Unifying energetic disorder from charge transport and band bending in organic semiconductors

Abstract

Characterizing the density of states (DOS) width accurately is critical in understanding the charge-transport properties of organic semiconducting materials as broader DOS distributions lead to an inferior transport. From a morphological standpoint, the relative densities of ordered and disordered regions are known to affect charge-transport properties in films; however, a comparison between molecular structures showing quantifiable ordered and disordered regions at an atomic level and its impact on DOS widths and charge-transport properties has yet to be made. In this work, for the first time, the DOS distribution widths of two model conjugated polymer systems are characterized using three different techniques. A quantitative correlation between energetic disorder from band-bending measurements and charge transport is established, providing direct experimental evidence that charge-carrier mobility in disordered materials is compromised due to the relaxation of carriers into the tail states of the DOS. Distinction and quantification of ordered and disordered regions of thin films at an atomic level is achieved using solid-state NMR spectroscopy. An ability to compare solid-state film morphologies of organic semiconducting polymers to energetic disorder, and in turn charge transport, can provide useful guidelines for applications of organic conjugated polymers in pertinent devices. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim