Effect of Au@MoS2 Contacted PEDOT:PSS on Work Function of Planar Silicon Hybrid Solar Cells

Solar cells formed by spin‐coating organic absorber layers on silicon have attracted widespread attention due to their simple processes and high photovoltaic conversion efficiency (PCE). In typical organic/Si solar cells, however, surface defects or unsatisfactory carrier separation are inadequate to yield excellent device performance. Here, the Au@MoS2 nanocomposites are well synthesized and doped into the organic layer of poly (3,4‐ethylenedioxythiophene)/polystyrene sulfonate (PEDOT:PSS) to improve its work function and the performance of PEDOT:PSS/Si HSCs consequently. By optimizing the doping level of Au@MoS2, the PCE significantly improved from 11.48% to 14.0% by tuning the work function of the PEDOT:PSS layer to more appropriate values. The calculated results based on the Mott–Schottky model indicate that the built‐in field in the PEDOT:PSS/Si interface of HSCs is significantly enhanced due to the increase of work function by the PEDOT:PSS thin films. The enhancement of the built‐in field results in the reduction of the electron–hole recombination loss effectively. The work provides a feasible method for preparing high‐performance PEDOT:PSS/Si HSCs.


Introduction
Organic-Si hybrid solar cells (HSCs) have attracted considerable research owing to their potential for high power conversion efficiency (PCE) and low fabrication cost. [1,2] In particular, as a DOI: 10.1002/admi.202300187 conducting polymer, poly(3,4ethylenedioxythiophene)/polystyrene sulfonate (PEDOT:PSS) plays a more and more important in promoting the performance of HSCs. [3,4] The PEDOT:PSS layer in the HSCs usually undertakes multiple functions, including charge extraction, hole-transporting, surface passivation, etc. [5][6] Therefore, the modification and optimization of PEDOT:PSS has become an effective means to acquire high-efficiency HSCs. [7] In pristine PEDOT:PSS solution, conductivity is extremely low (<1 S cm −1 ), but it can be significantly increased (≈1000 S cm −1 ) by adding co-solvents, such as dimethyl sulfoxide (DMSO), ethylene glycol (EG), or N,N-dimethylformamide (DMF). [8,9] In addition, there is still space to improve the film-forming properties of PEDOT:PSS by optimizing the interface contact between PEDOT:PSS and the Si substrate. Some surfactants, such as Triton-X100 and fluorinated surfactants, had introduced into the PEDOT:PSS solution to enhance its wettability. [10,11] However, if only high conductivity and good contact with Si substrates are achieved, the optimization abilities of the HSCs are insufficient. [12,13] The performance of the HSCs is heavily dependent on the work function (WF) of the PEDOT:PSS thin films. [14] The WF of PEDOT:PSS thin films can be improved well by two strategies. One is doping with high-WF materials, and the other is coating PEDOT:PSS thin films with high-WF materials. [15][16][17] The 2D transition-metal dichalcogenides (TMDs), especially molybdenum disulfide (MoS 2 ), are widely considered candidates due to their outstanding charge transportation. [12,15] W. Xing et al. used MoS 2 quantum dots to improve the work function of organic solar cells, resulting in a 12% improvement in PCE. [17] C-Y. Tu et al. demonstrated that a field-effect transistor (FET) with a single MoS 2 layer exhibits mobility as high as ≈200 cm 2 V −1 S −1 . [18] X. Gu et al. used few-layer MoS 2 acted as a hole transport layer and was employed in the inverted organic solar cells owing to its high transparency and tunable WF and creating the outstanding performance of the device. [19] Introducing the Au or Ag nanoparticles can improve the light emission and absorption of TMDs in visible bands by taking advantage of the localized surface plasmonic resonance (LSPR) effect. Xiaojuan Shen et al. prepared Si organic solar cells using Au nanoparticles doped PEDOT:PSS making an efficiency of 12.8%. [20] In addition to optical properties, Au nanoparticles also exhibit electrical properties. Ashim Chandra Bhowal et al. obtained better electrical properties by introducing Au nanoparticles into PEDOT:PSS matrices. [21] However, the properties of a single material are limited, which provides the opportunity to design a composite material. Here we synthesized Au@MoS 2 composites to explore the synergistic effects of the composites in PEDOT:PSS films. Au@MoS 2 -PEDOT:PSS as a multifunctional thin film layer embodies the common properties of two materials, which broadens the idea of preparing a hole transport layer with excellent properties. It also implies that Au@MoS 2 composites have a prominent potential to significantly contribute to high-efficiency PE-DOT:PSS/Si HSCs.

