Effect of film thickness and evaporation rate on co-evaporated SnSe thin films for photovoltaic applications

SnSe thin films were deposited by a co-evaporation method with different film thicknesses and evaporation rates. A device with a structure of soda-lime glass/Mo/SnSe/CdS/i-ZnO/ITO/Ni/Al was fabricated. Device efficiency was improved from 0.18% to 1.02% by a film thickness of 1.3 μm and evaporation rate of 2.5 Å S−1via augmentation of short-circuit current density and open-circuit voltage. Properties (electrical, optical, structural) and scanning electron microscopy measurements were compared for samples. A SnSe thin-film solar cell prepared with a film thickness of 1.3 μm and evaporation rate of 2.5 Å S−1 had the highest electron mobility, better crystalline properties, and larger grain size compared with the other solar cells prepared. These data can be used to guide growth of high-quality SnSe thin films, and contribute to development of efficient SnSe thin-film solar cells using an evaporation-based method.


Introduction
The extensive electronic transitions between delocalized and localized energy-band states ensure that chalcogenides based on transition metals have good optical and electrical properties. These attributes provide considerable convenience for solving the problem of energy shortages in different elds. Due to the excellent electronic and optical properties of binary compounds in transition metals, many scholars have conducted in-depth research in photovoltaic and solar-cell systems. [1][2][3][4][5] Among binary compounds in transition metals, SnSe has great application prospects because SnSe has an orthorhombic crystal structure, 6 which is suitable for solar cells. SnSe is a p-type semiconductor and its high absorption coefficient (10 5 cm À1 ) ensures that solar radiation is absorbed almost completely. 7,8 Besides, the maximum theoretical efficiency of 32%, rich content, and nontoxic nature in earth of SnSe thin-lm solar cells make them suitable for photovoltaic applications. 7,8 Four solid phases exist in a Sn-Se system (Se, Sn, SnSe and SnSe 2 ) to make it easier to control the phase composition during growth of SnSe compounds. 9,10 SnSe thin lms can be obtained by various growth methods: chemical-bath deposition, 7 thermal evaporation, 11 brush plates, 12 two-stage processes, 13 electro-deposition, 14 and spray pyrolysis. 15 The vacuum method is important because of its low cost and low complexity of material synthesis. Despite the extraordinary properties it shows for photovoltaic applications, only a few reports have been published on its layers and device studies, which have poor efficiency (<1%). [16][17][18][19][20] In the present study, Sn and Se were evaporated simultaneously in a vacuum by thermal evaporation to form SnSe thin lms. This method dictates that lm formation is dependent upon the substrate temperature, evaporation rate, lm thickness, and purity of the material. To improve optical and electrical properties, the evaporation rate and thickness of the SnSe thin lm are necessary for the synthesis of SnSe thin lms. However, several studies have not paid attention to the inuence of the evaporation rate and lm thickness on the structure, surface morphology, optical properties and electrical properties of SnSe thin lms, which limits the power-conversion efficiency of solar cells made from SnSe thin lms.
In this work, to enhance optical and electrical properties, SnSe thin lms were prepared under various evaporation rates and lm thicknesses by co-evaporation. The effects of evaporation rate and lm thickness on the properties of SnSe thin lms were studied. Finally, SnSe thin-lm solar cells with a soda-lime glass (SLG)/Mo/SnSe/CdS/i-ZnO/ITO/Ni/Al structure were prepared and showed optimal conversion efficiency of 1.02% at an evaporation rate of 2.5 A S À1 and lm thickness of 1.3 mm.
were deposited on a bare glass substrate and molybdenumcoated soda-lime glass substrate. The evaporation rate was controlled to 1, 2.5, 5 A S À1 to analyze the effects of evaporation rate on the properties of SnSe thin lms. To investigate the inuence of lm thickness on the properties of SnSe thin lms, lm thickness was set to 0.6, 0.9, and 1.3 mm. The experiments mentioned above were executed in a vacuum and an operating pressure of 10 À5 torr. Besides, all lms were deposited at 500 C and on the same substrate type (bare glass). SnSe thin lms prepared on a bare glass substrate were used for optical characteristics, electrical characteristics, structural analyses, and analyses of surface morphology. SnSe thin lms prepared on the SLG substrate were integrated into the solar cell. The SnSe thin-lm solar cells were fabricated by depositing a 70 nm CdS buffer layer in a pure argon atmosphere at 0.45 Pa, room temperature for 5 min, and deposition power of 100 W with the rf-sputtering method on SnSe absorber lms prepared on co-evaporation, followed by deposition of a 70 nm i-ZnO layer and 1500 nm ITO layer. Using an intrinsic ZnO target, deposition of the i-ZnO layer was by rf-sputtering at room temperature for 5 min, deposition power of 100 W, and in an atmosphere of pure argon at 0.45 Pa. The ITO layer was deposited by rf-sputtering using an ITO target at room temperature for 15 min, deposition power of 100 W, and in a pure argon atmosphere at 0.5 Pa. Nickel (60 nm) and aluminum (400 nm) top contacts were obtained by deposition using electron-beam evaporation. The total area of each SnSe solar cell was $0.42 cm 2 and etching, element doping, diffusion barrier or anti-reection coating were not used in the device fabrication. The structure of the SnSe thin-lm solar cell in this research is shown as Fig. 1.

