Exploring the microstructural, optoelectronic properties of deposition time dependent Cu2Sn(S,Se)3 thin film synthesized by non-vacuum arrested precipitation technique

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Highlights

  • Deposition time dependent Cu2Sn(S,Se)3 thin film synthesized by arrested precipitation technique.

  • Decrease in band gap energy from 1.84 to 1.62 eV was noticed with increase in deposition time.

  • progress in microstructure from bunch of nanospheres to aggregated nanospheres was observed.

  • A photoelectrochemical efficiency of 0.3 % was calculated for Cu2Sn(S,Se)3 thin film without post deposition treatments.

Abstract

A novel chemical approach is utilized to synthesize Cu2Sn(S,Se)3 (CTSSe) thin films by non-vacuum arrested precipitation technique (APT) via controlled release of ions from complexed solution. Effect of deposition time on the microstructural and optoelectronic properties of CTSSe thin films was studied comprehensively. The optical absorption studies demonstrate a decrease in band gap energy from 1.84 to 1.62 eV as the deposition time increases. Nanocrystalline thin films with improved grain size were illustrated from XRD studies. The FESEM micrographs depict progress in surface microstructure from a bunch of nanospheres to aggregated and densely packed nanospheres. EDS spectrum confirms the stoichiometric formation of Cu2Sn(S,Se)3 thin film. While, the XPS spectrum validates Cu1+, Sn4+, S2-, Se2-valence states of the elements in CTSSe thin film. Furthermore, the photoelectrochemical cell performance of 0.3 % for bare CTSSe thin film was elucidated with cell configuration glass-FTO/CTSSe/(I-/I3-)/graphite without any post deposition treatments. Also, the lowering of charge transfer resistance from EIS measurements corroborates for improved photoconversion efficiency. Finally, synthesis of CTSSe thin films by a simple APT finds a promising approach to fabricate efficient CTSSe photocathode for photovoltaic applications.

Introduction

Presently, clean energy production for commercial as well as domestic operation is a principal need to lessen the current conservational problems. The hunt for novel and economical material for energy conversion applications has been developing by leaps and bounds [1]. Presently, CuInGaSe2 (CIGS) and CdTe based TFSC's regardless of their conversion efficiencies greater than 22 %, experience scarcity of In, toxic nature of Cd and higher elemental cost of Ga and In [2,3]. Due to these reasons, a novel Cu-based quaternary CTSSe semiconductor can offer substitute for CdTe and CIGS compounds. Also, the combined benefit of S and Se in CTSSe in relation to band gap tunability becomes considerably attractive for reaching higher photovoltaic performance than other Cu-based compounds [4,5]. CTSSe is a combination of two semiconducting materials Cu2SnS3 (CTS) and Cu2SnSe3 (CTSe) belonging to I-IV-VI group chalcogenide compounds. The inherent tunable properties complimented with easy and reduced cost processing techniques make CTS an ecofriendly material that opens up new possibilities in industrial applications [6].

From past few years CTS compound has been comprehensively investigated for multifunctional applications such as; photovoltaic [7], photocatalysis [8], LPG gas sensing [9], electro-water splitting [10], Li-ion batteries [11], biomedical application [12], etc. Moreover, another I-IV-VI ternary chalcogenide compound CTSe has been investigated because of its inherent optostructural properties and being used widely in thin film solar cells (TFSC's). Further, in Cu–Sn–S system, the material phases such as Cu2SnS3, Cu2SnS4, Cu3SnS4, Cu4SnS4 has been studied extensively. While, Cu2SnSe3 and Cu2SnSe4 compounds have been broadly studied in nanocrystalline form in Cu–Sn–Se system [6]. Among these phases, Cu2SnSe3 and Cu2SnS3 are the most significant stoichiometric compounds widely used in photovoltaic application. Moreover, CTS and CTSe ternary thin films demonstrates an excellent physicochemical and optostructural properties such as; wide optical band gap (0.8–1.77 eV), p-type semiconductivity, high electron-hole mobility, elevated optical absorption coefficient (>105 cm−1) [3,7,13,14]. Thus, CTSSe quaternary mixed metal chalcogenide (MMC) thin films obtained by alloying CTS and CTSe ternary compounds may boost the properties of the parent material and can disclose unique properties.

