| 1. |
- Abramavicius, Vytautas, et al.
(författare)
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Carrier motion in as-spun and annealed P3HT:PCBM blends revealed by ultrafast optical electric field probing and Monte Carlo simulations
- 2014
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 16:6, s. 2686-2692
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Tidskriftsartikel (refereegranskat)abstract
- Charge transport dynamics in solar cell devices based on as-spun and annealed P3HT:PCBM films are compared using ultrafast time-resolved optical probing of the electric field by means of field-induced second harmonic generation. The results show that charge carriers drift about twice as far during the first 3 ns after photogeneration in a device where the active layer has been thermally annealed. The carrier dynamics were modelled using Monte-Carlo simulations and good agreement between experimental and simulated drift dynamics was obtained using identical model parameters for both cells, but with different average PCBM and polymer domain sizes. The calculations suggest that small domain sizes in as-spun samples limit the carrier separation distance disabling their escape from geminate recombination.
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| 2. |
- Andersson, Mattias, et al.
(författare)
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Unified Study of Recombination in Polymer:Fullerene Solar Cells Using Transient Absorption and Charge-Extraction Measurements
- 2013
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Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society. - 1948-7185 .- 1948-7185. ; 4:12, s. 2069-2072
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Tidskriftsartikel (refereegranskat)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.
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| 3. |
- Biswas, Abhijit, et al.
(författare)
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Control of the size and shape of TiO2 nanoparticles in restricted media
- 2013
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 24:19
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Tidskriftsartikel (refereegranskat)abstract
- Template-capped TiO2 nanostructures have been synthesized. In certain template conditions, TiO2 hexagons are found to form. These hexagonal structures can be effectively sensitized by fluorescein dye without any change in the protonation state of the dye. Bare TiO2 nanoparticles are not so useful for sensitization with dyes like fluorescein as they alter the dye protonation state. The novelty of this work is twofold-the hitherto elusive hexagonal phase of TiO2 nanoparticles has been stabilized and the synthesis of TiO2 in the rutile phase has been achieved under mild conditions.
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| 4. |
- Imahori, Hiroshi, et al.
(författare)
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Photoinduced Charge Carrier Dynamics of Zn-Porphyrin-TiO(2) Electrodes: The Key Role of Charge Recombination for Solar Cell Performance (†).
- 2011
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Ingår i: Journal of physical chemistry. A. - : American Chemical Society (ACS). - 1520-5215 .- 1089-5639. ; 115:16, s. 3679-3690
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Tidskriftsartikel (refereegranskat)abstract
- Time resolved absorption spectroscopy has been used to study photoinduced electron injection and charge recombination in Zn-porphyrin sensitized nanostructured TiO(2) electrodes. The electron transfer dynamics is correlated to the performance of dye sensitized solar cells based on the same electrodes. We find that the dye/semiconductor binding can be described with a heterogeneous geometry where the Zn-porphyrin molecules are attached to the TiO(2) surface with a distribution of tilt angles. The binding angle determines the porphyrin-semiconductor electron transfer distance and charge transfer occurs through space, rather than through the bridge connecting the porphyrin to the surface. For short sensitization times (1 h), there is a direct correlation between solar cell efficiency and amplitude of the kinetic component due to long-lived conduction band electrons, once variations in light harvesting (surface coverage) have been taken into account. Long sensitization time (12 h) results in decreased solar cell efficiency because of decreased efficiency of electron injection.
