Carsten Deibel, Daniel Rauh, Alexander Foertig
The charge carrier dynamics of organic solar cells are directly related to the loss mechanisms limiting the photovoltaic performance. In this study we derive and determine the recombination order {\Delta} of the mobile fraction of charge carriers, as it directly relevant to understand nongeminate losses of photocurrent. We compare it to the commonly reported order of decay for all charge carriers, {\delta}, which is often determined from transient measurements. For annealed P3HT:PCBM solar cells, we find that {\delta} is consistent with recombination of mobile with trapped charge carriers in tail states with a characteristic energy of about 40 meV, in analogy to previous studies. Furthermore, due to exposure of the devices to synthetic air for 30 h in the dark and, subsequently, 30 h under illumination, {\delta} increases strongly while {\Delta} decreases very slightly. The tail state energy rises to about 100 meV for 30 h oxygen exposure under illumination, implying that the tails become increasingly deeper, leading to more trapped charge carriers. The impact on the photocurrent, however, can be more easily judged by the recombination order {\Delta}, which decreases only weakly from 1.70 to 1.62, i.e. a slight shift towards trap-assisted recombination. Finally, we show an approach to reconstruct the time dependence of the photoinduced charge carrier density from 10 {\mu}s to 100 ms, based on charge extraction and open circuit voltage decay measurements. We confirm this reconstruction by comparison to transient absorption data, and show that the resulting charge carrier dynamics are consistent with the order of decay {\delta}. We discuss the interpretation of the recombination orders as figures of merit for charge carrier losses.
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http://arxiv.org/abs/1302.0367
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