서강대학교

초록/요약 도움말

Cu2ZnSn(S,Se)4 (CZTSSe) is drawing considerable interest for thin film solar cells [1-5]. It has a direct band gap and a high absorption rate (104 cm-1) [6]. CZTS and CZTSe have bandgap energies of 1.5 eV and 1.0 eV, respectively [6]. By controlling the composition ratio of S and Se, bandgap tuning may be possible to match ideal bandgap of 1.4 eV for a single junction solar cell [6]. Using earth abundant and non-toxic components Zn and Sn also has an advantage over similar thin film absorber materials such as CuInGaSe2 and CdTe [7]. The record efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is currently 12.6% [8], which is much lower than similar thin film chalcogenide solar cells based on Cu(In,Ga)Se2 (CIGS) with a maximum efficiency of 20.8% [9]. One of the possible reasons is that single-phase CZTSSe without secondary phases is difficult to make. If formations of secondary phases are unavoidable, it is also desirable to detect them and remove (e.g., by chemical etching) [10] it after it is made. Raman spectroscopy is a useful tool for detecting secondary phases in CZTSSe. Most studies on identifying secondary phases in CZTSSe samples using Raman spectroscopy are conducted by using green lasers (514.5 nm and 532 nm), but many secondary phases are not effectively identified. Each secondary phase has an optimal wavelength because of the resonance Raman condition [11]. The Raman signal of selenium measured by green laser is weak, and its main peak at 234 cm-1 [12] can be confused with the CZTSe peak at 233 cm-1 [13-15]. Using the laser wavelength dependence of the Raman intensity we are able to distinguish selenium in the CZTSSe layer with the 632.8 nm laser [12]. In this study we investigated CZTSSe cells (efficiency ~ 7.6%) made by sputtering followed by selenization. Laser ablation technique was used to remove selenium particles from the surface before making device [16-17]. Micro Raman mappings were compared with AFM images to show that selenium particles were present. Selenium particles that have a size of ~2.5 μm were randomly spread over the samples.