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

非技术性说明：基于小分子或聚合物的有机半导体目前正在研究用于彩色显示器、太阳能电池、化学传感器和柔性电子产品。尽管在纯化材料和生长高质量薄膜方面付出了相当大的努力，但这些半导体通常含有许多物理或化学缺陷，这些缺陷会影响其电学性能。这些缺陷通常是电荷的中心，这些电荷在器件中携带电流，被捕获。因此，在寻找消除这些缺陷或减轻其对设备的影响的方法方面存在相当大的激励。 在这个项目中，主要研究者开发了一种系统的方法，通过在半导体中引入少量选定的有机分子来中和缺陷。这些被称为掺杂剂的分子与缺陷交换电荷，使它们作为陷阱变得不活跃，并大大提高了半导体的电气质量。 掺杂剂密度必须精确地控制到每一万个基质分子一个掺杂剂分子的水平，以便在不向半导体添加过量自由电荷的情况下使陷阱失活。该项目通过合作为研究生和本科生提供先进电子材料以及化学，化学工程和电气工程的多学科研究培训。学生们还受益于普林斯顿大学与洪堡大学（德国）和东京大学（日本）的国际合作。技术说明：化学掺杂被认为是改变有机电子器件中分子和聚合物半导体的电子结构和/或改善其性能的关键工具。该项目采用分子n-和p-掺杂剂来解决分子和聚合物半导体膜中的基本问题，即，作为电子能隙态/陷阱的本征缺陷，以及它们对电荷载流子传输和界面操纵的负面影响。具体而言，这些间隙状态/陷阱减慢电荷载流子传输，导致体中和界面处的费米能级钉扎，并充当复合中心。这项研究旨在应用超低掺杂浓度来精确补偿和钝化有机半导体中的陷阱态，而不改变通常由“标准”掺杂工艺控制的主体基质中的电子结构和“自由”载流子密度。这项研究需要详细了解所涉及材料的电子结构，包括掺杂剂和半导体的电离能和电子亲合能、待失活的带隙态的分布和密度（通常在10^17-18 cm^-3范围内），以及电离掺杂剂对主体电子结构和这些半导体的输运性质的最终影响。各种表征工具，包括占据和未占据状态的直接和反向光电发射光谱、作为温度函数的载流子传输测量、用于掺杂剂分布和器件制造的二次离子质谱（例如，单异质结光伏电池）用于这项工作。

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.