Temperature Dependence of Field-Effect Thermoelectric Power in Rubrene Crystals

Abstract

Gate-induced carrier concentration dependence of electrical conductivity σ and thermoelectric power S of rubrene crystals is investigated down to low temperatures (T) using laser heating, and the relations between σ and S are discussed using the T dependence. With lowering T, σ decreases gradually, but S is basically metal like, though activated S ∝ E a S/T dependence appears at low V G. The logarithmic S ∝ ln σ relation is valid at respective T, where the slope is T independent and related to the ratio of S and σ activation energies E a S/E a σ. According to Lang’s method, the low-energy trap density of states is estimated from E a S. Following Kang and Snyder, the V G-dependent Fermi energy E F is determined from S in the S ∝ T region. When s in the power law S ∝ σ–1/s does not depend on T, the T-dependent transistor characteristics given by a two-variable function σ­(V G, T) is separated into a product of V G-dependent and T-dependent parts.

Realistic simulation of thermoelectric characteristics of organic semiconductors based on electronic structure calculations

“…(Color online) Calculation results of the Seebeck coefficients (left axis) and sheet conductivities (right axis) of pentacene (a) and rubrene (b) for the sheet carrier concentrations. Lines correspond to our calculation results and markers correspond to experimental results [ ▴,32) ▾, 33) ♦,34) •, 35) ×36) ].…”

Advances in Organic Thermoelectric Devices for Multiple Applications

“…By applying an electrical field in OTE device, the Fermi-level position as well as the charge carrier density in the organic semiconductor layer will be largely modified, which will significantly affect the device thermopower and conductivity, thus modulating the TE conversion. Moreover, in contrast to traditional chemical doping which is difficult to in situly control the doping level with a fixed Fermi level position, the utilization of field-effect provides a more controllable way to obtain more degrees of in situ charge transport information from both transistor characterizations and entropy measurements, [67][68][69][70][71][72][73][74][75][76][77][78][79][80][81] enabling the accurate comparison of experiments with various charge transport models for conjugated polymers including the variable range hopping, Mott mobility edge, (semi)metallic transport, etc. In particular, the electrical field-effect modulation can be realized by gating in the well investigated (organic field-effect transistor) OFET or OECT geometry which are fundamentally compatible with the OTE devices in terms of fabrication techniques and materials, while the temperature gradient is generally established along the horizontal direction by placing two parallel Peltier blocks below the insulating substrates of the vertical transistor architecture and the gate voltage tuned Seebeck signal can be measured from the source and drain.…”