| Over the past few years,mainly thanks to developments in non-fullerene small-molecule electron acceptor materials,substantial improvements in the power conversion efficiency（PCE）of organic solar cells have been reported in the scientific literature,with PCE approaching 20%for small-area single junction devices in laboratory.At the same time,the performance of the device is inextricably linked to the process of preparing the active layer.This thesis aims to investigate the conclusive influence of carrier processes on device performance through a theoretically guided experimental approach,focusing on the generation and recombination of carriers in high-efficiency organic solar cells,and thus deepen the understanding of these processes in the physical aspect.The main substance of this thesis is briefly described below.As the introduction of the thesis,the first chapter briefly describes the development of organic solar cells and the understanding of their photoelectric conversion process at the current stage.In addition to summarizing the progress of active layer materials,the necessary introduction to the relevant device test methods is also given.In the second chapter,the effects of different processing solvents（chlorobenzene,chloroform,and toluene）on the carrier recombination kinetics and energetics of PBQx-TCl:BTP-e C9-based organic solar cells are investigated.By reconstructing the current density-voltage characteristic curves of the devices,the study analyzes the degree of carrier loss due to different recombination mechanisms,such as bimolecular recombination and monomolecular recombination.It is confirmed that bimolecular recombination is the predominant recombination mechanism in high-efficiency devices,while monomolecular recombination is also evident in other devices besides bimolecular recombination.Having obtained the parameters related to carrier kinetics,a dimensionless parameterθis employed to quantify the competition between charge recombination and extraction,giving insight into why fill factors change withθ.The device processed from toluene has the smallestθvalue(2.2×10-3),which is slightly smaller than that of the device processed from chlorobenzene(2.7×10-3)and significantly smaller than that of the device processed from chloroform(9.5×10-3),indicating that using toluene as processing solvent in this study improves the carrier transport characteristics while reducing recombination.In general,a lowerθresults in a higher fill factor.The fill factor obtained from the experimental measurements follows the same trend as the simulated points.In addition to the effects of fabrication processes on device performance,the instinct properties of organic photovoltaic materials determine the upper limit of the power conversion efficiency of organic solar cells.In the third chapter,the study focuses on the effect of side chain fluorinated electron donor materials paired with different acceptor materials on device performance,linking the material composition of the active layer to the optoelectronic properties of the device within the framework of device physics.In this chapter,two representative donor materials,PBDB-T and PBDB-T-2F（also known as PM6）,are selected to be paired with nine electron acceptor materials,and the effect of side chain fluorination of the donor materials on various key parameters and power conversion efficiency is systematically investigated.It has been found that the introduction of fluorine substituents not only reduces the highest occupied molecular orbitals of the conjugated polymer but also enhances intra-and intermolecular interactions.Furthermore,the introduction of the fluorine substituent reduces the trap state density,thereby increasing carrier mobility and reducing the device voltage loss,resulting in a higher open circuit voltage and fill factor for the relevant device.In addition,organic semiconductor materials are mostly amorphous,band-tailed states and energetic disorders are unavoidable,resulting in large non-radiative voltage losses.A quantitative analysis of voltage losses shows that the PM6-based devices exhibit significantly lower voltage losses due to the suppressed non-radiative recombination than the PBDB-T-based devices.As trap-assisted recombination is a non-radiative recombination pathway,this result suggests that the reduced non-radiative voltage losses in PM6-based devices are directly related to the suppressed trap-assisted recombination.The FTPS-EQE result shows that the radiative voltage losses of the PM6devices are significantly lower than those of the PBDB-T devices,mainly due to the reduced disorder in the PM6 devices.The photovoltaic characteristics of several high-performance organic solar cells based on polymer donor D18 and non-fullerene acceptor blended films are investigated in the fourth chapter.Despite the relatively low carrier mobility on the order of 10-4 cm2 V-1 s-1,the power conversion efficiency of the best device is up to 18%.Organic solar cells based on D18:Y6,D18:BTP-e C9,and D18:L8-BO exhibit excellent photovoltaic performance.The outstanding performance originates from simultaneously increasing charge generation and a reduced recombination rate coefficient as low as 2×10-12 cm3 s-1.A refined drift-diffusion model simulation considering interfacial energy band bending is further employed to examine the impact of the optical bandgap,energy offset,mobility,and recombination rate coefficient on the performance of device.The simulated results reveal that increasing the mobility and decreasing the recombination rate coefficient,which can both enhance the fill factor and suppress the voltage loss,are key determinants to further improve the power conversion efficiency of organic solar cells.This study provides a comprehensive description of the relationship between fundamental material properties and device performance,offering important reference principles for the development of material systems and devices with better performance. |
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