Chemically Deposited CdS Buffer/Kesterite Cu₂ZnSnS₄ Solar Cells: Relationship between CdS Thickness and Device Performance

Earth-abundant, copper–zinc–tin–sulfide (CZTS), kesterite, is an attractive absorber material for thin-film solar cells (TFSCs). However, the open-circuit voltage deficit (<i>V</i>_oc-deficit) resulting from a high recombination rate at the buffer/absorber interface is one of the major challenges that must be overcome to improve the performance of kesterite-based TFSCs. In this paper, we demonstrate the relationship between device parameters and performances for chemically deposited CdS buffer/CZTS-based heterojunction TFSCs as a function of buffer layer thickness, which could change the CdS/CZTS interface conditions such as conduction band or valence band offsets, to gain deeper insight and understanding about the <i>V</i>_oc-deficit behavior from a high recombination rate at the CdS buffer/kesterite interface. Experimental results show that device parameters and performances are strongly dependent on the CdS buffer thickness. We postulate two meaningful consequences: (i) Device parameters were improved up to a CdS buffer thickness of 70 nm, whereas they deteriorated at a thicker CdS buffer layer. The <i>V</i>_oc-deficit in the solar cells improved up to a CdS buffer thickness of 92 nm and then deteriorated at a thicker CdS buffer layer. (ii) The minimum values of the device parameters were obtained at 70 nm CdS thickness in the CZTS TFSCs. Finally, the highest conversion efficiency of 8.77% (<i>V</i>_oc: 494 mV, <i>J</i>_sc: 34.54 mA/cm², and FF: 51%) is obtained by applying a 70 nm thick CdS buffer to the Cu₂ZnSn­(S,Se)₄ absorber layer

Self-standing 3D core-shell nanohybrids based on amorphous Co-Fe-Bi nanosheets grafted on NiCo2O4 nanowires for efficient and durable water oxidation

Here, a three-dimensional (3D) core–shell nanohybrid based on few-layer amorphous Co–Fe–Bi nanosheets directly grown on crystalline NiCo2O4 nanowires supported on the Ni foam (Co–Fe–Bi/NiCo2O4/NF) are facilely fabricated as highly efficient and durable electrocatalysts for water oxidation. This self-standing 3D core–shell nanohybrid design with unique materials chemistry and excellent interface engineering enhances the mass transport and stimulates the production of active sites during the oxygen evolution reaction. Serving as the anode catalysts, the resulting self-standing Co–Fe–Bi/NiCo2O4/NF nanohybrid electrocatalysts show a better electrocatalytic activity with an overpotential of 227 mV at 10 mA/cm2, a Tafel slope of 45 mV dec–1, excellent durability over 40 h, and the ability to deliver a current density of 200 mA/cm2 at an overpotential of ∼410 mV in an alkaline medium. Thus, the excellent electrocatalytic performance of the Co–Fe–Bi/NiCo2O4/NF nanohybrid demonstrates the importance of design and development of core–shell nanohybrids for large-scale practical applications in a multitude of energy conversion devices

Origin of Mechanoluminescence from Cu-Doped ZnS Particles Embedded in an Elastomer Film and Its Application in Flexible Electro-mechanoluminescent Lighting Devices

Mechanically driven light emission from particles embedded in elastomer films has recently attracted interest as a strong candidate for next-generation light sources on display devices because it is nondestructive, reproducible, real-time, environmentally friendly, and reliable. The origin of mechanoluminescence (ML) obtained from particles embedded in elastomer films have been proposed as the trapping of drifting charge carriers in the presence of a piezoelectric field. However, in this study, we propose a new origin of ML through the study of the microstructure of a Cu-doped ZnS particles embedded in an elastomer composite film with high brightness using transmission electron microscopy (TEM) to clearly demonstrate the origin of ML with respect to the microstructure of ML composite films. The TEM characterization of the ML composite film demonstrated that the Cu-doped ZnS particles were fully encapsulated by a 500 nm thick Al layer, which acts as an electron source for ML emission. Furthermore, we fabricated a flexible electro-mechanoluminescence (EML) device using a Cu-doped ZnS particles embedded in a flexible elastomer composite film. Our research results on a new emission mechanism for ML and its application in flexible light generating elastomer films represent an important step toward environmentally benign and ecofriendly flexible electro-mechanoluminescent lighting devices

