Carbon-Impurity Affected Depth Elemental Distribution in Solution-Processed Inorganic Thin Films for Solar Cell Application

A common feature of the inorganic thin films including Cu­(In,Ga)­(S,Se)₂ fabricated by nonvacuum solution-based approaches is the doubled-layered structure, with a top dense inorganic film and a bottom carbon-containing residual layer. Although the latter has been considered to be the main efficiency limiting factor, (as a source of high series resistance), the exact influence of this layer is still not clear, and contradictory views are present. In this study, using a CISe as a model system, we report experimental evidence indicating that the carbon residual layer itself is electrically benign to the device performance. Conversely, carbon was found to play a significant role in determining the depth elemental distribution of final film, in which carbon selectively hinders the diffusion of Cu during selenization, resulting in significantly Cu-deficient top CISe layer while improving the film morphology. This carbon-affected compositional and morphological impact on the top CISe films is a determining factor for the device efficiency, which was supported by the finding that CISe solar cells processed from the precursor film containing intermediate amount of carbon demonstrated high efficiencies of up to 9.15% whereas the performances of the devices prepared from the precursor films with very high and very low carbon were notably poor

Atom-Scale Chemistry in Chalcopyrite-Based Photovoltaic Materials Visualized by Atom Probe Tomography

Chalcopyrite-based materials for photovoltaic devices tend to exhibit complex structural imperfections originating from their polycrystalline nature; nevertheless, properly controlled devices are surprisingly irrelevant to them in terms of resulting device performances. The present work uses atom probe tomography to characterize co-evaporated high-quality Cu­(In,Ga)­Se2 (CIGS) films on flexible polyimide substrates either with or without doping with Na or doping with Na followed by K via a post-deposition treatment. The intent is to elucidate the unique characteristics of the grain boundaries (GBs) in CIGS, in particular the correlations/anti-correlations between matrix elements and the alkali dopants. Various compositional fluctuations are identified at GBs irrespective of the presence of alkali elements. However, [Cu-poor and Se/In,Ga-rich] GBs are significantly more common than [Cu-rich and Se/In,Ga-poor] ones. In addition, the anti-correlations between Cu and the other matrix elements are found to show not only regular trends among themselves but also the association with the degree of alkali segregation at GBs. The Na and K concentrations exhibited a correlation at the GBs but not in the intragrain regions. Density functional theory calculations are used to explain the compositional fluctuations and alkali segregation at the GBs. Our experimental and theoretical findings not only reveal the benign or beneficial characteristics of the GBs of CIGS but also provide a fundamental understanding of the GB chemistry in CIGS-based materials

Hierarchical Silver Network Transparent Conducting Electrodes for Thin-Film Solar Cells

Flexible metal network transparent conducting electrodes (TCEs) are expected to be the most promising candidates to replace indium tin oxide (ITO) due to their excellent electro-optical performance and mechanical flexibility. However, to successfully replace ITO with the metal network TCEs, more studies on their suitability for integration with real devices are needed. In this study, we developed a hierarchical silver network simultaneously meeting the requirements of (i) low sheet resistance, (ii) high optical transmittance, (iii) excellent mechanical flexibility, and (iv) good integration into a thin-film solar cell. The hierarchical silver network consists of a silver micromesh as the main framework and silver nanowires as the secondary framework. The hierarchical network provides a figure of merit similar to that of the individual micromesh and much higher than those of silver nanowires and ITO. When applied to Cu­(In, Ga)­Se2 thin-film solar cells, the hierarchical network achieved better device performance than the micromesh. In the hierarchical network, the micromesh enables low sheet resistance and the silver nanowires enable excellent integration with the device while maintaining high optical transmittance. Thus, considering the aforementioned requirements, the hierarchical network could be one of the best candidates as a TCE for Cu­(In, Ga)­Se2 thin-film solar cells

Hierarchical Silver Network Transparent Conducting Electrodes for Thin-Film Solar Cells

Flexible metal network transparent conducting electrodes (TCEs) are expected to be the most promising candidates to replace indium tin oxide (ITO) due to their excellent electro-optical performance and mechanical flexibility. However, to successfully replace ITO with the metal network TCEs, more studies on their suitability for integration with real devices are needed. In this study, we developed a hierarchical silver network simultaneously meeting the requirements of (i) low sheet resistance, (ii) high optical transmittance, (iii) excellent mechanical flexibility, and (iv) good integration into a thin-film solar cell. The hierarchical silver network consists of a silver micromesh as the main framework and silver nanowires as the secondary framework. The hierarchical network provides a figure of merit similar to that of the individual micromesh and much higher than those of silver nanowires and ITO. When applied to Cu­(In, Ga)­Se2 thin-film solar cells, the hierarchical network achieved better device performance than the micromesh. In the hierarchical network, the micromesh enables low sheet resistance and the silver nanowires enable excellent integration with the device while maintaining high optical transmittance. Thus, considering the aforementioned requirements, the hierarchical network could be one of the best candidates as a TCE for Cu­(In, Ga)­Se2 thin-film solar cells