Fabrication and investigation of the optoelectrical properties of MoS2/CdS heterojunction solar cells
© Gu et al.; licensee Springer. 2014
Received: 2 September 2014
Accepted: 25 November 2014
Published: 9 December 2014
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.
Single-layer (SL) and few-layer (FL) molybdenum disulfide (MoS2) recently became attractive alternative semiconductor materials for next-generation nanoelectronic applications due to their large electron mobility, large bandgap [1–5], excellent stability, and the absence of dangling bonds . MoS2 has been widely studied and applied in many areas, such as field-effect transistors [6–13], energy harvesting [14, 15], optoelectronics [16–18], cocatalysts [19–21], and counter electrodes [22, 23]. Moreover, single and multilayer MoS2 phototransistors have been demonstrated with an on/off ratio of approximately 103 and a carrier mobility of 80 cm2/Vs [17, 18], which indicates that MoS2 is a promising candidate for photovoltaic solar cells. Gourmelon et al. previously reported on the use of MoS2 in solar cells , but the report did not draw much interest until recently. Yu et al. reported a TiO2/MoS2/P3HT bulk heterojunction solar cell with a short-circuit current density of 4.7 mA/cm2, an open-circuit voltage of 560 mV, and a power conversion efficiency of 1.3%, as well as MoS2 nanomembrane-based Schottky-barrier solar cells with a power conversion efficiency of 0.7% for approximately 110-nm MoS2 and 1.8% for approximately 220-nm MoS2[24, 25]. Clearly, the optical current, voltage, and energy transfer efficiency of these cells are low, and further investigations of MoS2-based solar cells are significant and necessary.
It is well known that cadmium sulfide (CdS), with a large direct bandgap of 2.4 eV [26–28], is a viable material and widely used in solar cells as a window layer. Zhang et al. have demonstrated MoS2/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 MoS2/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 MoS2-based solar cells composed of p-MoS2 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 [31–33]. Additionally, CVD has been recognized as one of the best techniques for the fabrication of large-area homogeneous MoS2 films [12, 13, 34–36]. Moreover, we systematically analyzed the individual films' surface morphologies, structures and electrical and optical properties, as well as the photovoltaic properties of the MoS2/CdS films and heterojunction solar cells.
MoS2/CdS heterojunctions were formed by further CVD of a MoS2 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 MoS2 micro powder, 1 g analytical grade silver nitrate (AgNO3) powder, and 200 mL of diluted sulfuric acid (H2SO4) 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 cm3/min, carrying silver-doped MoS2 molecules into the furnace. The adsorption and deposition of MoS2 molecules onto the CdS films yielded MoS2/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 MoS2 films, MoS2 samples were deposited on quartz crystalline slides by the same method.
To construct a MoS2/CdS heterojunction solar cell, Ni electrodes were sputtered onto the corner of the MoS2/CdS thin films using magnetron sputtering. The surface morphologies and crystalline structures of MoS2 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 MoS2/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/cm2).
Results and discussion
Figure 7b displays the light-illuminated J-V characteristics of the fabricated MoS2/CdS heterojunction solar cell. The solar cell exhibits pronounced photovoltaic behavior, with an open-circuit voltage (Voc) of 0.66 V and a short-circuit current density (Jsc) of 0.227 × 10-6 A/cm2. We can see that Voc is much larger than the results obtained from other MoS2-based solar cells [24, 25], but Jsc 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 FF = JmVm/JscVoc, where Jm and Vm 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 . These results show that to improve the light energy efficiency of the MoS2/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.
We have fabricated heterojunction solar cells composed of p-MoS2 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 MoS2 film is homogeneous and continuous, with a thickness of around 10 nm, which is equal to a few layers of MoS2. The as-grown CdS film is continuous and compact. The optical absorption range of the MoS2/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 MoS2/CdS solar cell exhibits good rectification characteristics and pronounced photovoltaic behavior, with a short-circuit current density of 0.227 × 10-6 A/cm2 and an open-circuit voltage of 0.66 V, comparable to the results obtained from other MoS2-based solar cells.
WG is a graduate student major in fabrication of new semiconductor nanometer materials. FY, CW, YZ, and MS are undergraduates. XM is a professor and PhD-degree holder specializing in semiconductor materials and devices, especially expert in nanoscaled optical-electronic materials and optoelectronic devices.
This work was supported in part by the Innovation Program for Postgraduate of Suzhou University of Science and Technology (No. SKCX13S_053), the Priority Academic Program Development of Jiangsu Higher Education Institutions, the USTS Cooperative Innovation Center for Functional Oxide Films and Optical Information, and the Education Reform of Jiangsu (No. JGLX13_091).
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