1
|
Direct monitoring of bias-dependent variations in the exciton formation ratio of working organic light emitting diodes. Sci Rep 2015; 5:15533. [PMID: 26487499 PMCID: PMC4614446 DOI: 10.1038/srep15533] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/18/2015] [Accepted: 09/28/2015] [Indexed: 11/20/2022] Open
Abstract
In typical operation of organic light emitting diodes (OLEDs), excitons are assumed to generate with a ratio of 1:3 for singlet and triplet excitons, respectively, based on a simple spin statistics model. This assumption has been used in designing efficient OLEDs. Despite the larger generation ratio of triplet excitons, physical properties of fluorescent OLEDs are usually evaluated only through the electroluminescence (EL) intensity from singlets and the behaviors of triplets during the LED operation are virtually black-boxed, because the triplets are mostly non-emissive. Here, we employ transient spectroscopy combined with LED-operation for directly monitoring the non-emissive triplets of working OLEDs. The spectroscopic techniques are performed simultaneously with EL- and current measurements under various operation biases. The simultaneous measurements reveal that the relative formation ratio of singlet-to-triplet excitons dramatically changes with the magnitude of bias. The measurements also show that the generation efficiency of singlets scales with the bias, whereas that of triplets is nearly bias-independent. These features of the formation ratio and efficiency are compatibly explained by considering the yield of intersystem crossing and the energy separation of excitons from electron-hole pairs. The obtained findings via the spectroscopic measurements enable prediction of the formation pathways in OLEDs.
Collapse
|
2
|
Howard IA, Hodgkiss JM, Zhang X, Kirov KR, Bronstein HA, Williams CK, Friend RH, Westenhoff S, Greenham NC. Charge Recombination and Exciton Annihilation Reactions in Conjugated Polymer Blends. J Am Chem Soc 2009; 132:328-35. [DOI: 10.1021/ja908046h] [Citation(s) in RCA: 61] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Ian A. Howard
- Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom, Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom, and Department of Chemistry, Biochemistry & Biophysics, University of Gothenburg, Box 462, 40530 Gothenburg, Sweden
| | - Justin M. Hodgkiss
- Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom, Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom, and Department of Chemistry, Biochemistry & Biophysics, University of Gothenburg, Box 462, 40530 Gothenburg, Sweden
| | - Xinping Zhang
- Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom, Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom, and Department of Chemistry, Biochemistry & Biophysics, University of Gothenburg, Box 462, 40530 Gothenburg, Sweden
| | - Kiril R. Kirov
- Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom, Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom, and Department of Chemistry, Biochemistry & Biophysics, University of Gothenburg, Box 462, 40530 Gothenburg, Sweden
| | - Hugo A. Bronstein
- Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom, Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom, and Department of Chemistry, Biochemistry & Biophysics, University of Gothenburg, Box 462, 40530 Gothenburg, Sweden
| | - Charlotte K. Williams
- Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom, Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom, and Department of Chemistry, Biochemistry & Biophysics, University of Gothenburg, Box 462, 40530 Gothenburg, Sweden
| | - Richard H. Friend
- Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom, Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom, and Department of Chemistry, Biochemistry & Biophysics, University of Gothenburg, Box 462, 40530 Gothenburg, Sweden
| | - Sebastian Westenhoff
- Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom, Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom, and Department of Chemistry, Biochemistry & Biophysics, University of Gothenburg, Box 462, 40530 Gothenburg, Sweden
| | - Neil C. Greenham
- Cavendish Laboratory, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom, Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom, and Department of Chemistry, Biochemistry & Biophysics, University of Gothenburg, Box 462, 40530 Gothenburg, Sweden
| |
Collapse
|
3
|
Sohn Y, Stuckless JT. Characteristics of Photoexcitations and Interfacial Energy Levels of Regioregular Poly(3-hexythiophene-2,5-diyl) on Gold. Chemphyschem 2007; 8:1937-42. [PMID: 17676649 DOI: 10.1002/cphc.200700348] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
We have studied characteristics of photoexcitations and interfacial electronic structures of regioregular poly(3-hexlythiophene-2,5-diyl) (P3HT) on gold using two-photon photoemission (2PPE) spectroscopy. The vacuum level threshold is decreased by 1.3 eV from that of bare gold, attributable to interface dipole effects. The 2PPE spectral width narrows as the film thickness increases. We tentatively understand that this is due to destabilization of long-lived localized polaron, attributed to strong interchain interactions. On the basis of the analysis of the 2PPE distribution as a function of photon energy and laser power, the polaron level is located at 3.1 eV below the vacuum level. Using this value and a polaron level of 1.75 eV above the HOMO, we indirectly estimate an ionization potential of 4.85 eV for P3HT. An increase in two-photon photoemission yield with increasing photon energy is attributed to an enhanced electron-hole pair dissociation yield at higher photo-excitation levels. The decrease in power law slope with increasing film thickness is understood by Langevin recombination kinetics and saturation of photoexcitations.
Collapse
Affiliation(s)
- Youngku Sohn
- Chemistry, The University of British Columbia, Vancouver, British Columbia, Canada.
| | | |
Collapse
|
5
|
Sohn Y, Stuckless JT. Bimolecular recombination kinetics and interfacial electronic structures of poly[2-methoxy-5-(2-ethyl-hexyloxy)-p-phenylene vinylene] on gold studied using two-photon photoemission spectroscopy. J Chem Phys 2007; 126:174901. [PMID: 17492880 DOI: 10.1063/1.2727473] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Photoexcitation kinetics and interfacial electronic structures of poly[2-methoxy-5(2-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) film on gold have been investigated using two-photon photoemission spectroscopy (2PPE). The authors directly probed a fixed intermediate state located at 0.95 eV above the Fermi level (or 2.95 eV below the vacuum level), assigned to a charged polaron. Based on the power law slope and the 2PPE spectra with laser intensity, they found that the polaron follows a second order bimolecular annihilation process. The 2PPE yield dramatically increases with increasing photon energy. They attribute this to an enhanced dissociation of hotter excitons at higher excitation levels. The work function of MEH-PPV/Au is measured to be 3.9 eV, 1.2 eV downshift from the clean gold, attributable to interface dipole effects. The energy gap between the intermediate polaron state and the hole polaron level is estimated to be 2.45 eV.
Collapse
Affiliation(s)
- Youngku Sohn
- Department of Chemistry, AMPEL Laboratories, University of British Columbia, Vancouver, V6T 1Z1, Canada.
| | | |
Collapse
|