Department of Chemistry, Biochemistry and Pharmaceutical Sciences (DCBP), University of Bern, Freiestrasse 3, 3012 Bern, Switzerland.
Mater Horiz. 2022 Jan 4;9(1):482-491. doi: 10.1039/d1mh01343b.
Doping of organic semiconductor films enhances their conductivity for applications in organic electronics, thermoelectrics and bioelectronics. However, much remains to be learnt about the properties of the conductive charges in order to optimize the design of the materials. Electrochemical doping is not only the fundamental mechanism in organic electrochemical transistors (OECTs), used in biomedical sensors, but it also represents an ideal playground for fundamental studies. Benefits of investigating doping mechanisms electrochemistry include controllable doping levels, reversibility and high achievable carrier densities. We introduced here a new technique, applying terahertz (THz) spectroscopy directly to an electrochemically doped polymer in combination with spectro-electrochemistry and chronoamperometry. We evaluate the intrinsic short-range transport properties of the polymer (without the effects of long-range disorder, grain boundaries and contacts), while precisely tuning the doping level the applied oxidation voltage. Analysis of the complex THz conductivity reveals both the mobility and density of the charges. We find that polarons and bipolarons need to co-exist in an optimal ratio to reach high THz conductivity (∼300 S cm) and mobility (∼7 cm V s) of P3HT in aqueous KPF electrolyte. In this regime, charge mobility increases and a high fraction of injected charges (up to 25%) participates in the transport mixed-valence hopping. We also show significantly higher conductivity in electrochemically doped P3HT with respect to co-processed molecularly doped films at a similar doping level, which suffer from low mobility. Efficient molecular doping should therefore aim for reduced disorder, high doping levels and backbones that favour bipolaron formation.
有机半导体薄膜的掺杂可以提高其导电性，从而应用于有机电子学、热电学和生物电子学。然而，为了优化材料的设计，还有很多关于导带电荷的性质需要了解。电化学掺杂不仅是用于生物医学传感器的有机电化学晶体管 (OECT) 的基本机制，也是基础研究的理想场所。研究掺杂机制——电化学的好处包括可控制的掺杂水平、可逆性和可实现的高载流子密度。我们在这里引入了一种新技术，将太赫兹 (THz) 光谱直接应用于电化学掺杂聚合物，同时结合光谱电化学和计时安培法。我们评估了聚合物的固有短程传输特性（没有长程无序、晶界和接触的影响），同时精确调节掺杂水平——施加的氧化电压。对复杂 THz 电导率的分析揭示了电荷的迁移率和密度。我们发现，要达到 P3HT 的高 THz 电导率（约 300 S cm）和迁移率（约 7 cm V s），需要在最佳比例下同时存在极化子和双极化子。在这种情况下，电荷迁移率增加，并且注入的电荷（高达 25%）的很大一部分参与了混合价跳跃的输运。与具有相似掺杂水平的共加工分子掺杂膜相比，我们还显示了电化学掺杂的 P3HT 具有显著更高的电导率，而后者的迁移率较低。因此，有效的分子掺杂应该旨在减少无序、提高掺杂水平，并有利于双极化子形成的主链。
