Thin Film Solar

Despite the fact that silicon is the typical semiconductor in many electric units, including the photovoltaic cells that photovoltaic panels utilise to convert sunlight into electricity, it is hardly the most cost-efficient material available. For example, the semiconductor gallium arsenide and related ingredient semiconductors provide nearly two times the efficiency as silicon in solar devices, but they are rarely utilised in utility-scale applications mainly because of their excessive manufacturing value.

University of  Illinois (http://illinois.edu) professors J. Rogers and X. Li explored lower-cost techniques to produce thin films of gallium arsenide that also granted versatility in the kinds of devices they can be integrated into.

If you may minimise substantially the cost of gallium arsenide and other compound semiconductors, then you can increase their own range of applications.

Typically, gallium arsenide is placed in a single thin layer on a smaller wafer. Either the needed device is produced directly on the wafer, or the semiconductor-coated wafer is cut up into chips of the ideal dimension. The Illinois group made the decision to deposit numerous layers of the material on a one wafer, making a layered, “pancake” stack of gallium arsenide thin films.

If you increase ten levels in a single growth, you only have to fill the wafer a single time. If you do this in ten growths, loading and unloading with temp ramp-up as well as ramp-down take a lot of time. If you consider what is required for every growth – the equipment, the preparation, the time, the people – the overhead saving this solution presents is a substantial cost reduction.

The raw material: gallium arsenide

Following the scientists individually peel off the levels and shift them. To complete this, the stacks swap levels of aluminum arsenide with the gallium arsenide. Bathing the stacks in a solution of acid and an oxidising agent dissolves the levels of aluminum arsenide, freeing the single thin sheets of gallium arsenide. A soft stamp-like device picks up the levels, just one at a time from the top down, for shift to another substrate – glass, plastic material or silicon, depending on the application. Next the wafer could be used again for another growth.

By performing this it's possible to create considerably more material more quickly and more cost effectively. This process could make bulk amounts of material, as opposed to just the thin single-layer way in which it is typically grown.

Freeing the material from the wafer additionally starts the probability of flexible, thin-film electronics made with gallium arsenide or other high-speed semiconductors. To make devices which may conform but still maintain higher performance, which is significant.

In a paper released online 20 May 2010 in Nature (www.nature.com/nature/journal/v465/n7296/full/nature09054.html), the team details its techniques and displays three types of units making use of gallium arsenide chips manufactured in multilayer stacks: light products, high-speed transistors and photo voltaic cells. The authors also provide a detailed cost evaluation.

One more advantage of the multilayer technique is the release from area constraints, particularly crucial for photo voltaic cells. As the levels are taken out from the stack, they may be laid out side-by-side on one more substrate to produce a much larger surface area, whereas the standard single-layer process restricts area to the dimension of the wafer.

For photovoltaics, you want big area coverage to catch as much sunlight as possible. In an extreme situation we may grow enough layers to have ten times the area of the standard.

In the future, the team programs to explore more potential device applications and additional semiconductor resources that could adapt to multilayer growth.