2.3.1. P3HT:PC61BM OSCs

P3HT:PC61BM devices were fabricated on patterned ITO substrates (Xinyan Technology Ltd., Kwun Tong, Hong Kong, 15 Ω sq<sup>−</sup>1). The substrates were rinsed using soapy water, acetone, and IPA, followed by UV-ozone treatment for 20 min. 2 mg mL−<sup>1</sup> CCO and Mg:CCO suspensions were spin coated at 2000 rpm for 30 s on top of the ITO substrates. The thickness of CCO and Mg:CCO nanoparticle films are ~18 nm; see Section 3.2 for details. 23 mg mL−<sup>1</sup> P3HT and 23 mg mL−<sup>1</sup> PC61BM were dissolved in CB and stirred at 50 ◦C overnight. The P3HT:PC61BM active layer (~200 nm thick) was made by dispensing 35 μL P3HT:PC61BM solution on a spinning substrate at 1200 rpm for 60 s, followed by annealing at 170 ◦C in N2 for 10 min. Finally, 7 nm Ca and 100 nm Al were sequentially evaporated on top of the active layer. The current-voltage (*J-V*) measurements were carried out using a 2635A Keithley low-noise sourcemeter under AM 1.5G 100 mW cm−<sup>2</sup> illumination from a class AAA solar simulator (Abet Technologies) in a nitrogen filled glovebox. The diode area is 0.11 cm2 and the aperture area is 0.049 cm2.
