Multiple exciton generation

Breakdown of the causes for the Shockley-Queisser limit. The black height is Shockley-Queisser limit for the maximum energy that can be extracted as useful electrical power in a conventional solar cell. However, a multiple-exciton-generation solar cell can also use some of the energy in the green area (and to a lesser extent the blue area), rather than wasting it as heat. Therefore it can theoretically exceed the Shockley-Queisser limit.

Multiple exciton generation (MEG), or carrier multiplication, involves the generation of multiple electron-hole pairs from the absorption of a single photon. MEG may considerably increase the power conversion efficiency of nanocrystal based solar cells, though harvesting the energy may be difficult because of the short lifetimes of the multiexcitons.

MEG has been demonstrated in synthesized nanocrystals (quantum dots) including PbS, PbSe, PbTe, CdS, CdSe, InAs, and Si ^[1][2]. Recently, MEG has also been demonstrated in InP colloidal quantum dots^[3].

The quantum mechanical origin of MEG is still under debate and several possibilities have been suggested ^[4]:

• 1) Impact ionization: light excites a high-energy exciton (X) which decays irreversibly into a quasi-continuum of multiexciton (multi-X) states available at this energy. The model requires only the density of states of multiexcitons being very high, while the Coulomb coupling between X and multi-X can be quite small.
• 2) Coherent superposition of single and multiexciton states: the very first suggested model but oversimplified (high density of states of multi-X is not taken into account). Light excites an X (which is not a true eigenstate of the system) which can then coherently covert to multi-X and back to X many times (quantum beats). This process requires Coulomb coupling between them to be much stronger than the decay rate via phonons (which is usually not the case). The excitation will finally decay via phonons to a lower energy X or multi-X, depending on which of the decays is faster.
• 3) Multiexciton formation through a virtual exciton state. Light directly excites the eigenstate of the system (in this case, a coherent mixture of X and multi-X). The term "virtual" relates here to a pure X, because it is not a true eigenstate of the system (same for model 2).

All of the above models can be described by the same mathematical model (density matrix) which can behave differently depending on the set of initial parameters (coupling strength between the X and multi-X, density of states, decay rates).

References

1. ^ MEG and electrical extraction from a PbSe quantum dot photoconductor. Applied Physics Letters 92, 031107 (2008)
2. ^ MEG in colloidal silicon nanocrystals. Nano Letters 7, 2506 (2007)
3. ^ http://prb.aps.org/abstract/PRB/v81/i8/e081303
4. ^ Space-separated quantum cutting with silicon nanocrystals for photovoltaic applications. Nature Photonics 2, 105 (2008)