Organic photovoltaic devices. Bulk heterojunction organic solar cells with the conventional device structure were fabricated (glass|ITO|PEDOT:PSS|PCE10:EH-IDTBR|ZnO|Ag, see Fig. ). The ITO-coated glass substrates (1.3 × 1.3 cm 2 ) were first selectively etched with zinc dust and 2 M HCl, before thorough washing with acetone, ethanol, isopropyl alcohol and MilliQ water. Subsequently, the cleaned substrates were subjected to UV-Ozone treatment (BioForce Nanosciences UV/Ozone ProCleaner). To form the hole-transport layer, PEDOT:PSS was filtered through a Millex-GP 0.22 µm PES filter and spin-coated onto the substrates, before annealing on a hot plate in air at 383 K for 40 min. The PCE10:EH-IDTBR active layer was next spincoated onto the PEDOT:PSS layer, followed by zinc oxide nanoparticles. Finally, 100 nm of Ag as electrical contact was deposited by thermal evaporation through a patterned mask under vacuum (~10 -5 mbar), yielding an active area of ~0.5 × 0.5 cm 2 for each device.