Effect of Chlorine Doping on ZnSe Thin Films and Photovoltaic Devices

The compound semiconductor ZnSe is a promising n-type material of II-VI group with wide direct band gap (2.67 eV) and has zinc blend (cubic phase) and wurtzite (hexagonal phase) structures or the mixture of both, which depend on the deposition methods and technological conditions [1-6]. More progress has been achieved in fabrication of blue-green light emitting diodes [7,8], dielectric mirrors [9], filters [10], blue laser diode [11] and other optically sen sitive devices [12]. There are a number of reports on the different structural, optical and electrical properties of ZnSe polycrystalline thin films prepared by various techniques, such as chemical vapour deposition [13], metal organic chemical vapor deposition (MOCVD) technique [14], electrodeposition [15], photochemical deposition [16], chemical bath deposition (CBD) [17], pulsed laser deposition [18] and thermal evaporation [19]. The close space sublimation (CSS) method is considered one of the most promising techniques for A2B6 thin film deposition [20, 21]. Because of its large band gap, ZnSe has been used as window layer for the fabrication of photovoltaic solar cells. The use of ZnSe films as a heterojunction partner in II-VI thin-film solar cells has been explored by T. L. Chu at al. [22]. Therefore we have prepared the ZnSe thin films of different thicknesses by close space sublimation method at different substrate temperatures. In this paper, the different structural and optical parameters of as-deposited ZnSe thin films were estimated in dependence of the substrate temperature and after the immersion in a ZnCl₂:H2O solution and annealing. As absorbent, CdTe is recognized as promising absorbent for thin-film photovoltaic devices because of their near optimum direct band gap of ~1.5 eV and higher absorption coefficient. In this paper, we will study ZnSe/CdTe thin film solar cells based on doped ZnSe thin films by immersioninZn- Cl₂:H₂ solution and annealing.

ZnSe films were deposited by close space sublimation technique. ZnSe and CdTe of 99.999% purity were used as source materials. The ZnSe thin films were deposited at a system pressure of 10-6 Torr on glass substrate covered with SnO₂. The thickness of SnO₂ films, with a sheet resistance of 10 Ω/square, were about 500 nm. In order, to optimize the growth parameters, ZnSe thin films were deposited at different substrate temperatures. The substrate temperature was varied rom 500 K to 650 K. Then the ZnSe films was immersed in ZnCl₂:H₂O solution and annealed in the vacuum at 400oC, for 40 min. The ZnSe/CdTe thin-film photovoltaic heterojunctions were fabricated on glass substrates covered with a SnO₂ layer with an area of 2×2 cm². We used the optimized component films to fabricate ZnSe/CdTe thin-film solar cells. The ZnSe/CdTe thin-film solar cells were fabricated in a superstrate configuration. The samples with as-grown ZnSe thin films obtained at substrate temperatures 500 K, 550 K, 600 K and 650 K denoted as (M5.1), (M6.1), (M7.1) and (M8.1), respectively. The samples with ZnSe thin films activated inZnCl₂:H2O solution obtained at the same substrate temperatures were signed as (M5.2), (M6.2), (M7.2),(M8.2), respectively.

The structure of ZnSe films were performed with a Rigaku X-Ray Diffractometer (XRD) with CuKα1/40 kV/40 mA radiation source (λ=1.54056 Å), Ni filter, in the 2θ range of 10°- 90°, scanning speed of 0.5°/min. The XRD analysis was performed using Rigaku software PDXL. The energy dispersive spectroscopy (EDS) analysis of the samples was performed with a JEOL JSM-6390LV scanning electron microscope. The optical transmission spectra were recorded using a JASCO V-670 spectrophotometer. The photovoltaic characteristics of ZnSe/CdTe thin film solar cells were investigated by current-voltage characteristic through the wide band gap components at the room temperature (300 K) and 100 mW/cm² illumination.

The requirements for window layers in solar cells applications are high conductivity and adequate thickness in order to allow good transmission and uniformity which would avoid the effects of short cutting. Figure 1 shows the SEM image of as-grown and after ZnCl₂ annealing of ZnSe films deposited on SnO₂/glass substrates. From the SEM image it is clear that the ZnSe films are dense and pinhole free. The grain size is smaller than 1μm in both cases. Sample M5.2 with ZnSe after ZnCl₂ activation at 400°C shows a lot of round grains on the surface, and they did not coalesce together. The film lost compactness of the structure. No significant changes in morphology of the M6.2 and M7.2 samples, after activation in ZnCl₂:H₂O solution and annealing, were occurred.

Figure 2 shows XRD 2θ scans, between 20° and 80°, for ZnSe/ SnO₂/glass samples before and after ZnCl₂ activation. As seen, the XRD pattern exhibits intensive XRD peaks at 26.52°, 33.79°, 37.85°, 38.84, 42.55, 51.64, 54.51, 61.69, 71.04, 78.44 and 52° which are due to the X-ray diffraction from polycrystalline SnO₂ with tetragonal or orthorhombic structures [23]and four peaks with the smaller intensity for ZnSe phase presented in the Figure 2. The X-ray diffraction measurements on the as-deposited films and ZnCl₂ treated revealed, for both, a cubic ZnSe phase. The ZnSe phase for all samples corresponds to the 216: F- 3m space group. There is no evidence of formation of a new phase after ZnCl₂ annealing. The crystallite size was calculated using the Scherrer’s formula [10] from the full-width at half-maximum (FWHM) (β) :

Where λ is the X-ray wavelength, θ the diffraction angle, and k a constant usually equal to 1. The micro strain (ε ) was calculated from formula

ε = β cosθ/4 (2)

where β the full-width at half-maximum.

Table 1: Structural parameters of as-grown ZnSe thin films obtained at different substrate temperatures (T_s) and after ZnCl₂ activation.

As shown in Table 1, the ZnSe grains are smaller than those of SnO₂. For both, as-grown and activated ZnSe thin films, a slight lattice-parameter decrease is revealed with varying substrate temperature. In Figure 3 is presented EDS analysis of as-deposited and annealed ZnSe thin films obtained at Ts=600 K. The data for the other substrate temperatures are not shown as they all show very similar characteristics. Both, the as-deposited and annealed ZnSe films are Zn-deficient and agree well with as reported by other authors [24]. This is because the vapor pressure of Se is greater than that of Zn and their sticking coefficients are also different. The incorporation of chlorine in the activated ZnSe layers at substrate temperatures is confirmed by the detection of chlorine in a standardizes EDS analysis.

where d is the film thickness, R and T are the reflection and transmission coefficient, respectively. The thickness of the ZnSe layers was estimated using the following relation:

where n₁, and n₂ are the refractive indexes at two adjacent maxima (or minima) and λ₁ ,λ₂ the respective wavelength values. The analysis of the (αhν)2 = f (hν) plot of all ZnSe films is linear and it indicates a direct type of transition. The optical band gap values for as-deposited ZnSe films fluctuated between 2.68 eV (0.63 μm) and 2.69 eV (1.16 μm). The maximum value of E_g is connected with the very small size of crystallites in films. The thick films have a lower absorption value at the forbidden gap region of the ZnSe films. Thinner films have a high absorption value in the band-band absorption region. This effect may be explained by proposing that thicker films have bigger crystallites (grains) so they are closer to the crystalline ZnSe, but bigger grain sizes give results in larger unfilled inter-granular volumes so the absorption per unit thickness is reduced. One obvious result from the (αhν)² = f (hν) dependence (Figure 5) for ZnSe films grown at different substrate temperatures and ZnCl₂ activated at 400oC is that the energy gap decreases. The size of crystallites increases with the increase of the substrate temperatures and with ZnCl2 activation. This is confirmed by the XRD analysis (Table I) and is valid for all ZnSe films regardless of the substrate temperature.

Table 2: The optical parameters of as-grown and ZnCl₂ activated ZnSe thin films.

The photovoltaic characteristics of ZnSe/CdTe thin film solar cells were investigated through the wide band gap component at the room temperature (300 K) and 100 mW/cm2 illumination. The current density-voltage (J-V) characteristics of ZnSe/CdTe thin film solar cells with ZnSe activated at different substrate temperatures are illustrated in Figure 6. The photovoltaic parameters are presented in Table 3. The highest efficiency 6.6 % was achieved for ZnSe/ CdTe thin film solar cells with a thicker ZnSe activated buffer layer.

Table 3: Photovoltaic parameters of ZnSe/CdTe solar cells.

As one can see from Table 3 the value of the open circuit voltage (U_oc) and the current density (J_sc) achieve 0.75 V and 20.39 mA/cm2, respectively. The fill factor (FF) is low in general. According to the theory [19] the fill factor is determined by the series resistance, the saturated dark current density (J_o) and the diode quality factor (A). The dark Jo and A of the ZnSe/CdTe cells are in the range of 10-9 – 10^-7 A/cm² and 1.7-3.6, respectively. The low value of FF is mainly determined by the high value of the series resistance which is due to the high resistivity of both ZnSe and CdTe thin films and probably to the fact that the cells wet CdCl₂ treatment may contain oxide on the surface of CdTe.

In situ ZnCl₂ activation promotes the increasing of the crystallite’s sizes and improvement of the photovoltaic parameters in ZnSe/CdTe thin film solar cells. Also, with the increasing of the substrate temperature the crystallites sizes increase from 280 nm (500 K) to 337 nm (600 K). The analysis of the XRD spectra of the both, as-deposited and annealed ZnSe thin films, established a slight lattice-parameter decrease. The EDS analysis showed that the asgrown and annealed ZnSe films are Zn-deficient. In the activated ZnSe thin films, the presence of chlorine was depicted. The transmission and the band gap energy of the ZnCl₂ activated ZnSe thin films decrease. The efficiency of 6.6 % for ZnSe/CdTe thin film solar cells with a ZnSe activated window layer was achieved.

This publication is based upon work from COST Action OPERA – European Network for Innovative and Advanced Epitaxy – CA20116, supported by COST (European Cooperation in Science and Technology, www.cost.eu).

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