MoS2 Quantum Dots with a Tunable Work Function for High

Sep 20, 2016 - An efficient hole extraction layer (HEL) is critical to achieve high-performance organic solar cells (OSCs). ...... Sun , Y.; Takacs , ...
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MoS2 Quantum Dots with a Tunable Work Function for HighPerformance Organic Solar Cells Wang Xing,† Yusheng Chen,† Xinlong Wang,† Lei Lv,† Xinhua Ouyang,‡ Ziyi Ge,‡ and Hui Huang*,† †

College of Materials Science and Optoelectronic Technology & CAS Key Laboratory of Vacuum Physics, University of Chinese Academy of Sciences, Yanqi Lake, Huairou District, Beijing 101408, China ‡ Ningbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences (CAS), Ningbo 315201, China S Supporting Information *

ABSTRACT: An efficient hole extraction layer (HEL) is critical to achieve high-performance organic solar cells (OSCs). In this study, we developed a pinhole-free and efficient HEL based on MoS2 quantum dots (QDs) combined with UV−ozone (UVO) treatment. The optophysical properties and morphology of MoS2 QDs and their photovoltaic performance are investigated. The results showed that MoS2 QDs can form homogeneous films and can be applied as an interfacial layer not only for donors with shallow highest occupied molecular orbital (HOMO) but also for those with deep HOMO energy levels after UVO treatment (O-MoS2 QDs). The solar cells based on O-MoS2 QDs yield a power conversion efficiency (PCE) of 8.66%, which is 71% and 12% higher than those of the OSCs with pristine MoS2 QD and O-MoS2 nanosheets, respectively, and the highest PCEs for OSCs containing MoS2 materials. Furthermore, the stability of solar cells based on MoS2 QDs is greatly improved in comparison with state-of-the-art PEDOT:PSS. These results demonstrate the great potential of O-MoS2 QDs as an efficient HEL for highperformance OSCs. KEYWORDS: MoS2 quantum dots, pinhole free, UV−ozone treatment, tunable work function, hole extraction layer, organic solar cells

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INTRODUCTION Bulk heterojunction (BHJ) organic solar cells (OSCs) have attracted great attention in both academic and industrial fields for their low-cost fabrication, flexibility, light weight, and ease of processability through roll-to-roll coating.1,2 A typical BHJ structure includes electrodes for collecting charges, active layer for light absorption, and interfacial layers for electron and hole extraction.3 Both hole extraction layer (HEL) and electron extraction layer (EEL) play essential roles in solar cell performance. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)4 is the most used HEL material for OSCs, while its acidity and hygroscopicity5,6 is detrimental to device stability and hinders its practical applications. Thus, PEDOT:PSS-free HEL materials such as vanadium oxide (Va2O5),7 molybdenum oxide (MoO3),8 nickel oxide (NiO),9 tungsten oxide (WO3),10 etc., arouse numerous research interests. However, the fabrication of these materials generally involves vacuum deposition technique, in which harsh conditions and expensive facilities as well as complex processing © 2016 American Chemical Society

steps are required. Furthermore, some amorphous structures containing dangling bonds produced during vacuum deposition process can react with organic materials and decrease OSC performance.11,12 Recently, two-dimensional nanomaterials including graphene oxide13−15 and transition metal dichalcogenides (TMDs)16−25 have been used as HEL for OSCs because of their unique lamellar and electrical structures. TMD materials can be obtained by exfoliation,18 spray coating,22 lithium intercalation,20,23 and a hydrothermal process24 of the sulfide precursor. However, they suffer from structure defects or mixed phases,20 which is detrimental to the performance of OSCs. Furthermore, inevitable precipitation occurs in the solution within a short storage period of time,23 which is unfavorable for spin-coating or roll-to-roll solution process. Also, pinholes can be found in Received: May 21, 2016 Accepted: September 20, 2016 Published: September 20, 2016 26916

