Earth-abundant Absorber Material Research Articles

First-principles investigation of CZTS Raman spectra

${\mathrm{Cu}}_{2}{\mathrm{ZnSnS}}_{4}$ (CZTS) is an earth-abundant photovoltaic absorber material predicted to provide a sustainable solution for commercial solar applications. However, the efficiency of such solar cells is rather limited, cation disorder being often designated as the culprit. Raman spectroscopy has been widely used to characterize CZTS. Nonetheless, the interpretation of the spectra in terms of the atomic-scale disorder is precluded by the lack of consensus between theoretical and experimental results. In particular, there is a strong discrepancy in the relative intensities of the two prominent $A$ phonon peaks of the spectra. In the present study, we demonstrate that the internal parameters characterizing the position of the S atoms strongly influence these intensities. We show that agreement with experiments can be completely recovered when adopting the geometry computed using a hybrid exchange-correlation functional. Finally, using special quasirandom structures, we demonstrate that the disorder only leads to a change of the shape of the Raman peaks (tailing or leading edges, shouldering and splitting). This could be exploited to assess the quality of the sample in terms of how ordered they are.

Thermodynamic calculation of S−Sb system and Cu−S−Sb system

Sb2S3 and CuSbS2 have been proposed as alternative earth-abundant absorber materials for thin-film solar cells. However, no thermodynamic study of the S−Sb binary system and the Cu−S−Sb ternary system were investigated. In this paper, The S−Sb system and the Cu−S−Sb system are calculated utilizing the so-called CALPHAD (CALculation of PHAse Diagrams) technique. Using TEM-EDS and XRD, Cu0.9Sb1S2 is experimentally confirmed at the Cu1Sb1S2 and Sb2S3 two-phases region in the isothermal section at 673 K of the Cu−S−Sb ternary system. Given the asymmetric shape and miscibility gap of the liquidus in the S−Sb phase diagram, the associate solution model for the liquid phase is adopted. The solution phases (liquid, bcc, fcc) are treated with the Redlich–Kister equation. The compounds S3Sb2, Cu3SbS3, Cu12Sb4S13, CuSbS2, and Cu3SbS4 are described as a stoichiometric compound. A set of self-consistent thermodynamic parameters of the S−Sb binary system and the Cu−S−Sb ternary system are obtained. The calculated results are in good agreement with the experimental data. This study provides a set of reliable thermodynamic parameters to the Cu−Sb−S thermodynamic database, and a cost-effective tool to design material synthesis experiments and manufacturing processes.

Nanoscale Growth Initiation as a Pathway to Improve the Earth-Abundant Absorber Zinc Phosphide.

Growth approaches that limit the interface area between layers to nanoscale regions are emerging as a promising pathway to limit the interface defect formation due to mismatching lattice parameters or thermal expansion coefficient. Interfacial defect mitigation is of great interest in photovoltaics as it opens up more material combinations for use in devices. Herein, an overview of the vapor–liquid–solid and selective area epitaxy growth approaches applied to zinc phosphide (Zn3P2), an earth-abundant absorber material, is presented. First, we show how different morphologies, including nanowires, nanopyramids, and thin films, can be achieved by tuning the growth conditions and growth mechanisms. The growth conditions are also shown to greatly impact the defect structure and composition of the grown material, which can vary considerably from the ideal stoichiometry (Zn3P2). Finally, the functional properties are characterized. The direct band gap could accurately be determined at 1.50 ± 0.1 eV, and through complementary density functional theory calculations, we can identify a range of higher-order band gap transitions observed through valence electron energy loss spectroscopy and cathodoluminescence. Furthermore, we outline the formation of rotated domains inside of the material, which are a potential origin of defect transitions that have been long observed in zinc phosphide but not yet explained. The basic understanding provided reinvigorates the potential use of earth-abundant II–V semiconductors in photovoltaic technology. Moreover, the transferrable nanoscale growth approaches have the potential to be applied to other material systems, as they mitigate the constraints of substrate–material combinations causing interface defects.

