Photoinduced Carrier Generation and Recombination Dynamics of a Trilayer Cascade Heterojunction Composed of Poly(3-hexylthiophene), Titanyl Phthalocyanine, and C 60 (original) (raw)
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Physical Review B, 2004
The electrodeless flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to study the photogeneration of charge carriers in spin-coated films of regioregular poly(3hexylthiophene) ͑P3HT͒, over the photon energy range from 1.9 to 5.2 eV for incident light intensities from 10 13 to 10 16 photons/ cm 2 per ͑3 ns͒ pulse. The initial, single-photon quantum yield of photoionization, , has been estimated from the low-intensity limit to the photoconductivity based on a charge carrier mobility of 0.014 cm 2 /Vs (determined in separate pulse-radiolysis TRMC experiments on bulk P3HT). The value of is constant at ͑1.7± 0.4͒% within the range 1.9-3.0 eV, which encompasses the first electronic absorption band of P3HT. Above 3.0 eV, increases, up to a value of ͑7±2͒% at 5.2 eV. The activation energy of the photoconductivity was found to be approximately 50 meV at all photon energies. The high-intensity, sublinear dependence of the photoconductivity can be described by the occurrence of either exciton-exciton annihilation or diffusional charge recombination with rate coefficients of 2.3ϫ 10 −8 cm 3 / s and 1.1ϫ 10 −8 cm 3 /s.
Journal of Physical Chemistry C, 2012
In this work, we use the time-resolved microwave conductivity (TRMC) technique to study the dynamics of charge carrier generation in sequentially deposited conjugated polymer/fullerene layers. These layers are either fully solution-processed, using orthogonal solvents for the layers of the polymer poly(3-hexylthiophene) (P3HT) and the fullerene phenyl-C 61 -butyric acid methyl ester (PCBM), or prepared by thermally evaporating a C 60 layer onto P3HT films. Our work is motivated by the remarkable efficiency of organic photovoltaic (OPV) devices using a sequentially processed P3HT/PCBM active layer. Here we use an electrodeless photoconductivity probe, so we can photoexcite the sample either through the polymer or the fullerene layer. We use samples with extremely thick P3HT films (2.4 μm) and show that excitation from either side of both as-cast and thermally annealed sample yields virtually identical results, consistent with mixing of the PCBM into the polymer film. We also compare solution-deposited samples to samples made by thermally evaporating C 60 on P3HT, and find that we can distinguish between charge generation in bulk-P3HT and at the polymer/ fullerene interface. We show that, despite their morphological differences, the carrier dynamics in the sequentially processed samples resemble those of mixed, bulk heterojunction (BHJ) systems. All of this is consistent with the idea that PCBM readily mixes into the P3HT film in sequentially deposited P3HT/PCBM samples, although the total amount of fullerene mixed into the P3HT appears to be less than that typically used in an optimized BHJ. Finally, we discuss the implications for OPV device architectures prepared by sequential deposition from solution.
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 C, 2016
Charge generation and recombination dynamics in organic photovoltaic bulk heterojunction films comprising the donor polymer, PDPP-TNT (Poly {3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene}), blended with the fullerene acceptor, PC 71 BM ([6,6]-phenyl C 71-butyric acid methyl ester), have been studied. The charge carrier generation process was studied using femtosecond transient absorption and it was found that the efficiency of charge generation is not dominated by geminate recombination but rather is limited by exciton diffusion in the films. Highly sensitive nanosecond transient absorption (ns-TA) and time-resolved microwave conductivity (TRMC) were used to study charge recombination. From ns-TA measurements, we obtained a recombination rate constant of 1 x 10-9 cm 3 s-1 and found that charge recombination is limited by the diffusion of charge carriers (Langevin-type recombination). TRMC signals were 1 comparable with ns-TA on shorter time scales. However, in contrast to ns-TA, the TRMC signal contained an additional long-lived component. The fast decay at shorter time scales is attributed to the recombination of the majority of the charge carriers. The long-lived component is assigned to a small population of charge carriers with high mobility, suggesting they are located in isolated, crystalline domains within the bulk heterojunction. These findings are discussed in relation to the morphology of the blend film, fluorescence quenching properties, and device performance including photoinduced charge extraction by linearly increasing voltage (PhotoCELIV) measurements described in our previous publications.
Ultrafast Transient Spectroscopy of Polymer/Fullerene Blends for Organic Photovoltaic Applications
Materials, 2013
We measured the picoseconds (ps) transient dynamics of photoexcitations in blends of regio-regular poly(3-hexyl-thiophene) (RR-P3HT) (donors-D) and fullerene (PCBM) (acceptor-A) in an unprecedented broad spectral range of 0.25 to 2.5 eV. In D-A blends with maximum domain separation, such as RR-P3HT/PCBM, with (1.2:1) weight ratio having solar cell power conversion efficiency of ~4%, we found that although the intrachain excitons in the polymer domains decay within ~10 ps, no charge polarons are generated at their expense up to ~1 ns. Instead, there is a build-up of charge-transfer (CT) excitons at the D-A interfaces having the same kinetics as the exciton decay. The CT excitons dissociate into separate polarons in the D and A domains at a later time (>1 ns). This "two-step" charge photogeneration process may be typical in organic bulk heterojunction cells. We also report the effect of adding spin 1/2 radicals, Galvinoxyl on the ultrafast photoexcitation dynamics in annealed films of RR-P3HT/PCBM blend. The addition of Galvinoxyl radicals to the blend reduces the geminate recombination rate of photogenerated CT excitons. In addition, the photoexcitation dynamics in a new D-A blend of RR-P3HT/Indene C60 trisadduct (ICTA) has been studied and compared with the dynamics in RR-P3HT/PCBM.
