Gap State Compensation in Organic Semiconductors: The Ultra-Low Doping Regime

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Non-technical description: Organic semiconductors based on small molecules or polymers are currently being researched for applications in color displays, solar cells, chemical sensors and flexible electronics. Despite considerable efforts to purify materials and grow high quality films, these semiconductors generally contain a number of defects, either physical or chemical, which affect their electrical properties. These defects often are centers where charges, which carry the electric current in a device, get trapped. There is therefore considerable incentive in finding ways to either eliminate these defects or mitigate their effect on devices. In this project, the principal investigator develops a systematic method to neutralize the defects by introducing small amounts of selected organic molecules in the semiconductors. These molecules, called dopants, exchange a charge with the defects, rendering them inactive as traps and improving considerably the electrical quality of the semiconductors. The dopant densities must be precisely controlled down to levels of one dopant molecule per ten thousand host molecules in order to de-activate the traps without adding an excess of free charges to the semiconductor. The project provides multidisciplinary research training for graduate and undergraduate students in advanced electronics materials as well as Chemistry, Chemical Engineering and Electrical Engineering through collaboration. The students also benefit from the international collaboration through the Princeton partnerships with Humboldt University (Germany) and Tokyo University (Japan). Technical Description: Chemical doping is considered as a key tool to modify the electronic structure and/or improve the performance of molecular and polymer semiconductors in organic electronic devices. This project employs molecular n- and p-dopants to address a fundamental problem in molecular and polymer semiconductor films, i.e., intrinsic defects as electronic gap states/traps, and their negative impact on charge carrier transport and interface manipulation. Specifically, these gap states/traps slow charge carrier transport, cause Fermi level pinning in bulk and at interfaces, and act as recombination centers. This research seeks to apply ultra-low doping concentration to precisely compensate and de-activate trap states in organic semiconductors, without altering the electronic structure and 'free' carrier density in the host matrix that is typically controlled by 'standard' doping processes. This research requires detailed understanding of the electronic structure of the materials involved, including the ionization energy and electron affinity of dopants and semiconductors, the distribution and density of gap states to be de-activated (typically in the range of 10^17-18 cm^-3), as well as the eventual impact of ionized dopants on the host electronic structure and the transport properties of these semiconductors. Various characterization tools including direct and inverse photoemission spectroscopy of occupied and unoccupied states, carrier transport measurements as a function of temperature, secondary ion mass spectrometry for dopant distribution and device fabrication (e.g., single-heterojunction photovoltaic cells) are implemented for this work.