Comparative study of the CZTS, CuSbS2 and CuSbSe2 solar photovoltaic cell with an earth-abundant non-toxic buffer layer

In the present article, modeling outcomes of novel and third-generation heterojunctions (3G) thin-film solar cells (TFSCs): quaternary semiconductor compound CZTS and ternary semiconductor compound CuSbS2 and CuSbSe2 have been presented. Solar cells have been studied and analysed to investigate optimized structure with good efficiency. CuSbS2 and CuSbSe2 are promising earth-abundant photovoltaic absorber materials which are chemically simpler than the widely studied Cu2ZnSnS4. However, CuSbS2 and CuSbSe2 photovoltaic (PV) devices currently have relatively low efficiency and poor reproducibility, often due to suboptimal material quality and insufficient optoelectronic properties. To overcome these concerns, we carried out numerical analysis using the Solar Cell Capacitance Simulator-1 dimensional software (SCAPS-1D). The band alignment and several parameters such as thickness and doping concentration have been investigated. Further C-V, C-f and J-V characteristics are also studied to compare and optimize the performance of CZTS, CuSbS2 and CuSbSe2 solar cell. Understanding the constraints imposed by the CZTS, CuSbS2, and CuSbSe2 absorber layers could help this technology to overcome the lower efficiency barrier.

Microstructured ZnO-ZnS composite for earth-abundant photovoltaics: Elaboration, surface analysis and enhanced optical performances

In this paper, ZnO-ZnS composite structure is proposed as a new efficient and earth-abundant absorber material for thin-film solar cells (TFSCs). Promising elaboration strategy based on combining vacuum thermal evaporation technique and oxidation process under an annealing temperature of 500 °C was used to prepare ZnO-ZnS composite with high sun-light absorption capabilities. The fabricated microstructure was then characterized by Scanning Electron Microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and UV–Visible absorption spectroscopy. The influence of the annealing time on the structural and optical performances of the prepared samples was investigated. Surface analysis demonstrated the ZnO decoration of ZnS thin-film, where SEM images showed dense and pinhole-free ZnO-ZnS composite with micrometer-sized grains and a few voids visible at thin-films surface. Optical characterization showed that the prepared thin-film absorber exhibits an optical band-gap of 2.65 eV with a high Total Absorption Efficiency (TAE) of 62% and an absorption coefficient exceeding 2 × 104 cm−1. In addition, I-V characteristics under dark and 1-sun illumination of the microstructured ZnO-ZnS composite were extracted. It was revealed that the proposed absorber showcases a high visible photoresponse. Therefore, promoting effective light-scattering effects, this innovative ZnO-ZnS composite offers a sound pathway to prepare alternative low-cost absorbers for the future development of TFSCs.

Ternary Cu2SnS3: Synthesis, Structure, Photoelectrochemical Activity, and Heterojunction Band Offset and Alignment.

Ternary Cu2SnS3 (CTS) is an attractive nontoxic and earth-abundant absorber material with suitable optoelectronic properties for cost-effective photoelectrochemical applications. Herein, we report the synthesis of high-quality CTS nanoparticles (NPs) using a low-cost facile hot injection route, which is a very simple and nontoxic synthesis method. The structural, morphological, optoelectronic, and photoelectrochemical (PEC) properties and heterojunction band alignment of the as-synthesized CTS NPs have been systematically characterized using various state-of-the-art experimental techniques and atomistic first-principles density functional theory (DFT) calculations. The phase-pure CTS NPs confirmed by X-ray diffraction (XRD) and Raman spectroscopy analyses have an optical band gap of 1.1 eV and exhibit a random distribution of uniform spherical particles with size of approximately 15–25 nm as determined from high-resolution transmission electron microscopy (HR-TEM) images. The CTS photocathode exhibits excellent photoelectrochemical properties with PCE of 0.55% (fill factor (FF) = 0.26 and open circuit voltage (Voc) = 0.54 V) and photocurrent density of −3.95 mA/cm2 under AM 1.5 illumination (100 mW/cm2). Additionally, the PEC activities of CdS and ZnS NPs are investigated as possible photoanodes to create a heterojunction with CTS to enhance the PEC activity. CdS is demonstrated to exhibit a higher current density than ZnS, indicating that it is a better photoanode material to form a heterojunction with CTS. Consistently, we predict a staggered type-II band alignment at the CTS/CdS interface with a small conduction band offset (CBO) of 0.08 eV compared to a straddling type-I band alignment at the CTS/ZnS interface with a CBO of 0.29 eV. The observed small CBO at the type-II band aligned CTS/CdS interface points to efficient charge carrier separation and transport across the interface, which are necessary to achieve enhanced PEC activity. The facile CTS synthesis, PEC measurements, and heterojunction band alignment results provide a promising approach for fabricating next-generation Cu-based light-absorbing materials for efficient photoelectrochemical applications.

