Perovskite solar cells with porous carbon electrodes can be manufactured by low-cost screen printing and show promising operational stability owing to the absence of metals in the electrodes compared to other device configurations. Yet, power conversion efficiency has been stuck around 19% due to high charge recombination rates. Improving our understanding of the mechanisms underlying charge dynamics is not straightforward because of the complex structure of the device, which includes a porous scaffold comprising electrode, transport and insulation layers into which the perovskite material is infiltrated. Now, Hongwei Han, Anyi Mei, Furi Ling and team across China demonstrate that charge recombination primarily occurs at the electron transport layer/perovskite interface and is mediated by defects present in the titanium dioxide electron transport layer.

The researchers identify oxygen vacancies as the predominant defect at the surface of the titanium dioxide that trap charges, increasing the chance of recombination. The presence of oxygen vacancies is accompanied by reduction of titanium ions from Ti⁴⁺ to Ti³⁺. Han and team reason that, since Ti⁴⁺ is a hard Lewis acid cation, a hard Lewis base anion can oxidize Ti³⁺ back to Ti⁴⁺ while also neutralizing the oxygen vacancy. The anion can be added to the system in the form of a salt. The researchers look at the electrostatic potential of different anions and cations to identify the salt — ammonium phosphate — that has the strongest binding to titanium dioxide and hence better ability to suppress the oxygen vacancy defect.