Molecular Doping of Organic Semiconductors

Abstract

Molecular doping of organic semiconductors is becoming exceedingly important and has led to significant commercial developments in organic electronics, since it allows to overcome performance deficiencies and material limitations.

Increasing attention has recently been placed on using very low concentrations of dopants to eliminate the effect of gap states in organic semiconductors, in order to improve carrier mobility, adjust the energy level alignment at interfaces, and achieve overall better device performance. However, direct spectroscopic observations and quantitative analyses have not been done yet to study the impact of dopants on the density of states of organic semiconductors. Here, by using a combination of electron spectroscopy and carrier transport measurements, we investigate the distribution of valence and gap states in copper phthalocyanine (CuPc) upon the introduction of minute amounts of the p-dopant molybdenum tris[1,2-bis-(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3). We observe the progressive filling (and deactivation) of the deepest tail states accompanied by a decrease of the hopping transport activation energy by charges introduced by the dopants, as well as a significant broadening of the CuPc density of states. Simulations relate this broadening to the electrostatic and structural disorder induced by the dopant in the CuPc matrix.

Another challenge in this field is n-type doping. Although a variety of stable molecular p-dopants have been developed and successfully deployed in devices, air-stable molecular n-dopants suitable for materials with low electron affinity, which are exceedingly important in a range of applications, are essentially non-existent. We demonstrate a major advance to n-dope very low electron affinity organic semiconductors using cleavable air-stable dimeric dopants. Although the reduction potentials of these host materials are beyond the thermodynamic reach of the dimer's effective reducing strength, photo-activation of the doped system can result in kinetically stable and efficient n-doping. High-efficiency organic light-emitting diodes are fabricated by using electron-transport layers doped in this manner. Our strategy thus enables a new paradigm for using air-stable molecular dopants to improve conductivity in organic semiconductors with very low electron affinity and provide ohmic contacts to these materials regardless of the electrode work function, giving more freedom to device design and optimization.