Characterization
Raman spectroscopy was undertaken using a Raman spectrometer (Jobin-Yvon T64000). The structure was observed by Xray diffraction (XRD) using a Rigaku D/max 2550 V system with Cu Ka radiation. The atomic ratio of Sn/Se was estimated by Xray uorescence (XRF) using a Shimadzu EDX-7000 setup. The optical parameters were taken by Cary5000 UV-VIS-NIR spectrophotometer. Surface morphology was investigated by scanning electron microscopy (SEM) using a JEOL setup. Current-    voltage (J-V) characteristics were studied using a solar simulator (Xe lamp; Newport) with a source meter (2400; Keithley) at 100 mW cm À2 and AM 1.5 G illumination. The electrical properties of the lms were determined by measurement of the Hall effect (HMS3000; Ecopia).

Results and discussion
3.1 Crystalline structure Table 1 shows the composition of SnSe thin lms measured by XRF. The Sn/Se atomic ratio of all samples was close to 1. Fig. 2a shows the XRD patterns of the SnSe thin lms at a lm thickness of 0.6, 0.9, and 1.3 mm with an evaporation rate of 2.5 A S À1 to study the crystalline structure of these lms. . Compared with XRD patterns with a lm thickness of 0.6 and 0.9 mm, almost all diffraction peaks were sharper and stronger at a lm thickness of 1.3 mm. This observation was because of a reduction in stacking faults and lattice defects as the lm thickness increased. The (111) diffraction peak was signicantly stronger than the peaks at other diffractions, indicating that the SnSe thin lm had a preferential orientation of the (111) phase. The superiority of the (111) peak helps to improve the efficiency of solar cells. 21 Initially conclusions from Fig. 2a suggest that the SnSe thin lm with a thickness of 1.3 mm was superior to SnSe thin lms with a lm of 0.6 or 0.9 mm. Fig. 2b shows the XRD patterns of the SnSe thin lms at an evaporation rate of 1, 2.5, and 5 A S À1 with a lm thickness of 1 Compared with the XRD patterns with an evaporation rate of 1 or 5 A S À1 , the evaporation rate of 2.5 A S À1 exhibited stronger diffraction peaks, especially the (111) diffraction peak. Besides, the (111) diffraction peak was sharper and stronger than other peaks among all the diffraction peaks and all samples. Combined with the XRD data shown in Fig. 2a and b, SnSe thin lms prepared at an evaporation rate of 2.5 A S À1 and lm thickness of 1.3 mm may be more suitable for photovoltaic devices. The lattice parameters of the crystal were calculated using the following formula: where d is the distance between planes, h, k, l are Miller indices, and a, b, c are lattice constants. The lattice constants of these SnSe thin lms had the following values: a ¼ 11.50 A, b ¼ 4.15 A, and c ¼ 4.44 A. These values are in good agreement with those in the Joint Committee on Powder Diffraction Standards (JCPDS) database, as well as the literature for SnSe thin lms. 22 Detailed structural parameters of all lms are presented in Table 2. Fig. 3 shows the surface SEM images of SnSe thin lms deposited under different conditions. Although the microcrystals for all SnSe lms were distributed uniformly over the lm surface, the microstructure (crystallinity and grain size) of the samples was dependent upon the deposition conditions. Grain size could be enhanced by changing the thickness of the SnSe thin lm. Crystallinity and grain size could also be altered by changing the evaporation rate. The grain size of SnSe thin lms with a thickness of 1.3 mm and evaporation rate of 2.5 A S À1 was larger than that in other SnSe thin lms, which indicated enhanced grain growth and better crystallinity using this deposition condition. Usually, it is important to acquire absorber lms with better crystallinity or large grain size for high energy-conversion efficiency because smaller grains lead to recombination, which results in a decrease of open circuit voltage (V oc ) and reduction in current. 23

