Energy transfer processes in new conjugated polymer/6,6-phenyl-C61-butyric acid methyl ester systems for photovoltaic applications (original) (raw)
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The Journal of Physical Chemistry B, 2011
One of the major factors controlling the performance of organic bulk heterojunction solar cells is the yield of free charge pairs that results from the dissociation of a photoinduced exciton. In an agreed picture, generation of a singlet exciton by photon absorption is followed by diffusion and dissociation, or decay, of the exciton, and exciton dissociation results in geminate charge pairs, some of which escape their mutual attraction to contribute to the photocurrent. However, the mechanism of charge pair generation and the factors that control the charge generation efficiency are not yet understood. Disentangling the dynamics of the different processes is difficult because of the effect of disorder on exciton and charge dynamics. Exciton lifetime is influenced by the distance over which excitons may diffuse, i.e., by the size of polymer domains, which depends on processing and blend composition. Recombination of both geminate and nongeminate charges may take place over a range of time scales, resulting from the range of times for charge transfer in an energetically and structurally disordered medium. A model of charge and exciton dynamics should therefore be able to incorporate the effects of structural variations and disorder. Ultrafast transient absorption spectroscopy (TAS) provides a tool to study the dynamics and yield of charge pair generation under different conditions, provided that the transient absorption features can be correctly assigned. TAS can then be used together with a suitable model to investigate the influences of the blend film microstructure on charge generation.
Journal of Physics D: Applied Physics, 2009
We report on an experimental study of the charge transfer dynamics in a P3HT : PCBM blend by means of a femtosecond fluorescence up-conversion technique. Using two-photon excitation we probe the exciton dynamics in P3HT and a P3HT : PCBM blend with a weight ratio of 1 : 1 at excitation densities of up to 6 × 10 18 cm −3 . In both samples we find strongly nonexponential decay traces compatible with (i) diffusion-limited exciton-exciton annihilation and (ii) diffusion-limited donor-acceptor charge transfer in the polymer blend. Additionally, our results indicate that in the P3HT : PCBM blend about 50% of the photogenerated excitons undergo a prompt charge transfer process on a time scale of about 150 fs. Our study shows that fluorescence spectroscopy with femtosecond time resolution is a powerful technique for probing ultrafast charge transfer processes in solar cell materials.
Solution-processed bulk heterojunction (BHJ) based on electron-donor (D) polymer and acceptor (A) fullerene is a promising technology for organic photovoltaics. Geminate charge recombination is regarded as one of the main loss mechanisms limiting device performances. This stems from the dynamics of the initial charge transfer state (CTS), which depend on the blend morphology, the molecular conformation, and the energetics of the D:A interface. Here we study the photophysics of a crystalline phase-separated blend of regioregular poly(3-hexylthiophene) (P3HT) with [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) with a coarsened morphology, by mapping the transient absorption signal with submicrometer space and subpicosecond time resolution. At the P3HT:PCBM interface, we detect a long-lived photoinduced dynamic that we assign to a peculiar coherent CTS forming in ∼10 ps, not affected by geminate recombination and characterized by a different polarization with respect to the one in the usual polydispersed blend. Quantum chemical calculations on supramolecular P3HT:PCBM complexes confirm the presence of low-lying and highly polarized CTS, validating the experimental findings.
The Journal of Physical Chemistry B, 2012
The fluorescence decays of a stereoregular head-to-tail RR-HT poly(3-hexylthiophene), P3HT, in methylcyclohexane (MCH) are described by sums of three or four exponential terms, respectively above and below −10°C. In the high-temperature region, the polymer lifetime (ca. 500 ps) is accompanied by two shorter decay times (ca. 20 and 120 ps), which are assigned to intrachain energy transfer from high to lower energy excitons on the basis of temperature and wavelength dependence of the fluorescence decays. The absence of conformational (torsional) relaxation is attributed to the small dihedral angle between monomers that is predicted for the stereoregular polymer in the ground state. Below −10°C, the polymer forms excimer-like aggregates, showing vibrational structured absorption and emission bands similar to those observed in thin films. The vibrational structure is attributed to a deep minimum in the ground-state energy surface of the dimer or aggregate. Below −40°C, the fluorescence measured at the aggregate emission wavelength (670 nm) basically results from direct excitation of the aggregate and decays with a sum of three exponential terms (decay times of ca. 0.14, 0.6, and 1.5 ns, with similar weights). Because the spectral similarities between film and aggregates indicate similar electronic first singlet excited states (and oscillator strengths), the much shorter decay times (0.05, 0.15, and 0.43 ns) and lower fluorescence quantum yield of P3HT in films are assigned to efficient exciton dissociation and/or phonon-induced internal conversion competing with radiative decay (>1 ns).
