The Investigation On Mechanism And Performance Improvement Of Organic Photovoltaics By Optimizing Photon Harvesting And Morphology Of Active Layers

Organic photovoltaics(OPVs)have attracted extensive attention due to their advantages of low cost,environmental friendliness,flexibility and large-area preparation.At present,further improving the power conversion efficiency(PCE)and stability of OPVs is still major goal in this region.Indeed,organic semiconductor materials often possess a relatively narrow absorption spectral range.The full width at half maximum of absorption spectra is generally less than 150 nm,which should limit the photon harvesting range of one donor and one acceptor blend films.The photon harvesting of active layers can be enhanced by incorporating the third component with appropriate bandgap.The third component can also act as morphology regulator to improve phase separation of active layers,leading to more efficient charge transport and collection.Ternary strategy is employed to improve the performance of single-junction OPVs with a simple fabrication process.In this paper,adjusting the photon harvesting and morphology of active layers is the pointcut and fabricating highly efficient OPVs is the core goal.Because of different optical bandgap,energy level and charge mobility of used materials,the dynamic process in ternary active layers is more complex.The working mechanism and the reasons for performance improvement of ternary OPVs can be clarified by fabricating ternary OPVs based on various systems,investigating the dynamics processes of excitons and charges and characterizing the photoelectric characteristics of films and devices.A series of research work has been carried out on the active layer materials selection,fabrication process optimization and mechanism analysis.The main results and innovations of this paper can be summarized as follows:1.The various OPVs were investigated in this paper.The relationship among the material properties,preparation process,working mechanism and device performance was revealed.The complementary absorption spectra of used materials usually is the precondition for fabricating efficient OPVs.The performance of ternary OPVs is improved in both fullerene and nonfullerene system based on complementary absorption spectra of used materials.According to corresponding characterizations,the energy transfer relationship between two donors was proved and the influence of interaction between two donors on performance of OPVs was in-depth investigated.2.A new material selection method for highly efficient OPVs was proposed based on the complementary photovoltaic parameters of binary OPVs,besides the complementary absorption spectrum of the used materials.The material selection method has been verified in ternary system based on two donors or two acceptors.The energy loss can also reduced by employing ternary strategy,which should be a new way to improve the performance of ternary OPVs.3.To gain more insight into the influence of ternary strategy on photon harvesting of active layers,specific devices were prepared with the structure of ITO/PEDOT:PSS/PMMA/PDIN/Al to investigate the parasitic photon loss.The calculated absorption spectra of active layers in OPVs can really reflect the contribution of the third component on photon harvesting.4.The photogenerated excitons distribution of ternary active layers was explored,which provides more solid evidence for performance improvement of ternary OPVs.The calculated photogenerated excitons density of ternary active layers is higher than that of the corresponding binary active layers,indicating improved photon harvesting by employing ternary strategy.The improved photon harvesting should well support the enhancement of short circuit current density in OPVs.The hole and electron generated in the middle region of active layer are more easily collected by electrode simultaneously due to their similar transport distance in active layers.The calculated photogenerated exciton density values in the middle 50 nm region of the optimized ternary active layers is higher than that of corresponding binary active layers.The improved photogenerated exciton density in the middle 50 nm region can provide more potential for effective charge collection,which should support fill factor improvement of OPVs.The calculated photogenerated exciton density may give the more intuitive information on photon harvesting(number and distribution),which should be beneficial to better understand the effect of ternary strategy on PCE improvement of OPVs.5.The ternary strategy was also applied into semitransparent OPVs,leading to simultaneously improved PCE and average visible transmittance(AVT)of semitransparent OPVs.The CO_i8DFIC molecular arrangement can be adjusted by incorporating ultranarrow bandgap acceptor IEICO-4F as the third component,leading to the extended photon harvesting region in near-infrared(NIR)region.The extended photon harvesting in NIR region should contribute to performance improvement,especially for semitransparent OPVs without AVT loss.This paper has a total of 62 pictures,16 tables,and 205 references.