New device architectures for lead sulfide colloidal quantum dot photovoltaics utilizing metal oxide transport layers and electrodes
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Colloidal quantum dot photovoltaics are a class of third generation solar cell with the potential to harness greater amounts of solar energy than their silicon predecessors. These devices operate on the basis of electron transfer between two thin layers of semiconducting material. This project considers devices with semiconducting layers consisting of zinc oxide and lead sulfide quantum dots. Quantum dots are nanocrystalline semiconducting material, usually only a few hundred atoms in diameter. Their versatility lends them to wide ranging applications such as solar cells, light-emitting-diodes, and biological tagging. Their unique physical architecture motivates new inquiries into the physics of charge dynamics on the nanoscale. This project uses two new device architectures for lead sulfide colloidal quantum dot photovoltaics. The goal is to examine the effect of oxygen exposure and layer thickness on device performance. We fabricate two sets of devices using sputter deposition and thin-film spin casting techniques. The power efficiency is determined by measurements of device current-voltage characteristics under illumination from a white light-emitting-diode. Quantum efficiency measurements are taken with a spectrometer, chopper and a Xenon light source. It is observed that exposing the device to oxygen after deposition of the lead sulfide quantum dot layer has a detrimental affect on device power and quantum efficiency, but results in increased open circuit voltage. Decreasing the thickness of the zinc oxide transport layer also produces a significant increase in device power efficiency. Finally, an annealing process is shown to have a restorative effect on electron transport through thicker layers of zinc oxide.