Given that the combustion of carbon-based molecules for power is heavily polluting and is altering the planet's climate, environmentally-friendly sources of energy are urgently needed. Sunlight holds promise as one of these energy sources.
There are a number of problems here, one being that materials which convert sunlight into energy (the workhorse of solar cells) tend to be very expensive. Brian Krogel (University of Texas at Austin) and coworkers have addressed this issue by cheaply fabricating functioning (yet inefficient) solar cells from common materials.
Current state-of-the-art.
One of the most successful solar cells is based on a thin film of crystals comprised of copper, gallium, indium, and selenium. All of these metals, except for copper, are rare (expensive), yet power conversion efficiencies of almost 20% are possible.
Alternatively, solar cells based on a thin film of crystals comprised of copper, zinc, tin, and sulfur have been prepared. Although these components are cheap, power conversion efficiencies are less than 7%.
Beyond the issue of rare (expensive) components, a further challenge of fabricating thin film metal-based solar cells is that common methods of fabricating them are either expensive or harsh. Ideally, the metal crystals could be synthesized independently, and subsequently coated on the surface, a cheap and relatively gentle protocol.
Korgel and coworkers have taken such issues into consideration in their fabrication of thin film metal-based solar cells. Although the power efficiencies of their solar cells are well below commercial viability, further technical development will likely improve on their initial results.
Fabricating and evaluating the solar cells.
The scientists dispersed their metals (compounds of copper, zinc, tin, and sulfur) in a solvent, and heated it to 280°C for one hour in the absence of oxygen. The resulting metal crystals are roughly 11 nanometers in diameter.
There is even distribution of the metals throughout the crystals. This was confirmed by electron microscopy (a small-scale imaging technique) coupled with elemental imaging (discrimination between the different metals within the crystals).
The scientists then dispersed the crystals in another solvent, and sprayed the crystals onto a glass surface, without further processing. The power conversion efficiency of the thin film was 0.23%, meaning that roughly 1 out of every 435 units of light energy sent towards the thin film was converted into electrical energy.
Further development.
Clearly, a 0.23% power conversion efficiency is quite low, given that other technologies can reach almost 20%. However, the thin film was not processed in any way after fabrication, and it's easy to envision higher power conversion efficiencies if such processing were to be carried out.
The huge advantages of the solar cells described herein are that they are cheap, and the fabrication is far more versatile and gentle than most other techniques out there. If the power conversion efficiency can be improved significantly, these solar cells may easily replace the expensive devices currently in use that are fabricated from rare (expensive) metals.
for more information:
Steinhagen, C., Panthani, M. G, Akhavan, V., Goodfellow, B., Koo, B., & Korgel, B. A. (2009). Synthesis of Cu2ZnSnS4 Nanocrystals for Use in Low-Cost Photovoltaics
Journal of the American Chemical Society, 131 (35), 12554-12555 DOI: 10.1021/ja905922j