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The luminescence of a ladder-type methyl-poly(para-phenylene) (MeLPPP) doped with platinum-porphyrin dye PtOEP covering the concentration 10(-3)-5% by weight has been measured employing cw and transient techniques. Upon excitation into the range of absorption of the host, strong phosphorescence of the dopant is observed. Possible ways of populating the dopant triplet state are considered. (c) 2004 Elsevier B.V. All rights reserved
The delayed luminescence and phosphorescence of ladder-type methyl-poly(para-phenylene) (MeLPPP) doped with benzil at a concentration of 20% by weight has been measured. The introduction of benzil leads to a dramatic reduction of the polymer singlet emission. At the same time, a new band with maximum at 611 nm appears, corresponding to the phosphorescence of MeLPPP. The phosphorescence decay on the short time scale is close to an exponential law with a time decay of 15 ms. This indicates that benzil can efficiently sensitize the phosphorescence of the polymer. In addition, a broad and featureless emission is observed in the delayed luminescence spectra of benzil-doped MeLPPP, which is attributed to an exciplex formed between the polymer host and the dopant. We further observe that the delayed fluorescence is enhanced by the addition of benzil. It is concluded that the delayed fluorescence of benzil-doped MeLPPP is mainly due to the annihilation of triplet excitons on the polymer. Finally, efficient triplet-triplet energy transfer from the benzil-doped polymer to the red-emitting phosphorescent dye Pt(II)octaethylporphyrin is established. (C) 2004 American Institute of Physics
Materials for polymer electronics applications semiconducting polymer thin films and nanoparticles
(2004)
The paper presents two different approaches to nanostructured semiconducting polymer materials: (i) the generation of aqueous semiconducting polymer dispersions (semiconducting polymer nanospheres SPNs) and their processing into dense films and layers, and (ii) the synthesis of novel semiconducting polyfluorene-block-polyaniline (PF-b-PANI) block copolymers composed of conjugated blocks of different redox potentials which form nanosized morphologies in the solid state
The IR-based method of infrared transition moment orientational analysis (IR-TMOA) is employed to unravel molecular order in thin layers of the semiconducting polymer poly[N,N'-bis(2-octyldodecyl),-1,4,5,8-naphthalene-diimide-2,6-diyl]-alt-5-5'-(2,2'-bithiophene) (P(NDI2OD-T2)). Structure-specific vibrational bands are analyzed in dependence On polarization and inclination of the sample-With respect to the optical axis. By that the molecular Order parameter tensor for the respective molecular moieties with regard to the sample: coordinate system is deduced. Making use of the specificity of the IR spectral range, we are able to determine separately the orientation of atomistic planes defined through the naphthalenediimide (NDI) and bithiophene (T2) units relative to the substrate, and hence, relative to each other. A pronounced solvent effect is observed While chlorobenzene causes the T2 planes to align preferentially parallel to the substrate at an angle of 29 degrees, using a 1:1 chloronaphthalene:xylene mixture results in a reorientation of the T2 units from a face on into an edge on arrangement. In contrast the NDI unit remains unaffected. Additionally, for both solvents evidence is observed for the aggregation of chains in accord With recently published results obtained by UV-vis absorption spectroscopy.
The interplay between nanomorphology and efficiency of polymer-fullerene bulk-heterojunction (BHJ) solar cells has been the subject of intense research, but the generality of these concepts for small-molecule (SM) BHJs remains unclear. Here, the relation between performance; charge generation, recombination, and extraction dynamics; and nanomorphology achievable with two SM donors benzo[1,2-b:4,5-b]dithiophene-pyrido[3,4-b]-pyrazine BDT(PPTh2)(2), namely SM1 and SM2, differing by their side-chains, are examined as a function of solution additive composition. The results show that the additive 1,8-diiodooctane acts as a plasticizer in the blends, increases domain size, and promotes ordering/crystallinity. Surprisingly, the system with high domain purity (SM1) exhibits both poor exciton harvesting and severe charge trapping, alleviated only slightly with increased crystallinity. In contrast, the system consisting of mixed domains and lower crystallinity (SM2) shows both excellent exciton harvesting and low charge recombination losses. Importantly, the onset of large, pure crystallites in the latter (SM2) system reduces efficiency, pointing to possible differences in the ideal morphologies for SM-based BHJ solar cells compared with polymer-fullerene devices. In polymer-based systems, tie chains between pure polymer crystals establish a continuous charge transport network, whereas SM-based active layers may in some cases require mixed domains that enable both aggregation and charge percolation to the electrodes.
