Femtosecond excitation tuning and site energy memory of population transfer in poly(<i>p</i>-phenylenevinylene): Gated luminescence experiments and simulation

Sperling, Jaroslaw; Milota, Franz; Tortschanoff, A.; Warmuth, Ch.; Mollay, B.; Bäßler, H.; Kauffmann, H. F.

doi:10.1063/1.1519841

Cited by 31 publications

(35 citation statements)

References 58 publications

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“…In principle, higher energy excitation should lead to the initial formation of higher energy excitons, which have more probability to hop to lower energy localized states, so that exciton hopping in P3HT could be affected by the excitation wavelength, as was reported for the PPV polymer. 90 Judging from the normalized emission spectra, the spectral changes and time constants associated with exciton hopping are, however, quite independent of 400 or 500 nm excitation in solution. The wavelengthdependent spectral rise that we observe 1À2 ps after photoexcitation might have a contribution from the population of more red-shifted states by hopping.…”

Section: 49à51mentioning

confidence: 99%

Ultrafast Relaxation of the Poly(3-hexylthiophene) Emission Spectrum

et al. 2011

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The femtosecond-resolved evolution of the emission spectrum of the important conjugated polymer poly(3-hexylthiophene) (P3HT) is presented. Detailed fluorescence up-conversion spectroscopy was performed on P3HT solid-state films and on P3HT in chlorobenzene solution. Two excitation wavelengths and several emission wavelengths, covering the entire fluorescence spectrum, were used. The data were complemented by polarization-sensitive measurements. Our global analysis allowed a reconstruction of the time-resolved emission spectra with 200 fs temporal resolution, so that spectral changes due to the early relaxation processes following π–π* interband absorption in the pristine polymer could be comprehensively characterized. Absorption occurs in isolated polymer chains in solution and in the solid state (including interchain interactions) for the film. In both cases, we find evidence of delocalization of the electrons and holes formed in the energy bands directly after photoexcitation with excess energy. This is followed by ultrafast (~100 fs) self-localization of the primary photoexcitation and by relatively slow exciton formation (~1 ps). Further relaxation occurs with time constants ranging from hundreds of femtoseconds to tens of picoseconds, due to exciton hopping to sites with lower energy and to a slow conformational planarization of the polymer backbone. Depolarization, a spectral red shift, and important changes in the vibronic structure are observed as a consequence of this relaxation. Finally, relaxed intrachain and interchain singlet excitons are formed in solution and film, respectively, on a 100–200 ps time scale. They decay with a ~500 ps time constant, by intersystem crossing in solution and by nonradiative recombination in the film. Our results are consistent with and strongly support the conclusions we obtained from a similar time-resolved fluorescence study of the polymer PCDTBT (J. Am. Chem. Soc.2010, 132, 17459): ultrafast charge separation in polymer:fullerene blends seems to occur before localization of the primary excitation to form a bound exciton

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Section: 49à51mentioning

confidence: 99%

Ultrafast Relaxation of the Poly(3-hexylthiophene) Emission Spectrum

et al. 2011

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“…[64,65] This is clearly much less than the typical length scale of about 10-20 nm in optimized bulk heterojunctions, estimated by structural characterization in optimized morphologies. [27][28][29][30][31][32] Furthermore, point-like exciton diffusion by the hopping mechanism has been measured on numerous occasions to take 0.5-1 ps for a single hopping step, [39][40][41][42][44][45][46][49][50][51]88,89] which is again too slow to explain how excitons reach a fullerene interface in <100 fs. In agreement with this, an ultrafast transient absorption study of poly[2,7-(9,9-dioctylfluorene)-alt-5,5-(4,7′-di-2-thienyl-2′,1′,3-benzothiadiazole)] (APFO3) films with various concentrations studies on pristine polymers in our group, we have pointed out that the ultrafast charge separation can compete with relaxation processes such as excited-state localization and exciton diffusion.…”

Section: Photoexcitation and Charge Transfermentioning

confidence: 99%

“…[37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] Charge transfer at the donor/acceptor interface splits the excitation into a domain-segregated electron and hole. Many time-resolved studies have shown that the charge transfer to the fullerene materials is nearly quantitative and ultrafast (<100 fs for many optimized systems).…”

