Synthesis And Photovoltaic Properties Of Non-planar Polymeric Acceptors With Pendant Perylenediimide Units

In the past three decades,organic solar cells have attracted intensive attention,because of their enormous potential to fabricate the flexible and semitransparent devices.To date,most of the high-efficiency organic solar cells were fabricated using the architecture of bulk heterojunction type photoactive layer,in which p-type electron donors and n-type electron acceptors are separated spontaneously to form a nanoscale interpenetrating network.In recent years,non-fullerene acceptors have developed rapidly with the power conversion efficiencies exceeding 16% for single-junction non-fullerene small molecule acceptor based binary organic solar cells.Among the non-fullerene acceptors,those containing large and rigid ?-conjugated backbone such as perylenediimide usually tends to form large crystalline aggregate domains in the blend films,which is beneficial to achieve high carrier mobility but harmful to the efficient exciton dissociation.Therefore,perylenediimide-based small molecule acceptors are mostly designed as the non-planar or quasi three-dimensional structure to suppress the excessive aggregation.Meanwhile,polymeric acceptor based organic solar cells(or all-polymer solar cells)have received increasing attention due to their excellent mechanical properties.Among the polymeric acceptors,N2200 is most well known for its high electron mobility and high electron affinity.Via rational selection of the donor polymer,the power conversion efficiencies of all-polymer solar cells based on N2200 have been pushed to over 10%.Despite such a remarkable achievement,N2200 also suffers from the aforementioned disadvantage: the strong tendency to aggregate and hot processing is often applied to relieve this conundrum.In this contribution,we were inspirited by the design strategy which was widely used to construct perylenediimide-based small molecule acceptors and appllied it to construct polymeric acceptors.We designed and synthesized a series of analogous polymeric acceptors by introducing twisted perylenediimide in the side chains and highly electron-deficient naphthalenediimide in the main chains,respectively.The design rationale is to construct a more planar main chain with a deeper lowest unoccupied molecular orbital,which guarantees the transport of electrons along the polymer main chain.The twisted perylenediimide in the side chains endows the resulting copolymers with reduced tendency to aggregate,multi-dimensional electron transport properties and broadened absorption band.Chapter 2: We designed and prepared a series of random copolymers via linking perylenediimide to the polymeric backbone of(naphthalenediimide/quinoxaline)-(2,2'-bithiophene).The effects of the pendant perylenediimide on the optical,electrochemical and photovoltaic performance were tuned via varying ratios of quinoxaline and naphthalenediimide.Unfortunately,the power conversion efficiencies of the resulting copolymers reduced from 5.21% to 3.28% with increasing ratio of the quinoxaline units from 10% to 30% gradually.Chapter 3: We replaced 2,2'-bithiophene with thieno[3,2-b]thiophene.The power conversion efficiencies of the new copolymers increased with increasing ratio of the quinoxaline units.Atomic-force microscopic images showed that all random copolymers exhibited significantly improved miscibility with donor compared with an alternating copolymer reference,which coincided with the higher electron mobility measured by the space charge limited current method.The all-polymer solar cell based on the polymeric acceptor with high perylenediimide content achieved a power conversion efficiency of 4.81%.Although there is minimal conjugation between the perylenediimides and main chains,perylenediimides can also contribute to photocurrent generation.Chapter 4: We replaced quinoxaline with benzo[1,2-b:4,5-b']dithiophene.Moreover,we converted target copolymer with single-bond bridged perylenediimide into fused-perylenediimide based copolymer via photocyclization.We compared the optical,electrochemical and photovoltaic performance of them.The power conversion efficiency of the all-polymer solar cell based on the copolymer with pendant perylenediimide was about 3-fold higher than that of fused-perylenediimide based copolymer.This result can be understood from the atomic-force microscopic images of the latter in which separated isolated domains are not interconnected.