8. The Model System ZnPc:C\(_{60}\)

The topic of this chapter will be a detailed investigation of current-voltage data of the model system ZnPc:C\(_{60}\). We start by explaining the influence of the donor:acceptor mixing ratio on the open-circuit voltage. Then, we investigate a possible optimization of the morphology in the bulk heterojunction by using a vertical concentration gradient of donor and acceptor. The main focus of the further parts of this chapter is on the role of recombination and charge transport governing the power-conversion efficiency. By manipulating spatial absorption profiles in the bulk heterojunction we conclude that charge extraction and recombination between charges are the competing processes limiting the fill factor. An investigation of the open-circuit voltage as a function of illumination intensity allows us to discriminate between direct, indirect, and surface recombination. In particular, this chapter addresses the following questions: (a) What are possible explanations for different open-circuit voltages of devices consisting of the same active material system (e.g. when changing the mixing ratio)? (b) What are simple experiments to identify the correct reason? (c) Which explanations exist for a changed donor-acceptor gap in a bulk heterojunction based on the same materials? (d) Why is it interesting to investigate graded junctions? (e) What are possible explanations for the fill factor depending on the color of the illumination? What is the role of charge-carrier mobility? (f) Why does the internal quantum efficiency depend on the wavelength of the incident light and on the thickness of the optical spacer? What is the consequence for the spectral mismatch factor? (g) What point of the J-V curve is a good choice to investigate the dominating recombination mechanisms in a solar cell? How can this be done? (h) What does the diode ideality factor tell?