Doping ZnO Electron Transport Layers with MoS2 Nanosheets

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Doping ZnO Electron Transport Layers with MoS2 Nanosheets Enhances the Efficiency of Polymer Solar Cells Yi-Jiun Huang, Hsiu-Cheng Chen, Hsi-Kuei Lin, and Kung-Hwa Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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ACS Applied Materials & Interfaces

Doping ZnO Electron Transport Layers with MoS2 Nanosheets Enhances the Efficiency of Polymer Solar Cells Yi-Jiun Huang,† Hsiu-Cheng Chen,† Hsi-Kuei Lin,† and Kung-Hwa Wei

＊,†,‡

†

Department of Materials Science and Engineering, National Chiao Tung University, 300 Hsinchu, Taiwan ‡

Center for Emergent Functional Matter Science, National Chiao Tung University, 300 Hsinchu, Taiwan *E-mail: [email protected]

Keywords: photovoltaics, inverted solar cell, electron transport layer, ZnO:MoS2 nanocomposites, MoS2 nanosheets, surface morphology, synchrotron grazing-incidence small angle X-ray scattering

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ABSTRACT In this study, we incorporated molybdenum disulfide (MoS2) nanosheets into sol–gel processing of zinc oxide (ZnO) to form ZnO:MoS2 composites for use as electron transport layers (ETLs) in inverted polymer solar cells featuring a binary bulk heterojunction active layer. We could effectively tune the energy band of the ZnO:MoS2 composite film from 4.45 to 4.22eV by varying the content of MoS2 up to 0.5 wt%, such that the composite was suitable for use in bulk heterojunction photovoltaic devices based on poly[bis(5-(2-ethylhexyl)thien-2-yl)benzodithiophene–alt– (4-(2-ethylhexyl)-3-fluorothienothiophene)-2-carboxylate-2,6-diyl)] (PTB7-TH):phenyl-C71-butryric acid methyl ester (PC71BM). In particular, the power conversion efficiency (PCE) of the PTB7-TH:PC71BM (1:1.5, w/w) device incorporating the ZnO:MoS2 (0.5 wt%) composite layer as the ETL was 10.1%, up from 8.8% for the corresponding device featuring ZnO alone as the ETL—a relative increase of 15%. Incorporating a small amount of MoS2 nanosheets into the ETL altered the morphology of the ETL and resulted in enhanced current densities, fill factors, and PCEs for the devices. We used ultraviolet photoelectron spectroscopy, synchrotron grazing-incidence wide-/small-angle X-ray scattering, atomic force microscopy, and transmission electron microscopy to characterize the energy band structures, internal structures, surface roughness, and morphologies, respectively, of the ZnO:MoS2 composite films.


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Introduction Organic photovoltaics (OPVs) are devices that convert harvested sunlight into energy; they are among the most important technologies for renewable energy.1–3 After more than a decade of development, OPVs have many attractive properties, including light weight, flexibility, low manufacturing costs, and amenability to large-area fabrication.4,5 In an OPV system, optimizing the active layer is key for obtaining a high power convention efficiency (PCE), but so too is the development of an electron transport layer (ETL)—positioned between the active layer and the cathode—that has a suitable energy band structure such that it can tune the work function of the cathode, modify the morphology, and minimize carrier recombination in the active layer.6,7 Calcium (Ca) has been employed previously as the ETL in conventional device structures, but its use requires a high vacuum for deposition and, furthermore, it is a material that is subject to oxidation when exposed to air.8 To resolve these issues, the transport directions of carriers have been reversed using inverted structures along with a high-work-function metal anode.9 In inverted OPV devices, ETL with ZnO nanoparticles has often been used because of its high electron mobility, high transmittance to visible light, air-stability, and tunable electrical optical properties.10 Nevertheless, the presence of ZnO-nanoparticles ETL can result in a high series resistance as well as electron trapping—a result of the presence of defects/traps that were generated from adsorbed oxygen on the ZnO NP surfaces.11 Doping ZnO with a variety of different elements and then using the composites as ETLs can improve the conductivity and allow the use of various thick interlayers [between the indium tin oxide (ITO) electrode and bulk heterojunction (BHJ)] prepared through facile production techniques.12 For example, composites involving ZnO doped with perylene bisimides ,13–15 aluminum,16–19 fullerene derivatives,20,21 1,2-ethanedithiol,22 fulleropyrrolidine,23 and Sn24 have all displayed lower work functions by increasing the carrier concentration in the film, as well as improving electron transport into the ZnO layer by removing the electron trapping states associated with defects. Depending on the conjugated polymer types in the active layer, the PCEs of devices containing an ETL with a 3 ACS Paragon Plus Environment