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| 5. |
- Infahsaeng, Yingyot
(författare)
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Light Induced Charge Generation, Recombination, and Transport in Organic Solar Cells
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Light-to-electricity conversion processes in organic and dye sensitized solar cells have attracted much scientific effort, aiming to understand the fundamental nature of these processes and to improve solar cell efficiency. Power conversion efficiency of such solar cells has progressed steadily in the last few decades, suggesting applicability for commercial products in the nearest future. Yet, the overall light-to-charge carrier conversion processes as well as the factors controlling these processes are not fully understood. Dye sensitized semiconductor solar cells (DSSC) and bulk heterojunction (BHJ) organic solar cells are addressed in this Thesis. The main topics of the Thesis are light-induced charge generation, recombination, and transport in the active materials as well as solar cell devices. Electron injection and recombination in Zn-porphyrin sensitized TiO2 electrode were investigated by means of transient absorption (TA) spectroscopy and surface sensitive sum frequency generation. It was found that the sensitizer molecule is bound to the semiconductor (TiO2) surface at an angle, which is controlled by experimental factors such as natureof the sensitization solvent and time. This tilt angle controls the electron transfers (injection and recombination) between sensitizer and TiO2 and it was shown that transfer occurs through space rather than through the molecular spacer connecting the porphyrin core to the surface. A strong correlation between cell efficiency and electron injection and recombination dynamics was demonstrated. In particular, the role of the electron-Zn-porphyrin cation recombination was shown to be decisive for high solar cell efficiency – a very slow (> 50 ns) recombination resulted in the highest cell efficiency. In functioning BHJ solar cells, charge pair photo-generation and separation as well as recombination, transport and extraction of photo-generated charge carriers were studied by several techniques such as TA spectroscopy, electro-modulated differential absorption (EDA), time resolved electric field induced second harmonic generation (TREFISH), time-of-flight method (TOF), photo-carrier extraction by linearly increasing voltage (CELIV), transient photocurrent and photovoltage. Solar cells made of fullerene blends with several different polymers have been studied. Photo-induced dynamics has been observed over more than ten decades of time and three decades of photo-generated charge concentrations by a combination of various optical and electrical time-resolved methods. Light absorption has been shown to lead to formation of closely spaced bound rather than long-distance separated free charges. Mechanism of the charge pair separation dominated by fast three-dimensional diffusion at earlier time scale and by electric field-induced drift on later times has been purposed. Electric field dependence of charge pair separation and recombination in non-optimized and optimized solar cells was studied. The time dependent carrier mobility over picosecond to microsecond time scale was shown and considered in relation to charge pair separation and extraction in solar cells made of polymer:fullerene blends with different stoichiometry. Dominance of geminateand non-g eminate recombination has been shown for solar cell blends made of different polymers. Detail description of recombination mechanism over broad range of times and charge concentrations have been achieved by means of the combination of different optical and electrical techniques. A critical role of electrodes in functioning solar cells on the charge dynamics has been discussed.
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| 6. |
- Infahsaeng, Yingyot, et al.
(författare)
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Transient photocurrent of bulk heterojunction solar cell characterized by ns-laser and sub-ms LED
- 2015
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Ingår i: Proceedings of SPIE. - : SPIE. - 0277-786X .- 1996-756X. ; 9659
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Konferensbidrag (refereegranskat)abstract
- We measure the transient photocurrent of APFO3:PCBM bulk heterojunction solar cells illuminated with ns-laser and sub-ms LED light sources. The ratio of the number of collective charges to the number of excited photon (external quantum efficiency, EQE) and the transient photocurrent fall times have been carried out with difference pulse durations and fluences. The EQEs characterized by ns-laser source are shown to obey the bimolecular recombination at high excitation fluences. The increasing of transient photocurrent fall times suggests that the fall times of free charge carriers are effected by deep trap density of state (DoS) and thus the free charge carriers have a sufficient time for bimolecular recombination at short circuit condition. At the same fluences, however, the EQEs characterized by sub-ms LED sources exhibit an excitation fluences independence of EQE. The transient photocurrent fall times with sub-ms LED sources are rather constant when the excitation fluences increases indicating that the deep trap DoS has less effect at short circuit condition for longer pulse duration.
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| 7. |
- Pranculis, Vytenis, et al.
