Solution-processed MoS_x thin-films as hole-transport layers for efficient polymer solar cells

Abstract

Here we employed a simple solution-processed method to prepare MoS_x films as hole-transport layers (HTLs) in polymer solar cells (PSCs). Impressive performance was achieved with a power conversion efficiency (PCE) of 7.50%, which is comparable to those of the conventional devices with PEDOT:PSS HTLs. The surface and optoelectronic characteristics of MoS_x films obtained from different conditions were investigated. The results reveal that the annealing temperature of MoS_x is one of the most important factors on determining the device performance. PSCs based on MoS_x films exhibited much better device stability compared to the counterpart based on PEDOT:PSS, suggesting that the MoS_x film is a promising alternative HTL for PSCs.

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Introduction

Polymer solar cells (PSCs) have attracted increasing attention in the past decade, and significant breakthroughs in power conversion efficiencies (PCEs) have been achieved through extensive efforts from material design to device engineering.^1–4 Compared to the inorganic counterparts, PSCs have the advantages of low cost, light weight and good flexibility. In addition, organic materials are soluble in common organic solvents, and thus the roll-to-roll process at low cost can be used to make large area devices.⁵ In PSCs, interfaces between bulk heterojunctions (BHJs) and electrodes greatly affect the charge extraction and transport processes. Also, the bottom interface may influence the film morphology of the BHJs.⁶ As a result, interface engineering plays an important role on the device performance of PSCs.

In conventional PSCs the conducting polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), is widely used to modify the work function (WF) of the indium tin oxide (ITO) as an efficient hole-transport layer (HTL). PEDOT:PSS has good optical transparency and an appropriate WF of ∼5.1 eV which makes it easy to form good ohmic contact at the electrode/BHJ interlayer.⁷ Thus, PSCs with PEDOT:PSS buffer layers have shown good performance, however, the devices are relatively unstable owning to the possible reactions between the PEDOT:PSS and the electrode caused by its acidity and hygroscopic nature.^8,9 On the other hand, the electron leakage at the anode with PEDOT:PSS has been revealed to be another disadvantage.¹⁰ To overcome these defects, transition metal oxides have been investigated as efficient hole-transport materials to substitute PEDOT:PSS, such as V₂O₅, NiO_x, WO₃, and MoO₃.^11–14 For example, PSCs with high PCEs and good stability have been fabricated by using solution-processed MoO₃ films due to their good morphological and electronic properties.^15–17 Meanwhile, transition metal sulfides such as MoS₂, WS₂, and CuS have also come into sight due to their unique electrical properties and two-dimensional (2D) laminated structures.^18–21

With improved hole extraction and transport properties, MoS₂ films are usually fabricated as HTLs by a lithium intercalation and exfoliation process, which needs a relatively complicated procedure or other additional treatments.^22–26 Alternatively, MoS_x films can be prepared through a solution-processed method,²⁷ which is more promising for large-scale production of organic solar cells. However, only few reports have been focused on solution-processed MoS_x films for PSCs, and the highest PCE achieved so far is merely 3.90% based on the active layer of poly(3-hexylthiophene) and [6,6]phenyl-C61-butyric acid methyl ester (P3HT : PC₆₁BM). As a universal hole-transport material, solution-processed MoS_x films can be incorporated into other active layer systems for PSCs with better device performance. At the same time, it is worth to investigate the relationship between the structure of MoS_x film and the performance of the resulting PSCs.

In this work, MoS_x films were fabricated by spin-coating the MoS_x precursor solution onto ITO/glass substrates with an annealing in air. The device configuration is shown in Fig. 1, where the active layer is based on a blend of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th) and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM). Conventional devices with PEDOT:PSS as HTLs were also fabricated by the same experimental condition for comparison.

Fig. 1 Schematic illustration of device configuration and molecular structures of PTB7-Th and PC₇₁BM.

