Dichotomous Role of Exciting the Donor or the Acceptor on Charge Generation in Organic Solar Cells

Hendriks, Koen H.; Wijpkema, Alexandra S. G.; Franeker, Jacobus J. van; Wienk, Martijn M.; Janssen, Raj René

doi:10.1021/jacs.6b05868

Cited by 71 publications

(80 citation statements)

References 28 publications

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“…[7d,14] The LUMO–LUMO energy offsets between the donor and the acceptors (BTA2, 1, 3) are respectively 0.05, 0.21, 0.35 eV and the HOMO–HOMO energy offsets are respectively 0.11, 0.14, 0.17 eV. This gradual change of energy offsets will offer an excellent parameter for the study on the charge generation process …”

Section: Resultsmentioning

confidence: 99%

Simultaneously Achieved High Open‐Circuit Voltage and Efficient Charge Generation by Fine‐Tuning Charge‐Transfer Driving Force in Nonfullerene Polymer Solar Cells

Tang

Xiao

Wang

et al. 2017

Adv Funct Materials

187

154

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To maximize the short-circuit current density (J SC ) and the open circuit voltage (V OC ) simultaneously is a highly important but challenging issue in organic solar cells (OSCs). In this study, a benzotriazole-based p-type polymer (J61) and three benzotriazole-based nonfullerene small molecule acceptors (BTA1-3) are chosen to investigate the energetic driving force for the efficient charge transfer. The lowest unoccupied molecular orbital (LUMO) energy levels of small molecule acceptors can be fine-tuned by modifying the endcapping units, leading to high V OC (1.15-1.30 V) of OSCs. Particularly, the LUMO energy level of BTA3 satisfies the criteria for efficient charge generation, which results in a high V OC of 1.15 V, nearly 65% external quantum efficiency, and a high power conversion efficiency (PCE) of 8.25%. This is one of the highest V OC in the high-performance OSCs reported to date. The results imply that it is promising to achieve both high J SC and V OC to realize high PCE with the carefully designed nonfullerene acceptors.

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Section: Resultsmentioning

confidence: 99%

Simultaneously Achieved High Open‐Circuit Voltage and Efficient Charge Generation by Fine‐Tuning Charge‐Transfer Driving Force in Nonfullerene Polymer Solar Cells

Tang

Xiao

Wang

et al. 2017

Adv Funct Materials

187

154

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“…[21] A linear correlation between V oc and the HOMO and LUMO energies with slope 1 has also been seen in various other studies, [6,54] usually for a smaller set of materials, but deviations with slope <1 have also been reported. For solar cells of the polymers in Table 1, 2PyT (E g = 1.73 eV) gave the lowest short-circuit current density (J sc ) (7.00 mA cm −2 ), [56] while TDTPT (E g = 1.23 eV) gave the highest J sc (20.5 mA cm −2 ), [57] i.e., a factor of ≈3. Nevertheless, it is of interest to consider possible explanations for our finding of a slope <1.…”

Section: Correlation Between Homo Energies and V Ocmentioning

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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“…[47] Surprisingly, they found that excitation of the fullerene is accompanied by a substantially larger energy loss (at least 0.85 eV), while the threshold for efficient cells after excitation of the donor polymer can be slightly less than 0.6 eV. Although the microscopic origin for this significant difference is not clear yet, other groups have reported further examples of remarkably small energy losses using non-fullerene acceptors.…”

Section: Reducing the Driving Forcementioning

confidence: 98%

Energy Losses in Small‐Molecule Organic Photovoltaics

Linderl

Zechel

Brendel

et al. 2017

Advanced Energy Materials

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processible PV technology has recently appeared as well. [5] Comparing the different technologies in terms of PCE, which is specified as the product of the short-circuit current density j SC , the open-circuit voltage V OC and the fill factor FF divided by the incoming light intensity under standard AM 1.5G illumination conditions, OPVs can well compete with their inorganic counterparts in terms of j SC or, more precisely, the external quantum efficiency and also with minor trade-off in FF, but clearly suffer from lower V OC at a given energy gap E g of the light absorbing material. While this socalled bandgap-voltage offset can be as low as 0.3-0.4 eV in Si and GaAs [6] and only a little larger in perovskite cells, [7] OPV cells exhibit energy losses of at least 0.6 eV -in many cases, however, this offset can approach and even exceed 1 eV. [8] This is currently one of the main bottlenecks toward making OPVs competitive with inorganic PV cells.In this research news, we provide the required background information on the appearance of energy losses in OPV cells, by which we mean the difference between the equivalent of the optical gap and the measured open-circuit voltage that is frequently also denoted as voltage loss, and discuss recent progress toward better understanding their origin and strategies to reduce them. To keep focused, we will mainly address small molecules as active organic semiconductors, which are being processed into thin films by vacuum deposition techniques. Compared to frequently studied π-conjugated polymers, the synthesis of small molecules is more reproducible. Moreover, a rigorous purification of small molecules is easier, which gives the opportunity to reproducibly investigate well-defined systems. The application of vacuum deposition techniques prevents the use of solvents, which as a third component in wet chemical processing can strongly influence the morphology. [9] Thus, active layers of small molecules prepared by vacuum deposition methods mark a well-controlled model system for fundamental studies such as the origin of energy losses in OPV devices. However, we expect that most of the findings can be transferred to solution-processed OPVs as well, which have considerably higher complexity in terms of local morphology and phase behavior. Excitonic Organic Solar CellsIn order to properly address energy losses in OPVs it is useful to look at their working principles in more detail (see also [10] ).

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Dichotomous Role of Exciting the Donor or the Acceptor on Charge Generation in Organic Solar Cells

Cited by 71 publications

References 28 publications

Simultaneously Achieved High Open‐Circuit Voltage and Efficient Charge Generation by Fine‐Tuning Charge‐Transfer Driving Force in Nonfullerene Polymer Solar Cells

Simultaneously Achieved High Open‐Circuit Voltage and Efficient Charge Generation by Fine‐Tuning Charge‐Transfer Driving Force in Nonfullerene Polymer Solar Cells

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

Energy Losses in Small‐Molecule Organic Photovoltaics

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