Optical and Electrical Properties of Antimony and Fluorine Doped Tin Oxide Thin Films

The Sb:F:SnO2 layers (AR) were prepared by spray pyrolysis method. The anti-reflective layers (AR) heat-treated at 500°C for 30 min (solution amount 20 cc and 25 cc) have shown an improved crystallinity with crystallite size of 38-39 nm, high optical transmission of around 70 % at 450 nm. Incorporation of anti-reflective layer at cathode interface of SiO2/Si(N) devices increased the power conversion efficiency from 1.2% to 2.7% which is mainly contributed from photocurrent enhancement. The enhanced efficiency mainly contributed to the increase in Jsc. It is attributed to enhanced light absorption and better charge transport in the SiO2/Si (N) device with Sb:F:SnO2 AR layer. Results of optical and electrical studies show that the films are well suited for thin film solar cell as a window layer.


Introduction
In order to enhance the performance of a solar cell, it needs to reduce the reflection loss from the surface of the cell [1]. A flexible optical design for light collection is vital in achieving high performance solar cells. An ideal anti-reflective (AR) structure leads to zero reflection loss on solar cell surfaces over an extended solar spectral range for all angles of incidence. Thus, the perfect anti-reflective structure has been a subject of intensive research in thin film optics and most importantly, for solar cell applications [2].The interfacial layer in a Schottky barrier solar cell plays an important role in determining the short circuit current, open circuit voltage, fill factor and efficiency of the cell. The effects of interfacial oxide layer thickness on the open circuit voltage and efficiency of the cells have been increased [3]. Aluminum (Al) and Silver (Ag) metal layers were used in place of the counter and back electrodes [1,4]. Among these, Sb:F:SnO 2 is chosen as an anti-reflective (AR) layer because of its high electrical conductivity and its transparency in the visible and infrared light [5,6]. Further, its refractive index lies in between 1.7 and 2.0 and hence it can be used as an antireflection (AR) coating [7]. In addition, Sb:F:SnO 2 layer provides both high optical transmittance and high electrical conductivity simultaneously. The sol-gel processing exhibits good uniformity and better-controlled composition for thin film deposition by the spinning or spray pyrolysis method [6,8]. In this study, an attempt is made to find the effect of different layers on its optical and electrical properties by simultaneously doping antimony and fluorine. In addition, the effect of an anti-reflective layer on the photovoltaic efficiency of SiO 2 /Si solar cells is also highlighted.

Substrate Cleaning
The substrate cleaning is very important in the deposition of thin films. The substrates used in this work were phosphor doped N-type silicon with the resistivity value of 0.01~10Ωcm and orientation of (100) plane. The Si substrate was cut into the dimension of 1" x 1". Firstly the substrate was cleaned with HF: H 2 O (1:5) for 10 minutes to remove native oxide and immersed in acetone for 10 minutes. Then the substrate was immersed in methyl alcohol for 10 minutes to remove the impurities. After that, it was rinsed with deionized water (DI water) for a few minutes and dried with nitrogen blow at room temperature [9,10].

Deposition of Sb:F:SnO 2 Film
The Sb:F:SnO 2 thin films were prepared as per synthesis route shown in Figure 1. The SnCl 4 .5H 2 O (98 % purity), SbCl 3 (99 % purity), and NH 4 F (98 % purity) were used as precursors. SnCl 4 .5H 2 O (10g) was dissolved in ethanol (80ml) at room temperature under stirring. On the other hand, 2g of NH 4 F was dissolved in 10ml deionized water and 3g of antimony trichloride (SbCl 3 ) was separately dissolved in ethanol (20 ml). Over a period of exactly 10 minutes, NH 4 F solution and SbCl 3 solution were added to the SnCl 4 .5H 2 O solution. The mixture solution was continuously stirred for 12 hours at 60°C by solochemical method. Solution became transparent and filtered by 0.45 µm pole size filters. The clear solution of 20 cc, 25 cc and 30 cc were used to make anti-reflective (Sb:F:SnO 2 ) layers with working distance of about 20 cm and angle of 90° by spray pyrolysis method. The substrates temperature was kept at 500°C and monitored by digital temperature controller. Finally, the films were annealed at 500°C for 30 minutes and measured for their optical and electrical properties [1,11].

