The effect of temperature on the quantum yield for charge carrier photogeneration in P3HT−PCBM blend films was studied using ultrafast transient absorption and microwave photoconductance techniques. The quantum yield was found to be virtually independent of temperature for time scales up to tens of nanoseconds after photoexcitation of P3HT. Implications of this observation for the mechanism of free charge carrier generation are discussed. The decay of charges due to recombination and/or trapping on longer times becomes faster at higher temperature, as a result of thermally activated electron and hole mobilities. The magnitude of the quantum yield depends on the morphology of the blend film, which is determined by the spin-coating solvent and annealing conditions.

The morphology, optical properties, and photoconductance of blends of the poly(thienothiophene) derivatives poly (3,6-dialkylthieno[3,2-b]thiophene-co-bithiophene) (pATBT), poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT), and poly(2,5-bis(3-dodecylthiophen-2-yl)thieno[2,3-b]thiophene) (pBTCT) with [6,6]phenyl-C61-butyric acid methyl ester (PCBM) were studied. After thermal annealing, the pATBT:PCBM blend exhibits formation of phase-segregated polymer and PCBM domains. Annealing of pBTTT:PCBM and pBTCT:PCBM yields a layered structure with PCBM molecules intercalated between layers of π-stacked polymers. In the intercalated systems the photoluminescence is almost completely quenched, in contrast to the phase-segregated pATBT:PCBM blend. The higher degree of exciton quenching in the intercalated systems likely results in a higher initial yield of charges. However, on longer time scales (>10 ns), the microwave photoconductance for the layered systems is lower than for pATBT:PCBM blend systems. This is likely due to restricted motion of charges in intercalated systems, which reduces the yield of free charge carriers or enhances the charge carrier recombination.

The morphology and the optoelectronic properties of films of poly(dialkylthieno[3,2-b]thiophene-co-bithiophene) (pDA2T) and blends of this polymer with [6, 6]-phenyl C-61-butyric acid methyl ester (PCBM) is investigated. Upon spin-coating of the blend a thin film is formed containing both PCBM and the polymer in nanocrystalline form. On annealing, phase separation occurs, leading to formation of the PCBM-rich domains embedded in a polymer-rich matrix. The electrodeless time-resolved microwave conductivity technique is used to study the photogeneration of charge carriers and their decay over time. The photoconductance increases dramatically on adding PCBM to the polymer. Annealing of the blend reduces the photogeneration yield of charge carriers, due to the smaller interfacial area between pDA2T and PCBM. The phase separation of the polymer and PCBM after annealing retards recombination of charge carriers, which is beneficial for charge collection in a solar cell. The magnitude of the photoconductance of the pDA2T:PCBM blend is comparable to that for a P3HT:PCBM blend. The above findings, together with the smaller energy loss involved in electron transfer from pDA2T to PCBM, as compared to blends of P3HT and PCBM, make pDA2T a promising material for photovoltaic applications.

The optical and conductive properties of the liquid-crystalline organic semiconductor 5,5′′-bis (5-hexyl-2thienylethynyl)-2,2′:5′,2′′-terthiophene (TR5-C6) spin-coated on rubbed polyimide (PI) were studied. The absorption of light linearly polarized parallel to the rubbing direction largely exceeds that in the perpendicular direction. In contrast, a small anisotropy in the photoconductance was found from time-resolved microwave conductivity (TRMC) measurements. From analysis of the optical data, it is inferred that the terthiophene chains in the TR5-C6 molecules are oriented along the PI rubbing direction with a tilt angle of 53 degrees with respect to the plane of the substrate. The absence of a strong anisotropy in the mobility of charge carriers can be understood on the basis of calculated charge transfer integrals, which were found to be comparable for charge-transfer steps in different directions. This is due to arrangement of the molecules in a herringbone structure. Due to the relatively small values of the charge transfer integrals, it is likely that charge transport in TR5-C6 occurs via polaron hopping between localized states.

The relation between the morphology, optical, and photoconductive properties of thin-film bulk heterojunctions of poly(3-hexylthiophene) (P3HT) with a series of electron-accepting siloxanes with a different number (x ) 2, 4, 5) of pendant naphthalene diimide (NDIS) moieties is reported. All NDIS siloxanes show good electronaccepting properties, when blended with P3HT. Interestingly, the film (nano)morphology can be controlled by relatively small changes in the molecular structure of the NDIS siloxanes. Spin-coating from orthodichlorobenzene (ODCB) yields complex film (nano)morphologies, being correlated with the weight ratios of P3HT and NDIS siloxanes, and the molecular structure of the latter. On the other hand, all blends spincoated from chloroform (CHCl 3 ) show good mixing of the components at the molecular level. It is inferred that the nanomorphology of the blends can greatly influence their photoconductive properties: samples spincoated from ODCB invariably display a higher photoconductance than corresponding samples spin-coated from CHCl 3 . This is explained in terms of a higher mobility of holes in samples spin-coated from ODCB, as measured by time-resolved microwave conductivity measurements. These data are useful to delineate the conditions for the research in strives for efficient organic photovoltaics.