Principles and Applications of Extrinsic (Doped) Organic Semiconductors – Calvin Chan

Date: Mon. February 1st, 2010, 12:30 pm-1:30 pm
Location: Rockefeller 221

Organic semiconductors have garnered much attention for many promising applications, including organic light-emitting diodes, photovoltaic cells, thin-film transistors, thin-film batteries and spintronic devices. Despite this demand, robust and efficient organic electronic devices have been limited by the quality of organic semiconductor materials and a poor understanding of their underlying physics. Extrinsically doped organic semiconductors provide an avenue to overcoming the intrinsic material limitations that impede device performance. Moreover, fundamental doping studies provide insight into the nature of molecular charge transfer between organic moieties. For example, host molecules doped with highly electropositive donor species result in pronounced shifts of the Fermi-level towards the unoccupied molecular states [1]. The associated increase in electron concentration can be correlated to filling of a Gaussian distributed density of states via a Fermi-Dirac distribution [2]. Current densities increase by several orders of magnitude in these donor-doped films, and can be attributed to both enhanced electron injection and increased electron conductivity in the bulk. Inclusion of doped organic layers into real device structures yields significant increases in performance. In one example, planar heterojunction organic photovoltaic cells with doped interfacial layers exhibit increased open-circuit voltages, short-circuit currents, fill-factors, and power conversion efficiencies [3]. These implications and applications will also be extended to polymeric systems. Advances in doped organic semiconductors are critical to understanding charge transfer properties in organic films and enables the realization of organic electronic devices.
[1] C.K. Chan et al., Org. Electron. 9, 575 (2008).
[2] O. Tal, C.K. Chan et al., Phys. Rev. Lett. 95, 256405 (2005).
[3] C.K. Chan et al., Appl. Phys. Lett. 94, 203306 (2009).