1. Introduction

The CdTe thin film solar cells have attracted significant attention from the renewable energy community due to their direct band gap and broader coverage of the spectrum of solar irradiance in the visible region. The CdTe solar cells have reached a record efficiency of 22.5%^{[1, 2]}. The CdTe thin film solar cells are manufactured in CdTe/CdS/ITO/glass configuration, where CdS, which also acts as a window layer, is the junction counterpart of CdTe in the cell. The performance of the CdTe cell significantly depends on the properties of the window layer of CdTe thin film solar cells ^[3-6]. In the formation of the cell, the wider band gap, smaller lattice mismatch and good energy band alignment between the window and the absorber layer are the main requirements to select a certain material as a window layer in a heterojunction thin film solar cell^{[7, 8]}. CdS has a wider band gap (2.5 eV), a lattice mismatch of 0.066 nm with CdTe, and better conduction band alignment due to the smaller difference between the electron affinities (0.51 eV) of CdS and CdTe^[9]. A window material should be able to transmit the maximum number of incident photons to the absorber layer, which can further enhance the efficiency of the final device. The maximum of the solar energy incident on the cell can be transmitted to the absorber layer either by making the thinnest possible layer or by using a material with a larger band gap compared to that of CdS. In the Ⅱ-Ⅵ compounds ZnS, ZnO, and ZnSe have a larger band gap compared to that of CdS, however, the wider band gap (~2.7 eV), higher transparency, and small difference in the electron affinity values (~0.19 eV) which could help better alignment of the energy bands at the junctions, make ZnSe a viable alternative of CdS as a junction counterpart of CdTe. It is therefore, important to study the properties of ZnSe on a practical substrate such as In:SnO$_{2}$ (ITO) coated glass before using this material for solar cell application. In the present article we have studied the structural, optical and electrical properties of ZnSe thin films with 80, 90, and 100 nm thickness. One factor, which makes ZnSe inferior to CdS, when considering replacing CdS, is its lattice mismatch with CdTe. The lattice mismatch of ZnSe with CdTe is 0.081 nm, which is 0.015 nm larger than that of CdS. In the present work it is also observed that the deposition of ZnSe on ITO coated glass substrate have resulted in the reduction of lattice mismatch between ZnSe and CdTe.

2. Experimental

ZnSe thin films were deposited on ITO (200 nm thick) coated borosilicate glass substrate at 80 ℃ by thermal evaporation at the rate of 0.2 nm per second. The vacuum level was set to ~10$^{-6}$ Torr. The thickness of the films was controlled using a quartz crystal thickness monitor. The same procedure for the deposition of ZnSe thin films on glass substrate without ITO was adopted. The XRD spectra were taken between 2θ~20-60 using a D8 Bruker AXS diffractometer at a scan speed of 1 degree/min. The Rutherford Backscattering Spectroscopy (RBS) using a 2 MeV He$^{2+}$ beam from a tandem accelerator at NCP was used to determine the stoichiometry of ZnSe thin films. The transmission spectra of ZnSe/ITO/glass and ZnSe/glass thin films were taken using a Perkin-Elmer lambda-19 Uv-Vis-Nir spectrophotometer in the 200-1100 nm wavelength range. The annealing of one 100 nm thick ZnSe/ITO/glass thin film sample was carried out in air at 300 ℃ for 1 h. The samples with thickness 80, 90, and 100 nm were named as A, B, and C respectively, whereas the ZnSe/glass sample (100 nm thickness) was named as D in the text. The electrical measurements were carried out using a pico ampere meter and DC voltage source HP 4140B.

3. Results and discussion

The XRD patterns of ZnSe/ITO/glass thin film ($t\approx80$, 90, 100 nm) are shown in Figs. 1(a)-1(c), the XRD of ZnSe/glass thin film of 100 nm thickness is also included for comparison in Fig. 1(d). The XRD spectra of ZnSe thin films has shown a cubic structure with preferred orientation along the (111) plane observed at 2θ $\approx$ 27.22. However, there is a shift in the (111) peak position to lower 2θ values 26.465 and 26.45 degree respectively, with the increase in the thickness of the ZnSe films to 100 nm. The XRD spectrum of ZnSe thin film coated on glass film has also shown a cubic structure with (111) diffraction peak at $2\theta$ ~ 27.24 degree. We have calculated structural parameters i.e. lattice parameters, crystallite size, lattice strain etc. of ZnSe thin film samples using the (111) peak as shown in Table 1. It can be seen from this table that the lattice parameters have been increased to 5.83 and 5.834 Å with the increase in the film thickness, which shows that the lattice parameters are close to those of CdS, therefore minimizing the lattice mismatch between CdTe and ZnSe. Moreover, we did not observe any impurity peak from the ITO itself. The values of the lattice parameters for sample A and sample D were found to be 5.678 and 5.676 Å respectively, which are comparable to the already reported values for ZnSe^[10]. It was also observed that the crystallite size was decreased in thicker ZnSe thin film, whereas lattice strain and dislocation density have been increased, which is contrary to a similar work^[11] in which the crystallite size was increased with the increase of film thickness from 230 to 300 nm. We have also annealed sample C in air at 300 ℃ to check the role of the ITO layer diffusion in the increase of the lattice parameters of ZnSe. The XRD spectrum of the annealed sample C is shown in Fig. 2. It can be seen from this figure that although the lattice parameter has not been changed some extra peaks have appeared, which correspond to ITO and In₂O₃.This XRD analysis shows that the ZnSe films with thickness $>80$ nm grown on (ITO/glass) substrate at 80 ℃ could act as a suitable window material in CdTe solar cells because of the reduction in the lattice mismatch with CdTe and other superior optical properties as compared to that of CdS. The RBS analysis shows a nearly $1:1$ ratio of Zn to Se in ZnSe thin films. The RBS spectrum of the 80 nm thick ZnSe/ITO sample is shown in Fig. 3. The SRIM software (commercial software used for iob beam analysis) was used for the analysis of the RBS data. The percentage transmission spectra of A, B and C samples in the wavelength range 200-1200 nm are shown in Fig. 4. It was observed from these spectra that samples A and C have transmission close to 50%, whereas, sample B has less transmission. In order to further investigate the photo response of the ZnSe/ITO samples, the absorption coefficient $\alpha$(cm$^{-1})$ was calculated using Eq. (1)^[11-14].

