How to interpret absorption and fluorescence spectra of charge transfer states in an organic solar cell

Kahle, Frank‐Julian; Rudnick, Alexander; Bäßler, H.; Köhler, Anna

doi:10.1039/c8mh00564h

Cited by 64 publications

(141 citation statements)

References 60 publications

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“…[43] As shown in Figure S9 (Supporting Information), the aggregation domain size, surface roughness, and phase separation effect in the surfaces of the blends are comparable, so we cannot attribute the changes in the photovoltaic behaviors to phase separation. [43] As shown in Figure S9 (Supporting Information), the aggregation domain size, surface roughness, and phase separation effect in the surfaces of the blends are comparable, so we cannot attribute the changes in the photovoltaic behaviors to phase separation.…”

Section: Doi: 101002/aenm201901823mentioning

confidence: 96%

“…We carried out AFM and grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements to evaluate the morphology and molecular stacking behavior of the blend films. [43] As shown in Figure S9 (Supporting Information), the aggregation domain size, surface roughness, and phase separation effect in the surfaces of the blends are comparable, so we cannot attribute the changes in the photovoltaic behaviors to phase separation. Based on the 2D patterns obtained by GIWAXS (see Figure S10 in the Supporting Information for details), we can see that before the addition of NRM-1, the blend film has a strong (010) reflection at 1.68 Å −1 in the outof-plane direction corresponding to a π-π stacking distance of 3.74 Å −1 .…”

Section: Reducing Energy Loss (E Loss ) Is Of Critical Importance To mentioning

confidence: 96%

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Reduced Nonradiative Energy Loss Caused by Aggregation of Nonfullerene Acceptor in Organic Solar Cells

Qin

Zhang

et al. 2019

Advanced Energy Materials

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current. The latter is the value of qV OC a solar cell with that bandgap would have at a temperature of 300 K illuminated by 1 sun unconcentrated illumination and assuming emission of luminescence into the 2π halfspace above the solar cell. [16][17][18][19] 2) qV OC SQ − qV OC rad (ΔE 2 ) is the loss due to the absorption edge being nonabrupt and therefore primarily due to radiative recombination below the bandgap. [20][21][22][23][24] 3) The third loss is due to nonradiative recombination (ΔE 3 ). As the first term ΔE 1 is unavoidable for any solar cell, the main opportunities for reducing E loss to improve the V OC lie in the other two terms.The second term, ΔE 2 , can be reduced by elevating the charge transfer (CT) state. For instance, it has been demonstrated that when the ionic potential (IP) and electronic affinity (EA) of a donor are nearly aligned with those of an acceptor, the energetic offset from E g to E CT can be very small, resulting in a negligible ΔE 2 . [20,21,25,26] In contrast, when the IP and EA of a donor are much larger than those of an acceptor, as observed in many OSCs based on polymer donors and fullerene acceptors, ΔE 2 can be as large as over 0.60 eV. [17,27,28] The third term, ΔE 3 , can be varied from 0.25 to 0.48 eV. [29][30][31][32][33][34][35] As is known, for an OSC under open-circuit condition, an electron-hole pair at the CT state transits to the ground state by either radiative recombination or nonradiative recombination, so the electroluminescence (EL) emission can be used as a tool to monitor the nonradiative recombination. [21,28,[31][32][33] There have been many studies to investigate ΔE 3 for OSCs based on various photoactive materials. [28,30,36] For instance, for an OSC based on the polymer donor P3TEA and acceptor SF-PDI2, the electroluminescence quantum efficiency (EQE EL ) was as high as 5 × 10 −5 , corresponding to a ΔE 3 of 0.26 eV, [20] while for OSCs based on polymer donors and fullerene acceptors, ΔE 3 can surpass 0.40 eV. [30,31] Up to date, the methodology for tuning ΔE 3 has not yet been established. Considering that ΔE 1 is unavoidable and ΔE 2 can be reduced to a negligible value, the modulation of ΔE 3 is critical in the field of OSCs.Here, we report a new method to reduce E loss in an OSC based on the polymer donor PBDB-T and nonfullerene acceptor IT-4F by incorporating a small molecular material named NRM-1 in the photoactive layer. We selected the PBDB-T:IT-4F system to conduct the study because the IP and EA of PBDB-T are much larger than those of IT-4F and the energetic offset between E g and E CT , referring to ΔE 2 , can be obviously Reducing energy loss (E loss ) is of critical importance to improving the photovoltaic performance of organic solar cells (OSCs). Although nonradiative recombination ( E loss nonrad ) is investigated in quite a few works, the method for modulating E loss nonrad is seldom reported. Here, a new method of depressing E loss is reported for nonfullerene OSCs. In addition to ternary-blend bulk heterojunction (BHJ) solar cells...

