Earth-abundant Thin-film Solar Cells Research Articles

Evolution of large-grained CuSbS2 thin films by rapid sulfurization of evaporated Cu–Sb precursor stacks for photovoltaics application

Ternary chalcostibite (CuSbS2) is a non-toxic and earth-abundant thin-film solar cell material with an optimal band gap, high optical absorption coefficient, and p-type electrical conductivity. Large-grained CuSbS2 thin films are crystallized by sulfurizing thermally evaporated Cu/Sb/Cu precursor stacks at 450 °C for a short duration of 1–10 min. The precursor stacks sulfurized at 450 °C for 1 min resulted in the formation of a Cu-deficient (to a small extent) and Sb-rich large-grained thin-films of CuSbS2 without any secondary phases. Increasing the sulfurization duration from 5 min to 10 min resulted in the formation of near-stoichiometric CuSbS2 films with columnar growth and more uniformly sized grains. The direct band gap of the films is found to be 1.42 eV. The electrical resistivity decreases from 28.6 to 16.7 Ω cm upon increasing the sulfurization duration from 1 to 10 min, respectively, and the hole mobility is in the range of 0.44–0.56 cm2 V−1 s−1. The solar cells fabricated using the CuSbS2 films sulfurized for a duration of 5 min exhibited a maximum power conversion efficiency of 2.5% with an open-circuit voltage of 568.7 mV and a short-circuit current density of 13.8 mA/cm2. This study provides a pathway for the development of high-efficiency CuSbS2 solar cells.

Role of Intrinsic Defects in Enhancing the Photoabsorption Capability of CuZn2AlSe4.

As a promising candidate for low-cost and eco-friendly thin-film photovoltaics, the emerging quaternary chalcogenide based solar cells have experienced rapid advances over the past decade. Here, we propose quaternary semiconducting chalcogenides CuZn2AlSe4 (CZASe) through cross-substitutions (cation mutations). The nonexistence of imaginary modes in the entire Brillouin zone of CZASe represents the inherent dynamic stability of the system. The electronic, optical, and defect properties of stannite CZASe quaternary semiconducting material was systematically investigated using density functional theory calculations. We have found that the chemical-potential control is very important for growing good-quality crystals and also to avoid secondary-phase formations such as ZnSe, Al2ZnSe4, and Cu3Se2. The observed p-type conductivity is mainly due to antisite defect CuZn, which has the lowest formation energy with a relatively deeper acceptor level than that of the Cu vacant site (VCu). The electronic band structures of vacancies and antisite defects by means of hybrid functional calculations show energy band shifting and energy band narrowing or broadening, which eventually tunes the optical band gap and improves the solar energy-conversion performance of semiconducting CZASe. Our results suggest that the stannite CZASe quaternary chalcogenides could be promising candidates for the efficient earth-abundant thin-film solar cells.

Inter-relationship of the Structural Properties of Quaternary Chalcogenides CuZn2Ga(S/Se)4: A First-Principles Study

Quaternary chalcogenides based on stannite I-II $$_2$$ -III-VI $$_4$$ structure are the best potential candidates to overcome the current generation of solar harvesting materials. First-principles electronic structure simulations were performed on semiconducting CuZn $$_2$$ Ga(S/Se) $$_4$$ (CZGS/Se) to understand the inter-relationship of the structural properties. These structures contain a cubic close packing (ccp) array of S/Se-centered tetrahedrons, coordinated by one Cu, two Zn and one Ga atom occupying one half of the ccp tetrahedral voids. The remarkable variations in the crystal structures are explained by the influence of ionic radii of various atoms. The electronic and optical properties calculated using hybrid functional (Heyd, Scuseria and Ernzerhof, HSE06) show a suitable band gap of 1.59 eV for CZGSe with high optical absorption. The current density and maximum upper limit of theoretical energy conversion efficiency (P(%)) of CZGSe is enhanced when compared to that of CZGS. Our results suggest that the stannite CZGSe structure could be a promising candidate for efficient earth-abundant thin-film solar cell applications.

Reactive deposition growth of highly (001)-oriented BaSi2 films by close-spaced evaporation

Barium disilicide (BaSi2) is an emerging light-absorbing material for earth-abundant thin-film solar cells. Here, we report scalable and rapid deposition of BaSi2 films by close-spaced evaporation. In this method, BaAl4–Ni evaporation source generates Ba gas, which is deposited on a closely-placed Si(001) substrate. The reaction between deposited Ba atoms and Si substrates yields compositionally homogeneous BaSi2 films through the depth, as evidenced by X-ray diffraction and Auger electron spectroscopy. Although the present BaSi2 films suffer from cracking and exfoliation issues caused by thermal strain, exfoliation is successfully suppressed by lowering the substrate temperature using a reflector. Electron backscatter diffraction and X-ray diffraction analyses show that the dominant orientation of the BaSi2 films is (001). A considerably large single-orientation area more than 68μm was observed, which is the largest among previously reported BaSi2 films.

New Earth-Abundant Thin Film Solar Cells Based on Chalcogenides.

At the end of 2017 roughly 1.8% of the worldwide electricity came from solar photovoltaics (PV), which is foreseen to have a key role in all major future energy scenarios with an installed capacity around 5 TW by 2050. Despite silicon solar cells currently rule the PV market, the extremely more versatile thin film-based devices (mainly Cu(In,Ga)Se2 and CdTe ones) have almost matched them in performance and present room for improvement. The low availability of some elements in the present commercially available PV technologies and the recent strong fall of silicon module price below 1 $/Wp focused the attention of the scientific community on cheap earth-abundant materials. In this framework, thin film solar cells based on Cu2ZnSnS4 (CZTS) and the related sulfur selenium alloy Cu2ZnSn(S,Se)4 (CZTSSe) were strongly investigated in the last 10 years. More recently, chalcogenide PV absorbers potentially able to face TW range applications better than CZTS and CZTSSe due to the higher abundance of their constituting elements are getting considerable attention. They are based on both MY2 (where M = Fe, Cu, Sn and Y = S and/or Se) and Cu2XSnY4 (where X = Fe, Mn, Ni, Ba, Co, Cd and Y = S and/or Se) chalcogenides. In this work, an extensive review of emerging earth-abundant thin film solar cells based on both MY2 and Cu2XSnY4 species is given, along with some considerations on the abundance and annual production of their constituting elements.

