Water-Soluble SnO₂ Nanoparticles as the Electron Collection Layer for Efficient and Stable Inverted Organic Tandem Solar Cells

Combining wide bandgap and narrow bandgap organic materials to construct tandem organic solar cells (OSCs) offers opportunities for overcoming the efficiency limitation of single-junction OSCs. Commonly, the inorganic metal oxide of zinc oxide (ZnO) was considered as the most efficient electron collection layer (ECL) for the tandem device. Herein, the tin oxide (SnO2) nanoparticles are employed as the ECL in the inverted tandem devices due to their advantages of low-temperature solution processability, excellent transmittance, and high electron mobility. Thereby, the two tandem organic devices based on PBDB-T (or J71):IT-M-front cells and the PCE10:IEICO-rear cell with 10 nm-thick SnO2 ECLs in inverted structures provide power conversion efficiencies (PCEs) of 12.06% and 12.34%, respectively. Note that the J71:IT-M as the absorption layer in the front cell of the tandem device exhibits deep highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels and then contributes a higher open-circuit voltage (Voc) of 1.62 V and a lower Voc loss of 0.04 V for the corresponding tandem device.