How accurate are energies of HOMO and LUMO levels in small-molecule organic semiconductors determined from cyclic voltammetry or optical spectroscopy?

Sworakowski, J.

doi:10.1016/j.synthmet.2017.11.013

Cited by 147 publications

(118 citation statements)

References 45 publications

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“…Despite the general linear relationship, CV and UPS often yield significantly different results in terms of absolute values, and the slope and intercept of the linear relationships obtained by literature vary significantly . For instance, D'Andrade et al reported a slope ( α + ) of 1.4 (i.e., the IP scales at a factor of 1.4 with the E ox ) and an intercept ( β + ) of 4.6 eV, whereas Sworakowski and co‐workers using literature values of IP and E ox found slopes ranging from 0.9 to 1.5 and intercepts from 4.3 to 5.4 eV depending on the type of organic molecule studied, which was attributed to differences in polarization energies/solvation energy going from solid film to (ion‐containing) solution. In addition, the scatter in data points also vary significantly depending on type of molecule, with e.g., polycyclic aromatic hydrocarbon molecules closely following the linear fit ( R 2 = 0.98) while carbazole‐containing molecules less so ( R 2 = 0.81) .…”

Section: Summary Of Vertical Ip Eict+ D–a Interface Potential (δ) Cmentioning

confidence: 94%

“…The intercept of the linear relationship should be different when different reference redox couples are used since the formal potential of each redox couple relative to vacuum is different, e.g., smaller formal potential of Ag + /Ag compared to Fc + /Fc makes intercept smaller (Table S2 and Figure S8, Supporting Information). The slope of the linear relationship can be dependent on the types of organic semiconductors and the measurement processes because of the difference in phase situation (either in a solid or in solution) of molecules/polymers under study in these two technologies . In our studies we compared UPS IP and CV E ox , both measured for solid films on Pt substrates, whereas the D'Andrade's study on metal‐organic complexes compared UPS IP of solid films with CV E ox obtained for molecules dissolved in solution.…”

Section: Summary Of Vertical Ip Eict+ D–a Interface Potential (δ) Cmentioning

confidence: 99%

“…It has been found that for molecules dissolved in solution, the slope ( α + ) is solvent dependent and is smaller in low dielectric solvent . In general, it is reasonable to assume that polarization effects and hence the slope in the CV experiment differs for a solid polymer film compared to molecules/polymers dissolved in solution . Nevertheless, to better understand the processes involved, we take the data set from D'Andrade and fit the purely organic molecules and metal‐organic complexes separately, see Figure S9a–c, Supporting Information.…”

Section: Summary Of Vertical Ip Eict+ D–a Interface Potential (δ) Cmentioning

confidence: 99%

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Relationship of Ionization Potential and Oxidation Potential of Organic Semiconductor Films Used in Photovoltaics

Wang

Ouyang

et al. 2018

Solar RRL

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Ultraviolet photoelectron spectroscopy (UPS) and cyclic voltammetry (CV) are employed to measure energy levels for charge transport in organic semiconductor films. A series of classical molecules/polymers used in organic bulk heterojunction solar cells are deposited on platinum substrates/ electrodes to form thin films and a linear relationship of vertical ionization potential (IP) measured by UPS and relative oxidation potential (E ox ) obtained by CV is found, with a slope equal to unity. The intercept varies with the different reference redox couples and repeated potential sweep numbers during experiment processes. The relationship provides for an easy conversion of values obtained by the two techniques and correlates well with device parameters. The precision in the CV-derived IP values is not sufficient, however, to enable precise design of energy level alignment at heterojunction and the approach does not improve upon the current "best practice" for obtaining donor ionization potential-acceptor electron affinity gaps at heterojunctions.Bulk heterojunction donor/acceptor organic solar cells featuring properties of light weight, flexibility, semi-transparency, and easy solution-based processing for mass production have made great progress. [1][2][3] New donor materials, acceptors and developments in electrode design have improved the power conversion efficiency (PCE) of the organic solar cells but the PCE still is lagging behind the Si-technology, mainly due to larger radiative and non-radiative voltage losses. [4,5] According to Shockley-Queisser (S-Q) theory for solar cells, there is an unavoidable loss, ΔV SQ , in open circuit voltage, V oc , as compared to the band gap, E g , of the absorber. Effects such as non-radiative recombination at contacts, heterojunctions or in the bulk film also contribute to an additional voltage loss that tend to be significantly larger for the organic systems, [6] yielding a comparatively large total voltage loss even for current state-of-the-art devices. [4,7] In bulk heterojunction organic solar cells, the effective E g is thought to be related to the acceptor electron affinity (EA) and donor ionization potential (IP) difference, modified by possible vacuum level shifts at the heterojunction. Hence, the V oc should depend on the donor hole-acceptor electron charge transfer state, and indeed a linear dependence between the two is typically found. [4,[8][9][10][11] Tuning the energy level alignment at the bulk heterojunction thus is an effective way to reduce voltage loss and enhance PCE, [12][13][14][15] and results suggest the donor-acceptor junction can be tuned for a near-zero electron-or holetransfer driving energy while retaining a high photon to free charge generation. [6] Refined tuning of the energy level alignment at the heterojunctions is aided by understanding of the interface energetics at organic heterojunctions, [11,[16][17][18][19][20][21][22][23][24] but direct precise measurements of the energy levels at the heterojunctions still are problematic. First of al...