We introduce a new and simple method to quantify the effective extraction mobility in organic solar cells at low electric fields and charge carrier densities comparable to operation conditions under one sun illumination. By comparing steady-state carrier densities at constant illumination intensity and under open-circuit conditions, the gradient of the quasi-Fermi potential driving the current is estimated as a function of external bias and charge density. These properties are then related to the respective steady-state current to determine the effective extraction mobility. The new technique is applied to different derivatives of the well-known low-band-gap polymer PCPDTBT blended with PC70BM. We show that the slower average extraction due to lower mobility accounts for the moderate fill factor when solar cells are fabricated with mono- or difluorinated PCPDTBT. This lower extraction competes with improved generation and reduced nongeminate recombination, rendering the monofluorinated derivative the most efficient donor polymer.
We have applied time-delayed collection field (TDCF) and charge extraction by linearly increasing voltage (CELIV) to investigate the photogeneration, transport, and recombination of charge carriers in blends composed of PCPDTBT/PC70BM processed with and without the solvent additive diiodooctane. The results suggest that the solvent additive has severe impacts on the elementary processes involved in the photon to collected electron conversion in these blends. First, a pronounced field dependence of the free carrier generation is found for both blends, where the field dependence is stronger without the additive. Second, the fate of charge carriers in both blends can be described with a rather high bimolecular recombination coefficients, which increase with decreasing internal field. Third, the mobility is three to four times higher with the additive. Both blends show a negative field dependence of mobility, which we suggest to cause bias-dependent recombination coefficients.
We compare standard and inverted bulk heterojunction solar cells composed of PCPDTBT:PC70BM blends. Inverted devices comprising 100 nm thick active layers exhibited short circuit currents of 15 mA/cm(2), 10% larger than in corresponding standard devices. Modeling of the optical field distribution in the different device stacks proved that this enhancement originates from an increased absorption of incident light in the active layer. Internal quantum efficiencies (IQEs) were obtained from the direct comparison of experimentally derived and modeled currents for different layer thicknesses, yielding IQEs of similar to 70% for a layer thickness of 100 nm. Simulations predict a significant increase of the light harvesting efficiency upon increasing the layer thickness to 270 nm. However, a continuous deterioration of the photovoltaic properties with layer thickness was measured for both device architectures, attributed to incomplete charge extraction. On the other hand, our optical modeling suggests that inverted devices based on PCPDTBT should be able to deliver high power conversion efficiencies (PCEs) of more than 7% provided that recombination losses can be reduced.
A novel fluorinated copolymer (F-PCPDTBT) is introduced and shown to exhibit significantly higher power conversion efficiency in bulk heterojunction solar cells with PC70BM compared to the well-known low-band-gap polymer PCPDTBT. Fluorination lowers the polymer HOMO level, resulting in high open-circuit voltages well exceeding 0.7 V. Optical spectroscopy and morphological studies with energy-resolved transmission electron microscopy reveal that the fluorinated polymer aggregates more strongly in pristine and blended layers, with a smaller amount of additives needed to achieve optimum device performance. Time-delayed collection field and charge extraction by linearly increasing voltage are used to gain insight into the effect of fluorination on the field dependence of free charge-carrier generation and recombination. F-PCPDTBT is shown to exhibit a significantly weaker field dependence of free charge-carrier generation combined with an overall larger amount of free charges, meaning that geminate recombination is greatly reduced. Additionally, a 3-fold reduction in non-geminate recombination is measured compared to optimized PCPDTBT blends. As a consequence of reduced non-geminate recombination, the performance of optimized blends of fluorinated PCPDTBT with PC70BM is largely determined by the field dependence of free-carrier generation, and this field dependence is considerably weaker compared to that of blends comprising the non-fluorinated polymer. For these optimized blends, a short-circuit current of 14 mA/cm(2), an open-circuit voltage of 0.74 V, and a fill factor of 58% are achieved, giving a highest energy conversion efficiency of 6.16%. The superior device performance and the low band-gap render this new polymer highly promising for the construction of efficient polymer-based tandem solar cells.