Polyfluorene Conjugated Polyelectrolytes As Electron Transfer Layer For Polymer Solar Cells

Compared with inorganic solar cells,polymer solar cells(PSCs)have attracted a great deal of attention because of their light-weight,low-cost,good flexibility,and the general applicability.In the several decades,power conversion efficiency(PCE)of polymer solar cells has already made tremendous progress(the highest efficiency more than 12%).However,before industrialization and commercialization,PCE still need to improve.Therefore,it is important to optimize the performance of the device via novel materials of design and synthesis,and preparation of new device structure.In addition,it is also crucial that the photoelectric property and stability of the device can be improved by interface modification project.Excellent interface materials can form interface dipole at the electrode/active layer,which improve the charge of extraction and collection.So,it can not only reduce the energy barrier,but also can improve the interface contact between the active layer and the corresponding electrode,finally optimize the performance of solar cells.In this paper,a series of CPEs based on poly [(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-co-2,7-(9,9-dioctylfluorene)] derivates(PFNs)(PFN30,PFN50,PFN70 and PFN100)with different mole ratio of polar groups(-N(C2H5)2)were designed and synthesized to investigate the effect of the numbers of polar groups on the interfacial dipoles.As we all know,the polar groups in the conjugated polyelectrolytes(CPEs)can create the favorable dipoles at the electrode/active layer interface,which is critical for the CPEs to minimize the interfacial energy barrier in polymer solar cells(PSCs).Herein,controllably interfacial dipoles could be readily achieved by only tuning the numbers of-N(C2H5)2 in PFNs,as revealed by the work function of the PFNs modified ITO gradually reduced as the loadings of the-N(C2H5)2 increased.In addition,increasing the numbers of-N(C2H5)2 in PFNs were also favorable for developing the smooth and homogenous morphology of the active layer.As a result,the content of the polar amine in the PFNs exerted great influence on the performance of polymer solar cells.Increasing the numbers of the pendent-N(C2H5)2 could effectively improve the power conversion efficiency(PCE)of the devices.Among these PFNs,PFN100 with the highest content of-N(C2H5)2 polar groups delivered the device with the best PCE of 3.27%.It indicates tailoring the content of the polar groups in the CPEs interlayer is a facial and promising approach for interfacial engineering to developing high performance PSCs.In addition,In order to further optimize the p-type material,passivation its thickness sensitivity,we designed and synthesized a series of other conjugated polyelectrolyte PFB,PFf1 B and PFf4 B replacement Zn O as electron transport layer.Herein,novel alcohol-soluble CPEs by incorporation of fluorinate benzene onto the backbone,namely PFf1 B and PFf4 B,have been synthesized to modulate the interfacial dipoles and charge mobility.A favorable bi-dipole composed of ion-induced dipole and F hydrogen bond-induced dipole was discovered to be responsible for the tunable work function of indium tin oxide(ITO)electrode.Moreover,a desirable nanowires morphology of the upper active layer has also been obtained with the help of the self-assembly of fluorinated CPEs.More intriguingly,an unusual n-type doping favored by fluorine-induced electron transfer(FIET)was observed in these CPEs,leading to the improvement in the electron mobility.As a consequence,these fluorinated CPEs were demonstrated with a general application in the PSCs based on various active layers.Note that PFf4 B with the highest loading of F atoms can work efficiently in a thickness of up to 31.8 nm,which broke the thickness limitation of most reported CPEs interlayer.