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 work we study the internal electric field (Vint) 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 Vint, introducing LiF increases
Vint 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(para phenylene vinylene) (MDMO-PPV) based diodes, we observed two types of electromechanical actuation. In the forward direction, a significant current (up to several mA/cm2) 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.