Design Of Photovoltaic Donor Materials And Charge Carrier Dynamics Study For Organic Solar Cells

Solution-processed organic solar cells(OSCs)have several unique merits such as light-weight,easily manufactured into large-area flexible and semitransparent devices.In recent years,the innovative development of non-fullerene acceptors has significantly driven the power conversion efficiencies(PCEs)of single-junction OSCs to over 18%.The photovoltaic performance of classic bulk-heterojunction(BHJ)OSCs are mainly determined by the donor/acceptor photovoltaic materials and their blend morphology.Therefore,one of the key factors to achieve high-performance OSC is rationally paring donor and acceptor to get broad and strong absorption spectrum,suitable energy offset,well developed BHJ morphology with nanoscale phase-separation.Accordingly,this thesis has carried out a series of researches to improve the photovoltaic performance of OSCs,including polymer donors design,rational donors/acceptors paring and blend morphology optimization.Besides,the internal mechanism for high-performance OSCs is also explored from the perspective of device physics study.The main achievements are as follows.1.A new conjugated polymer donor material,PBN-S,is designed and synthesized by introducing the alkylthio side chain into the electron-deficient unit benzothiophenedione.PBN-S shows a strong absorbance and low-lying HOMO level,which matches well with non-fullerene acceptor IT-4F.Optimized OSC based on PBN-S:IT-4F demonstrates a high PCE of 13.10%with an active area of 4 mm2.When the active area is expanded to 100 mm2,remarkable PCEs of 10.21%and 10.69%are still realized from spin-coated or bladecoated OSCs,respectively.In addition,PBN-S shows an outstanding application characteristic of transparent photovoltaic devices.The PCE of the PBN-S:IT-4F blend based semitransparent OSC reaches 9.83%with an average visible transmittance of 32%.More importantly,the unencapsulated PBN-S based devices show excellent shelf-life stability,retaining over 80%of their initial PCE after 100 days storage in a nitrogen-filled glove-box or in air.2.The low-cost and easily synthesized polymer donor PTQ10 is blended with the narrow bandgap non-fullerene acceptor Y6 to enhance the light-harvesting capability of active layers.Compared with the absorption spectrum of PTQ10:IDIC,ca.130 nm redshifting is observed in the PTQ10:Y6 blend film,which notably enhances the short-circuit current density of the OSC to 26.65 mA cm-2.Meanwhile,the optimal PTQ10:Y6 blend shows more efficient charge separation and transportation,as well as reduced bimolecular recombination than PTQ10:IDIC,which leads to an excellent PCE of 16.53%.3.A strategy of designing third component photovoltaic material with similar molecular structure with host material is developed to optimize the blend morphology.As the third component for PBDB-TS:ITIC blend,PBDB-BT shows similar conjugated backbone and complementary absorption to PBDB-TS.Building the ternary OSCs with 10%(wt)PBDB-BT in o-xylene solvent,a higher PCE of 10.26%is realized compared to the binary device(PCE=9.81%).In-depth studies show that the enhanced PCE of the ternary device is attributed to the improved charge dissociation and charge carrier mobility,suppressed bimolecular recombination.4.The calculation method of charge carrier mobility in NF-OSCs and the relationship between Langevin prefactor and bimolecular recombination are investigated.(1)We have figured out the zero-field,zero-charge carrier density mobility,as well as the relevant simulation parameters(width of Gaussian density of state and so on)according to the driftdiffusion simulation based on PM6:IT-4F and PM6:Y6 blends.Compared with the average mobility results from other normally used techniques(time-resolved techniques,SCLC and so on),the zero-field mobility derived from drift-diffusion simulation better reflect the real properties of the materials.(2)We have investigated the charge recombination rate(defined as Langevin prefactor,γpre)based on five blends systems of PM6:PC61BM,PM6:IT-4F,PM6:Y6 and PBDB-T:O-IDTBR,PBDB-T:IE4F-S,respectively.The comparative study shows that the bimolecular recombination generally deviates from the classic Langevin recombination picture,for which the γpre plays an important role in describing the real bimolecular recombination.For some high-performance blends with low bimolecular recombination,the γpre can be as low as 10-2.In addition,the γpre is also inversely proportional to the Fill Factor of OSCs.The results show that the bimolecular recombination and the FF value of OSCs can be intuitively understood by measuring γpre.