Sun Kyung Hwang

The fabrication of kesterite Cu₂ZnSn(S,Se)₄(CZTSSe) thin‐film solar cells using the electrochemical deposition (ED), which is valued for its industrial feasibility, offers a cost‐effective and environmentally friendly approach to the carbon‐free and clean energy production. However, the reported power conversion efficiency of approximately 10 % for electrodeposited CZTSSe thin‐film solar cells is lower compared to the alternative methods like sputtering and spin‐coating, which is mainly attributed to the phase inhomogeneity and the rough morphology generated during the ED process. Ensuring the microscopic and macroscopic uniformity of the electrodeposited films is crucial for the improvement of the film quality and the device performances. In this review, strategies to address these challenges including the intrinsic film control such as the deposition mode, pH, concentration of metal ions, and complexing agents, as well as the extrinsic approaches such as doping, substitution of metal elements, and the introduction of interfacial layers. In addition, the prospects for the electrochemically deposited CZTSSe solar cells were presented, focusing on the promising applications in tandem, flexible, and solar water‐splitting devices. Finally, this review will provide technical insights into the ED process for preparing CZTSSe solar cells, outlining a perspective for the future development of highly efficient CZTSSe thin film solar cells.

Cu₂ZnSn(S,Se)₄ (CZTSSe) thin film solar cells are an attractive choice for a bottom cell of the low‐cost and environmental tandem solar cells with perovskite. However, the progress in developing efficient perovskite/CZTSSe tandem solar cells has been hindered by the lack of high performance of the CZTSSe bottom cell. Here, an efficient CZTSSe bottom cell is demonstrated by adopting a facile and effective CsF treatment process. It is found that the CsF treatment not only facilitates grain growth and improves phase homogeneity by suppressing the detrimental deep‐level defects and secondary phases, but also induces larger band bending and stronger drift force at the P‐N junction. As a result, the carrier extraction/transport can be effectively accelerated, while reducing the interfacial recombination. These combined effects eventually result in a significant performance enhancement from 8.38% to 10.20%. The CsF‐treated CZTSSe solar cell is finally applied to the mechanically‐stacked perovskite/CZTSSe 4‐terminal tandem cell by coupling a semi‐transparent perovskite top cell, which exhibits the highest reported tandem efficiency of 23.01%.

In spite of the high potential economic feasibility of the tandem solar cells consisting of the halide perovskite and the kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe), they have rarely been demonstrated due to the difficulty in implementing solution‐processed perovskite top cell on the rough surface of the bottom cells. Here, we firstly demonstrate an efficient monolithic two‐terminal perovskite/CZTSSe tandem solar cell by significantly reducing the surface roughness of the electrochemically deposited CZTSSe bottom cell. The surface roughness (R_rms) of the CZTSSe thin film could be reduced from 424 to 86 nm by using the potentiostatic mode rather than using the conventional galvanostatic mode, which can be further reduced to 22 nm after the subsequent ion‐milling process. The perovskite top cell with a bandgap of 1.65 eV could be prepared using a solution process on the flattened CZTSSe bottom cell, resulting in the efficient perovskite/CZTSSe tandem solar cells. After the current matching between two subcells involving the thickness control of the perovskite layer, the best performing tandem device exhibited a high conversion efficiency of 17.5% without the hysteresis effect.

Cu₂ZnSn(S,Se)₄ (CZTSSe) has generated considerable research interest owing to its composition of abundant elements and excellent light‐absorption properties. However, CZTSSe thin‐film solar cells suffer from a considerable deficit in the open‐circuit voltage (V_OC), which is mainly due to the severe interfacial recombination induced by the rough surface of CZTSSe and numerous physical defects. In this study, to improve the morphology and reduce the interfacial recombination, an In₂S₃ passivation layer was introduced between the CZTSSe and CdS layers via a chemical bath deposition process, and the effects of the In₂S₃ layer on the device performance were systematically examined by performing various electrodynamic analyses. The CZTSSe solar cells with thin In₂S₃ layers exhibited impressive increases in V_OC and conversion efficiency (from 7.33 to 9.24 %), due to the suppression of physical defects and the refined surface morphology resulting from filling the voids and pinholes. In addition, the nanoscale roughness of the In₂S₃/CZTSSe surface increased the number of nucleation sites for the CdS nuclei, which may reduce the activation energy of the heterogeneous nucleation. The presence of In₂S₃ layer resulted in uniform growth of CdS without macroscopic CdS agglomerates (i. e., reduced roughness of full devices), which improved the quality of the interface. These findings confirmed that the reduction of physical defects and the improved deposition of the CdS layer enabled by the added In₂S₃ passivation layer improved the device performance.

