Phase Separation Of All Conjugated Polymer Blends And Their Photovoltaic Performance

All-polymer organic solar cells have attracted more and more attention due to their advantages such as solution-processable,large-area preparation,good morphological stability,and excellent mechanical properties.The phase separation morphology of the active layer is a key factor to limit the performance of organic solar cells.The ideal all-polymer solar cell morphology is a bicontinuous network structure with a domain size of about 10-20 nm,higher phase purity,and larger interface area.However,the strong interaction between the components in all-polymer conjugated blends results in poor compatibility,which tends to form a large-scale phase-separated structure.Furthermore,the semi-crystalline nature of the conjugated polymer makes it difficult to control the competitive relationship between crystallization and phase separation.Moreover,the acceptor has a lower charge mobility.Those factors restrict the development of all-polymer solar cells.Therefore,we proposed to control the crystallization behavior by solution aggregation,polymer molecular weight,and intermolecular interaction and how the effects of relative rate between crystal nucleation and growth,the connection between crystals,and the local orientation of molecules on the phase separation structure are discussed.The main conclusions are as follows:1.By utilizing the temperature-dependent aggregation feature of PffBT4T-2OD,the process of crystal nucleation and growth during crystallization can be separated when select different temperatures.Furthermore,the effect of the relative rate between crystal nucleation and growth on the phase separation morphology was discussed exhaustively.The crystallization process is dominated by nucleation induce the liquid-solid phase separation happened.When the nucleation and growth processes match each other,crystallization and phase separation competing with each other,it belongs to a nucleation and growth mechanism of liquid-liquid phase separation.The crystallization process is dominated by crystal growth,the system undergoes spinodal decomposition of liquid-liquid phase separation.In this study,the relationship between crystallization and phase separation in polymer blends is discussed in depth from the perspective of crystallization kinetics caused by solution aggregation.It has a guiding role in regulating the phase separation morphology of the blend system.Moreover,the findings will be of interest in the regulation of the active layer morphology with temperature-dependent aggregation feature in other organic solar cells blend systems.2.By studying the characteristics of N2200 materials with different molecular weights(MWs),we found that,as the chain length increases,the chain conformation of N2200 can be divided into three ranges.The chain behaves like a rigid-rod structure for N220017k and N220028k.The molecular rigidity is weakened,and a rigid chain structure is observed for N220057k and N220096k.When MW is at N2200110k,which is larger than the critical MW,the molecular chains are tangled.Multi-chain aggregations appear when the MW is larger than the critical one,which providing opportunities for the formation of percolation networks.This structure can effectively decrease the size of phase separation and improve the purity of domain and the tendency of face-on orientation.It leads to an effective charge transport pathway.Thus,the charge transport between electron and hole mobility is more balanced.The power conversion efficiency is increased from 5.87%for J51:N220017k to 8.28%for J51:N2200110k.This research can guide the fabrication of high-performance all-polymer solar cells by choosing the suitable MW of polymer,achieving a percolation network structure.3.Adding a volatilizable solid additive 0.75 wt%Bipy to the J51:N2200 blend results in improved performance,with a PCE of 8.73%.Here,the Voc and FF increased from 0.81 V and 64.37%in control devices to 0.85 V and 69.44%in the device containing Bipy.The underlying working mechanism of the Bipy was investigated.This demonstrated that Bipy could enhance the π-π stacking of N2200 and increase the connectivity between crystalline lamellae.Besides,since the compatibility between Bipy and N2200 was weaker than that of J51,N2200 could be enriched on the top-surface during thermal annealing,which is energetically favorable to optimize the vertical phase separation,thus further resulting in better charge transport and higher charge collection efficiency in the devices.This work unveils that the molecular local orientation and optimal vertical separation structures are critical for charge transport.Understanding the correlation between the resulting microstructures and performance will provide useful guidance for the further development of photovoltaic devices.