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Ternary CuIn7Se11: Towards Ultra-Thin Layered Photodetectors and Photovoltaic Devices Sidong Lei, Ali Sobhani, Fangfang Wen, Antony George, Qizhong Wang, Yihan Huang, Pei Dong, Bo Li, Sina Najmaei, James Bellah, Gautam Gupta, Aditya D. Mohite, Liehui Ge,* Jun Lou, Naomi J. Halas,* Robert Vajtai, and Pulickel Ajayan* Since the discovery of graphene in 2004, atomically thin twodimensional (2D) materials have attracted great interest.[1–5] 2D materials research has led to a new, crosscutting frontier of fundamental physics, chemistry, material science, and device engineering. This has resulted in new device architecture and new methods for device fabrication that show great potential in next-generation electronics and optoelectronics. A few layered 2D materials, such as graphene, the transition metal dichalcogenides and the transition metal oxides, have been widely explored for these applications, including field effect transistors (FET)[3,6] with reasonable charge carrier mobilities and high ON/OFF ratios, photodetectors[1,5,7–12] with high quantum yield, and promising photovoltaic devices.[13,14] Efforts have also been devoted to the discovery of new 2D materials for better device fabrication platforms, to meet the requirements of future electronic and optoelectronic technologies. Among all possible applications of 2D materials, photodetectors and photovoltaics have drawn particular attention. In general, 2D materials show excellent optoelectronic properties: for example, MoS2 can exhibit a photoresponsivity as high as 880 A W–1[15] and the quantum efficiency of GaSe can be as high as 1367%.[8] However, most 2D-based photo-detectors with high quantum efficiencies also suffer from large dark currents
S. Lei, Dr. A. George, Q. Wang, Dr. P. Dong, Dr. B. Li, Dr. S. Najmaei, J. Bellah, Dr. L. Ge, Prof. J. Lou, Dr. R. Vajtai, Prof. P. Ajayan Department of Materials Science and Nano-Engineering Rice University Houston, Texas 77005, USA E-mail: [email protected]; [email protected] A. Sobhani, Prof. N. J. Halas Department of Electrical and Computer Engineering Rice University Houston, Texas 77005, USA E-mail: [email protected] F. Wen, Prof. N. J. Halas Department of Chemistry Rice University Houston, Texas 77005, USA Y. Huang, Prof. N. J. Halas Department of Physics and Astronomy Rice University Houston, Texas 77005, USA Dr. G. Gupta, Dr. A. D. Mohite MPA-11 Materials Synthesis and Integrated Devices Los Alamos National Laboratory Los Alamos, New Mexico 87545, USA
DOI: 10.1002/adma.201403342
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and low signal-to-noise ratios (S/N).[15–17] More importantly, the band gaps of these 2D materials are usually too large to cover a wide spectral range. Recently, InSe was reported to have a smaller band gap of 1.4 eV, making it appropriate for the near infrared (NIR) range.[18] But for photovoltaics and photodetectors, especially IR photodetectors, an even smaller band gap would be desirable to expand the spectral range of the response even further. Ternary copper indium selenide (CIS) may provide a solution to these problems. Because of its high photoresponsivity and wide spectral response range, the bulk CIS system has been extensively used in optoelectronics research and industry to serve as a new generation of flexible thin film solar cells.[19–21] CIS exhibits a variety of structures depending on the ratio between Cu and In, including chalcopytite, stannite, wurtzite structures, etc.[22,23] When the ratio of Cu to In lies between 1:5 to 1:9, it forms a layered phase, i.e., γ-phase as defined in earlier studies.[23] 2D layered CIS could potentially inherit the excellent optoelectronic properties of bulk CIS and also be a good candidate material for 2D based optoelectronic devices. The investigation of the ternary CIS compound system may also bring a new dimension to 2D material research. Current research on 2D materials has been almost exclusively focused on single elemental, binary systems or their alloys. Previous studies have shown, however, that elemental composition plays an important role in defining the physical properties of 2D materials. For example, binary systems and their alloys exhibit a variety of behaviors and modification spaces that can be optimized to meet the requirements of applications: the 1T-2H phase transition in transition metal dichalcogenides,[24–27] similar phase transitions in InSe,[28] and the structure of junctions between MoS2 and WS2,[29] are but a few examples. In sharp contrast, modification methods for the properties of single elemental systems are far more limited. Although pristine graphene has a unique band structure and a large charge carrier mobility,[30–32] it is still a challenge to tailor its band structure.[33,34] As a result, graphene FETs show poor on/off performance and graphene photodetectors always possess large dark currents and a relatively weak response.[1,5,35] Multi-elemental systems bring multiple degrees of freedom for controlling physical properties via stoichiometric variation, and as such, the layered ternary CIS system is an ideal platform for this line of research. Here we report the succcessful synthesis and isolation of few-layered CuIn7Se11, as a 2D atomically layered ternary compound. The ratio between Cu to In was determined to be 1:7, consistent with the range where the layered phase forms. The
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Figure 1. Structure characterization of CuIn7Se11 (CIS). a) Crystal structure model of CIS. A single-layered CIS contains 5 layers of metallic ions. The layer thickness of CIS is 1.6 nm. b) Scanning electron microscope (SEM) image of a CIS crystal synthesized by the melting and recrystallization of solid state precursors Cu2Se and In2Se3 (Cu:In = 1:7). The SEM image shows an obvious layered texture. c) High resolution transmission electron microscope (TEM) image of the CIS lattice plane. The electron beam is parallel to the z-axis of CIS lattice. The lattice constant is measured to be 0.40 nm along the a-axis which agrees well with the previously reported value.[36] d) Atomic force microscopy (AFM) height profile of mechanically exfoliated CIS flake. The inset in the upper left corner shows the optical image of the flake under study. The inset in the lower right corner shows the height profile of the exfoliated flake. It reveals a smooth surface of the flake and a thickness of 6 nm, corresponding to 3∼4 CIS layers. e) X-ray diffraction (XRD) pattern of synthesized CuIn7Se11. The strongest peaks are labeled according to previous study.[23]