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Section: Doi: 101002/aenm201901823mentioning

confidence: 96%

Section: Reducing Energy Loss (E Loss ) Is Of Critical Importance To mentioning

confidence: 96%

Reduced Nonradiative Energy Loss Caused by Aggregation of Nonfullerene Acceptor in Organic Solar Cells

Qin

Zhang

et al. 2019

Advanced Energy Materials

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“…In addition, the spectral line shape of the absorption coefficient for hv < E g is indicative of a variety of phenomena such as disorder in amorphous semiconductors and charge transfer absorption in organic semiconductors. Disorder‐induced and charge transfer state absorptions are related to loss mechanisms in organic solar cells, and have been studied extensively to improve solar cell efficiencies . Given the considerable interest in the sub‐bandgap absorption coefficients, sensitive methods of measuring it have been developed.…”

Section: Introductionmentioning

confidence: 99%

Determining Ultralow Absorption Coefficients of Organic Semiconductors from the Sub‐Bandgap Photovoltaic External Quantum Efficiency

Kaiser

Zeiske

Meredith

et al. 2019

Advanced Optical Materials

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Energy states below the bandgap of a semiconductor, such as trap states or charge transfer states in organic donor–acceptor blends, can contribute to light absorption. Due to their low number density or ultrasmall absorption cross‐section, the absorption coefficient of these states is challenging to measure using conventional transmission/reflection spectrophotometry. As an alternative, the external quantum efficiency (EQE) of photovoltaic devices is often used as a representative of the absorption coefficient, where the spectral line shape of the EQE is considered to follow the absorption coefficient of the active layer material. Here, it is shown that the sub‐bandgap EQE is subject to thickness dependent low finesse cavity interference effects within the device—making this assumption questionable. A better estimate for the absorption coefficient is obtained when EQE spectra corresponding to different active layer thicknesses are fitted simultaneously for one attenuation coefficient using an iterative transfer matrix method. The principle is demonstrated for two model acceptor–donor systems (PCE12‐ITIC and PBTTT‐PC71BM) and accurate subgap absorption coefficients are determined. This approach has particular relevance for both understanding subgap states and their utilization in organic optoelectronic devices.

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“…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.…”

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confidence: 99%

“…term 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 ( ) = − −        …”

mentioning

confidence: 99%

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

Zheng

Tummala

Wang

et al. 2019

Advanced Energy Materials

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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How to interpret absorption and fluorescence spectra of charge transfer states in an organic solar cell

Abstract: Energetic disorder is crucial to consistently model both absorption and photoluminescence spectra of charge transfer states in organic solar cells.

Cited by 64 publications

References 60 publications

Reduced Nonradiative Energy Loss Caused by Aggregation of Nonfullerene Acceptor in Organic Solar Cells

Reduced Nonradiative Energy Loss Caused by Aggregation of Nonfullerene Acceptor in Organic Solar Cells

Determining Ultralow Absorption Coefficients of Organic Semiconductors from the Sub‐Bandgap Photovoltaic External Quantum Efficiency

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

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