Steffi Sensfuss

Efficient large-area polymer solar cells on flexible substrates

Polymer solar cells based on poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene]:[6,6]-phenyl-C61-butyric acid methyl ester (1:3wt%) were fabricated using flexible 5×5cm2 polyester foils. These cells show a white-light power conversion efficiency of 3% comparable to smaller cells on glass published by other authors. Ten cells were arranged on one substrate with an active area of 25 and 175mm2 demonstrating a remarkably high reproducibility. It was found that the open-circuit voltage does not depend on the device area. An increase of the cell area caused a slightly lower short-circuit current, lower fill factor, and lower white-light power conversion efficiency.

Optical and ESR studies on poly(3-alkylthiophene)/fullerene composites for solar cells

Optical measurements (absorption coefficients, photoluminescence) and light-induced electron spin resonance ( LESR ) technique were used to study photoinduced charge generation and charge transfer in conjugated polymer / fullerene composites. The quenching of fluorescence by C 60 gives a first indication of an effective charge transfer from P3DDT to C 60 . At 77 K two kinds of LESR signals were identified, one of polaron (P +. ) on the polymer chain and one of radical anion (C 60 -.) of fullerene. The relaxation rates of generated paramagnetic centres were estimated by microwave power saturation studies. Moreover for the first centre (P +. ) two contributions to the LESR signal were observed: a permanent one and a photoinduced one. The light intensity dependence of the photoinduced P +. and C 60 -. ESR signals is of bimolecular type (I 0.5 ). Solar cells based on P3DDT/ C 60 and P3DDT/ PCBM are prepared and characterised.

Anisotropic optical properties of conjugated polymer and polymer/fullerene films

Abstract We have studied the optical properties of thin films of substituted polyphenylene ethynylene/polyphenylene vinylene copolymers. Two novel materials of this type were investigated: poly[1,4-phenyleneethylene-1,4-(2,5-didodecyloxylphenylene)-1,4-phenyleneethene-1,2-diyl-1,4-(2,5-didodecyloxyphenylene)ethene-1,2-diyl] (DE15) and poly[1,4-phenyleneethylene-1,4-(2,5-di(2-ethylhexyloxy)phenylene)-1,4-phenyleneethene-1,2-diyl-1,4-(2,5-di(2-ethylhexyloxy)phenylene)ethene-1,2-diyl] (DE42). Both pure and fullerene-doped films were prepared by spin coating onto silicon substrates. Their optical properties were obtained by determination of the dielectric function by means of spectroscopic ellipsometry. A large anisotropy of the dielectric function was found both for the pure materials and for their composites with 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C 61 (PCBM) with the optical axis normal to the film surface. The origin of this phenomenon and the influence of the anisotropy on the performance of plastic solar cells are discussed. The optical properties of such DE15/PCBM and DE42/PCBM composites are successfully described in terms of the Bruggemann effective medium approximation.