| 1. |
- Andersson, Mattias, et al.
(author)
-
Unified Study of Recombination in Polymer:Fullerene Solar Cells Using Transient Absorption and Charge-Extraction Measurements
- 2013
-
In: Journal of Physical Chemistry Letters. - : American Chemical Society. - 1948-7185 .- 1948-7185. ; 4:12, s. 2069-2072
-
Journal article (peer-reviewed)abstract
- Recombination in the well-performing bulk heterojunction solar cell blend between the conjugated polymer TQ-1 and the substituted fullerene PCBM has been investigated with pump-probe transient absorption and charge extraction of photo-generated carriers (photo-CELIV). Both methods are shown to generate identical and overlapping data under appropriate experimental conditions. The dominant type of recombination is bimolecular with a rate constant of 7 x 10(-12) cm(-3) s(-1). This recombination rate is shown to be fully consistent with solar cell performance. Deviations from an ideal bimolecular recombination process, in this material system only observable at high pump fluences, are explained with a time-dependent charge-carrier mobility, and the implications of such a behavior for device development are discussed.
|
|
| 2. |
- Bian, Qingzhen, 1988-, et al.
(author)
-
Vibronic coherence contributes to photocurrent generation in organic semiconductor heterojunction diodes
- 2020
-
In: Nature Communications. - : NATURE PUBLISHING GROUP. - 2041-1723. ; 11:1
-
Journal article (peer-reviewed)abstract
- Charge separation dynamics after the absorption of a photon is a fundamental process relevant both for photosynthetic reaction centers and artificial solar conversion devices. It has been proposed that quantum coherence plays a role in the formation of charge carriers in organic photovoltaics, but experimental proofs have been lacking. Here we report experimental evidence of coherence in the charge separation process in organic donor/acceptor heterojunctions, in the form of low frequency oscillatory signature in the kinetics of the transient absorption and nonlinear two-dimensional photocurrent spectroscopy. The coherence plays a decisive role in the initial 200 femtoseconds as we observe distinct experimental signatures of coherent photocurrent generation. This coherent process breaks the energy barrier limitation for charge formation, thus competing with excitation energy transfer. The physics may inspire the design of new photovoltaic materials with high device performance, which explore the quantum effects in the next-generation optoelectronic applications.
|
|
| 3. |
- Björk, Per, et al.
(author)
-
Oligothiophene Assemblies Defined by DNA Interaction : From Single Chains to Disordered Clusters
- 2009
-
In: SMALL. - : Wiley. - 1613-6810 .- 1613-6829. ; 5:1, s. 96-103
-
Journal article (peer-reviewed)abstract
- The organization of conjugated polyelectrolytes (CPEs) interacting with biomolecules sets conditions for the biodetection of biological processes and identity, through the use of optical emission from the CPE. Herein, a well-defined CPE and its binding to DNA is studied. By using dynamic light scattering and circular dichroism spectroscopy, it is shown that the CPE forms a multimolecule ensemble in aqueous solution that is more than doubled it? size when interacting with a small DNA chain, while single chains are evident in ethanol. The related changes in the fluorescence spectra upon polymer aggregation are assigned to oscillator strength redistribution between vibronic transitions in weakly coupled H-aggregates. An enhanced single-molecule spectroscopy technique that allows full control of excitation and emission light polarization is applied to combed and decorated;,DNA chains. It is found that the organization of combed CPE-lambda DNA complexes (when dry on the surface) allows considerable variation of CPE distances and direction relative to the DNA chain. By analysis of the polarization data. energy transfer between the polymer chains in individual complexes is confirmed and their sizes estimated.
|
|
| 4. |
- Björk, Per, et al.
(author)
-
Single molecular imaging and spectroscopy of conjugated polyelectrolytes decorated on stretched aligned DNA
- 2005
-
In: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 5:10, s. 1948-1953
-
Journal article (peer-reviewed)abstract
- DNA is the prototype template for building nanoelectronic devices by self-assembly. The electronic functions are made possible by coordinating electronic polymer chains to DNA. This paper demonstrates two methods for fabrication of aligned and ordered DNA nanowires complexed with conjugated polyelectrolytes (CPEs). The complex can be formed either in solution prior to stretching or after stretching of the bare DNA on a surface. Molecular combing was used to stretch the complexes on surface energy patterned surfaces, and PMMA for the bare DNA. Single molecular spectroscopy, in fluorescence, and microscopy, in atomic force microscopy, give evidence for coordination of the short CPE chains to the aligned DNA.
|
|
| 5. |
- Camacho Dejay, Rafael, et al.
