| Inorganic semiconductors have problems in preparing devices with complex process,high power consumption,large volume,and high integration,when it comes to preparing devices.In recent years,organic semiconductor materials,as one of the potential materials for the development of the semiconductor industry and the information technology industry,have received increasing attention from researchers because of their adjustable energy levels,low energy consumption,low cost and low temperature and easy processing.There is a practical need for organic semiconductor materials with higher sensitivity and faster response times.Whereas previous improvements in device performance have focused on improving mobility,more attention is now being paid to developing or synthesising new materials to investigate the microscopic transport mechanisms within their devices in order to see how the performance behaves in macroscopic devices.The performance of organic semiconductor devices depends to a large extent on the exciton diffusion length at the microscopic level,so it is important to study the exciton diffusion length of organic semiconductors.Based on this factor,this paper conducts research on the measurement of exciton diffusion length in organic semiconductor materials,establishes a mathematical model of exciton diffusion length and performs the relevant theoretical calculations,the main content of the thesis is as follows:(1)The exciton diffusion length in organic semiconductor materials is usually small,which limits the performance of organic optoelectronic devices.As exciton diffusion has been a key process in semiconductor photoelectric conversion,for the problem of exciton diffusion in organic semiconductor materials,considering carrier concentration as an important factor in the transport problem,a mathematical model based on the percolation theory and the F?rster resonance transfer rate was developed based on the diffusion model.The dependence on the exciton diffusion length is represented in the model by the temperature and the Gaussian density of statesσvalues.It was found that the exciton diffusion length decreases with increasing Gaussian standard deviationσ.Differences in Fermi energy levels cause significant changes in exciton diffusion lengths,with the higher the Fermi energy level the more dependent the exciton diffusion length is on theσvalue.The Fermi energy level indicates the level of the electron filling energy level.In addition,the exciton diffusion length decreases with increasing temperature,and at low Fermi energy levels the exciton diffusion length is more dependent on temperature.The above results match with Monte Carlo simulations.(2)Exciton transport in organic semiconductor materials is a complex microscopic process.In this action,there is a weak interaction between the donor/receptor and the transport is carried out by energy transfer.The weakly coupled F?rster energy transfer mechanism in energy transfer is mainly considered,as excitons can absorb or emit photons in the material,thus having an impact on the optical properties of the material.A model for the calculation of the one-dimensional steady-state exciton diffusion length based on the photoluminescence quenching spectrum has been developed to address the multi-state problem of exciton transport in organic semiconductors,taking into account the uncertainties in the observation of microscopic multi-states.To understand the microscopic process of F?rster energy transfer of excitons,the dependence of the exciton diffusion length on the photoluminescence efficiency and absorption coefficient is described,from which the value of the exciton diffusion length is estimated.(3)The external quantum efficiency of an organic phototransistor is related to its exciton diffusion length.Under the same conditions,the larger the exciton diffusion length,the higher the external quantum efficiency.The external quantum efficiency(EQE)currently in common use is equal to the ratio of the number of the photo-generated electrons or holes contributing to photocurrent to the number of incident photons.The EQE values calculated by this method are usually greater than 100%and do not reflect the actual photovoltaic conversion capability of organic phototransistors.Based on the photoelectric conversion effect and exciton diffusion theory,an external quantum efficiency EQE*model based on a single active layer and a planar heterojunction organic phototransistor excluding the photocurrent amplification effect is proposed,and its EQE~*value is estimated.It is shown that the external quantum efficiency is a function of the exciton diffusion length L_d,and under the same conditions,the longer the exciton diffusion length,the greater the external quantum efficiency. |
