摘要: 针对近年来实验发现的有机半导体电磁光现象中的同位素效应，本文基于强电子-声子耦合的紧束缚模型，利用非绝热近似分别研究了小分子晶体和聚合物链内的极化子的运动， 发现氢元素的氘化或碳13元素的存在都会降低有机材料迁移率。本文分析了各种可能的同位素效应并对其物理机理进行了分析。
Isotope Effect of Carrier Transport in Organic Semiconductors
- Received Date:
25 May 2020
Abstract: Isotopic substitution can effectively tune the device performance of organic semiconductors. Basing on the experimental results of isotope effects in electric, light and magnetic process in organic simiconductors, we adopt the tight-binding model with strong electron-phonon coupling to study the isotope effects on carrier transport. We try to give an quantificational explaination and show the physical origin of isotope effects on mobility in organic semiconductors in this work. Using polaron transport dynamics with diabatic approach, we simulate carrier transport in an array of small molecule crystal under weak bias. Because of strong electron-phonon coupling in organic materials, an inject electron will induce lattice distortion, and the carriers are no longer free electrons or holes, but elementary excitations such as solitons, polarons or bipolarons. Our simulating results indicate that the existing of deuterium and 13C element will decrease the mobility of orginic meterials, which means that isotopic substitution can be utilized to manifast organic devices proformance. Besides, we also find that isotope effects on mobility will increase with the increase of election-phonon coupling. This suggests that both the mass of lattice groups and election-phonon coupling should be taken into account to understand isotope effects in organic semiconductors. With the consideration of that, we derive the effective mass of polaron basing on continuum model, and verify that effective mass can successfully describe the isotope effects on mobility. For effective mass of carriers can also be measured to represent the property of a material, it can tell us whether we need isotopic substitution in organic layer or not to improve device performance. Then we present the microcosmic movement of a polaron in the moment when it encounters isotopic substituted molecules. We come to the conclusion that isotopic distribution will affect the instantaneous speed of the carrier, but has little effect on the mobility of the whole device when the substituted concentration remains constant.
In conclusion, after simulating various possible isotope effects in materials, analysing its physical mechanism and comparing calculation results with experiments, we provide a theoretical foundation for describing the isotope effects on mobility, which can be a basis of improvement of the performance of orgainc semiconducter devices.