The annealing temperature of MoS_x films significantly influenced the device performance, which may be ascribed to the modified interlayer characteristics (the annealing time is also an important factor as shown in Fig. S1†). Through optimization, the MoS_x-based PSCs showed an impressive PCE of 7.50%, which approached the PCE of PEDOT:PSS-based device (8.00%). It is worth mentioning that the device based on MoS_x was demonstrated to be more stable than that based on PEDOT:PSS.

Experimental section

Materials

(NH₄)₂MoS₄ obtained from Alfa Aesar was used to prepare MoS_x precursor solution. PTB7-Th was synthesized in our lab according to the published method¹ and PC₇₁BM was purchased from American Dye Source Inc. Other materials and reagents were purchased from Sigma-Aldrich Inc., Aladdin-Reagent Inc. or Adamas-beta Ltd. and used without further purification.

Preparation and characterization of MoS_x films

A sol–gel process was used to fabricate MoS_x films. To prepare the MoS_x precursor solution, (NH₄)₂MoS₄ was dissolved in deionized water with a concentration of 12 mg mL⁻¹, and then stirred at 60 °C in a nitrogen atmosphere. The ITO/glass substrates were cleaned by sequentially ultrasonication in detergent, deionized water, acetone, and isopropyl alcohol for 30 min each and then dried at 80 °C overnight. The precursor films were prepared by spin-coating the (NH₄)₂MoS₄ solution at 1200 rpm for 90 s on the ITO/glass substrates, after pretreating the substrates by UV-ozone for 15 min. Then the films were annealed at various temperatures (200, 250 and 300 °C) for 60 min in air and transferred into a N₂-filled glovebox directly.

UV-vis absorption and transmission measurements were recorded by a Lambda35 spectrophotometer at room temperature. The film thicknesses were determined by a Bruker Dektak XT surface profiler. Atomic force microscopy (AFM) images were performed in air by using a Bruker Nanoscope 8.15 atomic force microscope. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were used for surface chemical analysis through a Thermo Scientific ESCALAB 250Xi XPS/UPS system with a monochromatic Al Kα source. The samples were tested after argon-ion etching for 30 s on the surfaces.

Hole-only devices (ITO/MoS_x/PTB7-Th : PC₇₁BM/Au) were fabricated to estimate the hole-transport property of MoS_x films by the space charge limited current (SCLC) method. The hole mobilities were calculated by the equation:²⁸ J = 9ε₀ε_rμV²/8L³, where ε₀ is the permittivity of free space, ε_r is the dielectric constant of the materials, μ is the hole mobility, V is the voltage drop across the device (V = V_app − V_bi − V_r, where V_app is the applied voltage to the device, V_bi is the built-in voltage due to the difference in WF of the two electrodes, V_r is the voltage drop due to contact resistance and series resistance across the electrodes), L is the thickness of the organic film.

Device fabrication and measurements

PSCs were fabricated with various MoS_x films as the HTLs. PTB7-Th and PC₇₁BM with a 1 : 1.5 w/w ratio were dissolved together in a mixed solvent of chlorobenzene and 1,8-diiodooctane (100 : 3 by volume) to give an overall 25 mg mL⁻¹ solution. The active layer was fabricated in the glovebox by spin-coating the BHJ blend of PTB7-Th : PC₇₁BM on the MoS_x film. Then, the conjugated polymer poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN)²⁹ was dissolved in methanol with a small amount of acetic acid (6 mg mL⁻¹) to make a 0.3 mg mL⁻¹ solution, and spin-coated onto the active layer at 1500 rpm for 30 s as cathode modification layer.³⁰ Finally, the devices were transferred into a vacuum chamber and 100 nm of aluminum was deposited through shadow masks by thermal evaporation under a vacuum of ∼5 × 10⁻⁵ Pa. The effective area of the device was 4 mm². Solar cell characterization was performed under AM 1.5G irradiation (100 mW cm⁻²) by an Oriel Sol3A simulator (Newport) with a NREL-certified silicon reference cell. After the device encapsulation, the current density–voltage (J–V) characteristics were measured by a Keithley 2440 source measurement unit. The EQE spectra were measured by a Newport EQE measuring system.

