Water-Soluble 2D Transition Metal Dichalcogenides as the Hole

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Water-soluble 2D transition metal dichalcogenides as hole transport layer for high efficient and stable p-i-n perovskite solar cells Peng Huang, Zhao-Wei Wang, Yanfeng Liu, Kai-cheng Zhang, Ligang Yuan, Yi Zhou, Bo Song, and Yongfang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06403 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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ACS Applied Materials & Interfaces

Water-soluble 2D Transition Metal Dichalcogenides as Hole Transport Layer for High Efficient and Stable p-i-n Perovskite Solar Cells Peng Huang1, Zhaowei Wang1, Yanfeng Liu1, Kaicheng Zhang1, Ligang Yuan1, Yi Zhou1*, Bo Song1*, and Yongfang Li1,2 1

Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical

Engineering and Materials Science, Soochow University, Suzhou, 215123, China 2

Beijing National Laboratory of Molecular Sciences, CAS Key Laboratory of Organic Solids,

Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China

ABSTRACT As hole transport layer (HTL) material, PEDOT:PSS was often criticized on the intrinsic acidity and hygroscopic effect, which would in long-run affect the stability of perovskite solar cells (Pero-SCs). As alternatives, herein water-soluble 2D transition metal dichalcogenides (TMDs), MoS2 and WS2, were used as HTLs in p-i-n Pero-SCs. It was found that the content of 1T phase in 2D TMDs HTLs is centrally important to the power conversion efficiencies (PCEs) of Pero-SCs, and the 1T-rich TMDs (as achieved from exfoliation and without postheating) lead to much higher PCEs. More importantly, as PEDOT:PSS was replaced by 2D TMDs, both the PCEs and stability of Pero-SCs were significantly improved. The highest PCEs of 14.35% and 15.00% were obtained for the Pero-SCs with MoS2 and WS2, respectively, while for the Pero-SCs with PEDOT:PSS only showed a highest PCE of 12.44%. These are up to date the highest PCEs using 2D TMDs as HTLs. After being stored in glovebox for 56 days, PCEs of the Pero-SCs using MoS2 and WS2 HTLs remained 78% and 72%, respectively, whilst the PCEs of Pero-SCs with PEDOT:PSS almost dropped to 0 over 35 days. This study demonstrates that water-soluble 2D TMDs have great potential for the application as new generation of HTLs aiming at high performance and long-term stable PeroSCs. 1 ACS Paragon Plus Environment


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Keywords: p-i-n perovskite solar cells, 2D transition metal dichalcogenides, hole transport layer, 1T phase, improved stability

INTRODUCTION Perovskite solar cells (Pero-SCs) have seized wide research attention due to the simple fabrication procedure and the high record power conversion efficiency (PCE) of 22.1%.1-5 The advantage of perovskite is rooted in its superior nature, such as direct optical bandgap (about 1.5 eV), high carrier mobilities (1 - 10 cm2 V-1s-1), long carrier diffusion length (100 nm to 1 µm) and simple solution-processability.6-10 The above features make perovskite an ideal candidate for photoactive material in terms of effective charge generation and transportation. In comparison with the photoactive materials, interfacial engineering that can influence the carrier extraction and collection is still lagging behind and highly demanded.11-15 Poly(3,4-ethylenedioxythiophene) polystyrene-sulfonate (PEDOT:PSS), which is often applied as hole transporting layers (HTLs) in organic solar cells, works well in p-i-n Pero-SCs due to its good hole mobility and simple solution processability.16-19 However, its intrinsic acidity would be problematic to ITO electrode, and its hygroscopic effect would cause fast degradation of perovskite layer. These above features would affect the overall performance, especially the lifetime of the resulting devices.20-23 In this respect, alternatives are highly demanded, and many efforts have been made on exploring new materials. However, most of the presently developed HTLs either require toxic solvent or high temperature treatment that may restrict the application in large scale fabrication and application in flexible solar cells. For example, CuOx and 6-difluoro-N1,N1,N2,N2,N4,N4,N5,N5-octakis(4-methoxyphenyl) benzene-1,2,4,5-tetraamine showed good charge transportation ability, but toxic solvents (1,2dichlorobenzene or chlorobenzene) were required for processing.24-25 Nickel oxide and vanadium oxide were also good HTLs, however, they need high temperature post-heating (over 300 °C) after preparation.26 2 ACS Paragon Plus Environment

