**2. Excitonic polymer-based solar cells**

Polymer-based bulk-heterojunction solar cells (BHSC) have already shown certified efficiencies above 8 % demonstrating ability to compete with inorganic solar cell systems (eg. amorphous silicon cells fabricated on flexible substrates). Efficiencies exceeding 10% for solution processed solar cells are expected to be achieved soon.(Nayak et al., 2011) The power conversion efficiency of BHSCs is determined by the photophysical processes under operational conditions. A fundamental understanding of the relation between light absorption charge separation, charge transport, recombination, and film nanostructure as well as between the various thin film fabrication and processing parameters (such as solvent composition, solution concentration, deposition atmosphere and process temperature) is needed for further improvements. These important parameters can be controlled to some extent by adjusting the required film composition or device structure.(Gunes et al., 2007)

### **2.1 Solar cell device structure**

The typical device structure of most organic optoelectronic devices, including organic light-emitting diodes and solar cells, is shown in Fig. 1. The front electrode is based on a transparent conducting oxide, such as indium tin oxide (ITO), that serves as the high-work-function, positive electrode.(Brabec et al., 2001) To further improve the quality of the ITO electrode and aid hole (positive charge carrier) extraction from the active film, poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT-PSS) layer (tens of nanometers thick) is coated on top, forming a smooth surface which is essential in thin film devices. The photoactive film, the donor and acceptor bulk-heterojunction blend is deposited on top of the PEDOT-PSS layer. The whole device is finished by thermally evaporating the back contact (negative electrode) under high vacuum. To achieve the built-in electric field needed for most devices to operate, the back electrode must be made from a low-work-function metal that serves as the negative electrode. In the operation of a typical polymer-fullerene bulk-heterojunction solar cell, electrons generated in the active layer are collected by the back electrode (anode), and holes are collected at the opposite electrode (cathode).
