Studies Of Photovoltaic Properties Of Donor/acceptor Materials Constructed By End Group Strategy And Random Copolymerization Strategy

Organic solar cells have attracted widespread attention due to their unique properties,such as light weight,flexibility,semitransparent and processing capabilities for solution printing.Among them,the active layer material is the key component for high-performance organic solar cells.However,there are still few active layer materials with high efficiency and high stability.Herein,based on simple end-group strategy and ternary copolymerization strategies to regulate the molecular energy level,light absorption and morphology of active layer materials,a series of benzodithiophene（BDT）polymer donors,banana-type small molecule acceptors and polymer acceptors are prepared and applied to efficient organic solar cells.The specific research work is divided into the following aspects:（1）Basd on a simple end-capped strategy,a series of polymer donors（PM6,PM6-T,PM6-T-C1,PM6-T-C6,PM6-T-C12）were designed and synthesized,and the unreacted end group Br and Sn（Me）₃ in the polymer donor were removed by full-end-capping.It was found that the crystallinity of the polymer donor increased with the growth of the thiophene alkyl side chain of the end-capping groups,among which PM6-T had the most suitable phase separation,and the active layer formed by PM6-T-C12 with the longest alkyl side chain and acceptor Y6 had the worst morphology and the largest phase separation size.At the same time,the end-capping strategy effectively reduces the quenching of exciton in the active layer,increases the short-circuit current density(J_SC)and fill factor（FF）of the device,and improves the device performance.The PM6-T:Y6-based device achieved the highest power conversion efficiency（PCE）of 17.11%,with the open-circuit voltage(V_OC)of 0.850 V,J_SC of26.00 m A cm^-2,and a high FF of 77.40%,which is the highest value of the PM6:Y6-based devices.With the increase of alkyl chain length,the device of PCE decreases gradually,among which PM6-T:Y6 with 17.11%,PM6-T-C1:Y6 with 16.54%,PM6-T-C6:Y6 with 16.23%,and PM6-T-C12:Y6 with 15.98%.When PM6-T was used in ternary system（PM6-T:A-D-A:BTP-e C9）,PCE reaches 18.45%,which is one of the highest performances of BTP-e C9 devices.And the prepared end-capped devices have better storage stability,optical stability and thermal stability in N₂ environment.（2）Two novel small molecular acceptors of ZH1 and ZH2 were designed and synthesized through the end-capping strategy.Among them,ZH1 has a symmetrical structure,ZH2 has an asymmetric terminal group structure,and the molecular dipole moment of ZH2（5.55 Debye）is larger than that of ZH1（1.67 Debye）,and the dihedral angle of ZH2 was also larger than ZH1,which can effectively reduce the molecular planarity.The PM6:ZH1-based device of power conversion efficiency（PCE）was 3.86%with a open-circuit voltage(V_OC)of 1.057 V,and fill factor（FF）of52.83%.The PM6:ZH2-based device of PCE was achieved 10.04%,V_OC of 0.943V,and FF of 67.45%.When they were introduced into the PM6:BTP-e C9 system as the third component,which formed a complementary absorption range,formed an alloy with the acceptor BTP-e C9 effectively,reduced its stronger aggregation,regulated crystallization,optimized the molecular packing between acceptors,formed the interpenetrating network structure,enhanced exciton dissociation and charge transport efficiency,and reduced energy loss.Finally,the PM6:BTP-e C9:ZH2-based device obtained a remarkable champion PCE of 18.51%with a short-circuit current density(J_SC)of 27.10 m A cm^-2,V_OC of 0.860 V,and FF of 79.44%,which is significantly better than the binary system of PM6:BTP-e C9（PCE=16.92%）.The above results show that the asymmetric acceptor as the third component is an effective method to control the crystallinity and improve the performance of binary system.（3）Basd on the end-capped strategy,the asymmetric acceptor L8-BO-Br was introduced as the end-capped group into the polymer acceptor PY-IT,and a series of asymmetric end-groups polymer acceptors with different polymerization degrees（PY-IT-L8-6h,PY-IT-L8-24h,PY-IT-L8-48h,PY-IT-L8-72h）were constructed.The end-capped group can slightly regulate the energy level of polymer,but worse compatibility with the polymer donor PM6-T.Only PM6-T:PY-IT-L8-48h-based device reaches the similar efficiency with PM6-T:PY-IT-based device.As the content of end-capping groups increases,suggesting that the PM6-T:acceptors combitnation is more hypo-miscible in their blend.The devices of PM6-T:end-capped polymer acceptors-based has lower the exciton dissociation efficiency and more serious the charge recombination.However,this is a meaningful attempt to show that the effect of end groups on the performance of polymer acceptors is very significant,and should be paid more attention in material design and modification.（4）Through the ternary copolymerization strategy,3-cyanothiophene（T-CN-2Br）was introduced as the acceptor unit（A2）into the D-A copolymer donor PM6,and a series of D-A1-D-A2 polymer donors（PM6-CN-5%,PM6-CN-10%,PM6-CN-15%and PM6-CN-20%）were synthesized.Based on the electron-withdrawing characteristics of cyano-group,the polymer gains a deeper the highest occupied molecular orbital（HOMO）level and stronger crystallinity as the content of A2 unit increases.When the content of A2 unit reaches 20%,the polymer donor PM6-CN-20%has highest crystalline,resulting in poor compatibility and film morphology.As a results,the PM6-CN-15%:Y6-based device achieved a superior power conversion efficiency（PCE）of 16.79%,and the open-circuit voltage(V_OC)was increased to0.851 V,with short-circuit current density(J_SC)of 25.58 m A cm^-2,and fill factor（FF）of 77.15%.The results show that the ternary copolymerization was effective method in regulating the energy level and crystallinity of polymers.It can be used to obtained high-performance donor materials.