LiF Doping of C₆₀Studied with X-ray Photoemission Shake-Up Analysis

Abstract

We report our investigation of the chemical doping mechanism induced by LiF interaction with fullerene thin films. High resolution Xray photoelectron spectroscopy of the C1s shake-up satellites and F1s main core level, supported by density functional calculations, suggest the formation of a charge transfer complex between covalent LiF monomers and dimers and C 60 . This interaction was observed in both LiF/C 60 and C 60 /LiF depositions, suggesting that some charge transfer complexation can occur in these systems even without dissociation. hydrogen storage materials, 5 and even antibacterial treatments. 6,7 Due to the rich electronic properties in the ground and excited states, C 60 is most widely used as an organic semiconductor. With the ability to act as both an electron acceptor and donor, it is a standard material in organic light emitting diodes, solar cells, transistors and Schottky diodes.The interface with the metal contacts play a crucial role in the effectiveness of organic semiconductors in device performance and stability.8 Introducing a thin buffer layer of LiF between an organic film and an Al cathode has been seen to dramatically enhance the energy-level alignment and stability of the interface.9-13 Additionally, alternating stacks of LiF and C 60 14 and LiF doped C 60 nanocomposite buffer layers 15,16 have been shown to have high conductivities and stability in OLEDs and OPVs. Several mechanisms have been proposed for these behaviors including chemical reactions, dipole alignment, tunneling, and LiF dissociation requiring the additional presence of Al. 17In this contribution, we present results showing that in the absence of Al, there is a charge transfer interaction between LiF and C 60 , leading to the formation of LiF••C 60 complexes without dissociation. Using the well-developed and described X-ray photoelectron shakeup structure of C 60 and the strong feature from F1s XPS core level, we see evidence of a direction independent interaction. The spectroscopic features can be described by the semi-covalent interaction of LiF monomers or dimers with C 60 using density functional theory. Coupled with XPS core level shift calculations, using approximate Madelung energy, the predicted Millikan charges and bond lengths give values that approximate the observed core level shifts. Additionally, the small transfer of charge to C 60 predicted by the modelling is consistent with the minimal impact on the shake-up satellites of the C1s of C 60 . ExperimentalThe samples were produced using a Kurt J. Lesker OLED cluster attached to an X-ray photoelectron spectroscopy tool. For deposition of C 60 , ∼5Å of LiF on 100 nm sputter deposited Au/Si was used z E-mail: turaka@mcmaster.ca as a substrate. For deposition of LiF, the substrate was 350Å of C 60 thermally deposited onto cleaned Si with native oxide. C 60 and LiF were thermally evaporated from home built crucible sources at an average rate of 1 Å/min and 2-3 Å/hr as measured by oscillating quartz crystal microbalance (Inficon XTM/2) respectively onto previ...

On the Growth of Evaporated LiF on P3HT and PCBM

“…These were observed at 1.2 eV below and 1.3 eV above the respective ester CO and C–O–C peaks. As an interesting note, Turak et al deposited LiF onto in situ-evaporated C 60 and observed a charge complex formation occurring via the F 1s and C 1s conjugated π states in the fullerene cage; a high binding energy shoulder was present in the F 1s XP spectrum, and the conjugated π states were seen to increase in intensity. The air-curing of the PCBM and consequential oxidation in the present work could possibly have had an impact on the observed differences in LiF upon deposition on the fullerene.…”

“…Ideally, the concentration depth profiles of the components across the interface should also be determined to check for diffusion. There are many reports in the literature that have determined changes of the electronic structure for the near-surface area upon deposition of LiF onto organic interface materials ,,,− and the LiF layer formation on various organic layers, ,,,, but none as yet have addressed the changes in the electronic structure of the salt/organic layer interface upon salt deposition, whilst monitoring the chemical changes with salt deposition and also determining the vertical distribution of the salt.…”

LiF Nanoparticles Enhance Targeted Degradation of Organic Material under Low Dose X-ray Irradiation

“…The observed change in the LiF crystal texture was not consistent with that observed under other irradiation conditions [55] and, thus, warrants further study. The photoemission spectra of the damaged sample showed that the C 1s feature from DIP was intact, with only a slight shoulder at a higher binding energy, which may be consistent with F C bonding [56], but this is inconclusive.…”

Accelerating the formation of the conjugated ladder structure of Poly(acrylonitrile-co-vinyl acetate) by cross-linked poplar lignin doped with boron phosphate

“…After an electron is emitted during the photoionization process, valence electron will rearrange. When the conjugated ladder structure is formed, the valence electron from the highest occupied molecular orbital (HOMO) probably transits to an unoccupied lowest unoccupied molecular orbital (LUMO) [35], resulting in π-π * shake-up [36]. Besides, the gap to Femi energy level also decreases [37], so that multi valence electrons can transit to empty states in Femi energy level, which leads to asymmetric tailing.…”