Characterization of photovoltaics with In2S3 nanoflakes/p-Si heterojunction

We demonstrate that heterojunction photovoltaics based on hydrothermal-grown In2S3 on p-Si were fabricated and characterized in the paper. An n-type In2S3 nanoflake-based film with unique 'cross-linked network’ structure was grown on the prepared p-type silicon substrate. It was found that the bandgap energy of such In2S3 film is 2.5 eV by optical absorption spectra. This unique nanostructure significantly enhances the surface area of the In2S3 films, leading to obtain lower reflectance spectra as the thickness of In2S3 film was increased. Additionally, such a nanostructure resulted in a closer spacing between the cross-linked In2S3 nanostructures and formed more direct conduction paths for electron transportation. Thus, the short-circuit current density (Jsc) was effectively improved by using a suitable thickness of In2S3. The power conversion efficiency (PCE, η) of the AZO/In2S3/textured p-Si heterojunction solar cell with 100-nm-thick In2S3 film was 2.39%.


Background
Indium sulfide (In 2 S 3 ) is one of the important semiconductor materials with direct bandgap and attracts intense interest due to its high photosensitivity, photoconductivity, and photocatalyst characteristics at ambient conditions [1][2][3]. In In 2 S 3 , there are three polymorphic forms: defect cubic structure α-In 2 S 3 , defect spinel structure β-In 2 S 3 , and higher-temperature-layered structure γ-In 2 S 3 [4]. Among them, β-In 2 S 3 is an n-type semiconductor with superior photoelectric conversion function that can be employed in near-infrared to ultraviolet regions of solar energy absorption [5]. Hence, we may expect that β-In 2 S 3 will act as a good absorber in heterojunction thin film solar cells [6]. On the other hand, In 2 S 3 is a nontoxic semiconductor material which also offers potential advantage in process without Cd and Pb. A cell with ITO/ PEDOT:PSS/In 2 S 3 :P3HT/Al structure has been fabricated by Jia et al. [7], which showed the short-circuit current density (Jsc) of 0.68 mA cm −2 and a power conversion efficiency of 0.04%.
In recent years, In 2 S 3 thin films have been grown by a variety of deposition techniques such as chemical bath deposition (CBD) [8], thermal evaporation [9], solvothermal synthesis [10], and atomic layer chemical vapor deposition (ALCVD) [11]. Among them, chemical bath deposition is a desirable method because of its low cost, arbitrary substrate shapes, simplicity, and can be easily prepared in large areas. There have been many reports for the heterojunction solar cell with CBD grown In 2 S 3 . For example, In 2 S 3 was used for the n-type buffer layer of CIGS solar cells [12]. Crystalline silicon solar cells are presently the predominant photovoltaic devices among various solar cells due to their higher photovoltaic conversion efficiency, and long-term stability [13]. Recently, Abd-El-Rahman and Darwish et al. reported a p-Sb 2 S 3 / n-Si heterojunction photovoltaic that was fabricated by using thermal evaporation technique [14], which showed Jsc = 14.53 mA cm −2 , fill factor = 0.32, and η = 4.65%.
In this study, the In 2 S 3 thin films were deposited on a p-type silicon substrates via chemical bath deposition route. To our knowledge, works on In 2 S 3 film deposited on textured Si-based solar cell by CBD are few. In addition, the advantages of chemical bath deposition process are low temperature and low-cost synthesis. This fact motivates this work which discusses the structure and electrical property of the AZO/In 2 S 3 /textured p-Si heterojunction devices.

Methods
The In 2 S 3 nanoflakes were prepared according to the CBD procedure reported by Bai et al. [15]. Typically, aqueous solutions of 0.025 M InCl 3 , 0.048 M thioacetamide (CH 3 CSNH 2 ) (TAA), and 0.04 M acetic acid were mixed in a glass beaker under magnetic stirring. The beaker was maintained at a reaction temperature of 80°C using water bath. In addition, the samples of silicon wafer 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 [16].
The preparation process of In 2 S 3 /p-Si heterojunction solar cell was separated into three parts: First, the samples with 1.5 × 1.5-cm 2 square were cut from a (100)-oriented p-type silicon wafer with ρ = 10 Ω cm and 200-μm thickness. For ohmic contact electrodes, we used the DC sputtering technique to deposit 2-μm-thick Al onto the back of the Si substrates, followed by furnace annealing at 450°C for 1 h in Ar ambient conditions to serve Al as the p-ohmic contact electrodes. Second, 50~400-nm-thick n-type In 2 S 3 thin films were deposited on the prepared p-type Si substrates by chemical bath deposition route in order to form an In 2 S 3 /p-Si heterojunction structure. Finally, an AZO film and Al metal grid with thicknesses of 0.4 and 2 μm, respectively, were deposited by sputtering. The purpose of AZO deposition is to produce a transparent conductive film by RF magnetron sputtering using ZnO:Al (2 wt.% Al 2 O 3 ) target with a purity of 99.99% with 300-W power. All devices with the same AZO thickness (approximately 400 nm) were deposited at the same conditions. The single-cell size of photovoltaic device is about 0.4 cm 2 .
The phase identification of materials was performed by X-ray powder diffraction (Rigaku Dmax-33, Tokyo, Japan). The morphology and microstructure were examined by high-resolution transmission electron microscopy (HR-TEM; Hitachi HF-2000, Tokyo, Japan). The absorption and reflectance spectra were measured at room temperature using a Hitachi U-4100 UV-Vis-NIR spectrophotometer. The current density-voltage measurements (Keithley 2410 SourceMeter, Cleveland, OH, USA) were obtained by using a solar simulator (Teltec, Mainhardt, Germany) with an AM 1.5 filter under an irradiation intensity of 100 mW cm −2 .

