CHEMSUSCHEM COMMUNICATIONS DOI: 10.1002/cssc.201300615
Solution-Processed 2D Niobium Diselenide Nanosheets as Efficient Hole-Transport Layers in Organic Solar Cells Xing Gu, Wei Cui, Tao Song, Changhai Liu, Xiaoze Shi, Suidong Wang, and Baoquan Sun*[a] Thin-layer, two-dimensional NbSe2 nanosheets with lower trap density have been obtained and act as an alternative holetransporting layer to replace MoO3 in organic solar cells. If poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}):[6,6]-phenyl-C71-butyric acid methyl ester acts as an active layer, a power conversion efficiency of 8.10 % has been achieved without any further thermal treatment. The properties of this hole-transporting layer were investigated and the improvements in the devices are discussed.
Two-dimensional (2D) materials were discovered and intensely investigated in the late 1970s and early 1980s.[1] The 2D crystals of transition-metal dichalcogenides (TMDCs) of the MX2 type (M = Mo, W, Nb, Re, Ti, Ta, etc.; X = S, Se, Te) exhibit unique properties when they are well exfoliated and have attracted intensive research attention in recent years in fields ranging from catalysis[2] to nanometer-scale devices.[3] The electronic properties change with the composition of the MX2 material. For example, MoS2 displays typical semiconducting character, whereas NbSe2 and HfS2 exhibit metallic[4] and insulating properties, respectively. Different techniques have been applied to prepare the 2D structures, including bottom-up molecular beam epitaxy,[5] chemical synthesis,[6] and top-down exfoliation.[7] NbSe2 is an example of a TMDC. Scheme 1 b is an illustration of a monolayer of NbSe2 viewed along the [0001] direction. Theoretical calculations based on density functional theory have revealed that NbSe2 is metallic,[8] and the material exhibits specific anisotropic mechanical and electrical properties owing to weak bonding between adjacent layers. NbSe2 single crystals with a layered structure always grow along the direction of the plane and their surface consists of a selenide layer bound to the next one by van der Waals interactions. A perfect surface free of any dangling bonds is assumed to be self-passivated. This is true for thin- or single-layer NbSe2 because cleaving always occurs between selenide layers. These unique properties render them chemically inert, with few trapping states. The work function values of various layered materials are in [a] X. Gu, W. Cui, Dr. T. Song, C. Liu, X. Shi, Prof. S. Wang, Prof. B. Sun Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices Institute of Functional Nano & Soft Materials (FUNSOM) & Collaborative Innovation Center of Suzhou Nano Science and Technology Soochow University, 199 Ren-Ai Road, Suzhou Industrial Park, Suzhou Jiangsu 215123 (PR China) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201300615.
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Scheme 1. a) Schematic illustration of the inverted organic solar cells. A photoactive layer is located between a ZnO-modified indium tin oxide (ITO) cathode and a NbSe2 layer modified Ag-based anode. The active layer is a blend of poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). b) Schematic illustration of the monolayer flake of NbSe2 along the [0001] plane (top view). c) Schematic illustration of the layered structure of thin-layer NbSe2 (side view).
the range of 4.4–5.9 eV.[9] Based on these properties, NbSe2 is an ideal candidate for solar-cell devices. In organic solar cells, a hole-transporting layer (HTL) is used to create asymmetrical interfaces with respect to charge injection and collection. The HTL obtains an efficient hole-extraction capability from the organic materials with deep highest occupied molecular orbital levels in the active layer as well as reducing roughness.[10] Polyaniline,[11] poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)[12] and many transition-metal oxides (TMO; e.g., V2O5, MoO3, WO3, NiOx)[13] are prominent HTL materials; however, the acidity and hygroscopic nature of PEDOT:PSS have negative effects on device performance. In addition, the use of vacuum deposition techniques for TMOs may increase the cost of organic solar cells. Recently, HTLs based on TMOs from low-temperature solution processing have also been reported. These are very promising,[14] but their performances need to be improved compared with traditional HTLs. Alternatively, additional treatment may be indispensible to achieve a high performance. Although ChemSusChem 2014, 7, 416 – 420
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NbSe2 itself is not without toxicity concerns, a comparison of the material safety data sheets of NbSe2 and MoO3 indicates that it is preferable to MoO3. The advantages of easy processing, no need for annealing, high efficiency, and a low density of trap sites make layered materials good candidates for HTL layers.[3h] Thin-layered NbSe2 nanosheets were obtained through chemical exfoliation.[16] The nanosheets were dispersed in isopropanol (IPA) for film fabrication. Figure 1 a shows the UV/Vis spectrum of thin-layered NbSe2 in IPA. The solution is stable over weeks and a photograph of a typical dispersion of exfoli-
Figure 2. a) Scanning electron microscopy (SEM) image of bulk NbSe2. b) A low-resolution transmission electron microscopy (TEM) image of a typical NbSe2 nanosheet. c) A high-resolution TEM image of a NbSe2 nanosheet. Inset: selected-area electron diffraction (SAED) pattern of the single-layered NbSe2 nanosheets. d) Atomic force microscopy (AFM) height image of the NbSe2 nanosheets on the Si/SiO2 substrate.
Figure 1. a) Absorbance spectrum of exfoliated NbSe2 in IPA. Inset: photograph of typical dispersions of layered NbSe2. b) Grazing-incidence X-ray diffraction (GIXD) of the exfoliated NbSe2 nanosheet film and the bulk one on the Si/SiO2 substrate. Synchrotron radiation wavelength of l = 1.2438 with an incident angle of 0.28.
