Quantum dots (QDs) absorb light based on their size due to a strong energy band gap-size relationship. I have read that mixing multiples sizes of QDs together in one solar cell would not work because the charge transport in such a matrix is poor. Apparently, mixing sizes leads to charge trapping states?
There was no citation for this statement and I'm wondering if others in the field have information on charge transport in a system of mixed sized QDs. Does anyone know of any papers that discuss this or researchers that are exploring this area?
Actually, it is quite simple to understand.
Imagine a conduction band electron jumping from dot to dot, gving rise to charge transport through the material. Now, if you mix QDs with different sizes, there will be some whose conduction band (CB) lyes at lower energies, and some whose CB lies at higher energies (the energy position of the CB is size-dependent due to quantum confinement). Hence, once the random-walking electron falls, by chance, on an lower-lying state, belonging to a low-lying CB, it will be "trapped", because such a level will behave as a local trap, as any jump to nearby QDs will require an activation energy (the acceptor levels are all higher than the donor), which will ultimately hinder the mobility.
Thank you both for your responses. Fabrizio, if the conduction band electron had a specific amount of energy x when it entered the lower energy conduction band of a larger dot, wouldn't it still have the same energy x and be able to exit that conduction band with energy x? Or you are saying the electron will emit a photon as it "falls" to the lower energy conduction band of the larger dot and therefore have energy y (y
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