crystal structure and layered texture was further confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies. Photoconductivity measurements on the few-layered CIS showed a strong photoresponse throughout the visible range of the light spectrum. Few layered CIS photodetectors exhibited a good linear response range and a high S/N. Based on the excellent photo- and spectral-response, we also fabricated and characterized a few-layered CIS photovoltaic device prototype. With two different metal electrodes to serve as anode and cathode, photovoltaic effects were realized with a Schottky junction-based device, which may provide a possible route for the use of layered 2D CIS in actual photovoltaic applications. Figure 1a shows the side view of a model CIS lattice structure.[36] The inter-layer distance in this material is 1.6 nm and the lattice constant along the a-axis is 0.40 nm.[23,36] The crystal
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structure is similar to that of CuIn5Se8, and the deviation in stoichiometry of the CIS is compensated by ordered or disordered vacancies.[37] Based on previous studies of CuSe5Se8, sharing an identical structure with CuSe7Se11, the van der Waals gap for CIS is between Cu and the neighboring In atoms.[23] According to the phase diagram of Cu2Se-In2Se3, at least six different phases exist at high temperatures.[23] When the percentage of In2Se3 is in the range of 82% and 90%, a layered phase (γ-phase) dominates. Thus, CIS crystal was synthesized by mixing Cu2Se (Sigma-Aldrich, 99.95%) and In2Se3 (Alfa Aesar, 99.99%) with a molar ratio of 1:7. This ratio gives large layered crystal size for exfoliation and further study as discussed later. The mixture was then carefully ground in a mortar to produce a uniform solid state mixture and transferred to an evacuated quartz ampoule (1.6 V), the reverse bias Schottky junction breaks down and the dark current increases dramatically (inset of Figure 2d). The low breakdown voltage indicates that the Schottky barrier is not high (our later discussion of the PV effect will provide a more quantitative estimate of the Schottky barrier height). When the CIS MSMPD is illuminated uniformly by a 543 nm (He/Ne) laser with an intensity of 20 mW cm–2 and 50 mW cm–2, the device shows a strong photoresponse, as shown in Figure 2d. With a 2 V bias, the device exhibited a responsivity of 380 mA/W and an external quantum efficiency of 88.0%.(The external quantum efficiency is determined by the formula that η =
I ph E ph × × 100%, where Iph is current dife p
ference between currents without and with illumination, e is charge of electron, Eph is the photon energy, and P is light power shined onto the device). The photoresponse as a function of illumination intensity and bias voltage was also studied and is shown in Figure 2e. The photoresponse shows a linear relationship with illumination intensity ranging from 1 mW cm–2 to 200 mW cm–2, yielding a large linear response range of 70 dB. Figure 2f shows the time-resolved photoconductivity measurement on the few-layered CIS MSMPD. By fitting the time-resolved photoresponse with an exponential decay function (e −(t − t0 )/τ ) , a time constant of 24 ms is obtained. Compared to the previous reports of layered MSMPDs where Schottky junctions were avoided in order to improve the current level of the photodetector,[15] the Schottky contact is utilized here to minimize the dark current to ∼1 pA, and consequently, the device can yield a higher linear response range. A smaller dark current leads to a lower noise level and a higher S/N. As the dark current is generated by random charge-carrier injection and electron-hole pair generation, the noise level (variance) can be estimated by the following formula:
σ 2 = 2e × I d × BW ,
(1)
where e =1.6 × 10–19 C, Id is the average dark current level (1 pA in our case), and BW is the bandwidth of the detector. Since the response time is 24 ms, it is reasonable to set BW = 40 Hz for our device. The noise level is then determined to be 1.3 × 10−29 A2. The contact impedance due to the Schottky barrier in our devices does not significantly sacrifice its performance. The device still shows a high photoresponsivity under a low bias voltage. Under a low illumination intensity (1 mW cm–2), the
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Figure 2. Photoresponse study of mechanically exfoliated CIS few layered samples. a) Optical image of a 3–4 layered CIS flake photodetector with two Ti/Au electrodes spaced by a 4 µm gap. b) The photocurrent mapping of the as-fabricated CIS photo-detector. The photo-response is uniform across the whole CIS piece, indicating a long lifetime of charge carriers, and an effective electron-hole separation by the external electrical field. c) The photoconductivity spectrum of a 3–4 layered CIS sample with 2 µm electrode spacing indicates an indirect band-gap of 1.1 eV, while a 1–2 layered sample (inset) has a larger band gap of 1.4 eV. d) Photoconductivity IV curves of CIS photodetector (refer to Supporting Information for more details about the devices under study). The dark current is very low due to the Schottky barrier formed between the Ti/Au electrode and CIS flake (Inset shows a barrier breakdown voltage of 1.6 V, indicating a small barrier height). Under an illumination of 20 mW cm–2 and 50 mW cm–2, the CIS photo detector shows an increasing photoresponse, corresponding to an external quantum efficiency of 88%. The photoresponsivity is shown by the black dashed curve. With a 2 V bias, the photoresponsivity is 380 mA W–1. e) The photoresponse is linear as a function of illumination intensity with a 0.1, 1, and 2 V bias. The illumination intensity ranges from 0 to 200 mW cm–2 and the current varies from 1 pA (dark current) to 2.16 nA, yielding a linear response range larger than 70 dB. f) The time-resolved photoresponse measurement illustrates a stable photoresponse performance with a time constant of 24 ms.
device still produced a photocurrent of 0.2 nA. This yields 2 a signal-to-noise ratio (S/N, I photo /σ 2) of 3.1 × 109 or 95 dB, neglecting the quantum noise and Johnson noise. (For commercial silicon photodetectors, S/N can vary from 100 to 1000 dB.) These findings demonstrate that few-layered CIS is a promising candidate for the active material in atomically thin photodetectors, especially when both dark current level and photosensitivity are critical, as in the case of low-level signal photodetectors in spectrometers, portable integrated
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optoelectronic devices, and other applications. Besides photodetectors, photovoltaic devices are another important application for 2D materials, especially for on-chip power sources. Bulk CIS is widely used in flexible thin film photovoltaic devices, so it is reasonable to expect that few-layered CIS may also serve as a good candidate for 2D atomically thin photovoltaics. Also, the excellent photoresponse of the few-layered CIS MSMPD indicates good optical absorption and efficient photocarrier generation that meets the prerequisites of a good photovoltaic material.
