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Processing-Dependent Growth Mechanisms and Performance Improvement of Kesterite Solar Cells

  • Author(s): Hsu, Wan-Ching
  • Advisor(s): Yang, Yang
  • et al.
Abstract

As the global population and economy grow rapidly, energy has been an increasingly difficult issue. We not only have to set our minds to developing new technologies to deliver carbon-neutral energy to prevent disastrous interference in the global climate, we also have to consider if the scale of the new energy is large enough to fulfill the growing demands, which will reach 28 TW by year 2050. Most of today's commercially available PVs used one or more rare elements. It is expected that, based on today's commercially available PVs, the production will bump into the ceiling of natural resources even before the needed scale has been reached. Kesterite solar cell composed of earth abundant elements prevents this issue.

To make kesterite solar cells realistic and cost-effective, both the cell performance and the manufacturing processes have to be effective enough, i.e. high power conversion efficiency, low cost and high throughput. Co-evaporation, hydrazine-based processing and nanocrystal film selenization are three of the processes achieving the highest performance kesterite solar cells to-date (8% ~ 12%). This study aims for high power-conversion efficiency by playing to the strengths of these processes, while also taking realistic concerns, such as process scalability and simplicity, in to account. In-depth growth mechanisms, including the phase evolutions, reaction pathways, and morphology transitions, are studied to help to manage the challenges.

By understanding how to precisely control the composition/phases of films, 8-9% cell efficiencies are demonstrated by multiple co-evaporation deposition routes. A beneficial reaction pathway is suggested for the hydrazine-based processing, which may explain the high efficiency record set by the process. Through the analysis of the selenization effect and the engineering of the film surface composition, 8.6% cell efficiency has been obtained by the nanocrystal approach. The 8.6% device was finished by all-solution process all the way from kesterite deposition to transparent electrode deposition. The high performance of this all-solution processed device shows the great potential to manufacture low-cost, high-throughput, and high-performance kesterite solar cells in the future.

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