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polymer/metal oxide bilayer structure can be enhanced by approximately between 9 and 30%, as compared with the corresponding devices featuring only a single metal oxide ETL.25 The interface between the ETL and the electrode plays a key role affecting charge collection in an OPV device.26 Interfacial modification of ZnO ETLs with polymers has been effective for improving charge extraction—in particularly, for bilayer structures in which a thin layer of polymer is positioned on the ZnO layer. For example, when using bilayer ETL with a conjugated polyelectrolyte,27,28 poly(2-ethyl-2-oxazoline),29 perylene bisimide,30 light-harvesting complex II,31 1-butyl-3-methylimidazolium tetrafluoroborate,32 fullerene derivatives,33 a phosphine oxide– functionalized 1,3,5-triazine derivative,34 poly[(9,9-bis(3´-(N,N-dimethylamino)propyl)-2,7-fluorene)–alt–2,7-(9,9-dioctyfluorene)],35 polyethylenimine ethoxylated,36 poly(4-vinylpyridine),37 or lithium sulfonated polystyrene,38 on a sol–gel processed ZnO layer,39 the PCEs of these devices can increase by 23%, when they are compare to those of the corresponding devices with a single ZnO ETL. Instead of having a bilayer ETL structure in an inverted solar cell, a few reports on the devices using a composite ETL layer have appeared.40–42 MoS2 has been studied extensively because of its distinctive optical, electronic, and catalytic characteristics. In addition, the electronic properties of such devices change significantly depending on the number of MoS2 layers, with the band gap energy increasing from 1.29 for multilayer MoS2 to 1.9 eV for monolayer MoS2, while the band gap changes from an indirect band gap to a direct band gap as the number of layers decreases.43,44 MoS2 nanosheets have also been applied in OPV devices; for example, as efficient hole transport layers (HTLs);45 from ammonium thiomolybdate (NH4)2MoS4 to fabricate MoSx as an anode buffer layer;46 as a simple strategy to produce oxygen-incorporated HTLs;47 in the “salt-assisted liquid-phase exfoliation” method to form HTLs;48 through p- or n-doping treatment to obtain both HTL and ETL layers;49 with UV/ozone treatment to prepare HTLs;50,51 with chemical exfoliation to form interfacial layers and silver nanowire (AgNW)/MoS2 composites as transparent electrodes;52 in the form of MoS2 4 ACS Paragon Plus Environment