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Figure 3. Photovoltaic (PV) study of mechanically exfoliated CIS flake. a) SEM image of the atomically layered CIS PV device under study. The flake has a thickness of ≈3–4 layers. Two electrodes of different materials serve as the anode (Ti/Au) and cathode (Ge/Ag) respectively, with an interdigitated configuration and a spacing of ≈200 nm. The Ti/Au electrode forms a Schottky barrier with the CIS, and the Ge/Ag electrode forms an Ohmic contact. The built-in electrical field can help separate the photo-generated electron-hole pairs and generate photocurrent and photovoltage. b) The dark current of an as-fabricated CIS PV device. The IV curve shows an asymmetric feature under negative and positive biases. With a negative bias, the IV curve shows an obvious non-linear behavior. When the absolute bias is large enough, the Schottky barrier breaks down. On the positive bias branch with a bias larger than 0.5 V, the IV curve has a linear feature, indicating that the device is forward biased and the contact between the Ge/Ag and CIS is Ohmic. c) The IV curve of the CIS PV device working as solar cell. Under 150 mW cm–2 543 nm illumination, the device shows an open circuit voltage of 140 mV and a short circuit current of 38 pA. d) The working mechanism of a CIS Schottky PV device. The top diagram shows the band structure of the device without laser illumination. The CIS forms a Schottky barrier with the Ti/Au electrode and a built-in electrical field on one side, and on the other side, the CIS forms an Ohmic contact with the Ge/Ag. Under illumination (the bottom diagram), the electron-hole pairs are separated by the built-in electrical field of the Schottky barrier. Electrons move toward the Ge/Ag electrode making the potential more negative while holes move toward the Au electrode, resulting in a measurable photovoltage.
A built-in electrical field is essential in photovoltaic devices for the separation of photogenerated electrons and holes. Conventionally, semiconductor technologies rely on a pn junction to generate the built-in electrical field. Thus far, however, it is still a challenge to form a pn junction in a 2D layered material with a good interface. Recently, heterogeneous electrostatic doping has been successful in forming pn junction-like structures in WSe2 by gating and has demonstrated the potential of applying 2D materials in ultrathin photovoltaic devices.[14] However, this approach is still far from realistic applications since unrealistically large gate fields are required for device operation. Here we chose an alternative way to generate the photovoltaic effect in few-layered CIS without gating. Two different metals were used to form a Schottky contacts (Ti/Au) and an Ohmic contact (Ge/Ag) (refer to supporting information for more discussion), respectively, where the built-in electrical field generated by the Schottky contact was then utilized to separate
Adv. Mater. 2014, DOI: 10.1002/adma.201403342
the photogenerated electron-hole pairs. A similar configuration has also been demonstrated in MoS2-based devices.[13] This device strategy is not only limited to CIS, but can be applied to nearly all layered materials with an observable bandgap, providing a highly useful device architecture for 2D materials in photovoltaic applications. Figure 3a shows the SEM image of the CIS photovoltaic device fabricated on a flake with a thickness of 3–4 atomic layers. The Ti/Au (2 nm Ti, 38 nm Au) and Ge/Ag (2 nm Ge, 38 nm Ag, and 10 nm Au serving as anti-oxidation layer) serve as the anode and cathode, respectively, forming an interdigital configuration with a spacing of ≈200 nm between neighboring electrodes for an effective active area of 3.2 µm2. Since the layered CIS discussed here is an n-type semiconductor (refer to supporting information), the Ti/Au electrode forms a Schottky contact with CIS due to the large work function of Au. Ag tends to form an Ohmic contact with CIS. (Refer to supporting information
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for more discusssions.) Figure 3b shows an IV curve of the device in the dark, which shows an obvious asymmetric character. With a negative bias smaller than 0.5 V, the Schottky barrier between the Ti/Au and CIS is reverse biased, resulting in a small dark current. When the bias is large enough, the Schottky barrier undergoes the breakdown and current increases dramatically. With a positive bias, the Schottky barrier between the Ti/Au electrode and CIS is forward biased, so electrons can flow through the device. Assuming the forward biased current is a thermionic emission process, the height of the Schottky barrier is estimated to be 530 mV (see Supporting Information for calculation). When the device is illuminated with a 150 mW cm–2 543 nm laser, an obvious photovoltaic effect is observed. Figure 3c shows the IV curve of the photovoltaic device shown in Figure 3a. The blue curve is the IV of the as-fabricated device. The green curve represents the output power as a function of photovoltage. Without illumination, the configuration of the band structure of both contacts is shown in the upper part of the Figure 3d. The Schottky barrier between the Au contact and CIS forms the built-in electrical field and the drift current (current driven by the built-in electrical field) is balanced by the diffusion current (current due to thermodynamics). When illuminated, the