Revealing the Role of Potassium Treatment in CZTSSe Thin Film Solar Cells

Potassium (K) post-treatment on CIGSSe has been shown to yield the highest efficiency reported to date. However, very little is known on the effect of K doping in CZTSSe and the mechanism behind the efficiency improvement. Here we reveal the mechanism by which K enhances the charge separation in CZTSSe. We show that K accumulates at the CdS/CZTSSe, passivating the recombination at the front interface and improving carrier collection. K is also found to accumulate at the CZTSSe/Mo interface and facilitates the diffusion of Cd into the absorber which affects the morphology and grain growth of CZTSSe. As revealed by the <i>C</i>–<i>V</i>, external quantum efficiency, and color <i>J</i>–<i>V</i> test, K doping significantly increases the carrier density, improves carrier collection, and passivates the front interface and grain boundaries, leading to the enhancement of <i>V</i>_oc and <i>J</i>_sc. The average power conversion efficiency has been promoted from 5% to above 7%, and the best 7.78% efficiency has been achieved for the 1.5 mol % K-doped CZTSSe device. This work offers some new insights into the K doping effects on CZTSSe via solution-based approach and demonstrates the potential of facile control of K doping for further improvement of CZTSSe thin film solar cells

Superhydrophobic Surface-Enhanced Raman Scattering Platform Fabricated by Assembly of Ag Nanocubes for Trace Molecular Sensing

An analytical platform suitable for trace detection using a small volume of analyte is pertinent to the field of toxin detection and criminology. Plasmonic nanostructures provide surface-enhanced Raman scattering (SERS) that can potentially achieve trace toxins and/or molecules detection. However, the detection of highly diluted, small volume samples remains a challenge. Here, we fabricate a superhydrophobic SERS platform by assembling Ag nanocubes that support strong surface plasmon and chemical functionalization for trace detection with sample volume of just 1 μL. Our strategy integrates the intense electromagnetic field confinement generated by Ag nanocubes with a superhydrophobic surface capable of analyte concentration to lower the molecular detection limit. Single crystalline Ag nanocubes are assembled using the Langmuir-Blodgett technique to create surface roughness. To create a stable superhydrophobic SERS platform, an additional 25 nm Ag coating is evaporated over the Ag nanocubes to “weld” the Ag nanocubes onto the substrate followed by chemical functionalization with perfluorodecanethiol. The resulting substrate has an advancing contact angle of 169° ± 5°. Our superhydrophobic platform confines analyte molecules within a small area and prevents the random spreading of molecules. An analyte concentrating factor of 14-fold is attained, as compared to a hydrophilic surface. Consequently, the detection limit of our superhydrophobic SERS substrate reaches 10^–16 M (100 aM) for rhodamine 6G using 1 μL analyte solutions. An analytical SERS enhancement factor of 10¹¹ is achieved. Our protocol is a general method that provides a simple, cost-effective approach to develop a stable and uniform superhydrophobic SERS platform for trace molecular sensing

Enhancement of Open-Circuit Voltage of Solution-Processed Cu₂ZnSnS₄ Solar Cells with 7.2% Efficiency by Incorporation of Silver

Recently, considerable attention in the development of Cu₂ZnSnS₄ (CZTS)-based thin-film solar cells has been given to the reduction of antisite defects via cation substitution. In this Letter, we report the substitution of copper atoms by silver, incorporated into the crystal lattice through a solution processable method. We observe an increase in open-circuit voltage (<i>V</i>_OC) by 50 mV and an accompanying rise in device efficiency from 4.9% to 7.2%. The incorporation of Ag is found to improve the grain size, enhance the depletion width of the pn-junction, and reduce the concentration of antisite defect states. This work demonstrates the promising role of Ag in reducing the <i>V</i>_OC deficit of Cu-kesterite thin-film solar cells