Oligothiophenes and their derivatives, possessing attractive properties such as good environmental stability and excellent photoelectric properties, are important organic functional materials which can be developed greatly. In this review, the synthesis and properties of oligothiophenes and their derivatives with a focus on applications in organic photovoltaics in recent years are discussed. According to the structure, oligothiophenes and their derivatives are divided into two categories. One category includes oligothiophenes containing nonpolar groups or weak-polar groups, and the other one includes donor-acceptor (D-A) oligothiophenes and their derivatives. The D-A oligothiophene derivatives include one dimensional, two dimensional or star-shaped molecules, self-assemble molecules, donor-acceptor dyads or triads. The oligothiophene derivatives used as acceptors in photovaitic devices are also discussed. Due to the intramolecular charge transfer effect, the photophysical and electrochemical properties of D-A oligothiophene derivatives as donors are superior to those of containing weak-polar group derivatives. The D-A oligothiophene derivatives based devices have the highest photoelectric conversion efficiency (>10%) among small molecular photovoltaic materials. Finally, the main factors that affect the photovoltaic performance of devices are briefly analysised such as HOMO and LUMO energy levels, scope of spectral absorption and film morphology, etc. The open circuit voltage (V_oc) is related to the difference between the LUMO energy level of the acceptor and the HOMO energy level of the donor. The short-circuit current (J_sc) is related to the absorption range and intensity of the active layer. The morphology of the active layer can be a determining factor for the fill factor (FF) of photovoltaic devices.

Contents
1 Introduction
2 Synthesis of oligothiophene derivatives
3 Applitation of oligothiophene derivatives on organic photovoltaic devices
3.1 Low-polar oligothiophenes
3.2 Donor-accepter oligothiophene derivatives
4 Conclusion and outlook