photo-generated electron-hole pairs are separated by the built-in electrical field to prevent recombination. Electrons move to the Ag electrode (cathode) while the holes move to the Au electrode (anode). Due to the accumulation of electrons at the cathode and holes at the anode, a voltage appears across the device when no net current passes through the device (the open circuit voltage) which for our device is 140 mV. As the output current increases, the voltage drops due to the inner impedance of the device, and the short circuit current is 38 pA, yielding an incident photon-to-current efficiency (IPCE) of 1.7%. The output power (green curve) shows a maximum power of 2.04 pW at a voltage of 85 mV with a current of 24 pA, corresponding to a fill factor of 0.4 and an apparent power efficiency of 0.04%. The efficiency is not as high as the previously reported WSe2 pn junction.[14] However, in the previous report, the realization of the pn junction relies on a large gate voltage, limiting the applicability of the previously reported design. The photovoltaic device reported here eliminates this complication with a much simpler configuration. By optimizing the electrode separation distance, the open circuit voltage can be further improved to 300 mV, which is comparable to the Schottky barrier height (see supporting information). Because of the characteristic length of the effective built-in field by Schottky barrier, a small electrode separation comparable to this characteristic length cannot fully utilize the built-in field of the device. However, a large electrode separation leads to a higher internal impedance and a smaller short circuit current, in other words, the IPCE and the overall power efficiency is undermined. Therefore, to improve the performance of the device, one needs to optimize the channel length accordingly. The Schottky barrier height determines the voltage generated. The depletion length determines the electrode separation. Thus, the overall performance can be improved by selecting an optimal electrode material for forming a large Schottky barrier and by having a proper electrode separation for a full scale utilization of its built-in voltage. Additionally, the contact resistance
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on the Ohmic contact plays a role in the power output of the device, and should be made as small as possible: this also leaves room for further device optimization. In summary, we have successfully synthesized and isolated high quality few-layered flakes of ternary CuIn7Se11 for the first time. We confirm the layered crystal structure of the material using XRD, SEM and TEM analysis, and characterized its photocurrent properties using photoconductivity measurements. Our results indicate that an indirect bandgap of 1.1 eV exists for 3–4 layers of CuIn7Se11. Atomically layered CuIn7Se11 MSMPD with Ti/Au electrodes shows an excellent photoresponse with a low dark current of 1 pA, an S/N larger than 95 dB for 1 mW cm–2 543 nm illumination, a relatively fast response time of 24 ms, and a large linear dynamic range of 70 dB. These properties indicate that atomically layered CuIn7Se11 is a very good candidate for 2D photodetector applications. We have also demonstrated 2D CuIn7Se11 photovoltaic devices based on an asymmetric Schottky junction architecture. By choosing Ti/ Au and Ge/Ag to serve as the anode and cathode, respectively, we observed a 2D photovoltaic effect with a power efficiency of 0.04% and an IPCE of 1.7%. This device performance could be readily improved by optimizing the electrode materials and channel lengths. Our study shows that layered CuIn7Se11 is a highly promising ternary platform for atomically layered optoelectronic device development.
Experimental Section CuIn7Se11 Structure Characterization: The transmission electron microscopy (TEM) study was performed on a JEOL 2100F with 200 kV acceleration electrical field. A CIS bulk crystal was exfoliated into atomically layered flakes in isopropyl alcohol (IPA) for 24 h. Then a TEM grid with lacey carbon film was dipped into the IPA to prepare the sample for TEM study. Scanning electron microscopy study and energy dispersive spectrometry study were performed on a FEI Quanta 400 ESEM. Characterization of CuIn7Se11 Field Effect, Photoconductivity and Photovoltaics: The field effect, photoconductivity and photovoltaics studies were performed using a home-built probe-station under 2 × 10−5 torr. The devices were powered and the photocurrents were measured with a Keithley 6430B. A Newport CornerStone monochrometer was used to measure the photoresponse spectrum. A 543 nm He-Ne laser was utilized as the excitation source for the photoconductivity and photovoltaics studies.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the MURI ARO program, grant number W911NF-11–1–0362, by FAME, one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA, and by Netherlands organization for scientific research (NWO) under the framework of Rubicon program (project number 680-50-1205). This work was also supported by the Robert A. Welch Foundation under Grants C-1220, the National Security Science and
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Received: July 24, 2014 Revised: September 12, 2014 Published online:
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Engineering Faculty Fellowship (NSSEFF) N00244–09–1–0067, and the Office of Naval Research N00014–10–1–0989 .