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nanosheets decorated with 20-nm Au nanoparticles as HTLs for developing plasmonics; and in the use of UV-ozone treated MoS2 quantum dots as HTLs.53 MoS2 nanosheets exhibit a combination of ease of processing, cost-effective manufacturing, long-term environmental stability, and good compatibility with chemical functionalization for composite formation, allowing energy levels to be matched with those of other required material components. The MoS2 nanosheets that are produced with solution processing methods exhibit lower trap densities and surface dipoles, which provide the proper electrical field for charge transport and suppress charge recombination, and high charge mobility property. Using the MoS2 nanosheets alone, however, may not form a continuous layer structure, and we in turn incorporate them in the sol-gel processed ZnO layer for forming a suitable composite electron transport layer for high performance photovoltaic devices. In this study, we incorporated a small amount of MoS2 nanosheets, which had been produced within our laboratory,54 into sol–gel ZnO precursor solution, and then used the composite solution to form the ETL layer of inverted polymer solar cells. Doping different amount of MoS2 nanosheets into ZnO layer can reduce pristine ZnO’s work function to different extent, and can thus tune the work function of ZnO. We, therefore, expected the ZnO:MoS2 composites layers would enhance the device performances of inverted solar cells based on poly[4,8-bis(5-(2-ethylhexyl)thien-2-yl)benzo[1,2-b;4,5-b´]dithiophene-2,6-diyl–alt– (4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2,6-diyl)] (PTB7-TH):phenyl-C71-butryric acid methyl ester (PC71BM). The conduction band energy of the ZnO:MoS2 have been characterized using ultraviolet photoelectron spectroscopy (UPS). The nanostructures of ZnO:MoS2 composite ETLs have been deciphered with grazing-incidence small-angle X-ray scattering (GISAXS) and transmission electron microscopy (TEM) for discerning how the content of MoS2 influenced the morphology of the composite ETL.



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EXPERIMENTAL SECTION Materials and ZnO precursor solution MoS2 nanosheets were exfoliated using a liquid N2-quenching method. For preparation of the ZnO precursor solution, we dissolved zinc acetate dehydrate [Zn(CH3COO)2·2H2O] and ethanolamine in 2-methoxyethanol under stirring for one day and incorporated a small amount of MoS2 nanosheets to form the composite ETL layer. For preparation of the BHJ systems, PTB7-TH and PC71BM were purchased from 1-Material and FEM Technology, respectively. The PTB7-TH (10 mg mL–1) and PC71BM (15 mg mL–1) were dissolved in chlorobenzene (CB) containing 1,8-diiodoctane (DIO, 3 vol%) in a glove box, overnight at room temperature, to give the blend solution for device fabrication. Device Fabrication and Characterization We used detergent, deionized water, acetone, and isopropanol through ultrasonic treatment to clean the ITO substrate and then we dried the ITO glass substrate. The pure ZnO and ZnO:MoS2 blend solution was spin-coated onto ITO glass that had been subjected to UV-ozone treatment for 15 minute, and then annealed at 200 °C for 1 h. Prepared in a glove box, the active layers formed from the blending solution of PTB7-TH:PC71BM and CB were then spin-cast on the top of the ETL film; they were left for 30 min under ambient conditions prior to deposition of the anode. The thickness of each active layer film of PTB7-TH:PC71BM was approximately 110 nm. Device fabrication was complete after thermal evaporation of MoO3 and the Ag anode under high vacuum. Using a Keithley


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2400 source meter, we measured the characteristics of the current density–voltage (J–V). Meanwhile, the photocurrent was measured under simulated AM 1.5 G illumination at 100 mW cm–2 with a Xe lamp–based 150-W solar simulator. The dark J–V characteristics was also measured by a Keithley 2400 source meter, without the solar simulator. At last, we measured external quantum efficiencies (EQEs) using a QE-R system (Enlitech, Taiwan). We defined a device area of 0.1 cm2. Characterization of ZnO:MoS2 Composite Films ITO was first treated by UV-ozone for 15 min, and then the pure ZnO and ZnO:MoS2 blend solution were spin-coated on ITO and annealed at 200 °C for 1 h. To measure ZnO and ZnO:MoS2 composite films, the space charge limited current (SCLC) method was used to measure electron mobility within the structure of ITO glass/ZnO:MoS2/electrode (Al). The pure ZnO and ZnO:MoS2 blend solution were spin-coated on quart and annealed at 200 °C for 1 h. We used a UV–Vis spectrophotometer to acquire transmission spectra; and then we used a fluorescence spectrophotometer to acquire photoluminescence (PL) spectra. The pure ZnO and ZnO:MoS2 blend solution were spin-coated on ITO and annealed at 200 °C for 1 h. We used atomic force microscopy (AFM) in the tapping mode to determine the film morphology under ambient conditions. The water contact angle (WCA) on the film surface was measured using a commercial contact angle meter. The TEM samples, which are the pure ZnO and ZnO:MoS2 blend solution, were prepared on a carbon-covered Ni grid. The pure ZnO and ZnO:MoS2 blend solution



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were spin-coated on the wafer for the GISAXS and UPS measurement in the National Synchrotron Radiation Research Center.


