Investigation of ZnO nanotubes array as an antireflective coating for planar CZTS/ZTO solar cell

We present a theoretical investigation of ZnO nanotubes square periodic array as an antireflective coating for a planar solar cell including Kesterite as a p-type absorber and Zinc Tin Oxide as an n-type buffer. The method of the calculation is rigorous coupled wave analysis. We found that this coating leads to a considerable reduction of the loss of the ideal short circuit current density by about 80 %. The optimal values of the nanotube diameter, nanotube height, array period and the absorber thickness are 438 nm, 150 nm, 438 nm and 1.5 µm respectively. The study of the variation of the loss of the ideal short circuit current density with the incident angle of the sunlight shows that the NTs array still has good anti-reflection characteristics as long as the incident angle doesn’t exceed 60°.

can increase the absorption of the light, due to the trapping effect, by optimization of the geometrical parameters [8][9][10]. Furthermore, experimental research on InGaP/GaAs/Ge solar cell used ZnO nanotubes as antireflective coating (ARC) to improve the efficiency of the solar cells [11]. Indeed, the coating with a NT-film minimizes the reflected light at the front surface and thus increases the absorbed energy due to the gradual variation of the refractive index.
In this paper, we study the effect of ZnO NTs array as an antireflective film on the surface of thin films SC including Kesterite Cu2ZnSnS4 (CZTS) as a p-type absorber and Zinc Tin Oxide (ZTO) as an n-type buffer. Indeed, CZTS has outstanding electrical and optical features as a direct optical band gap of 1.5 [9] and ZTO has high transmittance in the visible range [8].
Furthermore, the p-CZTS-n-ZTO heterojunction has a favorable conduction band alignment and is only consisting of earth-abundant and non-toxic constituents. The use of NT array antireflective help To improve the CZTS thin film SC performance, especially the power conversion efficiency of this SC (9.3 % [12] is relatively weak compared to the one reported for CIGS thin film SC (22.9 %) [13].
The validation of the used code was made in previous work [8]. To minimize its value, we calculated the loss of the ideal short circuit current density, due to the reflection on the ZnO NTs array, as a function of the NTs diameter and the ratio between the period array and the NTs diameter for different values of NTs height. The maximal value obtained for the ideal short circuit current density (Jph) is about 41 % greater than that of a planar SC without NTs array.
A study of the variation of ideal current density with the incident angle of the light is also reported.
According to our best knowledge, an optical simulation of solar cell based on CZTS thin film with ZnO NTs array as antireflective has been reported for the first time in the present study.
In section 2, we described the structure of the SC simulated. Section 3 is devoted to the optimization of the geometrical parameters of NTs array to minimize the loss of the ideal short circuit current due to the reflection on the surface. The fourth section is reserved to study the variation of this current with the incident angle of the light.

Device setup
The structure of the SC covered by a ZnO square periodic array of NTs investigated in this article is shown in figure 1. The period, diameter and height of the NTs array are denoted by P, D and h respectively. The thickness of the wall of the NTs is 20 nm according to experimental measures [10]. The NTs are deposited on n-ZnO seed layer of 0.5 µm thickness which covers a Zn0.77Sn0.23O (buffer layer)-CZTS (absorber) heterojunction. The composition of Sn and the thickness of the buffer layer (10 nm) were chosen to obtain a good CZTS solar performance [8]. The thickness of the absorber layer was varied during the optical simulation. The p-CZTS layer is electrically connected to the device contact by a 0.3 µm FTO layer.

Optimization of the geometrical parameters of the NTs array
To highlight the effect of the NTs array on the reflectance by the SC surface, we presented on figure 2 the reflectance spectra obtained for SCs coated or not coated by NTs array. We note a significant decrease of the reflectance by the coating by a NTs array.   guides for the eyes to show the underlying nanowire structure described in Figure 1.

3-Variation of ideal current with the incident angle of the light
Taking the change in the sun's incidence angle into account, the relationship between Jph and JphR with the incident angle of light for different diameters of the NTs is illustrated in figure 6.
The angle 0 corresponds to the normal incidence. We remark that for large diameter and small It's worthy to note that figure 6a, which corresponds to the optimal geometrical parameters, shows that the NTs array still has good anti-reflection characteristics when the incident angle doesn't exceed 60°. The diameter of the NTs and the ratio P/D are 240 nm and 3 respectively

4-Conclusion
In this paper, we optically simulated a planar solar cell including Kesterite as a p-type absorber and Zinc Tin Oxide as an n-type buffer and coated by an antireflective coating composed of a ZnO square periodic nanotube array to optimize the structural morphology. An optimum value of 42 mA/cm 2 for the ideal short circuit current density and a minimum value of 3.5 mA/cm 2 for the loss of ideal short circuit current density were found for diameter NTs of 438 nm, NTs height of 150 nm, array period of 438 nm and absorber thickness of 1.5 µm. Furthermore, the ideal short circuit current density is independent of the incident angle below 60°. We conclude that the coating with a ZnO NWs enhances considerably the performance of the studied solar cell.