Organic and excitonic solar cells

Bulk heterojunction organic solar cells (OSCs) are an attractive alternative to silicon based solar cells due to low cost solution fabrication processes. The efficiencies and lifetimes of OSCs are not yet high enough for them to be commercially viable, but significant research effort has been devoted to improving the performance of OSCs through materials innovation and new device designs. It is known that the optical absorption in organic solar cells is limited due to the mismatch between the charge transport scale and optical absorption depth. There are two major research approaches aimed at improving the performance of OSCs. One approach is focused on the development of new organic compounds with tailored energy levels in order to increase the efficiency of OSCs through improved spectral response over the long wavelength region. The other is to realize broadband absorption enhancement in OSCs through light trapping effects. After a brief overview of basics of OSCs, the charge transport properties, different approaches including incorporating metal nanoparticles and surface plasmonic structures to enhancethe absorption in OSCs will be discussed. Apart from the encouraging results in achieving high efficiency for OSCs, realizing stabile OSC performance over a long operation lifetime also attracts a significant research effort. A comprehensive study on the degradation mechanisms of OSCs is also presented in the last part of the chapter.