Polymer Acceptors Containing B?N Unit:Charge-Transporting And Phase Separation Morphology

All-polymer solar cells(all-PSCs),which use the blend of polymer electron donor and polymer electron acceptor as the active layer,have show great potential for future application in flexible photovoltaic devices because of their advantages over other type organic solar cells(OSCs).These advantages include excellent thermal stability,morphological stability and mechanical stability.Till now,further development of all-PSCs are severely limited by the lack of polymer acceptors,and their power conversion efficiencies(PCEs)are still lags far behind that of other type OSCs,especially nonfullerene small molecule acceptor-based polymer solar cells.Most efficient polymer acceptors are designed based on imide unit.We have found that boron-nitrogen coordination bond(B?N)can dramatically decrease LUMO/HOMO energy levels of conjugated polymers and consequently can design polymer electron acceptors.Based on this idea,two types of polymer acceptors containing B?N unit have been developed,one is the"electron-acceptor-electron-acceptor(A-A)" type polymer acceptor,and the other is "electron-donor-electron-acceptor(D-A)" type polymer acceptor.However,B?N-containing polymer acceptors are still in their infancy,and their PCEs were only 0.1-3%.The poor device performance is mainly attributed to the low electron mobility of N based polymer acceptors and the non-ideal phase separation morphology blended with polymer donor.Research topic of this thesis is to develop high performance B?N-containing polymer acceptors,especially focuses on the electron mobility regulation and the phase separation morphology optimization.Through delicate molecular design,the electron mobility of the B?N-containing polymer acceptors increased from 10-7 cm2 V-1 s-1 to 10-3 cm2 V-1 s-1,which was increased by nearly four orders of magnitude.By increasing the crystallinity of the polymer acceptor,the phase separation morphology was optimized,and the unique phase separation behavior of the amorphous polymer acceptor was revealed.As a result,the PCE of the all-PSCs based on the new designed B?N-containing polymer acceptors increased from 0.1-3%to 10.1%.The representative research results are summarized as follows:1.Electron mobility regulation of A-A type B?N-containing polymer acceptors All-PSC device performance of A-A type B?N-containing polymer acceptor is poor because of its low electron mobility.This is attributed to the steric hindrance of the pendant bulky phenyl groups on the boron atom,which prevent close ?-stacking of conjugated polymer backbones.By extending the length of the planar copolymerization units,the steric hindrance effect of the pendant phenyl groups was alleviated.The ?-? stacking distance of the resulting polymer decreased from 0.46 nm to 0.38 nm and the electron mobility was dramatically increased from 3.4 × 10-7 cm2 V-1 s-1 to 2.2 × 10-1 cm2 V-1 s-1.As a result,the corresponding PCE of the all-PSCs was increased from 0.1%to 6.6%.2.Phase separation morphology of amorphous A-A type N-containing polymer acceptorCrystallization of polymer donor and/or polymer acceptor is one of the main driving forces for phase separation of polymer blends.Most of polymer acceptors are semicrystalline,and the active layer of the all-PSC is the blend of two semicrystalline polymers.Owing to the asymmetric B?N bridged thienylthiazole(BNTT)unit,the chemical structure of P-BN-DPy is random.The bulky phenyl groups on boron atom will prevent the close packing of polymer backbone,so the polymer chains cannot be aggregated in solution and the polymer does not crystallize in spin-coated film.We have revealed that this amorphous polymer acceptor can blend with various polymer donors to achieve excellent phase separation morphology and good device performance.We further proposed the film-forming process of the blend of amorphous polymer acceptor and aggregated polymer donor.This work provides not only a new direction to design polymer acceptors,but also a novel approach to control phase separation morphology of all-PSCs.3.Electron mobility regulation of D-A type B?N-containing polymer acceptorsThree molecular design strategies were proposed:1)two-dimensional conjugated side chains,to extend the conjugation,2)using 4,4-difluoro-4H-cyclopenta[2,1-b:3,4-b']dithiophene(fCDT)unit,to promote close packing of polymer chains and 3)inserting an extra A unit,using D-A1-D-A2 type polymer backbone to tune the crystallinity.As a result,the electron mobility was improved from 7.1 × 10-5 cm2 V-1 s-1 to 1.7 × 10-3 cm2 V-1 s-1,which was increased by nearly two orders of magnitude.Specifically,D-A1-D-A2 type polymer acceptors show high crystallinity and the electron mobility measured by organic field effect transistors(OFETs)is 0.24 cm2 V-1 s-1,proving the excellent electron-transporting ability of B?N-containing polymer.The all-PSCs based on this type of polymer acceptors exhibit excellent PCE of over 10%.4.Phase separation morphology optimization of crystalline D-A1-D-A2 type B?N-containing polymer acceptorsBased on the above-mentioned D-A1-D-A2 type B?N-containing polymer acceptors,we optimize the chemical structure of the polymer to explore the relationship between polymer structure,phase separation morphology and device performance.As indicated by the research:1)introducing fluorine atoms can enhance the compatibility of polymer donor and polymer acceptor,and thus can reduce the phase separation size;2)optimizing the length of the alkyl chain can regulate the aggregation behavior of the polymer;3)introducing siloxane groups at the end of the alkyl chains can adjust the polymer backbone orientation.