High-efficiency polymer solar cells with low temperature solution-processed SnO2/PFN as a dual-function electron transporting layer

Electron transporting layers (ETLs) existing between active layers and an electrode play a critical role in improving the performance parameters of polymer solar cells (PSCs). Traditional wide bandgap semiconductor metal oxides as ETLs usually require high temperature fabrication process, which is incompatible with flexible substrates as well as roll-to-roll manufacturing technology. Herein, we demonstrate high-efficiency PSCs with integrated low temperature solution-processed tin dioxide (SnO2) nanocrystals and a poly-[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)](PFN) stacked structure as an ETL with excellent photoelectric performance. A combination of characterizations including ultraviolet photoelectron spectroscopy, transient photovoltage and transient photocurrent measurements, and impedance spectroscopy were used to systematically study the interfacial effects induced by the SnO2/PFN ETL. It shows that SnO2 nanocrystals can serve as an efficient electron-selective buffer except for an unmatched energy level, while the PFN interlayer can intentionally reduce the energy misalignment of devices through forming dipoles at the interface and effectively reduce the work function. With these dual functions, the-state of the-art PSCs based on SnO2/PFN outperform those based on SnO2-only in power conversion efficiency, from 4.31% to 11.05%. We believe that the SnO2/PFN bilayer structure integrating the function of enhanced electron extraction and reduced charge recombination can be applied to produce higher performance devices by using a low temperature solution-processed technique.