ated NbSe2 is shown in the inset of Figure 1 a; these results indicate a well-dispersed suspension. According to zeta-potential measurements of NbSe2 in Figure S1 in the Supporting Information, dispersed NbSe2 is negatively charged. The zeta-potential value increases dramatically from 6.72 to 41.5 mV after exfoliation. The high surface charge induces a high electrostatic repulsion force between NbSe2 nanosheets, which allows them to disperse well in an alcohol solution. The layered structure was verified by GIXD and microscopic images. A comparison of the GIXD pattern of the exfoliated NbSe2 with that of the bulk sample (Figure 1 b) reveals that only one peak of the (002) plane remains clearly visible, whereas the other peaks decrease after chemical exfoliation; this confirms the layered NbSe2 structure. In addition, these nanosheets lie parallel with the substrate in a preferred orientation. The few-layered structure of NbSe2 was characterized by TEM. Figure 2 b shows a typical single-layered NbSe2 nanosheet in comparison with bulk NbSe2 characterized by SEM in Figure 2 a. The lattice spacing of 2.96 was assigned to the (100) 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
planes (Figure 2 b) determined by high-resolution TEM. The SAED patterns of flat areas of the nanosheets illustrate a hexagonal symmetry of the atomic arrangement and that individual sheet consists of a single-crystal domain (Figure 2 c). The layered structure of NbSe2 was also confirmed by AFM measurements (Figure 2 d). The thicknesses of these well-exfoliated nanosheets were approximately 1.6 nm, as estimated by the cross-section profile (white line in Figure 2 d), and the average flake sizes were around 200–400 nm. The X-ray photoelectron spectroscopy (XPS) results for the Nb 3d and Se 3d core peaks shown in Figure S2 in the Supporting Information were used to explore the phase compositions and elemental statuses. The XPS measurements were performed on both exfoliated NbSe2 thin films (Figure S2 a and b in the Supporting Information) and the bulk NbSe2 sample (Figure S2 c and d). Regarding exfoliated NbSe2 shown in Figure S2 a in the Supporting Information, the doublet with components at 204.5 and 207.2 eV was assigned to Nb4 + ions in NbSe2, whereas the small peak at 203.3 eV was attributed to Nb(4e) + in LieNbSe2 (e is the number of Li ions remaining in NbSe2 during the intercalation process). The two bands at 205.4 and 208.3 eV are ascribed to Nb5 + ions that are correlated with the formation of Nb2O5.[17] Regarding the bulk NbSe2 sample shown in Figure S2 c in the Supporting Information, these peaks are not observed, and therefore, highlight the absence of the Nb5 + ions. Here, partial oxidation may occur in the ultrasonic and the dialysis processes in water. The main composition of NbSe2 nanosheets is not changed. In this study, the NbSe2 HTL was formed by spin-coating within one day and the sample was kept in a glove box to avoid further aging effects. For the evolution of the Se 3d core peak (Figure S2 b and d in the Supporting Information), the doublet at 54.8 and ChemSusChem 2014, 7, 416 – 420
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55.6 eV corresponds to Se2 ions in NbSe2 both in the exfoliated films and in the bulk, which indicates that no selenium is oxidized.[18] In the inverted organic solar cell, the 2D NbSe2 layer is inserted between the active film and the metal anode. The device architecture of the inverted solar cells is shown in Scheme 1. The device is fabricated on the glass substrate coated with patterned ITO. The active layer is a blend of PTB7:PC71BM and ZnO is the cathode buffer layer. The NbSe2 layer was spin-coated twice to obtain a compacted layer. The current density–voltage (J–V) characteristics under simulated air mass (AM) 1.5 G irradiation at 100 mW cm2 are plotted in Figure 3 a. Table 1 summarizes the characteristics of the device parameters, such as the short-circuit current density (JSC), the
Figure 3. J–V curves (a) and external quantum efficiency (EQE) spectra (b) of the devices based on PTB7:PC71BM with different HTLs.
Table 1. Photovoltaic properties of inverted organic solar cells with an active layer of PTB7:PC71BM.[a] HTL
VOC [V]
JSC [mA cm2]
FF
PCE [%]
NbSe2
0.72 0.72 ( 0.01) 0.70 0.70 ( 0.01)
16.04 15.96 ( 0.07) 15.78 15.45 ( 0.29)
0.70 0.69 ( 0.007) 0.68 0.66 ( 0.012)
8.10 7.95 ( 0.08) 7.54 7.14 ( 0.21)
e-MoO3
[a] Data and statistics based on 20 cells of each type. Numbers in bold are the maximum recorded values.
open-circuit voltage (VOC), the fill factor (FF), and the power conversion efficiency (PCE). The NbSe2-based device yields the highest PCE of 8.10 %, a JSC of 16.04 mA cm2, a VOC of 0.72 V, and a FF of 70.0 %. The average PCE is 7.95 % for 20 cells. The devices display improved PCEs compared with those of the reference devices (average PCE: 7.14 %) based on an evaporated molybdenum oxide (e-MoO3 ; 6 nm; Figure 3 a) as the anode HTL. The enhanced device performance can partly be ascribed to the flake structure of this 2D material, which contains fewer traps. The charge recombination is effectively reduced and is discussed in detail later. The corresponding typical EQE spectra of the devices are presented in Figure 3 b. By using an AM 1.5 G solar photon flux spectrum, calculated values of JSC can be generated by integrating the EQE spectrum with the elementary charge. The maximum EQE value is 78.0 % and the calculated JSC is 15.42 mA cm2, which is consistent with the value obtained from the J–V characteristics (Table 1). In our experiment, one substrate controls six pixels, the yield of the device 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
is about 83 %: one bad device in six pixels would happen occasionally; this may be caused by a coverage ratio of less than 100 %, so silver would be directly in contact with the active layer or the thick NbSe2 nanosheets. In addition to the inverted device structure, the NbSe2 interlayer also functions well in the normal device structure based on an active layer of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). The device structure is illustrated and the J–V curves are shown in Figure S3. The electrical output characteristics are summarized in Table S1 in the Supporting Information. In comparison with PEDOT:PSS, the values of JSC and FF are improved. The NbSe2 layers can act as a suitable anode HTL for P3HT:PC61BM and PTB7:PC71BM to replace PEDOT:PSS and TMOs. In addition, the dispensability of an extra annealing process is compatible with the electrodes. The enhanced device performance also excludes the possibility that this arises from the vacuum and annealing process. We are also investigating adding a surfactant to optimize the surface energy of NbSe2 to improve wetting between these layers. Scanning Kelvin probe microscopy (SKPM) is used to characterize the surface potential change directly to identify the energy offset at the interfaces of the donor layer and the HTL. Through