DOI: 10.1021/acsami.6b06081 ACS Appl. Mater. Interfaces 2016, 8, 26916−26923

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the active layer under a pressure of 1 × 10−6 Torr. For comparison, 4,4-(((methyl(4-sulfonatobutyl)ammonio)bis(propane-3,1-diyl))bis(dimethylammoniumdiyl))bis(butane-1-sulfonate) (MSAPBS) was used to replace Ca. The blended solution of MSAPBS in the mixing solvent of methanol and acetic acid (methanol:acetic acid = 97:3, vol/ vol) was spun on the active layer at 4000 rpm for 30 s. All J−V characterizations were performed under AM 1.5 G using a Newport solar simulator. The effective area for the cells is 4 mm2. Characterization. Atomic force microscopy (AFM, Veeco) was used to characterize the morphology of MoS2. MoS2 solution in DMF was deposited on a silicon wafer by spin coating and drying under ambient conditions. An AFM image was obtained in a tapping mode. Different concentrations of MoS2 QDs solution were used to achieve different thicknesses deposited on SiO2/Si substrate. Scratches were intentionally made to confirm the thickness by AFM. A Fourier transform infrared (FTIR) transmittance spectrum of O-MoS2 QDs was recorded using a Thermo Fisher Scientific Nicolet iS50 research spectrometer. Transmission electron microscopy (TEM, FEI Tecnai G2 F20 S-TWIN) was conducted at an accelerating voltage of 200 kV. Samples were analyzed on Thermo Scientific ESCALab 250Xi using UPS. A gas discharge lamp was used for UPS, with helium gas admitted and the HeI (21.22 eV) emission line employed. The helium pressure in the analysis chamber during analysis was about 2 × 10−8 mbar. The data were acquired with −10 V bias. X-ray photoelectron spectroscopy (XPS) was performed on the Thermo Scientific ESCALab 250Xi using 200 W monochromated Al Kα radiation. The 500 μm X-ray spot was used for XPS analysis. The base pressure in the analysis chamber was about 3 × 10−10 mbar. Typically the hydrocarbon C 1s line at 284.8 eV from adventitious carbon is used for energy referencing.

HEL films based on these two-dimensional nanomaterials, which allows current leakage and decreases open circuit voltage.15,16 It has been reported that quantum dot (QD) materials such as PbS,26,27 TiO2,28 and graphene QDs29−32 can form pinhole-free and thickness-controllable layers by using the spin-coating approach. Among them, MoS2 QDs are widely used for electrochemical and photochemical hydrogen evolution reaction (HER) as well as bioimaging.33−37 The small size of QDs guarantees an excellent film-forming capability.30,31 Furthermore, the MoS2 QDs with nanometer size and few layers can improve the conductivity.37 However, MoS2 QDs have never been employed for HEL of OSCs, because their low work functions hinder them from forming ohmic contact with active layers. In the present work, we report that UV−ozone (UVO)treated MoS2 (O-MoS2) QDs with a tunable work function can be used as efficient HEL materials for high-performance OSCs. It is found that the work function of MoS2 QDs can be tuned with UVO treatment, which increases from 4.4 eV for pristine MoS2 QDs, to 4.9 eV after 8 min, to 5.2 eV after 30 min of UVO treatment. This offers an opportunity for O-MoS2 QDs to be an optimal HEL for OSCs with different donor materials, P3HT and PTB7-Th. The efficiencies of P3HT- and PTB7-Thbased solar cells can be tuned by the time of UVO treatment of MoS2 QDs. As a result, we could achieve a power conversion efficiency (PCE) of 8.7% for PTB7-Th-based OSCs, which is 71% and 12% higher than those of the OSCs with pristine MoS 2 QD and O-MoS 2 nanosheets, respectively, and comparable to the PCE (8.6%) of OSCs with PEDOT:PSS as the HEL. To the best of our knowledge, the PCE is the highest value for OSCs containing MoS2 materials. 16 Furthermore, the stability of solar cells is dramatically improved due to replacement of hygroscopic and acidic PEDOT:PSS with O-MoS2 QDs.

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RESULTS AND DISCUSSION MoS2 QDs were synthesized through sonication together with solvothermal treatment of MoS2 nanosheets.33 The successful synthesis of MoS2 QDs was evidenced with substantial characterizations. TEM and AFM were employed to characterize the morphology of the prepared MoS2 QDs. In the high

