Absorption In Thin-film Solar Cells Research Articles

Numerical Optimization of Thickness and Optical Band Gap of Absorber and Buffer Layers in Earth-Abundant Cu2ZnSnS4 Thin-Film Solar Cells

Numerical Optimization of Thickness and Optical Band Gap of Absorber and Buffer Layers in Earth-Abundant Cu2ZnSnS4 Thin-Film Solar Cells

Prebaking of an SnS source with sulfur for achieving higher photovoltaic performance in VTD-SnS thin films for solar cells

We prepared a prebaked (SnS + S) source absorber which results in suppressing the deep level and interface defects. The device showed a 42% enhancement in the performance compared to the reference absorber-based device without prebaking.

Influence of Hubbard U correction on the structural, electronic and optical properties of Kesterite Cu2XSnS4 (X= Zn, Fe)

The new materials like quaternary chalcogenides semiconductors is an approach towards environment-friendly photovoltaic materials due to their promising potential as thin film solar cell absorbers. This work investigates the density functional theory (DFT) and density functional theory plus Hubbard U (DFT + U) approach on the kesterite phase of sulfide-based chalcogenides, Cu2XSnS4, CXTS (X = Zn and Fe) materials. The inclusion of the potential correlation term, U, plays a vital role in aiding the understanding of the complex many-electron problem, which LDA and GGA from DFT might not adequately describe. It was found that, by applying Hubbard U terms on p and d orbital states, the value of electronic band gaps can be significantly fixed close to the experimental value (∼1.3 eV–1.5 eV). The parametrized dependence of the band gap was well explained. The investigation of optical properties associated with the thin film applications on imaginary parts of the dielectric function shows that both structures have greater absorption at the energy range of 0–5 eV. Moreover, the refractive index and optical absorption show good results for both kesterite CXTS in the most effective wavelength of light to absorb sunlight (visible spectrum), which could exhibit a better finding for a suitable candidate for cost-effective new thin film solar cell application.

A DFT insight into magnetic, optoelectronic and thermoelectric properties of h-Zn1-xCuxS monolayer

In this study, we used the density functional theory (DFT) method to analyze the effect of Cu insertion on the structural stability, electronic, magnetic, optical and thermoelectric properties of the hexagonal ZnS monolayer (h-ZnS-2D). Our findings show that Cu replaces zinc sites by giving a stable planar structure in Ferromagnetic (FM) phase with slight decrease in the lattice parameter. A p-type semiconductor with a direct band gap that decreases with increasing Cu concentration was obtained. The half-metallic character found for low Cu concentrations let consider the h-Zn1-xCuxS-2D for potential applications in spintronic devices. The optical properties show that the h-Zn1-xCuxS-2D monolayers have high absorption and low transmittance in the UV range. The improved figure of merit of h-Zn1-xCuxS-2D due to increased electrical conductivity and decreased thermal conductivity with Cu content, makes these nanostructures a promising absorber for thin-film solar cells and suitable for solar thermal energy conversion technologies.

Stabilized Perovskite Quantum Dot Solids via Nonpolar Solvent Dispersible Covalent Ligands.

The ligand exchange procedure of CsPbI3 perovskite quantum dots (PQDs) enables the fabrication of thick and conductive PQD solids that act as a photovoltaic absorber for solution-processed thin-film solar cells. However, the ligand-exchanged CsPbI3 PQD solids suffer from deterioration in photovoltaic performance and ambient stability due to the surface traps, such as uncoordinated Pb2+ sites on the PQD surface, which are generated after the conventional ligand exchange process using ionic short-chain ligands dissolved in polar solvents. Herein, a facile surface stabilization is demonstrated that can simultaneously improve the photovoltaic performance and ambient stability of CsPbI3 PQD photovoltaic absorber using covalent short-chain triphenylphosphine oxide (TPPO) ligands dissolved in a nonpolar solvent. It is found that the TPPO ligand can be covalently bound to uncoordinated Pb2+ sites and the nonpolar solvent octane can completely preserve the PQD surface components. Owing to their synergetic effects, the CsPbI3 PQD photovoltaic absorber stabilized using the TPPO ligand solution dissolved in octane exhibit higher optoelectrical properties and ambient stability than the control absorber. Consequently, CsPbI3 PQD solar cells composed of PQD photovoltaic absorbers fabricated via surface stabilization strategy provide an improved power conversion efficiency of 15.4% and an enhanced device stability.

