By applying the specific fabrication conditions summarized in the Experimental section and post‐production annealing at 150 °C, polymer solar cells with power‐conversion efficiency approaching 5 % are demonstrated. These devices exhibit remarkable thermal stability. We attribute the improved performance to changes in the bulk heterojunction material induced by thermal annealing. The improved nanoscale morphology, the increased crystallinity of the semiconducting polymer, and the improved contact to the electron‐collecting electrode facilitate charge generation, charge transport to, and charge collection at the electrodes, thereby enhancing the device efficiency by lowering the series resistance of the polymer solar cells.

The morphology and performance of bulk heterojunction solar cells comprised of poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C61butyric acid methyl ester (PCBM) have been investigated using P3HT with different molecular weights (MWs). It is concluded that the optimum annealing temperature for the bulk heterojunction material is related to the MW of P3HT. The best performance is obtained by using P3HT with an optimum ratio between high MW and low MW components. The corresponding ‘ideal morphology’ is comprised of highly ordered crystalline regions formed by low MW P3HT embedded and interconnected by a high MW P3HT matrix.

We report the origin of the strong UV-irradiation dependence, generally known as a “light-soaking” process, in inverted polymer solar cells (I-PSCs) using the interface of an sol-gel processed titanium sub-oxide (TiOx) and indium tin oxide (ITO) cathode. When I-PSCs incorporating TiOx as an electron-selecting layer were fabricated, the as-prepared devices exhibited an anomalous J-V curve with a kink shape, resulting in an extremely low efficiency. However, the kink shape disappeared after white light irradiation for considerable duration, after which the device parameters recovered the normal values expected for this class of devices. By using electrochemical impedance spectroscopy and by measuring the contact potential difference and transient photoconductivity of the TiOx layer, we found that the light-soaking process in I-PSCs originates from the photoinduced “rearrangement of the Fermi levels” at the sol-gel processed TiOx and ITO cathode interface together with trap sites existing in the TiOx layer. Based on our data, we optimized I-PSC devices with a high fill factor (FF) of ∼70%.

No abstract