Polymeric films from the alternating chemisorption of poly(vinylbenzylchloride) and a 4 '-hydroxystilbazole dye

A simple strategy was developed based on the alternating chemisorption of a reactive polymer and a dye bearing two different complementary reactive groups, to obtain stable thin dye-functionalized polymeric films. These films were grown on a precoating made by electrostatic self-assembly. Film growth is linear according to UV-VIS spectroscopy and ellipsometry. Films assembled from 10 deposition cycles show a thickness of 28 nm with a roughness of 4.5 nm (rms) as measured by small angle X-ray reflectivity, this means they are molecularly thin. The chromophores seem to realign upon adding a new layer of polymer as probed with UV-VIS spectroscopy. (C) 2001 Elsevier Science B.V. All rights reserved

Charge transport in MDMO-PPV:PCNEPV all-polymer solar cells

Charge transport properties are investigated of blends of poly [2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) and poly-[oxa-1,4-phenylene-(1-cyano-1,2-vinylene)-(2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene)-1,2-(2-cyanovinylene)-1,4-phenylene] (PCNEPV). The hole transport in the MDMO-PPV donor phase of the 1:1 weight ratio blend is trap-free space-charge limited, with a mobility identical to the pristine polymer. The electron current in the PCNEPV acceptor phase is strongly reduced by traps that are exponentially distributed in energy. The current in MDMO-PPV:PCNEPV bulk heterojunction solar cells is therefore unbalanced and dominated by the holes in the MDMO-PPV phase.

Ordered polyelectrolyte multilayers. Rules governing layering in organic binary multilayers.

We study the growth and internal structure of polyelectrolyte multilayers obtained by combining three polyanions with nine polycations of the ionene family, of systematically varied chemical architecture. We find that, contrary to a generally held belief, ordered organic multilayers are by no way exceptional, provided one of the polyelectrolytes bears groups which induce structure in water, such as long hydrophobic segments or mesogenic groups. However, this condition is not sufficient, as order will or will not emerge in the multilayer depending on the specific pairing of the polyelectrolytes. The results support the notion that layering in the multilayer results from some degree of prestructuring of a water-swollen layer adsorbed during each step of deposition. These findings pave the way to new possible uses of polyelectrolyte multilayers, for example, for applications requiring preferential alignment or strong confinement of specific functional groups

Improving polymer based photovoltaic devices by reducing the voltage loss at the donor-acceptor interface

The costs of large area, organic photovoltaic devices are stronly related to their module efficiency. Even for niche markets, such as consumer electronics, efficiency is imperative since the available area is limited. Therefore, if polymer photovoltaics is to become a mature technology, it is key to increase the power conversion efficiency of the devices. In our contribution an analysis is given of the energy loss factors in P3HT:[C6O]PCBM cells. The main loss occurs as a voltage loss at the donor-accpetor interface. Since this loss factor is linked to the HOMO-LUMO levels of the system, it is impossilble to reduce this loss using the same material combination. We present polymer: [C6O]PCBM cells with similar optical properties but with a reduced voltage loss at the interface, leading to enhanced open circuit Voltage of 1.0 V (compared to 0.62 V for P3HT:[C6O]PCBM devices). The polymer is an alternating copolymer with fluorence and benzothiadiazole units (PFTBT). Well-characterised devices yield already an AM 1.5 efficiency of 4%, thus competing with state-of-the-art P3HT:PCBM devices

Compositional and electric field dependence of the dissociation of charge transfer excitons in alternating polyfluorene copolymer/fullerene blends

The electro-optical properties of thin films of electron donor-acceptor blends of a fluorene copolymer (PF10TBT) and a fullerene derivative (PCBM) were studied. Transmission electron microscopy shows that in these films nanocrystalline PCBM clusters are formed at high PCBM content. For all concentrations, a charge transfer (CT) transition is observed with absorption spectroscopy, photoluminescence, and electroluminescence. The CT emission is used as a probe to investigate the dissociation of CT excited states at the donor-acceptor interface in photovoltaic devices, as a function of an applied external electric field and PCBM concentration. We find that the maximum of the CT emission shifts to lower energy and decreases in intensity with higher PCBM content. We explain the red shift of the emission and the lowering of the open-circuit voltage (VOC) of photovoltaic devices prepared from these blends with the higher relative permittivity of PCBM (?r = 4.0) compared to that of the polymer (?r = 3.4), stabilizing the energy (ECT) of CT states and of the free charge carriers in blends with higher PCBM concentration. We show that the CT state has a short decay time (? = ca. 4 ns) that is reduced by the application of an external electric field or with increasing PCBM content. The field-induced quenching can be explained quantitatively with the Onsager-Braun model for the dissociation of the CT states when including a high electron mobility in nanocrystalline PCBM clusters. Furthermore, photoinduced absorption spectroscopy shows that increasing the PCBM concentration reduces the yield of neutral triplet excitons forming via electron-hole recombination, and increases the lifetime of radical cations. The presence of nanocrystalline domains with high local carrier mobility of at least one of the two components in an organic heterojunction may explain efficient dissociation of CT states into free charge carriers. © 2008 American Chemical Societ