1. Introduction

Cadmium telluride (CdTe) is one of the major materials for thin film solar cells due to its short energy payback time, simple structure, long-term stability and high efficiency (up to 19.6%) [1, 2]. Many post-growth process steps are necessary to improve the performance of CdTe-based solar cells such as activation using chlorine containing compounds (CdCl₂ and HCl), depletion of Cd from the CdTe surface using nitric-phosphoric acid or bromine-methanol, and formation of good back-contact [3–7]. In particular, the post-growth CdCl₂ treatment process (also called the activation process), which involves thermal annealing in the presence of CdCl₂, has been widely used in most CdTe thin films to enhance cell performance because it is believed to result in higher short circuit current and open circuit voltage, as well as lower series resistance [8]. Qi et al. reported the effects of thermal annealing after deposition of a thin CdCl₂ layer on CdTe grown by electrodeposition [9]. In addition, an in situ vapor-phase CdCl₂ treatment during CdTe film growth has been reported [10]. Moutinho et al. found that the efficiency of CdTe solar cells with CdTe thin films deposited at low temperature in a close-spaced sublimation (CSS) system improved from 0.7% to 8.9% after the post-growth CdCl₂ treatment [11].

Most reports on CdTe-based solar cells have focused on the superstrate configuration, which requires a transparent substrate and transparent conductive electrodes [12–15]. As the superstrate structure suffers from limitations in the substrate material, stability of back contact, and lack of flexibility, the substrate configuration has attracted a great deal of attention. For example, metal foil and polymers as a substrate material have their advantages compared to glass substrates such as low price, lightweight, and shape adaptability to fit a curved surface in cases of substrate configuration. Additional advantages of the substrate configuration are (1) less limitations on the choice of the substrate, for instance, an opaque substrate (2) independent optimization of CdTe layer by post-growth treatment [16, 17]. Fabricating flexible CdTe solar cells on molybdenum (Mo) foil substrate has been reported because Mo is a flexible metal and is less liable to break due to its similar thermal expansion coefficient to that of CdTe. Williams et al. reported an in-depth study of substrate configuration CdTe solar cells on Mo with an efficiency of 8.01% [17]. They showed that the post-growth CdCl₂ treatment after CdTe deposition is critical to improve the cell efficiency in the substrate configuration. Romeo et al. developed a polymer-based flexible CdTe solar cell in a substrate configuration with 7.3% efficiency [18]. Graphene on SiO₂/Si is a strong candidate as a substrate for substrate configuration because CdTe thin films can be selectively deposited on graphene, which has outstanding mechanical, optical, and electrical properties. Additionally, the integration of the advanced silicon microelectronics with CdTe-based thin film solar cells has a great potential for next-generation high performance photovoltaic systems. In the present study, CdTe thin films grown on graphene were subject to CdCl₂ treatment to investigate the structural, optical, and morphological effects of post-growth treatment.

2. Experimental details

Figure 1 is a schematic of the CdTe growth on graphene and post-growth CdCl₂ treatments. A monolayer of graphene grown on Cu foil by the chemical vapor deposition method was used as a seed layer to grow CdTe. Bi-layer graphene was obtained by repeating the transfers onto SiO₂/Si substrates after chemical etching of Cu-foil in 1 wt% ammonium-persulfate solution (Sigma-Aldrich) [Fig. 1(a)]. UV-ozone treatment at room temperature was performed for 30 min to introduce defect sites on the graphene layer to increase the number of dangling bonds. The CdTe thin films were deposited on the defective graphene/SiO₂/Si substrate using a CSS system for 25 min under an ambient argon atmosphere [Fig. 1(b)]. The temperatures of the source and substrate were held at 600°C and 540°C during deposition, respectively. Details of CdTe growth on graphene can be found elsewhere [19]. The surface morphology of the CdTe was observed by scanning electron microscopy (SEM) (Hitachi, S-4700). A SEM image of a typical surface of a CdTe thin film is shown in Fig. 1(c), where CdTe structures with large grain sizes were observed with clear crystalline facets. Figures 1(d) and 1(e) describes the process of CdCl₂ treatment. Excess CdCl₂ powder (Sigma-Aldrich) was dissolved in methanol, followed by dipping the sample into the saturated CdCl₂ solution. Finally, thermal annealing was carried out in a furnace at 385°C in air for 15 to 90 min. To remove the oxide grown on the CdTe surface during thermal annealing, the samples were etched by 0.1 volume % bromine in methanol for 30 sec. Micro-photoluminescence (micro-PL) spectra of the CdTe thin films were obtained on graphene before and after the post-growth treatments using the 532 nm line of a single mode DPSS laser (Omicron). Micro-Raman spectroscopy using the 514 nm line of an Ar-ion laser (Horiba Jobin-Yvon, LabRam ARAMIS) and X-ray diffraction (XRD) (Cu target, 2theta mode, Rigaku, ATX-G) were used to characterize the crystallinity of the CdTe thin films before and after the post-growth treatments.

 

Fig. 1 Growth of CdTe thin films on graphene/SiO₂/Si substrate and the post-growth CdCl₂ treatment.

