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What is hopping transport? Why does this method affect the mobility of charge?
In the field of material science, charge transport mechanism is an important theoretical model for studying how electric current flows in a specific medium. These mechanisms can be divided into two main types: crystalline solids and molecular solids. These two extremes represent different characteristics of the materials in charge transport, which in turn lead to different charge flow behaviors.
In crystalline solids, charge transport is mostly carried out through "internal transport", also known as band transport; while in molecular solids, it is carried out through "hopping transport". This hopping nature of transport means that the charge "jumps" between molecules, a process that is affected by various factors, such as temperature, applied electric field, and the concentration of the local state.
"In disordered solids, the potential for localization leads to weak localization effects (fixed points), which reduce the mean free path of mobile charges and thus affect their mobility."
Basic Concepts of Jump Transmission
Hop transport is a widely used charge transport mechanism in amorphous or disordered materials. In this process, charge carriers (such as electrons) must overcome certain energy barriers to "jump" to surrounding molecules or atoms. This mechanism generally applies to molecular solids and polymer materials.
As the electric field changes, the mobility of charges may be affected. For example, high electric fields promote the thermal ionization of electrons, making it easier for them to overcome obstacles and jump. This phenomenon involves both thermal motion and the structural properties of the material, so it is very complex.
Concentration Effects of Localized States
Experimentally, it has been shown that when the concentration of localized states increases, the mobility of charges changes in a nonlinear manner. This is because in the nearest neighbor hopping model at low concentrations, the probability of charge hopping decays exponentially as the distance between atoms or molecules increases.
"In a disordered system, as the concentration of localized states increases, the mobility of charges is likely to decrease."
The influence of temperature on charge transport
Temperature is an important factor affecting charge mobility. When the temperature gets higher, the average kinetic energy of the charges also increases, which may facilitate electrons to overcome energy barriers and jump. However, in some cases, excessive temperature may also lead to the recombination of charge carriers with localized states, thus reducing mobility.
At low carrier density, the Mott equation for conductivity is used to describe the effect of temperature on hopping transport, indicating that within a certain parameter range, the conductivity changes with temperature. In addition, as the carrier density increases, activation conduction described by the Arrhenius law may also significantly affect the conductivity.
The influence of electric field
The applied electric field not only affects the driving force of the charge, but can also significantly affect the mobility of the charge. Studies have shown that under low voltage, the movement of charge carriers may follow an exponential law, which means that under strong electric fields, the speed of carrier movement is increased.
"The study shows that applying a high electric field increases the mobility of charge carriers, and this has been confirmed over a wide range of field strengths."
Experimental determination and future prospects
The charge transport mechanism is often determined by building a device and measuring its current-voltage characteristics. Additionally, fabricating these devices using micro-thin film deposition methods or further analyzing the transport mechanisms through other means may deepen our understanding of this phenomenon.
Currently, the academic community is still active in research on hopping transport and related charge mobility, which may bring innovative inspiration to the design of future electronic materials and devices. Understanding hopping transport can help us develop more efficient electronic devices, but how does this mobility at the microscopic level affect our daily lives?
