**2. Experimental Section**

As shown in Figure 1, for electron-only devices, ZnO films with different pattern sizes, single-junction OSCs, and tandem OSCs with a ZnO layer were fabricated and analyzed. The detailed fabrication methods are described below.

**Figure 1.** Device structures used in this study, with relevant energy levels of the different layers. (**a**) Electron only device, (**b**) single solar cell with a nano-ripple pattern, (**c**) tandem solar cell. ECL (electron collecting layer) and HCL (hole-collecting layer) are the ZnO nano-ripple layer and the PEDOT:PSS:Triton layer respectively.

#### *2.1. Preparation of Solutions*

A ZnOX sol-gel solution was prepared by the following method. Zinc acetate dehydrate (ZnO precursor) at 1.5 M concentration was prepared in 2-methoxyethanol, followed by adding 0.1 mL of ethanolamine. Two types of PEDOT:PSS (Al4083 and PH1000, respectively) with surfactant were used as the hole-collecting layer. A volume of 100 μL of surfactant Triton was mixed with 4 mL of 1:1 volume ratio of AI4083 and PH1000. The ratio of PEDOT:PSS was 1:6 for Al4083, and 1:2.5 for PH1000. PH1000 contains higher amount of PEDOT, and thus has a higher conductivity than AI4083. AI4083 and PH1000 were purchased from Bayer (Leverkusen, Germany). In solar cells, poly(3-hexylthiophene) (P3HT) and poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2- [(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) were used as the donor materials, while 1 ,1",4 ,4"-tetrahydro-di[1,4]methanonaphthaleno[1,2:2 ,3 ,56,60:2",3"][5,6]fullerene-C60 (ICBA), 6,6-phenyl C61-butyric acid methyl ester (PCBM), and 6,6-phenyl C71-butyric acid methyl ester (PC70BM) were used as acceptor materials. P3HT, PTB7, and fullerene derivatives were purchased from Ossila (Sheffield, UK). For a single solar cell (Figure 1b), a blend of P3HT and PCBM (1:1 weight ratio) was used as the active layer by dissolving 40 mg of P3HT:PCBM in 1 mL 1,2-dichlorobenzene (DCB, Sigma Aldrich, St. Louis, MO, USA). For the tandem solar cell (Figure 1c), the first sub-cell (bottom cell, Figure 1c) was based on a blend of P3HT and ICBA, and the second sub-cell (top cell, Figure 1c) was a blend of PTB7 and PC70BM. The P3HT:ICBA blend solution was prepared by dissolving 40 mg of P3HT:ICBA (1:1 weight ratio) in 1 mL of DCB. For second sub-cell active layer, the solution was prepared by dissolving 1:1.5 weight ratio of PTB7:PC70BM in DCB with a total concentration of 25 mg/mL. All prepared solutions were stirred for 12 h in a N2 glove box.