Enhanced Solar Water Oxidation Performance of TiO₂ via Band Edge Engineering: A Tale of Sulfur Doping and Earth-Abundant CZTS Nanoparticles Sensitization

We report the rational design and fabrication of earth-abundant, visible-light-absorbing Cu₂ZnSnS₄ (CZTS) nanoparticle (NP) in situ sensitized S doped TiO₂ nanoarchitectures for high-efficiency solar water splitting. Our systematic studies reveal that these nanoarchitectures significantly enhance the visible-light photoactivity in comparison to that of TiO₂, S doped TiO₂, and CZTS NP sensitized TiO₂. Detailed photoelectrochemical (PEC) studies demonstrate an unprecedented enhancement in the photocurrent density and incident photon to electron conversion efficiency (IPCE). This enhancement is attributed to the significantly improved visible-light absorption and more efficient charge separation and transfer/transport, resulting from the synergistic influence of CZTS NP sensitization and S doping, which were confirmed by electrochemical impedance spectroscopy (EIS). Moreover, density functional theory (DFT) calculations supported by the experimental evidence revealed that the gradient S dopant concentration along the depth direction of TiO₂ nanorods led to the band gap grading from ∼2.3 to 2.7 eV. This S gradient doping introduced a terraced band structure via upshift of the valence band (VB), which provides channels for easy hole transport from the VB of S-doped TiO₂ to the VB of CZTS and thereby enhances the charge transport properties of the CZTS/S-TNR photoanode. This work demonstrates the rational design and fabrication of nanoarchitectures via band edge engineering to improve the PEC performance using simultaneous earth-abundant CZTS NP sensitization and S doping. This work also provides useful insight into the further development of different nanoarchitectures using similar combinations for energy-harvesting-related applications

Colloidal Wurtzite Cu₂SnS₃ (CTS) Nanocrystals and Their Applications in Solar Cells

In the development of low-cost, efficient, and environmentally friendly thin-film solar cells (TFSCs), the search continues for a suitable inorganic colloidal nanocrystal (NC) ink that can be easily used in scalable coating/printing processes. In this work, we first report on the colloidal synthesis of pure wurtzite (WZ) Cu₂SnS₃ (CTS) NCs using a polyol-mediated hot injection route, which is a nontoxic synthesis method. The synthesized material exhibits a random distribution of CTS nanoflakes with an average lateral dimension of ∼94 ± 15 nm. We also demonstrate that CTS NC ink can be used to fabricate low-cost and environmentally friendly TFSCs through an ethanol-based ink process. The annealing of as-deposited CTS films was performed under different S vapor pressures in a graphite box (volume; 12.3 cm³), at 580 °C for 10 min using a rapid thermal annealing (RTA) process. A comparative study on the performances of the solar cells with CTS absorber layers annealed under different S vapor pressures was conducted. The device derived from the CTS absorber annealed at 350 Torr of S vapor pressure showed the best conversion efficiency 2.77%, which is the first notable efficiency for an CTS NCs ink-based TFSC. In addition, CTS TFSC’s performance degraded only slightly after 50 days in air atmosphere and under damp heating at 90 °C for 50 h, indicating their good stability. These results confirm that WZ CTS NCs may be very attractive and interesting light-absorbing materials for fabricating efficient solar-harvesting devices

Aqueous-Solution-Processed Cu₂ZnSn(S,Se)₄ Thin-Film Solar Cells via an Improved Successive Ion-Layer-Adsorption–Reaction Sequence