Gradient band gap CZTSSe prepared via sputtering from quaternary ceramic targets followed with annealing under different atmospheres

CZTSSe is a thin-film solar cell absorber material with long-time application potential, but its development is limited as its severe open-circuit voltage(Voc) deficit. To further improve the Voc, gradient band gap structure of absorber is expected to achieve this aim without large change of its overall band gap. In this work, a stack structure precursor was deposited by sputtering from quaternary ceramic targets. The top gradient band gap CZTSSe was achieved by annealing the precursor under pure Se or S/Se mix atmosphere. The effects of the atmosphere and the layers-stack precursor structure on the film properties and device performance were studied. Compared with selenization process, sulfo-selenization could effectively suppressed the severe reaction at the back contact interface. It would reduce Mo(S,Se)2 and secondary phases formation and improve solar cell efficiency. The Voc of the double-layers precursor solar cell successfully improved under both atmospheres as expected, representing this method could be a feasible direction to solve the large Voc,def problem. However, for higher efficiency sulfo-selenization cells, the Voc improvement in stack precursor samples did not bring obvious increase of efficiency due to the deterioration of fill factor, indicating that further optimization of device electrical properties should be applied in future study.

Comb-shaped Sb2S3 nanorod arrays on ZnO nanofibers for thin-film photovoltaics

A hierarchical composite of Sb2S3 nanorods grown on zinc oxide (ZnO) nanofiber was prepared, and the formation of comb-shaped Sb2S3 nanorod arrays on the ZnO nanofibers was confirmed. It was found that the size of the diameter and the density of the nanorods are regulatable by changing the concentration of polyvinyl pyrrolidone as an additive for the growth of Sb2S3 nanorod on ZnO nanofiber. The obtained Sb2S3 nanorod arrays were applied as a light absorber for thin-film solar cells composed of glass-fluorine-doped tin oxide/compact ZnO/ZnO nanofibers−ZnS/Sb2S3 nanorod arrays/poly(3-hexylthiophene-2,5-diyl)/MoOx/Ag. The rectification ratio and photocurrent generation efficiency of the comb-shaped Sb2S3 nanorod arrays were improved as compared with the heterojunction of randomly stacked Sb2S3 nanorods. Smaller series resistance (Rs) of 8.13 Ω cm−2 and an ideality factor (n) of 2.84 with the comb-shaped Sb2S3 nanorod arrays than those of the randomly stacked ones of Rs = 15.01 Ω cm−2 and n = 3.83 also indicated superior charge extraction property and suppressed recombination of the comb-shaped Sb2S3 nanorod arrays at the interface.

The suppressing of excitonic effects in Cu-chalcogenides for solar cell applications

Excitonic effect can usually significantly enhance the optical absorption coefficient in wide-gap insulators. Cu-chalcogenides are widely used as thin-film solar cell absorbers due to their high light utilization efficiency. Theoretical investigations on the excitonic absorption in Cu-chalcogenides are rare because of the complicated two-particle nature of an exciton. Here, we theoretically evaluate the continuum-excitonic effect in CuGaS2 and CuInS2 by solving the Bethe-Salpeter equation. Despite the large bandgap of 2.4 eV for CuGaS2, the excitonic effect induces moderate absorption enhancement when compared with the single-particle scenario. Moreover, the experimental results are better reproduced using the latter approach, indicating that the intrinsic excitonic effect is nearly suppressed in the practical experiments. The moderate intrinsic excitonic effect and its further suppressing in the experiments are explained by strong electronic screening effects in these materials. Justification of the simple single-particle approach would be convenient for future theoretical studies, which otherwise require more complex methods with more expensive computational efforts.

Investigation of Ag(Ga,In)Se2 as thin-film solar cell absorbers: A first-principles study

Investigation of Ag(Ga,In)Se2 as thin-film solar cell absorbers: A first-principles study

Development of an operando characterization stage for multi-modal synchrotron x-ray experiments.