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RESULTS and DISCUSSION Figure 1 displays the inverted device structure (glass/ITO/ZnO:MoS2/PTB7-TH:PC71BM/MoO3/Ag) and the molecular structures of PTB7-TH and PC71BM. The nanocomposite ZnO:MoS2 layer was the ETL, whereas the deposited molybdenum trioxide (MoO3) with high-work-function was the HTL. Figure 2a presents the current–voltage (J–V) characteristics of the PTB7-TH:PC71BM solar cells incorporating various ZnO:MoS2 ETLs as illuminated (AM 1.5G irradiation, 100 mW cm–2); Figure 2b provides the EQE curves with the integrated short-circuit current densities (Jsc) being close to the Jsc values that were measured directly from the optimized PTB7-TH/PC71BM systems.55 The integrated photocurrents from the EQE curves of the devices of the 0, 0.1, 0.3, 0.5 and 0.7 wt% MoS2 in ZnO cases are 16.8, 17.1, 17.3, 18.0 and 16.5 mA cm–2, respectively; these values are within 5% range deviation from those of measured Jsc, thus these two sets of data corresponding to each other reasonably. Table 1 lists the corresponding device performance, ZnO as the electron transport layer and incorporating PTB7-TH:PC71BM (1:1.5/w:w) as the active layer, displayed an open circuit voltage (Voc) of 0.79 V, a value of Jsc of 17.2 mA cm–2, a fill factor (FF) of 64.2%, and a PCE of 8.8%. The device performance improved considerably after applying the MoS2-incorporated ZnO ETL. The optimal PTB7-TH:PC71BM device was that prepared with an ETL comprising 0.5% MoS2 blended with ZnO. This particular device displayed a value of Voc of 0.8 V, a value of Jsc of 18.4 cm–2, an FF of 68.9%, and a PCE of 10.1%—a relative increase of 15%as compared to the PCE of the corresponding device incorporating pristine ZnO as the ETL. Increasing the MoS2 loading beyond 9 ACS Paragon Plus Environment


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0.5% worsened the performance of the device. For instance, when the MoS2 content was 0.7%, the PTB7-TH:PC71BM device displayed a lower value of Jsc (17.1 mA cm–2) and a lower FF (65.9%), resulting in a PCE of only 8.9%. We also applied focus ion beam (FIB) technique to prepare a thin slice of the device for observing the cross-sectional area under TEM (see Figure S7), and thus determined the thickness of ZnO:0.5 wt% MoS2 layer to be 25 nm. Table S1 shows the device performance with different ETL thickness of the inverted PTB7-TH:PC71BM cells. The Jsc increase slightly with the increasing thickness of ETL from 23 to 25 nm, and the Jsc decrease at 27 nm thick ETL(see Supporting Information Table S1). The best PCE occurred at device with ETL thickness of 25 nm. We used the slopes of the J-V curves from Figure 2a to calculate the series resistance (RS) and shunt resistance (RSH) of the inverted devices and determined the contact/interfacial resistance at the interface between ETLs and active layers.56 Figure S8 shows the dark current-voltage curves; we observed that the device with ZnO:0.5 wt% MoS2 show the lowest leakage current at negative voltage, and the diode characteristics indicated higher parallel resistance and lower serial resistance for the device with ZnO:0.5 wt% MoS2, as compared to the case of pristine ZnO case, leading to improved FF. The EQE intensity in the longer wavelength region (450-730 nm) is mainly owing to the PTB7-TH:PC71BM active layer. The effect of the ZnO:MoS2 ETL on the EQE is only to some extent—with the trend of slightly increasing EQE with the increasing amount of MoS2 when the amount is less than 0.5 wt%—that resulted from the reduced probability of recombination at the interface between the PTB7-TH:PC71BM active layer and the ETL. Figure 2c displays the SCLC 10 ACS Paragon Plus Environment