In an effort to produce CTSSe thin films few synthesis techniques are reported such as; ball milling [15], colloidal heat up [16,17], sputtering [5,18], hydrothermal [19], dip coating followed by annealing [13]. But most of the mentioned techniques require toxic organic solvents, harsh synthesis environments, synthesis at elevated temperature, sulfurization/selenization in toxic H2S medium, etc. There are only countable reports available on the synthesis of CTSSe thin films using low cost chemical synthesis techniques. Based on above discussion, in this study we have introduced a facile chemical synthesis technique i.e. an arrested precipitation technique (APT) to deposit CTSSe thin film using a tetradentate tartaric acid as a complexing agent. APT is a non-vacuum, low temperature, cost effective chemical synthesis technique which works on the principle of Ostwald's ripening process [20]. APT is promising thin film deposition technique to produce nanostructured CTSSe thin films. At the same time it offers desirable properties of material by tuning the basic preparative parameters [21]. Due to its versatile advantage APT is beneficial over other deposition techniques. The basic mechanism of crystal development in APT relies on the slow release of reacting metal and chalcogen ions from the complexed solution in reaction bath [22]. In the present study we have made an attempt to synthesize CTSSe thin film at room temperature using novel APT. The role of deposition time variation on the optostructural and optoelectronic properties of the synthesized CTSSe thin films is studied systematically for the first time. Further, the photoelectrochemical (PEC) cell performance of deposition time dependent CTSSe thin film was elucidated using two electrode cell configuration as glass-FTO/CTSSe/(I/I3)/graphite. Attaining the desired properties of the Cu-based chalcogenide thin film is the most challenging task due to differences in the reactivity of the reacting ions [23]. Thus, synthesis of CTSSe thin films by APT finds a favorable technique for the fabrication of photoactive CTSSe thin film for PEC application.

Section snippets

Materials and methods

The chemicals procured for the synthesis of time dependent CTSSe thin films were analytical grade and utilized as received. Copper chloride dihydrate (CuCl2·2H2O) [Sigma Aldrich, 99 %] and stannic chloride pentahydrate (SnCl4·5H2O) [Thomas Baker, 98 %] were used as precursors for Cu2+ and Sn4+ ions. Sodium selenosulphite (Na2SeSO3) was used as precursor for Se2− chalcogen ions which was obtained from refluxed solution of Na2SO3 [S D Fine Chem., 99 %] and Se metal powder for 8 h at 90°C as

CTSSe thin film deposition and characterization

To deposit time dependent CTSSe thin films by APT primarily, the Cu2+ and Sn4+ ions were complexed with tartaric acid in order to ensure controlled release of ions. Particularly a complexed solution of 10 ml of 0.1 M Cu(C4H4O6) and 10 ml of 0.1 M Sn(C4H4O6)2 was taken in a bath. With the help of aqueous ammonia the solution pH was maintained to 8±0.2. The disappearance of turbidity and change in color of the solution from light green to dark blue was noticed at this stage. To this pH stabilized

CTSSe thin film growth and reaction mechanism

From reaction kinetics point of view the growth and reaction mechanism of time dependent CTSSe thin films synthesized by APT is discussed here. Crystal growth, nucleation, microstructural properties of the material are some of the factors on which consistent thin film deposition depends. The various deposition conditions such as, deposition temperature, concentration of precursors, rate of substrate rotation, pH of the solution was finalized at the early phase of the reaction and is summarized

Thickness measurement study

Thickness of a thin film has a crucial role in determining the light absorption properties for photovoltaic application [29]. After each interval of deposition time the thickness of the deposited CTSSe thin film was measured using surface profilometer. Variation of film thickness with deposition time is represented in Fig. 2. It was noticed from Fig. 2 that the film thickness is in the range of 460–550 nm. The film thickness was increased up till a certain thickness called the ‘terminal

Conclusions

In the present study, a deposition time dependent Cu2Sn(S,Se)3 (CTSSe) thin films were successfully synthesized by APT. Variation of optoelectronic, microstructural and photoelectrochmical (PEC) properties of CTSSe thin films by varying deposition time was studied thoroughly. A red shift in optical absorption edge with reduction in band gap energy was observed. The crystallinity was improved along with crystallite size as the deposition time increased from 0.5 to 2.5 h. FESEM microstructures

Credit author statement

Monika P. Joshi, Data curation, Validation, Formal analysis, Writing – original draft. Popatrao N. Bhosale, Conceptualization, Methodology, Supervision, Resources, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

One of the authors, M. P. Joshi is thankful to Shivaji University, Kolhapur, India for assistance through Golden Jubilee Research Fellowship (GJRF). Also, the author is thankful to Dr. S. S. Mali and Prof. Chang Kook Hong, Polymer Energy Materials Laboratory, School of Applied Chemical Engineering, Chonnam National University, Gwangju, South Korea for providing XPS data.

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