characterization of the interfacial dipole upon the incorporation of NbSe2, the capability of NbSe2 as an HTL was evaluated. The sample was prepared by drop-casting a tiny drop of NbSe2 solution on the center of the P3HT film. The P3HT layer was partially covered by a nanosheet film upon drying. P3HT is used as a donor layer because it has the same function in the organic solar cell. It is noted that the thickness of the NbSe2 film is much thicker than the one used in real devices to obtain a clear interface, which is clearly visible and free of cracks. At the edge of the interface area, the topography shows an abrupt bump (Figure S4 a and b in the Supporting Information), however, the surface potential remains stable at the rest of the interface. Therefore, the difference in the surface potential is ascribed to the nature of the two films instead of the contrast in height. According to Figure S4 c and d in the Supporting Information, the surface potential with NbSe2 nanosheets is about 70 mV more negative than that of the P3HT layer. Figure S4 e in the Supporting Information shows a height and potential profile based on data extracted from the crosssection topography shown in Figure S4 a and c. These results display an electric dipole moment with the negatively charged end pointing toward the active film electrode and the positively charged end pointing toward the silver electrode. The generated electrode dipole moment reinforces the actual built-in potential across the device. In addition, it indicates that NbSe2 can be selected as an appropriate HTL with an energy offset of about 70 meV to facilitate hole collection. Transient photovoltage and photocurrent measurements are utilized to determine the steady-state carrier concentration (N) at open circuit by using a procedure known as “differential charging” to verify the assumption that the 2D NbSe2 nanosheets as an anode HTL can decrease the trap state of the device.[19] The perturbation light pulse can generate the transient voltage and photocurrent, as shown in Figure S5 a and b in ChemSusChem 2014, 7, 416 – 420
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the Supporting Information. The capacitance at each VOC can be estimated by using Equations (1) and (2):[18, 19] Z
t
Idt
ð1Þ
C ¼ DQ=DV 0
ð2Þ
DQ ¼
0
in which DQ is the differential charge generated by the same laser pulse with respect to time (t), I is the value of the transient photocurrent, DV0 is the maximum value of the voltage transient (10 mV), and C is the capacitance at each VOC (Figure S5 b in the Supporting Information). By tuning the whitelight intensity from 0.001 to 1 sun, the VOC values of the devices change from 0.20 to 0.70 V. The capacitance versus VOC curves of the devices with the different interface layer and without the interlayer are plotted in Figure 4 a. The device structures were ITO/ZnO/PTB7:PC71BM/Ag, ITO/ZnO/PTB7: PC71BM/e-MoOx/Ag, and ITO/ZnO/PTB7:PC71BM/NbSe2/Ag. The carrier concentration, N, for each open-circuit voltage is then obtained by integrating the capacitance over voltage, according to Equation (3): N¼
1 Aed
Z
Voc
CdV
ð3Þ
device without an interlayer exhibits larger values than those of the others; the device with the NbSe2 HTL displays the lowest capacitances. The different VOC versus C dependence may be explained as a change in the shape of the density of trap states (DOS) of the interface layer. From the plots of N versus VOC shown in Figure 4 b, the NbSe2-based device exhibits a smaller N value of approximately 1016 cm3, whereas the traditional e-MoO3 device is approximately 1017 cm3 ; this indicates that the NbSe2 layer can effectively suppress charge recombination. Additionally, to verify the point that this results from fewer trap states being present on the NbSe2 HTL, instead of a morphology change in the active layer, we further compared the carrier concentration of the active layers with/ without IPA solvent treatment. The C–V and N–V curves are shown in Figure S5 c and d in the Supporting Information. The corresponding electrical output characteristics are also summarized in Table S2 in the Supporting Information. The IPA solvent played a negligible effect on the performance of the device: NbSe2 as the HTL led to a lower trap density, owing to its intrinsic properties instead of a solvent effect. By fitting the transient voltage curves with a monoexponential decay [Eq. (4)]:[19] dV ¼ DV 0 expðt=TÞ
ð4Þ
0
in which A is the device area, e is the electronic charge, d is the film thickness, and N is the carrier concentration. It was observed that, at the same voltage, the capacitance of the devices differed with changes to the interface layer. The
in which DV0 is a constant that fit with the peak height, t is time, and T is the carrier lifetime. The carrier lifetime of the corresponding devices with different interface layers are plotted in Figure 4 c. The device with the NbSe2 HTL displays the longest carrier lifetime (T = 32 ms); thus, the photogenerated charges are efficiently collected by the electrodes and have a positive effect on the performance of the device. The carrier concentration and carrier lifetime described above were estimated and are summarized in Table 2. These results prove that the unique layered structures of the NbSe2 nanosheets with fewer traps are more appropriate for organic solar cells. In summary, we have demonstrated that NbSe2 nanosheets can act as excellent HTLs that are free of any thermal annealing processes in organic solar cells. A PCE of 8.10 %, which is superior to that of traditional vacuum-deposited e-MoO3, has been achieved based on PTB7:PC71BM. According to SKPM measurements and transient electrical output characteristics, the improved performance is due to the unique flake-like 2D structure, which exhibits a lower trap density and the existence of a surface dipole. The layered structure of the HTL provides a suitable electrical dipole for charge extraction and suppresses charge recombination. The well-behaved device further indicates that NbSe2 nanosheets could be a substitute for current Table 2. The carrier concentrations (N) and carrier lifetimes (T) of devices with different interlayers.
Figure 4. The measurements of the transient electrical output characteristics of PTB7:PC71BM devices with different buffer layers. a) Capacitance versus VOC for the PTB7:PC71BM system devices with NbSe2 nanosheets (blue), e-MoOx (black), and without a buffer layer (red). b) N versus VOC for the devices described in (a). c) Transient photovoltage for the devices described in a).
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Device structure
N [cm3]
T [ms]
ITO/ZnO/PTB7:PC71BM/Ag ITO/ZnO/PTB7:PC71BM/e-MoOx/Ag ITO/ZnO/PTB7:PC71BM/NbSe2/Ag
– ~ 1017 ~ 1016
9.39 11.0 32.0
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www.chemsuschem.org Natural Science Foundation of China (61176057, 91123005, 61211130358). We also acknowledge the BL14B station of the Shanghai Synchrotron Radiation Facility (SSRF).
