Successive surface engineering of TiO2 compact layers via dual modification of fullerene derivatives affording hysteresis-suppressed high-performance perovskite solar cellsElectronic supplementary information (ESI) available. See DOI: 10.1039/c6ta07876a

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 TiO 2 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-C 61 -butyric acid methyl ester (PC 61 BM) and an ethanolamine (ETA)-functionalized fullerene (C 60 -ETA) synthesized facilely via a one-pot nucleophilic addition reaction, the average power conversion efficiency (PCE) of the CH 3 NH 3 PbI 3 -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 TiO 2 interlayers. In contrast, single surface engineering of the TiO 2 layer with a PC 61 BM or C 60 -ETA layer alone results in only negligible changes in PCE, revealing the synergistic effect of these two fullerene derivatives: the PC 61 BM layer can passivate the traps on the TiO 2 surface, while the subsequent C 60 -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 TiO 2 ETL. The structural and morphological characterizations show that following dual surface modification of the TiO 2 layer with PC 61 BM and C 60 -ETA, both the surface coverage and crystallinity of the CH 3 NH 3 PbI 3 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 PC 61 BM may induce passivation of the traps on the TiO 2 surface and in the perovskite layer, remarkably, the hysteresis of the current-voltage response is dramatically suppressed following the dual surface modification. A new successive surface engineering method via a dual modification of TiO 2 compact layer by PC 61 BM and C 60 -ETA was developed, affording dr