Charge Transfer Dynamics and Molecular Orientation Probed by Core Electron Spectroscopies on thermal-annealed Polysilafluorene Derivative: Experimental and Theoretical Approaches

Abstract

Ultrafast charge delocalization dynamics in excited states of internal donor–acceptor copolymer poly[2,7-(9,9-bis(2-ethylhexyl)-dibenzosilole)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PSiF-DBT) was studied by resonant Auger spectroscopy (RAS) measured around silicon and sulfur K edges using the core-hole clock approach. The effect of the thermal annealing treatment at 100 and 200 °C on the charge transfer delocalization times and molecular orientation were probed. Exponential dependence of charge transfer times (τ CT) with photon energy was found at both absorption edges with a decreasing curve slope with the thermal treatment. Features characteristics of the Auger Resonant Raman effect were observed at sulfur KL2,3L2,3 Auger decay spectra. Edge-on and plane-on molecular orientations with respect to the substrate surface were measured for the thiophene and benzothiadiazole units, respectively, using angular-dependent NEXAFS spectra at the S K edge. Molecular orientation of silafluorene was also probed by NEXAFS at the Si K edge. The improvement of the polymer ordering with annealing was evaluated by NEXAFS. Differences in charge transfer times at Si and S K edges may be related to the localized–delocalized character of the molecular orbitals (MOs) involved in those excitation processes, which was corroborated by theoretical calculations at the Hartree–Fock (HF) level, with explicit relaxation of molecular orbitals due to the core-hole. The molecular orientation effect in the charge transfer process was also investigated through the nonadiabatic coupling matrix elements involving the first low-lying excited states in both K edge excitations.

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.…”

[1]Benzothieno[3,2-b]benzothiophene (BTBT) derivatives: Influence in the molecular orientation and charge delocalization dynamics

“…This result may be associated with the oligomer organization and ordering. The molecular orientation of the oligomers has direct influence in the charge transport process in the films, as already demonstrated for semiconducting polymers and copolymers containing the thiophene unit [1,2,16,17,30]. DitBu-BTBT and diiPr-BTBT films have preference to acquire a standing-up configuration.…”

“…The preferred molecular orientation of diiPr-BTBT and ditBu-BTBT films is the same at the surface as well as in the bulk: edge-on geometry of the organic backbone. For materials to be used in field effect transistors an upright position of the molecular plane would be the desired geometry, facilitating the source-drain current through the π overlap [30].…”

“…Through the well-established relationship between the intensities of the deconvoluted peaks of the RAS spectrum and the core-hole lifetime (S 1s, τ CH = 1.27 fs) [30], it is possible to calculate charge transfer times (τ CT ) [31]. Table 1 reports charge transfer times for the diiPr-BTBT and ditBu-BTBT oligomers.…”

Annealing effect on donor-acceptor interface and its impact on the performance of organic photovoltaic devices based on PSiF-DBT copolymer and C60

“…There is then a direct transition from an ohmic behaviour at low voltages to the SCLC regime at high V. This result agrees with angular dependent Near Edge X-Ray Absorption Fine Structure (NEXAFS) measurements that showed an improvement in film ordering after thermal annealing. 27 Using the MottGurney law (J ¼ 9ee o lV 2 /8L 3 ) to fit the SCLC regime (where e is the permittivity of the polymer, e o is the permittivity of vacuum, l is the effective mobility of holes, V is the applied voltage, and L is the film thickness), we found l $ 1.0 Â 10 À4 cm 2 /(V s) for holes in the PSiF-DBT layer. This result does not change significantly when one considers the built-in voltage of the HO devices as showed in Fig.…”