Photovoltaic Properties of PEDOT:PSS with Au@MoS 2 Thin Films
To comprehend the chemical ingredient of PEDOT:PSS with/without Au@MoS 2 films. Figure 1 shows the analysis with or without Au@MoS 2 (0.15 mg mL −1 ). A survey of the X-ray photoelectron spectrum (XPS) is in Figure 1a. Certificate Au@MoS 2 doped in the PEDOT:PSS. The S2p core energy levels of the comparison device and the PEDOT:PSS film doped with Au@MoS 2 for 0.15 mg mL −1 , are in Figure 1b. The S2p spectrum appears in both components. The lower binding energy component had attributed to the contribution of PEDOT, and the higher binding energy characteristic peak corresponds to the sulfur atom in PSS. [22] PEDOT and PSS had led to significant differences in bind energy values (over 4 eV). The displacement of binding energy after Au@MoS 2 doping indicates that Au@MoS 2 with PEDOT or PSS chains had good compatibility. The increase in the peak intensity of the binding energy curves of the composites confirms the successful doping of Au@MoS 2 .
The Raman spectrum in Figure 1c shows that the doping of Au@MoS 2 does not change the molecular structure of PEDOT. The comparison reveals that the structural vibrational changes occur mainly between 1500 and 1600 wave numbers of the PSS structure, corresponding to the asymmetric C = C stretching. The redshift of the characteristic peaks between this wave number after doping indicates that the negatively charged PSS chains interact with the positively charged Au nanoparticles, making the insulating PSS less wrappable to the highly conductive PEDOT. The change in the Raman spectrum also indirectly proves the successful doping of Au@MoS 2 . Figure 1d  Au@MoS 2 but also has little effect on the transmittance of the films. The transmittance of Au@MoS 2 is constant in the band from 450 to 900 nm, indicating that the dopant does not affect the light absorption of silicon. [4] By UV-vis absorbance measurements of Au@MoS 2 -PEDOT, the inset figure shows that the absorption peak of Au nanoparticles had at ≈520 nm observed, which is the characteristic surface plasmon resonance peak. Instruction the LSPR effect of Au@MoS 2 composite enhances the light scattering in PEDOT:PSS, which is beneficial for the light trapping of silicon and improving the device's performance.
To explore the electrical properties of Au@MoS 2 -PEDOT:PSS films, the resistivity ( ) and carrier mobility (μ) were measured, as shown in Figure 2. The resistivity in PEDOT:PSS films with different Au@MoS 2 concentrations (0, 0.05, 0.1, 0.15, 0.2, and 0.3 mg mL −1 ) exhibited a trend with first decrease and then increase. The concentration of Au@MoS 2 with 0.15 mg ml −1 , the low resistivity 0.00176 Ω cm was obtained. The electrical conductivity of PEDOT:PSS can be enhanced by doping optimum content Au@MoS 2 . The carrier mobility (μ) for different concentrations of Au@MoS 2 is shown in Figure 2b. The μ varies from 7.01 to 14.53 cm 2 V −1 s −1 . The level is high at 0.15 mg mL −1 . We suspect that the larger μ may result in the built-in electric field formed by the Au@MoS 2 composite material that allows the electron-hole to separate rapidly, increasing the mobility.

