SnS-based thin film solar cells: perspectives over the last 25 years (original) (raw)
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A review of tin (II) monosulfide and its potential as a photovoltaic absorber
Solar Energy Materials and Solar Cells, 2016
Research groups around the world are investigating tin (II) monosulfide (SnS) via various deposition methods and heterostructures for thin film solar cells. The maximum achieved efficiency has yet to reach 5% despite the promising properties of SnS. SnS devices have achieved high short-circuit current densities near 20 A/cm 2 , but open-circuit voltage and fill factor are significantly lower than models predict. The multi-valency of tin, complex Sn-S phase diagram, and layered nature of SnS result in a complex system and large variability in the microstructure of the material. Microstructure growth is largely dependent on the deposition method, thus impacting optoelectronic properties. As a result, thin film SnS made by different processing methods cannot be compared without first considering the differences in microstructure. This review evaluates SnS, including theoretical and experimental work, and its progress as a photovoltaic absorber. Single crystal and thin film growth methods as well as the properties of the material are reviewed. Solar cell structure, back contact metals and doping are evaluated to summarize the progress in implementing SnS into devices. The challenge of sulfur volatility is a serious issue for producing high-quality SnS, and is not weighted enough in the literature. In addition, the impact of device processing following SnS deposition is not considered in any studies. The best future approach for SnS-based devices will consider sulfur content a priority, and investigate the impacts of device processing on the SnS layer.
2018
Tin monosulfide (SnS) has been investigated as a solar cell absorber due to its suitable bandgap and high absorption. The optoelectronic properties of SnS are largely dependent on its microstructure and deposition methods. First, we review the experimental and theoretical studies on SnS since the mid-20th century and evaluates the results with a focus on the deposition methods and microstructures; we also summarize the major challenges facing the SnS solar cells. To advance our understanding of the material fundamentals, we study the ALD growth behavior and mechanisms of SnS using a new and highly reactive liquid tin (II) precursor. Pure and stoichiometric SnS films can be obtained in the range of 65 – 180 °C. Mechanistic studies suggest higher probability of a double ligand-exchange process (“Sn-bridge” formation) in the Sn-precursor exposure step and dissociative or associative chemisorption of H2S in the following step. Solar cells fabricated using the SnS film deposited using th...
ACS applied materials & interfaces, 2016
As novel absorber materials are developed and screened for their photovoltaic (PV) properties, the challenge remains to reproducibly test promising candidates for high-performing PV devices. Many early-stage devices are prone to device shunting due to pinholes in the absorber layer, producing "false-negative" results. Here, we demonstrate a device engineering solution toward a robust device architecture, using a two-step absorber deposition approach. We use tin sulfide (SnS) as a test absorber material. The SnS bulk is processed at high temperature (400 °C) to stimulate grain growth, followed by a much thinner, low-temperature (200 °C) absorber deposition. At a lower process temperature, the thin absorber overlayer contains significantly smaller, densely packed grains, which are likely to provide a continuous coating and fill pinholes in the underlying absorber bulk. We compare this two-step approach to the more standard approach of using a semi-insulating buffer layer dir...
3.88% Efficient Tin Sulfide Solar Cells using Congruent Thermal Evaporation
Advanced Materials, 2014
Tin sulfide (SnS) is an attractive material for thin-film photovoltaics due to its favourable optical properties, its ease of deposition, and its inexpensive and non-toxic constituent elements. Recently, we demonstrated SnS solar cells fabricated by atomic layer deposition (ALD) that achieved a certified power conversion efficiency (PCE) record of 4.36%. Now we replace ALD with thermal evaporation, which allows growth rates more than 25 times faster and is well suited for high-throughput industrial fabrication. We confirm that SnS evaporates congruently, which provides facile composition control akin to cadmium telluride. We demonstrate SnS heterojunction solar cell device performance up to 3.88% (certified), and we present an empirical loss analysis to guide further performance improvements.
Antimony-Doped Tin(II) Sulfide Thin Films
Chemistry of Materials, 2012
Thin-film solar cells made from earth-abundant, inexpensive, and non-toxic materials are needed to replace the current technologies whose widespread use is limited by their use of scarce, costly, and toxic elements. 1 Tin monosulfide (SnS) is a promising candidate for making absorber layers in scalable, inexpensive, and non-toxic solar cells. SnS has always been observed to be a p-type semiconductor. Doping SnS to form an n-type semiconductor would permit the construction of solar cells with p-n homojunctions. This paper reports doping SnS films with antimony, a potential n-type dopant. Small amounts of antimony (~1%) were found to greatly increase the electrical resistance of the SnS. The resulting intrinsic SnS(Sb) films could be used for the insulating layer in a p-in design for solar cells. Higher concentrations (~5%) of antimony did not convert the SnS(Sb) to low-resistivity n-type conductivity, but instead the films retain such a high resistance that the conductivity type could not be determined. Extended X-ray absorption fine structure analysis reveals that the highly doped films contain precipitates of a secondary phase that has chemical bonds characteristic of metallic antimony, rather than the antimony-sulfur bonds found in films with lower concentrations of antimony.