EXPERIMENTAL SECTION

Preparation of MoS2 QDs and MoS2 Nanosheets. For preparation of MoS2 QDs: typically, 2 g of MoS2 powder and 200 mL of DMF were mixed for 5 min by magnetic stirring. Then the resulting mixture underwent ice sonication with an output power of 150 W for 4 h to exfoliate MoS2, and the resulting suspension settled overnight without disturbance. The top 2/3 of the dispersion was decanted into flask, and vigorous stirring was maintained for 6 h at 140 °C. Afterward the suspension was centrifuged for 1 h at 10000 rpm to separate the sediment from supernatant. The light yellow supernatant was collected, and the solvent was removed under vacuum to afford a brown MoS2 QDs solid. Similarly, a suspension of 2 g of MoS2 powder and 200 mL of N-vinyl-2-pyrrolidone (NVP) was sonicated for 8 h and then centrifuged for 20 min at 10000 rpm to get a suspension of MoS2 nanosheets. The suspension was then vacuum-dried and dispersed in DMF at 2 mg/mL. Fabrication and Characterization of OSCs. The indium tin oxide (ITO)-coated glass substrates were cleaned sequentially with detergent water, deionized water, acetone, hexane, and 2-propanol with sonication assistance, followed by exposure to UVO treatment for 30 min. The next step is one of the following two. (1) The PEDOT:PSS was spun coated on ITO substrate at 4000 rpm for 40 s, followed by heating at 140 °C for 20 min. (2) MoS2 QD or MoS2 nanosheet solution in DMF (2 mg/mL) was spun coated at 2000 rpm for 60 s, followed by annealing at 150 °C for 20 min in glovebox. Afterward, the substrates were subjected to UVO treatment.18,22 The active layer (PTB7-Th:PC71BM with weight ratio 1:1.5 (10 mg/mL of PTB7-Th in chlorobenzene with 3 vol % 1,8-diiodooctane) or P3HT:PC61BM with weight ratio 1:1 (10 mg/mL of P3HT in chlorobenzene)) was spun coated on top of the HEL layer. Finally, the Ca (20 nm) and cathode Al (100 nm) were thermally deposited onto

Figure 1. Microstructure of the prepared MoS2 QDs: (a) The highresolution transmission electron microscopy (HRTEM) image of the prepared MoS2 QDs. The inset shows the highly crystallized lattice. (b) Selected area electron diffraction (SAED) of the prepared MoS2 QDs. (c) Histogram of particle size distribution of MoS2 QDs. (d) Atom force microscopy (AFM) of MoS2 QDs. The bottom of panel d is the height profile corresponding to the line shown in the AFM image. 26917

DOI: 10.1021/acsami.6b06081 ACS Appl. Mater. Interfaces 2016, 8, 26916−26923

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Figure 4. (a) Molecular structures of P3HT, PTB7-Th, PC61BM, and PC71BM. (b) UPS spectra for MoS2 QDs, and O-MoS2 QDs with UVO treatment for 8 min (O-MoS2 QD-8 min) and for 30 min (OMoS2 QD-30 min). (c) Energy levels for both P3HT:PC61BM and PTB7-Th:PC71BM systems. The work function for O-MoS2 nanosheets is cited from ref 18.

Figure 2. (a) The UV−vis absorption spectrum of MoS2 QDs and MoS2 nanosheets in DMF with a concentration of 0.2 mg/mL. (b) The UV−vis transmittance spectra of pure PEDOT:PSS (30 nm), pristine MoS2 QDs, MoS2 nanosheets, O-MoS2 QDs, and O-MoS2 nanosheets (3 nm) with UVO treatment for 30 min.

Figure 5. AFMs of the (a) ITO, (b) MoS2 QDs, (c) MoS2 nanosheets, (d) PEDOT:PSS, (e) O-MoS2 QDs, and (f) O-MoS2 nanosheets films deposited on ITO. The scale is 5 × 5 μm2.

(Figure 1d) showed that the thickness of MoS2 QDs is in the range of 0.5−2.5 nm, suggesting that the prepared MoS2 QDs have one or several layered structures. In contrast, the AFM image (Figure S1) showed that the thickness for the MoS2 nanosheets is also in the range of 0.5−2.5 nm but the lateral size is about several hundred nanometers. All these results are consistent with those reported by Xu et al.33 and Kim et al.,18 demonstrating that MoS2 QDs and MoS2 nanosheets are successfully prepared. Figure 2a shows the UV−vis absorption spectrum of MoS2 QDs and nanosheets in DMF solution. For MoS2 nanosheets, there are four characteristic peaks located at 410, 459, 613, and 665 nm, which is consistent with those reported by Xu et al.33 The distinct peaks at 410 and 459 nm arise from the direct transition from the deep valence band to the conduction band, while the peaks at 613 and 665 nm are attributed to the K point of the Brillouin zone.34,38 Because of the quantum size effect, the optical absorption of low-dimensional MoS2 QDs shows a strong blue shift when the lateral dimension of MoS2 is reduced to O-MoS2 nanosheets > O-MoS2 QDs > MoS2 nanosheets > PDOT:PSS

