Thomas Kietzke

Solution processable bulk-heterojunction solar cells using a small molecule acceptor

We report a small-molecule electron-acceptor based on 2-vinyl-4,5-dicyanoimidazole [Vinazene™] for use in solution processed organic solar cells. The material has a favourably located LUMO level of −3.6 eV and absorbs strongly in the visible spectrum up to 520 nm—attractive properties compared to the widely used acceptor (6,6)-phenyl-C60-butyric acid methyl ester (PCBM). The Vinazene derivative was blended with a poly(2,7-carbazole) donor—chosen for its complementary absorption range and comparatively high-lying HOMO level of −5.6 eV—and incorporated into bulk heterojunction devices. The influence of the donor/acceptor composition and annealing temperature on device performance were then investigated. The best performing devices exhibited reasonable power conversion efficiencies of 0.75% and open-circuit voltages of more than 1.3 V, substantially higher than previously reported devices using small molecule acceptors.

Photovoltaic properties and exciplex emission of polyphenylenevinylene-based blend solar cells

By studying the photoluminescence emission and photovoltaic properties of blends of polyphenylenevinylene (PPV)-based electron donating and accepting polymers, the authors observed a strict anticorrelation between relative exciplex emission in the solid state and photovoltaic efficiency of corresponding blend devices. Comparative studies on defined bilayer geometries showed that the overall shape of the current-voltage characteristics under illumination is independent of device geometry and layer thickness. Consequently, they conclude that the photocurrent in PPV-based blends is mainly determined by the efficiency to form free carriers rather than by free carrier recombination.

Optical enhancement in semitransparent polymer photovoltaic cells

This letter reports our efforts to improve the power conversion efficiency (PCE) and the optical transparency of semitransparent poly(3-hexylthiophene) (P3HT):1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C60 (PCBM) based photovoltaic (PV) cells. Semitransparent P3HT:PCBM based solar cells were prepared based on parameters obtained by optical admittance analysis. The performance of structurally identical polymer PV cells made with a reflective calcium/silver cathode and a semitransparent Ca(10nm)∕Ag(10nm)/indium tin oxide cathode are compared. For semitransparent polymer PV cells with a 75nm thick active P3HT:PCBM blend layer, an average transmission of more than 30% was achieved with a high PCE of 1.7% under simulated AM1.5G illumination of 100mW∕cm2.

Heterojunction topology versus fill factor correlations in novel hybrid small-molecular/polymeric solar cells

The authors present organic photovoltaic (OPV) devices comprising a small molecule electron acceptor based on 2-vinyl-4,5-dicyanoimidazole (Vinazene) 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.

Charge carrier generation and electron blocking at interlayers in polymer solar cells

The authors show that an electron-donating polymer interlayer and a spin coated layer of an electron-accepting polymer form a defined polymer-polymer heterojunction. Directional photoinduced charge transfer and efficient electron blocking at this heterojunction is clearly seen in Kelvin probe measurements. The photocurrent characteristics of this well-defined bilayer structure as well as of the respective blend device can be consistently fitted by models taking into account only the field dependence of charge carrier generation. Apparently, the efficiency to form free carriers is the determining process in both types of polymer-polymer solar cell structures.

Nanostructured solar cells based on semiconducting polymer nanospheres (SPNs) of M3EH-PPV and CN-Ether-PPV

Organic photovoltaic devices often show improved performances, if the active layer is made of a polymer blend. Due to the low miscibility of polymers the layer will phase separate and the lengthscale of the phase separation has a major influence on the device efficiency. We present a novel method to control the lengthscale of the phase separation, based on semiconducting polymer nanospheres (SPNs) forming the active layer. SPNs of M3EH-PPV (diameter 54nm) and CN-Ether-PPV (diameter 36nm) dispersed in water were produced by the miniemulsion process. Mono- and multilayers of these particles were fabricated by spincoating and photovoltaic devices utilizing these nanoparticles are shown to exhibit large external quantum efficiencies of up to 14%.

ITO-Based Cathode for Application in Semitransparent Organic Photovoltaic Cells

A high performance semitransparent cathode is one of the major impediments to the high performance semitransparent and tandem organic photovoltaic (OPV) cells. In this work, we discuss the possible designs of semitransparent cathode to improve the performance of semitransparent OPV cells. The optical properties of the poly(3-hexylthiophene) (P3HT): 1-(3methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM)-based OPV cells were studied by the optical admittance analysis. The performance of the OPV cells made with an opaque Ca(10nm)/Ag(100nm) cathode and a semitransparent Ca(10nm)/Ag(10nm)/ITO cathode are discussed. The interfacial properties at the cathode/organic interface were analyzed using the time-of-flight secondary ion mass spectroscopy (TOF-SIMS). The TOF-SIMS depth profile revealed that calcium oxide formed at the Ca/organic interface in semitransparent OPV cells, which was induced by the ITO sputtering process. It shows that the presence of calcium oxide at the organic/cathode interface could be the main reason resulting in a poor fill factor of a semitransparent OPV cell (~23%) as compared to that of a reference cell (~43%).

Polymer light emitting diodes based on LiF/Al composite cathode

We report on the performances of polymer light emitting diodes utilizing LiF/Al composite cathodes. Among the devices with the composite cathodes of different concentration of LiF, the layer by layer device is the most efficient, which is different from the results based on small molecule organic light diode devices. The different performances of the devices can be ascribed to the variation of the electron injection barrier heights at the polymer/cathode interface, as suggested by photovoltaic experiments.

Solution Processed Organic Solar Cells with High Open Circuit Voltages Approaching 1.3 V

Novel electron accepting materials based on 2-vinyl-4,5-dicyanoimidazoles (DCI) are blended with donor polymers to prepare efficient organic solar cells reaching external quantum efficiencies of 25% and open circuit voltages up to 1.3 V.