Reducing Burn-In Loss of Organic Photovoltaics by a Robust Electron-Transporting Layer

It is revealed that instability of interface between photoactive layer and electron-transporting layer (ETL) is one of the causes of the rapid degradation of organic photovoltaics (OPV) performance during initial operation (burn-in loss) under the light soaking. The stability of OPV is greatly enhanced by applying a robust ETL composed of TiO2 nanoparticles (TNPs). The TNPs bound with pi-pi interactive 3-phenylpentane-2,4-dione (TNP-Ph) form more robust ETLs than those bound with van der Waals interactive 3-methyl-2,4-pentanedione TNP (TNP-Me). The OPV with TNP-Ph maintains 73% of its initial power conversion efficiency (PCE) after 1000 h of light soaking, whereas the PCE of OPV with TNP-Me substantially reduces to 25% of initial PCE. The impedance analysis reveals that the burn-in loss is due to increase of resistance at the TNP ETL/photoactive layer interface during the light soaking. The transmission electron microscopy analysis shows that the TNP-Ph maintains most clear and robust interface with photoactive layer after the light-soaking test. This is attributed to the strong pi-pi interaction between phenyl rings of TNP-Ph. However, the TNP-Me bound with van der Waals interactive organic ligands penetrates the photoactive layer during the light-soaking test.