Photoinduced Charge Carrier Generation in a Poly(3-hexylthiophene) and Methanofullerene Bulk Heterojunction Investigated by Time-Resolved Terahertz Spectroscopy (original) (raw)
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The Journal of Physical Chemistry Letters, 2010
Herein we address the factors controlling photocurrent generation in P3HT:PCBM blend films as a function of blend composition and annealing treatment. Absorption, photoluminescence, and transient absorption spectroscopy are used to distinguish the role of exciton dissociation, charge pair separation, and charge collection. Variations in blend film microstructure with composition and annealing treatment are studied using X-ray diffraction. While the trend in photocurrent generation with composition and annealing [Muller, et al., Adv. Mater. 2008, 20, 3510] does not follow the trend in exciton dissociation, it closely follows the trend in charge pair generation. Moreover, charge pair generation efficiency is positively correlated to the degree of polymer crystallization and the appearance of large domains of both polymer and fullerene phases. We argue that larger domains assist charge pair separation by increasing the probability of escape from the P3HT:PCBM interface, thus reducing geminate charge recombination.
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.
The Disperse Charge-Carrier Kinetics in Regioregular Poly(3-hexylthiophene)
The Journal of Physical Chemistry B, 2004
The pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) is an electrodeless technique to measure the transient conductivity in bulk samples induced by a nanosecond high-energy electron pulse. By using the PR-TRMC technique, two commercial samples of regioregular poly(3-hexylthiophene) (P3HT), obtained from Merck and Sigma-Aldrich, were measured as a function of temperature (between 170 and 380 K) and radiation dose. The real part of the high-frequency GHz charge-carrier mobility sum was found to be 0.014 cm 2 /V s at room temperature with an activation energy of 28 meV. The conductivity in the Aldrich sample decayed rapidly, with a half-life of 4 ns, while the conductivity of the Merck sample had a half-life of 0.2 µs at room temperature. From measurements of the background conductivity under atmospheric conditions and using the charge-carrier mobility of 0.014 cm 2 /V s, a hole doping concentration of 5 × 10 17 cm -3 (with an activation energy of 61 meV) was found for the Aldrich sample, while it was only 2 × 10 16 cm -3 (with an activation energy of 98 meV) for the Merck sample. For radiation pulses generating a higher initial electron-hole pair concentration than the doping level, second-order electron-hole recombination was observed in the Merck sample, while in the Aldrich sample, the decay was first-order at all applied doses. This is attributed to the high doping concentration in the latter sample, which exceeded the highest possible pulse-generated electron-hole pair concentration. All transients were of the stretched exponential type (Kohlrausch law). The stretch parameter increased linearly with temperature in both samples, according to ) T/T 0 with T 0 ) 930 and 670 K for Merck and Aldrich P3HT, respectively. The linear increase of with temperature is in accordance with a model of dispersive hole transport with an exponential distribution of the activation energy of the hopping rates. A generalized version of the Kohlrausch law is derived to include both first-and second-order recombination processes at high radiation doses.
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.
Efficient, Stable Bulk Charge Transport in Crystalline/Crystalline SemiconductorâInsulator Blends
Advanced Materials, 2009
Multicomponent systems comprising both semiconducting and insulating constituents promise to broaden the technological potential of organic materials due to the fact that, with such architectures, a matrix of highly desirable characteristics may be realized, which is often difficult to obtain with one single species. Reassuringly, field-effect transistors (FETs) comprising active layers of such insulator/semiconductor blends have been demonstrated to approach the device performance of neat poly(3-hexylthiophene) (P3HT), which most often has been attributed to segregation of the semiconducting species to the gate dielectric interface. Unexpectedly, we find efficient ambipolar charge transport also in the bulk of crystalline/ crystalline P3HT/high-density polyethylene (HDPE) systems -a strong indication that an interpenetrating charge-transport network is generated not only at the thin-film architecture interfaces. This is evidenced by the fact that in time-of-flight (TOF) photoconductivity measurements on such P3HT/HDPE architectures, we observe high hole and electron bulk mobilities of up to 5 Â 10 À3 cm 2 V À1 s À1 even at higher HDPE content. Clearly, the use of crystalline/crystalline blends and corresponding copolymers is, thus, not only restricted to interface devices, such as field-effet transistors, but can also be employed in structures in which good bulk transport is required, broadening the range of applications such materials systems can be utilized for. This is strengthened by the fact that the environmental stability is significantly enhanced when the insulator HDPE is added to the semiconducting P3HT, despite the lack of a vertical-phase-separation mechanisms, which is known to result in ''self-encapsulation'' benefits. In fact, both hole and electron charge-carrier mobilities in P3HT:PE blends of different compositions were not significantly affected when stored in air over a period of four months, in strong contrast to neat P3HT architectures, in which charge transport deteriorated within hours.
Temperature-Resolved Local and Macroscopic Charge Carrier Transport in Thin P3HT Layers
Advanced Functional Materials, 2010
Previous investigations of the field-effect mobility in poly(3-hexylthiophene) (P3HT) layers revealed a strong dependence on molecular weight (MW), which was shown to be closely related to layer morphology. Here, charge carrier mobilities of two P3HT MW fractions (medium-MW: Mn = 7 200 g mol−1; high-MW: Mn = 27 000 g mol−1) are probed as a function of temperature at a local and a macroscopic length scale, using pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) and organic field-effect transistor measurements, respectively. In contrast to the macroscopic transport properties, the local intra-grain mobility depends only weakly on MW (being in the order of 10−2 cm2 V−1 s−1) and being thermally activated below the melting temperature for both fractions. The striking differences of charge transport at both length scales are related to the heterogeneity of the layer morphology. The quantitative analysis of temperature-dependent UV/Vis absorption spectra according to a model of F. C. Spano reveals that a substantial amount of disordered material is present in these P3HT layers. Moreover, the analysis predicts that aggregates in medium-MW P3HT undergo a “pre-melting” significantly below the actual melting temperature. The results suggest that macroscopic charge transport in samples of short-chain P3HT is strongly inhibited by the presence of disordered domains, while in high-MW P3HT the low-mobility disordered zones are bridged via inter-crystalline molecular connections.
The Journal of Physical Chemistry C, 2013
Surface photovoltage spectroscopy (SPS) was used to probe photon induced charge separation in thin films of regioregular and regiorandom poly(3-hexylthiophene) (P3HT) as a function of excitation energy. Both positive and negative photovoltage signals were observed under sub-band-gap (<2.0 eV) and super-band-gap (>2.0 eV) excitation of the polymer. The dependence of the spectra on substrate work function, thermal annealing, film thickness, and illumination intensity was investigated, allowing the identification of interface, charge transfer (CT), and band-gap states in the amorphous and crystalline regions of the polymer films. The ability to probe these states in polymer films will aid the development and optimization of organic electronic devices such as photovoltaics (OPVs), light-emitting diodes (OLEDs), and field effect transistors (OFETs). The direction and size of the observed photovoltage features can be explained using the depleted semiconductor model.