Solar conversion efficiency of photovoltaic and photoelectrolysis cells with carrier multiplication absorbers

Hanna, Melvin W.; Nozik, Arthur J.

doi:10.1063/1.2356795

Cited by 1,408 publications

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“…One strategy is to sensitize a low bandgap semicon ductor with a higher bandgap material that can generate additional photocurrent from absorbed high energy photons 6,9 . Such an opportunity is offered by singlet fission, which is the spin conserving process by which a spin singlet exciton on an organic chromophore splits to form a pair of lower energy triplet excitons on neighbouring molecules 10 .…”

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“…Transient optical absorption measurements suggest that this process can be highly efficient, as we have observed singlet fis sion occurring on an 80 fs timescale in pentacene 11 . The significance of singlet exciton fission for solar cells is that, if both triplets can be ionized, it possibly permits performance above the ShockleyQueisser limit on power conversion efficiency 4 without requiring a current matched tandem photovoltaic device 9 . However, this potential has not been realized to date 3 .…”

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In situ measurement of exciton energy in hybrid singlet-fission solar cells

et al. 2012

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singlet exciton fission-sensitized solar cells have the potential to exceed the shockley-Queisser limit by generating additional photocurrent from high-energy photons. Pentacene is an organic semiconductor that undergoes efficient singlet fission-the conversion of singlet excitons into pairs of triplets. However, the pentacene triplet is non-emissive, and uncertainty regarding its energy has hindered device design. Here we present an in situ measurement of the pentacene triplet energy by fabricating a series of bilayer solar cells with infrared-absorbing nanocrystals of varying bandgaps. We show that the pentacene triplet energy is at least 0.85 eV and at most 1.00 eV in operating devices. our devices generate photocurrent from triplets, and achieve external quantum efficiencies up to 80%, and power conversion efficiencies of 4.7%. This establishes the general use of nanocrystal size series to measure the energy of non-emissive excited states, and suggests that fission-sensitized solar cells are a favourable candidate for third-generation photovoltaics.

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In situ measurement of exciton energy in hybrid singlet-fission solar cells

et al. 2012

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“…[1][2][3][4][5][6][7][8][9] This finding is important because CM can potentially provide increased power conversion efficiency in low-cost, singlejunction photovoltaics via an enhanced photocurrent. 10,11 To obtain optimum photovoltaic power conversion efficiency using a single semiconductor material with energy gap E g , it is desirable that the CM onset energy is minimal and that the CM efficiency above the onset is as large as is allowed by energy conservation.…”

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Carrier Multiplication in InAs Nanocrystal Quantum Dots with an Onset Defined by the Energy Conservation Limit

2007

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Carrier multiplication (CM) is a process in which absorption of a single photon produces not just one but multiple electron−hole pairs (excitons). This effect is a potential enabler of next-generation, high-efficiency photovoltaic and photocatalytic systems. On the basis of energy conservation, the minimal photon energy required to activate CM is two energy gaps (2E g ). Here, we analyze CM onsets for nanocrystal quantum dots (NQDs) based upon combined requirements imposed by optical selection rules and energy conservation and conclude that materials with a significant difference between electron and hole effective masses such as III−V semiconductors should exhibit a CM threshold near the apparent 2E g limit. Further, we discuss the possibility of achieving sub-2E g CM thresholds through strong exciton−exciton attraction, which is feasible in NQDs. We report experimental studies of exciton dynamics (Auger recombination, intraband relaxation, radiative recombination, multiexciton generation, and biexciton shift) in InAs NQDs and show that they exhibit a CM threshold near 2E g .Carrier Multiplication Thresholds in Bulk and Nanocrystalline Semiconductors. It has recently been established that carrier multiplication (CM), the production of multiple excitons following absorption of a single photon of sufficient energy, is efficient within the range of solar photon energies in semiconductor nanocrystal quantum dots (NQDs). [1][2][3][4][5][6][7][8][9] This finding is important because CM can potentially provide increased power conversion efficiency in low-cost, singlejunction photovoltaics via an enhanced photocurrent. 10,11 To obtain optimum photovoltaic power conversion efficiency using a single semiconductor material with energy gap E g , it is desirable that the CM onset energy is minimal and that the CM efficiency above the onset is as large as is allowed by energy conservation.According to energy conservation, the apparent minimal photon energy that is required to activate CM (pω CM ) is simply 2E g . However, the CM thresholds observed in bulk materials are significantly larger than this value. 12,13 In bulk semiconductors, the generally accepted mechanism for multiexciton generation is impact ionization in which a highenergy electron (or a hole) transfers its energy to a valenceband electron that is excited across the energy gap. 14 The CM threshold in this case is determined not only by conservation of energy but also by conservation of translational momentum, which increases the threshold above 2E g . For example, using a free-particle approximation 15,16 and applying both energy and momentum conservation laws, one can obtain that the CM threshold is approximately 4E g (Figure 1a).In NQDs, translational momentum conservation is relaxed. Therefore, one might expect that the CM threshold could reach the 2E g limit as defined by energy conservation. However, available experimental data indicate that pω CM is still greater than twice the energy gap in such materials. For example, in PbSe and PbS NQDs, it ...

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“…In a semiconductor nanocrystal (NC) of a suitable material, multiple electron-hole pairs can be generated upon the absorption of a single high-energy photon, their number depending only on the photon energy 1 . Thanks to this process, called carrier multiplication (CM), or multiple exciton generation (MEG), a more efficient and complete conversion of solar energy into electric current could be achieved in a NC-based solar cell 2 and at the same time the detrimental heat generation that accompanies conventional conversion could be reduced.…”

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Origins of improved carrier multiplication efficiency in elongated semiconductor nanostructures

Sills

Califano

2015

Phys. Chem. Chem. Phys.

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Nanorod solar cells have been attracting a lot of attention recently, as they have been shown to exhibit a lower carrier multiplication onset and a higher quantum efficiency than quantum dots with similar bandgaps. The underpinning theory for this phenomenon is not yet completely understood, and is still the subject of ongoing study. Here we conduct a theoretical investigation into CM efficiency in elongated semiconductor nanostructures with square cross section made of different materials (GaAs, GaSb, InAs, InP, InSb, CdSe, Ge, Si and PbSe), using a single-band effective mass model. Following Luo, Franceschetti and Zunger we adopt the CM figure of merit (the ratio between biexciton and single-exciton density of states) as a measure of CM efficiency and investigate its dependence on the aspect ratio for both (a) constant cross section (i.e. varying the volume) and (b) constant volume (i.e., varying the cross section), by decoupling electronic structure effects from surface-related effects, increased absorption and Coulomb coupling effects. The results show that in both (a) and (b) cases elongation causes an increase in both single-and bi-exciton density of states, with the latter, however, growing much faster with increasing energy. This leads to the availability of more bi-exciton states than single-exciton states for photon energies just above the bi-exciton ground state and therefore suggests a higher probability of CM at these energies for elongated structures. Our results therefore show that the origin of the observed decrease of the CM threshold in elongated structures can be attributed purely to electronic structure effects, paving the way to the implementation of CM-efficiency-boosting strategies in nanostructures based on the lowering of the CM onset.

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Solar conversion efficiency of photovoltaic and photoelectrolysis cells with carrier multiplication absorbers

Cited by 1,408 publications

References 36 publications

In situ measurement of exciton energy in hybrid singlet-fission solar cells

In situ measurement of exciton energy in hybrid singlet-fission solar cells

Carrier Multiplication in InAs Nanocrystal Quantum Dots with an Onset Defined by the Energy Conservation Limit

Origins of improved carrier multiplication efficiency in elongated semiconductor nanostructures

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