Using vacuum process, we fabricated Cu 2 ZnSnS 4 solar cells with 8.4% efficiency, a number independently certified by an external, accredited laboratory. This is the highest efficiency reported for pure sulfide Cu 2 ZnSnS 4 prepared by any method. Consistent with literature, the optimal composition is Cu-poor and Zn-rich despite the precipitation of secondary phases (e.g., ZnS). Despite a very thin absorber thickness (~600 nm), a reasonably good short-circuit current was obtained. Time-resolved photoluminescence measurements suggest a minority carrier-diffusion length on the order of several hundreds of nanometers and relatively good collection of photo-carriers across the entire absorber thickness.
Thin films of tin sulphide for use in thin film solar cell devices
Thin Solid Films, 2009
SnS is of interest for use as an absorber layer and the wider energy bandgap phases e.g. SnS 2 , Sn 2 S 3 and Sn/S/ O alloys of interest as Cd-free buffer layers for use in thin film solar cells. In this work thin films of tin sulphide have been thermally evaporated onto soda-lime glass substrates with the aim of optimising the properties of the material for use in superstrate configuration device structures. The thin films were characterised using energy dispersive X-ray analysis (EDS) to determine the film composition, X-ray diffraction (XRD) to determine the phases present and structure of each phase, transmittance versus wavelength measurements to determine the energy bandgap and scanning electron microscopy (SEM) to observe the surface topology and topography. These properties were then correlated to the deposition parameters. Using the optimised conditions it is possible to produce thin films of tin sulphide that are pinhole free and conformal to the substrate that are suitable for use in thin film solar cell structures.
Thin film solar cells based on the ternary compound Cu2SnS3
Thin Solid Films, 2012
Alongside with Cu 2 ZnSnS 4 and SnS, the p-type semiconductor Cu 2 SnS 3 also consists of only Earth abundant and low-cost elements and shows comparable opto-electronic properties, with respect to Cu 2 ZnSnS 4 and SnS, making it a promising candidate for photovoltaic applications of the future. In this work, the ternary compound has been produced via the annealing of an electrodeposited precursor in a sulfur and tin sulfide environment. The obtained absorber layer has been structurally investigated by X-ray diffraction and results indicate the crystal structure to be monoclinic. Its optical properties have been measured via photoluminescence, where an asymmetric peak at 0.95 eV has been found. The evaluation of the photoluminescence spectrum indicates a band gap of 0.93 eV which agrees well with the results from the external quantum efficiency. Furthermore, this semiconductor layer has been processed into a photovoltaic device with a power conversion efficiency of 0.54%, a short circuit current of 17.1 mA/cm 2 , an open circuit voltage of 104 mV hampered by a small shunt resistance, a fill factor of 30.4%, and a maximal external quantum efficiency of just less than 60%. In addition, the potential of this Cu 2 SnS 3 absorber layer for photovoltaic applications is discussed.
Making Record-efficiency SnS Solar Cells by Thermal Evaporation and Atomic Layer Deposition
Journal of Visualized Experiments, 2015
Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent thermal evaporation, and it absorbs visible light strongly. However, for a long time the record power conversion efficiency of SnS solar cells remained below 2%. Recently we demonstrated new certified record efficiencies of 4.36% using SnS deposited by atomic layer deposition, and 3.88% using thermal evaporation. Here the fabrication procedure for these record solar cells is described, and the statistical distribution of the fabrication process is reported. The standard deviation of efficiency measured on a single substrate is typically over 0.5%. All steps including substrate selection and cleaning, Mo sputtering for the rear contact (cathode), SnS deposition, annealing, surface passivation, Zn(O,S) buffer layer selection and deposition, transparent conductor (anode) deposition, and metallization are described. On each substrate we fabricate 11 individual devices, each with active area 0.25 cm 2. Further, a system for high throughput measurements of current-voltage curves under simulated solar light, and external quantum efficiency measurement with variable light bias is described. With this system we are able to measure full data sets on all 11 devices in an automated manner and in minimal time. These results illustrate the value of studying large sample sets, rather than focusing narrowly on the highest performing devices. Large data sets help us to distinguish and remedy individual loss mechanisms affecting our devices.
Cubic-structured tin selenide thin film as a novel solar cell absorber
physica status solidi (a), 2016
Tin selenide thin film with a simple cubic crystalline structure (SnSe-CUB) of unit cell dimension a ¼ 11.9632 Åis obtained via chemical deposition on a tin sulfide (SnS-CUB) thin film base layer of simple cubic structure of a ¼ 11.5873 Å. The SnSe-CUB films obtained this way are thermally stable while heating to 300 8C. Its optical band gap is 1.4 eV. A thin film of 200 nm in thickness of this material in a solar cell may lead to a light generated current density of 23 mA cm À2 and a maximum of 29 mA cm À2. Thin film of SnSe-CUB possesses p-type electrical conductivity of 5 Â 10 À5 V À1 cm À1 , which is three orders of magnitude lower than that of SnSe films of orthorhombic crystalline structure. Overall, these characteristics make SnSe-CUB thin film a novel solar cell absorber material. ß 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2 Experimental 2.1 Thin film deposition Substrate layer thin film of SnS-CUB was deposited on Corning microscope glass