Experimental Section The NbSe2 nanosheets were prepared by a chemical lithium intercalation method.[16] The LixNbSe2 compound was retrieved and washed with hexane twice to remove excess lithium and organic residues. LixNbSe2 was ultrasonicated in water for 1 h, and dialyzed in water for 2 days. The sample was diluted with IPA to a concentration of about 1 mg mL1 for device fabrication. Absorbance spectra were recorded on a Jena SPECORD S600 instrument. A Tecnai G2 F20 transmission electron microscope was operated with an accelerating voltage of 200 kV. The GIXD and 2DGIXD measurements of exfoliated NbSe2 thin films (3 nm on the Si/ SiO2 substrate) were performed at the BL14B station of the Shanghai Synchrotron Radiation Facility (SSRF). SKPM (Veeco, Multimode V) was performed to measure the surface potential of the NbSe2 nanosheets. Zeta potentials were acquired by using a particle size analyzer (Malvern Nano-ZS90). The device architecture of the inverted solar cells is illustrated in Scheme 1 a. The patterned ITO-coated glass substrates were cleaned thoroughly with ethanol, acetone, and water and then they were exposed to UV–ozone for 20 min. The sol–gel ZnO precursor was deposited onto the ITO glass and annealed at 150 8C for 1 h in air as the electron-transport layer. The active layer of PTB7:PC71BM was spin-coated onto the ZnO layer. The weight ratio of PTB7:PC71BM (in chlorobenzene with 3 % 1,8-diiodooctane) was 1:1.5. Then NbSe2 films were deposited by spin-coating onto the active layer at 4000 rpm. Device fabrication was completed by thermal evaporation of 80 nm thick Ag as the anode in vacuo at a base pressure of approximately 1 106 torr (Kurt J. Lesker Mini Spectra). For the device with an e-MoO3 interlayer, e-MoO3 was thermally evaporated at a rate of 0.3 s1 (6 nm). The device area was 8.10 mm2 and strictly patterned by means of a shadow mask. The traditional device architecture was illustrated in Figure S3 in the Supporting Information, the polymer blend of P3HT:PC61BM at a 1:0.8 weight ratio was spin-casted at 700 rpm on a layer of PEDOT:PSS deposited on ITO-coated glass. After thermal annealing at 110 8C for 12 min, device fabrication was completed by thermal evaporation of 100 nm of Al doped with 8 LiF as the cathode. The device performance was characterized under simulated AM 1.5 G irradiation at 100 mW cm2 by using a Xe lamp solar simulator (Newport 91160). The light intensity was calibrated by using a Newport standard silicon solar cell 91150. In the transient electric output characteristic measurements, the devices were connected to a digital oscilloscope (Tektronix TDS 3012C) with an input impedance of 1 MW. The intensity of white light, referred to herein as a “light bias”, was used to control the VOC of the devices. A laser with a wavelength of l = 532 nm was used for optical perturbation, the pulse duration was set to 1 ms, and the frequency was 100 Hz; this resulted in a voltage transient with a peak value of 10 mV < VOC. The frequency, light intensity, and pulse duration were kept constant while the photocurrent transient was measured at an impedance of 50 W.
Acknowledgements This work was supported by the National Basic Research Program of China (973 Program; 2012CB932402) and the National
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Keywords: chalcogens · electrochemistry · niobium · organic solar cells · nanostructures [1] T. Ando, A. B. Fowler, F. Stern, Mod. Phys. 1982, 54, 437 – 672. [2] K. H. Hu, X. G. Hu, Y. F. Xu, X. Z. Pan, React. Kinet. Mech. Catal. 2010, 100, 153 – 163. [3] a) H. Hwang, P. Joo, M. S. Kang, G. Ahn, J. T. Han, B.-S. Kim, J. H. Cho, ACS Nano 2012, 6, 2432 – 2440; b) Y. Zhang, J. Ye, Y. Matsuhashi, Y. Iwasa, Nano Lett. 2012, 12, 1136 – 1140; c) B. Radisavljevic, M. B. Whitwick, A. Kis, ACS Nano 2011, 5, 9934 – 9938; d) T. Kim, H. Kim, S. W. Kwon, Y. Kim, W. K. Park, D. H. Yoon, A.-R. Jang, H. S. Shin, K. S. Suh, W. S. Yang, Nano Lett. 2012, 12, 743 – 748; e) H. Li, Z. Yin, Q. He, H. Li, X. Huang, G. Lu, D. W. H. Fam, A. I. Y. Tok, Q. Zhang, H. Zhang, Small 2012, 8, 63 – 67; f) Z. Yin, H. Li, H. Li, L. Jiang, Y. Shi, Y. Sun, G. Lu, Q. Zhang, X. Chen, H. Zhang, ACS Nano 2012, 6, 74 – 80; g) C. Sire, F. Ardiaca, S. Lepilliet, J.-W. T. Seo, M. C. Hersam, G. Dambrine, H. Happy, V. Derycke, Nano Lett. 2012, 12, 1184 – 1188; h) X. Gu, W. Cui, H. Li, Z. Wu, Z. Zeng, S. T. Lee, H. Zhang, B. Sun, Adv. Energy Mater. 