Inspired from Spiro-OMeTAD: developing ambipolar spirobifluorene derivatives as effective passivation molecules for perovskite solar cells

The imperfect perovskite/carrier selective layer (CSL) interface is the main reason for the unsatisfactory performance and poor stability of perovskite solar cells (PSCs). Therefore, introducing surface passivation molecules at selective interfaces has been widely used to improve the efficiency and stability of PSCs. In this work, we synthesize two kinds of surface passivation molecules, namely, 2-(4-ethylaminobenzene)-9,9 '-spirobifluorene (BSBF-NH2) and 2-(4-methylcarboxylicbenzene)-9,9 '-spirobifluorene (BSBF-COOH). The molecules are carefully designed with spirobifluorene cores and polar amino (carboxy) side groups. Compared with the control devices (18.27%), the PCE of BSBF-NH2 and BSBF-COOH passivated PSCs has been increased to 20.05% and 19.23%, respectively, mainly due to an improved V-oc and FF. The increased performance can be explained by the significantly accelerated hole extraction efficiency from the perovskite layer up to the hole transport material (HTM) after the surface treatment (BSBF-NH2 passivated), as confirmed from the femtosecond transient absorption (fs-TA) measurement. Besides, benefitting from the anchoring groups (-NH2/-COOH), the defect density of the devices is reduced. Moreover, thanks to the hydrophobic nature of the organic molecule, the stability of PSCs can be improved for BSBF-NH2 passivated devices. Finally, the results of this research are insightful for designing new organic passivation molecules for perovskite optoelectronic applications.