Jan Christoph Goldschmidt, the team head for novel solar cell concepts at German research institute the Fraunhofer ISE, has shown the results of a series of outdoor tests conducted to assess how high temperature affects the performance of perovskite and silicon tandem solar cells. “For emerging solar cells such as perovskite-based tandem cells, the influence of spectral and, to a larger extent, temperature variation on solar cell performance is not yet fully understood,” Goldschmidt explained.
The energy yield of two-terminal tandem cells is maximized when the two sub-cells produce the same current at the maximum power point. By contrast, when one of the two devices generates less current than the other, and current mismatch between the sub-cells occurs, the overall device’s current is affected. “The output current of a sub-cell is determined by the bandgap of the absorber but this may vary under operating conditions,” the scientist stated, adding that current mismatch is responsible for affecting the overall cell’s optimal bandgaps.
The Fraunhofer experts tested an encapsulated dummy tandem cell in an unspecified hot and sunny climate and, with the help of meteorological data, they simulated the device’s temperature over an entire year. These measurements showed, according to Goldschmidt, that the cell performance is not only affected by voltage losses, as in other kinds of solar cells, but also by the aforementioned current mismatch.
Reducing the bandgap of the perovskite sub-cell is pointed out as a possible measure to reduce the impact of temperature on this type of tandem cells. “They find that instead of the 1.73 electronvolt (eV) optimal bandgap value for the perovskite determined from idealized theoretical analysis, a bandgap value of 1.685 eV should lead to a higher energy yield under the temperature conditions experienced in the field,” Goldschmidt affirmed. Ideal bandgaps should be in the range of 1.65 electronvolts.
The findings of the tests are presented in the paper Tandem cells under the weather, published in nature energy.
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