On the Molecular Origin of Charge Separation at the Donor–Acceptor Interface

Sini, Gjergji; Schubert, Marcel; Risko, Chad; Roland, Steffen; Lee, Olivia P.; Chen, Zhihua; Richter, T.; Dolfen, Daniel; Coropceanu, Veaceslav; Ludwigs, Sabine; Scherf, Ullrich; Facchetti, Antonio; Fréchet, Jean M. J.; Neher, Dieter

doi:10.1002/aenm.201702232

Cited by 54 publications

(42 citation statements)

References 82 publications

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“…Equation underlines that both dynamic and static disorders can have a major impact on the nonradiative recombination rates and consequently on open‐circuit voltage losses. Therefore, there is increasing interest, triggered in particular by the rapid advance in the efficiency of organic solar cells, in better understanding the role of disorder on device performance . In the semiclassical approximation used to derive Equation , the CT absorption band is also characterized by a Gaussian distribution with a variance given by Equation ; the static energetic disorder can thus be extracted from temperature‐dependent measurements of the CT absorption or related photoconductivity spectral response .…”

Section: Introductionmentioning

confidence: 79%

Charge‐Transfer States at Organic–Organic Interfaces: Impact of Static and Dynamic Disorders

Zheng

Tummala

Wang

et al. 2019

Advanced Energy Materials

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105

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of this distribution in the semiclassical approximation is given by [15] 2 D 2 B k T σ λ =(1)where λ denotes the reorganization energy related to the electron transfer process between the CT state and the ground state; k B , the Boltzmann constant; and T, the temperature. The static disorder arises from the amorphous nature of the active layers and the positional inequivalency (even in the absence of vibrational motions) of the donor and acceptor (macro)molecules, which results in a time-independent distribution of the CT states with a standard deviation σ S . If both static and dynamic contributions have Gaussian distributions, the standard deviation of the overall (total) disorder σ T can be expressed as [12] T 2 S 2 D 2 σ σ σ = +The knowledge of the dynamic and static disorder contributions in D-A systems is important for an accurate determination of exciton dissociation rates, nonradiative recombination rates, charge transport, and charge-transfer optical transitions. In the semiclassical approximation, accounting for the impact of static disorder on the nonradiative recombination rate (k nr ) results in Equation (3), which is similar to the classical Marcus formula [16] except that the 2term appearing in the latter is replaced with T 2 σ [14,17,18] 2 1 2 exp 2 nr el 2 T 2 CT a 2 T 2 k V E  π σ λ σ ( ) = − −        where V el denotes the electronic coupling between the CT states and the ground state and CT a E , the adiabatic (relaxed) CT energy. Equation (3) underlines that both dynamic and static disorders can have a major impact on the nonradiative recombination rates and consequently on open-circuit voltage losses. Therefore, there is increasing interest, triggered in particular by the rapid advance in the efficiency of organic solar cells, in better understanding the role of disorder on device performance. [9,12,13,[17][18][19][20][21][22][23][24] In the semiclassical approximation used to derive Equation (3), the CT absorption band is also characterized by a Gaussian distribution with a variance given by Equation (2); [13] the static Molecular dynamics simulations are combined with density functional theory calculations to evaluate the impact of static and dynamic disorders on the energy distribution of charge-transfer (CT) states at donor-acceptor heterojunctions, such as those found in the active layers of organic solar cells. It is shown that each of these two disorder components can be partitioned into contributions related to the energetic disorder of the transport states and to the disorder associated with the hole-electron electrostatic interaction energies. The methodology is applied to evaluate the energy distributions of the CT states in representative bulk heterojunctions based on poly-3-hexyl-thiophene and phenyl-C 61 -butyric-acid methyl ester. The results indicate that the torsional fluctuations of the polymer backbones are the main source of both static and dynamic disorders for the CT states as well as for the transport levels. The impact of static and dynamic disorders on radia...

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Section: Introductionmentioning

confidence: 79%

Charge‐Transfer States at Organic–Organic Interfaces: Impact of Static and Dynamic Disorders

Zheng

Tummala

Wang

et al. 2019

Advanced Energy Materials

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105

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“…The functionality of OSCs is linked to the formation of charge-transfer (CT) states of a donor cation and an acceptor anion at their interface (Fig. 4a), mediating charge carrier dissociation and recombination in OSCs 7,8 . In the following, we demonstrate how molecular quadrupole moments can influence the CT state energy ( E CT ) as well as its difference to the energy of separated charges ( E CS ).…”

Section: Resultsmentioning

confidence: 99%

Impact of molecular quadrupole moments on the energy levels at organic heterojunctions

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The functionality of organic semiconductor devices crucially depends on molecular energies, namely the ionisation energy and the electron affinity. Ionisation energy and electron affinity values of thin films are, however, sensitive to film morphology and composition, making their prediction challenging. In a combined experimental and simulation study on zinc-phthalocyanine and its fluorinated derivatives, we show that changes in ionisation energy as a function of molecular orientation in neat films or mixing ratio in blends are proportional to the molecular quadrupole component along the π-π-stacking direction. We apply these findings to organic solar cells and demonstrate how the electrostatic interactions can be tuned to optimise the energy of the charge-transfer state at the donor−acceptor interface and the dissociation barrier for free charge carrier generation. The confirmation of the correlation between interfacial energies and quadrupole moments for other materials indicates its relevance for small molecules and polymers.

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“…A complete picture on charge generation, of course, has to involve both the effects of morphology, and the presence and influence of the charge-separated state, and is beyond the scope of this work. [67][68][69][70][71][72][73][74][75][76][77][78]…”

Section: Effect Of Hybridization On Device Performancementioning

confidence: 99%

Hybridization of Local Exciton and Charge-Transfer States Reduces Nonradiative Voltage Losses in Organic Solar Cells

et al. 2019

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A number of recent studies have shown that the non-radiative voltage losses in organic solar cells can be suppressed in systems with low energetic offsets between donor and acceptor molecular states, but the physical reasons underpinning this remain unclear. Here, we present a systematic study of 18 different donor:acceptor blends to determine the effect that energetic offset has on both radiative and non-radiative recombination of the charge transfer (CT) state. We find that for certain blends, low offsets result in hybridization between charge-transfer and lowest donor or acceptor exciton states, which leads to a strong suppression in the non-radiative voltage loss to values as low as 0.23V associated with an increase in the luminescence of the CT state. Further, we extend a two-state CT-state recombination model to include the interaction between CT and first excited states, which allows us to explain the low non-radiative voltage losses as an increase in the effective CT to ground state oscillator strength due to the intensity borrowing mechanism. We show that low non-radiative voltage losses can be achieved in material combinations with a strong electronic coupling between CT and first excited states, and where the lower band gap material has a high oscillator strength for transitions from the excited state to the ground state. Finally, from our model we propose that achieving very low non-radiative voltage losses may come at a cost of higher overall recombination rates, which may help to explain the generally lower FF and EQE of highly hybridized systems.

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On the Molecular Origin of Charge Separation at the Donor–Acceptor Interface

Cited by 54 publications

References 82 publications

Charge‐Transfer States at Organic–Organic Interfaces: Impact of Static and Dynamic Disorders

Charge‐Transfer States at Organic–Organic Interfaces: Impact of Static and Dynamic Disorders

Impact of molecular quadrupole moments on the energy levels at organic heterojunctions

Hybridization of Local Exciton and Charge-Transfer States Reduces Nonradiative Voltage Losses in Organic Solar Cells

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