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2D transition metal dichalcogenides (TMDs), MoS2 and WS2, have been applied in organic solar cells as HTLs due to the superior charge carrier mobilities.27-29 Very recently, Kim et al. firstly introduced MoS2 and WS2 as HTLs in p-i-n Pero-SCs through chemical vapor deposition method, and obtained moderate PCEs of 9.53% and 8.02%, respectively.30 Alternatively, Capasso et al. adopted solution-processable MoS2 as HTL material, which was exfoliated in N-methyl-2-pyrrolidone and re-dispersed in isopropanol by using a solventexchange process. They achieved an impressive PCE of 13.3% using composite of MoS2/Spiro-OMeTAD as HTL in n-i-p Pero-SCs, while obtained only a PCE of 4.5% when using MoS2 alone as HTL.31 These two primary publications indicate that the TMDs can be promising candidates as HTLs for Pero-SCs, however, some questions still need to be clarified. For example, how should we process the TMDs so that to get an optimal performance? Can the TMDs work alone as HTL for high efficient solar cells? More importantly, can the TMDs improve the stability of the resulting devices when PEDOT:PSS is replaced. In this study, water-soluble 2D MoS2 and WS2 were fabricated by exfoliating the corresponding bulk materials with lithium-intercalation reaction, and applied as HTLs for p-in Pero-SCs. It was found that the phase conversion of TMDs caused by exfoliation and postheating can greatly influence the performance of the corresponding solar cells. In this respect, the 1T-rich TMDs prepared without heating show higher priority than 1T-poor TMDs formed after post-heating. Under optimized conditions, greatly improved PCEs were acquired for the Pero-SCs based on MoS2 and WS2. In addition, the Pero-SCs based on 2D MoS2 and WS2 show higher stability comparing with that based on PEDOT:PSS. These results demonstrate that the 2D TMDs have great potential as easily processable and environmentally friendly HTLs for high efficient and stable Pero-SCs. EXPERIMENTAL SECTION

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Synthesis of 2D MoS2 and WS2. 0.3 g of bulk MoS2 (99%, Alfa Aesar) or WS2 (99.8%, Alfa Aesar) was dispersed in 3 mL of 2.4 M butyllithium solution in hexane (Amethyst) for 10 days in glovebox filled with nitrogen. The LixMoS2 or LixWS2 was obtained by washing with hexane (~ 300 mL) and filtration. Then LixMoS2 and LixWS2 in 60 mL of 45 vol% ethanol/water solvent were treated with ultrasonification (ultrasonic bath, 200W) for 120 min, and the solution was centrifuged to remove excess lithium ions by pouring out the upper fluid. The residual was added into 30 mL of water, and the solution was treated with ultrasonification for 10 min and the unexfoliated TMDs were removed by centrifugation. The resulting products is dissolved in water. Fabrication of Pero-SCs with PEDOT:PSS, 2D MoS2 and WS2 as HTLs. PEDOT:PSS, MoS2 and WS2 films were spin-coated on cleaned ITO pre-treated with UV ozone at 5000 rpm for 40 s. The ITO substrates with MoS2 and WS2 were dried in a vacuum oven at room temperature and ITO substrates with PEDOT:PSS was annealed at 150°C for 10 min in air. Perovskite layer was fabricated by fast deposition crystallization method. Briefly, PbI2 (0.62865 g) and PbCl2 (37.93 mg) were mixed with methylammonium iodide (0.23844 g) in N,N-Dimethylformamide (1.265 mL) at room temperature overnight to produce a clear perovskite solution with concentration of ~ 43 wt%. The perovskite solution was deposited on the top of HTLs at 4500 rpm for 30 s, and in the first 6 s, chlorobenzene (150 µL) was quickly dropped onto the substrate and as-spun films annealed at 90 °C for 5 min in a glovebox filled with nitrogen. Finally, it was sequentially deposited C60 (30 nm), BCP (8 nm) and Al (~ 80 nm) by thermal evaporation. Characterization. X-ray diffraction (XRD) data of bulk and 2D TMDs were collected by D2 PHASER. Raman spectra were tested on Renishaw inVia spectrometer mounted with 532 nm laser (UK). The thickness of 2D TMDs were evaluated by atomic force microscopy (AFM) and transmission electron microscope (TEM). 2D TMDs with different phases treated with different temperature in a glovebox filled with nitrogen were tested by X-ray photoelectron 4 ACS Paragon Plus Environment

