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Enhanced Photovoltaic Performances of Graphene/Si Solar Cells by Insertion of an MoS2 Thin Film

Atomically thin layered materials such as graphene and transition-metal dichalcogenides exhibit great potential as active materials in optoelectronic devices because of their high carrier-transporting properties and strong light-matter interactions. Here, we demonstrated that the photovoltaic performances of graphene/Si Schottky junction solar cells were significantly improved by inserting a chemical vapor deposition (CVD)-grown, large MoS2 thin-film layer. This layer functions as an effective passivation and electron-blocking/hole-transporting layer. We also demonstrated that the photovoltaic properties are enhanced with increasing number of graphene layers and decreasing thickness of the MoS2 layer. A high photovoltaic conversion efficiency of 11.1% was achieved with the optimized trilayer-graphene/MoS2/n-Si solar cell.

structures in MoS 2 can be changed by manipulation of the layer numbers, which has potential applications of various optical and electronic devices. [12][13][14] 2D materials are used as building blocks to fabricate artificial multi-junction nanostructures because layered 2D materials with an atomically flat surface can be easily stacked by van der Waals interactions. Artificial multi-junction nanostructures composed of various 2D materials with novel characteristics can be fabricated. Indeed, graphene on h-BN exhibits an extremely high carrier mobility that exceeds 60,000 cm 2 V −1 s −1 , 15 which is much larger than that of graphene on an SiO 2 /Si substrate. Very high photo-responsivity has also been reported for optoelectronic devices based on the stacking of graphene and MoS 2 (graphene/MoS 2 ). [16][17][18] The graphene/MoS 2 heterostructure is considered a promising platform for photovoltaic applications. [19][20][21][22] The photovoltaic properties of solar cells based on low dimensional carbon materials and Si heterojunctions have been intensively studied. [23][24][25][26] In the case of graphene/Si solar cell, the approaches used to improve photovoltaic performance have primarily involved carrier doping in the graphene layer, the insertion of a oxide layer, or the deposition of an antireflection layer onto the surface of a solar cell. [27][28][29][30][31][32][33] In this study, we investigate the photovoltaic properties of graphene/MoS 2 /n-Si solar cells. The photovoltaic properties were considerably enhanced by the insertion of chemical vapor deposition (CVD)-grown large MoS 2 thin film, which functions as an effective passivation and electron-blocking/hole-transporting layer. The optimized graphene/MoS 2 /n-Si solar cell exhibited a high photovoltaic conversion efficiency of 11.1%, which is a remarkable conversion efficiency in a photovoltaic device using MoS 2 thin film. Figure 1(a) shows a schematic of the graphene/MoS 2 /n-Si solar cell. It comprises the two types of 2D materials, CVD-grown MoS 2 film and graphene, stacked on a patterned n-type SiO 2 /Si substrate (with a  = 1 mm window). Large-scale MoS 2 films for solar cells were fabricated by the CVD method. [34][35][36] As a first step, an MoO 3 (99.98%, Sigma-Aldrich) film with the desired thickness was fabricated on an SiO 2 /Si substrate using the thermal-evaporation technique under vacuum conditions (~10 −3 Pa); this MoO 3 film was subsequently sulfurized. The sulfurization step of the MoO 3 film was conducted using the CVD furnace. The SiO 2 /Si substrate with an MoO 3 film was placed in the center of a quartz tube, and sulfur (>98%, Wako) was loaded at the upstream side. The substrate was heated by the CVD furnace to 750 °C at a heating rate of 15 °C/min and maintained at this temperature for 30 min. At the growth temperature (750 °C), sulfur was also heated to 190 °C using a ribbon heater, resulting in sulfur transport. All processes were performed under high-purity Ar gas flow flowing at 50 sccm under ambient conditions. We fabricated MoS 2 films with various thicknesses on SiO 2 /Si substrates, as shown in Figure S1.

