Synergetic effects of electrochemical oxidation of Spiro-OMeTAD and Li+ ion migration for improving the performance of n–i–p type perovskite solar cells. Issue 12 (26th February 2021) - Ding, Changzeng
Abstract : Oxidation of solar cells leads to Li + ion diffusion, which increases the conductivity of the Spiro-OMeTAD layer and the built-in potential within the cells. The synergetic effects improve charge injection at both interfaces and device performance. 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 H2 O, 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.