Electronic structure, molecular orientation, charge transfer dynamics and solar cells performance in donor/acceptor copolymers and fullerene: Experimental and theoretical approaches

Abstract

By combining experimental and theoretical approaches, the electronic structure, molecular orientation, charge transfer dynamics and solar cell performance in 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) films and blended with 6,6.-phenyl-C 61-butyric acid methyl ester (PSiF-DBT:PCBM) were investigated. Good agreement between experimental and theoretical PSiF-DBT UV-Vis absorption spectrum is observed and the main molecular orbitals contributing to the spectrum were determined using DFT single point calculations. Non-coplanar configuration was determined by geometric optimization calculation in isolated PSiF-DBT pentamer and corroborated by angular variation of the sulphur 1s near-edge X-ray absorption fine structure (NEXAFS) spectra. Edge-on and plane-on molecular orientations were obtained for thiophene and benzothiadiazole units, respectively. A power conversion efficiency up to 1.58%, open circuit voltage of 0.51 V, short circuit current of 8.71 mA/cm2 and a fill factor of 35% was obtained using blended PSiF-DBT:PCBM as active layer in a bulk heterojunction solar cell. Ultrafast electron dynamics in the low-femtosecond regime was evaluated by resonant Auger spectroscopy using the core-hole clock methodology around sulphur 1s absorption edge. Electron delocalization times for PSiF-DBT and PSiF-DBT:PCBM polymeric films were derived for selected excitation energies corresponding to the main transitions in the sulphur 1s NEXAFS spectra. The mixture of PSiF-DBT with PCBM improves the charge transfer process involving the π* molecular orbital of the thiophene units.

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

“…The intensity of peak A clearly decreases as the angle of incidence of the synchrotron radiation increases, a behaviour that suggests that the benzothiadiazole unit of the F8BT molecule is preferably oriented parallel to the substrate. Similar orientation was reported previously for the benzothiadiazole unit of PSiF‐DBT polymer measured at the N‐K and S‐K edge XAS …”

“…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.…”

“…Similar orientation was reported previously for the benzothiadiazole unit of PSiF-DBT polymer measured at the N-K and S-K edge XAS. 17,21,28 . It is worth mentioning that the suggested orientation represents an average orientation of the molecular plane with respect to the substrate from the volume sampled by the technique.…”

Sub-picosecond charge-transfer at near-zero driving force in polymer:non-fullerene acceptor blends and bilayers

“…Indeed, DFT calculations show that the electronic coupling and hence the transfer rate dramatically drop when the molecules are slightly further apart than in the equilibrium conformation (see discussion below). The slightly slower onset of HT in the other samples (1:1 blend and bilayer, 0.8 and 0.9 ps) can be explained by a less precise determination of the fastest time constant due to its low weight (14% and 17%), by a different molecular conformation between the donor and acceptor (we expect better coupling when m-ITIC is surrounded by J61 in the dispersed system) 48 , or by the influence of different molecular aggregation on the CT rate 10,49 . Nevertheless, the intrinsic time scale for HT remains surprisingly fast (<1 ps) no matter what phase morphology is present, in sharp contrast to previous observations (HT in ≈about 10 ps), where the influence of exciton diffusion was not accounted for 15,16,26,29 .…”

Comparing C60 and C70 as acceptor in organic solar cells: Influence of the electronic structure and aggregation size on the photovoltaic characteristics

“…contribute for the absorption spectrum of the PSiF-DBT which is showed in Fig. 3 [47] . The absorption band at 390 nm (3.18 eV) band has a strong π-π* character and mainly corresponds to electronic transitions involving orbitals which are delocalized along the PSiF-DBT.…”