2013, 3, 1262 – 1268. [4] L. Martin, R. Mamy, A. Couget, C. Raisin, Phys. Status Solidi B 1973, 58, 623 – 627. [5] P. P. Chow in Thin Film Processes II (Eds.: J. L. Vossen, W. Kern), Academic Press, San Diego, 1991, pp. 133 – 175. [6] a) M. B. Sigman, Jr., A. Ghezelbash, T. Hanrath, A. E. Saunders, F. Lee, B. A. Korgel, J. Am. Chem. Soc. 2003, 125, 16050 – 16057; b) S. Ithurria, B. Dubertret, J. Am. Chem. Soc. 2008, 130, 16504 – 16505. [7] J. N. Coleman, M. Lotya, A. O’Neill, S. D. Bergin, P. J. King, U. Khan, K. Young, A. Gaucher, S. De, R. J. Smith, Science 2011, 331, 568 – 571. [8] a) A. Kuc, N. Zibouche, T. Heine, Phys. Rev. B 2011, 83, 245213; b) V. V. Ivanovskaya, A. Zobelli, A. Gloter, N. Brun, V. Serin, C. Colliex, Phys. Rev. B 2008, 78, 134104; c) S. Lebgue, O. Eriksson, Phys. Rev. B 2009, 79, 115409. [9] a) J. Wilson, A. Yoffe, Adv. Phys. 1969, 18, 193 – 335; b) T. Shimada, F. S. Ohuchi, B. A. Parkinson, Jpn. J. Appl. Phys. Part 1 1994, 33, 2696 – 2698; c) J. C. McMenamin, W. E. Spicer, Phys. Rev. B 1977, 16, 5474 – 5487; d) G. L. Frey, K. J. Reynolds, R. H. Friend, H. Cohen, Y. Feldman, J. Am. Chem. Soc. 2003, 125, 5998 – 6007. [10] J. Meyer, R. Khalandovsky, P. Gçrrn, A. Kahn, Adv. Mater. 2011, 23, 70 – 73. [11] Y. Yang, A. Heeger, Appl. Phys. Lett. 1994, 64, 1245 – 1247. [12] F. Jonas, W. Krafft, B. Muys, Macromol. Symp. 1995, 100, 169 – 173. [13] a) Y. Yao, C. Shi, G. Li, V. Shrotriya, Q. Pei, Y. Yang, Appl. Phys. Lett. 2006, 89, 153507; b) V. Shrotriya, G. Li, Y. Yao, C. W. Chu, Y. Yang, Appl. Phys. Lett. 2006, 88, 073508. [14] a) Y. J. Lee, J. Yi, G. F. Gao, H. Koerner, K. Park, J. Wang, K. Luo, R. A. Vaia, J. W. Hsu, Adv. Energy Mater. 2012, 2, 1193 – 1197; b) J. J. Jasieniak, J. Seifter, J. Jo, T. Mates, A. J. Heeger, Adv. Funct. Mater. 2012, 22, 2594 – 2605; c) X. Li, W. C. Choy, F. Xie, S. Zhang, J. Hou, J. Mater. Chem. A 2013, 1, 6614 – 6621. [15] G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. Chen, M. Chhowalla, Nano Lett. 2011, 11, 5111 – 5116. [16] D. W. Murphy, F. J. Di Salvo, G. W. Hull, Jr., J. V. Waszczak, Inorg. Chem. 1976, 15, 17 – 21. [17] M. Bahl, J. Phys. Chem. Solids 1975, 36, 485 – 491. [18] G. Wertheim, F. Di Salvo, D. Buchanan, Solid State Commun. 1973, 13, 1225 – 1228. [19] C. Shuttle, B. O’Regan, A. Ballantyne, J. Nelson, D. Bradley, J. De Mello, J. Durrant, Appl. Phys. Lett. 2008, 92, 093311. [20] A. H. Ip, S. M. Thon, S. Hoogland, O. Voznyy, D. Zhitomirsky, R. Debnath, L. Levina, L. R. Rollny, G. H. Carey, A. Fischer, Nat. Nanotechnol. 2012, 7, 577 – 582. Received: June 27, 2013 Revised: October 30, 2013 Published online on January 24, 2014
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Solution-Processed 2D Niobium Diselenide Nanosheets as Efficient Hole-Transport Layers in Organic Solar Cells Xing Gu, Wei Cui, Tao Song, Changhai Liu, Xiaoze Shi, Suidong Wang, and Baoquan Sun*[a] Thin-layer, two-dimensional NbSe2 nanosheets with lower trap density have been obtained and act as an alternative holetransporting layer to replace MoO3 in organic solar cells. If poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}):[6,6]-phenyl-C71-butyric acid methyl ester acts as an active layer, a power conversion efficiency of 8.10 % has been achieved without any further thermal treatment. The properties of this hole-transporting layer were investigated and the improvements in the devices are discussed.
Two-dimensional (2D) materials were discovered and intensely investigated in the late 1970s and early 1980s.[1] The 2D crystals of transition-metal dichalcogenides (TMDCs) of the MX2 type (M = Mo, W, Nb, Re, Ti, Ta, etc.; X = S, Se, Te) exhibit unique properties when they are well exfoliated and have attracted intensive research attention in recent years in fields ranging from catalysis[2] to nanometer-scale devices.[3] The electronic properties change with the composition of the MX2 material. For example, MoS2 displays typical semiconducting character, whereas NbSe2 and HfS2 exhibit metallic[4] and insulating properties, respectively. Different techniques have been applied to prepare the 2D structures, including bottom-up molecular beam epitaxy,[5] chemical synthesis,[6] and top-down exfoliation.[7] NbSe2 is an example of a TMDC. Scheme 1 b is an illustration of a monolayer of NbSe2 viewed along the [0001] direction. Theoretical calculations based on density functional theory have revealed that NbSe2 is metallic,[8] and the material exhibits specific anisotropic mechanical and electrical properties owing to weak bonding between adjacent layers. NbSe2 single crystals with a layered structure always grow along the direction of the plane and their surface consists of a selenide layer bound to the next one by van der Waals interactions. A perfect surface free of any dangling bonds is assumed to be self-passivated. This is true for thin- or single-layer NbSe2 because cleaving always occurs between selenide layers. These unique properties render them chemically inert, with few trapping states. The work function values of various layered materials are in [a] X. Gu, W. Cui, Dr. T. Song, C. Liu, X. Shi, Prof. S. Wang, Prof. B. Sun Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices Institute of Functional Nano & Soft Materials (FUNSOM) & Collaborative Innovation Center of Suzhou Nano Science and Technology Soochow University, 199 Ren-Ai Road, Suzhou Industrial Park, Suzhou Jiangsu 215123 (PR China) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201300615.