[18] S. R. Tamalampudi, Y. Y. Lu, U. R. Kumar, R. Sankar, C. D. Liao, B. K. Moorthy, C. H. Cheng, F. C. Chou, Y. T. Chen, Nano Lett. 2014, 14, 2800. [19] B. M. Bas¸ol, V. K. Kapur, C. R. Leidholm, A. Halani, K. Gledhill, Sol. Energy Mater. Sol. Cells 1996, 43, 93. [20] D. Lincot, J. F. Guillemoles, S. Taunier, D. Guimard, J. Sicx-Kurdi, A. Chaumont, O. Roussel, O. Ramdani, C. Hubert, J. P. Fauvarque, N. Bodereau, L. Parissi, P. Panheleux, P. Fanouillere, N. Naghavi, P. P. Grand, M. Benfarah, P. Mogensen, O. Kerrec, Sol. Energy 2004, 77, 725. [21] B. J. Stanbery, Crit. Rev. Solid State Mater. Sci. 2002, 27, 73. [22] U. C. Boehnke, G. Kühn, J. Mater. Sci. 1987, 22, 1635. [23] N. Kohara, S. Nishiawaki, T. Negami, T. Wada, Jpn. J. Appl. Phys. 2000, 39, 6316. [24] G. Eda, T. Fujita, H. Yamaguchi, D. Voiry, M. Chen, M. Chhowalla, ACS Nano 2012, 6, 7311. [25] Y. C. Lin, D. O. Dumcenco, Y. S. Huang, K. Suenaga, Nat. Nanotechnol. 2014, 9, 391. [26] D. Voiry, H. Yamaguchi, J. Li, R. Silva, D. C. Alves, T. Fujita, M. Chen, T. Asefa, V. B. Shenoy, G. Eda, M. Chhowalla, Nat. Mater. 2013, 12, 850. [27] M. Kan, J. Y. Wang, X. W. Li, S. H. Zhang, Y. W. Li, Y. Kawazoe, Q. Sun, P. Jena, J. Phys. Chem. C 2014, 118, 1515. [28] V. Zólyomi, N. D. Drummond, V. I. Fal’ko, Phys. Rev. B 2014, 89, 205416. [29] S. Tongay, W. Fan, J. Kang, J. Park, U. Koldemir, J. Suh, D. Narang, K. Liu, J. Ji, J. Li, R. Sinclair, J. Wu, Nano Lett. 2014, 14, 3185. [30] Y. Zhang, Y. W. Tan, H. L. Stormer, P. Kim, Nature 2005, 438, 201. [31] M. Orlita, C. Faugeras, P. Plochocka, P. Neugebauer, G. Martinez, D. K. Maude, A. L. Barra, M. Sprinkle, C. Berger, W. A. de Heer, M. Potemski, Phys. Rev. Lett. 2008, 101, 267601. [32] Z. Jiang, Y. Zhang, H. L. Stormer, P. Kim, Phys. Rev. Lett. 2007, 99, 106802. [33] X. Fan, Z. Shen, A. Q. Liu, J. L. Kuo, Nanoscale 2012, 4, 2157. [34] X. Li, X. Wang, L. Zhang, S. Lee, H. Dai, Science 2008, 319, 1229. [35] B. Y. Zhang, T. Liu, B. Meng, X. Li, G. Liang, X. Hu, Q. J. Wang, Nat. Commun. 2013, 4, 1811. [36] L. D. Gulay, I. A. Ivashchenko, O. F. Zmiy, I. D. Olekseyuk, J. Alloys Compd. 2004, 384, 121. [37] S. Nomura, S.-i. Ouchi, S. Endo, Jpn. J. Appl. Phys. 1997, 36, L1075.
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Ternary CuIn7Se11: Towards Ultra-Thin Layered Photodetectors and Photovoltaic Devices Sidong Lei, Ali Sobhani, Fangfang Wen, Antony George, Qizhong Wang, Yihan Huang, Pei Dong, Bo Li, Sina Najmaei, James Bellah, Gautam Gupta, Aditya D. Mohite, Liehui Ge,* Jun Lou, Naomi J. Halas,* Robert Vajtai, and Pulickel Ajayan* Since the discovery of graphene in 2004, atomically thin twodimensional (2D) materials have attracted great interest.[1–5] 2D materials research has led to a new, crosscutting frontier of fundamental physics, chemistry, material science, and device engineering. This has resulted in new device architecture and new methods for device fabrication that show great potential in next-generation electronics and optoelectronics. A few layered 2D materials, such as graphene, the transition metal dichalcogenides and the transition metal oxides, have been widely explored for these applications, including field effect transistors (FET)[3,6] with reasonable charge carrier mobilities and high ON/OFF ratios, photodetectors[1,5,7–12] with high quantum yield, and promising photovoltaic devices.[13,14] Efforts have also been devoted to the discovery of new 2D materials for better device fabrication platforms, to meet the requirements of future electronic and optoelectronic technologies. Among all possible applications of 2D materials, photodetectors and photovoltaics have drawn particular attention. In general, 2D materials show excellent optoelectronic properties: for example, MoS2 can exhibit a photoresponsivity as high as 880 A W–1[15] and the quantum efficiency of GaSe can be as high as 1367%.[8] However, most 2D-based photo-detectors with high quantum efficiencies also suffer from large dark currents
S. Lei, Dr. A. George, Q. Wang, Dr. P. Dong, Dr. B. Li, Dr. S. Najmaei, J. Bellah, Dr. L. Ge, Prof. J. Lou, Dr. R. Vajtai, Prof. P. Ajayan Department of Materials Science and Nano-Engineering Rice University Houston, Texas 77005, USA E-mail: [email protected]; [email protected] A. Sobhani, Prof. N. J. Halas Department of Electrical and Computer Engineering Rice University Houston, Texas 77005, USA E-mail: [email protected] F. Wen, Prof. N. J. Halas Department of Chemistry Rice University Houston, Texas 77005, USA Y. Huang, Prof. N. J. Halas Department of Physics and Astronomy Rice University Houston, Texas 77005, USA Dr. G. Gupta, Dr. A. D. Mohite MPA-11 Materials Synthesis and Integrated Devices Los Alamos National Laboratory Los Alamos, New Mexico 87545, USA
DOI: 10.1002/adma.201403342
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and low signal-to-noise ratios (S/N).[15–17] More importantly, the band gaps of these 2D materials are usually too large to cover a wide spectral range. Recently, InSe was reported to have a smaller band gap of 1.4 eV, making it appropriate for the near infrared (NIR) range.[18] But for photovoltaics and photodetectors, especially IR photodetectors, an even smaller band gap would be desirable to expand the spectral range of the response even further. Ternary copper indium selenide (CIS) may provide a solution to these problems. Because of its high photoresponsivity and wide spectral response range, the bulk CIS system has been extensively used in optoelectronics research and industry to serve as a new generation of flexible thin film solar cells.[19–21] CIS exhibits a variety of structures depending on the ratio between Cu and In, including chalcopytite, stannite, wurtzite structures, etc.[22,23] When the ratio of Cu to In lies between 1:5 to 1:9, it forms a layered phase, i.e., γ-phase as defined in earlier studies.[23] 2D layered CIS could potentially inherit the excellent optoelectronic properties of bulk CIS and also be a good candidate material for 2D based optoelectronic devices. The investigation of the ternary CIS compound system may also bring a new dimension to 2D material research. Current research on 2D materials has been almost exclusively focused on single elemental, binary systems or their alloys. Previous studies have shown, however, that elemental composition plays an important role in defining the physical properties of 2D materials. For example, binary systems and their alloys exhibit a variety of behaviors and modification spaces that can be optimized to meet the requirements of applications: the 1T-2H phase transition in transition metal dichalcogenides,[24–27] similar phase transitions in InSe,[28] and the structure of junctions between MoS2 and WS2,[29] are but a few examples. In sharp contrast, modification methods for the properties of single elemental systems are far more limited. Although pristine graphene has a unique band structure and a large charge carrier mobility,[30–32] it is still a challenge to tailor its band structure.[33,34] As a result, graphene FETs show poor on/off performance and graphene photodetectors always possess large dark currents and a relatively weak response.[1,5,35] Multi-elemental systems bring multiple degrees of freedom for controlling physical properties via stoichiometric variation, and as such, the layered ternary CIS system is an ideal platform for this line of research. Here we report the succcessful synthesis and isolation of few-layered CuIn7Se11, as a 2D atomically layered ternary compound. The ratio between Cu to In was determined to be 1:7, consistent with the range where the layered phase forms. The
