Solar cells photoinduced charge transfer

It is the purpose of this chapter to introduce photoinduced charge transfer phenomena in bulk heterojunction composites, i.e., blends of conjugated polymers and fullerenes. Phenomena found in other organic solar cells such as pristine fullerene cells [11,12], dye sensitised liquid electrolyte [13] or solid state polymer electrolyte cells [14], pure dye cells [15,16] or small molecule cells [17], mostly based on heterojunctions between phthalocyanines and perylenes [18] or other bilayer systems will not be discussed here, but in the corresponding chapters of this book. 

Fig. 4 Photoinduced charge transfer from a donor (here PPV) to an acceptor (here Ceo) serves as a highly efficient charge separation mechanism in most polymer solar cells [26]...

Polymer-polymer solar cells employ two different polymers as donor and acceptor components in the photoactive layer. These two polymers require a molecular energy level offset between their HOMO and LUMO levels to enable a photoinduced charge transfer. Due to the close vicinity of the respective molecular energy levels, polymer-polymer solar cells allow high open circuit voltages to be reached. 

Working principles of organic solar cells are well described in a recent review and some monographs. More or less all types of organic solar cells described above comprise two components in the photoactive layer. One component serves as electron donor, whereas the other works as electron acceptor. Absorption of photons by the active layer components results in the electron transfer from donor to acceptor. This process called photoinduced charge transfer is a fundamental principle of operation of all known organic photovoltaic devices as well as the natural photosynthetic systems. In many cases, donor material is capable of efficient p-type transport and therefore can be called as p-type organic semiconductor. At the same time, electron acceptor material is denoted as n-type semiconductor in many cases. 

In this section we review theoretical and experimental results important for understanding the photophysics of polymer solar cells. We examine the states that are involved in the processes of photoinduced charge generation, and present a theoretical framework that describes the separation and recombination of charge-transfer states. Finally, we review the mechanisms for charge formation that operate in a fdm of a single conjugated polymer and the additional mechanisms, relevant to solar cell devices, that occur in binary donor acceptor blends. 

We present our recent research of excitation transfer and charge separation in conjugated polymers. We continue by summarizing our studies of ultrafast photoinduced processes in dye-sensitized nanocrystalline large band-gap semiconductor films - a key part of the GrStzel solar cell. Finally, our recent studies of energy transfer in transition metal supramolecular complexes, a kind of artificial antenna, are presented. 

---