An Electron and Ion Scattering Spectroscopic Study of Interfaces for Organic Photovoltaic Applications

Author: Natalya Schmerl

  • Thesis download: available for open access on 3 Jul 2020.

Schmerl, Natalya, 2018 An Electron and Ion Scattering Spectroscopic Study of Interfaces for Organic Photovoltaic Applications, Flinders University, College of Science and Engineering

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Abstract

The importance of interfacial phenomena in nanoelectronics device performance has become increasingly apparent in recent years. Organic photovoltaics is a field in nanoelectronics with the potential to provide cheaper, flexible alternatives to silicon based cells if their interfaces can be better understood and controlled, as this is where most issues with degradation or charge transport occur. As such, this dissertation has been dedicated to researching a range of materials, modifications and interfacial phenomena which have organic photovoltaic applications. In particular a focus has been on comparing the electronic structure at the outermost layer to the near surface area of each selected interface, as at this stage there are very few studies of this type in this field. Depth profiling was also performed in most instances to check for diffusion at the interface.

One chapter has also been dedicated to valence band spectral acquisition for insulating polymers. Insulating polymers can be embedded with conductors or semiconductors to create specific properties or morphology, and to properly characterize the electronic structure composite films the constituent film characterization is desirable. Three methods for charge compensation were compared, the most successful of these was embedding the polymer surface with nanotubes which had a rather featureless valence band spectrum. In this way the valence band features of the polymer were able to be successfully identified and compared to theoretical calculations in the literature.

Cleaning processes such as sputtering, plasma, and heating are common film treatments, all of which influence the electronic structure at the surface. As such, a comparative study was performed using ZnO as the case study. A range of Al doped films were likewise investigated and the band gap was also measured. The doping process was shown to incorporate AlOx into the ZnO lattice, increase the band gap, lower the conduction band and induce band bending in the valence band. Sputter cleaning with UHV heating was most effective for removing contamination, and plasma cleaning resulted in extra oxygen at the surface and surface dipole formation, this dipole was removed with UHV heating. When UHV heating was applied to both the sputter cleaned and plasma cleaned samples a surface conduction channel was exposed which could improve charge conduction in devices.

An interfacial layer inserted between the PCBM/P3HT photoactive layer and high workfunction electrode improves device performance but the mechanisms are not fully understood. LiF and NaF are two such interfacial materials, so a study was performed to gain insight into the differences in surface electronic structure induced by each salt on each organic material. Both salts induced a redistribution of electrons in the organics. NaF had a stronger influence which, with sufficient thickness fluorinated both PCBM and P3HT. Both salts induced an interfacial dipole on PCBM and P3HT, and it was found that altering the deposition method of salt from a single layer to sequential thinner depositions had an influence on the effect the dipole had on the valence band. The ability to alter the dipole via deposition method could be beneficial for OPV devices.

Keywords: Surface Analysis, XPS, UPS, MIES, NICISS, thin films, ARXPS, VBXPS, OPV, solar cells, interface analysis, nanoelectronics, electronic structure, electron spectroscopy, ion scattering spectroscopy

Subject: Physics thesis

Thesis type: Doctor of Philosophy
Completed: 2018
School: College of Science and Engineering
Supervisor: Gunther Andersson