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Scheme 1. a) Schematic illustration of the inverted organic solar cells. A photoactive layer is located between a ZnO-modified indium tin oxide (ITO) cathode and a NbSe2 layer modified Ag-based anode. The active layer is a blend of poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). b) Schematic illustration of the monolayer flake of NbSe2 along the [0001] plane (top view). c) Schematic illustration of the layered structure of thin-layer NbSe2 (side view).
the range of 4.4–5.9 eV.[9] Based on these properties, NbSe2 is an ideal candidate for solar-cell devices. In organic solar cells, a hole-transporting layer (HTL) is used to create asymmetrical interfaces with respect to charge injection and collection. The HTL obtains an efficient hole-extraction capability from the organic materials with deep highest occupied molecular orbital levels in the active layer as well as reducing roughness.[10] Polyaniline,[11] poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)[12] and many transition-metal oxides (TMO; e.g., V2O5, MoO3, WO3, NiOx)[13] are prominent HTL materials; however, the acidity and hygroscopic nature of PEDOT:PSS have negative effects on device performance. In addition, the use of vacuum deposition techniques for TMOs may increase the cost of organic solar cells. Recently, HTLs based on TMOs from low-temperature solution processing have also been reported. These are very promising,[14] but their performances need to be improved compared with traditional HTLs. Alternatively, additional treatment may be indispensible to achieve a high performance. Although ChemSusChem 2014, 7, 416 – 420
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NbSe2 itself is not without toxicity concerns, a comparison of the material safety data sheets of NbSe2 and MoO3 indicates that it is preferable to MoO3. The advantages of easy processing, no need for annealing, high efficiency, and a low density of trap sites make layered materials good candidates for HTL layers.[3h] Thin-layered NbSe2 nanosheets were obtained through chemical exfoliation.[16] The nanosheets were dispersed in isopropanol (IPA) for film fabrication. Figure 1 a shows the UV/Vis spectrum of thin-layered NbSe2 in IPA. The solution is stable over weeks and a photograph of a typical dispersion of exfoli-
Figure 2. a) Scanning electron microscopy (SEM) image of bulk NbSe2. b) A low-resolution transmission electron microscopy (TEM) image of a typical NbSe2 nanosheet. c) A high-resolution TEM image of a NbSe2 nanosheet. Inset: selected-area electron diffraction (SAED) pattern of the single-layered NbSe2 nanosheets. d) Atomic force microscopy (AFM) height image of the NbSe2 nanosheets on the Si/SiO2 substrate.
Figure 1. a) Absorbance spectrum of exfoliated NbSe2 in IPA. Inset: photograph of typical dispersions of layered NbSe2. b) Grazing-incidence X-ray diffraction (GIXD) of the exfoliated NbSe2 nanosheet film and the bulk one on the Si/SiO2 substrate. Synchrotron radiation wavelength of l = 1.2438 with an incident angle of 0.28.
ated NbSe2 is shown in the inset of Figure 1 a; these results indicate a well-dispersed suspension. According to zeta-potential measurements of NbSe2 in Figure S1 in the Supporting Information, dispersed NbSe2 is negatively charged. The zeta-potential value increases dramatically from 6.72 to 41.5 mV after exfoliation. The high surface charge induces a high electrostatic repulsion force between NbSe2 nanosheets, which allows them to disperse well in an alcohol solution. The layered structure was verified by GIXD and microscopic images. A comparison of the GIXD pattern of the exfoliated NbSe2 with that of the bulk sample (Figure 1 b) reveals that only one peak of the (002) plane remains clearly visible, whereas the other peaks decrease after chemical exfoliation; this confirms the layered NbSe2 structure. In addition, these nanosheets lie parallel with the substrate in a preferred orientation. The few-layered structure of NbSe2 was characterized by TEM. Figure 2 b shows a typical single-layered NbSe2 nanosheet in comparison with bulk NbSe2 characterized by SEM in Figure 2 a. The lattice spacing of 2.96 was assigned to the (100) 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
planes (Figure 2 b) determined by high-resolution TEM. The SAED patterns of flat areas of the nanosheets illustrate a hexagonal symmetry of the atomic arrangement and that individual sheet consists of a single-crystal domain (Figure 2 c). The layered structure of NbSe2 was also confirmed by AFM measurements (Figure 2 d). The thicknesses of these well-exfoliated nanosheets were approximately 1.6 nm, as estimated by the cross-section profile (white line in Figure 2 d), and the average flake sizes were around 200–400 nm. The X-ray photoelectron spectroscopy (XPS) results for the Nb 3d and Se 3d core peaks shown in Figure S2 in the Supporting Information were used to explore the phase compositions and elemental statuses. The XPS measurements were performed on both exfoliated NbSe2 thin films (Figure S2 a and b in the Supporting Information) and the bulk NbSe2 sample (Figure S2 c and d). Regarding exfoliated NbSe2 shown in Figure S2 a in the Supporting Information, the doublet with components at 204.5 and 207.2 eV was assigned to Nb4 + ions in NbSe2, whereas the small peak at 203.3 eV was attributed to Nb(4e) + in LieNbSe2 (e is the number of Li ions remaining in NbSe2 during the intercalation process). The two bands at 205.4 and 208.3 eV are ascribed to Nb5 + ions that are correlated with the formation of Nb2O5.[17] Regarding the bulk NbSe2 sample shown in Figure S2 c in the Supporting Information, these peaks are not observed, and therefore, highlight the absence of the Nb5 + ions. Here, partial oxidation may occur in the ultrasonic and the dialysis processes in water. The main composition of NbSe2 nanosheets is not changed. In this study, the NbSe2 HTL was formed by spin-coating within one day and the sample was kept in a glove box to avoid further aging effects. For the evolution of the Se 3d core peak (Figure S2 b and d in the Supporting Information), the doublet at 54.8 and ChemSusChem 2014, 7, 416 – 420
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55.6 eV corresponds to Se2 ions in NbSe2 both in the exfoliated films and in the bulk, which indicates that no selenium is oxidized.[18] In the inverted organic solar cell, the 2D NbSe2 layer is inserted between the active film and the metal anode. The device architecture of the inverted solar cells is shown in Scheme 1. The device is fabricated on the glass substrate coated with patterned ITO. The active layer is a blend of PTB7:PC71BM and ZnO is the cathode buffer layer. The NbSe2 layer was spin-coated twice to obtain a compacted layer. The current density–voltage (J–V) characteristics under simulated air mass (AM) 1.5 G irradiation at 100 mW cm2 are plotted in Figure 3 a. Table 1 summarizes the characteristics of the device parameters, such as the short-circuit current density (JSC), the
Figure 3. J–V curves (a) and external quantum efficiency (EQE) spectra (b) of the devices based on PTB7:PC71BM with different HTLs.
