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On the role of excess energy on free charge generation and recombination in polymer:fullerene blends

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Pi-conjugated materials are among the most promising classes of semiconductors for future thin film organic photovoltaics cells. Compared to the nowadays used inorganic semiconductors, organic semiconducting layers exhibit significantly higher absorption coefficients but also much lower mobilities, the latter origin from significant positional and energetic disorder in organic thin films. One prominent consequence of disorder is dispersive transport, where charge carrier motion progressively slows down due to occupation of states in the lower part of the density of states distribution. Given the fact that the average energy of incident photons under sun illumination is much larger than energy of the final electron-hole pair, the question needs to be addressed whether non-thermalized carrier contribute to free carrier generation and recombination under solar cell working conditions, and whether concepts based on local equilibrium considerations are applicable to organic solar cells.

In this talk I will present results of recent experiments regarding the generation and recombination of free charges in selected polymer:fullerene bulk heterojunction solar cells. Time-delayed collection experiments performed as function of field, excitation energy and temperature reveal an overwhelming effect of fullerene ordering on the efficiency of free charge generation, while excess energy seems to be of minor importance. On the other hand, free charge recombination is dispersive in some highly-disordered BHJ layers, while the steady state recombination seems to be dominated by thermalized carriers. We, therefore, propose that the charge generation and recombination mainly involves thermalized states. In accordance to this finding, the JV characteristics of most of our devices can be well explained by a modified Shockley-equation.

This talk is part of the Optoelectronics Group series.

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