Design And Synthesis Of High-efficiency Small Molecule Donors And Research On Device Performance

Organic solar cells(OSCs)are considered as promising solar energy conversion devices with several attractive advantages,including low cost,flexibility,and solution processability.In the past decade,power conversion efficiency(PCE)over 18%has been achieved for single-junction solar devices in polymer solar cells.However,the PCE records of all small molecule organic solar cells lag behind.How to further improve the PCE of all small molecule organic solar cells is still a hot topic..In this thesis,various strategies have been used(side-chain engineering,halogen substitution,core substitution,etc.)to synthesize a series of novel small molecule donors andfinely tune the spectral absorption,energy level,and crystallinity of these molecules..And we systematically characterized the material structure,film morphology,and photovoltaic property,and thus revealing the structure-performance relationship,guiding the structural design of high-performance materials.The main contents and results are listed as follows:1.Terminal group engineering for small molecular BTR is carried out to regulate its crystallinity,film morphologies and photovoltaic property,the work is divided into two parts:a)By introducing hydroxyl group into the rhodanine unit of BTR,we synthesized a small molecule donor derivative of BTR,BTR-OH,with similar chemical structure and absorption profile,yet weaker crystallinity.We added BTR-OH into the BTR:PC71BM binary host system to construct ternary ASM OSC and found that BTR-OH can reduce the crystallinity of the mixed film,thereby effectively improving the phase separation of the active layer,and significantly improving the device performance,the PCE of 10.14%is successfully obtained in?300 nm ternary devices.b)By terminal alkyl engineering,we designed and synthesized two new small molecule donors,namely,BTO and BTEH.The terminal alkyl chain can effectively regulate the crystallinity of the donors,thus showing different compatibility with the non-fullerene acceptor(NFA)IDIC and the fullerene acceptor PC₇₁BM.The results show that the strong crystallinity donor BTO can show better device performance with the strong crystallinity NFA IDIC,while the low crystallinity BTEH can show better performance with the amorphous PC₇₁BM.2.We designed and synthesized two novel medium bandgap molecular,T3 and T5,with selenophene-or thiophene-substituted benzodithiophene(BDT)as donor unit.The influence of different two-dimensional conjugated groups on the phase separation and the photoelectric properties of the device were investigated.The results show that the device of T3:Y6 can form a better interpenetrating network with higher carrier mobility,thus achieving higher PCE(13.22%).However,the open-circuit voltage(Voc)of the T3:Y6 device is only 0.79 V,giving a lot of space for improvement.Then,we further adopted side chain engineering and introduced sulfur atoms(T7)and silicon atoms(T8)respectively into the side chains of T3 in an attempt to lower its HOMO energy level to raise the Voc.Cyclic voltammetry(CV)show that the HOMO energy levels of T7 and T8 were lower than T3,however,both donor-based devices showed the opposite photovoltaic performance compared with T3:Y6.The PCE of T7:Y6 is just 10.63%,while the PCE of T8:Y6 is as high as 13.98%,The main reason for the different device performance may attributes to the different film morphology.3.By introducing chlorothiophene,fluorothiophene,and unsubstituted thiophene in the orthogonal position of the benzodifuran(BDF)unit,we designed and synthesized three small molecule donors,namely T9,T10,and T11.The influence of atom substitution on spectral absorption,molecular energy level,and charge mobility,charge recombination,and photoelectric properties of the device were systematically studied.Compared with chlorinated T9 or fluorinated T10(both the optical band gap are 1.78e V,HOMO energy levels are-5.35 and-5.42 e V,respectively),the optical band gap of T11 is reduced to 1.75 e V,and the HOMO energy level is increased to-5.30 e V.The blend films of T11:Y6 have more balanced charge mobility,lower bimolecular recombination,and trap recombination,and better phase separation,making the PCE as high as 10.54%,where Voc=0.76 V,Jsc=20.60 m A/cm²,FF=67.7%,while the PCEs of T9:Y6 and T10:Y6 is only 9.12%and 7.40%.4.Based on side chain alkyl engineering,two simple oligothiophene donors,DC5T and DC7T,were designed and synthesized,and the influence of alkyl arrangement on their photoelectric properties was systematically investigated.With IDIC-4F as a small molecule acceptor,the DC5T based device achieved a PCE of 7.03%,while the PCE of DC7T based device was only 1.30%.In order to lower the HOMO energy level of oligothiophene and increase the Voc,we designed and synthesized new oligothiophene donors 2FDC5T and 2Cl DC5T by halogenation.The introduction of halogen atoms could narrow the bandgap of the donors,but showed different photovoltaic performance.2Cl DC5T:IDIC-4F device could hardly work,while 2FDC5T:IDIC-4F device gave a high PCE of 9.03%.Mechanism study showed that the fluorinated donor 2FDC5T can better regulate the phase separation morphology of the active layer,reduce bimolecular recombination and defect recombination in the device,thereby enhance the efficiencie of the corresponding solar cell devices.