Cu₂ZnSn(S,Se)₄ (CZTSSe) thin‐film solar cells offer various advantages including excellent optical and electrical properties, nontoxic and earth‐abundant raw materials, and a simple fabrication process. However, these devices suffer from a high deficit of the open‐circuit voltage (V_OC), mainly caused by interface recombination, which increases with increasing surface roughness. In this study, to achieve a high V_OC and enhance the overall device performance, an additional heat treatment process was introduced during the fabrication of co‐electrodeposited rough CZTSSe solar cells, and its effect on the photovoltaic properties was systematically investigated using various characterization techniques including diode analysis, transient photovoltage decay measurement, evaluation of the temperature dependency of the open‐circuit voltage, current–voltage and drive‐level capacitance profile analysis, and electrochemical impedance spectroscopy. At the optimized post‐heat treatment (PHT) temperature of 200°C, a significant increase in the conversion efficiency (as high as 32%, from 7.11% to 9.40%) was observed owing to the change in the interfacial materials properties (i.e., higher conductivity and reduced interfacial nonradiative recombination), which in turn is a consequence of the diffusion of the Cd ions and the expansion of the Cu‐poor/Zn‐rich phase. The PHT‐applied CZTSSe device exhibited a high conversion efficiency close to the record‐high one reported for electrodeposited CZTSSe thin‐film solar cells. These findings confirm the potential of PHT to overcome the serious V_OC deficit of CZTSSe device; moreover, this approach could possibly be extended to other device fabrication processes to achieve higher device performance and adopted to commercialization.

Copper zinc tin sulfo-selenide (CZTSSe) is a promising light absorbing material of thin film solar cells due to its low materials cost especially when it is prepared by cost-effective processes like the electrochemical deposition process. The CZTSSe thin film solar cells, however, suffer from the relatively low efficiency mostly due to the significant charge recombination. Given that the surface recombination is one of the major recombination paths, controlling the surface roughness, and thus the interfacial area is one of the key factors for improving their device performances. In this study, we demonstrated a simple but effective strategy for reducing the surface roughness during the electrochemical deposition process of the CZTSSe thin films. By adopting an initial nucleation stage with higher deposition currents ahead of the steady-state galvanostatic deposition, the surface of the CZT precursor and CZTSSe thin films became significantly smoother and uniform (ΔR_rms: −43.8% for CZT, −28.9% for CZTSSe). The effects of the surface roughness on the photovoltaic properties of the CZTSSe thin film solar cells have been investigated systematically with various characterization techniques like the diode analysis, lifetime measurement, and the temperature dependency of the open-circuit voltage. The device with the smoother surface exhibited higher open-circuit voltage and fill factor mostly due to the significantly reduced charge recombination, leading to the high conversion efficiency of 8.64% (active).

To examine the effects of body position and time after feeding on gastric residuals in low birth weight infants(LBW). A repeated measures design was conducted. Twenty LBW infants being fed via indwelling nasogastric tubes were randomly assigned to one of 5 different position orderings. In each position, gastric residuals were measured at 30, 60, 90, 120, 150, and 180 minutes after feeding. In comparisons of gastric residuals with position and time, the main effects of position and time were statistically significant (F = 5.038, p = 0.001; F = 429.763, p < 0.001, respectively), but the interaction between position and time was found not to be significant. In pairwise comparisons, the gastric residuals were more reduced in the right anterior oblique and prone positions compared with the left lateral position (p < 0.05) and the differences across time periods were significant (p < 0.001). The body position after feeding has a significant affect on gastric residuals over time in LBW infants. The right anterior oblique or prone position is recommended rather than left lateral position after feeding. Knowledge of the proper position and the pattern of gastric emptying over time after feeding may lead to the development of evidence-based nursing care.