Multiple exciton generation

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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. ShockleyQueisserBreakdown2.svg
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.

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]

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]

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<span class="mw-page-title-main">Photoluminescence</span> Light emission from substances after they absorb photons

Photoluminescence is light emission from any form of matter after the absorption of photons. It is one of many forms of luminescence and is initiated by photoexcitation, hence the prefix photo-. Following excitation, various relaxation processes typically occur in which other photons are re-radiated. Time periods between absorption and emission may vary: ranging from short femtosecond-regime for emission involving free-carrier plasma in inorganic semiconductors up to milliseconds for phosphoresence processes in molecular systems; and under special circumstances delay of emission may even span to minutes or hours.

<span class="mw-page-title-main">Quantum dot</span> Zero-dimensional, nano-scale semiconductor particles with novel optical and electronic properties

Quantum dots (QDs) or semiconductor nanocrystals are semiconductor particles a few nanometres in size with optical and electronic properties that differ from those of larger particles via quantum mechanical effects. They are a central topic in nanotechnology and materials science. When a quantum dot is illuminated by UV light, an electron in the quantum dot can be excited to a state of higher energy. In the case of a semiconducting quantum dot, this process corresponds to the transition of an electron from the valence band to the conductance band. The excited electron can drop back into the valence band releasing its energy as light. This light emission (photoluminescence) is illustrated in the figure on the right. The color of that light depends on the energy difference between the conductance band and the valence band, or the transition between discrete energy states when the band structure is no longer well-defined in QDs.

<span class="mw-page-title-main">Quantum well</span> Concept in quantum mechanics

A quantum well is a potential well with only discrete energy values.

<span class="mw-page-title-main">Quantum efficiency</span> Property of photosensitive devices

The term quantum efficiency (QE) may apply to incident photon to converted electron (IPCE) ratio of a photosensitive device, or it may refer to the TMR effect of a magnetic tunnel junction.

Hybrid solar cells combine advantages of both organic and inorganic semiconductors. Hybrid photovoltaics have organic materials that consist of conjugated polymers that absorb light as the donor and transport holes. Inorganic materials in hybrid cells are used as the acceptor and electron transporter in the structure. The hybrid photovoltaic devices have a potential for not only low-cost by roll-to-roll processing but also for scalable solar power conversion.

<span class="mw-page-title-main">Quantum dot solar cell</span> Type of solar cell based on quantum dot devices

A quantum dot solar cell (QDSC) is a solar cell design that uses quantum dots as the captivating photovoltaic material. It attempts to replace bulk materials such as silicon, copper indium gallium selenide (CIGS) or cadmium telluride (CdTe). Quantum dots have bandgaps that are adjustable across a wide range of energy levels by changing their size. In bulk materials, the bandgap is fixed by the choice of material(s). This property makes quantum dots attractive for multi-junction solar cells, where a variety of materials are used to improve efficiency by harvesting multiple portions of the solar spectrum.

Third-generation photovoltaic cells are solar cells that are potentially able to overcome the Shockley–Queisser limit of 31–41% power efficiency for single bandgap solar cells. This includes a range of alternatives to cells made of semiconducting p-n junctions and thin film cells. Common third-generation systems include multi-layer ("tandem") cells made of amorphous silicon or gallium arsenide, while more theoretical developments include frequency conversion,, hot-carrier effects and other multiple-carrier ejection techniques.

Organic photovoltaic devices (OPVs) are fabricated from thin films of organic semiconductors, such as polymers and small-molecule compounds, and are typically on the order of 100 nm thick. Because polymer based OPVs can be made using a coating process such as spin coating or inkjet printing, they are an attractive option for inexpensively covering large areas as well as flexible plastic surfaces. A promising low cost alternative to conventional solar cells made of crystalline silicon, there is a large amount of research being dedicated throughout industry and academia towards developing OPVs and increasing their power conversion efficiency.

<span class="mw-page-title-main">Shockley–Queisser limit</span> Maximum theoretical efficiency of a solar cell

In physics, the radiative efficiency limit is the maximum theoretical efficiency of a solar cell using a single p-n junction to collect power from the cell where the only loss mechanism is radiative recombination in the solar cell. It was first calculated by William Shockley and Hans-Joachim Queisser at Shockley Semiconductor in 1961, giving a maximum efficiency of 30% at 1.1 eV. The limit is one of the most fundamental to solar energy production with photovoltaic cells, and is one of the field's most important contributions.

In condensed matter physics, biexcitons are created from two free excitons.

<span class="mw-page-title-main">Solar cell research</span> Research in the field of photovoltaics

There are currently many research groups active in the field of photovoltaics in universities and research institutions around the world. This research can be categorized into three areas: making current technology solar cells cheaper and/or more efficient to effectively compete with other energy sources; developing new technologies based on new solar cell architectural designs; and developing new materials to serve as more efficient energy converters from light energy into electric current or light absorbers and charge carriers.

<span class="mw-page-title-main">Nanocrystal solar cell</span>

Nanocrystal solar cells are solar cells based on a substrate with a coating of nanocrystals. The nanocrystals are typically based on silicon, CdTe or CIGS and the substrates are generally silicon or various organic conductors. Quantum dot solar cells are a variant of this approach which take advantage of quantum mechanical effects to extract further performance. Dye-sensitized solar cells are another related approach, but in this case the nano-structuring is a part of the substrate.