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Figure 1. Structure characterization of CuIn7Se11 (CIS). a) Crystal structure model of CIS. A single-layered CIS contains 5 layers of metallic ions. The layer thickness of CIS is 1.6 nm. b) Scanning electron microscope (SEM) image of a CIS crystal synthesized by the melting and recrystallization of solid state precursors Cu2Se and In2Se3 (Cu:In = 1:7). The SEM image shows an obvious layered texture. c) High resolution transmission electron microscope (TEM) image of the CIS lattice plane. The electron beam is parallel to the z-axis of CIS lattice. The lattice constant is measured to be 0.40 nm along the a-axis which agrees well with the previously reported value.[36] d) Atomic force microscopy (AFM) height profile of mechanically exfoliated CIS flake. The inset in the upper left corner shows the optical image of the flake under study. The inset in the lower right corner shows the height profile of the exfoliated flake. It reveals a smooth surface of the flake and a thickness of 6 nm, corresponding to 3∼4 CIS layers. e) X-ray diffraction (XRD) pattern of synthesized CuIn7Se11. The strongest peaks are labeled according to previous study.[23]
crystal structure and layered texture was further confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies. Photoconductivity measurements on the few-layered CIS showed a strong photoresponse throughout the visible range of the light spectrum. Few layered CIS photodetectors exhibited a good linear response range and a high S/N. Based on the excellent photo- and spectral-response, we also fabricated and characterized a few-layered CIS photovoltaic device prototype. With two different metal electrodes to serve as anode and cathode, photovoltaic effects were realized with a Schottky junction-based device, which may provide a possible route for the use of layered 2D CIS in actual photovoltaic applications. Figure 1a shows the side view of a model CIS lattice structure.[36] The inter-layer distance in this material is 1.6 nm and the lattice constant along the a-axis is 0.40 nm.[23,36] The crystal
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structure is similar to that of CuIn5Se8, and the deviation in stoichiometry of the CIS is compensated by ordered or disordered vacancies.[37] Based on previous studies of CuSe5Se8, sharing an identical structure with CuSe7Se11, the van der Waals gap for CIS is between Cu and the neighboring In atoms.[23] According to the phase diagram of Cu2Se-In2Se3, at least six different phases exist at high temperatures.[23] When the percentage of In2Se3 is in the range of 82% and 90%, a layered phase (γ-phase) dominates. Thus, CIS crystal was synthesized by mixing Cu2Se (Sigma-Aldrich, 99.95%) and In2Se3 (Alfa Aesar, 99.99%) with a molar ratio of 1:7. This ratio gives large layered crystal size for exfoliation and further study as discussed later. The mixture was then carefully ground in a mortar to produce a uniform solid state mixture and transferred to an evacuated quartz ampoule (1.6 V), the reverse bias Schottky junction breaks down and the dark current increases dramatically (inset of Figure 2d). The low breakdown voltage indicates that the Schottky barrier is not high (our later discussion of the PV effect will provide a more quantitative estimate of the Schottky barrier height). When the CIS MSMPD is illuminated uniformly by a 543 nm (He/Ne) laser with an intensity of 20 mW cm–2 and 50 mW cm–2, the device shows a strong photoresponse, as shown in Figure 2d. With a 2 V bias, the device exhibited a responsivity of 380 mA/W and an external quantum efficiency of 88.0%.(The external quantum efficiency is determined by the formula that η =
I ph E ph × × 100%, where Iph is current dife p
ference between currents without and with illumination, e is charge of electron, Eph is the photon energy, and P is light power shined onto the device). The photoresponse as a function of illumination intensity and bias voltage was also studied and is shown in Figure 2e. The photoresponse shows a linear relationship with illumination intensity ranging from 1 mW cm–2 to 200 mW cm–2, yielding a large linear response range of 70 dB. Figure 2f shows the time-resolved photoconductivity measurement on the few-layered CIS MSMPD. By fitting the time-resolved photoresponse with an exponential decay function (e −(t − t0 )/τ ) , a time constant of 24 ms is obtained. Compared to the previous reports of layered MSMPDs where Schottky junctions were avoided in order to improve the current level of the photodetector,[15] the Schottky contact is utilized here to minimize the dark current to ∼1 pA, and consequently, the device can yield a higher linear response range. A smaller dark current leads to a lower noise level and a higher S/N. As the dark current is generated by random charge-carrier injection and electron-hole pair generation, the noise level (variance) can be estimated by the following formula:
σ 2 = 2e × I d × BW ,
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Figure 2. Photoresponse study of mechanically exfoliated CIS few layered samples. a) Optical image of a 3–4 layered CIS flake photodetector with two Ti/Au electrodes spaced by a 4 µm gap. b) The photocurrent mapping of the as-fabricated CIS photo-detector. The photo-response is uniform across the whole CIS piece, indicating a long lifetime of charge carriers, and an effective electron-hole separation by the external electrical field. c) The photoconductivity spectrum of a 3–4 layered CIS sample with 2 µm electrode spacing indicates an indirect band-gap of 1.1 eV, while a 1–2 layered sample (inset) has a larger band gap of 1.4 eV. d) Photoconductivity IV curves of CIS photodetector (refer to Supporting Information for more details about the devices under study). The dark current is very low due to the Schottky barrier formed between the Ti/Au electrode and CIS flake (Inset shows a barrier breakdown voltage of 1.6 V, indicating a small barrier height). Under an illumination of 20 mW cm–2 and 50 mW cm–2, the CIS photo detector shows an increasing photoresponse, corresponding to an external quantum efficiency of 88%. The photoresponsivity is shown by the black dashed curve. With a 2 V bias, the photoresponsivity is 380 mA W–1. e) The photoresponse is linear as a function of illumination intensity with a 0.1, 1, and 2 V bias. The illumination intensity ranges from 0 to 200 mW cm–2 and the current varies from 1 pA (dark current) to 2.16 nA, yielding a linear response range larger than 70 dB. f) The time-resolved photoresponse measurement illustrates a stable photoresponse performance with a time constant of 24 ms.
