The synergistic influence of anionic bath immersion time on the photoelectrochemical performance of CZTS thin films prepared by a modified SILAR sequence

Abstract

A novel approach of successive ionic adsorption and reaction (SILAR) with a modified sequence is developed to synthesize Cu₂ZnSnS₄ (CZTS) films with outstanding photovoltaic characteristics. The influence of different anionic bath immersion times on the properties of CZTS films is investigated. The best PEC of 2.33% with a maximum J_sc of 12.88 mA cm⁻², V_oc of 0.42 V and FF of 0.43 is obtained for the lower value of anionic bath immersion time.

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Cu₂ZnSnS₄ (CZTS) is an emerging absorber that is structurally analogous to Cu(In,Ga)Se₂ (CIGS), but only contains earth abundant, non-toxic and low cost elements and has a near optical band gap of 1.4–1.5 eV and a high absorption coefficient of over 10⁵ cm⁻¹.¹ Various approaches have been reported to fabricate CZTS absorber layers, including vacuum-based and non-vacuum based deposition technologies.² The power conversion efficiency (PCE) of thin film solar cells (TFSCs) and the cost of the electricity are the two important parameters for developing the photovoltaic (PV) industry fields. Several attempts have, therefore, been focused on various methodologies that enable high PCE and low cost. While this CZTS PV field has been substantially advanced, the search for a particular methodology is still going on with incremental hopes. Most of the studies have been undertaken to override the challenges; (i) the problem of elemental losses as tin chalcogenides are volatile, (ii) complex reaction path during annealing in S or Se atmosphere at high temperature, (iii) existence of detrimental narrow band gap secondary phases, (iv) narrow range of compositional ratio required for high PCE of TFSCs devices. The reported PCE of the CZTS-based TFSCs fabricated by vacuum based sputtering technique has reached as high as 10.8%, which promotes further research on CZTS.³ On the other hand, to reduce manufacturing cost, several solution based approaches to fabricate CZTS absorber layer in solar cells were put forward.^4–6 The CZTSSe-based TFSC fabricated from the precursor containing hydrazine has exhibited the highest efficiency beyond 12%.⁴ However, specially designed equipment is required due to hydrazine toxicity, instability and easy explosivity. Another solution based approach based on nanocrystals without using toxic chemical has been employed and ∼8.5% conversion efficiency has been achieved.⁵ Yet many issues need to be resolved in this technique, such as carbon residue, phase segregation and unexpected intermediate products, weak adhesion between substrate and absorber layers, and so on.⁷ Among solution-based techniques, the successive ionic adsorption and reaction (SILAR) technique has extensively studied the synthesis of binary sulphide thin films because it has several merits such as simple, low cost.⁸ Recently, Mali and Shinde et al. reported the synthesis and characterization of CZTS thin films by SILAR technique.^9,10 Although they reported the initial stage researches for SILAR deposited CZTS thin films, the poor crystallinity, difficultly controlled compositional ratio, porous morphology, and poor deposition rate in the precursor issues were remained to realize the PCE in the CZTS solar cells.

Herein, we report a novel approach of SILAR with modified sequence to solve above-mentioned issues and to synthesize high quality Cu₂ZnSnS₄ (CZTS) films with PV outstanding characteristics. Previous SILAR processes reported the deposition of CZTS thin films using a single cationic bath, in which it is difficult to control the compositional ratio and suffered the poor deposition rate. In order to solve these issues, we modified SILAR sequence to well control the desire compositional ratio due to competitive adsorption between different metal ions (Cu⁺, Zn²⁺, Sn⁴⁺) in single cationic solution and to reduce growth rate in the Cu–Zn–Sn–S grain in the precursor thin films. Furthermore, effect of anionic bath immersion time on the properties of CZTS thin film and performance of solar cells were investigated. The fabricated PEC cells using these films exhibits the highest PCE of 1.87% for SILAR method so far.