Efficiency enhancement of CZTSSe solar cells via screening the absorber layer by examining of different possible defects

This study represents the investigation of earth-abundant and non-toxic CZTSSe absorber materials in kesterite solar cell by using the Finite Element Method (FEM) with (1) electrical, and (2) optical approaches. The simulated results have been validated with the experimental results to define guidelines for boosting the cell performance. For improving the cell efficiency, potential barrier variations in the front contact, and the effect of different lattice defects in the CZTSSe absorber layer have been examined. Controlling the defects and the secondary phases of absorber layer have significant influence on the cell performance improvement. Previous studies have demonstrated that, synthesis of CZTSSe:Na nanocrystals and controlling the S/(S + Se), Cu/(Zn + Sn), and Zn/Sn ratios (stoichiometry) have significant effects on the reduction of trap-assisted recombination (Shockley–Read–Hall recombination model). In this work, a screening-based approach has been employed to study the cell efficiency over a wide range of defect densities. Two categorized defect types including benign defects ({N}_{t}<{10}^{16} cm−3 , Nt defines trap density) and harmful defects {(N}_{t}>{10}^{16} cm−3) in the absorber bandgap in the CZTSSe solar cell, by analyzing their position changes with respect to the electron Fermi level (Efn) and the Valence Band Maximum positions have been identified. It is realized that, the harmful defects are the dominant reason for the low efficiency of the kesterite solar cells, therefore, reducing the number of harmful defects and also total defect densities lead to the power conversion efficiency record of 19.06%. This increment makes the CZTSSe solar cells as a promising candidate for industrial and commercial applications.

Investigation on the performance of SnS solar cells grown by sputtering and effusion cell evaporation

SnS is an earth-abundant, non-toxic, and low-cost absorber material for solar cell applications. In this work, the physical properties of SnS thin films and efficiency of SnS solar cells were investigated for different tin metal layer thicknesses of 300 nm (Sn-300), 500 nm (Sn-500), and 700 nm (Sn-700) deposited by DC sputtering followed by the sulfurization using effusion cell evaporation method. The XRD and Raman characterizations confirmed the formation of single-phase SnS compound with orthorhombic structure for the case of Sn-500. The sulfurized films of Sn-500 had remarkable and homogeneous morphology with the optical band gap energy of 1.35 eV. The fabricated device showed an efficiency of 0.74% with an open-circuit voltage of 267mV, short circuit current density of 8.47 mA/cm2, and fill factor of 54.16. By varying the different metal tin layer thicknesses, this is the first report with device efficiency for SnS solar cells grown via effusion cell evaporation technique.

Observation of photoluminescence edge emission in CuSbSe2 absorber material for photovoltaic applications

CuSbSe2 has great potential for being an earth-abundant absorber material in efficient thin film solar cells. In this study, the radiative recombination in CuSbSe2 polycrystalline material was studied by temperature and laser power dependent photoluminescence spectroscopy which revealed the band-to-band and band-to-deep acceptor transitions, with the last one involving a deep acceptor defect with an activation energy of 165 ± 6 meV. The observation of the edge emission in CuSbSe2 refers to the high quality of the synthesized material.

