Investigation of heterojunction interface of CZTSSe/Cd-free buffer layers for photovoltaic applications

Abstract

The optical band gap of ~1.45 eV and high absorption coefficient of ≥104 cm-1 have made Cu2ZnSn(S,Se)4 (CZTSSe) films as one of the most promising absorber materials in thin-film photovoltaics. The CZTSSe consists of elements that are abundant and non-toxic in nature makes it even more favorable while realizing cost-effective and environment-friendly solar cells. First of all, the optimization of substrate temperature (Tsub) of CZTSSe thin films deposited on soda-lime glass (SLG) substrate has been performed in a single-step route from a single quaternary sputtering target by dual ion beam sputtering (DIBS) system. The influence of Tsub on the compositional, structural, optical, and morphological properties has been investigated and the correlation of these properties has been established. The composition has beeeen evaluated using energy dispersive X-ray spectroscopy (EDX). The thin films deposited at Tsub = 200 and 300 °C shows Cu-poor and Zn-rich states(i.e. Cu/(Zn + Sn) < 1 and Zn/Sn > 1), which is not the case for films grown at other Tsub. However, the film grown at 200 °C is (S + Se) deficient. It is observed that the absorption coefficient of all thin films is >104 cm-1 in the visible region. The values of Eg in the fundamental absorption region of CZTSSe are found to be in the range of 1.23-1.70 eV depending upon the value of Cu/(Zn+Sn) ratio. From high-resolution X-ray photoelectron spectroscopy (XPS) analysis, it has been confirmed that theall five elements, Cu, Zn, Sn, S, and Se have been present in their expected chemical states. Therefore, all experimental characterizations determine 300 °C is the optimized substrate temperature to grow high-quality DIBS-grown CZTSSe materials, which can be used to realize cost-effective and high-performance photovoltaic applications. After optimization of CZTSSe absorber, XPS and spectroscopic ellipsometry (SE) analysis of the CZTSSe thin films with different Cu and Zn contents, as deposited onto Mo/SLG substrates is performed. We identify the bandgap variation in CZTSSe thin film at the surface and bulk and compare their optical constant using SE. The surface composition has been determined by quantification of the XPS data. The surface composition determined using XPS is compared to EDX. A prototype device grown with the optimized heterojunction is fabricated to validate the concept of improved device performance using CZTSSe having Cu-poor surface. Obtained performance parameters of the prototype device establish open circuit voltage (Voc) of 562 mV, which is comparable to the highestreported value of the Voc. The high value of the Voc can be associated to the Cu-poor surface layer with the higher bandgap at the surface of the CZTSSe. However, the observed values of short circuit current density (0.122 mA/cm2) and fill factor can be further improved by incorporating a window/transparent conducting oxide (TCO) layer combination, and different light manipulation techniques. The band alignment study of Cd-free buffer layers (Mg0.26Zn0.74O (MZ1), Mg0.30Zn0.70O (MZ2)) with CZTSSe has been performed. The values of valence band onset (VBOn) have been determined by linear extrapolation technique for MZ1, and CZTSSe films are 2.97 and 0.54 eV, respectively. The average values of valence band offset (ΔEV) and conduction band offset (ΔEC) is 2.12 and 0.29 eV, respectively. The positive value of ΔEC indicates that the conduction band minima of the buffer layer is above than that of the CZTSSe film. The photovoltaic performance is mainly affected byby (1) the small conduction band offset (CBO) spike (0–0.4 eV) prevents interface recombination, essentially recombination between electrons in the conduction band of the buffer MZ1 and holes in the valence band of CZTSSe; and (2) contrary to this, the CBO with cliff promote the interfacerecombination, specifically in the existence of interface defects and/or a high interface recombination velocity, bring significantly decreased Voc. From another point of view, the large positive value of ΔEC also, diminish the performance of the solar cell as it severely decreases the photocurrent. Similar anlaysis has been performed for MZ2/CZTSSe interface. The effect of DIBS sputtered 10 nm thin film of ZnS intermediate layer between Mo and CZTSSe has been performed. The structural, morphological, and optical properties of CZTSSe thin film with and without ZnS intermediate layer has been compared. It has been observed from the XRD analysis that CZTSSe with ZnS intermediate layer has a higher intensity of (112) peak. It has also been observed from FESEM analysis that the morphology of CZTSSe thin film with ZnS intermediate layer is improved.