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spectrometer (XPS, Escalab 250Xi). The thickness of MoS2 and WS2 films on ITO substrate were determined by spectroscopic ellipsometer. The current density-voltage (J-V) of devices with PEDOT:PSS, 2D MoS2 and WS2 were recorded using a Keithley 2400 source meter inside of a glove box filled with nitrogen. The performance of devices with PEDOT:PSS, 2D MoS2 and WS2 was measured under AM 1.5G solar illumination with an intensity of 100 mW cm-2. A shadow mask was clung to the substrate to define an active area of 7.57 mm2. Work functions (WFs) of ITOs modified with different materials were measured by Kelvin probe force microscope (KPFM) in air. The measurements of steady-state photoluminescence (PL) were conducted on FLS980. For transient state PL spectra, Lifespec II monitored the signal intensity of perovskite@PEDOT:PSS, perovskite@MoS2, perovskite@WS2 at 775 nm excited by a 477 nm laser. RESULTS AND DISCUSSION The water-soluble 2D TMDs, MoS2 and WS2, were prepared by lithium intercalation reaction. In brief, the bulk TMDs were lithiated by n-butyllithium, and thus the Li-intercalated TMDs were exfoliated into a few layers or even single layer via forced hydration.32-33 The reaction was conducted in mixed solvent of water and ethanol, and the resulting products were collected by centrifugation and then dispersed in water. The exfoliated TMDs show very good dispersity in water, and we hence define it as water-soluble 2D TMDs. XRD was employed to characterize the TMDs before and after exfoliation. As shown in Figure 1a & 1b, for both bulk MoS2 and WS2, multi-peaks were detected due to the diffraction of lattice planes. After reaction, only the peaks standing for (002) diffraction of MoS2 and WS2 remained in the XRD pattern, while the rest of peaks were no longer observed, which accords well with that reported in literature.34-36 These results suggest the bulk TMDs have been successful exfoliated to 2D structures. The formation of 2D TMDs was further confirmed by Raman spectra. For MoS2, the two peaks located at around 370 and 400 cm-1 corresponding to in(E12g) and out-plane (A1g) vibrations were often used to judge the state of the materials.37-38 As 5 ACS Paragon Plus Environment


shown in Figure 1c, the E12g and A1g peaks of bulk MoS2 were observed at 374.5 and 401.3 cm-1, and shifted to 379.2 cm-1 and 403.2 cm-1 after exfoliation, respectively, confirming the formation of 2D MoS2.39 Similar phenomena also happened to WS2 as shown in Figure 1d, and consist well with that reported in literature.40 002

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380

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Figure 1. XRD patterns of bulk and 2D (a) MoS2 and (b) WS2; Raman spectra of bulk and 2D (c) MoS2 and (d) WS2. The thicknesses of the 2D TMDs were determined by atomic force microscope (AFM). During the AFM measurement, the TMDs were cast on freshly cleaved mica substrates. As shown by the images and corresponding statistical data in Figure 2a-d, the thicknesses of MoS2 and WS2 were 3.6 - 8.0 nm (corresponding to 4 - 9 layers) and 2.5 - 4.0 nm (3 - 5 layers), respectively. The most probable layers for MoS2 and WS2 were 6 and 3, respectively. TEM images (Figure 2e & 2f) confirm the 2D structure of these two exfoliated TMDs. As shown in the high-resolution TEM (HR-TEM) images of MoS2 and WS2 in Figure 2g & 2h, different lattice domains were observed, corresponding to the 2H and 1T phases of these two TMDs.33, 41-42


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Figure 2. AFM height images of 2D (a) MoS2 and (b) WS2 cast on mica substrates; Statistical data of the thickness distribution for 2D (c) MoS2 and (d) WS2; TEM images of 2D (e) MoS2 and (f) WS2; HR-TEM images of 2D (g) MoS2 and (h) WS2. MoS2 and WS2 can exist in form of two phases, i.e. the natural semiconducting 2H and metallic 1T phases, which quantitatively determined by XPS.43 In Figure 3a, the Mo 3d signals of bulk MoS2 appear at ~ 229.5 and ~ 232.6 eV, corresponding to the binding energy of Mo4+ 3d5/2 and Mo4+ 3d3/2, respectively, both indicate the formation of pure 2H phase. The 2D MoS2 (exfoliated from the bulk material) shows two groups of peaks in the XPS spectra after deconvolution. Two new peaks appeared at ~ 228.6 and ~ 231.7 eV, corresponding to the binding energy of Mo4+ 3d5/2 and Mo4+ 3d3/2 of 1T phase, which takes 62% of the total 7 ACS Paragon Plus Environment