Results and discussion

Chemical analysis of MoS_x films

To study the surface chemical states of MoS_x, we performed XPS measurements. The samples on ITO/glass substrates were tested after surface cleaning. The photoelectron binding energy scale was calibrated to the C 1s peak for the C–C bonds at 284.8 eV. After fitting by Gaussian–Lorentzian curves, the Mo 3d XPS spectra of MoS_x films obtained at different annealing temperatures are shown in Fig. 2. The relative contents of different valence states of molybdenum are estimated from the cover of the Gaussian curves. For the MoS_x film obtained at 200 °C (Fig. 2a), the Mo 3d spectrum is fitted to the doublet peaks at 229.0 eV and 232.2 eV, corresponding to the Mo⁴⁺ peaks, and the doublet peaks at 231.2 eV and 234.4 eV, corresponding to the Mo⁵⁺ peaks. The peak at 226.2 eV belongs to the S 2s peak. The results reveal that two molybdenum oxidation states coexist in the film (31.28% Mo⁴⁺, 68.72% Mo⁵⁺). According to the literature,³¹ the spin–orbit doublet peaks at 228.8 eV and 231.9 eV belong to MoS₂. Our results show that the binding energy of Mo⁴⁺ increased about 0.2 eV. Because the MoS_x samples had been stored in air for a couple of days before XPS measurements, the presence of Mo⁵⁺ peaks may be ascribed to the partial oxidization of Mo from Mo⁴⁺ to Mo⁵⁺. This may also lead to a higher defect density and increase the binding energy. No prominent peak belongs to Mo⁶⁺ at about 236 eV can be observed, which proves that the MoS_x films haven't been oxidized to MoO₃.

Fig. 2 XPS spectra of the Mo 3d and S 2s core level peaks for MoS_x films obtained at (a) 200, (b) 250, (c) 300 °C. (d) S 2p peaks for different annealing temperatures.

For the MoS_x film obtained at 250 °C (Fig. 2b), the Mo 3d spectrum is fitted to the doublet peaks at 229.8 eV and 233.0 eV, corresponding to the Mo⁴⁺ peaks, and the doublet peaks at 231.7 eV and 234.9 eV, corresponding to the Mo⁵⁺ peaks. The peak at 226.4 eV belongs to the S 2s peak. The contents of Mo⁴⁺ and Mo⁵⁺ are 30.11% and 69.89%, respectively.

At 300 °C (Fig. 2c), the film component is 32.51% Mo⁴⁺ with doublet peaks at 229.5 eV and 232.7 eV, and 67.49% Mo⁵⁺ with doublet peaks at 231.2 eV and 234.4 eV. The S 2s peak is fitted at 226.0 eV. The results of valence states, peak positions and relative contents of different MoS_x films calculated from Mo 3d scan are summarized in Table S2.† At different annealing temperatures, we fabricated MoS_x films with different components. The slightly changed peak positions may be caused by different degrees of oxidization, which increase the density of defects in the films. When the annealing temperature increases from 200 °C to 300 °C, the peaks of S 2p at about 162.0 eV were observed for all three samples (Fig. 2d), further verifying the presence of MoS₂.³¹

We also measured MoS_x films by UPS, as shown in Fig. 3a. WFs of MoS_x films obtained at 200, 250, 300 °C are calculated to be 5.0, 5.1, 5.1 eV, respectively. In the literature, single-layered and undoped MoS₂ film has a WF of 4.4–4.5 eV.²³ The larger WF of MoS_x film may be ascribed to the presence of Mo⁵⁺, which increases the binding energy. As a result, the WF of MoS_x matches well with the HOMO of PTB7-Th (5.22 eV), which reduces the energy barrier between the active layer and the ITO anode, as illustrated in the flat energy band diagram (Fig. 3b). The relative increased WFs of MoS_x obtained at higher temperatures promote hole extraction and transport process, and improve the device performance.

