Active Layer Materials for Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 20

Scheme 2.1 Molecular structures of several representative organic photovoltaic materials

mobility, and therefore, this conjugated polymer exhibited much better photo-
voltaic properties. By using P3HT as donor material blending with PCBM as
acceptor, over 4 % PCE has been well repeated by different research groups, and
P3HT has attracted much attention in the OPV filed. Interestingly, photovoltaic
properties of P3HT/PCBM system are very sensitive to the morphology of the
blend, therefore, morphology study of this system has been well investigated. For
example, by using annealing [ 7 ], post-annealing [ 8 ] or solvent annealing process
[ 9 ], slow-growth process [ 10 ], additives [ 11 ], the morphology of the blend can be
well controlled and thus photovoltaic performance of the P3HT/PCBM system can
be improved. Small molecular organic semiconductors with appropriate properties
can also be used as electron donor materials in OPV devices. ZnPc is one of the
successful examples. Unlike conjugated polymers, ZnPc has poor solubility in
commonly used solvents, thus thermal evaporation method under high vacuum is
used to make ZnPc-based solar cells.
Fullerene and its derivatives are broadly used as electron acceptor materials in
OPVs. The solubility of unsubstituted fullerene is quite poor, which limits its
application in device fabrication process. Therefore, different substituents were
introduced onto fullerene. In 1995, Hummelen and Wudl et al. reported a feasible
approach to synthesize PCBM ([6,6]-phenyl-C61-butyric acid methyl ester), one of
the most successful fullerene derivative, which exhibits excellent photovoltaic
properties as electron acceptor material.
Although, *8 % PCEs have been achieved, efficiency of OPVs is still much
lower than inorganic photovoltaic cells. New organic materials with ideal prop-
erties are still requisite. Therefore, in the following section, OPV materials will be
sorted into different groups based on functions (donor and acceptor) and molecular
structures to give a sketch of the relationship between molecular structures and
photovoltaic properties.