Mixed 3D-2D passivation treatment for mixed-cation lead mixed-halide perovskite solar cells for higher efficiency and better stability

Abstract

Layered low‐dimensional perovskite structures employing bulky organic ammonium cations have shown significant improvement on stability but poorer performance generally compared to their 3D counterparts. Here, a mixed passivation (MP) treatment is reported that uses a mixture of bulky organic ammonium iodide (iso‐butylammonium iodide, iBAI) and formammidinium iodide (FAI), enhancing both power conversion efficiency and stability. Through a combination of inactivation of the interfacial trap sites, characterized by photoluminescence measurement, and formation of an interfacial energetic barrier by which ionic transport is reduced, demonstrated by Kelvin probe force microscopy, MP treatment of the perovskite/hole transport layer interface significantly suppresses photocurrent hysteresis. Using this MP treatment, the champion mixed‐halide perovskite cell achieves a reverse scan and stabilized power conversion efficiency of 21.7%. Without encapsulation, the devices show excellent moisture stability, sustaining over 87% of the original performance after 38 d storage in ambient environment under 75 ± 20% relative humidity. This work shows that FAI/iBAI, is a new and promising material combination for passivating perovskite/selective‐contact interfaces.

3D/2D Bilayerd Perovskite Solar Cells with an Enhanced Stability and Performance

“…Organic salt, composed of bulky organic cations and counter halide (see Section 4 . Organic salt for 2D perovskite for details), is dissolved in isopropyl alcohol (IPA) [ 24 , 25 , 26 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 ] or chlorobenzene (CB) [ 36 , 42 ] and dripped on the surface of a spinning 3D perovskite film, whose excess PbI 2 readily reacts with the organic salt, leading to the formation of a 2D capping layer on the surface during the post annealing process ( Figure 2 a). Alternatively, an immersion of the 3D perovskite film in the IPA solution containing the organic salt for 2D can be adopted, instead of dripping the solution, in order to induce the 2D capping layer ( Figure 2 b) [ 25 ].…”

“…Notably, a salt mixture of 3D and 2D is used in the in-situ growth method to form a 3D/quasi-2D layered structure. An IPA solution containing BAI and formamidinium iodide (FAI) with different ratios was employed in an attempt to form the capping layer on top of the 3D perovskite with excess PbI 2 , resulting in the highest photovoltaic performance when the same molar ratio between BAI and FAI was employed (BAI:FAI = 1:1) to form a quasi-2D layer—(BA) 2 (FA) n−1 Pb n I 3n+1 —as confirmed by x-ray diffraction (XRD) [ 39 ]. Similarly, penylethylammonium (PEA) was blended with FA at the same molar ratio, which led to a capping layer composed of a double phase comprised of (PEA) 2 PbI 4 and (PEA) 2 (FA)Pb 2 I 7 [ 33 ].…”

“…Figure 4 a shows the cross-sectional morphology of 3D/2D bilayered structures where a 30 nm-thick 2D capping layer (generally ranging from 9 mm [ 25 ] to 100 nm [ 31 ]) is grown on the surface of the 3D bulk film. Figure 4 b shows a surface image of the compact 3D film, where the white flake at grain boundaries indicates the excess PbI 2 exposed on the 3D surface [ 24 , 29 , 37 , 39 ]. On the other hand, a distinctive surface morphology is observed by the in-situ-grown 2D capping layer, which fully covers the 3D film ( Figure 4 c).…”

Halide perovskite nanotube toward energy applications: A first‐principles investigation

“…On the other hand, low‐dimensional halide perovskite materials are receiving much attention owing to their unique structures and properties. For instance, two‐dimensional (2D) halide perovskites and halide perovskite molecules without the corner‐shared repeating unit have been synthesized . Continuous efforts are spent on identifying new types of low‐dimensional halide perovskite materials with unique structures.…”

Structural and optical properties of 2D Ruddlesden‐Popper perovskite (BA)2(FA)n−1PbnI3n+1 compounds for photovoltaic applications

“…Recently, 3D‐2D passivation treatment on FA‐based perovskite solar cells has been reported for improving the efficiency and stability of devices . However, whether FA‐based 2D perovskites were formed or not during passivation remains unknown, and the crystal structure and nature of these materials for photovoltaic application remain unclear.…”