amount calculated by integration of the peaks, as shown in Table 1. Since post-heating to the devices at 90 °C to induce the crystallization of perovskite was conducted during the preparation of Pero-SCs, the XPS spectra of MoS2 was measured after the samples being treated at 90 °C for 5 min. Post-heating at 200 °C was also selected, because analogous conditions were often adopted in the literature.28, 44-45 The post-heating at 90 °C only cause a moderate change of percentage of 1T phase (decreased to 51%), while the post-heating at 200 °C resulted in a dramatic decrease to 17%. 1T-Phase

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2D MoS2

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Figure 3. XPS spectra of (a) bulk MoS2, 2D MoS2 (as prepared, treated by post heating at 90 and 200 °C) and (b) bulk WS2, 2D WS2 (as prepared, treated by post heating at 90 and 200 °C). Table 1. The content of 2H and 1T phases in different MoS2 and WS2 samples. MoS2 Bulk 2D Post-heating at 90 °C Post-heating at 200 °C

2H (%) 100 38 49 83

WS2 1T (%) 0 62 51 17

2H (%) 100 44 48 81

1T (%) 0 56 52 19

The XPS spectra and corresponding percentage of the two phases of WS2 were presented in Figure 3b and Table 1. The W 4f signals of 2H phase locate at ~ 32.4 and ~ 34.6 eV, which should be assigned to W4+ 4f7/2 and W4+ 4f5/2, and after exfoliation (i.e. formation of 2D WS2) the signals of 1T phase appeared at ~ 31.7 and ~ 33.7 eV. The percentage of 1T phase changed from 56% to 52% and then to 19%, as the 2D WS2 was treated by post-heating at 90 8 ACS Paragon Plus Environment

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and 200 °C, respectively. Interestingly, two peaks located at 36 and 38 eV appeared after exfoliation, which indicates the existence of W6+ species, possibly due to the formation of WO3 during the exfoliation process.46 The above results indicate that the 2D TMDs owns quite different percentage of 1T phase after being treated at different temperatures. For convenience of description, the 2D TMDs were differentiated by 1T-rich (> 50%) and 1T-poor (< 20%) according to the percentage of 1T phase. These results accord well with that reported in the previous literature.28, 33 In the following, the effect of percentage of 1T phase caused by post-heating on the performance of Pero-SCs were investigated. Herein, the device configuration was ITO/2D TMDs/perovskite/C60/Bathocuproine (BCP)/Al. The thickness of perovskite layer was controlled to ~ 320 nm, and both the thicknesses of 2D MoS2 and WS2 were optimized to 11 nm according to the PCEs. The detailed data for optimization refer to Figure S1 and Table S1. The typical J-V curves of the cells based on 2D MoS2 and WS2 with different phases were shown in Figure 4a, and the detailed photovoltaic characteristics are summarized in Table 2. Apparently, Pero-SCs with 1T-rich 2D TMDs as HTLs show much better photovoltaic properties comparing with those with 1T-poor TMDs. Taking 2D MoS2 for an example, the Pero-SCs with 1T-rich MoS2 as HTL presented a PCE of 13.62% with open circuit voltage (Voc) of 0.87 V, short circuit current density (Jsc) of 20.35 mA cm-2 and fill factor (FF) of 77.0%, while the device with 1Tpoor MoS2 showed a PCE of 7.64% with Voc of 0.62 V, Jsc of 18.70 mA cm-2 and FF of 66.3%. 5

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Figure 4. (a) J-V curves of devices with 1T-rich or 1T-poor TMDs; (b) PL intensities spectra of perovskite films on top of the 1T-rich or 1T-poor TMDs. Table 2. J-V characteristics of Pero-SCs with 1T-rich and 1T-poor TMDs. HTL 1T-rich MoS2 1T-poor MoS2 1T-rich WS2 1T-poor WS2

