Modified SnO₂ with Alkali Carbonates as Robust Electron-Transport Layers for Inverted Organic Solar Cells

We report for the first time that alkali carbonates (Li₂CO₃, K₂CO₃, and Rb₂CO₃) based on a low-temperature solution process can be used as interfacial modifiers for SnO₂ as robust electron-transport layers (ETL) for inverted organic solar cells (iOSCs). The room-temperature photoluminescence, the electron-only devices, and the impedance studies altogether suggested the interfacial properties of the alkali carbonates-modified SnO₂ ETLs, which were much better than those based on the SnO₂ only, provided efficient charge transport, and reduced the charge recombination rates for iOSCs. The iOSCs using the polymer donor poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl] and the fullerene acceptor phenyl-C₇₀-butyric acid methyl ester as the active layer showed the average power-conversion efficiencies (PCEs) based on ten devices of 6.70, 6.85, and 7.35% with Li₂CO₃-, K₂CO₃-, and Rb₂CO₃-modified SnO₂ as ETLs, respectively; these are more than 22, 24, and 33% higher than those based on the SnO₂ only (5.49%). Moreover, these iOSC devices exhibited long-term stabilities, with over 90% PCEs remaining after the devices were stored in ambient air for 6 weeks without encapsulations. We believe that alkali carbonates-modified SnO₂ approaches are an effective way to achieve stable and highly efficient iOSCs and might also be suitable for other optoelectronic devices where an ETL is needed, such as perovskite solar cells or organic light-emitting diodes.