Efficient organic photovoltaic devices show many interesting properties, but share a common drawback, namely their instability in atmosphere. We report on a shelf lifetime study of solar cells based on blends of two widely used polymeric semiconductors with 1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61 (PCBM), encapsulated in a new flexible and transparent poly(ethylene naphthalate) (PEN)-based ultra-high barrier material. The barrier coating is entirely fabricated by plasma enhanced chemical vapor deposition (PECVD). The conjugated polymers used are poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene-vinylene) (MDMO-PPV) and poly(3-hexyl)thiophene (P3HT). We have observed in this work that the encapsulation raises the shelf lifetime (50 % of the initial
efficiency) from a few hours into the range beyond 3,000 hours for MDMO-PPV based devices. Using the more stable P3HT, the lifetime could be increased to approximately 6,000 hours, or more than eight months.
In this study, we have investigated the possibility to realize different types of stacked, serially connected organic solar cells. First of all, we combined solution processed MDMO-PPV:PCBM or P3HT:PCBM and evaporated ZnPc-C60 bulk-heterojunction solar cells to achieved tandem cells exploiting the complementary absorption spectra of each single cells. Such devices exhibit open circuit voltages of 1V with a short-circuit current of approximately 5mA/cm<sup>2</sup> and a fill factor of 0.35 under simulated AM1.5 illumination. In the case of stacked, series connected cells with all active layers processed from solution, we observed a significant increase of the open circuit voltage in comparison with the single junction cells: Device fabricated from two bilayers comprising MDMO-PPV and PCBM as photoactive materials exhibit 1.28V open circuit voltage, 1.1mA/cm<sup>2</sup> short circuit current and a fill factor of 0.45 under simulated AM1.5 illumination.
Photo-induced phenomena were investigated in photoresposive organic field-effect transistors (photOFETs) based on conjugated polymer/fullerene solid-state mixtures as active semiconductor layer and divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) as gate dielectrics. The devices were characterized both in under dark showing n-type transistor behaviour with linear and saturated mobility of 1.7 x 10<sup>-3</sup> cm<sup>2</sup>/Vs and 2.7 x 10<sup>-2</sup> cm<sup>2</sup>/Vs respectively, and under white light illumination condition, where large shifts in the threshold voltage in the transfer characteristics were obtained. A typical phototransistor behaviour in a wide range of illumination intensities are observed in these devices.
In this work we study the internal electric field (<i>V<sub>int</sub></i>) present in devices based on an intrinsically semiconducting
polymer. Intermediate layers between the indium-tin-oxide and Al electrodes and the photoactive layer are
able to influence and alter this electric field. The two commonly used intermediate layers, namely poly(3,4-
ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) and LiF, are subject of this study.
Their influence is studied with Electroabsorption (EA) spectroscopy as well as transient photocurrent measurements
under applied bias. While PEDOT:PSS has no significant influence on <i>V<sub>int</sub></i>, introducing LiF increases
<i>V<sub>int</sub></i> close to the bandgap of the studied semiconducting polymer. However, using PEDOT:PSS directly influences
the spectral EA response. The interface between PEDOT:PSS and the conjugated polymer is studied by
impedance spectroscopy. We interpret the results in terms of the presence of charges at the interface.
Conjugated polymers are nowadays used in two different types of device. On the one hand, they act as electronically active semiconducting/conducting materials in organic electronic devices. On the other hand, one exploits them as electromechanically active materials since it has been observed that they can experience huge macroscopic strains upon electrochemical doping. We investigated the combination of these two effects by measuring the electromechanical behavior of typical polymeric electronic devices like rectifying (and/or light emitting) diodes. In the case of a poly(<i>para</i> phenylene vinylene) (MDMO-PPV) based diodes, we observed two types of electromechanical actuation. In the forward direction, a significant current (up to several mA/cm<sup>2</sup>) is flowing. Joule heating induces a thermo-electrostrictive bending of the device substrate. In the reverse direction, the diode behaves like a capacitor. Therefore the strains are induced by Maxwell forces. For poly(3-hexyl-thiophene) (P3HT) based diodes, displacement versus voltage in the reverse direction revealed a power law with an exponent of 1.5. This surprising result can be modeled by Coulombic attraction of the doped impurities present in the depletion zone and the charges present in the metal at the interface.