Cadmium and its growing use in CdTe photovoltaics
Discovered simultaneously in 1817 by Karl Hermann and Friedrich Stromeyer (a physician) in Germany, Cadmium was initially detected as an impurity in medicinal Zinc Oxide.
For centuries it was used in minor medicinal compounds (despite its slight toxicity), but in the twentieth century it was discovered that coating steel with Cadmium prevented corrosion. Later, Cadmium sulfides and selenides were used as very popular yellow and red pigments respectively. You can see this iconic colour on many household items, such as the glaze used on cooking pots (see below):
Nowadays, other organic pigments have replaced cadmium for these and other uses; however, Cadmium has seen increasing attention in a very different field.
Solar panels work by converting the incoming energy of photons into electrons. The first solar cell was happened upon by the Physicist Becquerel in the mid 19th century when he was just 19. It took, interestingly, another century for solar devices to become useable at producing power, the first of which was publicly demonstrated at Bell Labs in 1954. Many materials now exist which, when layered with other substances and conductors, produce electricity when incident light is shined on them.
One material of interest to Lipmann Walton is a crystalline compound called Cadmium Telluride. Research for this substance stretches back to the 50s, when it was spotted that it has a ‘bandgap’ optimally suited to converting sunlight into electricity.
Cadmium Telluride is a black, shiny substance formed from the transition metal Cadmium and the metalloid Tellurium. It is virtually transparent to Infrared light which has seen its use in special windows and glasses for scientific instruments. However, by far its most popular and prosperous use is in the solar cells aforementioned.
From the 50s onwards, the success of CdTe and CdSe photovoltaic cells was ensured, with over 8% of all solar cells installed in 2011 containing the material. This is an impressive figure, considering that over £7.3 bn worth of solar panels last year were installed in the US alone.
The company responsible for the proliferation of this compound seems to be First Solar, who in 2011 produced two Gigawatts worth of CdTe solar panels. In February 2014 First Solar inc reported that their panels had reached an efficiency of over 20.4% – a several percent increase in just a few months. Experts hint that since the efficiency of CdTe panels only seems to be increasing: perhaps the practical efficiency of the technology is not yet close to being saturated. In other words, these panels are only going to get better – all thanks to Cadmium Telluride.
One concern over this technology is the possible toxicity of the CdTe. However, this said, many have argued that as CdTe is completely insoluble in water and is not broken down by fire, these claims lack thought. Another problem is the scarcity of Tellurium, whose crustal abundance is similar to Platinum. This poses a problem if the technology is to increase its share from 8% upwards. In response to this some geologists have pointed out that there are more mineable deposits of Tellurium on the oceanic crust.