Towards high efficiency solution processable inverted bulk heterojunction polymer solar cells using modified indium tin oxide cathode

High efficiency inverted regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM)-based polymer solar cells (PSCs) using an indium tin oxide (ITO) cathode, modified with a solution-processed ionic liquid-functionalized carbon nanoparticles (ILCNs) layer, was demonstrated. ILCNs-modified ITO cathode has a very good thermal stability and a superior surface electronic property suited for application in polymer solar cells. A power conversion efficiency (PCE) of 3.2% was obtained for inverted PSCs measured under AM1.5G illumination of 100 mW/cm2, which is comparable to PCE of a reference P3HT:PCBM-based solar cell having a conventional structure. There was almost no deterioration in the performance of inverted PSCs that were aged over a post-annealing temperature range from 100 to 200 °C. The work function of ILCNs-modified ITO cathode annealed at different temperatures was also measured using ultraviolet photoelectron spectroscopy. It was found that the work function of 4.4 eV, measured for a bare ITO electrode, decreased to 3.8 eV when it was modified with ILCNs, and was not dependent on the post-annealing temperature.