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Insight into the Excited State Electronic and Structural Properties of the Organic Photovoltaic Donor Polymer Poly(thieno[3,4‑b]thiophene benzodithiophene) by Means of ab Initio and Density Functional Theory
journal contribution
posted on 2016-08-31, 00:00 authored by Itamar Borges, Elmar Uhl, Lucas Modesto-Costa, Adélia J. A. Aquino, Hans LischkaStructural and electronic
properties of the ground and lowest excited
states of the electron-donor conjugated copolymer poly(thieno[3,4-b]thiophene benzodithiophene) (PTB1) are reported based
on high-level theoretical investigations. PTB1 combined with phenyl-C61-butyric acid methyl ester (PCBM) results
in a bulk heterojunction blend with promising properties for use in
organic solar cells. The ab initio algebraic diagrammatic
construction method to second order, ADC(2), was used to obtain benchmark
data for excited state energies, oscillator strengths, and bond length
alternation (BLA) analysis. Time dependent density functional theory
(TDDFT) calculations using the exchange-correlation functionals PBE,
B3LYP, BHandHLYP, CAM-B3LYP, and LC-wPBE were also performed and compared
with ADC(2) calculations. It was shown that a minimum of 20% Hartree–Fock
exchange in the functional is necessary to reproduce the major features
of the ADC(2) results. Analysis of the BLA results indicates the possibility
of exciton trapping by geometry relaxation occurring in the middle
of the polymer chain. The corresponding exciton binding energy is
about 0.4 eV. Charge distributions in the ground and lowest excited
singlet state were analyzed as well. The natural population analysis
(NPA) confirms the electronegative character of the benozodithiophene
group and the corresponding positive one of the thienothiophene moiety
leading to an alternant chain polarization of positive and negative
charges. Electron density differences between the S0 and S1 states show a transfer
of electron density from double bond regions to areas of single bonds,
a feature which parallels nicely the lengthening of double bonds and
shortening of single bonds in the S1 state.
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thienothiophene moietyS 1 stateExcited State Electronicelectronegative characterelectron densitybenozodithiophene groupgeometry relaxationS 0alternant chain polarizationNPAsinglet stateexchange-correlation functionals PBEcharge distributionsexciton binding energy0.4 eVCAM-B 3LYPbulk heterojunction blendDensity Functional Theory Structuraloscillator strengthsbutyric acid methyl esterab initioTDDFTPCBMdiagrammatic construction methodstate energiesStructural Propertiesbond regionsbond length alternationPTB 1thiophene benzodithiopheneADCS 1 states showab Initiobenchmark datacalculationB 3LYP BHandHLYPelectron density differencespolymer chainC 61BLA resultspopulation analysis
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