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Tuning of the excited-state properties and photovoltaic performance in PPV-based polymer blends
(2008)
carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold viaweakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer: fullerene, small-molecule:C-60 and polymer: polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE higher than 90% without the need for excess electronic or vibrational energy.
Bulk electron transport and charge injection in a high mobility n-type semiconducting polymer
(2010)
Bulk electron transport in a high mobility n-type polymer is studied by time-of-flight photocurrent measurements and electron-only devices. Bulk electron mobilities of similar to 5 x 10(-3) cm(2)/Vs are obtained. The analysis of the electron currents suggests the presence of an injection barrier for all conventionally used low workfunction cathodes.
We explore the photophysics of P(NDI2OD-T2), a high-mobility and air-stable n-type donor/acceptor polymer. Detailed steady-state UV-vis and photoluminescence (PL) measurements on solutions of P(NDI2OD-T2) reveal distinct signatures of aggregation. By performing quantum chemical calculations, we can assign these spectral features to unaggregated and stacked polymer chains. NMR measurements independently confirm the aggregation phenomena of P(NDI2OD-T2) in solution. The detailed analysis of the optical spectra shows that aggregation is a two-step process with different types of aggregates, which we confirm by time-dependent PL measurements. Analytical ultracentrifugation measurements suggest that aggregation takes place within the single polymer chain upon coiling. By transferring these results to thin P(NDI2OD-T2) films, we can conclude that film formation is mainly governed by the chain collapse, leading in general to a high aggregate content of similar to 45%. This process also inhibits the formation of amorphous and disordered P(NDI2OD-T2) films.
Fullerene-based acceptors have dominated organic solar cells for almost two decades. It is only within the last few years that alternative acceptors rival their dominance, introducing much more flexibility in the optoelectronic properties of these material blends. However, a fundamental physical understanding of the processes that drive charge separation at organic heterojunctions is still missing, but urgently needed to direct further material improvements. Here a combined experimental and theoretical approach is used to understand the intimate mechanisms by which molecular structure contributes to exciton dissociation, charge separation, and charge recombination at the donor-acceptor (D-A) interface. Model systems comprised of polythiophene-based donor and rylene diimide-based acceptor polymers are used and a detailed density functional theory (DFT) investigation is performed. The results point to the roles that geometric deformations and direct-contact intermolecular polarization play in establishing a driving force ( energy gradient) for the optoelectronic processes taking place at the interface. A substantial impact for this driving force is found to stem from polymer deformations at the interface, a finding that can clearly lead to new design approaches in the development of the next generation of conjugated polymers and small molecules.
The authors present organic photovoltaic (OPV) devices comprising a small molecule electron acceptor based on 2- vinyl-4,5-dicyanoimidazole (Vinazene (TM)) and a soluble poly(p-phenylenevinylene) derivative as the electron donor. A strong dependence of the fill factor (FF) and the external quantum efficiency [incident photons converted to electrons (IPCE)] on the heterojunction topology is observed. As-prepared blends provided relatively low FF and IPCE values of 26% and 4.5%, respectively, which are attributed to significant recombination of geminate pairs and free carriers in a highly intermixed blend morphology. Going to an all-solution processed bilayer device, the FF and IPCE dramatically increased to 43% and 27%, respectively. The FF increases further to 57% in devices comprising thermally deposited Vinazene layers where there is virtually no interpenetration at the donor/acceptor interface. This very high FF is comparable to values reported for OPV using fullerenes as the electron acceptor. Furthermore, the rather low electron affinity of Vinazene compound near 3.5 eV enabled a technologically important open circuit voltage (V-oc) of 1.0 V.
Carrier transport and recombination have been studied in single component layers and blends of the soluble PPV- derivative poly[2,5-dimethoxy-1,4-phenylenevinylene-2-methoxy-5-(2-ethyl-hexyloxy)- 1,4-phenylenevinylene] (M3EH-PPV) and the small molecule acceptor 4,7-bis(2-(1-hexyl-4,5-dicyanoimidazole-2-yl)vinyl) benzo[c][1,2,5]-thiadiazole (HV-BT). Measurements on single carrier devices show significantly smaller electron mobility in the blend compared to the pure HV- BT layer, which is suggestive of the formation of isolated clusters of the acceptor in a continuous polymer matrix. The significant change in fill factor (FF) with increasing illumination intensity is consistently explained by a model taking into account bimolecular recombination and space charge effects. The decay of the carrier density after photoexcitation has been studied by performing photo-CELIV measurements on pure and blend layers. It is found that the decay at long delay times follows a power-law dependence, which is, however, not consistent with a Langevin-type bimolecular recombination of free charges. A good description of the data is obtained by assuming trimolecular recombination to govern the charge carrier dynamics in these systems.
To address the nature of charge transport and the origin of severe (intrinsic) trapping in electron-transporting polymers, transient and steady-state charge transport measurements have been conducted on the prototype donor/acceptor copolymer poly[2,7-(9,9-dialkyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PFTBTT). A charge-generation layer technique is used to selectively address transport of the desired charge carrier type, to perform time-of-flight measurements on samples with < 200 nm thickness, and to combine the time-of-flight and the photocharge extraction by linearly increasing voltage (photo-CELIV) techniques to investigate charge carrier dynamics over a wide time range. Significant trapping of free electrons is observed in the bulk of dioctyl-substituted PFTBTT (alt-PF8TBTT), introducing a strong relaxation of the charge carrier mobility with time. We used Monte-Carlo simulation to simulate the measured transient data and found that all measurements can be modeled with a single parameter set, with the charge transport behavior determined by multiple trapping and detrapping of electrons in an exponential trap distribution. The influence of the concomitant mobility relaxation on the transient photocurrent characteristics in photo-CELIV experiments is discussed and shown to explain subtle features that were seen in former publications but were not yet assigned to electron trapping. Comparable studies on PFTBTT copolymers with chemical modifications of the side chains and backbone suggest that the observed electron trapping is not caused by a distinct chemical species but rather is related to interchain interactions.
The recent development of donor–acceptor copolymers has led to an enormous improvement in the performance of organic solar cells and organic field-effect transistors. Here we describe the synthesis, detailed characterisation, and application of a series of structurally modified copolymers to investigate fundamental structure–property relationships in this class of conjugated polymers. The interplay between chemical structure and optoelectronic properties is investigated. These are further correlated to the charge transport and solar cell performance, which allows us to link their chemical structure to the observed physical properties.