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plots of electron mobility in the out-of-plane direction. The electron mobility increased significantly upon increasing the MoS2 content; for example, the electron mobility increased to 1.86 × 10–4 m2 V–1 s–1 for the device incorporating an ETL with 0.5 wt% MoS2 in ZnO from 1.59 × 10–4 m2 V–1 s–1 for the control device that had a ZnO ETL. When the MoS2 content increasing to 0.7 wt%, the electron mobility decreased to 1.48 × 10–4 m2 V–1 s–1.57 Figure 3 presents the energy level diagram of all layers in the device. Some of the conduction bands (4.45–4.22 eV) of the ZnO:MoS2 composite films were close to the lowest unoccupied molecular orbital (LUMO) of PC71BM (4.0 eV). In these cases, the good alignment of the conduction band of ZnO:MoS2 and the LUMO energy level of the fullerene leads to an ohmic-contact interface. Consequently, the ZnO:MoS2 layer could be finely tuned toward forming an ideal ETL. The energy level of the conduction band for the MoS2 is 4.2 eV, while the LUMO of ZnO:MoS2 composites films elevates from 4.45 eV for the pristine ZnO case to 4.22 eV for the 0.7 wt% MoS2 case with the increasing contents of MoS2.58 When incorporating 0.5 wt% MoS2 into ZnO, the LUMO become 4.23 eV, being quite suitable for the electronic transport. We calculated the band gaps of ZnO:MoS2 films blending with 0, 0.1, 0.3, 0.5, and 0.7% MoS2 from the transmittance spectra (Figure S1), obtaining values of 3.36, 3.38, 3.39, 3.42, and 3.40 eV, respectively. Although the transmittances of ZnO:MoS2 composites films decrease with the increasing contents of MoS2, the transmittances of visible-light area absorption wavelength still has up to more than 90%, and the active layer thus can absorb effectively. We recorded the PL spectra of 11 ACS Paragon Plus Environment


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the ZnO:MoS2 films for investigating the passivation effect of MoS2 modification on the defects of the ZnO films. We applied the formula to determine the valence band energy (Figure S2),

Φ = hν – ∆E where hν is equal to 21.21 eV, with ∆E = E1 – E2 The lower emission onset energy (E2) was determined by the intersection point of the two tangents of the UPS curves, where the secondary photoelectrons appearing in the valance band region. The upper emission onset energy (E1) of each of the ZnO:MoS2 films was determined with high-energy shoulder tangents in the cutoff region. From the difference of E1 and E2, the valence band energies for the ZnO:MoS2 films incorporating 0, 0.1, 0.3, 0.5, and 0.7% (w/w) MoS2 were 7.63, 7.71, 7.69, 7.66, and 7.62 eV, respectively.59,60 Figure S3 reveals that the emission intensity of PL peak reduced upon increasing the ratio of incorporated MoS2. At 374 nm appeared an emission peak that arises from the radiative annihilation of excitons or recombination of excitons. Because of trap emission or surface recombination, no appeared near at 534 nm61 very little oxygen defects were present in ZnO layers.62 Consequently, we suspected that the impact of oxygen defects in ZnO layers on the whole device performance would be negligible. The PL intensity of the ZnO film incorporated a small amount of MoS2 decreased apparently with ZnO:0.5 wt% MoS2 being the lowest that results in better electron transfer to ITO. The content of traps decrease in the ETL of the solar cell device, would reduce the possibility 12 ACS Paragon Plus Environment