Surface Topography of PEDOT:PSS Thin Films
The surface topography of PEDOT:PSS thin films with and without Au@MoS 2 had tested using the AFM. As shown in Figure  3a,c, the root means square roughness (RMS) had applied to evaluate the surface roughness of the PEDOT:PSS thin films. The RMS of the reference HSCs is 1.718 nm. Interestingly, the RMS of the HSCs with 0.15 mg mL −1 Au@MoS 2 is 1.871 nm. The consequence confirms that the roughness of PEDOT:PSS thin films does not change significantly when the Au@MoS 2 has doped. It also demonstrates that the Au@MoS 2 tiled in thin films effectively avoids short-circuiting due to direct contact between the Si substrate, Au@MoS 2 , and the Ag-grid electrode. The description of the composite size is less than the film thickness of PEDOT:PSS. The thicknesses with/without doped obtained by an FR-pRo UV-vis substrate film thickness tester. The PE-DOT:PSS film tested 83.1 nm and Au@MoS2-PEDOT:PSS film tested 97.4 nm. The thickness increased by ≈15 nm after doping, indicating that the increased thickness is likely to be the Au@MoS 2 material. In Figure 3b,d, the TEM images of the fewlayer MoS 2 nanosheets and the Au@MoS 2 , respectively. Illustrates that the few-layer MoS 2 nanosheets and the size of the Au nanoparticles loaded on the MoS 2 , 5-8 nm, do not impact the quality of the film. The results consistently demonstrate the small size of the composite, which can be completely encapsulated by the PEDOT:PSS film, as shown in Figure S1 (Supporting Information).

Photoelectric Characteristics of HSCs
The concentration of Au@MoS 2 in the PEDOT:PSS precursor is from 0 mg mL −1 to a value of 0.15 mg mL −1 . The open-circuit voltage (V OC ) and fill factor (FF) of the HSCs have increased at start. When passed 0.15 mg mL −1 , the device's performance declined. The current density-voltage (J-V) of HSCs had measured with and without Au@MoS 2 . As shown in Table 1 The ideality factor (n) and the reverse saturation current density (J 0 ) are derived from the J-V curves in dark conditions (Figure 4c; Figure S2, Supporting Information) and summarized in Table 2, based on Equation (1): [23,24] where k is 1.380649 × 10 −23 J K −1 , T is 298 K (kT = 0.025852 eV), and q is 1.6 × 10 −19 C. The ln J values in the dark condition of the HSCs with 0.15 mg mL −1 Au@MoS 2 are smaller than those of the reference HSCs, with better charge extraction at the Si/PEDOT:PSS/Ag port and yielding better charge collection efficiency. This result is consistent with previously reported literature. [24] It is possible to determine the n-values of HSCs by fitting the slopes of tangent lines of dark condition J-V curves in the range of 0.3-0.6 V. The HSCs with 0.15 mg mL −1 Au@MoS 2 displayed better ideal heterojunction quality of the PEDOT:PSS/Si interface with a smaller n and J 0 value, indicating that the charge recombination loss had reduced. The n-values of the reference device and the optimal performance PEDOT:PSS-Au@MoS 2 device are 2.03 and 1.35; J 0 are 2.57 × 10 −10 and 1.14 × 10 −9 mA cm −2 , respectively. The J 0 connected to the recombination loss of photo-generated charge carriers, with a smaller J 0 resulting in a greater J SC compared with the larger. Furthermore, the V OC also relies on the J 0 and is fitted to the following Equation (2): [21,23,25] A decrease in J 0 indicates enhanced PEDOT:PSS/Si heterojunction quality while limiting the formation of new defects, resulting in increased V OC . This V OC had calculated according to Equation (2), which reveals the reference HSCs at 619.7 mV (the experimental value is 624.4 mV) and the HSCs with 0.15 mg mL −1 Au@MoS 2 is 658.7 mV (the research value is 655.31 mV). To evaluate the rise in J SC , we investigated the EQE spectra at wavelengths ranging from 300 to 1100 nm, as shown in Figure 4b. In the visible wavelength region, the J SC had a significant increase in the EQE, which is attributed to the synergistic effect of Au nanoparticles. It is reasonable to explain that the increase in EQE by the doping of Au@MoS 2 was owing to an improvement in charge extraction efficiency in the visible light spectrum. We found that the higher J SC depends mainly on the Au@MoS 2 improving the heterojunction interface.
