Keith William Johnston

Fast, sensitive and spectrally tuneable colloidal-quantum-dot photodetectors

Solution-processed semiconductors are compatible with a range of substrates, which enables their direct integration with organic circuits, microfluidics, optical circuitry and commercial microelectronics. Ultrasensitive photodetectors based on solution-process colloidal quantum dots operating in both the visible and infrared have been demonstrated, but these devices have poor response times (on the scale of seconds) to changes in illumination, and rapid-response devices based on a photodiode architecture suffer from low sensitivity. Here, we show that the temporal response of these devices is determined by two components--electron drift, which is a fast process, and electron diffusion, which is a slow process. By building devices that exclude the diffusion component, we are able to demonstrate a >1,000-fold improvement in the sensitivity-bandwidth product of tuneable colloidal-quantum-dot photodiodes operating in the visible and infrared.

Schottky-quantum dot photovoltaics for efficient infrared power conversion

Planar Schottky photovoltaic devices were prepared from solution-processed PbS nanocrystal quantum dot films with aluminum and indium tin oxide contacts. These devices exhibited up to 4.2% infrared power conversion efficiency, which is a threefold improvement over previous results. Solar power conversion efficiency reached 1.8%. The simple, optimized architecture allows for direct implementation in multijunction photovoltaic device configurations.

Efficient Schottky-quantum-dot photovoltaics: The roles of depletion, drift, and diffusion

PbS colloidal quantum dot photovoltaic devices in a Schottky architecture have demonstrated an infrared power conversion efficiency of 4.2%. Here, we elucidate the internal mechanisms leading to this efficiency. At relevant intensities, the drift length is 10μm for holes and 1μm for electrons. Transport within the 150nm wide depletion region is therefore highly efficient. The electron diffusion length of 0.1μm is comparable to neutral region width. We quantitatively account for the observed 37% external quantum efficiency, showing that it results from the large depletion width and long carrier lifetime combined.

Schottky barriers to colloidal quantum dot films

We elucidate experimentally a quantitative physical picture of the Schottky barrier formed at the junction between a metallic contact and a semiconducting colloidal quantum dot film. We used a combination of capacitance-voltage and temperature-dependent current-voltage measurements to extract the key parameters of the junction. Three differently processed Al∕PbS colloidal quantum dot junction devices provide rectification ratios of 104, ideality factors of 1.3, and minimal leakage currents at room temperature. The Schottky barrier height is 0.4eV and the built-in potential 0.3V. The depletion width ranges from 90to150nm and the acceptor density ranges from 2×1016to7×1016cm−3.