Voc (V) 0.87 0.62 0.93 0.56

Jsc (mA cm-2) 20.35 18.70 20.64 18.02

FF (%) 77.0 66.3 72.4 64.5

PCE (%) 13.62 7.64 13.83 6.54

The difference of the Jsc and FF values can be accounted by the steady-state photoluminescence (PL) of the perovskite films on the 1T-rich or 1T-poor TMDs, respectively. As shown in Figure 4b, the PL of the perovskite films on the 1T-rich TMDs is almost completely quenched comparison with that on the 1T-poor TMDs, indicating 1T-rich TMDs presented much better extraction ability of charge carriers. It can be explained by the different conductivity of 1T and 2H phases, where the former possesses much higher conductivity comparing with the latter.33, 47 In order to clarify the change of the Voc values, work functions (WFs) of ITO modified with 1T-rich or 1T-poor TMDs were measured with KPFM. No clear change (~ 0.1 eV) of WFs of ITO modified with 1T-rich or 1T-poor TMDs was observed in Figure S2. It is possibly that the change of Voc values would related to interface recombination caused by difference of hole transport ability.48 The above results and analysis indicate that the formation of 1T phase during lithium-intercalation process is favorable for high performances, while the high temperature treatment will decrease the content of 1T phase, thus resulting in reduced performances. In the rest of this article, 1T-rich TMDs were employed as HTLs in Pero-SCs. As HTLs of Pero-SCs, 2D MoS2 and WS2 were compared with the conventional PEDOT:PSS. When using PEDOT:PSS as HTL (Figure 5a & Table 3), the highest PCE acquired was 12.44% with Voc of 0.84 V, Jsc of 19.02 mA cm-2 and FF of 78.0%, which is superior to that (11.0%) reported by other groups using similar perovskite precursor and device structure.49 In contrast, when using MoS2 and WS2 as HTLs, both the highest PCEs 10 ACS Paragon Plus Environment

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were significantly improved. For cells based on MoS2, the highest PCE obtained was 14.35% with Voc of 0.88 V, Jsc of 20.94 mA cm-2 and FF of 77.9%. For devices based on WS2, the highest PCE reached 15.00% with Voc of 0.97 V, Jsc of 21.22 mA cm-2 and FF of 73.0%. It is noteworthy that the devices with both these two kinds of 2D TMDs showed clear hysteresis (Figure S3 and Table S2) tested at a speed of 20V s-1, and the reverse scan always give better performance with high Voc values. All the Jsc values were confirmed by integrated current densities (Jint) based on IPCEs (Figure 5b). The ITO glass slides with different HTLs show comparable transparency, although in some regions the slides with TMDs showed relatively higher transmittance, and the rest showed relatively lower transmittance, as shown in Figure S4a. Because the absorption coefficient of perovskite is much bigger than the ITO glass and HTLs, the absorption difference of the ITO substrates with different HTLs have very little influence on the absorption of perovskite film, which can be reflected the normalized absorption spectra (Figure S4b). The Jsc values matched well with the respective Jint, suggesting that the J-V curves should reflect the photovoltaic performance of the devices. Moreover, the IPCE curves also indicate that the photocurrents of the devices based on MoS2 or WS2 are higher in almost the whole detected wavelength range, demonstrating the priority of 2D TMDs in the charge transportation. 100

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Figure 5. Representative (a) J-V and (b) corresponding IPCEs curves of Pero-SCs with PEDOT:PSS, MoS2 and WS2 as HTL.



Table 3. Characteristics of Pero-SCs based on PEDOT:PSS, MoS2 and WS2 and average values and standard deviations were analyzed from 50 cells. HTL

Voc (V)

PEDOT :PSS

0.84 0.75 ± 0.07 0.88 0.87 ± 0.02 0.97 0.93 ± 0.03

MoS2 WS2

Jsc (mA cm-2) 19.02 19.04 ± 0.69 20.94 20.51 ± 0.26 21.22 20.29 ± 0.47

FF (%)

PCE (%)

78.0 77.0 ± 3.1 77.9 74.4 ± 2.0 73.0 71.7 ± 1.7

12.44 10.96 ± 0.62 14.35 13.33 ± 0.38 15.00 13.65 ± 0.36

Jint (mA cm-2) 18.15

Rs (Ω cm-2) 0.9

Rp (kΩ cm-2) 8.9

20.87

0.6

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6.3

To check the reproducibility and acquire reliable assessment of the PCEs, 50 solar cells were statistically analyzed, and the results are presented in Figure 6 & Table 3. It is clearly shown that the PCEs of devices with MoS2 and WS2 have relatively narrower distribution, and the most probable PCEs are much higher comparing with that of device with PEDOT:PSS. The average PCEs of devices based on MoS2 and WS2 were 13.33% and 13.65%, respectively, while that based on PEDOT:PSS was only 10.96%. The improvement of PCE is mainly originated from the enhancement of Voc and Jsc values. PEDOT:PSS MoS2 WS2