Table 1. Photovoltaic Parameters of OSCs Based on Structure ITO/HEL/Active Layer/EEL/Al active layer

EEL

P3HT:PC61BM

Ca

PTB7-Th: PC71BM

Ca

PTB7-Th: PC71BM

MSAPBS

HEL

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

no HEL PEDOT:PSS MoS2 QDs O-MoS2 QDs no HEL PEDOT:PSS MoS2 QDs O-MoS2 QDs PEDOT:PSS MoS2 nanosheet O-MoS2 nanosheet MoS2 QDs O-MoS2 QDs

0.54 0.56 0.32 0.57 0.65 0.74 0.55 0.78 0.79 0.63 0.79 0.63 0.79

5.71 8.21 6.08 7.28 15.95 16.53 14.58 16.74 18.15 12.50 15.48 14.01 16.90

59.5 59.1 42.3 63.4 56.6 62.5 53.1 59.2 59.8 53.2 63.6 57.0 65.0

1.82 2.70 0.81 2.62 5.83 7.63 4.23 7.71 8.56 4.18 7.76 5.09 8.66

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QDs and O-MoS2 nanosheets, the smoother surface of O-MoS2 QDs can form better contact with the active layer, which is beneficial for charge transport43 and JSC and consistent with the photovoltaic performances as discussed below. PEDOT:PSS, MoS2 QDs, O-MoS2 QDs, MoS2 nanosheets, and O-MoS2 nanosheets were employed as HELs to construct solar cells with a structure of ITO/HEL/active layer/ Ca(MSAPBS)/Al. P3HT and PTB7-Th were chosen as two donors because they have different HOMO energy levels to probe the tunable work function properties of MoS2 QDs. The structures and energy levels of the compounds are shown in Figure 4. P3HT:PC61BM-based solar cells were first investigated to understand the properties of the different HEL materials. The typical J−V curves are shown in Figure 6, and the photovoltaic parameters are summarized in Table 1. The solar cells without HEL afford an efficiency of 1.81%, which is comparable to the reported results.44 PEDOT:PSS was used as the HEL for solar cells and dramatically increased the efficiency to 2.70%, because PEDOT:PSS helps to form an ohmic contact with P3HT. When MoS2 QDs were used an HEL, the efficiency was only 0.81% with Jsc =6.08 mA/mm2, VOC = 0.32 V, and FF = 42.3%. This is because the large mismatch between the work function of MoS2 QDs (4.4 eV) and the HOMO energy levels of P3HT (−5.0 eV), resulting in a Schottky contact between MoS2 QDs and P3HT layer, seriously limited the buildup of VOC and decreased the device performance. To form an ohmic contact with active layer, the MoS2 QDs film was treated with UVO to increase the work function. The effects of the time of UVO treatment on the device performance were investigated as shown in Figure 6a. It was observed that the efficiency continuously increased from 2.1% to 2.62% (Jsc =7.28 mA/ mm2, VOC = 0.57 V, and FF = 63.4%) with the increase of the time of the UVO treatment from 3 to 8 min. The efficiency is comparable to that of the PEDOT:PSS-based solar cells (2.70%), which indicates that an ohmic contact formed between O-MoS2 QDs and the P3HT active layer, because the work function of O-MoS2 QDs (4.9 eV) matched the HOMO energy levels of P3HT (−5.0 eV). When MoS2 was treated with UVO for longer time (>9 min), the efficiency of the solar cells dropped slightly due to the energy barrier between O-MoS2 (>5 eV) and P3HT (∼5 eV) and possible poor hole extraction. To evaluate the dependence of PCE on the thickness of MoS2 QDs, devices with different thicknesses of MoS2 QDs (Figure S3) with 8 min of UVO treatment were fabricated. Figure 6c shows the dependence of PCE on thickness, and the photovoltaic parameters are gathered in Table S1. It shows that when the thickness is smaller than 6 nm, the PCE almost stayed same with different thicknesses of HEL. However, when the thickness increases to 9 nm, the PCE decreases dramatically from 2.62% to 1.86% along with the decrease of JSC and FF. This may be ascribed to the increase of serial resistance (Table S1). To further investigate the tunable work function properties of MoS2 QDs and its differences from MoS2 nanosheets, PTB7Th,45−47 a highly efficient donor material with a deeper HOMO level (−5.4 eV), was chosen to construct solar cells with a structure of ITO/HEL/active layer/Ca/Al. The typical J−V curves are shown in Figure 7, and the photovoltaic parameters are summarized in Table 1. The solar cells with PEDOT:PSS as HEL afford an efficiency of 7.37%, comparable to the reported data.48 When pristine MoS2 QDs were employed as the HEL, the efficiency of solar cells dropped to

