Fabrication and investigation of the optoelectrical properties of MoS2/CdS heterojunction solar cells

Molybdenum disulfide (MoS2)/cadmium sulfide (CdS) heterojunction solar cells were successfully synthesized via chemical bath deposition (CBD) and chemical vapor deposition (CVD). The as-grown CdS film on a fluorine tin oxide (FTO) substrate deposited by CBD is continuous and compact. The MoS2 film deposited by CVD is homogeneous and continuous, with a uniform color and a thickness of approximately 10 nm. The optical absorption range of the MoS2/CdS heterojunction covers the visible and near-infrared spectral regions of 350 to 800 nm, which is beneficial for the improvement of solar cell efficiency. Moreover, the MoS2/CdS solar cell exhibits good current-voltage (I-V) characteristics and pronounced photovoltaic behavior, with an open-circuit voltage of 0.66 V and a short-circuit current density of 0.227 × 10-6 A/cm2, comparable to the results obtained from other MoS2-based solar cells. This research is critical to investigate more efficient and stable solar cells based on graphene-like materials in the future.

It is well known that cadmium sulfide (CdS), with a large direct bandgap of 2.4 eV [26][27][28], is a viable material and widely used in solar cells as a window layer. Zhang et al. have demonstrated MoS 2 /CdS heterojunction by photoelectrochemical methods and studied the photocatalytic and contact interface properties [15,19,29,30]. However, the photoelectric characteristics and conversion efficiency of MoS 2 /CdS heterojunction solar cells have not been demonstrated. And the complexity of these methods or the poor morphologies and structures of samples limited its use. Here, we present the fabrication of MoS 2 -based solar cells composed of p-MoS 2 and n-CdS by simply using chemical bath deposition (CBD) and chemical vapor deposition (CVD). CBD is considered to be a low-cost and simple approach, which can produce reproducible, uniform, and adherent CdS films [31][32][33]. Additionally, CVD has been recognized as one of the best techniques for the fabrication of large-area homogeneous MoS 2 films [12,13,[34][35][36]. Moreover, we systematically analyzed the individual films' surface morphologies, structures and electrical and optical properties, as well as the photovoltaic properties of the MoS 2 /CdS films and heterojunction solar cells.

Methods
MoS 2 /CdS heterojunction solar cells were synthesized, as shown in Figure 1, in a three-step process: (i) CBD of CdS on a fluorine tin oxide (FTO)-coated glass substrate using the reaction between CdAc 2 and H 2 NCSNH 2 , (ii) CVD of MoS 2 on CdS, and (iii) sputtering of Ni electrodes on MoS 2 . CdS thin films were firstly deposited via CBD on FTO substrates that had been ultrasonically cleaned with deionized water, and then dried at 80°C in a drying oven. The FTO substrates were immersed in a solution composed of 0.007 M cadmium acetate (Cd (CH 3 COO) 2 · 2H 2 O) and 0.05 M thiourea (H 2 NCSNH 2 ) and maintained at 80°C for 60 min with stirring to obtain uniform deposition. After deposition, the CdS films were ultrasonically washed to remove the loosely adhered CdS particles on the surface and subsequently dried and annealed at 400°C for 60 min in N 2 to improve the crystalline quality. Some CdS films were set aside as representative samples for characterization of surface morphologies and structures, and the others were used to synthesize MoS 2 /CdS heterojunction solar cells.
MoS 2 /CdS heterojunctions were formed by further CVD of a MoS 2 thin film on the pre-existing CdS film. The CVD experimental setup consisted of a horizontal quartz tube furnace, an intake system, a vacuum system, and a water bath. The substrates were placed in the center of the furnace, and subsequently, the furnace was pumped down to 10 −2 Pa and heated up to 550°C for 30 min. A mixed solution comprising 1 g analytical grade MoS 2 micro powder, 1 g analytical grade silver nitrate (AgNO 3 ) powder, and 200 mL of diluted sulfuric acid (H 2 SO 4 ) was formed by stirring for 5 min and maintained at 70°C via the water bath. Ar gas was then flowed through the mixed solution with a flow rate of 20 standard cm 3 /min, carrying silverdoped MoS 2 molecules into the furnace. The adsorption and deposition of MoS 2 molecules onto the CdS films yielded MoS 2 /CdS thin films. After the completion of the deposition, the samples were annealed at 600°C for 30 min in an Ar atmosphere. Furthermore, to investigate the material properties of MoS 2 films, MoS 2 samples were deposited on quartz crystalline slides by the same method.
To construct a MoS 2 /CdS heterojunction solar cell, Ni electrodes were sputtered onto the corner of the MoS 2 / CdS thin films using magnetron sputtering. The surface morphologies and crystalline structures of MoS 2 and CdS films were characterized using atomic force microscopy (AFM) and X-ray diffraction (XRD), respectively. The electrical properties of the samples were analyzed by a Hall Effect Measurement System (HMS-3000, Ecopia, Anyang, South Korea) at room temperature. The UV-visible absorption spectra of the samples were investigated by a UV-visible spectrophotometer (Shimadzu UV-3600, Kyoto, Japan). Photovoltaic measurements of the MoS 2 /CdS heterojunction solar cells were taken using a Keithley 4200 semiconductor characterization system (Keithley Instruments, Inc., Cleveland, OH, USA), both in the dark and under standard AM 1.5 illumination (100 mW/cm 2 ).   can effectively promote the absorption of light. Additionally, many MoS 2 quantum dots around 100 nm in diameter, shown in Figure 2b, are uniformly deposited on the surface of the MoS 2 film. Under the quantum dots, the MoS 2 film is homogeneous and continuous, with a uniform color and a thickness of about 10 nm, which is equal to a few layers of MoS 2 . This growth mode, called the layer-quantum dot mode, corresponds to the hexagonal crystalline structure of MoS 2 .

