Structural and optical study of CZTS-reduced graphene oxide composite towards photovoltaic device application

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ScienceDirect Materials Today: Proceedings 17 (2019) 131–137

www.materialstoday.com/proceedings

ICAMEES2018

Structural and optical study of CZTS-reduced graphene oxide composite towards photovoltaic device application Sonali Das a, Kadambinee Sa a, Injamul Alam a, Jagatpati Raiguru b, BVRS Subramanyam a, and Pitamber Mahanandia a* a

Department of Physics and Astronomy, National Institute of Technology Rourkela, Sundargarh769008, Odisha, India

b

Department of Electrical Engineering, National Institute of Technology Rourkela, Sundargarh769008, Odisha, India

Abstract The deficit in obtaining targeted photo conversion efficiency for (Cu2ZnSnS4) CZTS based photovoltaic cell results either from insufficient photo charge carrier generation or e-/h+ pair transportation towards the end of electrodes, which lowers the device Voc and Jsc. On this regard, semiconducting absorber material merged with highly mobile reduced graphene oxide (rGO) as filler form a bridging network within active layer facilitates both improvement in charge carrier separation as well as transportation to electrodes before recombination occurs. A simple solution casting approach for CZTS nanoparticle anchored rGO composite for photovoltaic device application is reported here. The presence and distribution of CZTS nanoparticles over the surface of rGO sheet is confirmed from XRD, Raman, SEM and UV-Visible analysis. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Advanced Materials, Energy & Environmental Sustainability, ICAMEES2018. Keywords: Cu2ZnSnS4 (CZTS); reduced graphene oxide (rGO); Photovoltaic device; XRD; Raman; SEM

1. Introduction Green energy is the need for the hour to control the pollution rate. Therefore, there is an increasing demand for the development of non-fossil fuel materials composed of non-toxic as well as cost effective materials for energy Corresponding author. Tel.: +91 0661 246 2730 Email address: [email protected], [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Advanced Materials, Energy & Environmental Sustainability, ICAMEES2018.

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generation. Among various materials chalcopyrite based photovoltaic cell (PVC) absorber material such as copper indium gallium sulphide (CIGS) shows greater efficiency for photocurrent production [1, 2]. Although the achieved energy conversion efficiency of chalcopyrite CIGS material is nearly 20% but the scarcity of In and Ga increases the cost of power generation and limits its further usages in solar market. Consequently, research is being carried out to identify the sustainable and affordable alternative solar energy harvesting material [3, 4]. Aiming towards this above problem, a p-type semiconductor Cu2ZnSnS4 (CZTS), made up of earths abundant materials has similar structure like CIGS along with bandgap energy suitable as absorber material for solar cell application [5, 6]. CZTS is made up of nontoxic elements having a band gap in the range of 1.4 to 1.6 eV along with a higher absorption coefficient of 104cm-1 and above can be used as a substitute for CIGS [7]. The diffusion among the nano particles in photoactive semiconductors generates photo electrons, greatly affect the photo conversion efficiency (PCE) of solar cell. Due to some surface effects in semiconducting nanocrystals, the efficiency of electron diffusion decreases resulting an increase in recombination of electron - hole pairs. Ultrafast carrier transfer material having higher mobility such as graphene can improve PCE by using the better light absorption material CZTS on it. CZTS nanocrystals on reduced graphene oxide sheet as nanocomposite is expected to act as sensitizing centres for efficient absorption of incident light and increased transfer of electrons and holes to the conductor, thereby increase in efficiency [8-9]. Better light absorption and charge transfer performance of graphene based hybrid systems with high quantum efficiency have been reported by Konstantatos et al. (∼25%) [10]. Enhancement in PCE (7.81%) has been observed in hybrid CZTS-rGO dye-sensitized solar cells (DSSCs) have been reported by Bai et al. [11] compared to CZTS (4.77%). Improve in optical properties of CZTS-rGO have been observed by Thangaraju et al. [12]. On this regard, in the present work, we report the preparation of CZTS anchored rGO composite by cost effective, scalable and non-toxic solution method. The synthesized CZTS-rGO composite shows enhanced optical properties which could be used as potential material for solar cell absorber.

2. Experimental There are multiple advantages of using solution process for fabrication such as low cost instrumentation, hand manipulation and easy deposition. The following materials copper (II) chloride dihydrate (99%) and zinc chloride (98%) were procured from HiMedia Laborataries Pvt. Ltd. whereas, stannous chloride dihydrate (97%) and thiourea (99.5%) were purchased from Fisher Scientific were used for making the solution precursor. The CZTS precursor was prepared using environmental friendly solution method by dissolving CuCl2.2H2O (0.01M), ZnCl2 (0.0078M), SnCl2.2H2O (0.0056M) and CH4.N2S (0.0444M) in distilled water act as solvent and reducing agent. The solution was stirred for 4hours to get a stable suspension. The solution is then dried at temperature 150°C in a controlled atmosphere furnace to obtain the desired phase pure CZTS. Graphite oxide (GO) was reduced to obtain reduced graphene oxide (rGO). The graphene oxide (GO) was prepared by modified Hummers method [13]. In brief, one gram of graphite powder was mixed with NaNO3 (500gm) and conc. sulfuric acid (H2SO4, 12mL). The mixture was

