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In organic solar cells there are two competing processes

In organic solar cells there are two competing processes, extraction and Langevin type recombination of charge carriers, which both are controlled by the charge carriers’ mobility. The maximum performance of organic BHJ solar cells is governed by the balance between transport and recombination of charge carriers. Both extremes of too low or too high mobility contribute to the losses that affect the efficiency through different mechanisms. An increase in carrier mobility would have a positive effect on transport, facilitating carrier extraction, but on the other hand it will increase the bimolecular recombination strength as well 36. Moreover, The behavior could be explained by both the hot electron and the collision theory presented in 37. Based on this reported theory, the high mobility results in high carrier velocity (v = ?V) at constant voltage V. On the other hand, according to the collision theory, the probability of collision and hence recombination will increase so that some energy will transform into thermal motion. Therefore, after a certain magnitude, increasing the mobility’s value can’t result in developing the solar cell performance any more. Therefore, an optimized carrier mobility is an important condition that must be fulfilled to obtain highly efficient organic solar cells. In addition, our results show that the mobility imbalance (between electrons and holes) can affect the performance of the solar cells. As shown in the figure 6, an increase in the electron mobility, by two orders of magnitude (10-8 to 10-6) with a constant hole mobility ( = 10-8 ) leads to an enhancement in the PCE. However, a further increase in the leads to a greater imbalance between the extracted hole and electrons which ultimately increases the more recombination and suppress more increase of PCE value.