Polymer/Macromolecule Semiconductor Building Blocks

Polyacetylenes exhibit semiconducting-insulating properties by a lack of free electrons (carriers), although there are conducting pathways through π-conjugations. Shirakawa, MacDiarmid and Heeger et al. in 1977 observed that polyacetylenes show high electrical conduction comparable with a metal when the insulating polyacetylenes obtain carriers by a bromine doping.¹⁾ After their observation, studies of conducting polymers were dramatically enhanced, and some of the representative polymers were practical for electronic equipment.

In 1990s, an organic light-emitting diode (OLED) of polyphenylene vinylene (PPV) was reported and then semiconducting polymers attracted us in this research area.²⁾ The PPV is one of the light-emitting polymers (LEP) and the functionality is due to the semiconducting property of a π-conjugated polymer without a chemical doping. A chemical modification of a π-conjugated polymer can create various colored emissions. For instance, polyfluorenes (PFO),³⁾ PPVs⁴⁾ and regiorandom poly(3-octylthiophene) (P3OT)⁵⁾ exhibit blue, green and red emissions, respectively.

Recent research on organic electronics developed sulfur-containing polymers such as polythiophene. Poly(3,4-ethylenedioxythiophene) (PEDOT) is useful for a capacitor, organic transistor, hole transport material for an OLED device, and secondary battery as well as for an actuator, sensor and thermoelectric conversion element.^6,7) PEDOT/PSS that is mixture of PEDOT and polystyrene sulfonate (PSS), is widely used as a hole transport material for organic photovoltaics (OPV).⁸⁾ Poly(3-hexyl)thiophene (P3HT) can be blended with fullerene derivatives (eg. PCBM) to form an efficient bulk heterojunction. The polymer for the OPV device works as a p-type semiconductor with light absorption.

On the other hand, some thiophene-based polymers exhibit a high energy level of the highest occupied molecular orbital (HOMO), because they are electron-rich. One problem of the thiophene-based polymers is their sensitivity for electrochemical oxidation. In order to solve the problem, one can combine an electron-rich monomer (donor) and an electron-deficient monomer (acceptor) to decrease the HOMO energy level and then stable donor-acceptor (DA type) polymers can be produced.^10-12) The DA type polymer usually shows a low band gap state which can absorb light of long wavelength. Furthermore, several DA type polymers are ambipolar enabling both p- and n-dopes.^13,14)