Focussing on solar cells

This week we are delighted to have Dr Diego Alonso Alvarez from the Department of Physics talking about his work on improving on how we harvest energy from sunlight.

The sun has been Earth’s main power source for literally billions of years. As we move towards a low carbon society generating electricity directly from the sun is seen as one of the best ways to meet our energy needs. High concentration photovoltaics (CPV) are the most efficient form of converting solar energy directly into electricity. It uses mirrors or lenses that concentrate the sunlight into tiny, but very sophisticated, multi-junction (MJ) solar cells. These solar cells can convert up to 46% of sunlight to electricity, around twice as efficient as flat PV panels. Additionally, given that a CPV panel tracks the position of the sun, the daily energy yield is much greater.

Despite these clear advantages, the technology still has cost issues that prevent it being more widely use. Mass solar cell production and high throughput will enable industry to reduce manufacturing cost and increase the technology’s share of the energy market.

Multi-junction solar cell testing. When electrical current is injected in the solar cell, it emits light – bright red in the case of a GaInP solar cell – that can be collected and analysed with a probe. Devices fabricated at the Fraunhofer Institute of Solar Energy Systems, Germany (http://www.ise.fraunhofer.de).
Multi-junction solar cell testing. When electrical current is injected in the solar cell, it emits light – bright red in the case of a GaInP solar cell – that can be collected and analysed with a probe. Devices fabricated at the Fraunhofer Institute of Solar Energy Systems, Germany (http://www.ise.fraunhofer.de).

I am a research associate in the Department of Physics, Imperial College London, and my work has been focussed on novel materials for solar cells, exploiting nanotechnology and quantum effects. Among others, I have worked with quantum dot intermediate band solar cells during my PhD in Spain and, more recently as a Marie Curie Fellow within the Quantum Photovoltaics Group on multi-quantum well solar cells, all of them aiming to push the efficiency limits of conventional devices.

Recently I joined the SolCell project, a collaboration of European companies and national research institutions studying cutting edge materials used in high efficiency multi junction solar cells. The overarching goal of the project is to provide the measurement infrastructures required by European industry and laboratories to accelerate the development of multi-junction solar cells. In other words, to provide the means of fabricating and characterising, fast and accurately, these type of advanced devices, reducing the production cost and the times between the device design and the final product.

My particular role in this project is focused on the fast, contactless solar cell characterisation in order to make a diagnosis of the quality of the material, and an estimation of the final solar cell efficiency, in the production line. An early diagnosis of the solar cell performance will help to identify fabrication issues as well as sorting them in to different product ranges, all for the benefit of the production throughput and reduction of costs.

The way of doing this analysis is by studying the light emitted by the solar cell: a good solar cell has to be as good emitter as absorber, and analysing such emission can give very valuable information of the internal voltages of the device, their spectral response or their working temperature, not accessible by other means unless a complete solar cell device is fabricated. My work will put together well known results used in the thin film industry and cutting edge research on electroluminescence imaging of multi-junction solar cells.

Further work on the manufacturing processes and, possibly, also on the actual design of the CPV modules will be necessary for reaching the widespread of this technology, but this project is a first step in that direction.

In close collaboration with the National Physical Laboratory, I am also developing modeling tools of the solar cell structure and performance. These models will support the research of NPL with Kelving Probe Microscopy in order to determine accurately several material parameters such as the electron affinity or doping levels. This knowledge will enable further optimisation of the multi junction solar cells and help reaching higher efficiencies.

On the teaching side, I have enjoyed, and learned a lot, being lab demonstrator in Physics as well as supervising BSc and MSc projects, including fundamental physics and materials science for solar cells and the design and fabrication of solar cell trackers, inverters and laboratory equipment.

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