Molecular Orientation in Polymer Films for Organic Solar Cells Studied by NEXAFS

Abstract

The application of new polymer materials for organic solar cells requires a detailed understanding of the orientation and the electronic properties of the system because these have a dramatic effect on the device performance and efficiency. The films were prepared by doctor blade casting, a simple preparation route from solution. We study the orientation of the low band gap polymers poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) and poly-[2,6-(4,4bis(2-ethyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene)-alt-5,8-(2,3-dioctylquinoxaline)] (PCPDTQ) as well as of the state of the art polymer poly(3-hexyl-thiophene) (P3HT) using near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Clear angular dependence in the NEXAFS spectra is observed for P3HT and PCPDTBT pointing to well-orientated films.

Surface, interface and electronic properties of F8:F8BT polymeric thin films used for organic light‐emitting diode applications

“…The three less intense peaks (labelled here A, B and C) have contributions from both the PEDOT and F8BT polymers. Similarly to previous XAS studies following S‐K edge photoexcitation spectra of PSiF‐DBT and PCPDTBT polymers, both containing the electron‐deficient benzothiadiazole and the electron‐rich thiophene units, and also the photoabsorption data obtained for neat PEDOT:PSS (see Fig. 1S), peak A (at 2471.3 eV) is better assigned as an S1s→π * transition from the benzothiadiazole unit.…”

Imaging Nanoscale Morphology of Semiconducting Polymer Films with Photoemission Electron Microscopy

“…A comprehensive overview is given by Huang et al [6] as well as McNeill. [19][20][21] These techniques allow spatially averaged insights into the overall orientation of polymer chains and the overall degree of order in the sample. A major drawback of this technique is that it does not deliver information about the orientation of polymer chains inside crystallites and the local degree of order.…”

“…[7] A commonly used technique is atomic force microscopy (AFM), [5] which allows for high-resolution imaging of the material surface and primarily delivers information about surface topography. [19][20][21] These techniques allow spatially averaged insights into the overall orientation of polymer chains and the overall degree of order in the sample. Therefore, AFM investigations are often combined with scattering techniques such as grazing-incidence X-ray scattering (GIXS), [12][13][14][15][16] resonant soft-X-ray scattering (R-SoXS), [12,13,17] or small-angle neutron scattering (SANS), [18] or with polarization-dependent absorption techniques such as near-edge X-ray absorption fine structure (NEXAFS).…”

Combining theory and experiment for X-ray absorption spectroscopy and resonant X-ray scattering characterization of polymers

“…Even though these K edges start to go beyond the energy range of many soft X-ray beamlines, there is much interest to increase capabilities in this "tender" X-ray (2 keV-7 keV) regime. Although typically less common due to the limited number of endstations that can perform XAS or RSoXS near the sulfur and phosphorus K edges, notable work has been done on sulfur edge XAS to probe molecular orientation of semiconducting oligothiophenes such as α-sexithiophene [80][81][82], polythiophenes [83][84][85]20], and low-bandgap donoracceptor polymers [84,20,86]. Assigning the transitions near the sulfur K edge is difficult not only because of the limited body of work published, but also due to challenges such as π * and σ * peaks being very close in energy.…”