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Large-scale implementation of solar power generation requires photovoltaic (PV) devices with efficiency-to-cost ratios better than existing silicon-based solar devices. Next generation devices depend on new materials and device structures. Thin-film PV devices, also called Second Generation devices, while becoming competitive with Si-based technologies, face unresolved technical challenges in achieving their potential for high efficiency. Additionally, the current generation of commercial thin-film solar cells based on cadmium telluride (CdTe) or copper indium gallium (di)selenide (CIGS) incorporate relatively rare elements that might become the limiting factor for wide scale deployment. New photovoltaic devices based on earth-abundant elements are actively being sought. All devices based on inorganic thin films feature sophisticated microscale and nanoscale material structure, grain boundaries, and defects that can be either beneficial or detrimental for the overall device performance.
In photovoltaic cells, light absorption creates free excitons that subsequently separate into free electrons and holes under the influence of the electric field of a p-n junction. In first generation cells, light absorption is achieved with an indirect bandgap material (Si) with low quantum absorption efficiency, whereas second generation cells rely on direct bandgap materials (including CdTe and CIGS) and consequently have significantly relaxed criteria on the required thickness for efficient light absorption. To generate useful power, the free carriers must leave the PV device and reach their respective contacts. Although the thickness of second generation cells, and consequently the required carrier diffusion lengths, are roughly two orders of magnitude smaller than Si-based cells, the efficiencies of these second generation cells remain below those of Si cells. This shortcoming is primarily associated with the enhanced charge trapping and reduced lifetime of carriers caused by the significant defect-induced recombination of compound semiconductors. Metrology capable of identifying charge trapping states inside thin-film PV materials, heterojunctions, surfaces and contact interfaces and of determining their impact on device performance could significantly contribute to PV development efforts.
Lead Organizational Unit:cnst
National Institute of Standards and Technology
Daniel Josell (NIST/MML)
National Renewable Energy Laboratory
Nikolai Zhitenev, Phone 301-975-6039NIST
100 Bureau Drive, MS 6204
Gaithersburg, MD 20899-6204