Synergetic effects of electrochemical oxidation of Spiro-OMeTAD and Li+ ion migration for improving the performance of n–i–p type perovskite solar cells

Abstract

n–i–p Type perovskite solar cells generally require air oxidation of the Spiro-OMeTAD layer to achieve high power conversion efficiency (PCE). However, the detailed oxidation mechanism is still not fully understood. In this paper, oxidation of Spiro-OMeTAD was demonstrated via a non-contact electrochemical route using UV-Vis absorption, laser beam induced current (LBIC) imaging and secondary ion mass spectrometry (SIMS) profiling of the Spiro-OMeTAD films. At the cathode, oxygen is reduced to form OH⁻ with the help of H₂O, while the anodic reaction is the oxidation of Spiro-OMeTAD to form Spiro-OMeTAD⁺. Diffusion of Li⁺ towards the surface of the Ag electrode completes the electrochemical cycle and increases the conductivity of the hole-transporting layer. SIMS analyses of the completed devices demonstrate that the oxidation of Spiro-OMeTAD also leads to migration of Li⁺ through the perovskite layer into SnO₂, which supposedly leads to an increase of the built-in voltage. We verify these results by incorporation of the experimentally measured Li⁺ concentration into a numerical drift-diffusion simulation, to replicate solar cell J–V-curves. This work provides a new insight into the oxidation of Spiro-OMeTAD in perovskite solar cells, and demonstrates that Li⁺ migration is involved in the oxidation of Spiro-OMeTAD.