Table 1. Photovoltaic properties of inverted organic solar cells with an active layer of PTB7:PC71BM.[a] HTL
VOC [V]
JSC [mA cm2]
FF
PCE [%]
NbSe2
0.72 0.72 ( 0.01) 0.70 0.70 ( 0.01)
16.04 15.96 ( 0.07) 15.78 15.45 ( 0.29)
0.70 0.69 ( 0.007) 0.68 0.66 ( 0.012)
8.10 7.95 ( 0.08) 7.54 7.14 ( 0.21)
e-MoO3
[a] Data and statistics based on 20 cells of each type. Numbers in bold are the maximum recorded values.
open-circuit voltage (VOC), the fill factor (FF), and the power conversion efficiency (PCE). The NbSe2-based device yields the highest PCE of 8.10 %, a JSC of 16.04 mA cm2, a VOC of 0.72 V, and a FF of 70.0 %. The average PCE is 7.95 % for 20 cells. The devices display improved PCEs compared with those of the reference devices (average PCE: 7.14 %) based on an evaporated molybdenum oxide (e-MoO3 ; 6 nm; Figure 3 a) as the anode HTL. The enhanced device performance can partly be ascribed to the flake structure of this 2D material, which contains fewer traps. The charge recombination is effectively reduced and is discussed in detail later. The corresponding typical EQE spectra of the devices are presented in Figure 3 b. By using an AM 1.5 G solar photon flux spectrum, calculated values of JSC can be generated by integrating the EQE spectrum with the elementary charge. The maximum EQE value is 78.0 % and the calculated JSC is 15.42 mA cm2, which is consistent with the value obtained from the J–V characteristics (Table 1). In our experiment, one substrate controls six pixels, the yield of the device 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
is about 83 %: one bad device in six pixels would happen occasionally; this may be caused by a coverage ratio of less than 100 %, so silver would be directly in contact with the active layer or the thick NbSe2 nanosheets. In addition to the inverted device structure, the NbSe2 interlayer also functions well in the normal device structure based on an active layer of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). The device structure is illustrated and the J–V curves are shown in Figure S3. The electrical output characteristics are summarized in Table S1 in the Supporting Information. In comparison with PEDOT:PSS, the values of JSC and FF are improved. The NbSe2 layers can act as a suitable anode HTL for P3HT:PC61BM and PTB7:PC71BM to replace PEDOT:PSS and TMOs. In addition, the dispensability of an extra annealing process is compatible with the electrodes. The enhanced device performance also excludes the possibility that this arises from the vacuum and annealing process. We are also investigating adding a surfactant to optimize the surface energy of NbSe2 to improve wetting between these layers. Scanning Kelvin probe microscopy (SKPM) is used to characterize the surface potential change directly to identify the energy offset at the interfaces of the donor layer and the HTL. Through characterization of the interfacial dipole upon the incorporation of NbSe2, the capability of NbSe2 as an HTL was evaluated. The sample was prepared by drop-casting a tiny drop of NbSe2 solution on the center of the P3HT film. The P3HT layer was partially covered by a nanosheet film upon drying. P3HT is used as a donor layer because it has the same function in the organic solar cell. It is noted that the thickness of the NbSe2 film is much thicker than the one used in real devices to obtain a clear interface, which is clearly visible and free of cracks. At the edge of the interface area, the topography shows an abrupt bump (Figure S4 a and b in the Supporting Information), however, the surface potential remains stable at the rest of the interface. Therefore, the difference in the surface potential is ascribed to the nature of the two films instead of the contrast in height. According to Figure S4 c and d in the Supporting Information, the surface potential with NbSe2 nanosheets is about 70 mV more negative than that of the P3HT layer. Figure S4 e in the Supporting Information shows a height and potential profile based on data extracted from the crosssection topography shown in Figure S4 a and c. These results display an electric dipole moment with the negatively charged end pointing toward the active film electrode and the positively charged end pointing toward the silver electrode. The generated electrode dipole moment reinforces the actual built-in potential across the device. In addition, it indicates that NbSe2 can be selected as an appropriate HTL with an energy offset of about 70 meV to facilitate hole collection. Transient photovoltage and photocurrent measurements are utilized to determine the steady-state carrier concentration (N) at open circuit by using a procedure known as “differential charging” to verify the assumption that the 2D NbSe2 nanosheets as an anode HTL can decrease the trap state of the device.[19] The perturbation light pulse can generate the transient voltage and photocurrent, as shown in Figure S5 a and b in ChemSusChem 2014, 7, 416 – 420
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the Supporting Information. The capacitance at each VOC can be estimated by using Equations (1) and (2):[18, 19] Z
t
Idt
ð1Þ
C ¼ DQ=DV 0
ð2Þ
DQ ¼
0
in which DQ is the differential charge generated by the same laser pulse with respect to time (t), I is the value of the transient photocurrent, DV0 is the maximum value of the voltage transient (10 mV), and C is the capacitance at each VOC (Figure S5 b in the Supporting Information). By tuning the whitelight intensity from 0.001 to 1 sun, the VOC values of the devices change from 0.20 to 0.70 V. The capacitance versus VOC curves of the devices with the different interface layer and without the interlayer are plotted in Figure 4 a. The device structures were ITO/ZnO/PTB7:PC71BM/Ag, ITO/ZnO/PTB7: PC71BM/e-MoOx/Ag, and ITO/ZnO/PTB7:PC71BM/NbSe2/Ag. The carrier concentration, N, for each open-circuit voltage is then obtained by integrating the capacitance over voltage, according to Equation (3): N¼
1 Aed
Z
Voc
CdV
ð3Þ
