As the article states, carrier multiplication due to impact ionization is an old and well known phenononon in semiconductors.
Basically, for a single-crystal semiconductor, the average number of minority carriers (N) that are generated by a single photon absorption is given by
N = Eph/w,
where w is a value given by the Klein Equation (roughly 3.37 times the bandgap of the semiconductor) and Eph is the energy of the photon, both expressed in eV.
So, for silicon with a bandgap of 1.1 eV, the threshold for impact ionization is 3.62 eV which corresponds to a wavelength of 343 nm - well into the UV - and twice that photon energy (172 nm) will average two minority carriers per photon.
The problem is that there is virtually no flux from sunlight at 350 nm and there is none at all at 172 nm.
PbSe has a bandgap of 0.27 eV, so the impact ionization threshold is 0.91 eV (1366 nm) and twice that energy is 683 nm (red light) which does have a reasonable amount of flux in the solar spectrum so you can expect a kick in performance.
Now, nanoparticle solar cells are weird. Standard solar cells collect minority carriers that manage to diffuse to a p-n junction before recombining and get swept across to provide the push to complete the circuit.
In nanoparticle devices, the difference in recombination rates between electrons and holes at the surface of the particle provides the push and the problem is getting the current out of the device.
In a standard cell, there is a well defined planar interface at the top of the device above which charges can be collected. In a nano-cell, the charges are separated at each particle and collection/connections are more difficult.
This work is interesting, but has a long way to go before it can become practical. |