RESULTS AND DISCUSSION
The graphene/MoS 2 /n-Si solar cells were fabricated by transferring CVD-grown MoS 2 films and graphene films (single-layer graphene, Graphene Platform) onto the n-type Si substrate (1-5 cm −1 ) with an ~300-nm-thick SiO 2 layer. A circular window with a diameter of 1 mm was patterned on the SiO 2 /Si substrate. This Si substrate was dipped into buffered hydrofluoric acid (HF) solutions for several seconds to remove the natural oxidation layer on the window, which obstructs the electrical contact. The MoS 2 film was then transferred via thermal release tape (REVALPHA; No. 3195MS, Nitto Denko). In this study, the transfer process using the thermal release tape was a more facile method compared with the conventional method of using a polymer support film. 37,38 In Figure S2, we show the MoS 2 transfer process in detail. For the fine transfer, the first key step is peeling the CVD-grown MoS 2 film from the growth substrate. The water droplet facilitates clear peeling because of the difference in hydrophobicities between the SiO 2 /Si substrate and the MoS 2 film. 39 In this transfer method, the process is completed quickly, because the MoS 2 film can be released from the tape by heating, thereby eliminating the need to expose the film to a solution for a prolonged period.
Subsequently, we transferred graphene layers onto MoS 2 /n-Si. Poly(bisphenol A carbonate) dissolved in chloroform (3 wt%) was spin-coated onto the single-layer graphene deposited onto copper foil for the transfer process. The graphene film was floated on FeCl 3 to etch out the copper foil and was subsequently transferred to the MoS 2 /n-Si substrate. 37 The graphene/MoS 2 /n-Si was baked at 120 °C for 5 min, and the polymer layer was then dissolved in chloroform.
Finally, Au was deposited as an electrode onto the surface of the graphene/MoS 2 /n-Si, with the exception of the window region, via the Ar-ion sputtering method. Indium (In), as the cathode, was soldered onto the back side of the solar cell. The process of fabricating the graphene/MoS 2 /n-Si solar cell is described in the Supporting Information.   These parameters of monolayer-and bilayer-graphene/MoS 2 /n-Si solar cells and the series resistance estimated from the slope of each J-V curve are shown in Table 1 of Supporting Information S4. The series resistance of the solar cells drastically decreased with increasing number of graphene layers, consistent with the previously reported results of decreased sheet resistance in graphene layers. 47 Our results suggest that the decrease in series resistance is responsible for the reduction of loss, which contributes to the enhancement of photovoltaic conversion efficiency in graphene/MoS 2 /n-Si solar cells.
The incident photon to current conversion efficiency (IPCE) spectra revealed that performance. 48 We fabricated a trilayer-graphene/MoS 2 /n-Si solar cell using a thinner (~9 nm) MoS 2 film. Figure 4 shows the J-V curve for the graphene/MoS 2 /n-Si solar cell. The optimized graphene/MoS 2 /n-Si solar cells exhibit V OC , J SC , and FF photovoltaic parameters of 0.56 V, 33.4 mA·cm −2 , and 0.60, which results in a high photovoltaic efficiency of 11.1%. This value of 11.1% is among the highest efficiencies reported for a solar cell containing an MoS 2 layer.
Thinner layer is suitable not only for passivation effect but also for shadow effect by MoS 2 layer.
This improvement indicates that more photons passed through the MoS 2 and were absorbed by the Si.
To better understand the photovoltaic process, we constructed band alignment diagrams for the graphene/n-Si and graphene/MoS 2 /n-Si solar cells, as shown in Figure 5. and the current density-voltage data were recorded using a source meter (Keithley 2400). The AM 1.5 condition for the solar simulator was confirmed using a standard cell (BS-500BK). For the incident-to-photon conversion efficiency (IPCE) measurements, the devices were tested using a monochromated xenon arc light system (Zolix LSP-X150).

Conflict of Interest:
The authors declare no competing financial interest.