Investigation on the Mechanism of Radical Intermediate Formation and Moderate Oxidation of Spiro-OMeTAD by the Synergistic Effect of Multisubstituted Polyoxometalates in Perovskite Solar Cells.

Conventional oxidation of 2,2',7,7'-tetrakis[ N , N -di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-OMeTAD) by air would bring various drawbacks for perovskite solar cells (PSCs), such as low power conversion efficiency (PCE) and poor stability. Here, a series of heteroatom-substituted Keggin-type polyoxometalates (POMs), H 4 PMo 11 VO 40 (PMo 11 V), H 5 PMo 10 V 2 O 40 (PMo 10 V 2 ), and H 6 PMo 9 V 3 O 40 (PMo 9 V 3 ) are prepared and applied as p-type dopants to realize quantitative and controllable oxidation of Spiro-OMeTAD under an inert condition. The possible mechanism and electron donor regions in the oxidation of Spiro-OMeTAD are investigated using two-dimensional nuclear magnetic resonance (NMR) spectra and the relationship between POM structures and the oxidation degree of Spiro-OMeTAD is proposed. In addition, the synergistic effect of heteroatoms makes V 2 -substituted PMo 10 V 2 exhibit appropriate oxidation of Spiro-OMeTAD and promoted the highest efficient hole extraction as well as the decreased charge recombination. Therefore, the champion device doped with PMo 10 V 2 shows a PCE of 20.41% and a superior open circuit voltage ( V oc ) of 1.133 V, surpassing that of the pristine device (18.61%). This work presents a fresh perspective to the controllable oxidation of Spiro-OMeTAD employing economical inorganic POM dopants, which would promote the commercialization of PSCs.