Photogeneration and Ultrafast Dynamics of Excitons and Charges in P3HT/PCBM Blends
The Journal of Physical Chemistry C, 2009
The photogeneration quantum yield and dynamics of charge carriers and excitons in thin films of neat regioregular poly(3-hexylthiophene) (P3HT) and blends with [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) were studied with ultrafast optical pump-probe spectroscopy. In neat P3HT the quantum yield for direct photogeneration of charge carriers amounts to 0.15 per absorbed photon. The remaining fraction of absorbed photons leads to formation of excitons. Recombination of charges reduces the quantum yield to about 25% of its initial value on a time scale of 100 ps followed by decay to a no longer observable yield after 1 ns. Addition of 50% PCBM by weight leads to ultrafast (<200 fs) formation of charge pairs with a total quantum yield of 0.5. The presence of 50% PCBM causes exciton decay to be about an order of magnitude faster than in neat P3HT, which is expected to be at least in part due to interfacial exciton dissociation into charge carriers. The yield of charges in the blend has decayed to about half its initial value after 100 ps, while no further decay is observed within 1 ns. The small fraction (∼1%) of excitons in neat P3HT that is probed by photoluminescence measurements has a lifetime of 660 ps, which significantly exceeds the 200 ps lifetime of nonfluorescent excitons that are probed by transient absorption measurements. The nonfluorescent excitons have a diffusion coefficient of about 2 × 10-4 cm 2 /s, which is an order of magnitude smaller than reported values for fluorescent excitons. The interaction radius for second-order decay of photoexcitations is as large as 8-17 nm, in agreement with an earlier result in the literature.
RSC Advances, 2020
Polymer-fullerene blends based on poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C61-butyric-acid methyl ester (PCBM) have been extensively studied as promising bulk heterojunction materials for organic semiconductor devices with improved performance. In these donor–acceptor systems where the bulk morphology plays a crucial role, the generation and subsequent decay mechanisms of photoexcitation species are still not completely understood. In this work, we use femtosecond transient absorption spectroscopy to investigate P3HT:PCBM diodes under the influence of applied static electric fields in comparison to P3HT:PCBM thin films. At the same time, we try to present a detailed overview about work already done on these donor–acceptor systems. The excited state dynamics obtained at 638 nm from P3HT:PCBM thin films are found to be similar to those observed earlier in neat P3HT films, while those obtained in the P3HT:PCBM devices are affected by field-induced exciton dissociation, result...
SPECTRALLY RESOLVED ANNEALING EFFECT IN P3HT: PCBM ORGANIC SOLAR CELLS
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Organic solar cells fabricated with blends of regioregular poly(3-hexylthiophene), P3HT, and 1-(3methoxycarbonyl)-propyl-1-phenyl-(6,6)C 61 , PCBM, were encapsulated and subjected to annealing at elevated temperatures. Using recently introduced Fourier transform photocurrent spectroscopy [2, 3] we report spectral measurements of photocurrent generation in the cells. We measure current response in the visible region and, simultaneously, a weak absorption in the near infrared (NIR), both as a function of annealing time and temperature (up to 210 ºC). Our dynamical range is 7 orders of magnitude of absorptance (1 -10 -7 ). Looking at a weak absorption shoulder in the NIR range, we observe shift of this shoulder into IR with annealing (quantitatively, the observed shift is 0.15 eV, this is one half of a typical V oc drop). We also observe two isospectic points, at 1.37 eV and 1.93 eV, which are common to all measured spectra until the annealing at 175 ºC.