device without an interlayer exhibits larger values than those of the others; the device with the NbSe2 HTL displays the lowest capacitances. The different VOC versus C dependence may be explained as a change in the shape of the density of trap states (DOS) of the interface layer. From the plots of N versus VOC shown in Figure 4 b, the NbSe2-based device exhibits a smaller N value of approximately 1016 cm3, whereas the traditional e-MoO3 device is approximately 1017 cm3 ; this indicates that the NbSe2 layer can effectively suppress charge recombination. Additionally, to verify the point that this results from fewer trap states being present on the NbSe2 HTL, instead of a morphology change in the active layer, we further compared the carrier concentration of the active layers with/ without IPA solvent treatment. The C–V and N–V curves are shown in Figure S5 c and d in the Supporting Information. The corresponding electrical output characteristics are also summarized in Table S2 in the Supporting Information. The IPA solvent played a negligible effect on the performance of the device: NbSe2 as the HTL led to a lower trap density, owing to its intrinsic properties instead of a solvent effect. By fitting the transient voltage curves with a monoexponential decay [Eq. (4)]:[19] dV ¼ DV 0 expðt=TÞ
ð4Þ
0
in which A is the device area, e is the electronic charge, d is the film thickness, and N is the carrier concentration. It was observed that, at the same voltage, the capacitance of the devices differed with changes to the interface layer. The
in which DV0 is a constant that fit with the peak height, t is time, and T is the carrier lifetime. The carrier lifetime of the corresponding devices with different interface layers are plotted in Figure 4 c. The device with the NbSe2 HTL displays the longest carrier lifetime (T = 32 ms); thus, the photogenerated charges are efficiently collected by the electrodes and have a positive effect on the performance of the device. The carrier concentration and carrier lifetime described above were estimated and are summarized in Table 2. These results prove that the unique layered structures of the NbSe2 nanosheets with fewer traps are more appropriate for organic solar cells. In summary, we have demonstrated that NbSe2 nanosheets can act as excellent HTLs that are free of any thermal annealing processes in organic solar cells. A PCE of 8.10 %, which is superior to that of traditional vacuum-deposited e-MoO3, has been achieved based on PTB7:PC71BM. According to SKPM measurements and transient electrical output characteristics, the improved performance is due to the unique flake-like 2D structure, which exhibits a lower trap density and the existence of a surface dipole. The layered structure of the HTL provides a suitable electrical dipole for charge extraction and suppresses charge recombination. The well-behaved device further indicates that NbSe2 nanosheets could be a substitute for current Table 2. The carrier concentrations (N) and carrier lifetimes (T) of devices with different interlayers.
Figure 4. The measurements of the transient electrical output characteristics of PTB7:PC71BM devices with different buffer layers. a) Capacitance versus VOC for the PTB7:PC71BM system devices with NbSe2 nanosheets (blue), e-MoOx (black), and without a buffer layer (red). b) N versus VOC for the devices described in (a). c) Transient photovoltage for the devices described in a).
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Device structure
N [cm3]
T [ms]
ITO/ZnO/PTB7:PC71BM/Ag ITO/ZnO/PTB7:PC71BM/e-MoOx/Ag ITO/ZnO/PTB7:PC71BM/NbSe2/Ag
– ~ 1017 ~ 1016
9.39 11.0 32.0
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www.chemsuschem.org Natural Science Foundation of China (61176057, 91123005, 61211130358). We also acknowledge the BL14B station of the Shanghai Synchrotron Radiation Facility (SSRF).
Experimental Section The NbSe2 nanosheets were prepared by a chemical lithium intercalation method.[16] The LixNbSe2 compound was retrieved and washed with hexane twice to remove excess lithium and organic residues. LixNbSe2 was ultrasonicated in water for 1 h, and dialyzed in water for 2 days. The sample was diluted with IPA to a concentration of about 1 mg mL1 for device fabrication. Absorbance spectra were recorded on a Jena SPECORD S600 instrument. A Tecnai G2 F20 transmission electron microscope was operated with an accelerating voltage of 200 kV. The GIXD and 2DGIXD measurements of exfoliated NbSe2 thin films (3 nm on the Si/ SiO2 substrate) were performed at the BL14B station of the Shanghai Synchrotron Radiation Facility (SSRF). SKPM (Veeco, Multimode V) was performed to measure the surface potential of the NbSe2 nanosheets. Zeta potentials were acquired by using a particle size analyzer (Malvern Nano-ZS90). The device architecture of the inverted solar cells is illustrated in Scheme 1 a. The patterned ITO-coated glass substrates were cleaned thoroughly with ethanol, acetone, and water and then they were exposed to UV–ozone for 20 min. The sol–gel ZnO precursor was deposited onto the ITO glass and annealed at 150 8C for 1 h in air as the electron-transport layer. The active layer of PTB7:PC71BM was spin-coated onto the ZnO layer. The weight ratio of PTB7:PC71BM (in chlorobenzene with 3 % 1,8-diiodooctane) was 1:1.5. Then NbSe2 films were deposited by spin-coating onto the active layer at 4000 rpm. Device fabrication was completed by thermal evaporation of 80 nm thick Ag as the anode in vacuo at a base pressure of approximately 1 106 torr (Kurt J. Lesker Mini Spectra). For the device with an e-MoO3 interlayer, e-MoO3 was thermally evaporated at a rate of 0.3 s1 (6 nm). The device area was 8.10 mm2 and strictly patterned by means of a shadow mask. The traditional device architecture was illustrated in Figure S3 in the Supporting Information, the polymer blend of P3HT:PC61BM at a 1:0.8 weight ratio was spin-casted at 700 rpm on a layer of PEDOT:PSS deposited on ITO-coated glass. After thermal annealing at 110 8C for 12 min, device fabrication was completed by thermal evaporation of 100 nm of Al doped with 8 LiF as the cathode. The device performance was characterized under simulated AM 1.5 G irradiation at 100 mW cm2 by using a Xe lamp solar simulator (Newport 91160). The light intensity was calibrated by using a Newport standard silicon solar cell 91150. In the transient electric output characteristic measurements, the devices were connected to a digital oscilloscope (Tektronix TDS 3012C) with an input impedance of 1 MW. The intensity of white light, referred to herein as a “light bias”, was used to control the VOC of the devices. A laser with a wavelength of l = 532 nm was used for optical perturbation, the pulse duration was set to 1 ms, and the frequency was 100 Hz; this resulted in a voltage transient with a peak value of 10 mV < VOC. The frequency, light intensity, and pulse duration were kept constant while the photocurrent transient was measured at an impedance of 50 W.
Acknowledgements This work was supported by the National Basic Research Program of China (973 Program; 2012CB932402) and the National
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