Active Layer Morphology Engineering of All-polymer Solar Cells by Systematically Tuning Molecular Weights of Polymer Donors/Acceptors

In all-polymer solar cells(APSCs),number-average molecular weights(Mns)of polymer donors and polymer acceptors play an important role in active layer morphology and photovoltaic performance.In this work,based on a series of APSCs with power conversion efficiency of approaching 10%,we study the effect of Mns of both polymer donor and polymer acceptor on active layer morphology and photovoltaic performance of APSCs.We select poly[4-(5-(4,8-bis(5-((2-butyloctyl)thio)thiophen-2-yl)-6-methylbenzo[1,2-b:4,5-b’]dithiophen-2-yl)thiophen-2-yl)-5,6-difluoro-2-(2-hexyldecyl)-7-(5-methylthiophen-2-yl)-2 H-benzo[d][1,2,3]triazole](CD1)as the polymer donor and poly[4-(5-(5,10-bis(2-dodecylhexadecyl)-4,4,9,9-tetrafluuoro-7-methyl-4,5,9,10-tetrahydro3 a,5,8,10-tetraaza-4,9-diborapyren-2-yl)thiophen-2-yl)-7-(5-methylthiophen-2-yl)benzo[c][1,2,5]thiadiazole](PBN-14)as the polymer acceptor.The Mns of polymer donor CD1 are 14.0,35.5 and 56.1 kg/mol,respectively,and the Mns of polymer acceptor PBN-14 are 32.7,72.4 and 103.4 kg/mol,respectively.To get the desired biscontinueous fibrous network morphololgy of the polymer donor/polymer acceptor blends,at least one polymer should have high or medium Mn.Moreover,when the Mn of polymer acceptor is high,the active layer morphology and APSC device performance are insensitive to the Mn of polymer donor.The optimal APSC device performance is obtained when the Mn of both the polymer donor and the polymer acceptor are medium.These results provide a comprehensive and deep understanding on the interplay and the effect of Mn of polymer donors and polymer acceptors in highperformance APSCs.

Progress of additives for morphology control in organic photovoltaics

The microstructure of the active layer in organic photovoltaics(OPVs),such as the size of phase separation,purity of the phases,and molecular packing within each phase,plays a crucial role in influencing the behavior of excitons and charge carriers within the active layer.It is also a key determinant of the photovoltaic performance of the device.During the optimization of OPV devices,the use of additives has been demonstrated to be an effective strategy in microstructure control,leading to enhanced performance.Therefore,the quest for stable and efficient novel additives,along with an exploration and summarization of the mechanisms underlying additive-induced microstructure control,is essential for a better understanding of the developmental trends of high-performance additives.In this review,we categorize additives based on their chemical structures and discuss their effects on the microstructure of the active layer from both thermodynamic and kinetic perspectives.Furthermore,we elaborate on the working mechanisms and their impact on the photovoltaic performance of the devices.This review provides an overview of recent advances in additives for OPVs,offering potential guidance for the future development of additives and further optimization of the active layer in photovoltaic devices.