Controlled thickness of a chemical-bath-deposited CdS buffer layer for a SnS thin film solar cell with more than 3% efficiency

Tin monosulfide (SnS) is considered a promising earth-abundant absorber material for thin film solar cells (TFSCs) with a theoretical power conversion efficiency (PCE) as high as 30%. However, the maximum PCE reported thus far is just 4.36%. This inferior device performance is attributed to low values of open-circuit voltage. In addition to the dense and pinhole-free surface morphology of the phase-pure p-type SnS thin film absorber, a controlled growth of the n-type buffer layer is a prerequisite for enhanced device performance. In this study, we explore the manner in which the thickness of a chemical-bath-deposited n-type CdS buffer layer influences the performance of SnS/CdS TFSCs. The thickness of the buffer layer was adjusted by varying the deposition time from 15 to 25 min, which resulted in thicknesses of 30–80 nm. Initially, the performance parameters of the TFSCs improved with the buffer layer thickness, but later deteriorated with thicker CdS layers. The highest PCE achieved was 3.05% at an optimized CdS buffer layer thickness of approximately 42 nm. This performance was accompanied by a significant decrease in reverse saturation current and shunt conductance, as evidenced by the current density–voltage characteristics under dark conditions.

First-principles DFT insights into the structural, elastic, and optoelectronic properties of α and β-ZnP2: implications for photovoltaic applications

Binary II–V semiconductors are highly optically active materials, possess high intrinsic mechanical and chemical durability, and have electronic properties ideal for optoelectronic applications. Among them, zinc diphosphide (ZnP2) is a promising earth-abundant absorber material for solar energy conversion. We have investigated the structural, mechanical, and optoelectronic properties of both the tetragonal (α) and monoclinic (β) phases of ZnP2 using standard, Hubbard-corrected and screened hybrid density functional theory methods. Through the analysis of bond character, band gap nature, and absorption spectra, we show that there exist two polymorphs of the β phase (denoted as β1 and β2) with distinct differences in the photovoltaic potential. While β1 exhibits the characteristics of metallic compounds, β2 is a semiconductor with predicted thin-film photovoltaic absorbing efficiency of almost 10%. The α phase is anticipated to be an indirect gap material with a calculated efficiency limited to only 1%. We have also analysed and gained insights into the electron localization function, projected density of states and projected crystal orbital Hamilton populations for the analogue bonds between the α and β-ZnP2. In light of these calculations, a number of previous discrepancies have been solved and a solid ground for future employment of zinc diphosphides in photovoltaics has been established.

Solvothermal Synthesis of Cu2SnSe3 Nanocrystals Using Elemental Precursors: Influence of Different Solvents on Growth and Morphology

Low cost earth abundant absorber materials are set to play a major role in future generation thin film solar cell technologies. Here, we have utilized a facile approach of dissolving elemental precursors, viz copper, tin and selenium, to synthesize earth abundant Cu2SnSe3 (CTSe) nano-crystals using the solvothermal method. All the metallic precursors were made to react with a mixture of thioglycolic acid and methylamine, and the reactants were dissolved in three different solvents—distilled water, ethylene glycol and oleic acid. The influence of the solvents on the structural, morphological and optical properties of the nano-crystalline CTSe particles was studied. X-ray diffraction and Raman studies reveal that the CTSe nanocrystals grow in the cubic phase with a lattice parameter ∼ 5.6 A. FESEM results reveal different morphologies including nanoflakes and nano-particles depending on the solvent. Selected area electron diffraction patterns confirm the crystalline nature of the products. Optical properties show that the optical gap of the nanocrystals is around 1.32–1.45 eV, which is in the band gap range reported in the literature and is optimum for solar cell applications. The photocatalytic activity of the CTSe nanocrystals was demonstrated by using the material for the degradation of methylene blue dye in the presence of UV–visible light. The results confirm that the CTSe nanocrystals exhibit good photocatalytic response, the best activity being seen in samples grown from water.

Identifying Short-Range Disorder in Crystalline Bulk Cu2SnS3 Phases: A Solid-State Nuclear Magnetic Resonance Spectroscopic Investigation

Cu2SnS3 has recently attracted attention as a low-cost, earth-abundant absorber material for thin-film solar cells. Unfortunately, multiple Cu2SnS3 structures exist and the conditions under which different structures form are poorly defined. The structures of tetragonal and monoclinic Cu2SnS3 phases are clarified by use of solid-state nuclear magnetic resonance spectroscopy (ssNMR). The monoclinic Cu2SnS3 phase is shown to form under Sn-rich, Cu-poor conditions and the tetragonal Cu2SnS3 phase forms under Cu-rich, Sn-poor conditions, indicating that there is a composition difference between the structures that has not previously been reported. Furthermore, the monoclinic phase is destabilized at temperatures between 200 and 345 °C, when the material becomes tetragonal.