<span class="mw-page-title-main">Photon upconversion</span> Optical process

Photon upconversion (UC) is a process in which the sequential absorption of two or more photons leads to the emission of light at shorter wavelength than the excitation wavelength. It is an anti-Stokes type emission. An example is the conversion of infrared light to visible light. Upconversion can take place in both organic and inorganic materials, through a number of different mechanisms. Organic molecules that can achieve photon upconversion through triplet-triplet annihilation are typically polycyclic aromatic hydrocarbons (PAHs). Inorganic materials capable of photon upconversion often contain ions of d-block or f-block elements. Examples of these ions are Ln3+, Ti2+, Ni2+, Mo3+, Re4+, Os4+, and so on.

Thermodynamic efficiency limit is the absolute maximum theoretically possible conversion efficiency of sunlight to electricity. Its value is about 86%, which is the Chambadal-Novikov efficiency, an approximation related to the Carnot limit, based on the temperature of the photons emitted by the Sun's surface.

<span class="mw-page-title-main">Core–shell semiconductor nanocrystal</span>

Core–shell semiconducting nanocrystals (CSSNCs) are a class of materials which have properties intermediate between those of small, individual molecules and those of bulk, crystalline semiconductors. They are unique because of their easily modular properties, which are a result of their size. These nanocrystals are composed of a quantum dot semiconducting core material and a shell of a distinct semiconducting material. The core and the shell are typically composed of type II–VI, IV–VI, and III–V semiconductors, with configurations such as CdS/ZnS, CdSe/ZnS, CdSe/CdS, and InAs/CdSe Organically passivated quantum dots have low fluorescence quantum yield due to surface related trap states. CSSNCs address this problem because the shell increases quantum yield by passivating the surface trap states. In addition, the shell provides protection against environmental changes, photo-oxidative degradation, and provides another route for modularity. Precise control of the size, shape, and composition of both the core and the shell enable the emission wavelength to be tuned over a wider range of wavelengths than with either individual semiconductor. These materials have found applications in biological systems and optics.

Blinking colloidal nanocrystals is a phenomenon observed during studies of single colloidal nanocrystals that show that they randomly turn their photoluminescence on and off even under continuous light illumination. This has also been described as luminescence intermittency. Similar behavior has been observed in crystals made of other materials. For example, porous silicon also exhibits this affect.

Intermediate band photovoltaics in solar cell research provides methods for exceeding the Shockley–Queisser limit on the efficiency of a cell. It introduces an intermediate band (IB) energy level in between the valence and conduction bands. Theoretically, introducing an IB allows two photons with energy less than the bandgap to excite an electron from the valence band to the conduction band. This increases the induced photocurrent and thereby efficiency.

Quantum dots (QDs) are semiconductor nanoparticles with a size less than 10 nm. They exhibited size-dependent properties especially in the optical absorption and the photoluminescence (PL). Typically, the fluorescence emission peak of the QDs can be tuned by changing their diameters. So far, QDs were consisted of different group elements such as CdTe, CdSe, CdS in the II-VI category, InP or InAs in the III-V category, CuInS2 or AgInS2 in the I–III–VI2 category, and PbSe/PbS in the IV-VI category. These QDs are promising candidates as fluorescent labels in various biological applications such as bioimaging, biosensing and drug delivery.

A quantum dot single-photon source is based on a single quantum dot placed in an optical cavity. It is an on-demand single-photon source. A laser pulse can excite a pair of carriers known as an exciton in the quantum dot. The decay of a single exciton due to spontaneous emission leads to the emission of a single photon. Due to interactions between excitons, the emission when the quantum dot contains a single exciton is energetically distinct from that when the quantum dot contains more than one exciton. Therefore, a single exciton can be deterministically created by a laser pulse and the quantum dot becomes a nonclassical light source that emits photons one by one and thus shows photon antibunching. The emission of single photons can be proven by measuring the second order intensity correlation function. The spontaneous emission rate of the emitted photons can be enhanced by integrating the quantum dot in an optical cavity. Additionally, the cavity leads to emission in a well-defined optical mode increasing the efficiency of the photon source.

<span class="mw-page-title-main">Perovskite nanocrystal</span> Class of semiconductor nanocrystals

Perovskite nanocrystals are a class of semiconductor nanocrystals, which exhibit unique characteristics that separate them from traditional quantum dots. Perovskite nanocrystals have an ABX3 composition where A = cesium, methylammonium (MA), or formamidinium (FA); B = lead or tin; and X = chloride, bromide, or iodide.

References

  1. 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.
  2. 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.
  3. Beard, MC; Knutsen, KP; Yu, P; Luther, JM; Song, Q; Metzger, WK; Ellingson, RJ; Nozik, AJ (2007). "MEG in colloidal silicon nanocrystals". Nano Letters. 7 (8): 2506–12. Bibcode:2007NanoL...7.2506B. doi:10.1021/nl071486l. PMID   17645368.
  4. 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.
  5. 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.
  6. 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.
  7. 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.
  8. Tielrooij, K.J.; Song, J C.W.; Jensen, S.A.; Centeno, A.; Pesquera, A.; Zurutuza Elorza, A.; Bonn, M.; Levitov, L.F.; Koppens, F.H.L. (24 February 2013). "Photoexcitation cascade and multiple hot-carrier generation in graphene". Nature Physics. 9 (4): 248–252. arXiv: 1210.1205 . Bibcode:2013NatPh...9..248T. doi:10.1038/nphys2564. S2CID   13999471.
  9. Congreve, D. N. (2013). "External Quantum Efficiency Above 100% in a Singlet-Exciton-Fission–Based Organic Photovoltaic Cell". Science. 340 (6130): 334–337. Bibcode:2013Sci...340..334C. doi:10.1126/science.1232994. PMID   23599489. S2CID   46185590.