**3.1. Device fabrication**

The AZO (~980 nm, 2 wt% Al)‐coated glass substrates purchased from Zhuhai Kaivo co. were prepared by radiofrequency magnetron sputtering process and they have a sheet resistance of about 7.34 ohm/square and average transmission over 80% in the visible light region. The P3HT:PCBM based bulk heterojunction IOSCs with the AZO transparent cathode was fabricated, and the obtained devices have the structure of Glass/AZO/Ca/P3HT:PCBM/MoO3/ Ag and an active area of 12.5 mm2 . The Ca interfacial modifier with different thicknesses (0, 1, 5, 10 nm) were thermally evaporated on AZO at a base pressure below 5.0 × 10−4 Pa. The detail fabrication process is the same as that in previous section. The J‐V characteristics were measured under AM 1.5G solar simulator spectrum before and after 15‐min light soaking. The stability of devices was investigated by measuring their J‐V characteristics once every 5 days in 1 month.
