Daniel J Farrell

Research interests

Improving the efficiency and driving down costs of solar cells are the goals of photovoltaic research. I have a general interest in how photonics and photon management can be applied to the solar energy conversion process to reach these goals. I am also interested in the possibilities that nanostructures may have in enabling new and interesting optical and electronic properties of semiconductor devices.

Hot-carrier solar cells

The hot-carrier solar cell is one of the most ambitious so-called "3rd generation" device concepts. An ideal device has the potential to reach 84% power conversion efficiency.

The device concept is simple: electrical power is extracted from a thermal gradient established between hot-carriers in an semiconductor absorber and cool carrier in external metal electrodes. A membrane (or filter, often called an energy selective contact) is placed between these two regions which only allows carriers to pass at a high energy. This makes a material that behaves optically as a low-bandgap semiconductor and electronically as a one with a high bandgap. Therefore the hot-carrier solar cell achieve both a high current and a high voltage at the same time, which allows the device to approach the ultimate thermodynamic limit of solar energy conversion.

However, a steady-state hot-carrier distribution must first be achieved if this high efficiency is to be reached. I have been developing a rate equation approach which simulates the semiconductor physics of a hot-carrier solar cell with the goal of identifying required material properties and designing interesting candidate structures.

Luminescent solar concentrators

One purely optical approach to the reduction of costs is the Luminescent Solar Concentrator (LSC).

Cost reduction is achieved by replacing the light harvesting area of conventional solar panels by low-cost, planar luminescent waveguides. A small strips of solar cells are attached to the edges therefore reducing the cost of the whole device.

What I call ‘first generation’ luminescent concentrators were devised in the late 1970s and offered realistic savings when the cost-per-Watt of flat-plate silicon was ~30$/W. Research into LSCs has continued to the present day; focusing on how new materials such as colloidal nano-structures and photonic structures can augment traditional designs. However, the best of these structures are still only capable of transporting ~10% of incident photons to the cells. In these devices, photon transport towards the cells is severely hindered by multiple reabsorption events that redirect photons out of waveguide modes, and limit devices to low efficiency.

Research interests are now focusing on ‘second generation’ approaches, which attempt to enhance the optical transport, and should enable high power conversion efficiency at low cost, provided the optical properties can be pushed to the limit. If this is achieved, the theoretical efficiency limits of luminescent concentrators are the same as conventional solar cells, ~30%.