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of interfacial recombination of carriers, and increase the Jsc and FF, thereby enhancing the PCE of the device. Figures 4a–e present AFM topography images of the ZnO:MoS2 ETLs containing 0, 0.1, 0.3, 0.5, and 0.7% MoS2, respectively. The MoS2 decreased the roughness to 3.6 nm for the blend comprising 0.5% MoS2 in ZnO from 5.2 nm for the pristine ZnO film; this behavior would potentially increase the degree of molecular contact that can introduce interfacial dipoles between the ITO and the active layer, leading to improved PCEs. This is because a smoother film would most likely feature few traps, thereby reducing the probability of recombination. As a result, decreasing the roughness of ZnO film would enhance the FF of the corresponding device. Furthermore, the roughness of the substrate would also affect the roughness of the active layer covering it, and a rough active layer would be unfavorable for device performance. We suspect that variations in the surface morphology of the active layer were caused by the changes in the surface morphology of the ZnO after incorporating MoS2. The RMS of ETL affects the probability of recombination at the interface between the PTB7-TH:PC71BM active layer and the ETL. When the incorporated MoS2 concentration increased to 0.7 wt%, the value of RMS increased to 4.4 nm from 3.6 nm, as compared to the case of ZnO:0.5 wt% MoS2, resulting in higher series resistance and leakage phenomenon. Therefore, the ZnO:0.5 wt% MoS2 composites layer shows the lowest RMS value and the highest electron mobility.63 Figure S9 shows the measured WCA images that were used to provide some evidence on the vertical phase separation of the pristine ZnO and ZnO blend with different ratio 13 ACS Paragon Plus Environment


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MoS2 as ETLs.64 The WCA of ZnO:MoS2 films indicate more hydrophilic than that of the pristine ZnO; the WCA of ZnO:MoS2 composites films decrease from 27.5o to 24.0o along with the increase contents of MoS2, and the ZnO:0.5 wt% MoS2 has the lowest contact angle. The experimental results of WCA are in well accordance with the RMS of AFM data (Figure 4). Figure S10 presents AFM images of PTB7-TH:PC71BM films on the ZnO:MoS2 composite layers (see Supporting Information page S-7); the RMS of AFM active layer surface slightly decrease from 2.2 nm to 1.9 nm along with the increase contents of MoS2, with the RMS for ZnO:0.5 wt% MoS2 being the lowest. The results are in well accordance with that of the WCA. The two results show that the RMS of AFM decrease with the increase contents of MoS2, and the lower RMS help the electron collection efficiency in inverted solar cells to enhance the PCE. Figure 5 displays TEM images of our ZnO:MoS2 films (Figure 5d), indicating that incorporated MoS2 improved not only the topology of the ZnO film but also its morphology. A relatively uniform distribution of MoS2 nanosheets on the ITO glass can be obtained after careful selection of the dispersion solvent;65 for example, 2-methoxyethanol (2ME), which was the main solvent of the ZnO precursor. The N2-quenched MoS2 nanosheets were dispersed well in the ZnO precursor solution (see Figure S5a); thus, spin-coating on the ITO glass provided a morphology of well-distributed MoS2 nanosheets dispersed in the ZnO precursor. Figures S5b and S5c present optical microscopy and AFM images of spin-coated ZnO:0.5 wt% MoS2 nanosheet composites on ITO glass. 14 ACS Paragon Plus Environment

