In solar cell research, carrier multiplication is the phenomenon wherein the absorption of a single photon leads to the excitation of multiple electrons from the valence band to conduction band. In the theory of a conventional solar cell, each photon is only able to excite one electron across the band gap of the semiconductor, and any excess energy in that photon is dissipated as heat. In a material with carrier multiplication, high-energy photons excite on average more than one electron across the band gap, and so in principle the solar cell can produce more useful work.
In quantum dot solar cells, the excited electron in the conduction band interacts with the hole it leaves behind in the valence band, and this composite uncharged object is known as an exciton. The carrier multiplication effect in a dot can be understood as creating multiple excitons, and is called multiple exciton generation (MEG). MEG may considerably increase the energy conversion efficiency of nanocrystal based solar cells, though extracting the energy may be difficult because of the short lifetimes of the multiexcitons.
The quantum mechanical origin of MEG is still under debate and several possibilities have been suggested:[1]
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 convert 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).
MEG was first observed in 2004 using colloidal PbSe quantum dots[2] and later was found in quantum dots of other compositions including PbS, PbTe, CdS, CdSe, InAs, Si,[3] and InP.[4] However, many early studies in colloidal quantum dots significantly overestimated the MEG effect due to undetected photocharging, an issue later identified and resolved by vigorously stirring colloidal samples.[5] Multiple exciton generation was first demonstrated in a functioning solar cell in 2011, also using colloidal PbSe quantum dots.[6] Multiple exciton generation was also detected in semiconducting single-walled carbon nanotubes (SWNTs) upon absorption of single photons.[7] For (6,5) SWNTs, absorption of single photons with energies corresponding to three times the SWNT energy gap results in an exciton generation efficiency of 130% per photon. The multiple exciton generation threshold in SWNTs can be close to the limit defined by energy conservation.
Graphene, which is closely related to nanotubes, is another material in which multiple exciton generation has been observed.[8]
Double-exciton generation has additionally been observed in organic pentacene derivatives through singlet exciton fission with extremely high quantum efficiency.[9]
^Timmerman, D.; Izeddin, I.; Stallinga, P.; Yassievich, I. N.; Gregorkiewicz, T. (2008). "Space-separated quantum cutting with silicon nanocrystals for photovoltaic applications". Nature Photonics. 2 (2): 105. Bibcode:2008NaPho...2..105T. doi:10.1038/nphoton.2007.279.
^Schaller, R.; Klimov, V. (2004). "High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion". Physical Review Letters. 92 (18): 186601. arXiv:cond-mat/0404368. Bibcode:2004PhRvL..92r6601S. doi:10.1103/PhysRevLett.92.186601. PMID 15169518. S2CID 4186651.
^Stubbs, Stuart K.; Hardman, Samantha J. O.; Graham, Darren M.; Spencer, Ben F.; Flavell, Wendy R.; Glarvey, Paul; Masala, Ombretta; Pickett, Nigel L.; Binks, David J. (2010). "Efficient carrier multiplication in InP nanoparticles" (PDF). Physical Review B. 81 (8): 081303. Bibcode:2010PhRvB..81h1303S. doi:10.1103/PhysRevB.81.081303.
^McGuire, John A.; Sykora, Milan; Joo, Jin; Pietryga, Jeffrey M.; Klimov, Victor I. (2010). "Apparent Versus True Carrier Multiplication Yields in Semiconductor Nanocrystals". Nano Letters. 10 (6): 2049–57. Bibcode:2010NanoL..10.2049M. doi:10.1021/nl100177c. PMID 20459066.
^Semonin, OE; Luther, JM; Choi, S.; Chen, HY; Gao, J.; Nozik, AJ; Beard, MC (2011). "Peak External Photocurrent Quantum Efficiency Exceeding 100% via MEG in a Quantum Dot Solar Cell". Science. 334 (6062): 1530–1533. Bibcode:2011Sci...334.1530S. doi:10.1126/science.1209845. PMID 22174246. S2CID 36022754.
^Wang, Shujing; Khafizov, Marat; Tu, Xiaomin; Zheng, Ming; Krauss, Todd D. (14 July 2010). "Multiple exciton generation in single-walled carbon nanotubes". Nano Letters. 10 (7): 2381–2386. Bibcode:2010NanoL..10.2381W. doi:10.1021/nl100343j. PMID 20507082.
an exciton. The carrier multiplication effect in a dot can be understood as creating multipleexcitons, and is called multipleexcitongeneration (MEG)...
(exciton) can be generated per incoming photon. Multipleexcitongeneration (MEG) is an exciton relaxation pathway which allows two or more excitons to...
Gamma experiment, a particle physics experiment in muon decay Multipleexcitongeneration, a concept in quantum electronics Musée d'ethnographie de Genève...
energy conversion efficiency value. Note that in the event of multipleexcitongeneration (MEG), quantum efficiencies of greater than 100% may be achieved...
high extinction coefficients and have shown the possibility of multipleexcitongeneration. In a QDSC, a mesoporous layer of titanium dioxide nanoparticles...
produce more than one exciton from one high-energy photon via the process of carrier multiplication or multipleexcitongeneration (MEG). This compares...
for electrons and holes to be bound into an exciton: indirect exciton and direct exciton. In indirect exciton, electrons and holes are in different quantum...
able to achieve up to 42% energy conversion efficiency due to multipleexcitongeneration (MEG). MIT researchers have found a way of using a virus to improve...
incorporate CNT-Silicon heterojunctions to leverage efficient multiple-excitongeneration at p-n junctions formed within individual CNTs. In the nearer...
material, it can either be absorbed (generating a pair of free carriers or an exciton) or it can stimulate a recombination event. The generated photon has similar...
of the phonon-bottleneck, and demonstrated a new mechanism of multipleexcitongeneration. The latter process was compared to singlet fission in molecular...
causes the formation of an exciton (an electron excited to a higher energy state) in the pigment molecule. The energy of the exciton is transferred to a chlorophyll...
supporting an excitonic state whose symmetry is such that in the course of the exciton relaxing, a radiation mode of non-zero topological charge is created directly...
produce a hydroxyl radical. The reaction starts by photo-induced excitongeneration in the metal oxide (MO) surface by photon (hv) absorption: MO + hν...
be regarded as an exciton, or an electron-hole pair bound together by electrostatic interactions. In photovoltaic cells, excitons are broken up into...
exciton states unlike inorganic and crystalline solar cells. The efficiency of the excitonic solar cells and inorganic solar cells (with less exciton-binding...
it comes to imaging the active layers of solar cells, Huang has studied exciton formation and charge transport at femtosecond timescales. She tracked the...
Hall states are excitons of composite fermions, that is, particle hole pairs of composite fermions. The energy dispersion of these excitons has been measured...
well as the only known particles whose electric charges are not integer multiples of the elementary charge. There are six types, known as flavors, of quarks:...
triplet state. Several effects have been suggested to play a role on the exciton fine structure such as electron-hole exchange interactions, crystal field...
difficulty in predicting HOMO and LUMO energy levels and the control of excitons through the material make it challenging to pinpoint which moieties will...
and exciton donor. The other material facilitates exciton dissociation at the junction. Charge is transferred and then separated after an exciton created...
the ordinary particles. The 12 fundamental fermions are divided into 3 generations of 4 particles each. Half of the fermions are leptons, three of which...
array of single particles between the electrodes, each separated by ~1 exciton diffusion length, was proposed to improve the device efficiency and research...