$\alpha = \frac{1}{{d\ln (1/T)}},$

(1)

where d = film thickness, T = transmission.

The absorption coefficients of A, B and C samples are compared in Fig. 5. The calculated absorption coefficient shows that it has decreased in the 600-1200 nm wavelength range, which has started to increase to 1.3 × 10$^{8}$ cm$^{-1}$ in the shorter wavelength up to 200 nm in the UV region. These results show that in the visible region of the electromagnetic spectrum the ZnSe/ITO thin films have shown the possibility of becoming the partner of CdTe in the solar cell. For the sake of comparison, the transmission spectra of ZnSe thin film on glass substrate were also taken, see Fig. 6. The result shows that all the samples with 80, 90, and 100 nm thickness have higher %T as compared to that of ZnSe/ITO thin films, possibly due to the absence of ITO coating, which is otherwise present in ZnSe/ITO film. It is also observed that the ZnSe/ITO sample with 90 nm thickness has shown a higher absorption coefficient as compared to that of the other two samples. The higher absorption coefficient can be correlated with the smaller band gap as compared to other two samples. The energy gap $E_{\rm g}$ of the films was calculated from ($\alpha h\nu )^{2}$ versus $h\nu$ plots as shown in Fig. 7 by assuming a direct band gap material. Intercepts were drawn from the linear portion of the ($\alpha h\nu )^{2}$ versus $h\nu$ curves. The x-intercepts values are selected as the energy gap of each sample. The calculated band gap values are 2.43, 2.2, and 2.6 eV for samples A, B, and C respectively. The smaller band gap of 90 nm thick ZnSe/ITO thin film supports higher absorption of light in these samples. The band gap in poly crystalline thin films depends on various factors, which include crystallite size, structural parameters, deviation from stoichiometry and the lattice strain^[14]. The smaller crystallites result in quantum confinement, therefore, the increase in the band gap in sample C is due to the decrease in the crystallite size^{[14, 15]}. Considering an ideal abrupt ZnSe/CdTe heterojunction and assuming nondegenerate doping, the band positions in the ZnSe/CdTe junction are illustrated in Fig. 8. The figure shows that the same $\Delta {E_{\rm{c}}}$ and $\Delta$$E_{\rm v}$ discontinuities will exist at the heterojunction interface as before the formation of the junction.

The results of the electrical resistance measurement are given in Table 2. The resistance was measured in the dark and under the illumination of a halogen lamp. The results show that ZnSe samples have very small photosensitivity (photo-to-dark current ratio), which shows that there is very small loss of light energy within ZnSe film. However, the 90 nm thick sample has a relatively higher photosensitivity, which is gain correlated with its narrow energy gap as compared to the other two samples. The behavior of the photo decay in ZnSe thin films was also studied. The results of photo decay measurements are shown in Fig. 9 for the as-made thin film samples, 80, 90, and 100 nm. At $t=0$ when the light was switched on, there is a rise in the current through A, B and C thin films. The excess carriers have been produced due to the illumination of the sample which is called the photocurrent. A gradual decay in photocurrent was observed when the light was switched OFF; it took five minutes for the current to decay to its initial values (at $t=0$). The rise of current in the light is called photocurrent whereas the decrease in photocurrent when illumination was removed is called photo decay. This behavior which is observed in semiconductors is due to the presence of defects in the crystals, which may form electronic states (called trapping/recombination centers) in the forbidden energy gap. In the presence of a high density of recombination centers the rise time and fall time of the photocurrent is decreased whereas the trapping centers act to prolong the decay of photocurrent^[15-17]. As shown in Fig. 5, sample A has very slow decay of the photo current and it did not reach its initial value within the experiment time, which shows that the emission rate from the trapping centers is very small. The decay of photo current in sample B and C is fast. Since the recombination rate depends on the density of excess carriers and the recombination centers, a fast decay of the photo current suggests that sample B and C have a larger number of recombination centers in the forbidden energy gap^[18]. Moreover, sample B has a narrower energy gap as compared to that of A and C, therefore, the probability of band-to-band recombination is also high for this sample.

4. Conclusions

The following conclusions can be drawn from the above studies:

(1) The ZnSe thin films with thickness greater than 80 nm on ITO substrate have unit lattice parameters close to those of CdS.

(2) The optical band gap of ZnSe thin films is close to the standard measured values of 2.7 eV for the samples with higher thickness i.e. 100 nm, which removes the limitation on the window layer thickness to get the maximum transmission of the incident solar spectrum.

(3) The measurements of the decay of the photo generated current in ZnSe thin films show that all three samples have recombination as well as trapping centers manifested in the behavior of the decay of the photo generated current. Moreover, the thinnest sample has shown the longest decay time, which might be due to the larger number of defects present in the forbidden gap acting as recombination/trapping centers.

(4) The ZnSe could be a possible candidate as the window layer in CdTe thin film solar cells.