| 1. |
- De, Swati, et al.
(författare)
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Exciton Dynamics in Alternating Polyfluorene/Fullerene Blends
- 2008
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Ingår i: Journal of Chemical Physics. - College Park, MD, United States : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 350:1-3, s. 14-22
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Tidskriftsartikel (refereegranskat)abstract
- Exciton dynamics in alternating copolymer/fullerene solar cell blends have been investigated using femtosecond transient absorption spectroscopy. The acceptor concentrations have been varied over a wide range. Experimental data, kinetic modeling and simulations, all indicate that the efficiency of exciton conversion to charges is 100% even at acceptor concentrations as low as 20 wt%. The reported dependence of solar cell efficiency on fullerene concentration may thus arise from other factors. However, there exists an acceptor concentration threshold (5 wt%) below which a substantial fraction of the excitations remain unquenched. The results, we believe are very relevant to optimization of performance efficiency by clever manipulation of morphology. We have also observed exciton–exciton energy transfer in these blends at low acceptor concentrations.
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| 2. |
- De, Swati, et al.
(författare)
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Geminate charge recombination in alternating polyfluorene copolymer/fullerene blends
- 2007
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 129:27, s. 8466-8472
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Tidskriftsartikel (refereegranskat)abstract
- By measuring excited state and charge dynamics in blends of an alternating polyfluorene copolymer and fullerene derivative over nine orders in time and two orders in light intensity, we have monitored the light-induced processes from ultrafast charge photogeneration to much slower decay of charges by recombination. We find that at low light intensities relevant to solar cell operation relatively fast (∼30 ns) geminate recombination is the dominating charge decay process, while nongeminate recombination has a negligible contribution. The conclusion of our work is that under solar illumination conditions geminate recombination of charges may be directly competing with efficient charge collection in polymer/fullerene solar cells. © 2007 American Chemical Society.
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| 3. |
- Jespersen, Kim, et al.
(författare)
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Charge formation and transport in bulk-heterojunction solar cells based on alternating polyfluorene copolymers blended with fullerenes
- 2006
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Ingår i: Organic Electronics. - : Elsevier BV. - 1566-1199 .- 1878-5530. ; 7:4, s. 235-242
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Tidskriftsartikel (refereegranskat)abstract
- We investigate charge formation in bulk-heterojunction solar cells based on conjugated polymers in the form of alternating polyfluorene copolymers and the methanofullerene PCBM. Using transient absorption spectroscopy we show that optimal charge formation is obtained with 20-50 wt% PCBM. This is in contrast to the maximum short circuit current density obtained at similar to 80 wt% PCBM as determined by steady state current density-voltage characterization. Hence, we show explicitly that the solar cell performance of these interpenetrating polymer networks containing PCBM is limited by charge transport rather than by formation of charges. (c) 2006 Elsevier B.V. All rights reserved.
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| 4. |
- Nemec, Hynek, et al.
(författare)
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Charge carrier dynamics in alternating polyfluorene copolymer : Fullerene blends probed by terahertz spectroscopy
- 2008
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 112:16, s. 6558-6563
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Tidskriftsartikel (refereegranskat)abstract
- Time-resolved terahertz spectroscopy is used for investigation of photoinduced charge carrier dynamics in blends of a polyfluorene copolymer (poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl- 2',1',3-benzo-thiadiazole)]) and an electron acceptor ([6,6]-phenyl-C61-butyric acid methyl ester). The transient far-infrared response appears instantaneously after photoexcitation. We show that the transient conductivity spectrum is dominated by two major contributions: response of separated charge carriers and response of coupled polaron pairs. © 2008 American Chemical Society.
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| 5. |
- Nemec, Hynek, et al.
(författare)
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Ultrafast conductivity in a low-band-gap polyphenylene and fullerene blend studied by terahertz spectroscopy
- 2009
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969 .- 1098-0121. ; 79:24, s. 245326 (art no)-
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Tidskriftsartikel (refereegranskat)abstract
- Time-resolved terahertz spectroscopy and Monte Carlo simulations of charge-carrier motion are used to investigate photoinduced transient conductivity in a blend of a low-band-gap polyphenylene copolymer and fullerene derivative. The optical excitation pulse generates free holes delocalized on polymer chains. We show that these holes exhibit a very high initial mobility as their initial excess energy facilitates their transport over defects (potential barriers) on polymer chains. The conductivity then drops down rapidly within 1 ps, and we demonstrate that this decrease occurs essentially by two mechanisms. First, the carriers loose their excess energy and they thus become progressively localized between the on-chain potential barriers-this results in a mobility decay with a rate of (180 fs)(-1). Second, carriers are trapped at defects (potential wells) with a capture rate of (860 fs)(-1). At longer time scales, populations of mobile and trapped holes reach a quasiequilibrium state and further conductivity decrease becomes very slow.
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| 6. |
- Zhang, Fengling, et al.
(författare)
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Influence of solvent mixing on the morphology and performance of solar cells based on polyfluorene copolymer/fullerene blends
- 2006
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Ingår i: Advanced Functional Materials. - : Wiley. - 1616-3028 .- 1616-301X. ; 16:5, s. 667-674
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Tidskriftsartikel (refereegranskat)abstract
- The influence of the solvent on the morphol. and performance of polymer solar cells was studied in devices based on blends of the polyfluorene copolymer, poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2', 1',3'-benzothiadiazole)), and [6,6]-phenyl-C61-butyric acid Me ester. The blends are spin-coated from CHCl3 or from CHCl3 mixed with small amts. of xylene, toluene, or chlorobenzene. The devices are characterized under monochromatic light and AM1.5 solar illumination. An enhancement of the photocurrent d. was obsd. in diodes made from CHCl3 mixed with chlorobenzene, and a decreased photocurrent d. is obsd. in diodes made from CHCl3 mixed with xylene or toluene, compared to diodes made from neat CHCl3. The open-circuit voltages are almost the same for all diodes. The surfaces of the active layers were imaged with AFM. Height images indicate that a finer and more uniform distribution of domains corresponds to the diodes with enhanced photocurrent that are made from CHCl3 mixed with chlorobenzene, while a structure with larger domains is assocd. with the lower photocurrents in the diodes made from CHCl3 mixed with xylene or toluene. The influence of the morphol. on the excited-state dynamics and charge generation was studied using time-resolved spectroscopy. Fast formation of bound charge pairs followed by their conversion into free charge carriers was resolved, and excitation-intensity-dependent non-geminate recombination of free charges was obsd. A significant enhancement in free-charge-carrier generation was obsd. on introducing chlorobenzene into CHCl3. Imaging photocurrent generation from the solar cells with a light-pulse technique shows an inhomogeneous photocurrent distribution, which is related to the undulations in the thickness of the active layer. Thicker parts of the diodes yield higher photocurrent values. [on SciFinder (R)]
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