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Figure 6. Histograms of PCEs of Pero-SCs with PEDOT:PSS, MoS2 and WS2 as HTLs. Each group of data were analyzed from 50 cells. The enhanced Voc could be explained by better interfacial energy alignment between the two 2D TMDs and the perovskite layer.50-51 WFs of ITO substrates modified with MoS2, WS2 and PEDOT:PSS were determined with KPFM and the data were summarized in Figure 7. The WFs of ITOs modified with PEDOT:PSS, MoS2 and WS2 were 5.02, 5.13 and 5.18 eV, respectively. The differences of energy levels between the WF of ITOs modified with MoS2 / WS2 and the valence band of perovskite are less than 0.2 eV, suggesting smoother hole 12 ACS Paragon Plus Environment

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transfer and higher Voc can be realized.52 Moreover, between MoS2 and WS2, the latter shows a bit higher WF, closer to the valence band of perovskite, implying higher Voc, which consist with the values provided by J-V curves. Valence Band of Perovskite 5.40

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5.00 5.01 4.80

4.75

Figure 7. WFs of ITO substrates modified with PEDOT:PSS, 2D MoS2 and 2D WS2. To study the charge extraction ability of the three HTLs, the steady and transient state PL of the perovskite films were measured. As shown in Figure 8a, the PL is strongly quenched in the presence of MoS2 and WS2, which explains the facilitated charge transportation between perovskite and ITO.53 Transient state PL measurements were used to examine the ability of MoS2 and WS2 HTL on the charge transportation and recombination. The curves were fitted with a double exponential decay function including a fast (life time τ1, weight fraction f1) and a slow (life time τ2, weight fraction f2) decay processes in Figure 8b. The fast decay component (τ1, f1) stems from the quenching of photo-generated of free carrier of perovskite through the charge transportation from perovskite to HTLs and electrode. The slow decay component (τ2, f2) can be attributed to the process of radiative recombination in perovskite.10, 54

The detailed parameters extracted from the decay curves were summarized in Table 4. In

case of perovskite@PEDOT:PSS, τ1 and τ2 were 6.9 (f1 = 37.6%) and 57.6 ns (f2 = 65.4%), respectively. In case of perovskite@MoS2 and perovskite@WS2, τ1 and τ2 were similar to those in case of PEDOT:PSS/perovskite, but f1 increased to 49.0% (for perovskite@MoS2) and 48.5% (for perovskite@WS2). The higher f1 suggest that the charges should be more effectively dissociated and transported when using MoS2 or WS2 as HTLs. 13 ACS Paragon Plus Environment


PEDOT:PSS MoS2

PL Intensity

(a)

(b)

PEDOT:PSS MoS2

103

WS2 Fitted curves

Intensity

WS2

102

720

760

800

0

840

20

40

60

80

100

Times (ns)

Wavelength (nm)

Figure 8. (a) PL intensities and (b) transient state PL spectra of perovskite films on PEDOT:PSS, 2D MoS2 and 2D WS2. Table 4. Lifetimes and weight fractions fitted from the transient state PL. HTL PEDOT:PSS MoS2 WS2

τ1 (ns) 6.9 6.0 5.7

f1 (%) 37.6 49.0 48.5

τ2 (ns) 57.6 56.9 51.4

f2 (%) 62.4 51.0 51.5

To further compare the charge transportation abilities of the three HTLs in Pero-SCs, transient photocurrent was measured under a white bias light, as shown in Figure 9a. The charge-extraction time was taken when the photocurrent decays to 1/e. The values were 2.3, 1.2 and 1.9 µs for Pero-SCs based on PEDOT:PSS, MoS2 and WS2, respectively. The shorter charge-extraction time indicates that as using 2D TMDs as HTLs the dissociation and transportation become easier, thus leading to the improved Jsc. PEDOT:PSS MoS2

1.0

WS2

0.5

40

Rsc (Ω cm2)

(a) Photocurrent (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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PEDOT:PSS MoS2

(b)

WS2 35

30

25

0.0

0

1

2 3 Time (µs)

4

5

0.0

0.2

0.4

0.6

0.8

Vapp (V)

