| 1. |
- Aarnio, Harri, et al.
(författare)
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Recombination studies in a polyfluorene copolymer for photovoltaic applications
- 2005
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Ingår i: Synthetic Metals. - : Elsevier BV. - 0379-6779. ; 155:2, s. 299-302
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Tidskriftsartikel (refereegranskat)abstract
- We present detailed continuous wave (cw) and transient photoinduced absorption (PA) measurements in thin films of a novel alternating polyfluorene copolymer, poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3-benzo-thiadiazole)] (DiO-PFDTBT), and its blends with the sol. fullerene deriv. [6,6]-phenyl-C61-butyric acid Me ester (PCBM) in wt. ratios of 1:0, 4:1 and 1:4. We measure the frequency, intensity and temp. dependence of the PA signal in the frequency domain, and compare with the results obtained from the transient PA decay measurements in the time domain. In all blends, the PA spectrum shows a broad high energy PA band ranging from .apprx.1 eV to 2 eV as well as a low energy band peaking at .apprx.0.35 eV. We attribute the low energy band to the P1 transition of polarons and part of the high energy band to the correlated P2 transition of polarons. Both frequency and time domain measurements show that the high energy band has two decay components, a faster component in the microsecond time regime and a slower component in the millisecond time regime. The slow component is strongly dispersive, whereas the fast component is practically non-dispersive. [on SciFinder (R)]
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| 2. |
- Aarnio, Harri, et al.
(författare)
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Spontaneous Charge Transfer and Dipole Formation at the Interface Between P3HT and PCBM
- 2011
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Ingår i: Advanced Energy Materials. - : Wiley-VCH Verlag Berlin. - 1614-6832 .- 1614-6840. ; 1:5, s. 792-797
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Tidskriftsartikel (refereegranskat)abstract
- In the pursuit of developing new materials for more efficient bulk-heterojunction solar cells, the blend poly (3-hexylthiophene):[ 6,6]-phenyl-C(61)-butyric acid methyl ester (P3HT:PCBM) serves as an important model system. The success of the P3HT: PCBM blend comes from efficient charge generation and transport with low recombination. There is not, however, a good microscopic picture of what causes these, hindering the development of new material systems. In this report UV photoelectron spectroscopy measurements on both regiorandom-(rra) and regioregular-(rr) P3HT are presented, and the results are interpreted using the Integer Charge Transfer model. The results suggest that spontaneous charge transfer from P3HT to PCBM occurs after heat treatment of P3HT: PCBM blends. The resulting formation of an interfacial dipole creates an extra barrier at the interface explaining the reduced (non-)geminate recombination with increased charge generation in heat treated rr-P3HT: PCBM blends. Extensive photoinduced absorption measurements using both above-and below-bandgap excitation light are presented, in good agreement with the suggested dipole formation.
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| 3. |
- Bao, Qinye, et al.
(författare)
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Trap-Assisted Recombination via Integer Charge Transfer States in Organic Bulk Heterojunction Photovoltaics
- 2014
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Ingår i: Advanced Functional Materials. - : Wiley-VCH Verlag. - 1616-301X .- 1616-3028. ; 24:40, s. 6309-6316
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Tidskriftsartikel (refereegranskat)abstract
- Organic photovoltaics are under intense development and significant focus has been placed on tuning the donor ionization potential and acceptor electron affinity to optimize open circuit voltage. Here, it is shown that for a series of regioregular-poly(3-hexylthiophene): fullerene bulk heterojunction (BHJ) organic photovoltaic devices with pinned electrodes, integer charge transfer states present in the dark and created as a consequence of Fermi level equilibrium at BHJ have a profound effect on open circuit voltage. The integer charge transfer state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor-acceptor interface, even though there is a large variation in electron affinity for the fullerene series. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of trap-assisted recombination via integer charge transfer states. Based on the results, novel design rules for optimizing open circuit voltage and performance of organic bulk heterojunction solar cells are proposed.
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