Fig. 3 (a) UPS spectra of MoS_x films on ITO/glass substrates obtained at different temperatures. (b) Flat energy band diagram of the components used in the PSCs.

Surface morphology of MoS_x films

Since the device performance can be immensely influenced by the surface morphology of the interlayer, we further studied the surface characteristics of MoS_x films by AFM. Fig. 4 shows the height images, three-dimensional (3D) surface topography images and surface profiles of MoS_x films (on ITO substrates) obtained at different annealing temperatures. The bare ITO surface image was also captured as a comparison. The ITO shows a relatively rough surface with a root-mean-square (RMS) roughness of 0.66 nm, while the roughness of the MoS_x film obtained at 300 °C is only 0.50 nm. Different from the ITO with a regularly arranged morphology, the MoS_x films exhibit undulating topographies and smoother surfaces, which promote better interface contact and reduce the surface defects. The MoS_x film processed at 200 °C shows a smaller particle packing surface, while those processed at higher temperatures show a sheet stacked surface. The special morphology of MoS_x obtained at 200 °C may affect the electrical properties of the interface and the ohmic contact with the active layer, leading to the poor device performance.

Fig. 4 Tapping-mode AFM images of MoS_x films on ITO and bare ITO surface, including height images, 3D topography images and surface profiles of (a) bare ITO (b) 200 °C MoS_x (c) 250 °C MoS_x (d) 300 °C MoS_x.

Optical transmittance spectra of bare ITO and ITO/MoS_x films are depicted in Fig. 5. High optical transparency can be observed in the visible region. Although MoS_x films fabricated at different annealing temperatures exhibit some differences in film characteristics, they show similar optical transparencies.

Fig. 5 Linear transmission spectra of MoS_x films obtained at different temperatures.

Device performance

Fig. 6a shows the J–V characteristics under AM 1.5G irradiation (100 mW cm⁻²). The PCEs of devices with MoS_x HTLs obtained at different annealing temperatures show great difference. For the device with MoS_x film obtained at 200 °C, the PCE is only 3.55%. The poor performance can be ascribed to the low open-circuit voltage (V_oc) of 0.55 V, short-circuit current density (J_sc) of 16.36 mA cm⁻² and fill factor (FF) of 39.21%. The device performance is improved as the annealing temperature increased from 200 °C to 300 °C. For the device with MoS_x film obtained at 300 °C, the PCE is significantly improved to 7.50%, with increased V_oc (0.77 V), J_sc (18.16 mA cm⁻²) and FF (53.56%). The improved device performance is mainly due to the modified interface characteristics between the ITO and active layer, which avoid the direct contact and enhance the charge extraction and transport processes. As the annealing temperature increased from 200 to 300 °C, reduced series resistance (R_s) and increased shunt resistance (R_sh) of the device were observed which is in agreement with the improved charge-transport process for the device with high temperature annealed MoS_x. The high PCE is close to that of PEDOT:PSS-based devices fabricated in the same experimental condition for comparison (8.00%). The results are summarized in Table 1. Compared to the PEDOT:PSS-based device, a significantly increased J_sc is achieved for the MoS_x-based device. The PCE distribution of devices with MoS_x obtained at 300 °C is depicted in Fig. S2.†

Fig. 6 (a) J–V characteristics of PSCs with MoS_x obtained at different temperatures and PEDOT:PSS as HTLs and (b) the corresponding EQE spectra (hollow symbols are the integrated photocurrents with the AM 1.5G spectrum).