The CZTS films were fabricated by sulfurization of CZTS precursor films under H₂S (5%) + N₂ (95%) atmosphere at 580 °C for 1 h. The CZTS precursor films were prepared on Mo coated glass substrate using SILAR technique with modified sequence for 80 cycles. The detailed experimental procedures are given in the ESI (see S1).†

Fig. 1(a) shows the XRD patterns for sulfurized CZTS thin films prepared with different anionic bath immersion time. The CZTS precursor thin films exhibit broad diffraction peaks, indicating that the precursor films composed of small size grains, which confirm the nanocrystalline nature (Fig. SI-1†). However, the crystal structure of CZTS precursor thin films cannot be distinguished by XRD patterns because the diffraction peaks are broad and the 2θ position of CZTS structure is quite similar to that of secondary phases such as ZnS and Cu₂SnS₃.¹¹ XRD patterns for all sulfurized films (Fig. 1(a)) exhibit three major diffraction peaks along (112), (220) and (312) planes located at 28.59°, 47.48° and 56.36°, which corresponds to kesterite CZTS (JCPDS no. 26-0575) and indicates polycrystalline nature. The full with half at maximum (FWHM) value of CZTS thin films decrease with decreasing anionic bath immersion time, indicating that the crystal quality of CZTS thin films are improved with decreasing anionic bath immersion time. (ESI Table SI-1†) In addition, the peak intensities of (112) plane for CZTS thin films are enhanced with decreasing anionic bath immersion time.

Fig. 1 XRD patterns (a) and Raman spectra for CZTS-5, CZTS-10 and CZTS-15 samples.

However, XRD alone is often cannot confirm the phase purity of CZTS thin films. In particular, the diffraction pattern is indistinguishable from that of several related sulphides such as CuS, SnS, and Cu₂SnS₃.^11,12 Raman spectroscopy is able to detect the possible secondary phases. Fig. 1(b) shows the Raman spectra for sulfurized CZTS thin films prepared with different anionic bath immersion time. For all the samples, the major peak appears at 338 cm⁻¹, which is consistent with the previously reported values.¹² In addition, the shoulder peaks at 292 and 371 cm⁻¹, which could be attributed to kesterite CZTS. No observation of any Raman peaks from Cu₂SnS₃, Cu_2−xS, and Sn_2−xS_x, indicating that the precursor thin films are completely synthesized as a pure kesterite CZTS after the sulfurization process.

Table SI-2† shows the compositional ratio of Cu/(Zn + Sn), Zn/Sn and S/(metal) for CZTS-5, CZTS-10 and CZTS-15 samples before and after sulfurization. The ratio of S/(metal) are 0.76, 0.88 and 0.98, respectively, while those after sulfurization are 1.07, 1.03 and 1.02.

For the CZTS-5 sample (Fig. 2(a)), the cross-section image clearly reveals the heterogeneous compounds with no clear indication of grains formation. The morphology is porous and unreacted agglomerated particles can be seen. This peculiar arrangement favours effective incorporation of ‘S’ during the sulfurization process to fabricate growth of compact CZTS with increased thickness and grain size. The cross-section images (Fig. 2(b)) further confirms this growth mechanism. In Fig. 2(e), for CZTS-15 sample, the layer and compact grains in the as-deposited sample are clearly discernible with less or no unreacted particulate matter. An intermediate growth of combination of grains and unreacted particulate matter can be easily identified from Fig. 2(b) for CZTS-10 sample. The incorporation of ‘S’ species during the sulfurization at high temperature becomes increasingly difficult for CZTS-10 and CZTS-15 samples due to aforementioned phenomena, that affects overall growth and film thickness of CZTS thin films The more detailed description of surface morphology are given in the ESI (Fig. SI-2 & 3).† The growth mechanism is graphically illustrated in the schematics (Fig. SI-4†).

Fig. 2 Enlarged view cross-sectional image of precursor (a, c and e) and sulfurized CZTS-5, CZTS-10 and CZTS-15 samples (b, d and f).