Results and discussion
XRD patterns of various In 2 S 3 films with thicknesses of 50 to 300 nm are shown in Figure 1. The In 2 S 3 films were formed directly from the amorphous precursors by using chemical bath deposition method. All of the peaks for various thicknesses were identified to be the tetragonal β-In 2 S 3 phase (JCPDS card no. 25-0390) [17]. It can be seen that the crystallinity of In 2 S 3 increases as the thickness of In 2 S 3 film increases. The peaks of (206), (0012), and (2212) was observably seen while the thickness of In 2 S 3 film was increased up to 300 nm. In this experiment, In 3+ ions could form a variety of complexes in a solution. As InCl 3 is dissolved in water, it is hydrolyzed and finally form In(OH) 3 . The possible chemical reactions for the synthesis of In 2 S 3 nanocrystals can be expressed as following [18]: During the reaction processes, sulfide ions were slowly released from CH 3 CSNH 2 and reacted with indium ions. Consequently, the In 2 S 3 nanoflakes were formed via an in situ chemical reaction manner. Equation (4) indicates that In 2 S 3 is produced by the reaction of S 2− and In 3+ .
TEM analysis provides further insight into the structural properties of as-synthesized nanoflakes In 2 S 3 . Figure 2a shows the low-magnification TEM image, and the nanoflakes can be clearly observed. The crystalline In 2 S 3 nanoflakes are identified by electron diffraction (ED) pattern in the inset of Figure 2a, which exhibits diffusing rings, indicating that the In 2 S 3 hollow spheres are constructed of polycrystalline In 2 S 3 nanoflakes. The   concentric rings can be assigned to diffractions from (101), (103), and (116) planes of tetragonal In 2 S 3 , which coincides with the XRD pattern. It is possible that the assembled effect arising from the nanocrystals results in the decrease of surface energy. A representative HRTEM image for such a tetragonal In 2 S 3 nanostructure is shown in Figure 2b. It was found the interplanar distance of the crystal fringe is 3.3 Å, corresponding to the spacing of the (109) plane of tetragonal In 2 S 3 [19]. Figure 3a,b shows the side-view and top-view SEM images of the textured p-Si substrate by using wet etching process. The uniform pyramids had been made on the surface of the p-Si, which was defined as the antireflective structures for incident sunlight. The various thicknesses of In 2 S 3 films were grown on the surface of the textured p-Si substrate; the thicknesses of the In 2 S 3 films were about 50, 100, and 300 nm, respectively, as shown in Figure 3c,d,e. The images of the In 2 S 3 /textured p-Si substrate exhibit a rough surface. The EDS line profiles indicate that the film consists of indium and sulfur. The atomic concentrations of In = 56.6% and S = 43.4% are calculated from the EDS spectrum, as shown in Figure 3f. The In 2 S 3 films were grown not only in the lateral direction, but also randomly in the vertical direction. In the inset of Figure 3f, we can see that the surface of the In 2 S 3 film is with a cross-linked network structure.
We have measured the UV-Vis absorption spectra of the various thicknesses of the In 2 S 3 film and estimated the bandgap energy from the absorption onset of data curves in Figure 4a. For a direct bandgap semiconductor, the absorbance in the vicinity of the onset due to the electron transition is given by where α is the absorption coefficient, C is the constant, hν is the photon energy, and E g is the bandgap energy. The inset of Figure 4a reveals the relationship of (αhν) 2 and hν gives a bandgap energy of 2.5 eV by the extrapolation of the linear region. The result was similar to previous report that 120-and 68-nm thicknesses of thermal-evaporated tetragonal In 2 S 3 are with the bandgap of 2.54 and 2.52 eV, respectively [20]. Figure 4b shows the transmittance spectra of the 400-nm-thick AZO films on In 2 S 3 films with various thicknesses. While the pure 400-nm AZO film on the glass showed 90.2% of transmittance, the transmittance values of 400-nm-thick AZO on In 2 S 3 with 50-, 100-, and 300-nm thickness were about 86.2%, 75.5%, and  Figure 5 Reflectance spectra of the planar p-Si, textured p-Si, and the In 2 S 3 film with various thicknesses on textured p-Si substrate.