Fabrication of CZTSSe absorbers by optimized selenization of one-step co-electrodeposited CZTS precursors

Cu2ZnSn(S,Se)4 (CZTSSe) absorber layers are fabricated by co-electrodeposited Cu–Zn–Sn–S (CZTS) precursors followed by selenization. In order to complex the Cu ions and shift its reduction potential to more negative, trisodium citrate is applied as the complexing agent during the co-electrodeposition. After the co-electrodeposition, CZTS precursors are preliminary annealed at 280 °C in order to improve the homogeneity by intermixing of the elements. The influences of selenization temperatures on the chemical compositions, crystal phases, optical properties and photovoltaic device performances are also systematically investigated by X-ray energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), Raman spectroscopy, UV–vis absorption spectroscopy, and J–V measurements. EDS results reveal that the CZTSSe absorber layers are all in a Cu-poor and Zn-rich nature, which is beneficial for fabricating cells with high efficiency. It is found that the crystalline quality and morphology of the CZTSSe absorber can be greatly improved by post-selenization at 550 °C. Photovoltaic devices are fabricated with standard soda-lime glass (SLG)/Mo/CZTSSe/CdS/i-ZnO/ITO/Ag grid structures. Champion cell demonstrates 2.81% efficiency based on the optimized CZTSSe absorber. These results validate co-electrodeposition as a promising strategy for developing earth-abundant thin film solar cells at low-cost.

Effects of deposition rate on the structure and electron density of evaporated BaSi2 films

In order to control the electrical properties of an evaporated BaSi2 film, which is an emerging candidate for the absorber-layer material of earth-abundant thin-film solar cells, we have investigated the effects of deposition rate on the produced phases, microstructure, and carrier density of the thin films grown by thermal evaporation of BaSi2. X-ray diffraction results show that a high substrate temperature is necessary for BaSi2 formation at a high deposition rate, which is discussed from viewpoints of vapor composition and diffusion time. Microstructural characteristics such as grain size of 30–120 nm, oxide particle arrays present around the interface, and partial oxidation at a low substrate temperature are revealed by cross-sectional transmission electron microscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy combined with an energy-dispersive X-ray spectroscopy. With increasing deposition rate, the crystalline quality of BaSi2 is found to improve, as evidenced by a decrease in full-width at half maximum of a [Si4]4− vibration band in Raman spectra. At the same time, electron density, which is determined by Hall measurement, decreases with deposition rate. The variation of electron density is discussed on the basis of microstructural characteristics and BaSi2 formation mechanism. The most probable reason is concluded to be composition deviation from stoichiometry.

Enhanced thermoelectric properties of earth-abundant Cu2SnS3 via In doping effect

Ternary copper tin sulfide (Cu–Sn–S) compounds have been investigated for decades as promising candidates of earth-abundant thin film solar cell absorbers. Cu2SnS3 is a representative member of this material system, and is highly expected to be a potential thermoelectric material by the first-principles calculations. In this work, Cu2SnS3 bulk materials were synthesized by the mechanical alloying (MA) combined with spark plasma sintering (SPS), and their crystal structures, electronic transport properties and enhanced thermoelectric performance were systematically investigated. We found that pristine Cu2SnS3 is a p-type thermoelectric material with a relatively high thermopower up to 300 μV/K and low thermal conductivity below 1.0 W/m K above 700 K. It revealed that In doping is an effective means to improve the electrical transport properties, which leads to a figure of merit (ZT) of Cu2Sn0.9In0.1S3 close to 0.6 at the relatively low temperature (773 K).

Iron Pyrite Thin Films Synthesized from an Fe(acac)3 Ink

Iron pyrite (cubic FeS2) is a promising candidate absorber material for earth-abundant thin-film solar cells. Here, we report on phase-pure, large-grain, and uniform polycrystalline pyrite films that are fabricated by solution-phase deposition of an iron(III) acetylacetonate molecular ink followed by sequential annealing in air, H2S, and sulfur gas at temperatures up to 550 °C. Phase and elemental compositions of the films are characterized by conventional and synchrotron X-ray diffraction, Raman spectroscopy, Auger electron spectroscopy, secondary ion mass spectrometry, and X-ray photoelectron spectroscopy (XPS). These solution-deposited films have more oxygen and alkalis, less carbon and hydrogen, and smaller optical band gaps (E(g) = 0.87 ± 0.05 eV) than similar films made by chemical vapor deposition. XPS is used to assess the chemical composition of the film surface before and after exposure to air and immersion in water to remove surface contaminants. Optical measurements of films rich in marcasite (orthorhombic FeS2) show that marcasite has a band gap at least as large as pyrite and that the two polymorphs share similar absorptivity spectra, in excellent agreement with density functional theory models. Regardless of the marcasite and elemental impurity contents, all films show p-type, weakly activated transport with curved Arrhenius plots, a room-temperature resistivity of ~1 Ω cm, and a hole mobility that is too small to measure by Hall effect. This universal electrical behavior strongly suggests that a common defect or a hole-rich surface layer governs the electrical properties of most FeS2 thin films.