ITO-free inverted organic solar cells fabricated with transparent and low resistance ZnO/Ag/ZnO multilayer electrode

• The opto-electrical properties of ZnO/Ag/ZnO multilayer films are investigated.
• ZnO/Ag/ZnO films show low sheet resistances in the range of 3.6–3.9 Ω/sq.
• Transmittance of ZnO(40 nm)/Ag(19 nm)/ZnO(40 nm) films is ∼95% at 550 nm.
• Organic solar cell with ZnO/Ag/ZnO show a conversion efficiency similar to OSC with ITO.

Inverted organic solar cells (OSCs) based on poly (3-hexylthiophene) (P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) bulk heterojunctions (BHJ) were fabricated with optimized ZnO/Ag/ZnO multilayer and conventional indium–tin oxide (ITO) cathode electrodes and their performance was compared. The ZnO/Ag/ZnO multilayer films showed sheet resistances in the range 3.6–3.9 Ω/sq, while ITO exhibited 14.2 Ω/sq. On the one hand, the carrier concentration gradually decreased from 1.74 × 1022 to 4.33 × 1021 cm−3 as the ZnO thickness increased from 8 to 80 nm, respectively. The transmittance of the ZnO(40 nm)/Ag(19 nm)/ZnO(40 nm) films was ∼95% at 550 nm, which is comparable to that of ITO (∼96%). The multilayer films were smooth with a root mean square (RMS) roughness of 0.81 nm. OSCs fabricated with the ZnO(40 nm)/Ag(19 nm)/ZnO(40 nm) film showed a power conversion efficiency (2.63%) comparable to that of OSCs with a conventional ITO cathode (2.71%).