(author)
-
Polarization Imaging of Emissive Charge Transfer States in Polymer/Fullerene Blends
- 2014
-
In: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 26:23, s. 6695-6704
-
Journal article (peer-reviewed)abstract
- Photoexcitation of conjugated polymerfullerene blends results in population of a local charge transfer (CT) state at the interface between the two materials. The competition between recombination and dissociation of this interfacial state limits the generation of fully separated free charges. Therefore, a detailed understanding of the CT states is critical for building a comprehensive picture of the organic solar cells operation. We applied a new fluorescence microscopy method called two-dimensional polarization imaging to gain insight into the orientation of the transition dipole moments of the CT states, and the associated excitation energy transfer processes in TQ1:PCBM blend films. The polymer phase was oriented mechanically to relate the polymer dipole moment orientation to that of the CT states. CT state formation was observed to be much faster than energy transfer in the polymer phase. However, after being formed an emissive CT state does not exchange excitation energy with other CT states, suggesting that they are spatially and/or energetically isolated. We found that the quantum yield of the CT emission is smaller for CT states spatially located in the highly oriented polymer domains, which is interpreted as the result of enhanced CT state dissociation in highly ordered structures.
|
|
| 6. |
- Chen, Miaoxiang, 1962-, et al.
(author)
-
High carrier mobility in low band gap polymer-based field-effect transistors
- 2005
-
In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 87:25, s. 252105-1-252105-3
-
Journal article (peer-reviewed)abstract
- A conjugated polymer with a low band gap of 1.21 eV, i.e., absorbing infrared light, is demonstrated as active material in field-effect transistors (FETs). The material consists of alternating fluorene units and low band gap segments with electron donor-acceptor-donor units composed of two electron-donating thiophene rings attached on both sides of a thiadiazolo-quinoxaline electron-acceptor group. The polymer is solution-processable and air-stable; the resulting FETs exhibit typical p-channel characteristics and field-effect mobility of 0.03 cm2 V−1 s−1.
|
|
| 7. |
- De, Swati, et al.
(author)
-
Exciton Dynamics in Alternating Polyfluorene/Fullerene Blends
- 2008
-
In: Journal of Chemical Physics. - College Park, MD, United States : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 350:1-3, s. 14-22
-
Journal article (peer-reviewed)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.
|
|
| 8. |
- De, Swati, et al.
(author)
-
Geminate charge recombination in alternating polyfluorene copolymer/fullerene blends
- 2007
-
In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 129:27, s. 8466-8472
-
Journal article (peer-reviewed)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.
|
|
| 9. |
- Herland, Anna, et al.
(author)
-
Alignment of a conjugated polymer onto amyloid-like protein fibrils
- 2007
-
In: Small. - : Wiley. - 1613-6810 .- 1613-6829. ; 3:2, s. 318-325
-
Journal article (peer-reviewed)abstract
- The amyloid-like fibril is a biomolecular nanowire template of very high stability. Here we describe the coordination of a conjugated polyelectrolyte, poly(thiophene acetic acid) (PTAA), to bovine insulin fibrils with widths of <10 nm and lengths of up to more than 10 m. Fibrils complexed with PTAA are aligned on surfaces through molecular combing and transfer printing. Single-molecule spectroscopy techniques are applied to chart spectral variation in the emission of these wires. When these results are combined with analysis of the polarization of the emitted light, we can conclude that the polymer chains are preferentially aligned along the fibrillar axis.
|
|
| 10. |
- Herland, Anna, et al.
(author)
-
Decoration of amyloid fibrils with luminescent conjugated polymers
- 2008
-
In: Journal of Materials Chemistry. - : Royal Society of Chemistry (RSC). - 0959-9428 .- 1364-5501. ; 18:1, s. 126-132
-
Journal article (peer-reviewed)abstract
- In this work we report the coating of a biological template with a polar, but uncharged, luminescent conjugated polymer, soluble in organic solvents but not in water, to produce a nanowire. Amyloid fibrils from bovine insulin were decorated with an alternating polyfluorene derivative. Decorated fibrils were partially aligned on hydrophobic surfaces as separate and bundled fibrils, by means of molecular combing. The single molecule spectroscopy technique utilizing excitation by rotating linearly polarized light and fluorescence detection through a rotating polarizer showed a high degree of anisotropy of the polymer chains on the individual fibrils. The high degree of polarization indicated highly oriented polymer chains with the preferential orientation of the polymer backbone along the fibrils. The anisotropy ratios are comparable with those of well-oriented polymer chains in films. © The Royal Society of Chemistry.
|
|
