The Effect Of Small-molecule Ionic Squaraine Interfacial Layer On The Performance Of Organic Solar Cells

Interfacial engineering is an important study region of polymer solar cells(PSCs),which plays a key role in improving charge transportation and collection.Among all kinds of cathode interfacial layers(CIL),alcohol-soluble materials have attracted considerable attention and exhibited comparable performance in PSCs.In this paper,a series of novel alcohol-soluble ionic small-molecule squaraine dyes(BBSQNEt,BBSQNEtOH,TBuSQNEtOH,TBuSQNEtOHl)were synthesized for PTB7:PC71BM based PSCs,and the effect of these squaraine dyes as the anode interfacial layer(AIL)and CIL and the work mechanism have been studied.In order to enhance the characteristics of PSCs,the squaraine dyes are introduced in PTB7:PC71BM PSCs as AIL and CIL,respectively.Compared with traditional AIL PEDOT:PSS and CIL PFN,squaraine dyes could work as effective CIL but has negative effect on devices,as AIL.A high power conversion efficiency(PCE)of 7.47%was obtained when BBSQNEt was inserted between the active layer and the Al cathode in conventional PSCs.It is observd that the open voltage(Voc)increases from 0.67V to 0.75V,the short circuit current(Jsc)increases from 11.09 mA cm-2 to 14.98 mA cm-2 and the fill factor(FF).increases from 56.8%to 66.1%,resulting in a 78%improvement in PCE relative to the devices without CIL.Kelvin probe microscopy(SKPM)indicated that the BBSQNEt layer lifts the surface potential of the active layer,which suggest the fomation of a strong dipole layer.On one hand,the dipole layer could reduce the work function of Al cathode and reduce electron barrier;on another hand,it strengthens the build-in electrical field and facilitates the electron transportation and collection,thus resulting in the excellent performance of PSCs.According to the space charge limited current(SCLC)method,the electron mobility is improved by two orders of magnitude when BBSQNEt was incorporated as CIL.According to the photoluminescence and Time-resolved transient photoluminescence(TRPL)spectra,the exciton recombination at the surface of the active layer is weakened when BBSQNEt was coated on the surface.The photocurrent density(Jph)versus the effective voltage(Kff)characteristics show that the exciton dissociation probabilities IP(E,T)is improved by the BBSQNEt layer from 81.43%to 94.22%and the related saturated photocurrent density is enhanced as well.