Investigating electron transport layers in organic solar cells and the effects of doping, through electronic and spectroscopic characterisation techniques

Thesis

Efficient extraction of charge carriers from a solar cell is essential to producing well-performing photovoltaic devices, with the aim of achieving current outputs upward of 20 mA/cm2 and power conversion efficiencies competing with the best photovoltaic technologies available. Incorporation of thin interfacial films of selective electron and hole conductive materials, positioned either side of the photoactive layer and before the respective electrodes, can aid in this charge extraction. The energy levels of such thin films and their respective alignment within the stack can be measured using photoelectron spectroscopy techniques to ensure charge carriers can readily transport across the interfaces for improved transport to the electrodes. As such, the maximum power conversion efficiency that a photovoltaic device achieves heavily depends on its component materials, and these “charge transport” layers play a key role in achieving the best device efficiencies.

This thesis explores the physical and optoelectronic properties of an organic electron transport layer (ETL), deposited between the photoactive layer and the cathode, comprising the fullerene-derivative material phenyl-C61-butyric acid benzocyclobutene ester (PCBCB). Data from both neat films and device stacks are compared to data from those fabricated from the commonly used ETL material, zinc oxide (ZnO). This work aims to explore how this organic ETL could result in more stable devices than with metal oxide ETLs, by eliminating photocatalysis at material interfaces. The work dives into the processing of PCBCB films and its effects on film energetics, surface and bulk morphology, and consequent effects on active layer morphology. The electronic tunability of PCBCB is further investigated by photoelectron spectroscopy, with particular focus on its work function and Fermi level position, via addition of organic n-type dopants, primarily 4-(2,3-Dihydro-1,3-dimethyl-1H-benzimidazol-2-yI)-N,N-dimethylbenzenamine (N-DMBI) and its derivatives. Furthermore, enhanced device performance is exhibited with improved processing, and with the use of doped PCBCB layers. The design and integration of a new ultraviolet photoelectron spectroscopy tool in Oxford for spectroscopic analysis of thin film and material energetics is also detailed. Photoelectron spectroscopic characterisation of other materials to be used within the fields of both organic and perovskite optoelectronic devices are also explored as an example of other investigations conducted using these techniques. Ultimately, this work hopes to advance our understanding of how processing and optimisation of the ETL can have a significant impact on organic photovoltaic performance.

Authors: Evans, N

• DOI: 10.5287/ora-xqvpkmj9x
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: electron transport; spectroscopic characterisation; organic photovoltaics; semiconductor doping; ultraviolet photoelectron spectroscopy; solar cells; OPV
• Subjects: Semiconductor doping; Organic photovoltaic cells; Solid state physics; Photoelectron spectroscopy