A facile improved successive ionic-layer adsorption and reaction (SILAR) sequence is described for the fabrication of Cu₂ZnSn­(S,Se)₄ (CZTSSe) thin-film solar cells (TFSCs) via the selenization of a precursor film. The precursor films were fabricated using a modified SILAR sequence to overcome compositional inhomogeneity due to different adsorptivities of the cations (Cu⁺, Sn⁴⁺, and Zn²⁺) in a single cationic bath. Rapid thermal annealing of the precursor films under S and Se vapor atmospheres led to the formation of carbon-free Cu₂ZnSnS₄ (CZTS) and CZTSSe absorber layers, respectively, with single large-grained layers. The best devices based on CZTS and CZTSSe absorber layers showed total area (∼0.30 cm²) power conversion efficiencies (PCEs) of 1.96 and 3.74%, respectively, which are notably the first-demonstrated efficiencies using a modified SILAR sequence. Detailed diode analyses of these solar cells revealed that a high shunt conductance (<i>G</i>_sh), reverse saturation current density (<i>J</i>_o), and ideality factor (<i>n</i>_d) significantly affected the PCE, open-circuit voltage (<i>V</i>_oc), and fill factor (FF), whereas the short-circuit current density (<i>J</i>_sc) was dominated by the series resistance (<i>R</i>_s) and <i>G</i>_sh. However, the diode analyses combined with the compositional and interface microstructural analyses shed light on further improvements to the device efficiency. The facile layer-by-layer growth of the kesterite CZTS-based thin films in aqueous solution provides a great promise as an environmentally benign pathway to fabricate a variety of multielement-component compounds with high compositional homogeneities

A Simple Aqueous Precursor Solution Processing of Earth-Abundant Cu₂SnS₃ Absorbers for Thin-Film Solar Cells

A simple and eco-friendly method of solution processing of Cu₂SnS₃ (CTS) absorbers using an aqueous precursor solution is presented. The precursor solution was prepared by mixing metal salts into a mixture of water and ethanol (5:1) with monoethanolamine as an additive at room temperature. Nearly carbon-free CTS films were formed by multispin coating the precursor solution and heat treating in air followed by rapid thermal annealing in S vapor atmosphere at various temperatures. Exploring the role of the annealing temperature in the phase, composition, and morphological evolution is essential for obtaining highly efficient CTS-based thin film solar cells (TFSCs). Investigations of CTS absorber layers annealed at various temperatures revealed that the annealing temperature plays an important role in further improving device properties and efficiency. A substantial improvement in device efficiency occurred only at the critical annealing temperature, which produces a compact and void-free microstructure with large grains and high crystallinity as a pure-phase absorber layer. Finally, at an annealing temperature of 600 °C, the CTS thin film exhibited structural, compositional, and microstructural isotropy by yielding a reproducible power conversion efficiency of 1.80%. Interestingly, CTS TFSCs exhibited good stability when stored in an air atmosphere without encapsulation at room temperature for 3 months, whereas the performance degraded slightly when subjected to accelerated aging at 80 °C for 100 h under normal laboratory conditions

Band Tail Engineering in Kesterite Cu₂ZnSn(S,Se)₄ Thin-Film Solar Cells with 11.8% Efficiency

Herein, we report a facile process, i.e., controlling the initial chamber pressure during the postdeposition annealing, to effectively lower the band tail states in the synthesized CZTSSe thin films. Through detailed analysis of the external quantum efficiency derivative (<i>d</i>EQE/<i>d</i>λ) and low-temperature photoluminescence (LTPL) data, we find that the band tail states are significantly influenced by the initial annealing pressure. After carefully optimizing the deposition processes and device design, we are able to synthesize kesterite CZTSSe thin films with energy differences between inflection of d­(EQE)/dλ and LTPL as small as 10 meV. These kesterite CZTSSe thin films enable the fabrication of solar cells with a champion efficiency of 11.8% with a low <i>V</i>_oc deficit of 582 mV. The results suggest that controlling the annealing process is an effective approach to reduce the band tail in kesterite CZTSSe thin films