Optical properties
Optical transmittance has an important role in semiconductors because it reects the absorption coefficient and band gap energy. Transmittance suddenly drops off near the absorption edge due to band-to-band transitions. Therefore, the band gap of a semiconductor can be calculated based on the value of transmittance, which can enable this material to be applied in different devices. The optical properties of SnSe thin lms deposited under different conditions were studied using a Cary5000 UV-VIS-NIR spectrophotometer. For photons of wavelength between 400 and 1400 nm, transmittance of 0.01-24.8% (deposited with different lm thicknesses) and 0.01-17.4% (deposited with different evaporation rates) were obtained. Transmittance varied with different lm thicknesses and evaporation rates ( Fig. 4a and b). This observation was because, as the lm thickness and evaporation rate changed, the strain changed because the inuence of the mismatch between glass and SnSe lattice implied generation of defects in the SnSe layer close to the glass substrate. From Fig. 4 we can conclude that, photons with a wavelength of 400-1400 nm, SnSe thin lm of thickness of 1.3 mm, and evaporation rate of 2.5 A S À1 obtained the lowest transmission of 0.01-4.8%. We used the transmittance data to calculate the absorption coefficient and optical band gap. The absorption coefficient (a) can be obtained from the transmittance and eqn (1) where d is the path length, T is the transmittance of the SnSe thin lm, and a is the absorption coefficient. The band gap of SnSe can be obtained by eqn (2) (ahn) n ¼ A(hn À E g ) (2) where the value of E g is dependent upon the type of band transition of the material. The band gap obtained when n is 1/2 represents the indirect band gap (Davis-Mott model). The band gap obtained when n is 2 represents the direct band gap (Tauc's model). 24 Fig. 4c and e show the relationship between Tauc's curve (ahn) 2 and the photon energy (hn) for determining the direct band gap of different lm thicknesses and evaporation rates, respectively. Fig. 4d and f show the relationship between the Davis-Mott curve (ahn) 1/2 and the photon energy (hn) to determine the indirect band gap of different lm thicknesses and evaporation rates, respectively. Fig. 4c and d reveal that the value of the direct band gap E g of SnSe deposited with different lm thicknesses was 0.98-1.13 eV and the value of the indirect band gap E g was 0.87-1.13 eV. Besides, Fig. 4e and f show the value of the direct band gap to be 0.98-1.12 eV and indirect band gap to be 0.87-0.97 eV. The change of direct and indirect band gap was due to the difference in stacking faults and lattice defects as the lm thickness or evaporation rate changes. 25 The obtained values for the band gap are in accordance with theoretical expectations given by Singh and colleagues. 26 Our results are in good agreement with those of reports of SnSe lms prepared using other methods. 27 The optical properties of SnSe thin lms were analyzed further by Raman spectroscopy. The excitation wavelength of the Raman spectrum was 542 nm, and the measurement range was 50 to 250 cm À1 . Fig. 5 shows the Raman spectra of prepared SnSe thin lms with different lm thicknesses and evaporation rates. All Raman spectra show vibration modes at 74, 105.8, 130, and 152 cm À1 , which correspond to A 1 g , B 3g , A 2 g and A 3 g modes, respectively, 28,29 and which conform closely to the characteristic pattern of SnSe. A g and B 3g are two rigid shear modes of the layer relative to its adjacent layers, and they determine the characteristic planar vibration modes of the orthorhombic phase of SnSe. When considering the relationship between

