**2.1 Organic semiconductors**

Organic semiconductors, which are generally considered as intrinsic wide band gap semiconductors (band gap>1.4 eV), have many advantages to be used in solar cells. For example, organic semiconductors of which electronic band gap can be engineered by chemical synthesis with low-cost (Günes et al., 2007) have generally high absorption coefficients.

Organic semiconductors consist of different chemical structures (Nunzi*,* 2002) including polymers, oligomers, dendrimers, dyes, pigments, liquid crystals (Yilmaz Canli et al., 2010) etc. In carbon-based semiconductors, conductivity is obtained by conjugation, which single and double bonds between the carbon atoms alternate (Pope & Swenberg, 1999).

Conjugated organics are challenging materials for solar cells owing to their semiconducting and light absorbing features. As a compound of organic solar cells, organic semiconductors can be processed by thermal evaporation techniques or solution based coating or printing techniques at low temperatures (Deibel & Dyakonov, 2010).
