were cleaned by sonicating sequentially with water, acetone and finally isopropanol for 10 minutes each. The cleaned substrates were then dried with nitrogen and plasma treated (Harrick Plasma PDC-001-HP) for 6 minutes at 300 mTorr of dry air and high power (30 W). To deposit the electron transporting layer, ZnO solution was prepared by dissolving 1 g of zinc acetate dihydrate (Sigma Aldrich) and 0.28 g ethanolamine (Sigma Aldrich) in 10 mL of 2-methoxyethanol (Sigma Aldrich) under vigorous stirring for over 12 hours in air. This solution was then filtered with a 0.45 µm PTFE syringe filter before deposition. The filtered solution was deposited on the cleaned ITO substrates by spin-coating for 30 seconds at 4000 rpm in ambient atmosphere which resulted in an active layer thickness of ~20-30 nm measured by Bruker Dektak XT profilometer. After spin-coating, the ZnO layer was annealed in air at 200 ˚C for 30 min followed by slow cooling to room temperature for active layer deposition. The photoactive layer solution for all devices were prepared by dissolving the polymer and ITIC-4F (Sigma Aldrich) at 1:1 (wt/wt) ratio in chlorobenzene without any additives. The resulting blend solution is stirred at 40 °C overnight and brought to room temperature 1-2 hours before deposition. The photoactive layer is then blade coated on the ZnO coated ITO-glass substrate at coating temperatures of 25 °C, 65 °C, and 105 °C at 5 mm/s coating speed. To ensure same film thickness when coating at different temperatures, the blade angle was varied from 10° to 55° and the film thickness was measured by using Dektak XT profilometer. The optimal film thickness for all devices was ~ 80 nm. All thin films were then annealed in a nitrogen filled glovebox at 100 °C for 10 minutes. Finally, 8 nm hole transporting layer and 120 nm of Ag electrode were deposited by using a Thermal Evaporator (Kurt J. Lesker Nano36) by slowly ramping the rate of deposition to 0.1 Å/s and 1 Å/s respectively.