Journal of the American Chemical Society, 2008
We report herein a comparison of the photophysics of a series of polythiophenes with ionization potentials ranging from 4.8 to 5.6 eV as pristine films and when blended with 5 wt% 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]C 61 (PCBM). Three polymers are observed to give amorphous films, attributed to a non-planar geometry of their backbone whilst the other five polymers, including poly(3-hexylthiophene), give more crystalline films. Optical excitation of the pristine films of the amorphous polymers is observed by transient absorption spectroscopy to give rise to polymer triplet formation. For the more crystalline pristine polymers, no triplet formation is observed, but rather a short-lived (~ 100 ns), broad photoinduced absorption feature assigned to polymer polarons. For all polymers, the addition of 5 wt% PCBM resulted in 70-90% quenching of polymer photoluminescence (PL), indicative of efficient quenching of polythiophene excitons. Remarkably, despite this efficient exciton quenching, the yield of dissociated polymer + and PCBM − polarons, assayed by the appearance of a long-lived, powerlaw decay phase assigned to bimolecular recombination of these polarons, was observed to vary by over two orders of magnitude depending upon the polymer employed. In addition to this power-law decay phase, the blend films exhibited short-lived decays assigned, for the amorphous polymers, to neutral triplet states generated by geminate recombination of bound radical pairs and, for the more crystalline polymers, to the direct observation of the geminate recombination of these bound radical pairs to ground. These observations are discussed in terms of a two-step kinetic model for charge generation in polythiophene/PCBM blend films analogous to that reported to explain the observation of exciplex-like emission in poly(p-phenylenevinylene)-based blend films. Remarkably, we find a excellent correlation between the free energy difference for charge separation (ΔG CS rel) and yield of the long-lived charge generation yield, with efficient charge generation requiring a much larger ΔG CS rel than that required to achieve efficient PL 3 quenching. We suggest this observation is consistent with a model where the excess thermal energy of the initially formed polarons pairs is necessary to overcome their coulomb binding energy. This observation has important implications for synthetic strategies to optimize organic solar cell performance, as it implies that, at least devices based on polythiophene/PCBM blend films, a large ΔG CS rel (or LUMO level offset) is required to achieve efficient charge dissociation.
Journal of Physical Chemistry B, 2006
We report on the ultrafast photoinduced charge separation processes in varying compositions of poly(3hexylthiophene) (P3HT) blended with the electron acceptor [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). Through the use of time-resolved terahertz spectroscopy, the time-and frequency-dependent complex photoconductivity is measured for samples with PCBM weight fractions (W PCBM ) of 0, 0.2, 0.5, and 0.8. By analysis of the frequency-dependent complex conductivity, both the charge carrier yield and the average charge carrier mobility have been determined analytically and indicate a short (<0.2 nm) carrier mean free path and a suppressed long-range transport that is characteristic of carrier localization. Studies on pure films of P3HT demonstrate that charge carrier generation is an intrinsic feature of the polymer that occurs on the time scale of the excitation light, and this is attributed to the dissociation of bound polaron pairs that reside on adjacent polymer chains due to interchain charge transfer. Both interchain and interfacial charge transfer contribute to the measured photoconductivity from the blended samples; interfacial charge transfer increases as a function of increasing PCBM. The addition of PCBM to the polymer films surprisingly does not dramatically increase the production of charge carriers within the first 2 ps. However, charge carriers in the 0.2 and 0.5 blended films survive to much longer times than those in the P3HT and 0.8 films.
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.
Physical Review B, 2008
Using optical-pump terahertz-probe spectroscopy, we have investigated the time-resolved conductivity dynamics of photoexcited polymer-fullerene bulk heterojunction blends for two model polymers: poly͓3-hexylthiophene͔ ͑P3HT͒ and poly͓2-methoxy-5-͑3,7-dimethyloctyloxy͒-1,4-phenylenevinylene͔ ͑MDMO-PPV͒ blended with ͓6,6͔-phenyl-C 61 butyric acid methyl ester ͑PCBM͒. The observed terahertzfrequency conductivity is characteristic of dispersive charge transport for photoexcitation both at the − ء absorption peak ͑560 nm for P3HT͒ and significantly below it ͑800 nm͒. The photoconductivity at 800 nm is unexpectedly high, which we attribute to the presence of a charge-transfer complex. We report the excitationfluence dependence of the photoconductivity over more than four orders of magnitude, obtained by utilizing a terahertz spectrometer based upon on either a laser oscillator or an amplifier source. The time-averaged photoconductivity of the P3HT:PCBM blend is over 20 times larger than that of P3HT, indicating that longlived hole polarons are responsible for the high photovoltaic efficiency of polymer:fullerene blends. At early times ͑ϳps͒ the linear dependence of photoconductivity upon fluence indicates that interfacial charge transfer dominates as an exciton decay pathway, generating charges with mobility of at least ϳ0.1 cm 2 V −1 s −1 . At later times, a sublinear relationship shows that carrier-carrier recombination effects influence the conductivity on a longer time scale ͑Ͼ1 s͒ with a bimolecular charge annihilation constant for the blends that is approximately two to three orders of magnitude smaller than that typical for neat polymer films.