Active Layer Materials for Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 30

Scheme 2.9 Molecular structures of DPP-based polymers

based polymers, but also DPP-based small molecular compounds exhibited very
potential photovoltaic properties. For instance, several solution-processable DPP-
based small molecules (1.5-1.8 eV) with PCEs better than 4 % have been
reported.

2.2.1.4 Benzo[1,2-b;4,5-b 0 ]dithiophene-Based Polymers

As reported, three kinds of functional groups, including alkoxy, alkyl, and alkyl-
thiophene, were used as side groups on the 4- and 8-positions of the BDT unit to
make solution-processable polymers. The alkoxy-substituted BDT can be syn-
thesized easily through a one-pot two-step reaction [ 55 ]. Typically, the yield of the
alkoxy substituted BDT was between 70 and 90 %. The alkyl-substituted BDT
compounds can be synthesized by the three-step reaction as reported [ 56 ]. The
alkylthiophene-substituted BDT compounds were synthesized by the similar
reaction for alkynyl-substituted BDTs, but 2-lithium-5-alkyl-thiophene was used
instead of alkynyl lithium [ 57 ]. BDT has a symmetric and planar conjugated
structure, and hence tight and regular stacking can be expected for the BDT-based
conjugated polymers. Therefore, BDT-based polymers were first used in organic
field effect transistors (OFETs). In 2007, a BDT-thiophene-based polymer was
reported and exhibited a hole mobility of 0.25 cm 2 V -1 s -1 , which was one of the
highest values for polymer-based OFET [ 58 ] (Scheme 2.10 ).
Band gap and molecular energy level of BDT-based polymers can be readily
tuned in a wide range by copolymerizing with conjugated building blocks with
different electron withdrawing effect. Hou et al., first designed and synthesized