My initial guess was that the valence shells of the QDs were partially filled (I haven't examined any in particular; this is a general conjecture) as in a p orbital with six positions for electrons and perhaps 5 occupied after first excitation by photon. With 5 e remaining, it would still be beneficial, provided there's enough energy from the incoming photon, to promote another e and thereby lower repulsive Coloumbic interactions and increase exchange energy of the remaining e. I'm not sure if this is on the right track. Any help would be greatly appreciated.
I think the mechanism itself is similar to bulk materials. One change is loss of excess exciton energy by thermalisation (so via phonons) may be reduced in QDs because the DoS is lower. Also, a second exciton can be generated in the same QD, or in an adjacent QD provided there is some mechanism of energy transfer. I can recommend the following refs. for Si QDs:
http://www.nature.com/nphoton/journal/v6/n10/full/nphoton.2012.206.html
http://www.nature.com/nphoton/journal/v2/n2/full/nphoton.2007.279.html
http://www.nature.com/nnano/journal/v6/n11/abs/nnano.2011.167.html
I'm not sure the orbital-based argument is correct because QDs are not yet isolated atoms but have a size-dependent DoS between isolated atoms and bulk material.
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