Figure 7. (a) Dependence of PCE of devices based on PTB7Th:PC71BM on UVO treatment time. (b) J−V curves of devices based on PTB7-Th:PC71BM using PEDOT:PSS, MoS2 QDs, and O-MoS2 QDs as HEL and Ca as EHL, respectively. (c) J−V curves of devices based on PTB7-Th:PC71BM using PEDOT:PSS, MoS2 QDs, O-MoS2 QDs, MoS2 nanosheets, and O-MoS2 nanosheets and MSAPBS as EEL.

in Figure 5. The root-mean-square (RMS) roughness of ITO is 1.89 nm. After the deposition of PEDOT:PSS, the RMS decreases to 0.88 nm, smoother than the surface of ITO, which is beneficial to forming a better contact with the active layer. In the case of MoS2 QDs film, large dots with sizes of tens of nanometers are observed in Figure 5b. This is possibly because MoS2 QDs are self-assembled to form aggregates on ITO, which were also observed by Wang et al.38 The MoS2 QDs film has an RMS of 1.31 nm, lower than that of pristine ITO. In contrast, the MoS2 nanosheets film has an RMS of 2.27 nm, which is the largest and is detrimental for good contact with the active layer. After UVO treatment for 30 min, the RMS of OMoS2 QDs and O-MoS2 nanosheets further decreases to 1.19 and 2.03 nm, respectively, which may originate from the incorporation of O oxygen atoms.22 In comparison to the MoS2 26920

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Figure 8. Stability of devices using PTB7-Th:PC71BM and Ca as active layer and EEL, respectively: (a) normalized PCE; (b) normalized JSC; (c) normalized VOC; (d) normalized FF variations upon keeping time in air.

morphology of the films, which is supported by the AFM results. The external quantum efficiency (EQE) spectra of devices with different HELs are shown in Figure S5 and Table S2. The calculated photocurrent of device increases from 14.5 mA/cm2 for MoS2 QDs to 17.2 mA/cm2 for O-MoS2 QDs, which are consistent with those from J−V curves. The stability of the O-MoS2 QDs and PEDOT:PSS-based solar cells without encapsulation was studied in the air at 25 °C and about 30% humidity. Figure 8a−d shows the normalized PCE (a) and other parameters including JSC (b), VOC (c), and FF (d) on the basis of the time in air. As observed, the PCE of PEDOT:PSS-based solar cells decreased to about half of the original value after 8 days. In comparison with PEDOT:PSS, the O-MoS2 QDs can effectively extend the stability of the solar cells. After 47 days, the PCE of O-MoS2 QD-based solar cells still maintained 64%. Regarding VOC and FF, no significant changes are found with extending the time in both O-MoS2 QDs and PEDOT:PSS systems. However, JSC decreased in both systems and dropped much faster in the PEDOT:PSS system. It was documented that the acidity and hygroscopicity of PEDOT:PSS can damage both the PEDOT:PSS/ITO surface and itself, resulting in increased series resistance. Therefore, it is concluded that the difference in device degradation between OMoS2 QDs and PEDOT:PSS systems is ascribed to the interface degradation.18 These results demonstrated that replacement of PEDOT:PSS with O-MoS2 QDs can dramatically extend the stability of solar cells.

