Optimization Of Energy Loss,Defect State Density And Morphology In High Efficiency Organic Solar Cells

Owing to many unique advantages of organic materials,organic solar cells（OSCs）have been fast developed and become one of the most dynamic research fields in recent years.Due to improvements in material design,device engineering and device physics,the power conversion efficiency（PCE）of OSCs has been rapidly improved,which is about to exceed 20%.However,the PCE of OSCs is still low compared to inorganic and organic-inorganic hybrid perovskite solar cells,which mainly comes from the large energy loss and defect density in OSCs.Therefore,how to reduce the energy loss and defect density of the device is becoming a focus and difficulty in the research field of OSCs.In this thesis,we aim to address this issue and achieve improved device performance through both material and device engineering.In addition to the development of novel photovoltaic materials through efficient molecular design strategies to reduce the energy loss in the device,fine tuning of the morphology of the active layers via layer-by-layer deposition methods has also been applied to reduce the density of defect states.The details of the study are as follows.In Chapter 2,we focus on a wide band gap polymer donor PBDB-TF.By introducing a strong electron-withdrawing unit 3,4-dicyanothiophene（DCT）with structurally and synthetic simplicity to the PBDB-TF conjugated skeleton,the energy level,molecular orientation,crystallinity and charge transport properties of the polymer donor are optimized.A narrow band gap non-fullerene acceptor Y6-BO with matched energy levels and complementary spectra was selected to study the photovoltaic properties.When 30%DCT is introduced into PBDB-TF,the energy loss of OSCs can be reduced from an initial 0.60 e V to 0.57 e V,with a minimum non-radiative recombination energy loss of 0.22 e V.The low dielectric constant of organic semiconductor materials makes them impossible to produce free carriers after photoexcitation,resulting in electron and hole pairs（excitons）.Additional energy is needed to promote exciton dissociation,which would lead to large energy loss in OSCs.In Chapter 3,we design and synthesize a new acceptor molecule named BTP-OE by introducing a branched polar OE side chain on the Y6-BO pyrrole unit.On the one hand,the introduction of polar side chains increases the dielectric constant of both the acceptor molecule BTP-OE and the blend film.On the other hand,the high dielectric constant acceptor BTP-OE leads to reduced electron mobility,increased trap density,significant first-order recombination,larger energetic disorder,and higher non-radiative energy loss in OSCs.As a result,OSCs based on BTP-OE with high dielectric constant have low V_oc and FF.This work demonstrates the complex relationship between the dielectric constant of organic semiconductors and the device properties of OSCs,and the large non-radiative recombination energy loss due to increasing defect density.Based on the previous two works,we find that the morphology of the active layer has a great influence on the energy loss and density of defect states in OSCs.In Chapter 4,we regulate the morphology of the active layer by optimizing the device processing method.We selected all-polymer solar cells（all-PSCs）with advantages in stability and mechanical properties as the research object.PBDB-T was selected as the polymer donor and PYT as the polymer acceptor.In all-PSCs,the morphology of the active layer is difficult to control due to the excessive entanglement between polymer chains,which introduces more defects.Hence,morphological regulation of the active layer is the key to improving the performance of all-PSCs.We used layer-by-layer（Lb L）deposition method and synergistic effect of solvent additives and thermal annealing to subtly regulate the microstructure of the polymer donor and acceptor.It turns out that the crystal coherence length of the donor material is significantly increased,and the acceptor material forms nanofiber structure.At the same time,the donor and acceptor form a favorable vertical gradient distribution,which promotes exciton dissociation and charge transport,and reduces the density of defect states as well as charge recombination losses.Based on the above advantages,the all-PSCs achieved a PCE of 16.05%and a record FF（77%）.Building on the work of Chapter 4,in Chapter 5 we turn our attention to a polymerized small molecule PY-OT with isomeric purity as an acceptor.The higher LUMO energy level of the PY-OT is expected to increase the V_oc and reduce the energy loss in all-PSCs.Again,we use the Lb L deposition method to optimize the active layer morphology of PY-OT based all-PSCs.The device performance of PBDB-T/PY-OT based all-PSCs is greatly improved due to the correct choice of energy level matched donor.The PCE increased from 10.45%（BHJ）to15.15%,which was the highest PCE value of all-PSCs based on the polymerized small molecule acceptor（PSMAs）with the polymerization site at theδposition.The work in Chapters 4 and 5demonstrates that the Lb L deposition method is a simple and feasible method to reduce the density of defect states in the active layer and has great potential in the preparation of high-performance all-PSCs.