New Developments in the Photonic Applications of Conjugated Polymers

Abstract

I. Semiconducting Polymers as Materials for “Plastic” Photonics Devices Solid-state photonic devices are a class of devices in which the quantum of light, the photon, plays a role. Because the interband optical transition (absorption and/or emission) is involved in photonic phenomena and because photon energies from near-infrared to near-ultraviolet are of interest, the relevant materials are semiconductors with band gaps in the range from 1 to 3 eV. Typical inorganic semiconductors used for photonic devices are Si, Ge, and Group III-V and Group II-VI alloys.1 Conjugated polymers are a novel class of semiconductors that combine the optical and electronic properties of semiconductors with the processing advantages and mechanical properties of polymers. Important examples of polymers within this class include poly(p-phenylenevinylene) (PPV), poly(p-phenylene) (PPP), and polyfluorene (PF) derivatives whose molecular structures are shown in Figure 1. The relative simplicity with which high photoluminescence (PL) efficiency polymers of different colors can be achieved is in stark contrast to inorganic semiconductors, where, for example, bright blue light emitting diodes (LEDs) were not available until recently because of the difficulties in growing InGaN films.2 Most of the photonic phenomena known in conventional inorganic semiconductors have been observed in these semiconducting polymers. The dream of using such materials in high-performance “plastic” photonic devices is rapidly becoming reality: high-performance photonic devices fabricated from conjugated polymers have been demonstrated, including diodes,3 light-emitting diodes,4 photodiodes,5 field-effect transistors,6 polymer grid triodes,7 light-emitting electrochemical cells,8 and optocouplers,9 i.e., all the categories that characterize the field of photonic devices. These polymer-based devices have reached performance levels comparable to or even better than those of their inorganic counterparts. For a recent review of progress in plastic photonic devices fabricated with semiconducting polymers, see ref 10.