In 2 S 3 (300nm)/AZO(400nm)
68.6%, respectively. It can be seen that the transmittance is decreased as the thickness of In 2 S 3 film increases. Figure 5 shows the reflectance spectra of the planar p-Si, textured p-Si, and the In 2 S 3 film with various thicknesses on textured p-Si substrate in the range of 2001 ,100 nm. The average reflectance was about 11.3%, 10.9%, and 8.7% for the In 2 S 3 film on the textured p-Si substrate with 50-, 100-, and 300-nm thicknesses, respectively. These values are lower than the average reflectance of planar p-Si and textured p-Si (32.0% and 16.2%, respectively). Therefore, the reflectance is obviously reduced by the nanoflake In 2 S 3 and decreased as the thickness of In 2 S 3 film increases. It could be attributed to the decreasing reflectance for In 2 S 3 film at short wavelengths because the nanotexturization was on the surface [21]. Figure 6a displays the schematic structure of the heterojunction solar cell in which the nanotextured In 2 S 3 / p-Si was the photoactive layer of such a device. Photovoltaic performance of the AZO/In 2 S 3 /p-Si heterojunction solar cell with various In 2 S 3 thicknesses is given in Table 1. All samples for the electrical measurement were performed with AZO film of about 400 nm. Characterization of the AZO/In 2 S 3 film deposited on the textured p-Si substrate was studied for the first time. Figure 6b shows a SEM image of an inclined angle of the AZO/In 2 S 3 /p-Si heterojunction structure. The AZO deposited on the In 2 S 3 (100 nm)/p-Si substrate exhibits a well coverage and turns into a cylinder-like structure with a hemispherical top as shown in the inset of Figure 6b. The deposition thickness of the AZO was estimated to be 400 nm. Jiang et al. [22] revealed that they had fabricated the SnS/α-Si heterojunction photovoltaic devices, which the junction exhibited a typical rectified diode behavior, and the short-circuit current density was 1.55 mA/cm 2 . Hence, the AZO/In 2 S 3 /p-Si structure in the study was suitable for solar cell application.
The current-voltage (J-V) characteristics of the fabricated photovoltaic devices were measured under an illumination intensity of 100 mW/cm 2 , as shown in Figure 6c. Such result shows that the short-circuit currents (Jsc) were increased while the In 2 S 3 films were deposited onto the p-Si. The power conversion efficiency (PCE) of the devices can be obviously improved from 0.47% to 2.39% by employing a 100-nm-thick In 2 S 3 film.  The photovoltaic condition is AM 1.5 G at 100-mW/cm 2 illumination.
It was also found that the highest open-circuit voltage (Voc) and short-circuit current density are 0.32 V and 23.4 mA/cm 2 , respectively. Therefore, the optimum thickness of the In 2 S 3 film is 100 nm, with PCE of 2.39%. When the thickness of the In 2 S 3 film increases, the efficiency decreased because of the decrease in Jsc and FF, as shown in Figure 6d. A similar phenomenon was also observed in the In 2 S 3 /CIGS heterojunction thin film solar cell [23]. It is possible that some defects on the interface of the AZO/In 2 S 3 /p-Si heterojunction with thicker In 2 S 3 films will decrease the PCE. The cell performance improved markedly as the thickness of the In 2 S 3 layer was increased to 100 nm. This improved cell performance is attributed to the reduction of possible shunt paths by the inclusion of a high-resistivity In 2 S 3 buffer layer between the transparent conducting ZnO:Al and the p-Si layers. The cell performance, however, deteriorated in devices with 200-and 300-nm-thick In 2 S 3 layers since the series resistance of the solar cell increased due to the high resistance of the In 2 S 3 layer. Therefore, the 100-nm In 2 S 3 sample shows the best performance.

Conclusions
In summary, we have successfully synthesized the nanoflake In 2 S 3 by a chemical bath deposition route in the study. The well-crystallized single phase of tetragonal In 2 S 3 that can be obtained at 80°C and deposited on p-Si substrate was investigated for the first time. The visible light absorption edge of the as-grown In 2 S 3 film corresponded to the bandgap energy of 2.5 eV by UV-Vis absorption spectra. It can be seen that the lower reflectance spectra occurred while the thickness of In 2 S 3 film on the textured p-Si was increased. The photovoltaic characteristics of the AZO/In 2 S 3 /textured p-Si heterojunction solar cells with various In 2 S 3 thicknesses were also given in the investigation, and the PCE of such device with 100-nm-thick In 2 S 3 film is 2.39% under 100-mW/cm 2 illumination.

Competing interests
The authors declare that they have no competing interests.
Authors' contributions YJH and LWJ carried out the design of the study and drafted this manuscript. CHL and THM conceived of the study and participated in its design and coordination. YLC and HPC carried out the preparation of the samples and characteristic measurements. All authors read and approved the final manuscript.