The Single Phase Region in Cu2ZnSnS4 Thin Films from Theory and Combinatorial Experiments

Cu2ZnSnS4 (CZTS) is hoped to be a future, earth-abundant absorber material for thin film solar cells, but performance remains below the level needed for commercialization. In this work, the size of the single phase region of CZTS obtained from thin film synthesis methods is explored, to determine the scope available for defect engineering and thereby future improvements in solar cell performance. A chemical model for the single phase region is presented, based on equilibria between defect complexes in the CZTS phase and the other solid- and gas-phase components present during synthesis. The model predicts a variable single phase region size, depending on the partial pressures of SnS and S2. The model is verified by analysis of combinatorial thin-film CZTS samples prepared with different synthetic conditions and characterized by Raman and compositional mapping. We conclude that typical synthesis strategies for CZTS are not capable of accessing the full range of the CZTS single phase region since the required partial pressure of S2 is very large. The important implication is that our understanding of CZTS defect chemistry from experimental studies is incomplete and that scope exists for tuning the defect properties toward better solar cell performance.

Sustainable p-type copper selenide solar material with ultra-large absorption coefficient.

Earth-abundant solar absorber materials with large optical absorption coefficients in the visible enable the fabrication of low-cost high-efficiency single and multi-junction thin-film solar cells. Here, we report a new p-type semiconductor, Cu4TiSe4 (CTSe), featuring indirect (1.15 eV) and direct (1.34 eV) band gaps in the optimal range for solar absorber materials. CTSe crystallizes in a new noncentrosymmetric cubic structure (space group F4[combining macron]3c) in which CuSe4 tetrahedra share edges and corners to form octahedral anionic clusters, [Cu4Se4]4-, which in turn share corners to build the three-dimensional framework, with Ti4+ ions located at tetrahedral interstices within the channels. The unique crystal structure and the Ti 3d orbital character of the conduction band of CTSe give rise to near-optimal band gap values and ultra-large absorption coefficients (larger than 105 cm-1) throughout the visible range, which are promising for scalable low-cost high-efficiency CTSe-based thin-film solar cells.

Band Edge Positions and Their Impact on the Simulated Device Performance of ZnSnN2-Based Solar Cells

ZnSnN2 (ZTN) has been proposed as a new earth abundant absorber material for photovoltaic (PV) applications. While carrier concentration has been reduced to values suitable for device implementation, other properties such as ionization potential, electron affinity, and work function are not known. Here, we experimentally determine the value of ionization potential (5.6 eV), electron affinity (4.1 eV), and work function (4.4 eV) for ZTN thin film samples with Zn cation composition Zn/(Zn+Sn) = 0.56 and carrier concentration $n$ = 2 × 1019 cm−3 . Using both experimental and theoretical results, we build a model to simulate the device performance of a ZTN/Mg:CuCrO2 solar cell, showing a potential efficiency of 23% in the limit of no defects present. We also investigate the role of band tails and recombination centers on the cell performance. In particular, device simulations show that band tails are highly detrimental to the cell efficiency, and recombination centers are a major limitation if present in concentration comparable to the net carrier density. The effect of the position of the band edges of the p- type junction partner was assessed too. Through this study, we determine the major bottlenecks for the development of ZTN-based solar cell and identify avenues to mitigate them.

Atomic-layer-deposited buffer layers for thin film solar cells using earth-abundant absorber materials: A review

Atomic layer deposition (ALD) is not just a thin film deposition technology limited to the semiconductor IC industries to grow high-k gate dielectric or a Cu diffusion barrier layer. In recent times, it has found plenty of applications in the field of renewable energy due to its precise thickness control up to few angstroms and its unique feature of conformal and uniform coating on any randomly shaped 3D structure. ALD has far-reaching applications in this field, including electrochemical storage, fuel cells, solar photovoltaics (PV), and catalysis for water splitting to produce H2 as a green fuel. In solar PV technology, ALD is now being extensively used as an efficient tool to deposit surface passivation layers, absorber or sensitizer, transparent conducting oxide, and barrier and buffer layers in several kinds of solar cells. Out of all the different layers associated with a solar cell, ALD is majorly used for the development of a very thin n-type buffer layer. This review article presents a systematic chronological study on such ALD-grown buffer layers for thin film solar cells (TSFCs). The study is carried out in detail based on different earth-abundant absorber materials, such as Cu2ZnSn(S,Se)4 (CZTSSe), Cu2O and SnS, for which ALD is successfully used to deposit the buffer layer.

