ABSTRACT
The use of organic materials for photovoltaics has been started for several decades. In the starting age, the device is usually prepared by simply inserting one single kind of molecular ›lm between two metal electrodes.3 Due to poor charge separation and collection, the ef›ciency of the device is extremely low. The ›rst breakthrough of the development of organic solar cells was the use of a heterojunction structure by Tang in 1986, in which two kinds of molecules were used as electron transporter and hole transporter, respectively.4 The charge separation and collection ef›ciency was signi›cantly improved by separating the electron and hole transporting function into two different kinds of materials. Based on the planar heterojunction structure, an optimized edition, that is, a bulk heterojunction structure, was introduced for small molecules by Hiramoto et al.5 By increasing the interface area between the donor and acceptor materials, the charge separation ef›ciency was much improved. Meanwhile, the use of fullerene derivatives as the electron acceptor signi›cantly improved the electron transport of the active ›lm.6 Following the device structure revolution, the advancement of the molecular design and ›lm morphology brought the next big jump in device performance. Up to now, the highest ef›ciency of single-junction organic polymer solar cells is up to 11.1% (Mitsubishi), while for tandem solar cells based on small molecules, the highest ef›ciency is 12% (Heliatek). The highest ef›ciency of organic solar cells is now close to the performance of generally used thin-›lm solar cells, and commercialization is already in progress.
