Energy distributions ͓density-of-states ͑DOS͔͒ of defects in the effective band gap of organic bulk heterojunctions are determined by means of capacitance methods. The technique consists of calculating the junction capacitance derivative with respect to the angular frequency of the small voltage perturbation applied to thin film poly͑3-hexylthiophene͒ ͑P3HT͒ ͓6,6͔-phenyl C 61 -butyric acid methyl ester ͑PCBM͒ solar cells. The analysis, which was performed on blends of different composition, reveals the presence of defect bands exhibiting Gaussian shape located at E Ϸ 0.38 eV above the highest occupied molecular orbital level of the P3HT. The disorder parameter , which accounts for the broadening of the Gaussian DOS, lies within the range of 49-66 meV. The total density of defects results of order 10 16 cm −3 .

We report organic photovoltaic devices in which the standard ITO transparent electric contact has been substituted by lower cost ultrathin metallic electrodes. Solution and vacuum processable n and p-type semiconductors provide the electrode with the rectifying behavior of the diode. We are in this way able to invert the built-in electric field at wish and make the device deliberately either top or bottom sensitive with the same efficiency depending on the application. Taking advantage of these new generation electrodes we furthermore report devices with fill factors over 70%-to our knowledge, the largest published to date for an organic photovoltaic cell-and power conversion efficiencies over the state-ofart with 3.5% in inverted P3HT:PCBM devices, ITO free designs over 2.5% and (semi)transparent photovoltaic devices with conversion efficiencies close to 2.6%. This breakthrough could once and for all trigger the fabrication of organic tandem solar cells and photovoltaic windows.

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