**1. Introduction**

Thin-film photovoltaic cells have generated enormous attention since the reliable efficiencies of CuInGaSe2 (CIGSe) and CdTe have exceeded 20% [1–4]. While there are numerous benefits of CIGSe and CdTe photovoltaic cells, such as consuming less material and high efficiency, the constituent elements of the materials inevitably have high cost and toxicity. Compared with CIGSe and CdTe, the Cu2ZnSn(S,Se)4 (CZTSSe) compound has a high absorption coefficient and adjustable bandgaps, and the constituent elements are inexpensive and environmentally friendly [5,6]. Hence, CZTSSe is considered to be a potential absorber material. However, the conversion efficiency of CZTSSe photovoltaic cells hasonly achieved 12.6%, which is still far lower than the conversion efficiency of CIGSe-based solar cells (22.9%) [7–9]. In order to improve conversion efficiency and make CZTSSe industrially viable, a lot of researches are still needed. Recently, studies have shown that enhancing the open-circuit voltage (Voc) and improving the crystal quality and optical–electrical characteristics of the CZTSSe layer are the main challenges that CZTSSe photovoltaic cells must face [10–12]. Considerable researches have been carried out to explore the reason of the lower Voc. It was found that the reasons are varied, and one of the frequently mentioned ones is the unsuitable conduction band offset (CBO) [10–12]. The CBO of CdS/CZTSSe is affected by the bandgaps of CZTSSe. Therefore, engineering an adjustable

bandgap is a practical method to break through the current limit of Voc. In order to achieve the goal of tuning CZTSSe bandgaps, a large number of experiments have been carried out [13–18].

Recent studies show that adjusting the bandgaps of CZTSSe by partial metal cation replacement is probably an effective approach. Among them, taking the place of Zn with Mg can adapt the bandgaps and enhance the crystallinity of CZTSSe films. In our previous studies, the Cu2MgxZnxSn(S,Se)4 samples with different Mg concentrations were successfully synthesized by the sol–gel method [19]. It was found that the bandgaps of Cu2MgxZnxSn(S,Se)4 samples can be tuned in the ranges of 1.12 to 0.88 eV as Mg concentration varied from x = 0 to 0.6 [19]. In addition, we investigated the effects of Mg content on the properties of Cu2MgxZnxSn(S,Se)4 films in detail. The results of the study showed that the Cu2MgxZnxSn(S,Se)4 films with adjusted bandgaps, high crystallinity, and high carrier concentration will be a potential high-efficiency photovoltaic cell absorber material [19]. Furthermore, the realization of bandgap regulation by Mg instead of Zn has the following advantages: Firstly, compared to Cd and Ge, the Mg element is more abundant, low cost, and environmentally friendly [20]. Moreover, the formation of other binary and ternary phases may be eliminated or reduced in the process of synthesizing Cu2MgxZnxSn(S,Se)4 films, because ZnS and ZnSe are present, while MgS and MgSe are unstable in the process of synthesizing the precursor solution [20]. Therefore, it is concluded that Cu2MgxZnxSn(S,Se)4 is worthy of study as a potential absorbing layer material.

As we all know, for the sake of improving the properties of CZTSSe and obtaining CZTSSe films with a single phase and large crystal size, a heat treatment of the precursor films at an elevated temperature (>500 ◦C) is usually required [21,22]. A large number of studies have shown that the selenization temperature and selenization time significantly influence the properties of the films, including the crystal quality, and optical and electrical properties [23–25]. The influence of selenization treatment on the physical performance of CZTSSe has been investigated by extensive researches [21–25]. However, the effects of selenization temperature and selenization time on the phase evolutions, crystal quality, and optical–electrical characteristics of Cu2MgxZnxSn(S,Se)4 have not been reported so far. Hence, in the present work, Cu2Mg0.2Zn0.8Sn(S,Se)4 (CMZTSSe) samples annealed at different selenization conditions are synthesized, and the influences of selenization temperature and selenization time on the structure, and optical and electrical properties of CMZTSSe samples are investigated in detail.
