Liquid Crystalline Blends in Organic Photovoltaics

Bulk heterojunction active layer in organic thin film solar cells is a typical example of mixture for functional blends. The mixture of p- and n-types of semiconductors have to form an appropriate geometry of two compounds to make effective charge transport paths for hole and electron as well as effective interfaces at which charges are generated with less efficient recombination. In addition, excitons generated by photon abosorption have to migrate for a certain distance to reach to the interface and it also needs an appropriate path. 

Most of BHJ active layers studied so far for conjugated polymers are not of well-controlled geometry for p-n junction in a layer. Some results have indicated for the systems of alkylated conjugated polymers with fullerene derivatives that fullerene molecules are positioned in forests of alkyl chains as schematically drawn in Fig. 8.9 [39,40]. In small molecule system, it is reasonably expected that one could obtain the better clearance of nano-scale periodical structures and more detailed information of structure-property relationship [41]. In particular, by use of mesomorphic state, one could control such structures and thus, could obtain more useful information to improve the performance. 

The first application of discotic liquid crystal to organic thin film solar cells is of a binary blend of liquid crystalline phthalocyanine (Pc) as p-type of semiconductor and non- mesogenic PTCDI derivative as n-type [45], where the active layer was not a BHJ type, but had a p-i-n junction (Fig. 8.11). The Pc layer was fabricated by spin-coating and heated up to the isotropic phase followed by cooling to obtain a spontaneously formed homeotoropic alignment on ITO-coated substrate. The n-type layer was deposited by sublimation on Pc layer and finally the counter A1 electrode was deposited in vacuo. The performance was shown simply as short circuit current, Isc = 0.4 mA cm , open circuit voltage, Vqc = 0.3 V and external quantum efficiency, EQE 0.5 %. 

A simple BHJ solar cell was fabricated with C6PCH2 and PCBM by spin coating. For a 60 40 wt% blend of C6PCH2 and PCBM, power conversion efficiency (PCE) showed 3.1 % and 70 % of EQE was observed at Q-band region. Studies on the binary mixtures of C6PCH2 and PCBM indicate that the addition of PCBM into C6PCH2 leads to deformation of columnar lattice in the Col mesophase with decrease of isotropization temperature. At ca. 25 mol % of PCBM, one can see the eutectic point where the Col mesophase disappears. This molar ratio implies in the crystal blend that chain-like arrangement of PCBM is positioned 

On the other hand, porphyrin is an analogous compound to Pcs. Therefore, porphyrins have also strong absorption bands in both UV and visible region such as Soret and Q bands. Recently, mesogenic porphyrins were synthesized for thin 

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