Transparent conductors as solar energy materials: A panoramic review

This review captures the synthesis, assembly, properties, and applications of copper chalcogenide NCs, which have achieved significant research interest in the last decade due to their compositional and structural versatility. The outstanding functional properties of these materials stems from the relationship between their band structure and defect concentration, including charge carrier concentration and electronic conductivity character, which consequently affects their optoelectronic, optical, and plasmonic properties. This, combined with several metastable crystal phases and stoichiometries and the low energy of formation of defects, makes the reproducible synthesis of these materials, with tunable parameters, remarkable. Further to this, the review captures the progress of the hierarchical assembly of these NCs, which bridges the link between their discrete and collective properties. Their ubiquitous application set has cross-cut energy conversion (photovoltaics, photocatalysis, thermoelectrics), en...

Compound Copper Chalcogenide Nanocrystals

The creation of a suitable inorganic colloidal nanocrystal ink for use in a scalable coating process is a key step in the development of low-cost solar cells. Here, we present a facile solution synthesis of chalcopyrite CuInSe2 nanocrystals and demonstrate that inks based on these nanocrystals can be used to create simple solar cells, with our first cells exhibiting an efficiency of 3.2% under AM1.5 illumination. We also report the first solution synthesis of uniform hexagonal shaped single crystals CuInSe2 nanorings by altering the synthesis parameter.

Development of CuInSe2 nanocrystal and nanoring inks for low-cost solar cells.

Copper indium gallium selenide (CIGS) based solar cells are receiving worldwide attention for solar power generation. They are efficient thin film solar cells that have achieved 22.8% efficiency comparable to crystalline silicon (c-Si) wafer based solar cells. For a production capacity of 1000 MW y−1 with 15% module efficiency, the CIGS module production cost is expected to be $0.34 W−1. For CIGS cells over glass, a graded bandgap high temperature deposition process has been established, however, this process has not been established for CIGS over flexible polymer substrates which is a low temperature process. For small area devices, the main focus is precise control over CIGS film stoichiometry and efficiency. For industrial production, apart from stoichiometry and efficiency, low-cost, reproducibility, high-throughput and process tolerance are of much importance in commercializing the technology. Due to process complexity, CIGS module production is lagging behind that of cadmium telluride (CdTe) modules. In this review article, the working mechanism of CIGS solar cells with a back surface field, the importance of developing CIGS solar cells, and the limitations for their commercialization are discussed. CIGS solar cells are compared with c-Si solar cells. After briefly reviewing the history of the chalcopyrite alloy system, graded bandgaps, effects of sodium distribution in CIGS layers, growth of CIGS layers using various techniques, role of buffer layer/transparent conducting oxides, CdS free buffer layers, concerns related to flexible solar cells, and factors affecting the cell efficiency are reviewed. Further efficiency improvement options are discussed. Cell stability, challenges, solutions and future prospects of CIGS solar cells are outlined.

Copper indium gallium selenide based solar cells – a review

Recent developments in the colloidal synthesis of high quality nanocrystals have opened up new routes for the fabrication of low-cost efficient photovoltaic devices. Previously, we demonstrated the utility of CuInSe2 nanocrystals in the fabrication of CuInSe2 thin film solar cells. In those devices, sintering the nanocrystal film yields a relatively dense CuInSe2 film with some void space inclusions. Here, we present a general approach toward eliminating void space in sintered nanocrystal films by utilizing reactions that yield a controlled volume expansion of the film. This is demonstrated by first synthesizing a nanocrystal ink composed of Cu(In1−xGax)S2 (CIGS). After nanocrystal film formation, the nanocrystals are exposed to selenium vapor during which most of the sulfur is replaced by selenium. Full replacement produces a ∼14.6% volume expansion and reproducibly leads to good dense device-quality CIGSSe absorber films with reduced inclusion of void space. Solar cells made using the CIGSSe absorber fi...

Sulfide Nanocrystal Inks for Dense Cu(In1−xGax)(S1−ySey)2 Absorber Films and Their Photovoltaic Performance

Solar cells based on copper ternary chalcogenide compounds and alloys have emerged over the last 20 years as a promising solution to the problem of high-cost solar cells. Solar power conversion efficiencies exceed 21% in laboratory devices using thin films of these materials, and their characteristic thinness results in negligible direct materials costs per unit area compared with wafers. Photovoltaic devices made from these materials have also been shown to be intrinsically stable, circumventing the historical disadvantage of degradation typical of earlier thin film solar cell technologies. However, these copper chalcogenide devices and materials are relatively complex. This article provides an overview of the current state of our scientific understanding and technological development of them.

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Billy J. Stanbery

Papers

Copper Indium Selenides and Related Materials for Photovoltaic Devices