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Recently, two‐dimensional (2D) structure on three‐dimensional (3D) perovskites (graded 2D/3D) has been reported to be effective in significantly improving both efficiency and stability. However, the electrical properties of the 2D structure as a passivation layer on the 3D perovskite thin film and resistance to the penetration of moisture may vary depending on the length of the alkyl chain. In addition, the surface defects of the 2D itself on the 3D layer may also be affected by the correlation between the 2D structure and the hole conductive material. Therefore, systematic interfacial study with the alkyl chain length of long‐chained alkylammonium iodide forming a 2D structure is necessary. Herein, the 2D interfacial layers formed are compared with butylammonium iodide (BAI), octylammonium iodide (OAI), and dodecylammonium iodide (DAI) iodide on a 3D (FAPbI3)0.95(MAPbBr3)0.05 perovskite thin film in terms of the PCE and humidity stability. As the length of the alkyl chain increased from BA to OA to DA, the electron‐blocking ability and humidity resistance increase significantly, but the difference between OA and DA is not large. The PSC post‐treated with OAI has slightly higher PCE than those treated with BAI and DAI, achieving a certified stabilized efficiency of 22.9%.

Efficient perovskite solar cells (PSCs) are mainly fabricated by a solution coating processes. However, the efficiency of such devices varies significantly with the aging time of the precursor solution used to fabricate them, which includes a mixture of perovskite components, especially methylammonium (MA), and formamidinium (FA) cations. Herein, how the inorganic–organic hybrid perovskite precursor solution of (FAPbI3)0.95(MAPbBr3)0.05 degrades over time and how such degradation can be effectively inhibited is reported on, and the associated mechanism of degradation is discussed. Such degradation of the precursor solution is closely related to the loss of MA cations dissolved in the FA solution through the deprotonation of MA to volatile methylamine (CH3NH2). Addition of elemental sulfur (S8) drastically stabilizes the precursor solution owing to amine–sulfur coordination, without compromising the power conversion efficiency (PCE) of the derived PSCs. Furthermore, sulfur introduced to stabilize the precursor solution results in improved PSC stability.

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The use of inexpensive, highly efficient, and long‐term stable hole‐transporting layers (HTLs) while facilitating the fabrication process has become a critical issue for PSC commercialization. Among organic HTLs, copper phthalocyanine (CuPc) has been increasingly studied owing to its low cost and excellent thermal stability. Nevertheless, CuPc has a low energy level in the conduction band, resulting in low efficiency due to a poor electron barrier. In this study, an efficient and stable PSC is fabricated by combining CuPc with an ultrathin poly(methyl methacrylate) (PMMA) interlayer, which is deposited on a [(FAPbI3)0.95(MAPbBr3)0.05] absorption layer (here, FAPbI3 and MAPbBr3 denote formamidinium lead triiodide and methylammonium lead tribromide, respectively). PMMA in perovskite has been found to reduce perovskite surface defects and series resistance as well as the electronic barrier to HTL. The optimum concentration of PMMA allows for the fabrication of the PSC with a PCE of 21.3%, which is the highest PCE for PSCs featuring metal phthalocyanines as the HTL reported to date. The stability of the encapsulated PSC exceeds 80% after 760 h at 85 °C under 85% RH conditions.

it provides a larger contact area between the TiO 2 and perovskite. Almost all of the best PSC efficiencies published so far have been obtained by using TiO 2 photoelectrodes. [15-18] Moreover, excellent long-term photo-stability has also been reported in PSCs fabricated using TiO 2 electrodes. [19] mp-TiO 2 layers for PSCs can be formed using spin-coating or screen-printing methods with TiO 2 pastes containing organic binders and surfactants (e.g., ethyl cellulose and terpineol), as well as organic solvents. [20,21] Subsequently, the TiO 2 layers require a follow-up such as high-temperature process to burn out the organic components contained in the TiO 2 paste. The biggest advantage of a hightemperature process would be a lowering of the series resistance of the layer, due to the strong contact between the TiO 2 particles and the TCO substrate. Therefore, in terms of efficiency, high-temperature-processed TiO 2 layers are very desirable; nevertheless, their use may be a large limiting factor in the fabrication of PSCs using a low-temperature continuous coating process or substrates based on an organic material (e.g., polyethylene naphthalate (PEN)). [22] Even PEN-based flexible substrates can be problematic, because they can bend if subjected to a heat treatment at 150 °C. Although recent studies have shown that SnO 2 can be used as an ETLs to fabricate PSCs at relatively low temperature, a heat treatment at temperature ≥150 °C is required to reach the best performances. [23,24] Meanwhile, formamidinium lead triiodide (FAPbI 3)-based perovskites are generally heat-treated at 150 °C to obtain an α-phase. If all the processes for high-efficiency PSCs can be conducted at ≈100 °C, their value will be very high in terms of using flexible substrates and facilitating large-area continuous processes. Currently, most high-efficiency PSCs are fabricated using spin coating, a simple and reliable technique that can produce very uniform thin films. However, since it is problematic to apply spin coating in large-area processes, other methods including blade coating, [25] slot die coating, [26] inkjet printing, [27] and spray coating [28] have been actively studied as scalable deposition techniques. Among these, spray coating has been already widely used in several industry fields and offers great advantages for the continuous production of PSC modules with large areas and a reduced material consumption. [29] So far, several groups reported the deposition of perovskite thin films or of entire ETLs, perovskites, and HTLs (required for the fabrication TiO 2 is one of the most efficient and widely used materials for electrontransporting layer (ETLs) in perovskite solar cells (PSCs). The formation of efficient TiO 2 layers is generally carried out at high temperature by baking at a temperature >400 °C or by vacuum deposition (e.g., atomic layer deposition and E-beam). In this study, the preparation of a TiO 2 ETL for PSCs is reported with excellent properties at low temperatures based on the synthesis of a stable TiO 2 colloidal ...