Lead oxides for photovoltaics

Thesis

This thesis investigates lead oxides as photovoltaic materials. Vacuum deposition methods and ex-situ annealing are used to produce different stoichiometries of lead oxide. The relationship between structure and the optoelectronic properties is then investigated. Following this, a number of photovoltaic devices are prototyped and a Kelvin probe used to determine and understand the band structure of devices.

Thin films of PbO produced via air annealing of thermally evaporated lead consist of a mixture of two phases, orthorhombic and tetragonal, that determine the materials properties and effectiveness as absorber layer in a Schottky device. Films of higher tetragonal content are more photoactive, showing lower series resistance. Kelvin probe reveals that with an increasing work function of the PbO with increasing duration of the annealing, the Schottky barrier between PbO and Al increases, which results in a higher VOC. This trend is inverted when the Fermi level of PbO drops below that of ITO, creating an opposing junction.

Reactively sputtered PbO2 films are highly conductive degenerate semiconductors. Increasing oxygen flow rate during deposition leads to increased resistivity and decreased mobility, resulting from a decrease in grain size. Alongside this an increase in carrier concentration is observed as the material gets less ordered at higher oxygen flow rates, which results in an increase in Fermi level. Due to its high conductivity the material is not photoactive, and the high work function between -5.6 and -5.8 eV does not allow the formation of a Schottky junction or a p-n junction with the evaporated p- type PbO.

Post deposition annealing of the sputtered films leads to the formation of the more resistive Pb3O4 phase. This material shows lower carrier concentration and mobility, however, work functions are similarly high. The changes induced by the heat treatment are not substantial enough to be able to create a junction between the as-deposited and the annealed material, as is revealed by Kelvin probe and Hall Effect measurements.

Heterojunctions between P3HT and Pb3O4 were made to test predictions made by KP measurements. A heat treatment on P3HT improved its electronic properties and raised the Fermi level, resulting in the transformation of a diode in to a photovoltaic device and a decrease in dark current.

Authors: Droessler, LM

• Publication date: 2014
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: photovoltaics thin film metal oxide thermal evaporation sputtering
• Subjects: Materials Sciences