Successive surface engineering of TiO2 compact layers via dual modification of fullerene derivatives affording hysteresis-suppressed high-performance perovskite solar cells

Abstract

Interfacial engineering is critical for highly efficient charge carrier transport in perovskite solar cells (PSCs). Herein, we developed a new method, called successive surface engineering, that affords PSCs with enhanced efficiency and dramatically suppressed current–voltage hysteresis. Upon modifying the TiO₂ compact layer, which is commonly used as an electron transport layer (ETL) in regular-structure (n–i–p) planar heterojunction (PHJ) PSCs, by successively incorporating [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) and an ethanolamine (ETA)-functionalized fullerene (C₆₀-ETA) synthesized facilely via a one-pot nucleophilic addition reaction, the average power conversion efficiency (PCE) of the CH₃NH₃PbI₃-based PHJ-PSC devices increased from 13.00% to 16.31%; the best PCE attained was 18.49%, which, to our knowledge, represents the highest PCE reported to date for regular-structure PHJ-PSCs devices based on fullerene-modified TiO₂ interlayers. In contrast, single surface engineering of the TiO₂ layer with a PC₆₁BM or C₆₀-ETA layer alone results in only negligible changes in PCE, revealing the synergistic effect of these two fullerene derivatives: the PC₆₁BM layer can passivate the traps on the TiO₂ surface, while the subsequent C₆₀-ETA layer not only improves the wettability of the perovskite film on the ETL but also facilitates electron transport across the interface between the perovskite and the TiO₂ ETL. The structural and morphological characterizations show that following dual surface modification of the TiO₂ layer with PC₆₁BM and C₆₀-ETA, both the surface coverage and crystallinity of the CH₃NH₃PbI₃ perovskite film are improved. Steady-state photoluminescence decay and electrochemical impedance spectroscopic studies manifest that the dual surface modification substantially improves the charge extraction efficiency and suppresses charge recombination. As a consequence, this dual surface modification leads to an obvious increase of the short-circuit current density (J_sc), which contributes primarily to the PCE enhancement. Additionally, because PC₆₁BM may induce passivation of the traps on the TiO₂ surface and in the perovskite layer, remarkably, the hysteresis of the current–voltage response is dramatically suppressed following the dual surface modification.