(författare)
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Charge Carrier Generation and Transport in Different Stoichiometry APFO3:PC61BM Solar Cells
- 2014
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society. - 0002-7863 .- 1520-5126. ; 136:32, s. 11331-11338
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we studied carrier drift dynamics in APFO3:PC61BM solar cells of varied stoichiometry (2:1, 1:1, and 1:4 APFO3:PC61BM) over a wide time range, from subpicoseconds to microseconds with a combination of ultrafast optical electric field probing and conventional transient integrated photocurrent techniques. Carrier drift and extraction dynamics are strongly stoichiometry dependent: the speed of electron or hole drift increases with higher concentration of PC61BM or polymer, respectively. The electron extraction from a sample with 80% PC61BM takes place during hundreds of picoseconds, but slows down to sub-microseconds in a sample with 33% PC61BM. The hole extraction is less stoichiometry dependent: it varies form sub-nanoseconds to tens of nanoseconds when the PC61BM concentration changes from 33% to 80%. The electron extraction rate correlates with the conversion efficiency of solar cells, leading to the conclusion that fast electron motion is essential for efficient charge carrier separation preventing their geminate recombination.
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| 8. |
- Vithanage, Dimali, et al.
(författare)
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Visualizing charge separation in bulk heterojunction organic solar cells.
- 2013
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 4
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Tidskriftsartikel (refereegranskat)abstract
- Solar cells based on conjugated polymer and fullerene blends have been developed as a low-cost alternative to silicon. For efficient solar cells, electron-hole pairs must separate into free mobile charges that can be extracted in high yield. We still lack good understanding of how, why and when carriers separate against the Coulomb attraction. Here we visualize the charge separation process in bulk heterojunction solar cells by directly measuring charge carrier drift in a polymer:fullerene blend with ultrafast time resolution. We show that initially only closely separated (<1 nm) charge pairs are created and they separate by several nanometres during the first several picoseconds. Charge pairs overcome Coulomb attraction and form free carriers on a subnanosecond time scale. Numerical simulations complementing the experimental data show that fast three-dimensional charge diffusion within an energetically disordered medium, increasing the entropy of the system, is sufficient to drive the charge separation process.
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| 9. |
- Ye, Shen, et al.
(författare)
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Role of Adsorption Structures of Zn-Porphyrin on TiO2 in Dye-Sensitized Solar Cells Studied by Sum Frequency Generation Vibrational Spectroscopy and Ultrafast Spectroscopy
- 2013
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:12, s. 6066-6080
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Tidskriftsartikel (refereegranskat)abstract
- Several Zn-porphyrin (ZnP) derivatives were designed to build highly efficient dye-sensitized solar cells (DSC). It was found that solar cell efficiencies normalized for surface coverage (eta(rel)) are affected by the molecular spacer connecting the porphyrin core to the TiO2 surface, the sensitization conditions (solvent and time), and, to a lesser extent, the nature of the terminal group of the ZnP. Ultrafast transient absorption spectroscopy shows that electron transfer rates are strongly dependent on spacer and sensitization conditions. To understand this behavior at a molecular level, surface-sensitive vibrational spectroscopy, sum frequency generation (SFG), has been employed to investigate the adsorption geometries of these ZnP derivatives on the TiO2 surface for the first time. The average tilt angles and adsorption ordering of the ZnP molecules on the TiO2 surface were measured. A simple linear correlation between adsorption geometry of the adsorbed ZnP molecules, eta(rel), and the concentration of long-lived electrons in the conduction band of TiO2 was shown to exist. The more perpendicular the orientation of the adsorbed ZnP (relative to the TiO2 surface), the higher the concentration of long-lived electrons in the conduction band, which contributes to the increase of photocurrent and solar cell efficiency. This result indicates that the electron transfer between ZnP and TiO2 occurs "through-space" rather than "through the molecular spacer". It is also revealed that the sensitization solvent (methanol) may affect adsorption geometry and adsorption ordering through coadsorption and modify the electron transfer dynamics and consequently solar cell efficiency. Aggregation effects, which were observed for the longer sensitization times, are also discussed in relation to adsorption geometry and radiationless quenching processes. With the work reported here we demonstrate a novel strategy for DSC material characterization that can lead to design and manufacturing of photoactive materials with predictable and controlled properties.
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