It is widely accepted that micro- and nanoscale inhomogeneities govern the performance of many thin-film solar cell absorbers. These inhomogeneities yield material properties (e.g., composition, structure, and charge collection) that are challenging to correlate across length scales and measurement modalities. The challenge is compounded if a correlation is sought during device operation or in conditions that mimic aging under particular stressors (e.g., heat and electrical bias). Correlative approaches, particularly those based on synchrotron x-ray sources, are powerful since they can access several material properties in different modes (e.g., fluorescence, diffraction, and absorption) with minimal sample preparation. Small-scale laboratory x-ray instruments have begun to offer multi-modality but are typically limited by low x-ray photon flux, low spatial resolution, or specific sample sizes. To overcome these limitations, a characterization stage was developed to enable multi-scale, multi-modal operando measurements of industrially relevant photovoltaic devices. The stage offers compatibility across synchrotron x-ray facilities, enabling correlation between nanoscale x-ray fluorescence microscopy, microscale x-ray diffraction microscopy, and x-ray beam induced current microscopy, among others. The stage can accommodate device sizes up to 25 × 25mm2, offering access to multiple regions of interest and increasing the statistical significance of correlated properties. The stage materials can sustain humid and non-oxidizing atmospheres, and temperature ranges encountered by photovoltaic devices in operational environments (e.g., from 25 to 100 °C). As a case study, we discuss the functionality of the stage by studying Se-alloyed CdTe photovoltaic devices aged in the stage between 25 and 100 °C.

Low temperature assisted phase transitions in thermally evaporated Cu2Se/Ga3Se2/In3Se2 multilayer thin film structure: Raman signatures of a pristine chalcopyrite CuInxGa1-xSe2 structure

Copper Indium Gallium Selenide (CIGS) thin film solar cell absorber layers are fabricated by the conventional co-evaporation of metal stack precursors followed by a selenization process at a high-temperature range. The article deals with an alternative approach to fabricate the CIGS layer by sequential evaporation of metal selenides followed by a low-temperature treatment (250 °C) during pre-and post-deposition processes. Our post annealing treatment on substrate heat treated film reported a first-ever pristine CuIn0.7Ga0.3Se2 thin film structure without selenization was reported at a very low temperature. A better microstructure with an enhanced grain growth, preferential (112) plane orientation, and Raman signatures of pristine chalcopyrite without a secondary Cu2-xSe phase at the surface was observed. Electron microscopy studies affirm the polycrystalline nature of film structure. Optical analyses have shown near-optimum values as of standard absorber layers. Crucial optical parameters such as refractive index, static and high-frequency dielectric constants were estimated by different relations. The values are concurrent with the semiconductor scale. These features in a low-temperature range optimistically land them as promising candidates in flexible electronics.

Antiperovskite Sr3mn and Ba3mn (M=Sb or Bi) as Promising Photovoltaic Absorbers for Thin-Film Solar Cells: A First-Principles Study

Antiperovskite Sr3mn and Ba3mn (M=Sb or Bi) as Promising Photovoltaic Absorbers for Thin-Film Solar Cells: A First-Principles Study

Bifacial semi-transparent ultra-thin Cu(In,Ga)Se2 solar cells on ITO substrate: How ITO thickness and Na doping influence the performance

Ultra-thin Cu(In,Ga)Se2 (CIGSe) is a promising absorber for thin-film solar cells, as it combines the advantages of low raw material consumption and high conversion efficiency. In addition, ultra-thin absorbers on transparent back contacts bring the advantage of semitransparency, which is essential for e.g. tandem or bifacial solar cells. This work optimizes ultra-thin CIGSe on In2O3:Sn (ITO) for application in bifacial semi-transparent ultra-thin (BSTUT) CIGSe solar cells. Firstly, 100–400 nm ITO were coated onto glass substrates, and it was revealed that the thickness of ITO influences its optical bandgap Eg due to the Burstein-Moss (B-M) shift. The band gap of 400 nm ITO increased by 0.14 eV compared to the 100 nm thick ITO, and the Voc of the related BSTUT CIGSe solar cells raised by 0.043 V as a result of the diminished Schottky barrier Φb at the ITO/CIGSe interface. Secondly, 0–8 mg of NaF used for post deposition treatment (PDT) of the CIGSe were applied to the BSTUT solar cells. Compared to the reference without NaF, 8 mg NaF PDT enhanced the carrier density NA from 2 × 1015cm−3 to 1.2 × 1016cm−3 and diminished the ITO/CIGSe Schottky barrier Φb by 0.21 eV. In conclusion, we found that NaF PDT can tune the carrier density of ultra-thin CIGSe on ITO, and both thicker ITO and higher NaF PDT dose can reduce the ITO/CIGSe Schottky barrier. These discoveries enable future optimization of BSTUT CIGSe solar cells.

Spiers Memorial Lecture: Next generation chalcogenide-based absorbers for thin-film solar cells.

