. As a result, the photodetector based on the heterojunction exhibited outstanding photodetection capabilities, including a decent photoresponsivity of 20.1 A/W, a faster response time of less than 8.3 ms, and a broad response range from 370 to 1550 nm. These parameters were superior to those of the pure In2Se3 device and better than those of other heterojunction-based photodetectors. Moreover, under illumination, obvious photovoltaic-like behaviors were observed from the heterojunction device. Therefore, these results demonstrated a new approach to design and construct novel optoelectronic devices.

Figure 7. (a) Band structures of In2Se3 and CuInSe2 in the heterostructures. (b) Schematic energy band diagram of the heterostructures under light irradiation. Electrons are injected from the CuInSe2 to In2Se3, and holes are injected from the In2Se3 to CuInSe2.

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form a type-II-like junction. In this bandgap model, photoexcited electron−hole pairs (blue arrows in Figure 7b) seem to be separated readily (orange arrows in Figure 7b).37,38 In addition, the photoresponse of our heterojunction detector up to 1550 nm, which is the forbidden optical absorption region for CuInSe2 and In2Se3, arose from the photoinduced electron transfer from the valence band of CuInSe2 to the bottom of the conduction band of In2Se3 (green arrow in Figure 7b). To further verify the photoinduced charge transfer in In2Se3/ CuInSe2, Kelvin probe force microscopy (KPFM) was utilized to quantitatively analyze the changes in surface potential under illumination.39 The synchronous measurements for AFM topography and their corresponding contact potential difference (CPD) distribution at the interface of the In2Se3/CuInSe2 heterojunction in the dark and under white light illumination are shown in Figure S9. The CPD between the AFM tip and the local area of In2Se3 or CuInSe2 can be given by

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b16323. Summary of reported photodetectors, morphology, structure, and photodetection performances of the pure In2Se3 photodetector; EDS spectrum of the heterojunction; I−V curves and broadband photodetection of the In2Se3/CuInSe2 photoconductor; and KPFM measurements at the interface of the In 2 Se 3 /CuInSe 2 heterojunction (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

CPDIn2Se3 = Φtip − ΦIn2Se3

(1)

ORCID

CPDCuInSe2 = Φtip − ΦCuInSe2

(2)

Guowei Yang: 0000-0003-2141-6630 Notes

where Φti, ΦIn2Se3, andΦCuInSe2 are the work functions of the tip, In2Se3, and CuInSe2, respectively.40,41 Therefore, the Fermi level difference (ΔEf) between In2Se3 and CuInSe2 can be obtained by ΔEf = ΦIn2Se3 − ΦCuInSe2 = CPDCuInSe2 − CPDIn2Se3

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant 50902097) and State Key Laboratory of Optoelectronic Materials and Technologies.

(3)

As shown in Figure S8c, the ΔEf between In2Se3 and CuInSe2 is determined to be about 386.7 mV in the dark. Under white light illumination, the ΔEf increases to about 405.4 mV. This significant change of ΔEf between dark and light illumination can be ascribed to an obvious optoelectric effect occurring in the In2Se3/CuInSe2 heterostructure under light illumination.42 Under illumination, a large number of photoexcited electron− hole pairs are generated in the In2Se3 and CuInSe2 films. Subsequently, the type-II band alignment of the In2Se3/CuInSe2 heterostructure will lead to electron flow into the In2Se3 side, while holes are injected into the CuInSe2 side. As a result, the quasi-Fermi levels of these two materials shift in opposite directions, as shown in Figure S10. Therefore, the Fermi level difference between In2Se3 and CuInSe2 widens under illumination, corresponding to increase in ΔEf. On the basis of the above discussions, we clearly demonstrate the formation of type-II band alignment in the In2Se3/CuInSe2 heterojunction. The synergistic effects between In2Se3 and

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