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• A complex gradient passivation strategy was first proposed for inverted PSCs. • In(SCN 2 H 4) 3 Cl 3 effectively passivates defect states in bulk perovskite layer. • In(SCN 2 H 4) 3 Cl 3 facilitates hole extraction by modulating interface energy level alignment. • The efficiency and stability of inverted PSCs are significantly improved. To address the interfacial and bulk defects and mismatched energy band alignment at perovskite/hole transport layr (HTL) interface in inverted perovskite solar cells (PSCs), here a novel In(SCN 2 H 4) 3 Cl 3 complex diffusion doping strategy was proposed to construct CH 3 NH 3 PbI 3 :In(SCN 2 H 4) 3 Cl 3 composite layer for poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA)-based inverted PSCs. The In(tu) 3 Cl 3 effectively promotes the quality of perovskite films with larger perovskite crystal grains and flatter and denser surfaces. Both theoretical and experimental results reveal that the In(SCN 2 H 4) 3 Cl 3 can not only passivate interfacial and bulk defects in perovskite layer, but also form a spatial separation of photogenerated charges through CH 3 NH 3 PbI 3 :In(SCN 2 H 4) 3 Cl 3 heterojunction interface, which effectively decreases the charge recombination and enhances charge separation and transport property in PSCs. Moreover, the In(SCN 2 H 4) 3 Cl 3 optimizes energy band alignment at perovskite/PTAA interface and creates a polarization electric field at the PTAA/In(SCN 2 H 4) 3 Cl 3 interface directing toward PTAA layer, which promotes the hole transfer from the CH 3 NH 3 PbI 3 to the PTAA and increases the open-circuit voltage (V oc). As a consequence, optimized inverted PSCs with CH 3 NH 3 PbI 3 :In(SCN 2 H 4) 3 Cl 3 layer achieve high efficiency of 21.25 % and also show a significantly improved long-term stability under different aging test conditions. Our work provides a valuable guide for designing efficient and stable inverted PSCs. [ABSTRACT FROM AUTHOR]

[Display omitted] • The stable α-phase pure FAPbI 3 is prepared by imidazolium-based ionic liquid. • The efficiency enhances to 20.76% due to the high properties of pure FAPbI 3. • The unencapsulated pure FAPbI 3 devices show the excellent stability. The stable black α-phase is the key to attaining high-efficiency formamidinium lead triiodide (FAPbI 3) perovskite solar cells (PSCs). There have been some reports using different sizes of ions to stabilize α-phase FAPbI 3 ; unfortunately, they often affect the bandgap, stability and carrier dynamics, in addition to increasing the number of carrier trap sites. Herein, we present 1-allyl-3-methylimidazolium-chloride (AMICl) ionic liquid to stabilize the α-phase of pure FAPbI 3 perovskite without encountering the above adversities. It is found that the thermodynamic temperature of α-FAPbI 3 is significantly reduced from 150 °C to 145 °C by using AMICl, and the absorption edge of FAPbI 3 remains at 850 nm. The efficiency of a FAPbI 3 device with AMICl is increased from 18.47% for the control device to 20.76%, one of the highest values among pure FAPbI 3 solar cells. The high efficiency is attributed to the low trap density, long carrier lifetime and suppressed carrier recombination, thanks to the large grain size and good crystallinity of FAPbI 3 generated by the formation of intermediate products from PbI 2 and AMICl, demonstrating by density functional theory (DFT) calculation. Meanwhile, FAPbI 3 solar cells with AMICl exhibit the excellent environmental stability. The bare device without any encapsulation maintains about 87% of its initial value after being exposed to ambient for 24 days, significantly improved over the control device, which retains about 53% of its initial efficiency under the same conditions. [ABSTRACT FROM AUTHOR]