Figure 9. (a) TPC decay and (b) Rsc versus Vapp extracted from EIS of Pero-SCs based on PEDOT:PSS, MoS2 and WS2. In addition, the J-V curves measured in the dark were shown in Figure S5. The series resistance (Rs) of the Pero-SCs with 2D TMDs was reduced comparing with that of the PeroSCs with PEDOT:PSS, while the shunt resistances (Rp) were comparable, as shown in Table 3. The reduction in Rs would improve the charge transportation, and thus led to increase of Jsc. 14 ACS Paragon Plus Environment

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EIS was employed to study the charge transporting process.14, 55 The measurement was carried out under 1 sun AM irradiation, and the applied voltages (Vapp) varied from 0 to 0.9 V with step of 0.1 V. The EIS curves and the details of data processing are presented in Figure S6 and Table S3. To simplify the discussion, only the selective contact resistances (Rsc, fitted from the arcs of higher frequency regions) that reflect the impact of interface between HTL and perovskite layers were extracted and plotted versus the corresponding Vapp in Figure 9b. Clearly, the Rsc values of Pero-SCs based on MoS2 or WS2 were much smaller than that based on PEDOT:PSS, which also explains the enhancement of the corresponding Jsc. In the last part of this study, the stability of the cells with different HTLs was investigated. Since the cells were not capsulated, they were kept in the glovebox filled with nitrogen during the measurement. As shown in Figure 10, the average PCEs of the devices based on 2D MoS2 and 2D WS2 remained 78% and 72% after being stored for 56 days, respectively, whereas that based on PEDOT:PSS decayed quickly, and dropped down to almost 0 in 35 days. To track down the reasons for the difference of stability, a group of control experiments were conducted by observing the degradation of perovskite film on different HTLs under parallel storage conditions (~ 15 °C, 20% - 45% humidity) (scanning electron microscopy (SEM) images are presented in Figure S7). The results indicate that perovskite@PEDOT:PSS degraded faster than perovskite@MoS2 and perovskite@WS2. We assume that the hygroscopic feature of PEDOT:PSS should be responsible to the accelerated degradation of perovskite, thus resulting in the quicker decay of the PCEs.21, 56 1.0

Normalized PCE

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0.8 0.6

PEDOT:PSS MoS2

0.4

WS2

0.2 0.0 0

10

20

30

40

50

60

Time (days)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 10. Plots of PCEs of Pero-SCs based on PEDOT:PSS, MoS2 and WS2 versus the storage time. For each group of measurement, 20 cells were adopted for analysis. CONCLUSIONS Two water-soluble 2D TMDs, MoS2 and WS2, were employed as HTLs in Pero-SCs, and showed superior photovoltaic performance compared with the conventionally PEDOT:PSS. (1) The content of 1T phase that generated from lithium intercalation reaction is centrally important to the performance of solar cells. The post-heating at high temperature causes decrease of 1T phase, thus resulting in reduced PCEs. (2) As PEDOT:PSS was replaced by MoS2 and WS2, the PCEs of Pero-SCs were elevated from 12.44% to 14.35% and 15.00%, respectively. The improvement of the PCEs mainly originated from the enhanced Voc and Jsc, which could be resulted from the better match of energy levels between modified ITOs and valence band of perovskite and the facilitated charge dissociation and transportation caused by MoS2 and WS2. (3) Pero-SCs based on MoS2 and WS2 show greatly improved stability comparing with that based on PEDOT:PSS. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:. J-V curves of devices with different thickness of 2D TMDs; WFs; J-V curves of devices at both forward and reverse scans; transmittance and absorption spectra; J-V curves measured in the dark; EIS data and SEM images.

AUTHOR INFORMATION Corresponding Authors

*E-mail:[email protected]; [email protected].

ACKNOWLEDGEMENTS 16 ACS Paragon Plus Environment

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This work were supported by the National Natural Science Foundation of China (51673139), A Priority Academic Program Development of Jiangsu Higher Education Institutions, State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials.

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Table of Contents PEDOT:PSS

MoS2 PCE (%)

Al C60/BCP MAPbI3-xClx

10 5

10 5

0 50 1T phase (%)

Normalized PCE

15

WS2

15

2D MoS2 / WS2 ITO

PCE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

0 0

30 60 Time (days)


Water-Soluble 2D Transition Metal Dichalcogenides as the Hole

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