Structural Properties of Sb:F:SnO 2 Films
Structural studies of Sb:F:SnO 2 films with solution amount of 20 cc, 25 cc and 30 cc heat-treated at 500°C for 30 minutes were done using X-ray diffraction (XRD) technique. The crystalline sharp peaks in the diffraction patterns were identified by using the International Centre for Diffraction Data (ICDD). The crystalline size was calculated by using Scherer's equation; D = (kλ)/(B cosθ) where, B is the peak width measured at half intensity (radian), λ is the wavelength measured in Å, k is the particle shape factor or Scherer constant (k = 0.9) and D is the diameter of the crystallites (Å).

Surface Morphology of Sb:F:SnO 2 Films
Surface morphology of Sb:F:SnO 2 were examined by scanning electron microscopy (SEM). Figure 3 shows the SEM images of Sb:F:SnO 2 films (solution amount 30 cc). It is observed that the Sb:F:SnO 2 films were fully covered on the substrate and the particle-like features with submicron size ranging from 0.2 µm to 0.5 µm were mostly found on the film surface.

Optical Properties
The optical transmission spectra of Sb:F:SnO 2 films using solution amount of 20 cc, 25 cc and 30 cc were recorded in the wavelength of 300 to 800 nm using Shimadzu UV-1800 spectrophotometer. The transmission spectra of SnO 2 :F:Sb layers were shown in Figure 4-a. The transmittance value of 40% to 70% was found in the visible region of 400-700 nm. As clearly seen, the optical transmittance decreased (absorption increased) upon increasing solution amount which is ascribed to thicker film formation with higher solution amount. The thickness of the films was determined by surface profiler (Tencor Alpha step IQ) and the values are listed in Table 1. In addition, the optical band gaps of SnO 2 :F:Sb anti-reflective layers were estimated from the plot of (αhv) 2 versus hv as shown in Figure 4

Electrical Properties of Sb:F:SnO 2 Films
Electrical conductivity (mobility) of anti-reflective layer is also important to be measured since it would determine the chare transport. The electrical resistance values of Sb:F:SnO 2 films were measured by using digital multimeter and are listed in Table 1. The electrical resistance increased with increasing film thickness.

Impact of Sb:F:SnO 2 Anti-reflective Layers on Photocurrent Generation
The Sb:F:SnO 2 anti-reflective layer has been integrated at the cathode interface of the Si/SiO 2 photovoltaic device. Figure 5 shows the schematic diagram of SiO 2 /Si (N) device with and without anti-refractive layer together with their photographs. Figure 6 shows the current-voltage (I-V) characteristics of SiO 2 /Si(N) photovoltaic device with and without anti-reflective layer. The critical parameters for the device with and without anti-reflective layer are listed in Table (2).

Conclusions
The Sb:F:SnO 2 layers (AR) were prepared by spray pyrolysis method. The anti-reflective layers (AR) (solution amount 20 cc and 25 cc) have shown an improved crystallinity with crystallite size of 38-39 nm, high optical transmission of 70% at 450 nm. Incorporation of anti-reflective layer at cathode interface of SiO 2 /Si(N) devices increased the power conversion efficiency from 1.2% to 2.7% which is mainly contributed from photocurrent enhancement. The enhanced efficiency mainly contributed to the increase in J sc . It is attributed to enhanced light absorption and better charge transport in the SiO 2 /Si(N) device with Sb:F:SnO 2 AR layer. Results of optical and electrical properties show that the films are well suited for window layer on thin films solar cells.