On the reliability of determination of energies of HOMO and LUMO levels in organic semiconductors from electrochemical measurements. A simple picture based on the electrostatic model

Sworakowski, J.; Lipiński, Józef; Janus, Krzysztof

doi:10.1016/j.orgel.2016.03.031

Cited by 83 publications

(92 citation statements)

References 125 publications

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“…Due to the ambiguity on the offset between the electrochemical potentials and the estimated FMO energy levels, we only report the raw data of electrochemical potentials in this work. Nevertheless, the trends in the electrochemical potentials can be rationally related to the energy levels of the FMOs , …”

Section: Resultsmentioning

confidence: 99%

Tuning the NIR Absorption Properties of 1,3,4,6,9b‐Pentaazaphenalene Derivatives Through the Spatially Separated Frontier Molecular Orbitals

et al. 2020

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Strategical modulation of energy levels of molecular electronic orbitals and resulting near‐infrared (NIR) absorption properties of 2,5,7,9‐tetrasubstituted 1,3,4,6,9b‐pentaazaphenalene (5AP) derivatives are reported. Recent reports on NIR‐absorbing materials mainly rely on donor–acceptor electronic interactions or macrocyclic π‐conjugated systems. We achieved the narrow HOMO–LUMO gap based on spatially separated frontier molecular orbitals (FMOs) of 5APs. The FMOs of 5APs enabled us to selectively tune either their HOMO or LUMO by choosing the position of the substituent.

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Section: Resultsmentioning

confidence: 99%

Tuning the NIR Absorption Properties of 1,3,4,6,9b‐Pentaazaphenalene Derivatives Through the Spatially Separated Frontier Molecular Orbitals

et al. 2020

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“…The CV technique obtains oxidation/reduction potentials ( E ox /E red , referenced to a redox couple) and these potentials can then be converted to oxidation/reduction energies ( qE ox /qE red ). Studies have shown a linear relationship between UPS IP and CV E ox and IPES EA and CV E red , respectively, such that IP = α + × qE ox + β + and EA = α − × qE red + β − . 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 .…”

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

confidence: 99%

“…Studies have shown a linear relationship between UPS IP and CV E ox and IPES EA and CV E red , respectively, such that IP = α + × qE ox + β + and EA = α − × qE red + β − . 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.…”

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

confidence: 99%

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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“…Fc/Fc + is one of the redox couples recommended by the International Union of Pure and Applied Chemistry for reporting energy levels in nonaqueous solvents. [36][37][38][39][40] This would alter the proportionality constant between E CT and (E HOMO,D − E LUMO,A ). [35] Recently, several experimental studies of the oxidation potential as obtained from CV (E ox,CV ) and ionization potential from UPS (IP) have revealed that both quantities are linearly correlated, i.e., IP = α + qE ox,CV + β + , but that the best-fit slope varies considerably from α + = 0.9 to 1.5, depending on the material class.…”

Section: Introductionmentioning

confidence: 99%

Relating Frontier Orbital Energies from Voltammetry and Photoelectron Spectroscopy to the Open‐Circuit Voltage of Organic Solar Cells

Willems

Weijtens

Vries

et al. 2019

Advanced Energy Materials

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For 19 diketopyrrolopyrrole polymers, the highest occupied molecular orbital (HOMO) energies are determined from i) the oxidation potential with square-wave voltammetry (SWV), ii) the ionization potential using ultraviolet photoelectron spectroscopy (UPS), and iii) density functional theory (DFT) calculations. The SWV HOMO energies show an excellent linear correlation with the open-circuit voltage (V oc ) of optimized solar cells in which the polymers form blends with a fullerene acceptor ([6,6]-phenyl-C 61 -butyl acid methyl ester or [6,6]-phenyl-C 71 -butyl acid methyl ester).Remarkably, the slope of the best linear fit is 0.75 ± 0.04, i.e., significantly less than unity. A weaker correlation with V oc is found for the HOMO energies obtained from UPS and DFT. Within the experimental error, the SWV and UPS data are correlated with a slope close to unity. The results show that electrochemically determined oxidation potentials provide an excellent method for predicting the V oc of bulk heterojunction solar cells, with absolute deviations less than 0.1 V.

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On the reliability of determination of energies of HOMO and LUMO levels in organic semiconductors from electrochemical measurements. A simple picture based on the electrostatic model

Cited by 83 publications

References 125 publications

Tuning the NIR Absorption Properties of 1,3,4,6,9b‐Pentaazaphenalene Derivatives Through the Spatially Separated Frontier Molecular Orbitals

Tuning the NIR Absorption Properties of 1,3,4,6,9b‐Pentaazaphenalene Derivatives Through the Spatially Separated Frontier Molecular Orbitals

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

Relating Frontier Orbital Energies from Voltammetry and Photoelectron Spectroscopy to the Open‐Circuit Voltage of Organic Solar Cells

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