4.23% with JSC = 14.58 mA/cm2, VOC = 0.55 eV, and FF = 53.1%, which indicates that a Schottky contact formed between MoS2 QDs and the PTB7-Th active layer, harmful for solar cell performance. The MoS2 QDs that were treated with UVO increase the work function and reduce the roughness for a better contact with the active layer. Figure 7a shows the relationship between the efficiency of the solar cells and the time of UVO treatment. It was observed that the solar cells with MoS2 QDs treated with 30 min of UVO afforded the highest PCE of 7.71% with JSC = 16.74 mA/cm2, VOC = 0.78 eV, and FF = 59.2%. The dramatic improvement in VOC and efficiency are ascribed to an ohmic contact between O-MoS2 QDs and active layer, rooted from the well-matched work function of O-MoS2 QDs (5.2 eV) and HOMO energy level of PTB7-Th (−5.4 eV) and the reduced roughness of O-MoS2 QDs as studied with UPS and AFM. MSAPBS is a nonconjugated small-molecule EEL that was employed to enhance OSCs performance.49 When MSAPBS was used to replace Ca, the PCE of solar cells with a structure of ITO/O-MoS2 QDs/active layer/MSAPBS/ Al further increased to 8.66% with JSC = 16.90 mA/cm2, VOC = 0.79 eV, and FF = 65.0%, which is the highest PCE for OSCs containing MoS2 materials. To understand the difference between MoS2 QDs and MoS2 nanosheets, MoS2 and OMoS2 nanosheets were used as HELs to construct solar cells with a structure of ITO/HEL/active layer/MSAPBS/Al. The efficiency of MoS2 nanosheet-based solar cells is 4.18% with VOC of 0.63 V, JSC of 12.5 mA/cm2, and FF of 53.2%. When the O-MoS2 nanosheets were used as HEL, the efficiency dramatically increased to 7.76% with VOC of 0.79 V, JSC of 15.5 mA/cm2, and FF of 63.6%. Again, this improvement is ascribed to the well-matched work function of O-MoS2 nanosheets (5.2 eV)18 and HOMO energy level of PTB7-Th (−5.4 eV) and the reduced roughness of O-MoS2 nanosheets. However, this efficiency (7.76%) is obviously lower than that of the OSC based on O-MoS2 QDs (8.66%). Because their work functions are similar and the transparency of O-MoS 2 nanosheets is higher than that of O-MoS2 QDs, the discrepancy of the efficiency should originate from the roughness and

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CONCLUSIONS We have systematically studied the application of MoS2 and OMoS2 QDs, MoS2 and O-MoS2 nanosheets, and PEDOT:PSS as HEL for OSCs. The results demonstrated that O-MoS2 QDs film is the optimal HEL layer due to its high transparency, tunable work function, and optimal morphology. After systematic optimization, the PTB7-Th-based solar cells with O-MoS2 QDs as the HEL layer reached an efficiency of 8.7%, the highest efficiency for solar cells containing MoS2 materials. Furthermore, the stability of solar cell devices is improved 26921

DOI: 10.1021/acsami.6b06081 ACS Appl. Mater. Interfaces 2016, 8, 26916−26923

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ACS Applied Materials & Interfaces

