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

Abstract

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...

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

“…In this case, Deming regression gives a slope of 1.07 ( Figure 8). [40] The spread in the data around the best-fit line in Figure 8 can be partially attributed to the experimental uncertainty of the data points: about ±0.05 eV for E HOMO,SWV and about ±0.10 eV or in some cases slightly more for E HOMO,UPS . Energy Mater.…”

“…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.…”

“…[34] In the literature, there is little consensus about the correct vacuum energy level of the Fc/Fc + redox couple, as values between −4.4 and −5.4 eV have been reported and used. [40] It is therefore at present not clear which experimental technique for determining (E HOMO,D − E LUMO,A ) is most suitable for accurately predicting V oc . [36][37][38][39][40] This would alter the proportionality constant between E CT and (E HOMO,D − E LUMO,A ).…”

“…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 . This would alter the proportionality constant between E CT and ( E HOMO,D − E LUMO,A ).…”

WITHDRAWN: Near 14 cm2 V-1 s-1 Charge Carrier Mobility Enabled by Amorphous Indacenodithiophene-based Polymers with Controlled Regioregularity

“…2c), evidencing the relatively weaker electron-donating nature of the IDT unit. 37,38 With theoretical understanding of the structural features of axisymmetric CDT vs. centrosymmetric IDT Figures S10-13). Four regioregular polymers with the FBT units facing the same regular pattern along the backbones yet different backbone compositions were synthesized from the corresponding monomers under the same conditions (Ph(PPh 3 ) 4 as the catalyst in o-xylene under microwave heating at 220°C), as shown in Scheme 1.…”