Critical review on sputter-deposited Cu₂ZnSnS₄ (CZTS) based thin film photovoltaic technology focusing on device architecture and absorber quality on the solar cells performance

Abstract

Thin film photovoltaic Cu₂ZnSnS₄ (copper zinc tin sulfide or CZTS) is one of the most promising sustainable solar cell absorber material. The CZTS absorber layer containing earth-abundant materials such as copper, zinc, tin and sulfur can be an alternative to existing materials for thin film solar cells. Recently, there has been an increased interest to step-up the efficiency and step-down the manufacturing cost of CZTS-based solar cells. This review critically addresses the advantages and challenges associated with sputter-deposited CZTS solar cells, since sputtering is an industry compatible and relatively low-cost vacuum deposition technique. Various approaches to fabricate CZTS thin films by sputtering are discussed. In addition, the single target quaternary CZTS sputtering technique has been discussed in detail. Current state-of-the art device architectures and methods to improve the quality of interfaces are discussed. This review is intended to highlight current trends and challenges in the field to realize the opportunity of CZTS thin film solar cells for large scale application.

Introduction

Humanity needs to meet the power generation requirement of 30 TW (1 TW = 10¹² W) by 2050 without carbon emissions associated with the expected increase of global energy demand [1]. Solar photovoltaics (PV) has great potential to meet future large-scale electricity supply with low-carbon emission [2]. Fig. 1 illustrates the power conversion efficiency (η) of the different champion solar cells developed worldwide. Crystalline-silicon (c-Si) is the most dominant PV technology. Silicon-based photovoltaic devices are preferable due to their durability, compatibility with the technology of microelectronics and high efficiency. However, this technology is material intensive owing to the low absorption of Si, which acts as a bottle-neck for further reduction in cost. Besides, the processing cost of c-Si solar cells is high, it also requires complex processes including high temperature treatment and ion implantation.

Chalcogenide-based thin film PV technologies have the potential to reduce the cost of the PV technology. CdTe and Cu(In,Ga)(S,Se)₂ (CIGS) technologies have shown tremendous progress and finally reached commercial production. However, scarcity of In, Ga, and Te as well the toxicity of Cd are issues of concern. In addition, according to the study of Wadia et al., both these PV technologies are unable to meet annual world electricity demand [3]. Besides, raw material cost of CIGS and CdTe is more expensive and their annual electricity potential is lower compared to other promising alternative PV materials indicated in Fig. 2.

Kesterite semiconductors consisting of copper, zinc, tin and sulfur and/or selenium are considered as promising alternatives to CdTe and CIGS owing to the abundance and non-toxicity of its constituents. Fig. 3a shows the growth of the number of publications on kesterite semiconductors since 2006. Kesterite thin films have been prepared using various vacuum and non-vacuum based techniques including sputtering [4], [5], [6], [7], thermal evaporation [8], [9], [10], pulsed laser deposition [11], [12], electrodeposition [13], [14], [15], spray pyrolysis [16], [17], [18], [19], sol-gel [20], [21], [22] and hydrazine solution approach [23], [24], [25].

The champion kesterite solar cell based on Cu₂ZnSn(S,Se)₄ absorber achieved record η of 12.6% in 2014. This PV device was fabricated by the hydrazine pure solution approach. However, hydrazine is both highly toxic and reactive [28] which limits its application in the industry. Sputtering is a suitable method for high volume manufacturing owing to the following advantages: uniformity of deposited films on large scale, high deposition rate and reproducibility of the process [33], [34], [35], [36]. Besides, sputtering enables interface engineering, tuning of crystallinity and composition of the films [37], [38], [39], [40]. Fig. 3b depicts the annual number of publications on selenium free sputter-deposited CZTS thin films which clearly indicate growing interest in CZTS thin film solar cells made by sputtering.

High abundance and non-toxicity make pure sulfide CZTS preferable to Cu₂ZnSn(S,Se)₄. CZTS exists in two crystalline forms: stannite and kesterite, of which the latter is more useful owing to its enhanced stability [41]. CZTS is a quaternary p-type semiconductor showing direct bandgap of about ~1.5 eV, carrier concentration similar to CIGS and high absorption coefficient of order 10⁴ ${\mathrm{cm}}^{-1}$ for visible wavelengths [42], [43], [44], [45], [46]. P-type conductivity is achieved owing to intrinsic defects such as copper vacancies [46]. The bandgap of CZTSSe can be tuned from 1.0 eV to 1.5 eV by increase of S/(Se+S) ratio from 0 to 1 [47], [48] providing a degree of flexibility in device fabrication with the material. Similar to CIGS, the grain boundaries of CZTS provide an enhanced minority carrier collection [49]. Carrier mobility and resistivity of CZTS thin films are in the range of 0.1 – 35 cm²/V s and 3.4 × 10⁻³ – 600 Ω cm, respectively [50]. According to the Shockley-Queisser limit, CZTS PV devices have the potential to reach a η of 28% [51].

Although a number of reviews on kesterite solar cells have been published to date [43], [46], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72], there is no review focused on the sputter-deposited pure sulfide (selenium free) CZTS thin film solar cells, which have shown significant progress recently. In addition, the η of these cells is still too low compared to the best CIGS and CdTe thin film solar cells. Fig. 4 presents η of the sputter-deposited CZTS thin film solar cells versus publication year. The wide dispersion of η can be attributed to differences in the quality of CZTS absorber layer, interface quality and device architecture.

The low power conversion performance of CZTS thin film solar cell is mainly due to poor fill factor (FF) and low open-circuit voltage (V_oc). As can be seen from Fig. 5, typical FF of the best CZTS solar cells is about 0.6 while FF of the best CIGS and CdTe solar cells are close to 0.8 [120]. Moreover, the performance of sputter-grown CZTS solar cells suffers from huge V_oc deficit (E_g/q – V_oc), where E_g is the bandgap and q is the electron charge (Fig. 6). Obviously, there is plenty of room for improvement to reach the high η of CIGS and CdTe PV technologies. Bulk recombination and recombination at the absorber/buffer interface significantly affect FF and V_oc of CZTS PV devices [58]. Bulk recombination is attributed to the presence of various defect states and defect complexes within the CZTS absorber layer [43]. Presence of low bandgap secondary phases also negatively affects V_oc of CZTS solar cells. Careful control of film composition as well as adjustment of sulfurization parameters is crucial during fabrication of high quality CZTS thin films. Various sputtering techniques used to prepare CZTS thin films and their influence on PV properties of CZTS PV devices are presented in subsections 2.1–2.3. Recombination at the absorber/buffer interface is caused by cliff-like nature of CZTS/CdS heterojunction and defects at the interface, which favor intensive recombination at the interface [58], [121]. Although CdS is a widely used material in fabrication of CZTSSe and CIGS solar cell it is not ideal for CZTS PV devices. Alternative device architectures and their effect on device performance are discussed in Section 3. In addition, poor quality back interface reduces FF and device performance. A summary of various approaches to improve the quality of absorber/back contact interface through intermediate layers is given in Section 4.

The aim of the paper is to show current trends and present status in the field to not only enlighten challenges related to the CZTS thin film solar cells but also provide the insights for the further development of CZTS thin film solar cells.