The material used to absorb the solar light in organic solar cells, is an organic material such as a conjugated polymer. The basic principle behind both the polymer solar cell and other forms of solar cells, however, is the same, namely the transformation of the energy in the form of electromagnetic radiation (light) into electrical energy (a current and a voltage), i.e. a physical phenomenon called the photovoltaic effect. This energy conversion is possible with the use of semiconductors. Semiconductors are a group of materials which are in-between an insulator and a conductor. Silicon is a classic example of a semiconductor and is also the material that is currently used in most solar cells, i.e. the first generation solar cells.
Source : http://www.plasticphotovoltaics.org/lc/lc-polymersolarcells.html