*2.3. Organic Solar Cell Fabrication and Characterization*

OSCs were fabricated on ITO substrates (sheet resistance 20 Ω/square) purchased from Xinyan Technologies (Kowloon, Hong Kong). Pre-patterned ITO coated glass substrates were cleaned in an ultrasonic cleaner with deionized water and isopropyl alcohol (IPA) for 20 min each.

For single solar cell (Figure 1b), prepared ZnO sol-gel solution was deposited onto the ITO layer by spin coating (3000 rpm for 60 s) through a 0.45 μm pore PVDF filter in a N2 glove box. The substrates were directly transferred outside the glove box and dried in ambient air during different times (0, 5, 10, 30, 60, and 180 min, respectively) and then thermally annealed at 125 ◦C over 10 min, in order to form nano-ripple structures. The final thickness of the fabricated film was 200 nm. The samples were then transferred into N2 glove box to deposit the following layers. A P3HT:PCBM solution was filtered through a 0.45 μm pore polytetrafluoroethylene (PTFE) filter, and spin-coated in two steps (500 rpm for 30 s, and then 1000 rpm for 45 s). The films were thermally annealed at 145 ◦C for 10 min. The thickness of this layer was approximately 100 nm after annealing. Then, PEDOT:PSS:Triton solution was deposited at 1000 rpm for 60 s. The thickness of this layer was 120 nm after annealing at 120 ◦C for 2 min.

For tandem cells (Figure 1c), the ZnO nano-ripple layer (ECL), the first sub-cell active layer, and PEDOT:PSS:Triton layer (HCL) were fabricated in the same way as reported above. PTB7:PC70BM was used as active layer in the second sub-cell. The PTB7:PC70BM solution was filtered through a 0.45 μm pore PTFE filter, and spin-coated at 700 rpm for 60 s. The layer was annealed in DCB solvent vapor for 5 min. The thickness of this layer was approximately 80 nm. Finally, a gold electrode (anode) was thermally evaporated through a shadow mask, defining a 0.28 cm<sup>2</sup> active surface area. The deposition rate of Au was 0.5 Å/s under 5 <sup>×</sup> <sup>10</sup>−<sup>6</sup> Torr process pressure.

The current density-voltage (J-V) characteristics of the OSCs were measured in a N2 glove box using a source meter (Keithley 2635, Tektronix, Beaverton, OR, USA) in the dark and under illumination. An Air-Mass 1.5 (AM 1.5) solar simulator with 100 mW cm−<sup>2</sup> was used as the light source.