device still produced a photocurrent of 0.2 nA. This yields 2 a signal-to-noise ratio (S/N, I photo /σ 2) of 3.1 × 109 or 95 dB, neglecting the quantum noise and Johnson noise. (For commercial silicon photodetectors, S/N can vary from 100 to 1000 dB.) These findings demonstrate that few-layered CIS is a promising candidate for the active material in atomically thin photodetectors, especially when both dark current level and photosensitivity are critical, as in the case of low-level signal photodetectors in spectrometers, portable integrated
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optoelectronic devices, and other applications. Besides photodetectors, photovoltaic devices are another important application for 2D materials, especially for on-chip power sources. Bulk CIS is widely used in flexible thin film photovoltaic devices, so it is reasonable to expect that few-layered CIS may also serve as a good candidate for 2D atomically thin photovoltaics. Also, the excellent photoresponse of the few-layered CIS MSMPD indicates good optical absorption and efficient photocarrier generation that meets the prerequisites of a good photovoltaic material.
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Figure 3. Photovoltaic (PV) study of mechanically exfoliated CIS flake. a) SEM image of the atomically layered CIS PV device under study. The flake has a thickness of ≈3–4 layers. Two electrodes of different materials serve as the anode (Ti/Au) and cathode (Ge/Ag) respectively, with an interdigitated configuration and a spacing of ≈200 nm. The Ti/Au electrode forms a Schottky barrier with the CIS, and the Ge/Ag electrode forms an Ohmic contact. The built-in electrical field can help separate the photo-generated electron-hole pairs and generate photocurrent and photovoltage. b) The dark current of an as-fabricated CIS PV device. The IV curve shows an asymmetric feature under negative and positive biases. With a negative bias, the IV curve shows an obvious non-linear behavior. When the absolute bias is large enough, the Schottky barrier breaks down. On the positive bias branch with a bias larger than 0.5 V, the IV curve has a linear feature, indicating that the device is forward biased and the contact between the Ge/Ag and CIS is Ohmic. c) The IV curve of the CIS PV device working as solar cell. Under 150 mW cm–2 543 nm illumination, the device shows an open circuit voltage of 140 mV and a short circuit current of 38 pA. d) The working mechanism of a CIS Schottky PV device. The top diagram shows the band structure of the device without laser illumination. The CIS forms a Schottky barrier with the Ti/Au electrode and a built-in electrical field on one side, and on the other side, the CIS forms an Ohmic contact with the Ge/Ag. Under illumination (the bottom diagram), the electron-hole pairs are separated by the built-in electrical field of the Schottky barrier. Electrons move toward the Ge/Ag electrode making the potential more negative while holes move toward the Au electrode, resulting in a measurable photovoltage.
