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

Abstract

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...

Thermally Activated Delayed Fluorescence Sensitization for Highly Efficient Blue Fluorescent Emitters

“…The dynamic disorder is related to electron-vibration interactions that lead to a time-dependent variation in the S 1state energies with a standard deviation denoted σ D ; the static disorder is associated with the glassy and amorphous nature of the emissive film, which results in a time-independent variation in the S 1 -state energy with a standard deviation σ S . The results obtained from ensemble-and time-average calculations [42][43][44][45] give σ D [σ S ] values of 75 [50] meV and 125 [156] meV for TBPe and 4CzDPO, respectively. Thus, both static and dynamic disorders are much higher in 4CzDPO, which can be attributed to its less rigid molecular structure in comparison to TBPe.…”

Radiative and Nonradiative Recombinations in Organic Radical Emitters: The Effect of Guest–Host Interactions

“…As was shown recently, σ T arises from contributions related to both dynamic disorder and static disorder. [ 59–62 ] The dynamic disorder is associated with electron‐vibration interactions that results in a time‐dependent variation in D 1 ‐state energies with a standard deviation σ D ; the static disorder is related to the amorphous and glassy nature of the TTM‐3NCz/CBP film, which leads to a time‐independent variation in the D 1 ‐state energy with a standard deviation σ S . In the case of Gaussian distributions, the total standard deviation σ T can be expressed as .…”

“…It is interesting to recall that σ D in the semiclassical approximation is related to λ via the expression: . [ 60 ] Based on this relation, the reorganization energy is estimated to be λ = 134 meV; this value is about 30% smaller than that estimated from the geometry optimizations of the isolated emitter. Thus, these results imply that the host matrix is constraining the vibrational motions of the radical molecules in their ground and excited states and effectively reduces the vibrational coupling and consequently the dynamic disorder.…”

The Bulk Heterojunction in Organic Photovoltaic, Photodetector, and Photocatalytic Applications

“…Moreover, in the case when no low-energy CT states are not detected with the common techniques such as photothermal deflection spectroscopy (PDS) and Fourier transform photocurrent spectroscopy (FTPS), the use of a three-state. [179,180] Similarly, introducing the effect of the wave function overlap between singlet and CT state, explained by the oscillator strength of the CT to GS transition, ΔV OC,non rad has been predicted to approach values as low as 0.17 V. [181] It is then not surprising that with these design rules, novel organic semiconductors materials with an almost zero energy offset between the D and A materials for maximizing V OC have been developed. Between those blends, the discovery of PM6:Y6 boosted the performances of OPV devices with values over 17%.…”