Section: Introductionmentioning

confidence: 99%

Charge Formation, Recombination, and Sweep‐Out Dynamics in Organic Solar Cells

Cowan

Banerji

Leong

et al. 2012

Adv Funct Materials

313

252

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This article presents a critical discussion of the various physical processes occurring in organic bulk heterojunction (BHJ) solar cells based on recent experimental results. The investigations span from photoexcitation to charge separation, recombination, and sweep‐out to the electrodes. Exciton formation and relaxation in poly[N‐9″‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and poly‐3(hexylthiophene) (P3HT) are discussed based on a fluorescence up‐conversion study. The commonly accepted paradigm describing the conversion of incident photons into charge carriers in the BHJ material is re‐examined in light of these femtosecond time‐resolved measurements. Transient photoconductivity, time‐delayed collection field, and time‐delayed dual pulse experiments carried out on BHJ solar cells demonstrate the competition between carrier sweep‐out by the internal field and the loss of photogenerated carriers by recombination. Finally, an emerging hypothesis is discussed: that bimolecular recombination accounts for the majority of recombination from short circuit to open circuit in optimized solar cells, and that bimolecular recombination is bias‐ and charge‐density‐dependent. The study of recombination loss processes in organic solar cells leads to insights into what must be accomplished to achieve the “ideal” solar cell.

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“…At longer times, λ ex has no influence on the dynamics of exciton hopping, although such an effect might be expected since selective excitation of low energy sites prevents further downhill relaxation. 36 Most probably, our quite broadband femtosecond laser excitation always leads to some inhomogeneity in excited chromophores. Finally, the results show that neither the exciton decay nor the yield of long-lived states depends on λ ex .…”

Section: ■ Introductionmentioning

confidence: 99%

Charge Separation Pathways in a Highly Efficient Polymer: Fullerene Solar Cell Material

Paraecattil

Banerji

2014

J. Am. Chem. Soc.

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PBDTTPD is one of the best conjugated polymers for solar cell applications (up to 8.5% efficiency). We have investigated the dynamics of charge generation in the blend with fullerene (PCBM) and addressed highly relevant topics such as the role of bulk heterojunction structure, fullerene excitation, and excess energy. We show that there are multiple charge separation pathways. These include electron transfer from photoexcited polymer, hole transfer from photoexcited PCBM, prompt (<100 fs) charge generation in intimately mixed polymer:fullerene regions (which can occur from hot states), as well as slower electron and hole transfer from excitons formed in pure PBDTTPD or PCBM domains (diffusion to an interface is necessary). Very interestingly, all the charge separation pathways are highly efficient. For example, the yield of long-lived carriers is not significantly affected by the excitation wavelength, although this changes the fraction of photons absorbed by PCBM and the amount of excess energy brought to the system. Overall, the favorable properties of the PBDTTPD:PCBM blend in terms of morphology and exciton delocalization allow excellent charge generation in all circumstances and strongly contribute to the high photovoltaic performance of the blend. ■ INTRODUCTIONOrganic solar cells based on electron-donating conjugated polymers blended with electron-accepting fullerene derivatives have reached high efficiencies beyond 8%.1−3 In order to further enhance their performance, it is essential to deeply understand the underlying photophysical processes. Highly relevant topics that need to be addressed include the role of delocalization and exciton diffusion for efficient charge generation at the donor:acceptor interface, the role of hot neutral and charge transfer states in the formation of free carriers, and the role of the fullerene as light harvester.4 Both the nanoscale structure of the bulk heterojunction (BHJ) and the photophysical dynamics must be considered to gain answers to those questions. Femtosecond transient absorption (TA) spectroscopy is an undeniably useful tool to study the dynamics of neutral excited states and photoinduced charges in the materials of interest. 5−9Here, we will show that this purely spectroscopic technique additionally yields detailed morphological insights. Our study leads to a comprehensive picture of how different charge generation pathways relate to BHJ structure.We focus the investigation on PBDTTPD (poly(benzo[1,2-b:4,5-b′]dithiophene-alt-thieno[3,4-c]pyrrole-4,6-dione), and its blend with PCBM ([6,6]-phenyl C 60 butyric acid methyl ester). The polymer is among the best performing for photovoltaic applications, with up to 8.5% efficiency.1 Recent reports on morphology control, 1,10−12 device design, 13 charge trapping, 14 long-term stability, 15 quantum calculations, 16,17 and spectroscopy 7,18−20 confirm high interest for the material. PBDTTPD also distinguishes itself from many other conjugated polymers through its very planar structure in both the ground and ex...

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Femtosecond excitation tuning and site energy memory of population transfer in poly(p-phenylenevinylene): Gated luminescence experiments and simulation

Cited by 31 publications

References 58 publications

Ultrafast Relaxation of the Poly(3-hexylthiophene) Emission Spectrum

Ultrafast Relaxation of the Poly(3-hexylthiophene) Emission Spectrum

Charge Formation, Recombination, and Sweep‐Out Dynamics in Organic Solar Cells

Charge Separation Pathways in a Highly Efficient Polymer: Fullerene Solar Cell Material

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