Energy-level alignment at organic and hybrid organic-inorganic photovoltaic interfaces

Thesis

Organic and hybrid organic-inorganic photovoltaic (PV) devices have the potential to provide low-cost, large scale renewable energy. Despite the tremendous progress that has been made in this field, device efficiencies remain low. This low efficiency can be partly attributed to the low open-circuit voltages (Voc) generated by organic and hybrid organic-inorganic PV devices. The Voc is critically determined by the energy-level alignment at the interface between the materials forming the device. In this thesis we use first-principles methods to explore the energy-level alignment at the interfaces between the conjugated polymer poly(3-hexylthiophene) (P3HT) and three electron acceptors, zinc oxide (ZnO), gallium arsenide (GaAs) and graphene. We find that Voc reported in the literature for ZnO/P3HT devices is significantly lower than the theoretical maximum and that the interfacial electrostatic dipole plays an important role in the physics underlying the charge transfer at the heterojunction. We note significant charge transfer from the polymer to the semiconductor at GaAs/P3HT interfaces, and use this result to help interpret experimental data. Our findings support the conclusion that charge transferred from P3HT to GaAs nanowires can passivate the surface defect states of the latter and, as a result, account for the observed decrease in photoluminescence lifetimes. Finally, we explore the energy-level alignment at the graphene/P3HT interface and find that Voc reported for experimental devices is in line with the theoretical maximum. The effect of functionalised graphene is also examined, leading to the suggestion that functionalisation might have important consequences for device optimisation.

Authors: Noori, K

• Publication date: 2013
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: energy-level alignment; photovoltaics; density functional theory; solar cells
• Subjects: Condensed Matter Physics; Materials modelling; Atomic scale structure and properties; Materials Sciences