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Functional Inorganic Materials and Devices
Post-deposition Processing of SnS Thin Films and Solar Cells: Prospective Strategy to Obtain Large, Sintered and Doped SnS Grains by Recrystallization in the Presence of a Metal Halide Flux Nicolae Spalatu, Jaan Hiie, Reelika Kaupmees, Olga Volobujeva, Juri Krustok, Ilona Oja Acik, and Malle Krunks ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03213 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 19, 2019
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ACS Applied Materials & Interfaces
Post-deposition Processing of SnS Thin Films and Solar Cells: Prospective Strategy to Obtain Large, Sintered and Doped SnS Grains by Recrystallization in the Presence of a Metal Halide Flux
Nicolae Spalatu,*,† Jaan Hiie,† Reelika Kaupmees,† Olga Volobujeva,† Juri Krustok,‡ Ilona Oja Acik,† Malle Krunks†
†
Department of Materials and Environmental Technology, Tallinn University of Technology,
Ehitajate tee 5, 19086 Tallinn, Estonia ‡
Division of Physics, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia
*Author to whom correspondence should be addressed:
[email protected] Keywords: SnS, thin film solar cells, annealing, melted phase recrystallization, close-spaced sublimation
Abstract Post-deposition treatments (PDTs) are common technological approaches to achieve high-efficiency chalcogenide solar cells. For SnS, a promising solar cell material, most PDT strategies to control the SnS properties are overwhelmingly based on an annealing in sulfur-containing ambient atmosphere that is described by condensed state reactions and vapor phase transport. In this work, a systematic study of the impact of PDTs in a N2 atmosphere, ampules at temperatures between 400 and 600 °C and a SnCl2 treatment at 250-500 °C on the properties of SnS films and SnS/CdS solar cells prepared by close-spaced sublimation is reported. The ampule and N2 annealing conditions do not affect the grain size of the SnS layers but significantly impact the concentration of intrinsic point defects, carrier density and mobility. Annealing at 500-600 °C strongly enhances the hole concentration and decreases the carrier mobility, having detrimental impacts on the device 1 ACS Paragon Plus Environment
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performance. SnCl2 treatment promotes grain growth, sintering and doping by mass transport through the melted phase, it adjusts the hole density and improves the carrier mobility in the SnS layers. SnS/CdS solar cells with an efficiency of 2.8% are achieved in the SnCl2 treatment step, opening new possibilities to further improve the performance of SnS-based devices. Introduction The demand for electricity production with negligible environmental impacts is increasing globally.1-3 Photovoltaics (PVs) are a nonpolluting and low-maintenance route for generating electricity from the limitless supply of sunlight. However, for PVs to have a considerable impact, lowering the cost of the production of solar cells is necessary. For decades, thin film solar cells (TFSCs) have been recognized as important sources of low-cost and environmentally friendly energy for the future.4 Inorganic TFCSs have achieved very high conversion efficiencies; copper– indium–gallium–selenide (CIGS) technology reached a record efficiency of 22.9% in 2017,5 while CdTe technology obtained a remarkable result of 22.1%6 in a small area. However, these technologies have some limitations due to the use of toxic or scarce elements as the absorber compounds and difficulties in scaling the automatic fabrication process. Considering these aspects, new absorbers composed of inexpensive, nontoxic, and earth-abundant elements have been investigated, and tin-based chalcogenides have been identified as promising candidates. Cu2(ZnSn)(SSe)4 (CZTSSe) is one of the most investigated chalcogenide materials, with a record efficiency of 12.6%.7 However, theoretical studies have revealed several critical issues with CZTS, most of which originate from the multivalence of Sn and the narrow homogeneity range of this multicomponent material.8 Orthorhombic tin monosulfide (SnS) has recently emerged as a promising green alternative to CdTe and CIGS solar cells due to its very attractive optoelectronic properties, such as a suitable bandgap of approximately 1.3 eV, a high absorption coefficient (105 cm−1)9,10 and an effective onset of optical absorption that coincides with the optimum band gap for maximum efficiency according to the Shockley−Queisser limit within the AM 1.5 illumination of the solar spectrum.11 In addition, 2 ACS Paragon Plus Environment
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ACS Applied Materials & Interfaces
because SnS is a binary compound with a high vapor pressure, a fast, low-temperature, vacuumbased deposition technique can be employed similar to those established in CdTe photovoltaics. Furthermore, the earth-abundant nontoxic constituents and the easy fabrication of these cells promise low-cost manufacturing. All these attributes suggest the great potential of SnS for highefficiency TFSCs and commercial applications. However, in practice, the highest certified power conversion efficiency of SnS devices is still only 4.36%,12 even though the first SnS PV device was reported in 1997 with a 0.5% efficiency.13 The current record efficiency has been achieved by following three important approaches: (i) growing p-type SnS by atomic layer deposition (ALD) and subsequent annealing of the absorber in a H2S ambient atmosphere, (ii) tuning the band offset and carrier concentration in the ALD-Zn(O,S):N buffer layer, and (iii) reducing the interface recombination by the formation of a thin layer (
