Organic Solar Cells

Abstract

This chapter starts with a description of the characteristic electronic structure and charge transport properties of organic semiconductors. These introductory parts are followed by detailed elaborations on the working principles of organic solar cells based on the considerations of Chap. 2 . The discussions focus on processes which limit the quantum efficiency and the maximum extractable energy. Beginning with a description of the general steps involved in energy conversion, the chapter shifts focus towards real structures and materials. It concludes with more experimental aspects concerning fabrication and characterization. Whereas the first sections review very basic chemistry knowledge, the solar-cell section gives a detailed evaluation of currently used models and ideas to explain the current-voltage characteristics of organic solar cells. Some basic questions addressed are: (a) What are the major prerequisites for organic materials to show semiconducting properties? (b) What happens when molecules form a solid? What are the main differences between organic and crystalline inorganic semiconductors? (c) What is the role of the reorganization energy considering optical and electrical properties of organic semiconductors? (d) What approaches exist to describe the charge-carrier mobility in organic solids? (e) What are the steps of energy conversion in an excitonic solar cell? What is the role of the bulk heterojunction? (f) How can different recombination mechanisms be distinguished experimentally? (g) What is (the role of) the charge-transfer state? (h) Can the open-circuit voltage be related to other optical and electrical quantities accessible by experiment? (i) Does a stricter limit exist for the maximum power-conversion efficiency of organic solar cells compared to their inorganic counterparts? (j) Why does a single-layer organic solar cell show diode behavior? (k) What determines the fill factor? (l) What is the role of metal electrodes? What kind of energetic situations are possible when a metal-organic contact is formed? (m) What are common solar-cell stacks? How can they be fabricated? (n) What are the remaining challenges for making organic solar cells competitive?