Table 1 Device characteristics of PSCs with MoS_x HTLs (obtained at 200, 250, 300 °C) and PEDOT:PSS HTLs

HTL	Voc (V)	Jsc (mA cm^−2)	FF (%)	PCEmax^a (%)	Rs (Ω cm^2)	Rsh (Ω cm^2)
a The data in parentheses represent average PCEs obtained from eight devices. The Rs and Rsh were calculated by the inverse of the slope of the corresponding J–V curves under illumination at J = 0 and V = 0, respectively.
MoSx 200 °C	0.55	16.36	39.21	3.55 (3.14 ± 0.25)	19.65	207.49
MoSx 250 °C	0.64	16.89	35.65	3.86 (3.72 ± 0.11)	35.08	257.59
MoSx 300 °C	0.77	18.16	53.56	7.50 (7.36 ± 0.11)	14.11	491.90
PEDOT:PSS	0.79	16.05	62.70	8.00 (7.84 ± 0.15)	9.69	773.36

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The corresponding EQE spectra of PSCs incorporating MoS_x and PEDOT:PSS are shown in Fig. 6b. High EQEs across the visible region can be observed for devices with MoS_x HTLs. The MoS_x-based device (300 °C) shows obviously higher EQEs especially from 450 to 600 nm compared to those of the PEDOT:PSS-based device. Integrating the EQE data with the AM 1.5G spectrum, we achieve the J_sc of 16.10, 16.60, 17.74, and 15.94 mA cm⁻² for devices with MoS_x obtained at 200, 250, 300 °C and PEDOT:PSS, respectively. Calculated J_sc values show less than 3% error compared to the measured values in Table 1.

The hole-transport abilities of MoS_x and PEDOT:PSS films were studied by the SCLC method. From J–V characteristics of the hole-only devices (Fig. 7), hole mobilities of devices with MoS_x obtained at 200, 250, 300 °C and PEDOT:PSS are estimated to be 4.52 × 10⁻⁶, 1.14 × 10⁻⁵, 5.97 × 10⁻⁵, and 2.63 × 10⁻⁵ cm² V⁻¹ s⁻¹, respectively (the thicknesses of active layers are measured to be about 128, 144, 168, and 137 nm, respectively). The much higher hole mobility of MoS_x obtained at 300 °C, even higher than PEDOT:PSS, may contribute to the better photovoltaic performance of the resulting device (Table 1).

Fig. 7 (a) J–V characteristics in the dark for hole-only devices with MoS_x and PEDOT:PSS HTLs; (b) the corresponding J^0.5–V characteristics. The solid lines are fits of the data points.

Device stability

Device stability also plays a key role in PSCs. To study the long-term stability of the devices with MoS_x, the normalized PCEs as a function of storage time in air are depicted in Fig. 8. The device with MoS_x HTL showed much better stability than the device with PEDOT:PSS HTL. After encapsulation, PEDOT:PSS exhibited a large degradation in three weeks. And after five weeks, only 13% of its original PCE value maintained. However, the device with MoS_x was more stable. Even after two months, the device with MoS_x still remained more than 52% of the initial efficiency. As a result, using MoS_x to substitute PEDOT:PSS as HTL in conventional PSCs is proved to be an effective strategy to improve the long-term device stability.

Fig. 8 Device stability of PSCs with MoS_x or PEDOT:PSS.

Conclusion

In summary, MoS_x films were fabricated as HTLs for PSCs by a simple solution-processed method and their characteristics were examined. XPS analysis illustrated that MoS_x films consisted of multiple valence states with Mo⁴⁺ and Mo⁵⁺. PSCs incorporating MoS_x HTLs showed good performance with a highest PCE of 7.50%, which is close to those of the traditional devices with PEDOT:PSS HTLs. The results showed that the photovoltaic properties of MoS_x films significantly depended on their annealing temperature. All of the photovoltaic parameters were improved as the annealing temperature increased from 200 to 300 °C, which can be attributed to the smoother surface, higher mobility, and a proper WF of 5.1 eV. Additionally, the MoS_x-based device remained more than 50% of the initial PCE value even after two months, which was much more stable than the PEDOT:PSS-based conventional device. In general, this study demonstrates that solution-processed MoS_x films are promising HTLs for high performance and stable PSCs.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NOs. 61325026 and 51173186), and the CAS/SAFEA International Partnership Program for Creative Research Teams.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The PCE distribution of devices with MoS_x obtained at 300 °C, summary of XPS analysis calculated from Mo 3d scan and device performance of different experimental conditions. See DOI: 10.1039/c6ra01204c

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