Solution-processed CuSbS2 solar cells based on metal–organic molecular solution precursors

Developing earth-abundant and low-toxic light absorber materials is crucial for next-generation photovoltaics. In this article, a simple solution-process method for the deposition of CuSbS2 thin film is presented. An equimolar mixture of Cu2O and Sb2O3 was dissolved in butyldithiocarbamic acid, forming a thermally degradable metal–organic molecular solution. Uniform and phase-pure CuSbS2 thin films can be obtained by spin coating this precursor solution followed by a fast annealing process in an inert gas environment. Preferential crystal growth of (111) lattice planes was observed in films prepared at annealing temperature of more than 290 °C. The films possess a direct band gap of 1.6 eV with high absorption coefficient. CuSbS2 planar heterojunction solar cells of FTO/CdS/CuSbS2/Au structure were assembled achieving a power conversion efficiency of 0.68% under one sun illumination. This metal–organic molecular solution precursor provides a simple method to fabricate multicomponent inorganic chalcogenide films for photovoltaic application.

Structural, optical and electrical properties of copper antimony sulfide thin films grown by a citrate-assisted single chemical bath deposition

Copper antimony sulfide (CAS) has been proposed as low toxicity and earth abundant absorber materials for thin film photovoltaics due to their suitable optical band gap, high absorption coefficient and p-type electrical conductivity. The present work reports the formation of copper antimony sulfide by chemical bath deposition using sodium citrate as a complexing agent. We show that by tuning the annealing condition, one can obtain either chalcostibite or tetrahedrite phase. However, the main challenge was co-deposition of copper and antimony as ternary sulfides from a single chemical bath due to the distinct chemical behavior of these metals. The as-deposited films were subjected to several trials of thermal treatment using different temperatures and time to find the optimized annealing condition. The films were characterized by different techniques including Raman spectroscopy, X-ray diffraction (XRD), profilometer, scanning electron microscopy (SEM), UV–vis spectrophotometer, and Hall Effect measurements. The results show that the formation of chalcostibite and tetrahedrite phases is highly sensitive to annealing conditions. The electrical properties obtained for the chalcostibite films varied as the annealing temperature increases from 280 to 350°C: hole concentration (n)=1017–1018cm−3, resistivity (ρ)=1.74–2.14Ωcm and carrier mobility (μ)=4.7–9.26cm2/Vseg. While for the tetrahedrite films, the electrical properties were n=5×1019cm−3, μ=18.24cm2/Vseg, and ρ=5.8×10−3Ωcm. A possible mechanism for the formation of ternary copper antimony sulfide has also been proposed.

Ligand-Exchanged CZTS Nanocrystal Thin Films: Does Nanocrystal Surface Passivation Effectively Improve Photovoltaic Performance?

Nanocrystal (NC) Cu2ZnSnS4 (CZTS) solar cells, composed of a nontoxic and earth abundant absorber material, have great potential in low-cost solar energy harvesting. However, CZTS NC films typically must be thermally annealed at elevated temperatures and under harsh environments to produce high-efficiency devices. The efficiencies of unannealed CZTS NC solar cells have been hampered by low open circuit potentials (Voc, <325 mV) and low short circuit current densities (Jsc, <2 mA), primarily because of the incomplete passivation of the crystal surface. Although great progress has been made in understanding the surface chemistry of II–VI and IV–VI semiconductor NCs, the surface chemistry of complex quaternary CZTS NCs is largely unexplored. Here, for the first time, we report a comprehensive study of the surface chemistry of CZTS NCs focusing on depositing ligand-passivated, uniform NC thin films to address the issue of large Voc deficit and low current collection efficiency typically observed for CZTS NC s...