Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV16)
Publication date: 28th March 2016
The ideality factor of solar cells, typically determined using the light intensity vs open circuit voltage method (suns - Voc), is often used to understand the nature of charge recombination in devices. Hybrid perovskite solar cells frequently show hysteresis in their current-voltage characteristics which is thought to be related to the migration of ionic defects in the material. We show that the suns-Voc ideality factor determined from steady-state Voc values at different light intensities is not a meaningful quantity because the distribution of ionic defects in the device reaches a different equilibrium condition for each light intensity. The differing ionic distributions result in series of spatially different conduction and valence band profiles with correspondingly different charge recombination characteristics. We demonstrate that a meaningful value of the ideality factor can be determined by measuring the instantaneous Voc immediately following a given pre-biasing condition at a range of light levels. Changing the pre-biasing conditions results in measurement of the device in a different defect distribution state. Using this method we find that the ideality factor of devices varies according to the pre-biasing conditions as well as cell architecture and composition. The results show that many devices show an ideality factor closer to unity following forward biasing than after reverse biasing. This helps explain the higher fraction of radiative recombination observed under these conditions, as well as the higher nominal efficiency values measured following forward bias conditions.
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International Conference on Hybrid and Organic Photovoltaics (HOPV) is celebrated yearly in May. The main topics are the development, function and modeling of materials and devices for hybrid and organic solar cells. The field is now dominated by perovskite solar cells but also other hybrid technologies, as organic solar cells, quantum dot solar cells, and dye-sensitized solar cells and their integration into devices for photoelectrochemical solar fuel production.
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