Multiple exciton generation (MEG), or carrier multiplication, involves the generation of multiple electron-hole pairs from the absorption of a single photon

Photon

In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

. MEG may considerably increase the power conversion

Power conversion

In electrical engineering, power conversion has a more specific meaning, namely converting electric power from one form to another. This could be as simple as a transformer to change the voltage of AC power, but also includes far more complex systems...

 efficiency of nanocrystal

Nanocrystal

B. D. Fahlman has described a nanocrystal as any nanomaterial with at least one dimension ≤ 100nm and that is singlecrystalline.-Summary:More properly, any material with a dimension of less than 1 micrometre, i.e., 1000 nanometers, should be referred to as a nanoparticle, not a nanocrystal...

 based solar cells, though harvesting the energy

Energy

In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 may be difficult because of the short lifetimes of the multiexcitons.

MEG has been demonstrated in synthesized nanocrystals (quantum dots) including PbS

Lead(II) sulfide

Lead sulfide is an inorganic compound with the formula Pb. It finds limited use in electronic devices. PbS, also known as galena, is the principal ore and most important compound of lead....

, PbSe

Lead(II) selenide

Lead selenide , or lead selenide, a selenide of lead, is a semiconductor material. It forms cubic crystals of the NaCl structure; it has a direct bandgap of 0.27 eV at room temperature...

, PbTe

Lead(II) telluride

Lead telluride is a compound of lead and tellurium ; it is a narrow gap semiconductor. It occurs naturally as the mineral altaite.- Properties :* Dielectric constant ~1000.* Electron Effective mass ~ 0.01me...

, CdS

Cadmium sulfide

Cadmium sulfide is the inorganic compound with the formula CdS. Cadmium sulfide is a yellow solid. It occurs in nature with two different crystal structures as the rare minerals greenockite and hawleyite, but is more prevalent as an impurity substituent in the similarly structured zinc ores...

, CdSe

Cadmium selenide

Cadmium selenide is a solid, binary compound of cadmium and selenium. Common names for this compound are cadmium selenide, cadmium selenide, and cadmoselite ....

, InAs, and Si

Silicon

Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 .
Recently, MEG has also been demonstrated in InP colloidal quantum dots.

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

• 1) Impact ionization
  Impact ionization
  Impact ionization is the process in a material by which one energetic charge carrier can lose energy by the creation of other charge carriers...
  
  : 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
  Density of states
  In solid-state and condensed matter physics, the density of states of a system describes the number of states per interval of energy at each energy level that are available to be occupied by electrons. Unlike isolated systems, like atoms or molecules in gas phase, the density distributions are not...
  
   of multiexcitons being very high, while the Coulomb coupling between X and multi-X can be quite small.
• 2) Coherent superposition
  Superposition principle
  In physics and systems theory, the superposition principle , also known as superposition property, states that, for all linear systems, the net response at a given place and time caused by two or more stimuli is the sum of the responses which would have been caused by each stimulus individually...
  
   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
  Quantum beats
  In physics, quantum beats are simple examples of phenomena that cannot be described by semiclassical theory, but can be described by fully quantized calculation, especially quantum electrodynamics. In semiclassical theory , there is an interference or beat note term for both V-type and \Lambda-type...
  
  ). 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).