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stirred for two and half hour in an ice bath. The temperature was maintained at less than 5°C throughout the mixing. Then KMNO4 (3gm) was added slowly and again was stirred for another one and half hour. Remove the ice bath and the mixture were cooled down to room temperature. Then add 100ml of distilled water (gas evolved) and then again stirred for two and half hour and add H2O2 (10mL), 300 ml of distilled water and again 500mL of distilled water was added. With the addition of distilled water, oxidation was initiated. To terminate the oxidation reaction, H2O2 (10mL) was added to the above mixture. The colour of the mixture turns down mud brown colour. The mixture was further washed with excess water until the PH of the solution was nearly neutral (6-7). Generally, vacuum lifted and dried under reduced, pressure overnight to yield GO. If during stirring the liquid is too concentrated, then add extra amount of hydrogen peroxide. Then GO was sonicated for 6hour. The sonicated GO was centrifuged with 8000rpm for 10min and dried in oven for half an hour and GO was obtained as grey–yellow powder. After vigorous ultra sonication the exfoliated GO sheets were obtained. The exfoliated GO was then reduced to form rGO by using ferrocene and HNO3 previously reported by Sa et al. [14]. The prepared rGO has been neutralized and washed with deionized water followed by drying at 100°C under vacuum condition to obtain rGO flakes. The as synthesized rGO flakes and CZTS nanoparticles were dispersed in N, N- and dimethyl formaldehyde (DMF) simultaneously and separately using ultra sonication and magnetic stirring process. The two stable dispersions were mixed and again sonicated for 2hours and centrifuged with an equal volume of ethanol and the mixed dispersion was dried at 80°C under vacuum to obtain the CZTS nanoparticle anchored rGO composite material. The as prepared rGO, CZTS and composite were characterized using X-ray diffraction (XRD) to ascertain the phases. Other characterization involves, Raman spectroscopy, SEM and optical absorption analysis by UV-Vis spectrometer. 3. Results and discussion Identification of phase and crystallinity are important factors that affect the electrical and optical properties of CZTS. The diffraction patters of the as synthesized and cured CZTS nanoparticles show peaks at 2θ = 28.48°, 32.75°, 47.51°, 56.30°, 69.23°, 76.31° corresponding (112), (200), (220), (312), (008) and (332) associated to singlephase kesterite of CZTS (JCPDS 26-0575) shown in Fig. 1(a). In composite XRD pattern the diffraction orientations of CZTS and rGO confirms the presence of both CZTS, rGO in composite (Fig. 1(b)). The diffraction peak of CZTS-rGO composite indicates the overlapping of the CZTS reflection over rGO broad peaks due to the presence of both CZTS and rGO. In composite diffraction pattern less intense rGO peaks compared to CZTS is observed due to exfoliated and reduced nature of rGO. It is also important to synthesize phase pure CZTS as the presence of secondary phases hampers the electrical properties of the solar cell absorber material. It is not possible to distinguish between the secondary phases (ZnS, SnS and Cu2S) and CZTS using XRD due to its sensitivity to secondary phases is limited [15]. Raman spectroscopy analysis helps in determining the secondary phases present in the CZTS prepared by sulphurizations. In the present case, CZTS has been prepared without sulphurizations, the presence of secondary phases if any has been analysed by Raman spectroscopy using light source of 514 nm Ar+ laser. Fig. 1(c)

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shows the Raman spectra of the CZTS nanoparticles which confirm the formation of phase pure CZTS as the peak at 330cm-1 corresponding to the characteristic A1 mode of CZTS kesterite phase with copper pure condition is more prominent and sharp. The raman spectra for CZTS nanoparticle anchored rGO composite material shows the D and G bands at 1347cm-1 and 1586cm-1 respectively corresponding to the presence of rGO in the composite samples (Fig. 1(c)). The D band corresponds to disordered structure of sp2 carbon whereas its E2g vibration mode is depicted by the G band. The comparable intensities of both D and G bands show the orderedness of the synthesized composite material by the solvothermal solution method.

(b)

(a)

(c)

Fig. 1. XRD pattern of (a) CZTS, (b) CZTS anchored rGO composite and (c) Raman spectra of CZTS, CZTS anchored rGO composite.

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(b)

Fig. 2. (a) SEM micrograph of CZTS nanoparticle and (b) CZTS anchored rGO composite material.

The SEM image (Fig. 2(a)) of the CZTS nanoparticles shows the formation of uniform spherical particles which form clusters due to its small sizes. Fig. 2(b) shows the SEM micrograph of CZTS anchored rGO composite. The morphology shows a uniform structure with CZTS particles covering the rGO sheets. The room temperature UVVisible absorption spectra for the CZTS nanoparticle and the composite shows higher absorbance for the CZTS anchored rGO composite as compared to CZTS shown in Fig. 3(a). The increased absorption cannot be attributed to the incorporation of rGO in CZTS only as this can result from the scattering from rGO layers to CZTS. The optical bandgap was calculated using tauc plot for the CZTS as well as the composite. The bandgap for the CZTS anchored rGO composite (1.58eV) is slightly higher than bare CZTS (1.52eV) which can be corroborated to the blue shift in the absorption spectra for the CZTS composite.

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(a)

(b)

(c)

Fig. 3. (a) Absorption spectra of CZTS nanoparticle CZTS anchored rGO composite and band gap calculation using Tauc plot for (b) CZTS and (c) CZTS anchored rGO composite

4. Conclusions Phase pure kesterite CZTS are prepared by cost effective solution method. The phase pure CZTS nanoparticles were prepared without sulphurizations using environment friendly materials. Moreover, a homogeneous CZTS nanoparticle anchored rGO composite with CZTS nanoparticles uniformly distributed on the rGO sheets where also synthesized using solution method which was confirmed using SEM. The bandgap for the CZTS anchored rGO composite is found to be 1.58eV which is in the optimal range for solar energy conversion. The absorbance of the composite are also found to increase with the incorporation of rGO. The obtained results depicts the potential application of the as prepared composite material as the absorber layer in CZTS based photovoltaic device. Acknowledgement The authors acknowledge financial support by DST/INSPIRE Fellowship/2016/IF160157.

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