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3. Results and discussion

Figure 2 shows SEM images of the CdTe surface after CdCl₂ treatment for 15 min [Fig. 2(a)], 30 min [Fig. 2(b)], 60 min [Fig. 2(c)], and 90 min [Fig. 2(d)], respectively, where the difference in the surface morphology between the as-deposited [Fig. 1(c)] and the CdCl₂-treated CdTe thin films [Figs. 2(a)–2(d)] was clear. Small new grains (bumps) appeared on the CdCl₂-treated CdTe surface. The number of small grains on the CdTe films increased until 30 min of post-growth CdCl₂ treatment. However, the SEM image shows that the number of small grains decreased after 60 min CdCl₂ treatments, compared with that after 30 min of treatment. Many small grains formed again after the 90 min treatment. This cycle can be explained by the recrystallization of the CdTe structures during the CdCl₂ treatment. One of the other well-known effects of CdCl₂ treatment is grain growth of CdTe, which was prominent for columnar CdTe structures with small grain sizes observed at low substrate temperature growth (420–474°C) in the CSS system [11]. Nominal grain growth was previously reported on CdTe thin films grown at a high substrate temperatures > 600°C, which is in good agreement with our SEM images because our CdTe thin film already had large globular grains due to the high deposition temperature. Figure 3(a) shows SEM images of the CdTe surface treated for 90 minutes. As shown in the enlarged SEM images [Figs. 3(b)- 3(d)], the formation of small new grains depended on the crystalline plane of the CdTe structure, suggesting different reactivity in each crystalline plane of the multi-crystalline CdTe structures.

 

Fig. 2 Scanning electron microscopy images of the CdTe surface after the post-growth CdCl₂ treatment for (a) 15 min, (b) 30 min, (c) 60 min and (d) 90 min.

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Fig. 3 Scanning electron microscopy images of CdTe thin films that were CdCl₂-treated for 90 min.

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XRD measurements were performed to analyze crystallinity of the CdTe thin films. The XRD data before and after CdCl₂ treatment are summarized in Fig. 4(a). As-deposited CdTe thin film was preferentially oriented in the (111) direction, which indicates that the orientation of CdTe was affected by the underlying highly oriented two-dimensional carbon array of the graphene. A randomized XRD pattern was observed in the CdCl₂-treated samples due to recrystallization of the CdTe thin film. The ratio of peak intensities at (220) and (311) with respect to (111) was determined to quantify the degree of change in CdTe orientation according to the post-growth treatment [Fig. 4(b)]. A value of zero indicates that the film is completely oriented along (111) orientation. An increasing intensity ratio indicates more randomization of CdTe thin films. Figure 4(c) shows the full width at half maximum (FWHM) values of each peak at (111) and (220) for different treatment times. FWHM values with a variation < 10% indicates that the crystalline quality of the CdTe thin films was less affected by the CdCl₂ treatment. These XRD data are well matched with the SEM images. Peak energy of 1.45–1.50 eV was observed in the PL spectra measured at room temperature. Three peaks were observed in the Raman spectra at 141, 164 and 330 cm⁻¹ presented in Fig. 5.Both the pure E mode of the tellurium phase and the transverse optical phonon of CdTe were found at ~141 cm⁻¹. The last two Raman shifts (164 and 330 cm⁻¹) indicate longitudinal-optical (LO) and 2LO phonons of CdTe, respectively. The position and shape of the Raman spectra indicate that high quality CdTe film was maintained after CdCl₂ treatment. In addition, a chemical etch of the oxide using a bromine-methanol solution did not change the crystalline structure of CdCl₂-treated samples based on the XRD results (data not shown), which indicates that the re-orientation of CdTe thin films by CdCl₂ treatment is not related to the oxidation reaction [20]. We believe that our results of the post-growth CdCl₂ treatment on the CSS-grown CdTe / graphene films are helpful to improve the performance of the CdTe-based thin film solar cells with the substrate configuration. Also, our results hold a promise for a new form of CdTe solar cell structure integrated with the graphene and the advanced Silicon microelectronics.

 

Fig. 4 (a) X-ray diffraction (XRD) data before and after various duration of CdCl₂ treatment (b) normalized XRD peak ratios (c) full width at half maximum of (111) and (220) XRD peaks at different CdCl₂ treatment times.

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Fig. 5 Micro-Raman spectra at room temperature before and after the post-growth CdCl₂ treatments.

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4. Conclusion

CdCl₂ treatments were performed for various times at 385°C on CdTe thin films grown on a defective bi-layer graphene using a CSS system. The treated samples had new small grains on the original CdTe surface produced by the recrystallization process during CdCl₂ treatment. The formation and number of small new grains (bumps) on the surface varied depending on treatment time and the crystal plane of the CdTe structure. The changes in the XRD intensity ratios of both (311)/(111) and (220)/(111) peaks confirmed the randomization of the crystallinity on the CdTe thin film by the post-growth CdCl₂ treatments, which are consistent with SEM images. Post-growth treatment which is an essential step for the superstrate configuration CdTe solar cells is investigated for the substrate configuration of CdTe / graphene films / SiO₂ / Si substrate.