Full Length ArticleHigh SnS phase purity films produced by rapid thermal processing of RF-magnetron sputtered precursors
Introduction
In recent years, photovoltaic (PV) solar energy has established itself as a major player in the energy sector. PV technology has become increasingly competitive due to a consistent price reduction in the last decades. Although there have been some technological advances, this reduction in costs is mainly due to the increase of manufacturing scale factors and due to the global trend of concentration of companies, allowing the control of the whole value chain. The energy costs associated with producing high quality Si wafers has been an obstacle to a further reduction. Thin film technology is also maturing to competitive levels and is expected to contribute to further cost reductions. Several materials have been developed for the application to PV technology. The CuInGaSe2 (CIGS) and CdTe stand out from other competitors for the performance achieved by devices based on these compounds. Although the devices have reached interesting conversion efficiencies, these materials exhibit significant intrinsic problems. They are made up of rare elements such as In, Ga and Te, and a toxic element, Cd. Alternative compounds, such as, the kesterites ZnSn(S,Se)4, are now starting to present interesting conversion efficiencies for commercial applications. The main problem associated with the latter compounds is their narrow chemical potential stability region, making at difficult to grow films without secondary phases. This motivates the study of less complex compounds, such as tin monosulphide, SnS, for thin film solar cell (TFSC) applications due not only to a high optical absorption coefficient ( ) above the photon energy threshold of 1.3 eV [1], [2], [3], [4] but also to the non-toxicity and earth abundance. SnS shows an intrinsic p-type conductivity with majority carrier density ranging from to and mobilities above 100 /Vs [3], [4], [5]. A wide variety of synthesis methods have been reported for the growth of tin sulphide. High vacuum growth techniques, such as thermal evaporation [6], [7], [8], electron beam evaporation [9], [10], DC and RF-magnetron sputtering [11], [12] have been employed in the growth of SnS thin films. Atmospheric pressure processes such as spray pyrolysis [13], electrodeposition [14], [15], [16], chemical bath deposition (CBD) [17], [18], [19], solgel [20] are also reported in the literature. However, the highest reported photovoltaic conversion efficiency is 4.4%. The main reasons for the low efficiency is considered to be the poor quality of the SnS layer, such as, the occurrence of secondary phases (, and others), which have detrimental effects in solar cell devices [12] and a high grain boundary density. This implies that the thermodynamic growth conditions must be tightly controlled during this process. Nevertheless, recent works performed by Steinmann et al. [21] and Sinsermsuksakul et al. [22] show important improvements in the conversion efficiencies of TFSC based on tin monosulphide. In Table 1 the record efficiencies of CIGS, CdTe and CZTSSe TFSC are compared with the champion cells based on SnS for different growth methods.
The aim of the work reported here was to explore routes to enhance SnS phase purity of films produced by rapid thermal processing (RTP) of RF-magnetron sputtered precursors. Focusing on the effect of the annealing thermodynamics, we were able to establish a growth route yielding single phase SnS films free from the detrimental -Sn [12] and phases observed in previous works.
Section snippets
Sample preparation
In this work, the method used for the growth of tin sulphide thin films consisted on the annealing of RF-magnetron sputtered precursors. The precursor layers were deposited on bare soda lime glass (SLG) and on Mo coated SLG, cleaned in successive ultrasonic baths of acetone/ethanol/deionised water and finally dried with a gas jet. The deposition of Mo layer was performed by DC-magnetron sputtering from a Mo target with purity of 99.9%. The base pressure of the RF-sputtering
RTP annealing of samples on a graphite susceptor covered with a glass dome
A new approach to the annealing, which consists, in covering the graphite susceptor with a glass dome was employed, to confine the additional vapour to a smaller volume around the samples, thus reducing the evaporation losses. Several sets of annealing conditions were tested. The maximum temperature and heating rate were varied, from 450 to 570 C and from 0.2 to 8 C/s, respectively. Also, two different variants were studied. The first, consisted on the annealing without additional tin
Conclusions
The work reported above aimed at establishing a suitable route to prepare single phase SnS thin films deemed as essential for optimized photovoltaic applications. The growth of SnS thin films with good quality depends strongly on the annealing conditions. In order to minimize the material loss through evaporation, improve the films’ morphology and eliminate the residual -Sn and phase, we added a transparent glass dome over the sample to confine the vapour in a smaller volume over
Acknowledgements
The authors acknowledge the financial support with National Funds through FCT – Portuguese Foundation for Science and Technology, grant references UID/CTM/50025/2013, SFRH/BD/102807/2014 and FEDER funds through the COMPETE 2020 Programme under the project number POCI-01-0145-FEDER-007688 and by the project CENTRO-01-0145-FEDER-000005: SusPhotoSolutions: Soluçoˇes Fotovoltaicas Sustentáveis.
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