The series resistance (R s ) is connected to the quality of the HSCs and has a major impact on the FF. It was calculated based on the J-V curve under dark conditions (follows Equation (3)).  The R s of undoped HSCs was 9.76 Ω cm −2 , 0.15 mg mL −1 Au@MoS 2 doped is 4.73 Ω cm −2 (Table 1). For reference HSCs, the lower built-in field is less capable of charge separation and extraction, leading to poor contact properties of the PEDOT:PSS/Si interface and increased contact resistance. The built-in field increases, accelerating charge extraction, upgrading the passivation quality of the PEDOT:PSS thin films to the junction region, and improving the hole-transport capability of the thin films. As a result, this leads to a decrease in contact resistance, which is inevitably related to improving the solar cell filling factor. [24,26] In this equation, A d is the effective area of the hybrid solar cell (1.065 cm 2 ), and the slope of the d(V)/d(lnJ)-J curve is the value of R s . The Au@MoS 2 device and reference HSCs had researched by the capacitance-voltage (C-V) measurement to evaluate the The V bi denotes the built-in field, V denotes the applied voltage, and N A denotes the concentration of acceptor impurities (1 × 10 17 cm −3 ). As shown in Figure 6b, V bi had extracted according to the intersection of voltage and 1/C 2 -V curves (the voltage intercept). For the HSCs doped 0.15 mg mL −1 Au@MoS 2 , reaching a larger V bi of 839 mV, an enhancement of 68 mV compared with the reference HSCs (V bi of 771 mV). The V bi of the HSCs treated the Au@MoS 2 at 0.05, 0.1, 0.2, and 0.3 mg mL −1 , depicted in Figure S3 (Supporting Information) using the same conditions. The above evidence demonstrated that doping the Au@MoS 2 into PEDOT:PSS thin films enhanced the built-in field. Greater V bi is essential for separating the electron-hole pairs at the PEDOT:PSS/Si interface. Figure 4d shows the energy band structure diagram of PEDOT: PSS/Si HSCs.
For this study, the UPS had applied to evaluate the WF of PE-DOT:PSS thin films with and without Au@MoS 2 . The WF of the PEDOT:PSS thin films can get by Coordinate transformation using the formula: where Φ is the work function, hv is the incident photon energy (here it is 21.22 eV), E cutoff is the cutoff edge, and E Fermi is the Fermi level. All HSCs have identical fermi edges after being calibrated with metallic Au. As shown in Figure 5a, the WF of the reference PEDOT:PSS thin films is 4.71 eV, and a higher WF of 4.93 eV for the PEDOT:PSS/Au@MoS 2 thin films had achieved. The corresponding HOMO level of the reference PEDOT:PSS thin films and the PEDOT:PSS/Au@MoS 2 thin films is O.546 eV and 0.420 below the Fermi level, respectively, shown in Figure 5b. The above confirms that the reduction of recombination loss in the PEDOT:PSS/Si interface is attributed to the higher electron barrier created by doping Au@MoS 2 .
Moreover, for the reference HSCs, a V bi of 670 mV is calculated. The calculation result is 100 mV smaller than that computed by the Mott-Schottky model (771 mV), explained by PSS un-enrichment at the PEDOT:PSS/Si interface increasing the WF, but PSS enrichment at the surface of PEDOT:PSS thin films. As illustrated in Figure 6b, the HSCs with 0.15 mg mL −1 Au@MoS 2 completed the V bi up to 890 mV. As illustrated in Figure 5c,d, With Au@MoS 2 , the WF of PEDOT:PSS thin films was enhanced by 0.22 eV, resulting in more powerful band bending at the PEDOT:PSS/Si contact. The HOMO level of the Au@MoS 2 -PEDOT:PSS thin films was increased at the PEDOT:PSS/n-Si interface. The above confirms that the reduction of recombination loss in the PEDOT:PSS/Si interface is attributed to the higher electron barrier created by doping Au@MoS 2 . The WF enhancement of the PEDOT:PSS is the decisive factor for the HSCs' improvement.

