Graphene transparent conductors for tandem photovoltaic cells

Thesis

Transparent conducting electrodes (TCEs) are a major source of efficiency loss in perovskite/silicon tandem solar cells. With the best-performing TCEs containing indium, they also compromise the sustainability of this promising renewable energy technology. Graphene, owing to its broadband transmittance and high mobility, offers great potential as an alternative TCE. However, its sheet resistance remains too high to be competitive against conventional TCE materials. Despite low sheet resistance being attainable via chemical doping, such techniques have poor stability and reduce transmittance, limiting their applicability in optoelectronic devices. This thesis explores how ion-charged dielectrics (ICDs) can be used to modulate the conductivity of graphene to achieve sufficiently low Rsheet to make it competitive with state-of-the-art TCOs in tandem solar cells.

In this thesis, a proof-of-concept for the ICD doping of graphene is first developed. The impact of a corona-charged dielectric on the properties of a graphene layer interfaced with it is characterised, demonstrating a graphene sheet resistance reduction of >75% to ~400 Ω/□. This is expanded upon by investigating the impact of migrated potassium ions in SiO2/Si on graphene sheet resistance. A record 117.9 Ω/□ sheet resistance is achieved – among the lowest reported in the literature for CVD monolayer graphene wet transferred to SiO2. Field-effect transistor characterisation indicates a charge concentration >6×10^12 /cm2 was generated on the graphene. The ability and stability of ionic charge in glass to dope graphene are also investigated, with sheet resistance <300 Ω/□ observed in vacuum after Na+ migration to the graphene/glass interface. This is contrasted with graphene encapsulation with corona-charged PMMA and AlOx. Although they can dope graphene, they have poor stability over time. Finally, the suitability of ICD-doped graphene for perovskite/silicon tandem cells is assessed via modelling. It is calculated that an efficiency gain of >1%abs may be achieved if ICD-doped graphene is used instead of state-of-the-art indium-based TCOs.

Authors: O'Sullivan, J

• DOI: 10.5287/ora-xrwp6815z
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: graphene; transparent conducting electrodes; photovoltaics
• Subjects: Materials science; Graphene; Two-dimensional materials; Photovoltaic cells