A built-in electrical field is essential in photovoltaic devices for the separation of photogenerated electrons and holes. Conventionally, semiconductor technologies rely on a pn junction to generate the built-in electrical field. Thus far, however, it is still a challenge to form a pn junction in a 2D layered material with a good interface. Recently, heterogeneous electrostatic doping has been successful in forming pn junction-like structures in WSe2 by gating and has demonstrated the potential of applying 2D materials in ultrathin photovoltaic devices.[14] However, this approach is still far from realistic applications since unrealistically large gate fields are required for device operation. Here we chose an alternative way to generate the photovoltaic effect in few-layered CIS without gating. Two different metals were used to form a Schottky contacts (Ti/Au) and an Ohmic contact (Ge/Ag) (refer to supporting information for more discussion), respectively, where the built-in electrical field generated by the Schottky contact was then utilized to separate
Adv. Mater. 2014, DOI: 10.1002/adma.201403342
the photogenerated electron-hole pairs. A similar configuration has also been demonstrated in MoS2-based devices.[13] This device strategy is not only limited to CIS, but can be applied to nearly all layered materials with an observable bandgap, providing a highly useful device architecture for 2D materials in photovoltaic applications. Figure 3a shows the SEM image of the CIS photovoltaic device fabricated on a flake with a thickness of 3–4 atomic layers. The Ti/Au (2 nm Ti, 38 nm Au) and Ge/Ag (2 nm Ge, 38 nm Ag, and 10 nm Au serving as anti-oxidation layer) serve as the anode and cathode, respectively, forming an interdigital configuration with a spacing of ≈200 nm between neighboring electrodes for an effective active area of 3.2 µm2. Since the layered CIS discussed here is an n-type semiconductor (refer to supporting information), the Ti/Au electrode forms a Schottky contact with CIS due to the large work function of Au. Ag tends to form an Ohmic contact with CIS. (Refer to supporting information
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for more discusssions.) Figure 3b shows an IV curve of the device in the dark, which shows an obvious asymmetric character. With a negative bias smaller than 0.5 V, the Schottky barrier between the Ti/Au and CIS is reverse biased, resulting in a small dark current. When the bias is large enough, the Schottky barrier undergoes the breakdown and current increases dramatically. With a positive bias, the Schottky barrier between the Ti/Au electrode and CIS is forward biased, so electrons can flow through the device. Assuming the forward biased current is a thermionic emission process, the height of the Schottky barrier is estimated to be 530 mV (see Supporting Information for calculation). When the device is illuminated with a 150 mW cm–2 543 nm laser, an obvious photovoltaic effect is observed. Figure 3c shows the IV curve of the photovoltaic device shown in Figure 3a. The blue curve is the IV of the as-fabricated device. The green curve represents the output power as a function of photovoltage. Without illumination, the configuration of the band structure of both contacts is shown in the upper part of the Figure 3d. The Schottky barrier between the Au contact and CIS forms the built-in electrical field and the drift current (current driven by the built-in electrical field) is balanced by the diffusion current (current due to thermodynamics). When illuminated, the photo-generated electron-hole pairs are separated by the built-in electrical field to prevent recombination. Electrons move to the Ag electrode (cathode) while the holes move to the Au electrode (anode). Due to the accumulation of electrons at the cathode and holes at the anode, a voltage appears across the device when no net current passes through the device (the open circuit voltage) which for our device is 140 mV. As the output current increases, the voltage drops due to the inner impedance of the device, and the short circuit current is 38 pA, yielding an incident photon-to-current efficiency (IPCE) of 1.7%. The output power (green curve) shows a maximum power of 2.04 pW at a voltage of 85 mV with a current of 24 pA, corresponding to a fill factor of 0.4 and an apparent power efficiency of 0.04%. The efficiency is not as high as the previously reported WSe2 pn junction.[14] However, in the previous report, the realization of the pn junction relies on a large gate voltage, limiting the applicability of the previously reported design. The photovoltaic device reported here eliminates this complication with a much simpler configuration. By optimizing the electrode separation distance, the open circuit voltage can be further improved to 300 mV, which is comparable to the Schottky barrier height (see supporting information). Because of the characteristic length of the effective built-in field by Schottky barrier, a small electrode separation comparable to this characteristic length cannot fully utilize the built-in field of the device. However, a large electrode separation leads to a higher internal impedance and a smaller short circuit current, in other words, the IPCE and the overall power efficiency is undermined. Therefore, to improve the performance of the device, one needs to optimize the channel length accordingly. The Schottky barrier height determines the voltage generated. The depletion length determines the electrode separation. Thus, the overall performance can be improved by selecting an optimal electrode material for forming a large Schottky barrier and by having a proper electrode separation for a full scale utilization of its built-in voltage. Additionally, the contact resistance
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on the Ohmic contact plays a role in the power output of the device, and should be made as small as possible: this also leaves room for further device optimization. In summary, we have successfully synthesized and isolated high quality few-layered flakes of ternary CuIn7Se11 for the first time. We confirm the layered crystal structure of the material using XRD, SEM and TEM analysis, and characterized its photocurrent properties using photoconductivity measurements. Our results indicate that an indirect bandgap of 1.1 eV exists for 3–4 layers of CuIn7Se11. Atomically layered CuIn7Se11 MSMPD with Ti/Au electrodes shows an excellent photoresponse with a low dark current of 1 pA, an S/N larger than 95 dB for 1 mW cm–2 543 nm illumination, a relatively fast response time of 24 ms, and a large linear dynamic range of 70 dB. These properties indicate that atomically layered CuIn7Se11 is a very good candidate for 2D photodetector applications. We have also demonstrated 2D CuIn7Se11 photovoltaic devices based on an asymmetric Schottky junction architecture. By choosing Ti/ Au and Ge/Ag to serve as the anode and cathode, respectively, we observed a 2D photovoltaic effect with a power efficiency of 0.04% and an IPCE of 1.7%. This device performance could be readily improved by optimizing the electrode materials and channel lengths. Our study shows that layered CuIn7Se11 is a highly promising ternary platform for atomically layered optoelectronic device development.
Experimental Section CuIn7Se11 Structure Characterization: The transmission electron microscopy (TEM) study was performed on a JEOL 2100F with 200 kV acceleration electrical field. A CIS bulk crystal was exfoliated into atomically layered flakes in isopropyl alcohol (IPA) for 24 h. Then a TEM grid with lacey carbon film was dipped into the IPA to prepare the sample for TEM study. Scanning electron microscopy study and energy dispersive spectrometry study were performed on a FEI Quanta 400 ESEM. Characterization of CuIn7Se11 Field Effect, Photoconductivity and Photovoltaics: The field effect, photoconductivity and photovoltaics studies were performed using a home-built probe-station under 2 × 10−5 torr. The devices were powered and the photocurrents were measured with a Keithley 6430B. A Newport CornerStone monochrometer was used to measure the photoresponse spectrum. A 543 nm He-Ne laser was utilized as the excitation source for the photoconductivity and photovoltaics studies.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the MURI ARO program, grant number W911NF-11–1–0362, by FAME, one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA, and by Netherlands organization for scientific research (NWO) under the framework of Rubicon program (project number 680-50-1205). This work was also supported by the Robert A. Welch Foundation under Grants C-1220, the National Security Science and
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Received: July 24, 2014 Revised: September 12, 2014 Published online:
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Adv. Mater. 2014, DOI: 10.1002/adma.201403342
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