Aggregate formation in poly(3-hexylthiophene) depends on molecular weight, solvent, and synthetic method. The interplay of these parameters thus largely controls device performance. In order to obtain a quantitative understanding on how these factors control the resulting electronic properties of P3HT, we measured absorption in solution and in thin films as well as the resulting field effect mobility in transistors. By a detailed analysis of the absorption spectra, we deduce the fraction of aggregates formed, the excitonic coupling within the aggregates, and the conjugation length within the aggregates, all as a function of solvent quality for molecular weights from 5 to 19 kDa. From this, we infer in which structure the aggregated chains pack. Although the 5 kDa samples form straight chains, the 11 and 19 kDa chains are kinked or folded, with conjugation lengths that increase as the solvent quality reduces. There is a maximum fraction of aggregated chains (about 55 6 5%) that can be obtained, even for poor solvent quality. We show that inducing aggregation in solution leads to control of aggregate properties in thin films. As expected, the field-effect mobility correlates with the propensity to aggregation. Correspondingly, we find that a well-defined synthetic approach, tailored to give a narrow molecular weight distribution, is needed to obtain high field effect mobilities of up to 0.01 cm 2 /Vs for low molecular weight samples (=11 kDa), while the influence of synthetic method is negligible for samples of higher molecular weight, if low molecular weight fractions are removed by extraction. V C 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: [442][443][444][445][446][447][448][449][450][451][452][453] 2012

Despite the central role of light absorption and the subsequent generation of free charge carriers in organic and hybrid organicÀinorganic photovoltaics, the precise process of this initial photoconversion is still debated. We employ a novel broadband (UVÀVisÀNIR) transient absorption spectroscopy setup to probe charge generation and recombination in the thin films of the recently suggested hybrid material combination poly(3-hexylthiophene)/silicon (P3HT/Si) with 40 fs time resolution. Our approach allows for monitoring the time evolution of the relevant transient species under various excitation intensities and excitation wavelengths. Both in regioregular (RR) and regiorandom (RRa) P3HT, we observe an instant (<40 fs) creation of singlet excitons, which subsequently dissociate to form polarons in 140 fs. The quantum yield of polaron formation through dissociation of delocalized excitons is significantly enhanced by adding Si as an electron acceptor, revealing ultrafast electron transfer from P3HT to Si. P3HT/Si films with aggregated RR-P3HT are found to provide free charge carriers in planar as well as in bulk heterojunctions, and losses are due to nongeminate recombination. In contrast for RRa-P3HT/Si, geminate recombination of bound carriers is observed as the dominant loss mechanism. Site-selective excitation by variation of pump wavelength uncovers an energy transfer from P3HT coils to aggregates with a 1/e transfer time of 3 ps and reveals a factor of 2 more efficient polaron formation using aggregated RR-P3HT compared to disordered RRa-P3HT. Therefore, we find that polymer structural order rather than excess energy is the key criterion for free charge generation in hybrid P3HT/Si solar cells.

The ratio of the 0-0 to 0-1 peak intensities in the photoluminescence (PL) spectrum of red-phase poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], better known as MEH-PPV, is significantly enhanced relative to the disordered blue-phase and is practically temperature independent in the range from T = 5 K to 180 K. The PL lifetime is similarly temperature independent. The measured trends are accounted for by modeling red-phase MEH-PPV as disordered π-stacks of elongated chains. Using the HJ-aggregate Hamiltonian expanded to include site disorder amongst electrons and holes, the absorption and PL spectra of cofacial MEH-PPV dimers are calculated. The PL 0-0/0-1 line strength ratio directly responds to the competition between intrachain interactions which promote J-aggregate-like behavior (enhanced PL ratio) and interchain interactions which promote H-aggregate-like behavior (attenuated PL ratio). In MEH-PPV aggregates, J-like behavior is favored by a relatively large intrachain exciton bandwidth--roughly an order of magnitude greater than the interchain bandwidth--and the presence of disorder. The latter is essential for allowing 0-0 emission at low temperatures, which is otherwise symmetry forbidden. For Gaussian disorder distributions consistent with the measured (inhomogeneous) line widths of the vibronic peaks in the absorption spectrum, calculations show that the 0-0 peak maintains its dominance over the 0-1 peak, with the PL ratio and radiative lifetime practically independent of temperature, in excellent agreement with experiment. Interestingly, interchain interactions lead only to about a 30% drop in the PL ratio, suggesting that the MEH-PPV π-stacks--and strongly disordered HJ-aggregates in general--can masquerade as single (elongated) chains. Our results may have important applications to other emissive conjugated polymers such as the β-phase of polyfluorenes.

The low‐bandgap polymer poly{[4,4‐bis(2‐ethylhexyl)‐cyclopenta‐(2,1‐b;3,4‐b′)dithiophen]‐2,6‐diyl‐alt‐(2,1,3‐benzo‐thiadiazole)−4,7‐diyl} (PCPDTBT) is widely used for organic solar cell applications. Here, we present a comprehensive study of the optical properties as a function of temperature for PCPDTBT in solution and in thin films with two distinct morphologies. Using absorption and photoluminescence spectroscopy as well as Franck‐Condon analyses, we show that PCPDTBT in solution undergoes a phase transformation at 300 K from a disordered to a truly aggregated state on cooling. The saturation value of aggregates in solution is reached in PCPDTBT thin films at any temperature. In addition, we demonstrate that the photophysical properties of the aggregates in films are similar to those in solution and that a low percentage of thermally activated excimer states is present in the films at temperatures above 200 K. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1416–1430