Competition between the Charge Transfer State and the Singlet States of Donor or Acceptor Limiting the Efficiency in Polymer:Fullerene Solar Cells

Faist, Mark A.; Kirchartz, Thomas; Gong, Wei; Ashraf, Raja Shahid; McCulloch, Iain; Mello, John C. de; Ekins-Daukes, N. J.; Bradley, Donal D. C.; Nelson, Jenny

doi:10.1021/ja210029w

Cited by 249 publications

(317 citation statements)

References 77 publications

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“…This observation suggests that the small LUMO energy offset may not be the limiting factor, but instead a small HOMO offset limits the performance, possibly by enabling a recombination channel through the fullerene exciton, as others have proposed. 40 The problem is exacerbated in cells using ICBA since this material reduces the HOMO band offset, and the ICBA exciton is also at lower energy than the PC 60 BM exciton.…”

Section: Background and Measurement Methodologymentioning

confidence: 99%

Electronic Structure and Transition Energies in Polymer–Fullerene Bulk Heterojunctions

et al. 2014

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Photocurrent spectroscopy is used to measure both the charge transfer and exciton optical absorption spectra of various bulk heterojunction organic solar cells. The energy difference between the polymer HOMO energy and the fullerene LUMO energy is obtained from the spectra, along with the disorder energy. Combining information from cells with several different polymers and fullerenes allows measurements of the energy differences between HOMO or LUMO energies for about 10 different polymers and fullerenes, with an estimated uncertainty of 50 meV. Heterojunction band offsets are obtained for the various cells, distinguishing between the excitonic and the single-carrier band offsets. The cell open-circuit voltage is shown to be closely correlated with the interface band gap. The exciton disorder energy is directly correlated to the band-tail disorder and we also consider the effects of exciton thermalization on the charge generation mechanism. The data indicate that an energy offset between the polymer exciton and the charge transfer ground state below about 0.25 eV adversely affects the cell performance, while a HOMO band offset below about 0.2−0.3 eV also degrades cell performance but by a different mechanism.

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Section: Background and Measurement Methodologymentioning

confidence: 99%

Electronic Structure and Transition Energies in Polymer–Fullerene Bulk Heterojunctions

et al. 2014

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“…CT state EL was first reported by Kim et al, 48 and then also observed by other groups (Figure 2c). 46,47,49 Although the EL of the CT state is in the same spectral range as the PL, the EL peak usually shows slight redshift compared with the PL peak. 50 In addition, the EL peak could also show slight blueshift with increasing injection current or applied voltage.…”

Section: Electroluminescencementioning

confidence: 99%

Charge generation in polymer–fullerene bulk-heterojunction solar cells

Gao

Inganäs

2014

Phys. Chem. Chem. Phys.

204

252

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Charge generation in organic solar cells is a fundamental yet heavily debated issue. This article gives a balanced review of different mechanisms proposed to explain efficient charge generation in polymer-fullerene bulk-heterojunction solar cells. We discuss the effect of charge-transfer states, excess energy, external electric field, temperature, disorder of the materials, and delocalisation of the charge carriers on charge generation. Although a general consensus has not been reached yet, recent findings, based on both steady-state and transient measurements, have significantly advanced our understanding of this process.2

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“…The driving force for charge separation for these excitons on the fullerene, Δ E HOMO , is smaller than Δ E LUMO in this case and could be insuffi cient for charge generation. [ 36 ] In blends with high-bandgap polymers, the fullerene singlet can also act as an additional recombination channel through activation of the fullerene singlet that appears at Δ E HOMO < 0.35 eV and can reduce fi ll factor as well as open-circuit voltage. [ 36 ] …”

Section: Full Papermentioning

confidence: 99%

“…[ 36 ] In blends with high-bandgap polymers, the fullerene singlet can also act as an additional recombination channel through activation of the fullerene singlet that appears at Δ E HOMO < 0.35 eV and can reduce fi ll factor as well as open-circuit voltage. [ 36 ] …”

Section: Full Papermentioning

confidence: 99%

Understanding the Reduced Efficiencies of Organic Solar Cells Employing Fullerene Multiadducts as Acceptors

Faist

Shoaee

Tuladhar

et al. 2013

Advanced Energy Materials

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132

140

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The use of fullerenes with two or more adducts as acceptors has been recently shown to enhance the performance of bulk‐heterojunction solar cells using poly(3‐hexylthiophene) (P3HT) as the donor. The enhancement is caused by a substantial increase in the open‐circuit voltage due to a rise in the fullerene lowest unoccupied molecular orbital (LUMO) level when going from monoadducts to multiadducts. While the increase in the open‐circuit voltage is obtained with many different polymers, most polymers other than P3HT show a substantially reduced photocurrent when blended with fullerene multiadducts like bis‐PCBM (bis adduct of Phenyl‐C61‐butyric acid methyl ester) or the indene C60 bis‐adduct ICBA. Here we investigate the reasons for this decrease in photocurrent. We find that it can be attributed partly to a loss in charge generation efficiency that may be related to the LUMO‐LUMO and HOMO‐HOMO (highest occupied molecular orbital) offsets at the donor‐acceptor heterojunction, and partly to reduced charge carrier collection efficiencies. We show that the P3HT exhibits efficient collection due to high hole and electron mobilities with mono‐ and multiadduct fullerenes. In contrast the less crystalline polymer Poly[[9‐(1‐octylnonyl)‐9H‐carbazole‐2,7‐diyl]‐2,5‐thiophenediyl‐2,1,3‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl (PCDTBT) shows inefficient charge carrier collection, assigned to low hole mobility in the polymer and low electron mobility when blended with multiadduct fullerenes.

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Competition between the Charge Transfer State and the Singlet States of Donor or Acceptor Limiting the Efficiency in Polymer:Fullerene Solar Cells

Cited by 249 publications

References 77 publications

Electronic Structure and Transition Energies in Polymer–Fullerene Bulk Heterojunctions

Electronic Structure and Transition Energies in Polymer–Fullerene Bulk Heterojunctions

Charge generation in polymer–fullerene bulk-heterojunction solar cells

Understanding the Reduced Efficiencies of Organic Solar Cells Employing Fullerene Multiadducts as Acceptors

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