Results and discussion
The crystal structures of the samples were characterized by XRD. The XRD pattern of the CdS film is illustrated in Figure 3a. Only the (111) diffraction peak, appearing at 26.2°, belongs to cubic CdS; the others, located at 24.8°, 28.2°, 43.7°, and 50.8°, correspond to the (100), (101), (110), and (112) diffraction planes of a hexagonal CdS, respectively, which is more suitable to be an n-type window layer for solar cells, due to its high transmission and electrical conductivity [37]. Moreover, these observed diffraction peaks are rather sharp, especially the (111) and (101) peaks, which indicate good crystallinity. Figure 3b shows the XRD pattern of the MoS 2 film. Six sharp diffraction peaks are located at 14.7°, 29.3°, 33.1°, 47.8°, 54.6°, and 56.4°, corresponding to the (002), (004), (100), (105), (106), and (110) crystal planes of MoS 2 , respectively, which show that the MoS 2 film exhibits a variety of crystal structures. In addition, it has to be noted that no silver diffraction peaks are observed, indicating that the silver doping does not change the crystal structure of the MoS 2 film. Figure 4 shows the ultraviolet-visible (UV-vis) absorption spectra of the CdS, MoS 2 , and MoS 2 /CdS samples in the wavelength region of 350 to 800 nm. The CdS film has a strong optical absorption peak at 490 nm, and the optical absorption covers the wavelength region of 350 to 510 nm, consistent with the previously reported findings [29,38]. Over the region 510 to 800 nm, the absorptivity of the CdS film decreases abruptly, and no other absorption peaks are observed, indicating that the CdS film is transparent to light in this range. However, there is an absorption peak observed for the MoS 2 film located at 735 nm, which corresponds to the MoS 2 bandgap of about 1.69 eV. The optical absorption range of the MoS 2 film almost covers the range that the CdS film does not absorb light, demonstrating that MoS 2 /CdS solar cells enhance the absorption of light, compared with silicon-based solar cells. Moreover, the optical absorption range of the MoS 2 /CdS sample covers the visible and near-infrared spectral regions of 350 to 800 nm, which is beneficial for the improvement of solar cell efficiency.
We measured surface current-voltage (I-V) properties, carrier mobilities, and Hall coefficients of the MoS 2 and CdS samples using a Hall Effect measurement system.   applied current, indicating that the MoS 2 and CdS films have good conductivity, with few surface defects or impurities. The electron mobilities in the MoS 2 and CdS films are 1.579 × 10 3 cm 2 /Vs and 7.68 × 10 2 cm 2 /Vs, respectively. Note that the mobility value for the MoS 2 film is higher than previously reported [39,40], which may be due to lower phonon and lattice scattering. Furthermore, the Hall coefficients of the MoS 2 and CdS films are 6.379 × 10 6 cm 3 /C and −3.257 × 10 2 cm 3 /C, respectively, showing that MoS 2 is a p-type semiconductor, and it can form a p-n junction with n-type CdS, as demonstrated in previous studies [15,19,29,[41][42][43]. Figure 6 displays the energy band diagram of the fabricated MoS 2 /CdS heterojunction solar cell. E C1 , E C2 , E V1 , and E V2 denote the conduction bands and valence bands of CdS and MoS 2 , respectively. E F is the Fermi level energy. χ 1 and χ 2 are