Contact Resistance in Ambipolar Organic Field-Effect Transistors Measured by Confocal Photoluminescence Electro-Modulation Microscopy

Although it is theoretically expected that all organic semiconductors support ambipolar charge transport, most organic transistors either transport holes or electrons effectively. Single-layer ambipolar organic field-effect transistors enable the investigation of different mechanisms in hole and electron transport in a single device since the device architecture provides a controllable planar pn-junction within the transistor channel. However, a direct comparison of the injection barriers and of the channel conductivities between electrons and holes within the same device cannot be measured by standard electrical characterization. This article introduces a novel approach for determining threshold gate voltages for the onset of the ambipolar regime from the position of the pn-junction observed by photoluminescence electro-modulation (PLEM) microscopy. Indeed, the threshold gate voltage in the ambipolar bias regime considers a vanishing channel length, thus correlating the contact resistance. PLEM microscopy is a valuable tool to directly compare the contact and channel resistances for both carrier types in the same device. The reported results demonstrate that designing the metal/organic–semiconductor interfaces by aligning the bulk metal Fermi levels to the highest occupied molecular orbital or lowest unoccupied molecular orbital levels of the organic semiconductors is a too simplistic approach for optimizing the charge-injection process in organic field-effect devices

尽管理论上所有有机半导体均支持双极性电荷传输，但绝大多数有机晶体管仅能高效传输空穴或电子。单层双极性有机场效应晶体管可在单一器件中研究空穴与电子的不同传输机制，因其器件结构可在晶体管沟道内构建可控的平面pn结。然而，通过标准电学表征手段无法直接比较同一器件内电子与空穴的注入势垒及沟道电导率。本文提出一种全新方法，可通过光致发光电调制（Photoluminescence Electro-Modulation, PLEM）显微镜观测到的pn结位置，确定双极性工作区起始的阈值栅极电压。实际上，双极性偏置工况下的阈值栅极电压对应沟道长度趋近于零的情况，因此与接触电阻相关联。光致发光电调制显微镜是一种可在同一器件中直接对比两种载流子的接触电阻与沟道电阻的有效工具。本研究结果表明，通过将体相金属费米能级与有机半导体的最高占据分子轨道（Highest Occupied Molecular Orbital, HOMO）或最低未占据分子轨道（Lowest Unoccupied Molecular Orbital, LUMO）对齐来设计金属/有机半导体界面，对于优化有机场效应器件的电荷注入过程而言，这一思路过于简化。