The Bandgap Challenge

The internet is littered with publications that show not only promise, but results. Every so often an invention is reinvented, the clock is rewound, all steps retraced. This is the effect of commercially dependend universities. Recently I was investigating a possible solar panel based on Titanium and Carbon. Not because I heard about it, but because Carbon is black, which means it absorbs light energy, otherwise it would be white. In solar cells energy is extracted because the cell absorbs photons and increases the energy state of an electron, moving it to a ‘higher’ band. The energy of the light (visible by it’s color) has to match the energy needed to move to a higher band, which in many cases means only part of the spectrum is used. The term that is used to indicate the sweet spot is called the band gap. 

The rise in efficiency is due to better electron extraction, better choice of mix of materials, light concentration and multi junction design.

If you take common color dies you will find they absorb light so as to reflect the color they have. By the color you can recognize the band gap voltage. Most dies are non conductive, so if you want to havest the energy of the electrons in the higher state you need to pair them with a conductor and other materials. These are so called die sensitized solar panels, usually made of titaniom oxide with an electrolyte that also contains the die, and a tinoxide transparent front. When organic dies are used (like flower pigment) they degrade in a couple of weeks. Also the yeild is not that good (7%). On the plus side they are very cheap to make. One can combine dies to make multi junction versions, like it is done with the cells based on other materials. There are also possibilities for combinations of the two.