Inorganic-based thin-film photovoltaics (TFPV) represents an important component of the growing low-carbon energy market and plays a vital role in the drive toward lower cost and increased penetration of solar energy. Yet, commercialized thin-film absorber technologies suffer from some non-ideal characteristics, such as toxic or non-abundant element use (e.g., CdTe and Cu(In,Ga)(S,Se)2, which bring into question their suitability for terawatt deployment. Numerous promising chalcogenide, halide, pnictide and oxide semiconductors are being pursued to bridge these concerns for TFPV and several promising paths have emerged, both as prospective replacements for the entrenched technologies, and to serve as partner (i.e., higher bandgap) absorbers for tandem junction devices-e.g., to be used with a lower bandgap Si bottom cell. The current perspective will primarily focus on emerging chalcogenide-based technologies and provide both an overview of absorber candidates that have been of recent interest and a deeper dive into an exemplary Cu2BaSnS4-related family. Overall, considering the combined needs of high-performance, low-cost, and operational stability, as well as the experiences gained from existing commercialized thin-film absorber technologies, chalcogenide-based semiconductors represent a promising direction for future PV development and also serve to highlight common themes and needs among the broader TFPV materials family.

Enhanced light harvesting in GaAs thin-film solar cells using plasmonic gold (Au) nanoparticle absorbers

Surface texturing methods are often used in thin-film solar cells (SCs) to achieve effective photon trapping. However, such approaches have a detrimental impact on cell performance and are not suitable for thin-films. Plasmonic light trapping using metal nanoparticles (NPs) is one of the promising technique for achieving superior optical absorption in thin-film SCs with less material usage. In this article, to improve the optical performance of GaAs thin-film SCs, a uniformly distributed Gold (Au) nanoparticle (NP) array is introduced at the top of transparent indium-tin-oxide (ITO) electrode. The Au NP array functions as an antireflection layer, increasing light harvesting in active layer for both visible and infrared (IR) wavelength regions. By performing 3-D finite-different time-domain (FDTD) analysis, we have presented a comparative optoelectronic study of the GaAs SCs with and without Au NPs. Photovoltaic parameters such as optical absorption (A (λ)), photogeneration density (Goptical rate), and photocurrent density (Jsc) are used to evaluate performance of the structures. The proposed thin-film SCs with Au NPs achieved an optical absorption of 60% (planar) and 77% (NW array) with a remarkable photocurrent of 22.8 mA/cm2 (planar) and 26.7 mA/cm2 respectively.

Cerium-based lead-free chalcogenide perovskites for photovoltaics

Inorganic-organic hybrid lead halide perovskites have shown great prospects in solar cells in recent years, and their power conversion efficiency (PCE) has reached up to $25.5%$. However, Pb toxicity and poor ambient stability have remained two unsolved urgent problems. To address these problems, more and more researchers have begun to focus on the application of inorganic lead-free perovskites in the photovoltaic field. Herein, several cerium-based lead-free inorganic chalcogenide perovskites $A{\mathrm{CeTe}}_{3}$ $(A=\mathrm{Ca},\mathrm{Sr},\mathrm{Ba})$ are proposed as suitable optical absorbers for thin-film solar cells based on first-principles calculations. The distorted-phase $A{\mathrm{CeTe}}_{3}$ perovskites are predicted as direct-band-gap semiconductors with a gap value of 1.02--1.13 eV. Moreover, the distorted $A{\mathrm{CeTe}}_{3}$ not only possess a strong optical absorption coefficient $({10}^{5}--{10}^{6}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1})$, superior to ${\mathrm{MAPbI}}_{3} ({\mathrm{MA}}^{+}={\mathrm{CH}}_{3}{\mathrm{NH}}_{3}^{+})$, but also have excellent PCE, especially for ${\mathrm{BaCeTe}}_{3}$, whose PCE can reach $\ensuremath{\sim}29%$ even in a thin film $(1\ensuremath{\mu}\mathrm{m})$. All these characteristics render distorted $A{\mathrm{CeTe}}_{3}$ perovskites promising photovoltaic materials.

Growth and photovoltaic device using Cu3VS4 films prepared via co-sputtering from Cu–V and V targets

CVS (Cu3VS4) materials have the potential to become a new generation of thin film solar cell absorber materials, due to the availability of the constituent materials and a nearly ideal energy gap (1.3–1.5eV). In this study, we investigated the effects of co-sputtering using targets of Cu–V and V and various sulfurization temperatures to produce thin films of various Cu/V ratios. In experiments, CVS films with a high Cu/V ratio presented large surface grains of Cu2-XS with the appearance of secondary phases (CuS, Cu1.8S, Cu2S, and Cu0.7V2.3S4) during H2S sulfurization process. A low Cu/V ratio with high sulfurization temperature produced nearly single-phase CVS. The optimal CVS absorber layer was prepared using a Cu/V ratio of 2.76 with sulfurization for 20 min at a temperature of 550 °C. A prototype CVS thin film solar cell achieved a maximum power conversion efficiency of 3.11%.