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We measured the surface electronic structures of the film with X-ray photoelectron spectroscopy (XPS).66 Figure S4 presents the XPS spectra of the Zn, O, Mo, and S atoms; the spectra indicate Zn 2p3/2 peaks near 1021 eV; O 1s peaks near 530.5 and 532.0 eV; Mo 3d3/2 and Mo 3d5/2 peaks near 228 and 231 eV; and S peaks near 168 eV. As we had discussed in a previous report25 that these signals suggest the Zn atoms existed in the completely oxidized state despite the fact that the Zn binding energy reduced in the presence of MoS2. The lower-binding-energy O 1s peak indicates that the O atoms of O2– ions on the ZnO have a wurtzite structure of a hexagonal Zn2+ ion array.67 Moreover, the O atoms were surrounded by Zn atoms with their full components, and the higher-binding-energy component represents the O atoms associated with the chemisorbed O2– ions on the ZnO surface.68 A very large change in the type of the bonding of the O atoms after incorporating the MoS2 nanosheets was not observe. Increasing the ratio of MoS2, however, can lead to a large increase in the signal of the Mo and S atoms. Figures 6a–e presents 2-D GIWAXS patterns of thin films of ZnO:x wt% MoS2 nanosheets (x = 0, 0.1, 0.3, 0.5, and 0.7, respectively); Figure 6f presents the one-dimensional (1-D) profiles along the out-of-plane direction integrated in the red circular-sector regions. For the ZnO:0.1 wt% MoS2 composite film, a small peak appeared at 1.0 Å–1 and a large sharp peak at 1.12 Å–1. When the MoS2 content increased to 0.3 wt%, the peak intensity at 1.0 Å–1 increased and the other decreased.69 We attribute the peak at 1.0 Å–1 to the MoS2; it matches the X-ray diffraction (002) peak at a value of 2θ of 14.4°, according to the correlation q = (4π/λ) sin (θ). Increasing the MoS2 content further to 0.5 15 ACS Paragon Plus Environment


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and 0.7 wt% resulted in the MoS2 peak turning from sharp peak to broader humps, consistent with small MoS2 crystallite domains existing in the ZnO:x wt% MoS2 (x = 0.1, 0.3, 0.5, and 0.7) composite films. We applied GISAXS technique for probing the nanoscale-phase separation in the ZnO:MoS2 composite films. Figure 7a–e presents 2-D GISAXS images of ZnO:x wt% MoS2 (x = 0, 0.1, 0.3, 0.5 and 0.7) composite thin films, and also the line-cut 1-D profiles along the in-plane direction (Figure S6). In these scattering images, the specular high intensity peak is known as the Yoneda peak, appeared in the green rectangular.70 For those composite films with x wt% MoS2 (x = 0, 0.1, 0.3, 0.5 and 0.7), the 1-D profiles presents almost similar line shapes. When 0.5% MoS2 was blended in ZnO, the highest PCE may indicate that an improvement in structural order in the direction perpendicular to the substrate.


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CONCLUSION We have demonstrated that simply solution-processed ZnO:MoS2 composite layers can be efficient ETLs that enhance the efficiency of inverted solar cells. Through systematic characterizations of these ZnO:MoS2 composite ETL films using UPS, GISAXS, AFM, and TEM, we found that the physical, optical, electronic, morphological, and inner structural properties of the ZnO:MoS2 composites are superior to that of the pristine ZnO layer for serving as the ETL for the devices. Incorporating MoS2 nanosheets into ZnO layer as the ETL―positioned between the active layer and the ITO electrode―improved not only the alignment of the energy band and interfacial smoothness, but also the electron transport in the devices. Such blending treatment of ZnO formed ETLs that were in near-ohmic contact between the ITO electrode and the active layer. As the MoS2 content increased up to 0.5 wt%, the film surface became smoother and denser, thereby decreasing the interfacial resistance and suppressing the leakage current. For the PTB7-TH:PC71BM (1:1.5, w/w) device featuring the composite of ZnO:0.5 wt% MoS2 as the ETL, the PCE improved to 10.1% from a value of 8.8% for the corresponding device featuring pure ZnO as its ETL-a relative increase almost of 15%. Consequently, incorporating a small amount of these MoS2 nanosheets into ZnO and using the composite as the ETL allowed effective tuning of the ETL morphology, improved the FF, and enhanced the device efficiency, indicating ZnO:MoS2 composites can be effective ETLs within organic photovoltaics.



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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website Transmittance spectra, UPS, PL, XPS, Digital camera image, Optical microscope image, and AFM topographic image

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Yi-Jiun Huang: 0000-0001-8494-9106 Hsiu-Cheng Chen: 0000-0003-1625-7508 Hsi-Kuei Lin: 0000-0001-7134-4247 Kung-Hwa Wei: 0000-0002-0248-4091

Conflicts of Interest There are no conflicts of interest to declare.


