Nano/micro-structure Regulation And Performance Degradation Of Polymer/fullerene-based Organic Solar Cells

In recent years,organic solar cells(OSCs),as the third-generation photovoltaic technology,have been developed rapidly due to the potential merits of ease-fabrication,light-weight,low cost and flexibility.The power conversion eficiency(PCE)and stability are two key factors in realizing the commercial application of OSCs.The key to improve PCE of the device is to make full use of the solar spectrum energy,such as material light absorption,exciton dissociation and carrier transport;and most of the work on the stability of the device is focused on the degradation phenomenon and degradation mechanism under various stresses.On the one hand,it is necessary to continue to improve the performance of organic solar devices and extend the service life of the devices.On the other hand,it is necessary to further explore its complicated working mechanism.In this dissertation,organic solar cells based on polymer/fullerene were chosen as the research system.Post-additive soaking treatment effect on the performance of ternary organic solar cells and the UV-cross-linkable small molecule effect on the high temperature stability of organic photovoltaic devices were studied by various means to control the microstructures in the blend films.In addition,we have analyzed the photophysical properties of the exciton dynamics in the blend films,and tried to establish a quantitative correlation between the microstructures and the thermal stability,which reveals the mechanism of the device degradation.The main works are to prepare high-performance and high-stability devices as the core goal,and to optimize the morphology as the research emphasis,to explore the device performance degradation mechanism as the foothold,by means of device preparation techniques,morphology characterization techniques,time-resolved spectroscopy and microscopy research methods,synchrotron radiation facility,electrical testing to carry out work.Overall,this book contains three partsChapter 1 described the development history and current situation of organic solar cells,working mechanism and characterization parameters,device structures and processing technology,current problems and challenges.In the meantime,the researchers found that morphological control of bulk heteroj unction active layers plays an important role in improving the performance of organic photovoltaic devices.Chapter 2 details the various methods for the application in characterization of active layer morphology.In chapter 3,the nano/micro-structure of active layer in efficient ternary organic solar cells with large thickness is regulated by post-additive soaking(PAS)treatment,and the molecular arrangement between dual-donor molecules and the D/A vertical phase separation are optimized.PTB7-Th:PCDTBT:PC7iBM ternary system was selected based on the fluorescence resonance energy transfer mechanism.By detecting the influence of the third component PCDTBT on the morphology of the active layer and the device performance,we have identified that the third donor driven by surface energy will lead to the deterioration of the morphology and the degradation of the device performance.As contrast,the morphology of the ternary system was optimized and the device performance was greatly improved by using a precisely configured solution(DIO:cyclohexane:n-hexane 6:29:71)to soak the active layer.PAS-processed thick active layer enables the PCE to be more than 8%at thickness window of 289 nm.The main reasons for improving the PCE of ternary solar cells with large thickness active layer are as follows:(ⅰ)the more π-π interaction and intermixing among donor polymer molecules,which is beneficial to energy transfer and hole transport;(ⅱ)the improved the surface smoothness of the active layer with the reduced size of fullerene aggregates and the optimized 3D compositional homogeneity,which can favorable to the dissociation of excitons.(ⅲ)DIO can permeate the whole active layer following by redissolve and redistribute the acceptor molecules.During solvent evaporation process,PC71BM concentrated at the bottom will migrate to the top surface of the active layer,formed a bicontinuous interpenetrating network for carrier transportation to the two electrodes.In chapter 4,we study the performance degradation of organic photovoltaics especially an initial,rapid degradation-period,termed as a Burn-in loss.In order to explore the mechanism of device aging,we observe the effect of a certain factor on the aging performance by artificially strengthening it,and obtain the factors that alTect the PCE degradation.Organic solar cells based on PffBT4T-20D as donor and PC61BM as acceptor were fabricated.By controlling the PC61BM crystallization kinetics in the films by ultraviolet light,we track the changes of PCE and internal microstructures of the films after annealing at 80 ℃(ISOS-D-2 protocol).The results show that the Burn-in loss is due to the thermodynamic instability of the mixed amorphous phase in the active layer.The active layer experienced two main changes during the Burn-in loss stage:(i)the PC61BM molecules migrated rapidly from the fine mixed amorphous region to the crystalline or ordered region,forming a compact aggregate.The short circuit current has obvious Burn-in loss,which leads to the exponential attenuation of the device perfonnance.(ii)the number of PffBT4T-20D grains increases with the annealing time,but the size remains unchanged.Two changes lead to less contact area of D/A,which is unfavorable for the dissociation of excitons and the transport of polaron.Thus,the stability of nano/micro-structure in the active layer fabrication process is the key to obtain high performance and high stability photovoltaic devices.The physical mechanism of aging loss proposed in this work is helpful to understand the degradation process of organic solar cells and can provide some guidance for the development of new materials and the optimization of devices.In chapter 5,a new UV-cross-linkable small molecule,BPA2EODMA,is introduced into P3HT:PC6iBM bulk heterojunction to overcome the thermal instability issue.The B PA2EODM A in the active layer can be crosslinked by heating or ultraviolet radiation to form an insoluble insulating network skeleton,which can obviously inhibit the thermal induced aggregation of fullerene at high temperature and reduce the scale of phase separation.Thus,the thermal stability of the organic solar cells is improved.The effects of the insulation network on the morphology of active layer,device performance,exciton separation and charge transport,carrier mobility are investigated.It was found that the lateral movement and crystallization rate of PC61BM in the blend film were obviously limited with 10 wt%BPA2EODMA during 150℃ thermal annealing.At higher annealing temperature and longer annealing time,the initial micromorphology of P3HT:PC61BM blend films was frozen,and the phase separation can still be maintained.After heating for 5 h,the crosslinker-modified devices retained 50%of the initial PCE,whereas the devices without any cross-linker showed 15%PCE retention.At the same time,the bending test shows that the method can also improve the flexural endurance characteristics,making it possible to apply in roll-to-roll flexible organic solar cells.