# CZTSe-Based Solar Cell Performance Improvement Using the CSLO Technique
## 1 Introduction  
Cu\({}_{2}-\)II\(-\)IV\(-\)VI4 quaternary compounds have been of great interest for many years in sustainable photovoltaics due to their suitable bandgaps, their abundance on Earth and because they only require cheap fabrication techniques [1]. Among these multi-layer configurations, Copper Zinc Tin Sulfide (CZTS) is an emerging solar cell absorber that, in terms of lattice structure, it is similar to chalcopyrite semiconductors such as CuGaSe\({}_{2}\), CuInSe\({}_{2}\), and Cu (Ga,In)Se\({}_{2}\), but contains only earth-abundant, nontoxic elements and has a near-optimal direct bandgap energy of E\({}_{8}\)-1.5 eV, theoretically determined with high optical absorption coefficients over 10\({}^{4}\) cm\({}^{-1}\)[2]. Therefore, CZTSSe-based solar cells such as Cu\({}_{2}\)ZnSnS\({}_{4}\) (CZTS), Cu\({}_{2}\)ZnSnSe\({}_{4}\) (CZTSe), and Cu\({}_{2}\)ZnSn(S,Se)\({}_{4}\) (CZTSSe) have many advantages due to their special properties, which include being eco-friendly, cost-effective, and highly efficient [3, 4]. In addition, CZTSSe-based solar cells have received attention as excellent absorber layers of photovoltaic cells, since they have nontoxic materials, a simple fabrication process, and reasonable optoelectronic properties for thin-film solar cells [5]. For instance, their direct bandgap and high absorption coefficient in the visible region are important factors towards choosing them for the absorber layer in solar cells [6, 7, 8]. In fact, solar cells are considered an important alternative source of green and clean energy [9]. Hence, numerous techniques and solutions have been reported for the optimization of solar cells and overcoming problem related to this. Improving the performance of solar cells is generally based on layer-optimizing techniques [10], material selection [11], light-trappingmethods [12], the use of solar concentrators [13], and the solar cell structure design [14; 15]. Regarding this, more works are introduced as follows. [PERSON] et al. [10] suggested the use of a silicon Schottky solar cell with a passivation layer to improve the performance of the solar cell. In another study, [PERSON] et al. [11] used a Cu (In\({}_{1\sim}\)Ga\({}_{\text{x}}\)) Se\({}_{2}\) solar cell to enhance cell efficiency. According to the reported results, the carrier transport mechanism and conduction band alignment at the CdS/CIGS interface have led to an increase in efficiency from 12% to 14%. Furthermore, light trapping methods are used to absorb a significant fraction of the incident sunlight to minimization reflection [12]. In addition, many types of research show that CuSbS\({}_{2}\) absorber-based solar heterostructure cells show enhanced electrical properties [14]. Moreover, a device architecture with three terminals has been proposed by [PERSON] et al. [15]. The proposed solar cell is heterojunction bipolar transistor-based with a simple structure in comparison to multi-junction designs. A model of a GaInP material-based p+/n+ single junction solar cell was simulated by [PERSON] et al. [16] with two passivation layers to increase the efficiency up to 22%. In this direction, the multiple roles of conjugated polymers in enhancing the environmental stability and performance of perovskite solar cells were introduced in [17; 18]. The most common hybrid perovskite materials used as absorber layers in perovskite solar cells (PSC) are methylammonium lead iodide (CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\) or MAPbI\({}_{3}\)) and methylammonium lead bromide (CH\({}_{3}\)NH\({}_{3}\)PbBr\({}_{3}\) or MAPbBr\({}_{3}\)), which enhance the efficiency of cells [19]. Moreover, research works [20; 21] have investigated the role of i-aSi:H layers in aSi:H/cSi heterojunction solar cells, with the implementation of non- or lightly-doped hydrogenated amorphous silicon layer by [PERSON] et al. [20] and the use of overlapped p/i and n/i a-Si:H layers by [PERSON] et al. [21], which improved cell performance. Furthermore, another important factor in the design of solar cells is the surface layer shaping [22].