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Section: Summary Of Vertical Ip Eict+ D–a Interface Potential (δ) Cmentioning

confidence: 94%

Section: Summary Of Vertical Ip Eict+ D–a Interface Potential (δ) Cmentioning

confidence: 99%

Section: Summary Of Vertical Ip Eict+ D–a Interface Potential (δ) Cmentioning

confidence: 99%

See 1 more Smart Citation

Relationship of Ionization Potential and Oxidation Potential of Organic Semiconductor Films Used in Photovoltaics

Wang

Ouyang

et al. 2018

Solar RRL

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“…And HOMO energy levels of Ta‐CMP, Ta‐CMP‐N, and Ta‐CMP‐CN are calculated to be −5.03, −4.99, and −5.10 eV with similar band gap of 3.18, 3.19, and 3.20 eV, respectively. Electrochemical band gaps of Ta‐CMPs are about 0.3 eV higher than their optical band gaps, which may be due to the exciton binding energy …”

Section: Resultsmentioning

confidence: 94%

Terminal Group Effect of Conjugated Microporous Polymers for Photocatalytic Water‐Splitting Hydrogen Evolution

Ding

Zhang

et al. 2019

Macro Chemistry & Physics

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hydrogen production from water splitting under visible light. [11] Since then, much effort has been concentrated on the relationship between molecular structure and the photo catalytic activity of CMPs. It is found that the structural factors, such as linkers geometry, conjugation length, and functional groups, play important roles in determining the wettability, charge transport and separation, light adsorption, and band gap of CMPs, thus affecting hydrogen production performance of CMPs-based photo catalysts. [9,[12][13][14][15] Among them, triazine-based CMPs are particularly interesting thanks to their abundant nitrogen content, high electron deficiency, and spatial coplanarity, thus they have been effectively applied in photocatalytic hydrogen evolution from water. [16,17] However, the influence of the terminal groups on hydrogen evolution performance of CMPs is still scarcely reported.In this work, three triazine-based CMPs (Ta-CMP, Ta-CMP-N, and Ta-CMP-CN) were synthesized by Suzuki-Miyaura cross-coupling reaction and end-capping strategy, among which Ta-CMP was synthesized without addition of any end-capping agent, while Ta-CMP-N and Ta-CMP-CN were end-capped with electron-donating dimethylaniline group and electronwithdrawing benzonitrile group, respectively. A study on the relationship between the terminal groups and the photocatalytic performances was also conducted. The result revealed that terminal groups have little effect on the morphology, specific surface area, and band gap, but have an impact on the electron and hole separation ability. Electron-withdrawing group can enhance the electron-hole separation efficiency, which is beneficial for the enhancement of the H 2 evolution performance of the photocatalyst, while the electron-donating group exhibits an opposite effect. Therefore, Ta-CMP-CN shows the best photocatalytic activity of 698 µmol g −1 h −1 under visible light, while hydrogen evolution rate (HER) of Ta-CMP-N is only 99 µmol g −1 h −1 . Results and Discussion Synthesis and CharacterizationAs shown in Scheme 1, Ta-CMP, Ta-CMP-N, and Ta-CMP-CN were synthesized by Suzuki-Miyaura cross-coupling reaction. Conjugated Microporous PolymersConjugated microporous polymers (CMPs) have attracted more and more attention as active materials for photocatalytic water-splitting hydrogen evolution, however, the correlation between terminal group and hydrogen evolution performance of CMPs is rarely studied. Herein, three triazine-based CMPs (Ta-CMPs) with the same polymer backbone and different terminal groups (Ta-CMP, Ta-CMP-N, and Ta-CMP-CN) are synthesized via Suzuki-Miyaura coupling reaction using different end-capping agents. Although the Ta-CMPs show similar porous structure and comparable band gaps, Ta-CMP-CN with electron-withdrawing terminal group exhibits the highest hydrogen evolution rate (HER) of 698 µmol g −1 h −1 under visible light, while Ta-CMP-N with electron-donating terminal group exhibits the lowest HER of 99 µmol g −1 h −1 . It is found that the addition of electron-withdrawing termi...

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“…The so-called "optical gap" of each material can be derived from each spectrum through the onset of the long-wavelength absorption maxima. 4,58 Four main absorption peaks, centred at z240, 293, 325 and 340 nm, respectively, are observed ( Fig. 3A) for each carbazole derivative studied (mCP, CBP and TCB), without any signicant bathochromic shi being perceived.…”

Section: Optical Propertiesmentioning

confidence: 97%

The impact of phenyl–phenyl linkage on the thermodynamic, optical and morphological behavior of carbazol derivatives

et al. 2020

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How accurate are energies of HOMO and LUMO levels in small-molecule organic semiconductors determined from cyclic voltammetry or optical spectroscopy?

Cited by 147 publications

References 45 publications

Relationship of Ionization Potential and Oxidation Potential of Organic Semiconductor Films Used in Photovoltaics

Relationship of Ionization Potential and Oxidation Potential of Organic Semiconductor Films Used in Photovoltaics

Terminal Group Effect of Conjugated Microporous Polymers for Photocatalytic Water‐Splitting Hydrogen Evolution

The impact of phenyl–phenyl linkage on the thermodynamic, optical and morphological behavior of carbazol derivatives

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