Fig. SI-5† shows plot of (αhν)² versus hν, illustrating the direct optical band gap values of 1.52, 1.44 and 1.37 eV for samples CZTS-5, CZTS-10 and CZTS-15, respectively.

The J–V characteristics for samples CZTS-5, CZTS-10 and CZTS-15 under dark and upon illumination are shown in Fig. 3(a) & (b), respectively. Fig. 3(a) shows the rectifying behaviour originated by formation of a junction between the CZTS photoelectrodes and electrolyte. Table SI-3† summarizes the short circuit current density (J_sc), open circuit voltage (V_oc), fill factor (FF) and conversion efficiency (η), ideality factor (A), series resistance (R_s) and shunt resistance (R_sh) for the CZTS solar cells. Interestingly, the device parameters such as η, J_sc, V_oc, and FF are higher as the anionic bath immersion time decreases. The significant enhancement in the J_sc from 6.49 to 12.88 mA cm⁻² and V_oc from 0.38 to 0.42 V is observed with decreasing in anionic bath dipping time from 15 to 5 s upon illumination as shown in Fig. 3(b). The augmentation of these solar cell parameters could be attributed to the improved the morphology of CZTS thin films, which subsequently affect; (i) quality of CZTS-electrolyte junction and (ii) dynamics of charge carrier recombination via surface states or bulk defects. The best CZTS PEC device exhibited the highest J_sc of 12.88 mA cm⁻², V_oc of 0.42 V, FF of 0.43 and η of 2.33% for lower anionic bath immersion time of 5 s. For the lower anionic bath immersion time, the increase in the grain size with compact morphology has enabled the collection of a higher fraction of incident photons, increasing useful current generation and thereby the conversion efficiency.

Fig. 3 J–V characteristics of CZTS-5, CZTS-10 and CZTS-15 samples under dark (a) and upon illumination (b).

Table SI-3† shows the ideal factor values for sulfurized CZTS-5, CZTS-10 and CZTS-15 samples. Samples CZTS-10 and CZTS-15 have the value of A larger than 2. In PEC devices based on these photoelectrodes, suggest relatively larger recombination for trapped electrons and holes. The value of A for CZTS-5 sample is only 1.95, suggesting the recombination is relatively lower. Fig. SI-6† shows a semi-logarithmic plot of J against V. The values of J_o are calculated by extrapolating the plot to y-axis and listed in the Table SI-3.† The values of J_o for CZTS-5, CZTS-10 and CZTS-15 are 1.29 × 10⁻⁵, 2.21 × 10⁻⁵ and 7.95 × 10⁻⁵ mA cm⁻², respectively. Overall J_o value decreases due to the decreased defect density and less carrier recombination by improving the surface morphology by adjust varying anionic bath immersion time. To exploit full potential of SILAR to obtain high efficiency CZTS, several investigations like (i) variation of ‘Cu’ content to yield Cu-poor (ii) variation of ‘Zn’ content to yield Zn-rich compounds, are underway.

Conclusions

SILAR technique with modified cycle sequencing has been developed to synthesize good PV quality CZTS thin films. The controlled tuning of the surface morphology of CZTS thin films has been investigated by simply varying the anionic bath immersion times that has profound effect on CZTS morphology. The enhancement in the conversion efficiencies from 0.96 to 2.33% has been achieved. The efficiency of 2.33% with J_sc of 12.88 mA cm⁻², V_oc of 0.42 V and FF of 0.43 has been achieved for CZTS sample with lower anionic bath immersion time, which is the highest efficiency for SILAR deposited CZTS films.

Acknowledgements

This work is supported by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy (no.: 20124010203180) and supported partially by the University Grant Commission (UGC), New Delhi through major research project F. no. 41-945/2012 (SR).

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedure, XRD pattern for CZTS precursor, elemental composition, FE-SEM results (top and cross-sectional view), optical absorption, solar cell parameters. See DOI: 10.1039/c4ra01208a

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