The Studies On Property Optimization Of Organic Solar Cells By Fluorinated Terpolymer Donors

Recently, polymer solar cells (PSCs) have been the subject of extensivestudy because of their potential applicability in light weight, flexible, colorfuland/or transparent large-area devices. The power conversion efficiencies (PCE)of organic bulk heterojunction solar cells have recently surpassed10%. Thedevelopment of organic polymer solar cells commercial application is imminent.High efficiency solar cells can be obtained by designing and synthesizing newpolymers.(1) The three random terpolymers were carried out by Stillepolycondensations reacting, which benzo[1,2-b:4,5-b′]dithiophene (BDT) asdonor unit and3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-octyldodecyl)-pyrrolo-[3,4-c]pyrroledione (DPP),2,5-difluorobenzene (DFB) as acceptor units.PBDT-DFB3-DPP7, PBDT-DFB1-DPP1and PBDT-DFB7-DPP3had betterthermal stability. The decomposition temperatures (5%weight loss) of thesethree polymers are405oC,408oC and409oC, respectively. All of the randomterpolymers have broad absorption bands ranging from300to900nm. TheHOMO level of polymers can be adjusted by changing the proportion of donorand acceptor units. The ratio of DFB unit is higher, both the HOMO and LUMO levels are lower. The PSC devices were fabricated with the conventionalstructure, ITO/PEDOT:PSS/active layer/LiF/Al, which the conjugatedterpolymers as donor material and PCBM as acceptor material, respectively. Theoptimization of the devices ability by adjusting annealing temperature, theproportion of the additive, the concentration of the additives, etc. In AM1.5G,power of100mW cm-2simulated sunlight, the best PCE value of2.67%wasobtained.(2) The four conjugated polymers were carried out by reacting, whichadjust the different ratio of donor and acceptor. Which BDT as donor unit andDPP, tetrafluorobenzene (TFB) as acceptor units. PBDT-TFB,PBDT-TFB3-DPP7, PBDT-TFB1-DPP1and PBDT-TFB7-DPP3had betterthermal stability. The decomposition temperatures (5%weight loss) of thesethree polymers are420oC,401oC,342oC and356oC, respectively. Theabsorption bands of PBDT-TFB is ranging from300to600nm. In this structure,the introduction of different ratio DPP unit can lead to the long wavelengthabsorption band (600-900nm). The optical band gaps of the four polymers wereestimated to be2.07eV,1.38eV,1.50eV and1.51eV, respectively. They allhave low-lying HOMO energy levels. Which can provide a new terpolymersdonor material for solar cell applications.(3) Based on the researching of difluorobenzene unit of terpolymer(PBDT-DFB1-DPP1) and tetrafluorobenzene unit of terpolymer(PBDT-TFB1-DPP1), we had synthesized and perfected terpolymer PBDT-B1-DPP1and PBDT-FB1-DPP1, and discussed the different number offluorine atoms substituted benzene as the properties of acceptor units. The seriesof terpolymers exhibited broad absorption bands ranging from300to900nm.With the increased of the number of fluorine atoms, the optical absorption of thetetrafluoro unit of polymer is significantly higher than other three polymers.And the four terpolymers have good thermal stability and low HOMO levels.Their decomposition temperatures (5%weight loss) are range of407-432oC,which enable them to become better donor material in organic polymer solarcells.