Study On Performance Improvement Of Organic Solar Cells By Optimizing Morphology Of Active Layer

In recent years,organic solar cells(OSCs)have been developed rapidly.However,the power conversion efficiency(PCE)and stability of OSCs are still low compared with traditional inorganic solar cells.The OSCs can not meet the requirements of industrialization.Not only the performances of OSCs need to be improved,but also the complex work mechanisms need further study.In this paper,preparating the OSCs with high PCE and high stability is core goal;optimizing the morphology of active layer is research emphasis;using receptors with different molecular structures,optimizing the self-assembly process of active layer and developing effective post-processing technology are breakthrough points;the characterization techniques of thin film morphology,optical and spectroscopic,electrical and surface composition are supporting points;exploring the key scientific problems for optimizing the morphology of active layer to improve the device performance is foothold.The main research contents are summarized as follows:(1)The impact of different receptor molecular structure on the OSCs performance are explored through studying the different molecular structure effects on donor/acceptor(D/A)molecular arrangement.The champion PCEs are 7.05%and 2.54%for the OSCs with SMPV1:PC71BM and SMPV1:ICBA as active layer,respectively.The short circuit current(Jsc)of the OSCs with ICBA receptor is much lower than that of the OSCs with PC7,BM as acceptor,the main reasons as following two key points:(?)ICBA lack of effective absorption in the visible range compared with PC71BM;(?)the vertical cross-intercalation molecular arrangement between SMPV1 and ICBA molecules will limit the exciton dissociation,charge transport and collection.The D/A intermolecular arrangement can be purposely optimized according to the chemical structure of donor and acceptor,which may provide a new way to improve the performance of OSCs.(2)The change trends of donor and acceptor molecular arrangement in the active layer self-assembly process are explored by adjusting active layer self-assembly time with alcohol solution.The self-assembly process of active layer can be rapidly terminated by spin-coating PFN methanol solution onto the top surface of blend films along with the fabrication of PFN interface modification layer.The self-assembly time of blend film(PffBT4T-2OD:PC71BM)was regulated from 20 min to 120 min with a step of 20 min increment.The PCEs were markedly improved from 6.74%to 8.66%for OSCs with 80 min active layers self-assembly process and PFN interfacial layer.The active layers have two trends in the self-assembly process:(?)PffBT4T-2OD molecular arrangement can switch from edge-on to face-on,which can favorable to hole transport;(?)the degree of D/A phase separation can be gradually increased.The longer self assembling time will lead to a large degree of D/A phase separation,which is not conducive to the exciton dissociation.It is necessary to regulate the self-assembly time of the active layer,which plays an important role in the optimization of donor molecular arrangement and D/A phase separation.Spin alcohol solution on the top surface of active layer is an effective method to control the self assembling time of active layer.(3)The self-assembly process of active layer can be controlled by upside-down drying method,and the molecular arrangement and D/A vertical phase separation can be optimized.The PCE and stability of OSCs can be improved by employing active layer upside-down drying method,which was solidly confirmed from four kinds of cells with PTB7-Th:PC71BM,PTB7:PC7iBM,PffBT4T-2OD:PC71BM or PBDT-TS1:PC71BM as active layers.The reasons have two points for enhanced PCEs of OSCs with the active layers undergoing upside-down drying process:(?)the more ordered and tightly packed?-? stacking of polymer molecules;(?)the optimized vertical phase separation;(?)the more ordered and tightly packed ?-? stacking of polymer may froze PC71BM distribution in polymer network.The stability test shows that the encapsulation method with parafilmcan can effectively prevent the diffusion of water and oxygen into the device,and improve the stability of OSCs.(4)The D/A vertical phase separation of the active layer can be optimized by methanol soaking active layer due to the mutual solubility of methanol and solvent additives.During the evaporation process of solvent additive DIO and methanol,the PC71BM molecules may migrate to the top surface of active layer,and the degree of D/A vertical phase separation can be optimized by adjusting the methanol soaking time.The working mechanisms of PFN interface modification layer are further clarified by the method of PFN methanol solution soaking:(?)the strong interfacial dipole of ultrathin PFN layer can improve the built-in potential across the active layer;(?)methanol treatment active layer can optimize D/A vertical phase separation.The two step strategy of methanol soaking active layer and then spin-coating PFN methanol solution was used to synergistically optimize the D/A vertical phase separation and obtain the ultrathin PFN interfacial dipole layer,which provides a simple and effective method for improving the performance of OSCs.A step-by-step optimization strategy was carried by using the two step strategy after adjusting active layer self-assembly time.The step-by-step optimization strategy is firstly proposed to adjust donor molecular packing during self-assembly process and acceptor redistribution during methanol soaking process,respectively.This step-by-step optimization strategy can further improve the PCE of OSCs by synergistically optimizing the self-assembly time of active layer and methanol soaking time.