The effect of in-situ and post deposition annealing towards the structural optimization studies of RF sputtered SnS and Sn2S3 thin films for solar cell application
Introduction
Recent semiconductor investigations highly focused on developing eco-friendly absorber layer materials for the application of thin film solar cells (TFSCs) with reduced cost and improved efficiency. TFSCs mainly includes CIGS, CZTS and CdTe where the availability (In & Ga) and toxicity (Se & Te) of the constituent elements are the concerning factors in further development (Jackson et al., 2011, Ahmed et al., 2012, Ananthoju et al., 2019, Hossain et al., 2019). And in the case of CZTS, less toxic environmentally abundant constituent elements make it a suitable alternative, however the multi-valence state of Sn and the narrow chemical potential stability of CZTS easily leads to the formation of several binary and ternary secondary phases (Scragg et al., 2012, Nagoya and Asahi, 2010). Also its considerable open circuit voltage (Voc) deficiency which arise due to the cation disordering and interface recombination is an important factor which motivates the technologist to investigate on less complex binary compounds (Park et al., 2018). In this perspective, as a group IV-VI binary semiconductor with p type conductivity, direct energy band gap value of 1.38 eV, very high optical absorption coefficient (α > 105 cm−1), less toxic and environmentally abundant constituent elements makes SnS a highly potential alternative for the existing absorber layer materials in TFSCs (Baby and Mohan, 2017a).
Even though SnS is exhibiting promising characteristics towards the application of TFSCs, the highest efficiency achieved so far is 4.4%, which is far below the predicted theoretical efficiency of 24% (Sinsermsuksakul et al., 2014, Loferski, 1956). This low efficiency is (a) due to the presence of secondary phases such as SnS2 and Sn2S3, (b) high electrical resistivity and low carrier concentration of SnS and (c) due to the band alignment mismatch between SnS and buffer layer interface (Baby and Mohan, 2018, Burton et al., 2013, Skelton et al., 2017, Sun et al., 2013). Band alignment studies of different tin sulphide phases reveals that SnS/Sn2S3 interface would form p-n junction with type II nature with a small positive conduction band offset (CBO) of 0.09 eV which is desirable to reduce the interface recombination without any loss and for SnS/SnS2 p-n junction interface has a large negative CBO of −0.51 eV and it could act as a recombination centre (Park et al., 2015, Whittles et al., 2016). Hence the observed low CBO and also due to the presence of common constituents at the interface, the formation of SnS/Sn2S3 p-n hetrojunction can be a suitable choice for photovoltaics application. But the chemical stability of Sn2S3 phase is a huge concern (Whittles et al., 2016).
Hartman et al., 2011, Ikuno et al., 2013 reported the preparation of SnS thin films using single target RF magnetron sputtering and the phase formation is studied using XRD analysis. However, XRD cannot confirm the complete formation of single phase SnS as it is difficult to investigate any minor crystalline phase formed with less than ~5%. Banai et al. (2014) reported the presence of multiple phases of tin sulphide using Rietveld analysis for as deposited films and also for the films with in-situ substrate temperature (ISST) between 100 and 300 °C. Arepalli et al (2018) prepared as deposited and ISST at 300 °C Sn-S films with varying the working pressure and SnS phase formation is studied from Raman analysis in the range of 75–600 cm1 with an excitation wavelength of 532 nm. Whereas it is difficult to excite Raman modes of Sn2S3 with excitation wavelengths at 514 and 532 nm, in films having less percentage of Sn2S3 phase (Baby and Mohan, 2017a, Baby and Mohan, 2018). Baby and Mohan (2018) also reported the presence of Sn2S3 secondary phase from XPS analysis for films which showed a less intense Raman peak at ~63 cm−1. Arepalli et al. (2019) reported the effect of substrate temperature in the sputtered SnS films by varying the ISST from as deposited to 420 °C and the phase formation studies through Raman analysis revealed the presence of Sn2S3 phase (63 cm−1). Banai et al., 2016a, Patel et al., 2018 optimized the single phase formation of SnS films using RF sputtering of SnS2 target followed by the post deposition annealing (PDA) at 400 °C. Zhao et al (2016) reported in situ growth of SnS by reactive sputtering of Sn target and optimized the single phase formation of SnS with a reaction temperature of 400 °C. Sousa and Cunha (2019) reported SnS single phase formation by rapid thermal processing of RF sputtered SnS2-x precursors.
