TRANSPORT OF CARRIERS IN A SEMICONDUCTOR POLYMER RANDOM CHAIN - A RENORMALIZATION APPROACH

We present a theoretical study for the quantum transport of carriers i n a random semiconductor polymer poly(p-phenylene vinylene) (PPV) chai n with the benzoid rings partially replaced by quinoid rings. A renorm alization approach maps the polymeric chain into an effective one-dime nsional lattice and the transmission coefficient as a function of ener gy is computed by transfer matrix techniques. In the case of randomly- distributed quinoid rings the spectrum shows narrow peaks in the condu ction and the valence Lands which indicate the presence of special res onant electron and holes states, lying within a larger fraction of loc alized states. The obtained density of states (DOS) shows a bandgap sm aller than the transmission gap between the electron and the hole reso nance peaks, which implies that an electric voltage larger than the ba nd gap is required to inject electrons and holes into the polymer. Thi s could explain why the operating voltage of PPV light-emitting diodes (LEDs) is larger than the semiconductor gap, in sharp contrast to con ventional semiconducting LEDs.