Friday, December 11, 2009

A talk by Arvind Shah

Arvind Shah, professor and founder of the Photovoltaics (PV) Laboratory at IMT, University of Neuch√Ętel, also happens to be one of the co-founders of our very own Centre for Electronics Design and Technology (CEDT) at the Indian Institute of Science, Bangalore. He was here at CEDT yesterday to give a talk titled "Thin-film silicon solar cells/modules : prospects and bottlenecks" and I was one of the privileged ones in the audience. I'm not much into thin-film technology or for that matter into any of device physics or materials research. But I still garnered a few interesting take-home points from his discourse and the subsequent deliberations in my mind and some related information gathered from the internet. I have listed them down here in bulleted form for easy assimilation.

  • The classification of the various solar cell technologies are shown in the following figures:
Mono-crystalline silicon solar cell

Poly-crystalline or multi-crystalline silicon solar cell

Amorphous silicon solar cell
  • The major obstacles in the wide-spread use of solar energy are
  1. Availability of raw materials
  2. "Grey energy" or the energy investment that goes into the production of solar cells
  3. Energy conversion efficiencies of all existing solar cell technologies are low hence requiring large areas for tapping substantial amounts of energy
  4. Energy storage problems (Batteries simply aren't good enough)
  • Talking of raw materials, it is interesting to know that the raw material that is the major bottle-neck in the wide use of wind energy happens to be Neodymium. A blog post titled "Wind and Neodymium" is certainly worth a read in this context.
  • There exists a slow degradation mechanism in amorphous silicon (a-Si) solar cells because of which the energy conversion efficiency drops to two-thirds of its original value after about a hundred hours of exposure to sunlight. Subsequently the efficiency stabilizes and there is no further degradation. This phenomenon is referred to as the Staebler-Wronski effect or Light Induced Degradation (LID) in literature. Putting it in numbers, if I bought an a-Si cell/module/panel that gives me 12% efficiency initially, after about three weeks its efficiency would drop to 8%. Of course the good manufacturers should quote this final lower value on their products.
  • Every solar cell technology has an associated temperature coefficient of efficiency. For most technologies, the temperature coefficient is negative which means that the efficiency decreases with increase in temperature. In the case of a-Si, it's the other way round i.e. they perform better as temperature rises. So, when they are getting cooked up by the sun's heat, while crystalline silicon (c-Si) cells might give an efficiency worse than their quoted values, a-Si cells might still be doing better that their quoted values.
  • A "tandem cell" refers to an approach where two cells (or two junctions) are made to work in tandem by absorbing energy from different parts of the spectrum of the incident light. A typical tandem cell structure
  • The major factors on which the efficiency of a solar cell depends are:
  1. Intensity of light
  2. Temperature
  3. Spectral content of incident light
Most, if not all, of the records set in terms of solar cell efficiencies are under ideal laboratory conditions. The commercially achieved efficiencies tend to be a notch lower for any given technology.

Dr. Shah also discussed about indirect and direct band gap semiconductors, Transparent Conducting Oxides (TCOs), light trapping methods among other issues and answered several questions posed by an eager audience.