Diverse simulations of time-resolved photoluminescence in thin-film solar cells: A SnO2/CdSeyTe1−y case study

Time-resolved photoluminescence (TRPL) is widely used to measure carrier lifetime in thin-film solar cell absorbers. However, the injection dependence of data and frequent non-exponential decay shapes complicate the interpretation. Here, we develop a numerical model to simulate injection-dependent TRPL measurements in a SnO2/CdSeyTe1−y solar cell structure, considering parameters of interest to researchers in industry and academia. Previous simulations have shown that in low injection, excess electrons and holes injected by the laser pulse are rapidly separated in the electric field formed by the pn junction. As a result, at early times, the PL signal can decay faster than the Shockley–Read–Hall lifetime in the absorber bulk (τbulk). Prior simulations have shown that the charge stored in the junction can slowly leak out to affect decays at late times. However, it has not been clear if and to what degree charge storage can affect the slopes extracted from TRPL decays—τ2—commonly cited as the TRPL-measured lifetime. Here, we show that charge storage can, in some cases, result in τ2 values that substantially overestimate τbulk. Previous simulations indicate that high-injection conditions can screen the junction field and minimize charge separation. Here, we show that continued injection increases can drive down τ2 below τbulk as radiative recombination becomes dominant. We catalog charge storage and radiative recombination impacts for a diverse set of material parameters and compare results to double-heterostructure models.

Impact of substrate temperature during NaF and KF post-deposition treatments on chemical and optoelectronic properties of alkali-free Cu(In,Ga)Se2 thin film solar cell absorbers

The substrate temperature TPDT during the alkali fluoride post-deposition treatment (PDT) of Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorbers is a critical process parameter. So far, TPDT is optimized empirically for a particular alkali distribution, and literature only reports on the effects of high TPDT (300–350 °C). To better understand the influence of TPDT and to close the gap for TPDT below 300 °C, a temperature series from 100 to 315 °C was studied. Furthermore, most studies on KF PDTs are performed with alkali-containing soda-lime glass substrates, in which the electrical effects of alkalis from the substrate and the PDT on the final device cannot be separated. To circumvent this obstacle, we use alkali-free substrates in our study and compare electrical and chemical effects of NaF and KF PDTs at different TPDT using current-density-voltage and capacitance-voltage measurements, as well as time-of-flight secondary-ion mass spectrometry and ultraviolet photoelectron spectroscopy. The alkali concentrations are quantified as a function of TPDT and discussed in relation to the respective cell parameters. Both, NaF and KF PDTs, improve the cell efficiency significantly, with the highest conversion efficiency of 16.3% obtained for a NaF PDT at TPDT = 315 °C. A given TPDT leads to a larger K concentration (after KF PDT) than Na concentration (after NaF PDT). For TPDT above 150 °C, similar conversion efficiencies are achieved for NaF and KF PDTs. In contrast, the KF PDT results in a significant decrease of the conversion efficiency at TPDT = 105 °C, likely caused by the formation of a Cu-Se secondary phase at the CIGSe surface, which is accompanied by a shift of the valence band maximum towards the Fermi energy.

Effect of plasmonic Aluminum nanoparticles shapes on optical absorption enhancement in silicon thin-film solar cells

Scattering from metal nanoparticles near their localized plasmon resonance; especially, the resonances of noble metals which are mostly in the visible or infrared part of the electromagnetic spectrum; is a way of improving light absorption in thin-film solar cells. The surface plasmon resonance can be affected by different factors such as the type, size, shape, and dielectric properties of the surrounding medium. Here we investigate, using the Finite Difference Time Domain (FDTD) method, how different shapes of aluminum nanoparticles affect absorption enhancement in silicon thin-film solar cells. Our results show that using these particles more than 30% conversion efficiency for plasmonic solar cells can be achieved compared to a cell without particles. We have also found that although the spherical particles have the highest absorption peak, optimization of some parameters such as the height of the cylinder or disk-shaped particles and their distance from the substrate can increase the absorption. The results can provide more information and insight to understand and optimize plasmonic particles for solar cell applications.