Annealing effect and photovoltaic properties of nano-ZnS/textured p-Si heterojunction
© Ji et al.; licensee Springer. 2013
Received: 19 July 2013
Accepted: 14 October 2013
Published: 9 November 2013
The preparation and characterization of heterojunction solar cell with ZnS nanocrystals synthesized by chemical bath deposition method were studied in this work. The ZnS nanocrystals were characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Lower reflectance spectra were found as the annealing temperature of ZnS film increased on the textured p-Si substrate. It was found that the power conversion efficiency (PCE) of the AZO/ZnS/textured p-Si heterojunction solar cell with an annealing temperature of 250°C was η = 3.66%.
Recently, 2D nanostructure P-N junctions have attracted a great deal of attention for their potential applications in photovoltaic device . Zinc sulfide (ZnS) was one of the first semiconductors discovered  and is also an important semiconductor material with direct wide band gaps for cubic and hexagonal phases of 3.72 and 3.77 eV, respectively . It has a high absorption coefficient in the visible range of the optical spectrum and reasonably good electrical properties . This property makes ZnS very attractive as an absorber in heterojunction thin-film solar cells [5, 6]. Furthermore, ZnS also offers the advantage of being a nontoxic semiconductor material (without Cd and Pb). A cell with ITO/PEDOT:PSS/P3HT:ZnS/Al structure was obtained by Bredol et al. , which showed a very high open-circuit voltage (Voc) value of 1.2 V and a power conversion efficiency of 0.2%.
In recent years, ZnS thin films have been grown by a variety of deposition techniques, such as chemical bath deposition , evaporation , and solvothermal method . Chemical bath deposition is promising because of its low cost, arbitrary substrate shapes, simplicity, and capability of large area preparation. There are many reports of successful fabrication of ZnS-based heterojunction solar cells by the chemical bath deposition method, such as with CIGS used for the n-type emitter layer .
This study aimed to grow ZnS thin films on a p-type silicon wafer using chemical bath deposition method. Crystalline silicon solar cells are presently due to their higher photovoltaic conversion efficiency, long-term stability, and optimized manufacturing process . n-ZnS/textured p-Si heterojunctions were produced, and their photovoltaic properties were investigated under various annealing temperatures.
ZnS nanocrystals were prepared using the chemical bath deposition (CBD) procedure. Aqueous solutions of 0.15 M ZnSO4, 0.5 M thiourea (NH2)2CS, and 0.2 M ammonia NH3 were mixed in a glass beaker under magnetic stirring. The beaker was maintained at a reaction temperature of 80°C using a water bath for 30 min. In addition, the silicon wafer samples were cleaned using a standard wet cleaning process. Subsequently, KOH was diluted to isotropically etch the silicon wafer to form a surface with a pyramid texture .
The preparation process of ZnS/textured p-Si solar cells has three parts: Firstly, square samples of 1.5 × 1.5 cm2 were cut from a (100)-oriented p-type silicon wafer with ρ = 1–10 Ω cm and thickness of 200 μm. For ohmic contact electrodes, DC sputtering was used to deposit about 2 μm of Al onto the back of the Si substrates, followed by furnace annealing at 450°C for 1 h in Ar ambient to serve as the p-ohmic contact electrodes. Secondly, a 200-nm n-type ZnS thin film was deposited on the prepared p-type Si by chemical bath deposition in order to form a ZnS/p-Si heterojunction. Finally, an AZO film and Al metal grid with a thickness of about 0.4 and 2 μm, respectively, were deposited by sputtering.
The phase identification was performed by X-ray powder diffraction (Rigaku Dmax-33, Rigaku Corporation, Tokyo, Japan). The morphology and microstructure were examined by high-resolution transmission electron microscopy (HRTEM) (HF-2000, Hitachi, Tokyo, Japan). The reflectance spectra were measured at room temperature using a JASCO UV-670 UV–vis spectrophotometer (Jasco Analytical Instruments, Easton, MD, USA). The current–voltage measurements (Keithley 2410 source meter, Keithley Instruments Inc., Cleveland, OH, USA) were obtained using a solar simulator (Teltec, Mainhardt, Germany) with an AM 1.5 filter under an irradiation intensity of 100 mW/cm2.
Results and discussion
During the reaction processes, sulfide ions release slowly from CH3CSNH2 and react with zinc ions. Consequently, ZnS nanocrystals form via an in situ chemical reaction manner. Equation 4 indicates that ZnS is produced by the reaction of S2- and Zn2+.
Photovoltaic performance of the AZO/ZnS/textured p -Si heterojunction solar cell with various annealing temperatures
The current density-voltage (J-V) characteristics of the finished photovoltaic devices were measured under an illumination intensity of 100 mW/cm2 and shown in Figure 6b. The measurements show that the ZnS film deposited onto the p-Si results in increased Voc. The power conversion efficiency (PCE) of the devices improved significantly from 0.89% to 3.66% when the ZnS film annealing temperature was 250°C. The highest Voc was 0.32 V and the highest current density was 29.1 mA/cm2. Therefore, the best annealing temperature of the ZnS film is 250°C, with a PCE of 3.66%. When the annealing temperature of the ZnS film increased to 300°C, the efficiency decreased because of a large percentage decrease in Voc. The possible reason is that the ZnS film included impurities or defects originating from high-temperature process. In addition, the value of Rsh has relatively changed, resulting in element composition instability. Therefore, Voc and cell performance deteriorated with a 300°C annealing process. A similar phenomenon was also observed in the ILGAR-ZnO layers to cover the rough CIGSSe absorber heterojunction thin-film solar cells . Therefore, the interface of the AZO/ZnS/textured p-Si heterojunction may have some defects at higher annealing temperature of ZnS films, and this decreases the PCE. The external quantum efficiency (EQE) spectra for the photovoltaic devices of the AZO/ZnS/ textured p-Si heterojunction solar cell are shown in Figure 6c. All EQE spectra are similar in shape, except for the sample without ZnS, and the EQE value for the optimal annealing temperature of the ZnS film (250°C) is higher than that of most wavelengths. The differences in the EQE spectra are due to the increase in leakage current that occurs by decreasing the FF, and therefore, the interface of the AZO/ZnS/textured p-Si heterojunction may have some defects for ZnS films annealed at higher temperature.
A chemical bath deposition method for the synthesis of ZnS nanocrystals is reported in this work. The cubic ZnS film was deposited on p-Si substrate and obtained a well-crystallized single phase with various annealing temperatures. Lower reflectance spectra were found as the annealing temperature of ZnS film increased on the textured p-Si substrate. The photovoltaic characteristics of the AZO/ZnS/textured p-Si heterojunction solar cells with various annealing temperatures of the ZnS film were examined, and the In2S3 film with an annealing temperature at 250°C had η = 3.66% under an illumination of 100 mW/cm2.
The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under contract nos. NSC 100-2221-E-492-021, NSC 101-2221-E-024-015, and NSC 101-2221-E-150-045.
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