Conclusion
We demonstrated the Au@MoS 2 nanocomposites were mixed successfully in PEDOT:PSS/Si HSCs. Au nanoparticles introduction induced an (LSPR) effect to improve the absorption in the visible band of Au@MoS 2 , increase photocurrent, and assist MoS 2 in regulating the band gap and carrier mobility. The synergistic effect of the two materials had reflected in the Au@MoS 2 composite material. The UPS testing indicates the WF enhanced from 4.71 to 4.93 eV at 0.15 mg mL −1 . Au@MoS 2 nanocomposite doping deepens the work function of PEDOT:PSS thin films and then induces a stronger built-in field in the PEDOT:PSS/Si interface. As a result, the separation behavior of electron-hole pair gets an extra boost recombination loss of carriers depresses effectively. Finally, the PCE from 11.48% to 14.0%., have also been well-optimized. Our work emphasizes that the PEDOT:PSS layer decorated with Au@MoS 2 nanocomposites exhibits exceptional hole-extraction capability in the PEDOT:PSS/Si HSCs. The layer can be an alternative anode hole-extraction layer in other new types of photovoltaic devices.
Fabrication of MoS 2 Nanosheets and Au@MoS 2 Nanocomposites: 50 mg of the purchased molybdenum disulfide (MoS 2 ) joined in a 100 mL beaker. Then, 50 mL of N,N-Dimethylformamide (DMF) add as the exfoliation and dispersion solvent. The mixtures were sonicated in a watercooled bath for an hour to get out their parts. Furthermore, to eliminate the larger particle size of MoS 2 , the suspensions were centrifuged at 8000 rpm for 5 min at room temperature. Additionally, this suspension percolated using a 0.22 μm filter head. Finally, the MoS 2 nanosheets had prepared by vacuum drying at 80°C for 12 h. Take the prepared MoS 2 nanosheets and HAuCl 4 aqueous solution (25 mm) as precursors. The Au@MoS 2 had manufactured using a microwave-assisted hydrothermal technique, as reported previously. [27] The analysis of obtained materials shown in Supporting Information.
HSCs Fabrication: A standard RCA clean procedure cleans the Si wafers (size at 1.1 × 1.1 cm 2 ). After completing all the operations, the Si substrates were immersed for 1 min in a diluted hydrofluoric acid solution (10 vol.%) to eliminate any remaining native oxide. The PEDOT:PSS mixed with 5 wt.% DMSO to boost conductivity, and 0.2 wt.% of Triton X-100 to increase the wettability. After dispersing in absolute ethanol to prepare solutions of different concentrations and ultrasonic treatment for an hour, the generated solution dop into the PEDOT:PSS precursor solution with mixing ratios of 0, 0.05, 0.1, 0.15, 0.2, and 0.3 mg mL −1 , respectively. The prepared PEDOT:PSS solution was then coated on the Si substrates at a speed of 2300 rpm for 60 s and annealed at 130°C for 30 min. Next, the PC 61 BM solution (20 mg mL −1 of chlorobenzene solution) was spin-coated on the rear side at 3000 rpm for 40 s and annealed at 130°C for 10 min. Finally, using thermal evaporation, the Ag-grid front electrode (200 nm, which represents 12% of the device surface) and Al rear electrode (80 nm) were deposited in sequence under vacuum conditions. Characterization: The solar simulator (AM 1.5) with an intensity of 100 mW cm −2 had used to imitate natural sun illumination and assess the current density-voltage (J-V) relationship (using the Zolix solar test equipment). The quantum efficiency measurement system (Zolix SCS150) had employed for EQE. The micro-morphology and valence state of Au@MoS 2 nanosheets had characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), and X-ray photoelectron spectroscopy (XPS). The surface morphology of the PEDOT:PSS thin films had measured by the atomic force microscope (AFM). An unfiltered HeI(21.22 eV) gas discharge lamp (PHI5000 Versa Probe III analysis system) had used to measure the ultraviolet photoelectron spectrometer (UPS). The capacitance-voltage (C-V) curves had obtained using a Keithley 4200-SCS instrument. FR-pRo UV/VIS substrate film thickness tester.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.