Towards the preparation of Sn2S3 thin films, Burton et al. (2013) prepared single crystal Sn2S3 through chemical vapour transport method. Avellaneda et al. (2019) prepared SnS films using chemical bath deposition method followed by annealing in S + N2 atmosphere at 400 and 450 °C for 1 hr which resulted in the formation of single phase of SnS2 and Sn2S3 respectively as evident from Raman and XPS analyses. Price et al. (1999) prepared films with central region having Sn2S3 phase while edges traces the presence of SnS2 phase using atmospheric pressure chemical vapour deposition method with a deposition temperature of 525 °C. Guneri et al. (2012) reported single phase Sn2S3 thin films having a band gap value of 2 eV using chemical bath deposition method with a deposition time of 24 hr. By using physical vapour deposition methods, Reddy et al. (2017) prepared Sn2S3 films using co-evaporation technique by varying the Sn/S ratio and Raman analysis confirmed the formation of Sn2S3 phase with a minor presence of SnS phase (220 cm−1).
As per the literatures survey made, 1. The preparation of single phase SnS thin films without having any trace belongs to the secondary phases such as SnS2 and Sn2S3 by employing single SnS target based RF magnetron sputtering was not investigated so far 2. The preparation of single phase Sn2S3 thin films using any physical vapour deposition methods was not reported so far, even though it is a promising n-type semiconductor from SnS family.
Hence, one of the primary criteria to enhance the efficiency of SnS based thin film solar cells is to maintain the phase purity of both p and n type characters. This work is aimed at the following, 1. To optimize the various experimental factors such as RF power, ISST and PDA towards the formation of single phase of SnS by RF magnetron sputtering using SnS as the source target; 2. To investigate the phase stability of SnS structure with the better optimization conditions by varying method of heat treatment as ISST and PDA; 3. To explore the phase transition from p-type SnS to n-type Sn2S3 and also to tailor the experimental factors for achieving n-type Sn2S3 with better phase stability; 4. To examine SnS and Sn2S3 films using XRD, Raman, EDAX, XPS and UV–Vis-NIR Spectrophotometer. 5. Finally, the electrical properties of SnS and Sn2S3 films using Hall measurement are studied.
Section snippets
Materials and methods
SnS and Sn2S3 thin films were prepared from SnS (purity (99.99%) target using RF magnetron sputtering by varying RF power from 30 to 60 W and ISST up to 500 °C. Initially the sputtering chamber was evacuated to a high vacuum of 4 × 10−6 mbar using turbo molecular pump which was backed by rotary pump. The working distance i.e. the distance between the substrate and the target was kept as 8 cm. The working pressure during the deposition was maintained as 8.5 × 10−3 mbar and the deposition time
Characterization techniques
The chemical structure of the prepared films was studied using confocal Raman spectrometer (Reinshawin Via Raman Microscope, UK) with an excitation wavelength of 785 nm using semiconductor laser in the scan range of 50–350 cm−1 with acquisition time and power as 150 s and 0.0025 mW respectively. The phase purity of the prepared films was further analysed from the oxidation states of Sn and S and the chemical bonding of SnS using X-ray photoelectron spectroscopy (XPS) (PHI 5000 Versa Probe II,
Chemical structure analysis
The chemical stability of SnS and its phase transition to Sn2S3 is studied by varying the RF power, ISST and PDA through micro Raman analysis as shown in Fig. 1. Fig. 1(a) shows the Raman spectra of films sputtered by varying the RF power from 30 to 60 W with fixed ISST as 400 °C. The formation of Sn2S3 (66, 159, 177 and 203 cm−1) along with SnS (68, 96, 164, 192, 222 and 286 cm−1) is observed with RF power of 30 W (Baby and Mohan, 2017a, Baby and Mohan, 2017b, Baby and Mohan, 2018,
Conclusion
Phase optimization towards the formation of single phase SnS and Sn2S3 thin films is successfully studied from a single SnS target RF magnetron sputtering by varying RF power, ISST and PDA. Raman, XPS and XRD analyses confirmed the formation of single phase of SnS films with RF power of 40 W and ISST of 400 °C only, whereas mixed phases of SnS and Sn2S3 formed in all other cases. The phase transition from SnS to Sn2S3 is observed with either ISST or PDA at 500 °C. The crystallite size
Acknowledgements
The authors acknowledge SERB-DST, India (SR/FTP/PS-131/2012 & EEQ/2016/000228) for the financial support. Authors thank ACMS-IIT Kanpur for providing XPS facility. Authors are grateful to Dr. D. Selvakumar, Scientist-F, DEBEL (DRDO), Bangalore for SEM and EDAX analysis. Authors thank CeNSE –IISC, Bangalore for providing Fe-SEM, GA-XRD and Hall Measurement facilities. BHB sincerely thanks UGC-BSR for providing research fellowship.
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