Zwitterion‐Functionalized SnO₂ Substrate Induced Sequential Deposition of Black‐Phase FAPbI₃ with Rearranged PbI₂ Residue

Abstract

Up to now, most of the highly efficient and stable black-phase FAPbI 3 perovskites are fabricated through one-step deposition using antisolvent, which is not convenient for large-scale fabrication because of its narrow processing windows. [5][6][7] Another commonly used method of two-step sequential deposition has wide processing windows, which offers several advantages for the production of larger modules. But it is somehow difficult to fabricate FAPbI 3 perovskite using this method, which can be ascribed to the difficulty of intercalating FA + with large ionic radius into the [PbI 6 ] 4framework in sequential deposition. This process could also introduce defects in black-phase FAPbI 3 perovskite film, which would lead to undesirable nonradiative recombination. [8] To overcome these drawbacks, methylammonium (MA + ), cesium (Cs + ), and bromide (Br -) ions have been doped into FA-based perovskites via sequential deposition, which can reduce the formation energy of blackphase FAPbI 3 and inhibit the defects, [9][10][11] but these doping strategies introduce some side effects such as enlarged bandgap and potential phase segregation issues. [12][13][14] The double-edged sword effect of lead iodide (PbI 2 ) residue is another issue in sequential deposition. Moderate residual PbI 2 at the grain boundary of perovskite can passivate defects and reduce hysteresis, [15] whereas sometimes the residual PbI 2 can deteriorate the photostability of PSCs. [16,17] Thus, good control on the concentration and distribution of residual PbI 2 also plays a crucial role in the formation of high-quality black-phase FAPbI 3 perovskite on the photovoltaic performance. Herein, we report employing a zwitterion of formamidine sulfinic acid (FSA) to functionalize tin(IV) oxide colloid solution (SnO 2 ) layer for sequential deposition of black-phase FAPbI 3 for highly efficient and stable perovskite photovoltaics. Besides helping with the crystallization of black-phase FAPbI 3 perovskite, the incorporation of FSA zwitterion can rearrange the stack direction, orientation, and distribution of residual PbI 2 to reduce its side effect. The FSA zwitterion also acts as interfacial dipole to enhance the conductivity and decrease the work function of SnO 2 electron-transport layer (ETL). All these advantages Black-phase formamidinium lead iodide (FAPbI 3 ) with narrow bandgap and high thermal stability has emerged as the most promising candidate for highly efficient and stable perovskite photovoltaics. In order to overcome the intrinsic difficulty of black-phase crystallization and to eliminate the lead iodide (PbI 2 ) residue, most sequential deposition methods of FAPbI 3based perovskite will introduce external ions like methylammonium (MA + ), cesium (Cs + ), and bromide (Br -) ions to the perovskite structure. Here a zwitterion-functionalized tin(IV) oxide (SnO 2 ) is introduced as the electrontransport layer (ETL) to induce the crystallization of high-quality black-phase FAPbI 3 . The SnO 2 ETL treated with the zwitterion of formamidine sulfinic ...

Modulation of nucleation and crystallization in PbI2 films promoting preferential perovskite orientation growth for efficient solar cells

“…In addition, the –NH 3 + and –SO 3 − terminal groups in PFAT can further passivate inherent defects in perovskite films. 13,35,53 The –NH 3 + passivates positively charged iodine vacancies by delivering a lone pair of electrons, and the –SO 3 − can interact with undercoordinated Pb 2+ ions.…”

“…2–9 FAPbI 3 -based PSCs deliver extremely efficient PCEs, and their perovskite absorbers are generally prepared by a one-step antisolvent or a two-step sequential deposition method. 10–16 In particular, the two-step approach has excellent operability and vast potential for commercialization, thanks to the possibility of prior regulation of the lead iodide (PbI 2 ) templates. However, the heavy dependence of perovskite crystal growth on PbI 2 substrates and organic ammonium salt infiltration means that perovskite grains can easily become disordered and introduce unwanted non-radiative recombination sites, which drastically compromise the photovoltaic efficiency and stability of PSCs.…”

“…Therefore, the growth of PbI 2 films is crucial for the subsequent intercalation of organic ammonium salts and thus the conversion to the perovskite phases in the typical two-step sequential deposition method. 13,17,19–22…”

Reduced Surface Hydroxyl and Released Interfacial Strain by Inserting CsF Anchor Interlayer for High‐Performance Perovskite Solar Cells

“…The work function (WF) can be calculated by the equationwhere E F is the Fermi level and E cut off is the cutoff of the UPS spectrum in high‐binding energy range. [ 21 ] The WF of SnO 2 and CsF–SnO 2 were calculated to be 4.39 and 4.19 eV and the conduction band minimum (CBM) was calculated to be 4.53 and 4.24 eV, respectively. The Kelvin probe measurement in Figure 1C also confirmed the variation of WF, and the illustration of energy band arrangement of SnO 2 and CsF–SnO 2 is plotted in Figure S4, Supporting Information.…”

Myth behind Metastable and Stable n-Hexylammonium Bromide-Based Low-Dimensional Perovskites

“…Several strategies have been explored to minimize non-radiative recombination losses at interfaces and thus improve the V oc of devices. Many chemicals, such as organic halide salts, − ionic liquids, , and zwitterions, − can be used to post-treat on the perovskite surface to decrease the trap density. Lead iodide (PbI 2 ) is a commonly used wide-band gap semiconductor that can reduce the occurrence of non-radiative recombination at perovskite grain boundaries by the formation of type-I band alignment. , At present, it is generally considered to form either an amorphous or a mixed-halide two-dimensional (2D) passivation layer. ,, Such type-I band alignment at grain boundaries can also be constructed by converting PbI 2 into low-dimensional perovskite in situ. ,,− Long-chain alkylammonium bromides, such as n -butylammonium bromide (BABr) and n -octylammonium bromide (OABr), are some widely used passivation compounds.…”