We investigate thin poly(3‐hexylthiophene‐2,5‐diyl)/[6,6]‐phenyl C61 butyric acid methyl ester (P3HT/PCBM) films, which are widely used as active layers in plastic solar cells. Their structural properties are studied by grazing‐incidence X‐ray diffraction (XRD). The size and the orientation of crystalline P3HT nanodomains within the films are determined. PCBM crystallites are not detected in thin films by XRD. Upon annealing, the P3HT crystallinity increases, leading to an increase in the optical absorption and spectral photocurrent in the low‐photon‐energy region. As a consequence, the efficiency of P3HT/PCBM solar cells is significantly increased. A direct relation between efficiency and P3HT crystallinity is demonstrated.

The monochromatic external quantum efficiency of a bulk heterojunction photodetector based on a blend of poly-3(hexylthiophene) with a methanofullerene is reported to be as high as 76% at the peak maximum at 25 °C. Analysis of the temperature dependence, the illumination intensity dependence together with absorption measurements in reflection geometry, allow calculation of the internal quantum efficiency of the device close to 100% at the peak maximum. Recombination of photoinduced carriers is negligible or even absent in these photodetectors when operated in the photovoltaic mode. Optical losses in these bulk heterojunction devices are analyzed.

The challenge to reversing the layer sequence of organic photovoltaics (OPVs) is to prepare a selective contact bottom cathode and to achieve a suitable morphology for carrier collection in the inverted structure. The authors report the creation of an efficient electron selective bottom contact based on a solution-processed titanium oxide interfacial layer on the top of indium tin oxide. The use of o-xylene as a solvent creates an efficient carrier collection network with little vertical phase segregation, providing sufficient performance for both regular and inverted solar cells. The authors demonstrate inverted layer sequence OPVs with AM 1.5 calibrated power conversion efficiencies of over 3%.

The efficiency of poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C61 butyric acid methyl ester (PCBM) bulk heterojunction solar cells dramatically depends on the molecular weight of the donor polymer P3HT. Only for P3HTs with a number-average molecular weight (M n) of >10 000 have high power conversion efficiencies of >2.5% been reached. This behavior is caused by distinctly reduced charge carrier (hole) mobility in the donor phase of the devices built from lower M n P3HT samples. The reduced performance of such devices is related to a reduced intermolecular ordering (π-stacking) of the P3HT phase. These findings highlight the important role of the molecular weight in semicrystalline semiconducting polymers for the device performance and may help in the further development of novel, efficient, lower band gap donor polymers for photovoltaic applications.