the electron affinities of CdS (3.8 eV) [38] and MoS 2 (4.0 eV), respectively. V 0 is the built-in potential, and E, with the direction from n-CdS to p-MoS 2, is the built-in electric field. Because of the Fermi level difference between n-CdS and p-MoS 2 , electrons diffuse from n-CdS to p-MoS 2 , and simultaneously, holes in p-MoS 2 move to n-CdS, leading to the formation of a space-charge region and built-in electric field with the direction from n-CdS to p-MoS 2 at the contact interface. The built-in electric field, E, prevents carriers from diffusing and makes them drift in the opposite direction, and finally, the heterojunction comes to thermal equilibrium with a unified Fermi level. Under light illumination, the photogenerated electrons and holes are quickly separated and driven into n-CdS and p-MoS 2 , respectively, under the acceleration of E, which gives rise to the generation of the photocurrent. Figure 7a shows the dark current density-voltage (J-V) characteristics of the fabricated MoS 2 /CdS heterojunction solar cell. Remarkably, the current curve of the device shows an exponential dependence on the applied positive voltage, and tends to be almost zero under the reverse voltage, indicating that the MoS 2 /CdS solar cell exhibits good rectification characteristics, and forms a well-defined p-n junction, as demonstrated by the previous reports [15,19,29]. Figure 7b displays the light-illuminated J-V characteristics of the fabricated MoS 2 /CdS heterojunction solar cell. The solar cell exhibits pronounced photovoltaic behavior, with an open-circuit voltage (V oc ) of 0.66 V and a short-circuit current density (J sc ) of 0.227 × 10 −6 A/cm 2 . We can see that V oc is much larger than the results obtained from other MoS 2 -based solar cells [24,25], but J sc is much lower than that of common solar cells [24,25], which is likely attributed to the large resistances for the device. The fill factor (FF) can be obtained based on the relationship of Vacuum level  = J m V m /J sc V oc , where J m and V m are the current density and voltage at the maximum power output, respectively. In this instance, FF is approximately 0.22, comparable to previously reported values [25]. These results show that to improve the light energy efficiency of the MoS 2 /CdS heterojunction solar cells it is necessary to lower the contact resistance of the cell, which is also critical to solar cells based on graphene-like materials.

Conclusions
We have fabricated heterojunction solar cells composed of p-MoS 2 and n-CdS films using CBD and CVD methods and studied the surface morphologies, structures, and electrical and optical properties, as well as the photovoltaic properties. The MoS 2 film is homogeneous and continuous, with a thickness of around 10 nm, which is equal to a few layers of MoS 2 . The as-grown CdS film is continuous and compact. The optical absorption range of the MoS 2 /CdS film covers the visible and near-infrared spectral regions of 350 to 800 nm, which is beneficial for improving solar cell efficiency. Moreover, the MoS 2 /CdS solar cell exhibits good rectification characteristics and pronounced photovoltaic behavior, with a short-circuit current density of 0.227 × 10 −6 A/cm 2 and an open-circuit voltage of 0.66 V, comparable to the results obtained from other MoS 2 -based solar cells.