Research On Photoelectric Properties Of Organic Semiconductor Materials And Devices

Organic semiconductor has recently emerged as a promising and versatile material for photoelectric devices.There are many models researching the charge transport in semiconductors for improving their performance.Most of them give good descriptions of experimental data,but it is still an open question which model is correct.In particular,the research and debate on excitons are constantly going on.However,the performance of organic photoelectric devices is limited by excitons.This dissertation shows numerical calculations based on three modified versions of a classical model,and compares to experimental data for typical devices at room and low temperatures.Although their results are similar to each other at room temperatures,only the version considering exciton effects by using hydrogen-like model can give qualitative descriptions to recent experimental data at low temperatures.Therefore,when modeling organic solar cells,exciton effects should be considered at low temperature,while it can be ignored at normal temperature.Based on this exciton dissociation model,this dissertation proposes a unified model by combining the exciton diffusion and dissociation.This model can reduce to the classical model of exciton diffusion or dissociation.Moreover,it can explain one meaningful experimental data of the short-circuit current characteristic at different active layers quantitatively,which can not be fitted by two classical models.Furthermore,the performance of organic photoelectric devices is limited by the exciton splitting at the organic interface which is determined by the exciton binding energy.However,the binding energy is controversial and hard to directly measure.We demonstrate the binding energy can be accurately calculated,requiring only lowest-energy absorption peak in a model proposed here.This model is established with a rational dispersion relation of organic material,which is verified by the reported absorption spectrum of 19 kinds of materials.The numerical results of all the lowest-energy absorption peaks are consistent with the experimental data,and all the obtained exciton binding energy are within the range of reported results.Especially,the binding energy of perovskite is 5 meV,which is the same as the reported experimental data under50 Tesla and 2 K condition.Additionally,we find that the more visible the absorption shoulder,the less binding energy.Furthermore,this model allows estimating the effective mass,dielectric constant and excition radius.By using Fermi-Dirac statistics,this dissertation demonstrates that both ideality factor and Einstein coefficient will up to 2,even 6,when the energy disorder of the organic layer is large enough.By comparing the numerical values of 15 materials with experimental data,the ideality factor is almost the same as the degeneracy value when the light intensity gets near to 10 W/m~2.Additionally,both degeneracy and ideality factor have positive relation with the energy disorder.However,the power conversion efficiency will decrease as the energy disorder increasing.These results provide a direction to increase the power conversion efficiency of the organic solar cells.At the same time,this work demonstrates that the mechanism of light ideality factor equals to the Einstein coefficient by using the generalized Einstein coefficient in the drift-diffusion model.Moreover,this work proposes that the light ideality factor will satisfy the relation of 4:2:1:0 as the intensity of illumination increases.By extracting the mutation points of the intermediate state and the final state from the experimental data,the theoretical calculation is used to fit the experimental data.This work challenges the generally accepted interpretation of the ideality factor:Langevin recombination will dominate if the ideality factor gets near to 1,while SRH recombination becomes important when the deality factor gets near to 2.However,the generally accepted theoretical results fit the experimental data far worse than that of the analytical model proposed in this dissertation.This dissertation shows that Langevin recombination is predominant in the active layer when the optical ideal factor tends to be stable as the light intensity increasing.Einstein coefficient,the theoretical index to characterize the degree of carrier degeneracy,will equal to the ideal factor by analytical and numerical solutions.Therefore,this dissertation proposes that the applicability of Boltzmann statistics to unknown materials can be judged by using the stable light ideal factor which is obtained near 100 W/m~2:when the ideal factor gets near to 1,it can be used;the larger the ideality factor,the more obvious the difference between the results obtained by Fermi statistics and Boltzmann statistics.Thus,this work proposes an experimentally accessible criterion to judge the feasibility of Boltzmann statistics in(both organic and inorganic)semiconductors.Our work provides a prerequisite for studying optoelectronics of semiconductors by statistical method,which allows more accurate understandings of their photophysics properties.