Abstract
Cu2ZnSnS4 nanocrystals are annealed in a Se-rich atmosphere inside a transmission electron microscope. During the heating phase, a complete S-Se exchange reaction occurs while the cation sublattice and morphology of the nanocrystals are preserved. At the annealing temperature, growth of large Cu2ZnSnSe4 grains with increased cation ordering is observed in real-time. This yields an annealing protocol which is transferred to an industrially-similar solar cell fabrication process resulting in a 33% increase in the device open circuit voltage. The approach can be applied to improve the performance of any photovoltaic technology that requires annealing because of the criticality of the process step.
Highlights
The current energy challenge facing our society is exemplified in Shell’s Sky scenario which projects that global electricity generation by 2070 will be nearly 5 times greater than today’s level.[1]. This increased capacity will be driven, in part, by the proliferation of new and distributed applications in healthcare, transportation, construction, aerospace, and consumer devices. The growth in these applications defines a new era for photovoltaics because of the demand for innovative and sustainable power sources that can be integrated with high value products
This creates a significant opportunity for inorganic thin film photovoltaics because of their stability and compatibility with a variety of flexible substrates including foils, plastics, and ultrathin glass.[2]
A critically important step to achieve high performance in the fabrication of all thin film photovoltaic absorbers, including CZTS, is an annealing step performed at high temperature
Summary
Radiative recombination of photogenerated charge carriers.[10,11] Recently, this process has been studied using in situ X-ray diffraction;[12−14] all these studies were performed on bulk films prepared from stacked nanocrystals. The grain growth is accompanied by an improvement in the crystallinity of the material and cation ordering within the kesterite structure To promote this ordering and reduce the density of defects in a CZTS thin film photovoltaic absorber, we transferred the insights obtained from the in situ TEM experiment to a photovoltaic device fabrication step that more closely resembles an industrial process. The insights obtained at the nanometer scale in the TEM were subsequently exploited to increase the open circuit voltage of kesterite solar cells via reduction of cation disorder These experiments demonstrate a new approach to improve the performance of any photovoltaic technology that requires thermal annealing in the device fabrication.
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