The polymer-based concept for building OPVs presents a good balance between technical complexities and production costs as compared to the high-vacuum deposited small molecule and Gratzel-device concepts; presented earlier. However, the conjugated or resonating double-bonds of conducting polymers, which are polar in nature, are very sensitive to the shorter wavelength of the solar spectrum and these bonds can be broken if the energy of the solar light lies within a certain ultraviolet (UV) range. Therefore, new types of organic systems are required to overcome this limitation, and make efficient use of a wide part of the solar spec­trum. Discotic liquid crystals (DLCs) are expected to fill this role as an alternative to the conducting polymers. The name “discotic” describes materials formed from disk-like molecules (discogens) which possess a central planar aromatic core to which peripheral aliphatic chains are attached. Thermal fluctuations of the chains, or tails, are sufficient to suppress inhomogeneously-distributed structural traps, giving rise to a “liquid-like” dynamic disorder of the tails. The cores can be formed by triphenylene, coronene or phthalocyanine molecules (Fig. 6.6).

When Chandrasekhar, observed the thermotropic DLCs for the first time in 1977 [12] he explained that the term “liquid crystal” signifies a state of aggregation that is intermediate between the crystalline solid and the amorphous liquid [13]. Materials that form liquid-crystalline phases are called mesogens and different phases can be distinguished within the liquid-crystalline state. These are nematic, smectic, cho­lesteric, and discotic phases. Figure 6.7 depicts schematically the nematic and columnar phases. The nematic phase has a one-dimensional orientational ordering and is subject to positional disorder. The columnar hexagonal phase however, exhibits both orientational and positional ordering, where the molecular core-on-

Fig. 6.6 Molecular schematics of triphenylene (left), coronene (middle) and phthalocyanine (right)

core stacking forms a hexagonal array of columns. The conductivity in this phase is due to the stacking between neighbouring disc-shaped molecules as a consequence of the n-n overlap originating from the delocalized п-orbitals above and below each aromatic core and interactions between the aliphatic chains. These chain-chain interactions ensure self-assembly of the discotic phase rather than a herringbone arrangement that characterizes systems with short chains. The n-n overlap, which defines the co-facial distance/separation between two discs in a stack, provides a one-dimensional pathway for charge migration (conductivity) along the column direction (stacking axis) and is normally thought to be the only part of the charge — transport system. The distance between the columns on the other hand is controlled by the length of the aliphatic chains forming the molecular tails. The highly anisotropic character of the electrical conductivity having its axial component greater than the in-plane makes these molecular wires attractive, offering features such as charge transfer for molecular-conducting devices including photovoltaic applications. Further, the columnar hexagonal positional ordering can be enhanced towards a helical type ordering (Fig. 6.7) which improves further the conductivity.