INJECTION AND BIPOLARONIC CHARGE SPECIES AT METAL-ORGANIC SEMICONDUCTOR CONTACT INTERFACES

Restricted (Penn State Only)
Author:
Dhanker, Rijul
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 20, 2017
Committee Members:
  • Noel Christopher Giebink, Dissertation Advisor
  • Noel Christopher Giebink, Committee Chair
  • Thomas Nelson Jackson, Committee Member
  • Xingjie Ni, Committee Member
  • Enrique Daniel Gomez, Outside Member
Keywords:
  • injection
  • internal photoemission
  • surface plasmons
  • OMAR
  • Bipolarons
  • charge modulation spectroscopy
  • organic magentic field effects
  • organic semiconductors
Abstract:
Organic electronics have gained wide spread popularity due to their highly tunable electronic and optical properties while retaining relatively simple device fabrication, finding applications commonly in displays and lighting. Despite rapid commercialization we are limited by our understanding of the fundamental device properties that play a major role in charge injection at contacts, transport in the device and subsequent radiative recombination. This first part of this thesis deals with an adaptation of internal photoemission (IPE), using surface plasmon polariton (SPP) modes, in order to selectively control absorption in the device allowing for up to 8x enhancement in injection from the desired contact. In addition, we have been able to isolate true IPE photocurrent from disorder related spurious (almost 50\% at 3/4 of bandgap) background signals demonstrating a reliable and repeatable technique for in-situ measurements. The second part of this work focuses bipolaron states, in which two electrons or two holes occupy a single molecule or conjugated polymer segment which are typically considered to be negligible in organic semiconductor devices due to Coulomb repulsion between the two charges. We use charge modulation spectroscopy to reveal a bipolaron sheet density >10$^{10}$ cm$^{-2}$ at the interface between an indium tin oxide anode and the common small molecule organic semiconductor. In our work we have been able to provide definite evidence in support of the origin of the magnetic response while for the first time demonstrating the presence of bipolaronic charge species in unipolar organic semiconducting devices. We find that the magnetocurrent response of hole-only devices correlates closely with changes in the bipolaron concentration, supporting the bipolaron model of unipolar organic magnetoresistance and suggesting that it may be more of an interface than a bulk phenomenon. These results are understood on the basis of a quantitative interface energy level alignment model, which indicates that bipolarons are generally expected to be significant near contacts in the Fermi level pinning regime and thus may be more prevalent in organic semiconductor devices than previously thought.