Rational design on n-type organic materials for high performance organic photovoltaics

The demand for further optimization of the photovoltaic efficiency has stimulated an intensive research effort both for new low-band-gap polymeric materials as electron donors and for new efficient electron-accepting materials. As regards the latter, less attention has been observed on the optimization of the electron acceptor material compared to the extensive studies on the electron donor polymer in organic photovoltaic (OPV) cells. The majority of the acceptor materials used so far in solar cells are organic (carbon based) materials, however other traditional acceptor materials include inorganic semiconductors, such as cadmium selenide (CdSe) nanocrystals, titanium oxide (TiO2) and zinc oxide (ZnO) nanoparticles. From the implemented organic materials high power conversion efficiencies PCEs (>9.0%) are observed in OPVs utilizing fullerene derivatives and especially [6,6]-phenyl-C-61 butyric acid methyl ester (PC60BM) as the acceptor. Recently, very promising efficiencies of similar to 5.0% have been also obtained by Polyera corporation utilizing soluble conjugated polymers as both the donor and the acceptor components. It is therefore expected that through improved materials design for enhancing the electron mobility, better tuning of the energy levels and absorption profile of organic materials, significant improvements in the device performance can still be expected. In this work, the current trends on the most promising solution processable n-type organic materials (fullerene derivatives, small molecules and conjugated polymers) will be presented in detail, emphasizing on the correlation between structure/optoelectronic properties/morphology characterization and device performances.