5. Simulation Study on Single-Layer Bulk-Heterojunction Solar Cells

This chapter contains a detailed simulation study on a single-layer bulk heterojunction sandwiched between two metal contacts. We apply the simulation approach discussed in the previous chapter and compare the results to the predictions of approximated analytical solutions introduced in Chap. 3. In particular we investigate the interplay between mobilities and different recombination mechanisms and their effect on the fill factor and the open-circuit voltage. In the calculations we combine various properties of the bulk heterojunction with characteristics of the contacts. We point out the role of injection barriers, built-in potential, bending of energy levels, and contact selectivity. The quest for an optimum mobility will be discussed as well. In the final part of the chapter we analyze the photocurrent (dark current subtracted from current under illumination) as a function of voltage to identify processes that limit the photocurrent. In particular, we want to clarify the following points: (a) Is there an optimum mobility for charge carriers in the active layers of an organic solar cell? (b) What order of magnitude for the mobility is required for a high-performance solar cell? (c) Does the mobility influence the open-circuit voltage? (d) What does selectivity of the contacts mean and why is it important? (e) What is the role of injection barriers at the electrodes? (f) Which processes reduce the open-circuit voltage and which do not? (g) What governs the temperature dependence of the open-circuit voltage? (h) What can be learned from the intensity dependence of the photocurrent-voltage relation? (i) What is the meaning of a point of intersection between the J-V curves in dark and under illumination? (j) What does the fill factor as a function of device thickness tell?