Effect of Acceptor Strength on Optical and Electronic Properties in Conjugated Polymers for Solar Applications

Abstract

Four new low-bandgap electron-accepting polymers-poly(4,10-bis(2-butyloctyl)-2-(2-(2-ethylhexyl)-1,1-dioxido-3-oxo-2,3-dihydrothieno[3,4-d]isothiazol-4-yl)thieno[2',3':5,6]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H-dione) (PNSW); poly(4,10-bis(2-butyloctyl)-2-(5-(2-ethylhexyl)-4,6-dioxo-5,6-dihydro-4H-thieno[3,4-c]pyrrol-1-yl)thieno[2',3':5,6]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-dione) (PNTPD); poly(5-(4,10-bis(2-butyloctyl)-5,11-dioxo-4,5,10,11-tetrahydrothieno[2',3':5,6]pyrido[3,4-g]thieno[3,2-c]isoquinolin-2-yl)-2,9-bis(2-decyldodecyl)anthra[2,1,9-def:6,5,10-d'e'f']diisoquinoline-1,3,8,10(2H,9H)-tetraone) (PNPDI); and poly(9,9-bis(2-butyloctyl)-9H-fluorene-bis((1,10:5,6)2-(5,6-dihydro-4H-cyclopenta[b]thiophene-4-ylidene)malonitrile)-2-(2,3-dihydrothieno[3,4-b][1,4]dioxine)) (PECN)-containing thieno[2',3':5',6']pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-dione and fluorenedicyclopentathiophene dimalononitrile, were investigated to probe their structure-function relationships for solar cell applications. PTB7 was also investigated for comparison with the new low-bandgap polymers. The steady-state, ultrafast dynamics and nonlinear optical properties of all the organic polymers were probed. All the polymers showed broad absorption in the visible region, with the absorption of PNPDI and PECN extending into the near-IR region. The polymers had HOMO levels ranging from -5.73 to -5.15 eV and low bandgaps of 1.47-2.45 eV. Fluorescence upconversion studies on the polymers showed long lifetimes of 1.6 and 2.4 ns for PNSW and PNTPD, respectively, while PNPDI and PECN showed very fast decays within 353 and 110 fs. PECN exhibited a very high two-photon absorption cross section. The electronic structure calculations of the repeating units of the polymers indicated the localization of the molecular orbitals in different co-monomers. As the difference between the electron affinities of the co-monomers in the repeating units decreases, the highest occupied and lowest unoccupied molecular orbitals become more distributed. All the measurements suggest that a large difference in the electron affinities of the co-monomers of the polymers contributes to the improvement of the photophysical properties necessary for highly efficient solar cell performance. PECN exhibited excellent photophysical properties, which makes it to be a good candidate for solar cell device applications.

Evaluating the Effect of Heteroatoms on the Photophysical Properties of Donor–Acceptor Conjugated Polymers Based on 2,6-Di(thiophen-2-yl)benzo[1,2-b:4,5-b′]difuran: Two-Photon Cross-Section and Ultrafast Time-Resolved Spectroscopy

“…This increase in the emission intensity is likely due to a weaker electronic interactions between the BDF and DPP. 58 On the other hand, the lower emission intensity near 550 nm by the polymers with a furan as the linker suggest that a more efficient energy transfer process could be happening. The TPA cross-section were measured and the data shows a larger TPA cross-section by the polymer with a furan molecule in their main-chain.…”

Heteroatom and Side Chain Effects on the Optical and Photophysical Properties: Ultrafast and Nonlinear Spectroscopy of New Naphtho[1,2-b:5,6-b′]difuran Donor Polymers

“…The PTB7 [19][20][21] and PTB7-Th donor polymers have been widely studied and as a result there is detailed information regarding structure-function relationships within these polymers and efficiencies as high as >10% have been reported 1,22 . However, there remains some particular questions regarding the mechanism (electronic, charge related, photon or energy transfer) of the photovoltaic effect in this system.…”

“…To date, there have been only a few detailed reports comparing the structure-function relationships of thiophene-and furan-based CPs using laser spectroscopic techniques 21,25,26 . In one report, the photophysical properties of four polymers based on benzo[1,2-b:4,5-b′]difuran (BDF) donor moiety and diketopyrrolopyrrole (DPP) acceptor moiety (flanked by either furan or thiophene spacers) was investigated 26 .…”

Series of Quinoidal Methyl-Dioxocyano-Pyridine Based π-Extended Narrow-Bandgap Oligomers for Solution-Processed Small-Molecule Organic Solar Cells

“…Localization of the HOMO and LUMO molecular orbitals in different moieties can provide necessary driving force for the delocalization of excited electrons and benefit to form quinoidal structures. 49 Quinoidal structure can enhances electron transfer from the electron-rich BDT to the electron-deficient MDP, leading to an enhanced ICT transition. These data agree well with the results obtained from the film absorption spectra (see Figure 1a), in which the absorption at the near infrared region were enhanced for the 3T and 5T mediated molecules.…”