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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology, Taiwan, for financial support (MOST 103-2221-E-009-211-MY3, MOST 104-2119-M-009-013, MOST 105-2119-M-009-006). We also appreciate the SPROUT Project of Ministry of Education, Taiwan, financially supported by the Center for Emergent Functional Matter Science of National Chiao Tung University from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan, and Prof. Yu-Wei Su in the Department of Chemical Engineering of Feng Chia University, Taiwan for discussing GIWAXS/GISAXS data.

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Figure 1. (a) Schematic representation of an inverted solar cell device structure. (b) Structures of PTB7-TH and PC71BM, blended as the active layer.


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Figure 2. (a) J–V characteristics of inverted solar cells containing PTB7-TH:PC71BM as the active layer, with and without ETLs containing MoS2 at various contents. (b) EQE data of devices based on PTB7-TH:PC71BM and ZnO films incorporating MoS2 at various contents. (c) J–V curves of electron-only devices having the structure ITO/ETL/Al, with the ETL featuring ZnO incorporating MoS2 at various contents; inset: sample configuration. 27 ACS Paragon Plus Environment


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Table 1. Device performance of the inverted PTB7-TH:PC71BM cells having the structure ITO/ZnO:x wt% MoS2/PTB7-TH:PC71BM/MoO3/Ag. All data are averaged from 10 devices. ZnO:x wt%

Voc

Jsc

FF

PCE

µe

Rs

Rsh

MoS2

(V)

(mA cm–2)

(%)

(%)

(cm2 V–1 s–1)

(Ωcm2)

(Ωcm2)

x=0

0.79 ± 0.004

17.4 ± 0.2

64.2 ± 1.3

8.8 ± 0.1

1.59 × 10–4

2.4

159.3

x = 0.1

0.80 ± 0.004

17.5 ± 0.3

66.4 ± 1.4

9.3 ± 0.1

1.66 × 10–4

2.2

195.3

x = 0.3

0.80 ± 0.004

17.6 ± 0.2

67.2 ± 1.1

9.5 ± 0.1

1.75 × 10–4

2.1

201.6

x = 0.5

0.80 ± 0.004

18.4 ± 0.3

68.9 ± 0.8

10.1 ± 0.1

1.86 × 10–4

2.0

212.3

x = 0.7

0.79 ± 0.004

17.1 ± 0.3

65.9 ± 1.4

8.9 ± 0.1

1.48 × 10–4

2.6

155.6

Figure 3. Energy level diagram of ZnO in the presence of MoS2 at various weight ratios, determined from UPS and UV–Vis spectroscopic measurements; energy levels for ITO, PTB7-TH, PC71BM, and MoO3 have been taken from the literature.


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Figure 4. AFM topographic images (5 × 5 µm) of ZnO:MoS2 blend films incorporating MoS2 at contents of (a) 0, (b) 0.1, (c) 0.3, (d) 0.5, and (e) 0.7 wt%.

Figure 5. TEM images of ZnO:MoS2 blend films incorporating MoS2 at contents of (a) 0, (b) 0.1, (c) 0.3, (d) 0.5, and (e) 0.7 wt%. 29 ACS Paragon Plus Environment


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Figure 6. (a–e) 2D GIWAXS patterns of ZnO:x wt% MoS2 films; x: (a) 0, (b) 0.1, (c) 0.3, (d) 0.5, and (e) 0.7. (f) Line-cut 1-D profiles of these images along the out-of-plane direction.

Figure 7. (a–e) 2D GISAXS patterns of ZnO:x wt% MoS2 (x = 0, 0.1, 0.3, 0.5, and 0.7). 30 ACS Paragon Plus Environment

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TOC


Doping ZnO Electron Transport Layers with MoS2 Nanosheets

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