Electrical properties
The electrical properties of SnSe thin lms were measured by the Hall-effect measurement system using the van der Pauw method and four-point probes at room temperature. Table 3 shows the electrical properties of the SnSe thin lms with different lm thicknesses and evaporation rates. All lms were p-type semiconductors and electrical properties varied with different lm thicknesses and evaporation rates. Although the resistivity of the SnSe thin lm deposited with a lm thickness of 1.3 mm and evaporation rate of 2.5 A S À1 was slightly higher than that under other deposition conditions, it exhibited the highest electron mobility (36.65 cm 2 V À1 s À1 ). The highest electron mobility obtained in this condition may have been due to the better crystallinity and large crystallite size which may, ultimately, decrease inter-crystalline barriers. Also, high carrier mobility is required for improving the efficiency of solar-cell conversion. Hence, the SnSe lm (obtained with a lm thickness of 1.3 mm and evaporation rate of 2.5 A S À1 ) with higher carrier mobility may be more appropriate for application in photovoltaic devices. Fig. 6 shows the J-V curves measured for SnSe thin-lm solar cells prepared under different conditions under AM 1.5 G sunirradiation condition at room temperature. The lm thickness and evaporation rate exhibit had a signicant effect upon the power-conversion efficiency of the SnSe thin-lm solar cell due to the effects on crystalline structure, microstructure, electrical properties and optical properties. Table 4 shows the device parameters of the fabricated solar cells. The devices fabricated with SnSe thin lms were deposited and showed the highest efficiency (h) of 1.02% with a V oc of 172 mV, short-circuit current density (J sc ) of 18.87 mA cm À2 , and ll factor (FF) of 31.27% with a lm thickness of 1.3 mm and evaporation rate of 2.5 A S À1 . The FF was almost identical for a solar cell with a lm thickness of 1.3 mm and evaporation rate of 2.5 A S À1 compared with SnSe thin-lm solar cells deposited under different conditions. However, V oc and J sc were improved obviously ( Table 4). The improvement in V oc and J sc for the sample of lm thickness 1.3 mm and evaporation rate of 2.5 A S À1 was attributed mainly to the better crystallinity, larger grain size, and higher carrier mobility obtained in this work.

Conclusions
The structure, optical properties, morphology and electrical properties of SnSe thin lms with different lm thicknesses and evaporation rates were investigated. The crystallinity and grain size of SnSe thin lms could be altered by changing the lm thickness and evaporation rate. A larger grain size was obtained for SnSe thin lms of lm thickness 1.3 mm and evaporation rate of 2.5 A S À1 . A direct band gap of SnSe thin lms of 0.98-1.13 eV and indirect band gap of SnSe thin lms of 0.87-1.13 eV were obtained. All lms exhibited p-type conductivity, and a relatively low resistivity of 1.92 U cm and maximum electron mobility of 36.65 cm 2 V À1 s À1 were obtained with a lm thickness of 1.3 mm and evaporation rate of 2.5 A S À1 . All lms had an orthogonal structure and preferential (111) planar orientation. The manufactured SnSe solar cells exhibited a high powerconversion efficiency of 1.02%.

Conflicts of interest
There are no conicts to declare.