(12) Anwar, M.; Hogarth, C. A.; Lott, K. A. K. Electron Spin Resonance Study of Thermally Evaporated Amorphous Thin Films of MoO3/SiO. Phys. Status Solidi A 1989, 114, 225−231. (13) Chen, S.; Yu, X.; Zhang, M.; Cao, J.; Li, Y.; Ding, L.; Shi, G. A Graphene Oxide/Oxygen Deficient Molybdenum Oxide Nanosheet Bilayer as a Hole Transport Layer for Efficient Polymer Solar Cells. J. Mater. Chem. A 2015, 3, 18380−18383. (14) Li, S. S.; Tu, K. H.; Lin, C. C.; Chen, C. W.; Chhowalla, M. Solution-Processable Graphene Oxide as an Efficient Hole Transport Layer in Polymer Solar Cells. ACS Nano 2010, 4, 3169−3174. (15) Liu, J.; Xue, Y.; Gao, Y.; Yu, D.; Durstock, M.; Dai, L. Hole and Electron Extraction Layers Based on Graphene Oxide Derivatives for High-Performance Bulk Heterojunction Solar Cells. Adv. Mater. 2012, 24, 2228−2233. (16) Gu, X.; Cui, W.; Li, H.; Wu, Z.; Zeng, Z.; Lee, S. T.; et al. A Solution-Processed Hole Extraction Layer Made from Ultrathin MoS2 Nanosheets for Efficient Organic Solar Cells. Adv. Energy Mater. 2013, 3, 1262−1268. (17) Kwon, K. C.; Kim, C.; Le, Q. V.; Gim, S.; Jeon, J. M.; Ham, J. Y.; et al. Synthesis of Atomically Thin Transition Metal Disulfides for Charge Transport Layers in Optoelectronic Devices. ACS Nano 2015, 9, 4146−4155. (18) Le, Q. V.; Nguyen, T. P.; Jang, H. W.; Kim, S. Y. The Use of UV/Ozone-Treated MoS2 Nanosheets for Extended Air Stability in Organic Photovoltaic Cells. Phys. Chem. Chem. Phys. 2014, 16, 13123− 13128. (19) Niu, L.; Li, K.; Zhen, H.; Chui, Y. S.; Zhang, W.; Yan, F.; Zheng, Z. Salt-Assisted High-Throughput Synthesis of Single- and Few-Layer Transition Metal Dichalcogenides and Their Application in Organic Solar Cells. Small 2014, 10, 4651−4657. (20) Yang, X.; Fu, W.; Liu, W.; Hong, J.; Cai, Y.; Jin, C.; et al. Engineering Crystalline Structures of Two-Dimensional MoS2 Sheets for High-Performance Organic Solar Cells. J. Mater. Chem. A 2014, 2, 7727−7733. (21) Yun, J. M.; Noh, Y. J.; Lee, C. H.; Na, S. I.; Lee, S.; Jo, S. M.; et al. Exfoliated and Partially Oxidized MoS2 Nanosheets by One-Pot Reaction for Efficient and Stable Organic Solar Cells. Small 2014, 10, 2319−2324. (22) Qin, P.; Fang, G.; Ke, W.; Cheng, F.; Zheng, Q.; Wan, J.; et al. In Situ Growth of Double-Layer MoO3/MoS2 Film from MoS2 for Hole-Transport Layers in Organic Solar Cell. J. Mater. Chem. A 2014, 2, 2742−2756. (23) Liu, W.; Yang, X.; Zhang, Y.; Xu, M.; Chen, H. Ultra-Stable Two-Dimensional MoS2 Solution for Highly Efficient Organic Solar Cells. RSC Adv. 2014, 4, 32744−32748. (24) Li, X.; Zhang, W.; Wu, Y.; Min, C.; Fang, J. Solution-Processed MoSx as an Efficient Anode Buffer Layer in Organic Solar Cells. ACS Appl. Mater. Interfaces 2013, 5, 8823−8827. (25) Ibrahem, M. A.; Lan, T. W.; Huang, J. K.; Chen, Y. Y.; Wei, K. H.; Li, L. J.; Chu, C. W. High Quantity and Quality Few-Layers Transition Metal Disulfide Nanosheets from Wet-Milling Exfoliation. RSC Adv. 2013, 3, 13193−13202. (26) Ip, A. H.; Thon, S. M.; Hoogland, S.; Voznyy, O.; Zhitomirsky, D.; Debnath, R.; et al. Hybrid Passivated Colloidal Quantum Dot Solids. Nat. Nanotechnol. 2012, 7, 577−582. (27) Gao, J.; Perkins, C. L.; Luther, J. M.; Hanna, M. C.; Chen, H. Y.; Semonin, O. E.; et al. n-Type Transition Metal Oxide as a Hole Extraction Layer in PbS Quantum Dot Solar Cells. Nano Lett. 2011, 11, 3263−3266. (28) Tu, Y.; Wu, J.; Zheng, M.; Huo, J.; Zhou, P.; Lan, Z.; et al. TiO2 Quantum Dots as Superb Compact Block Layers for High-Performance CH3NH3PbI3 Perovskite Solar Cells with an Efficiency of 16.97%. Nanoscale 2015, 7, 20539−20546. (29) Li, M.; Ni, W.; Kan, B.; Wan, X.; Zhang, L.; Zhang, Q.; et al. Graphene Quantum Dots as the Hole Transport Layer Material for High-Performance Organic Solar Cells. Phys. Chem. Chem. Phys. 2013, 15, 18973−18978.

greatly compared to those based on conventional PEDOT:PSS. Therefore, O-MoS2 QDs can be employed as a universal HEL for high performance OSCs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06081. AFM image of MoS2 nanosheets; EQE curves of devices based on PTB7-Th:PC71M with different HELs and MSAPBS (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the NSFC (51303180 and 21574135), Beijing Natural Science Foundation (2162043), One Hundred Talents Program of Chinese Academy of Sciences, and University of Chinese Academy of Sciences for financial support. The authors are also grateful for the help from Prof. Jianhui Hou and Prof. Xiao Lin.

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