Kinetic Stabilization of the Sol–Gel State in Perovskites Enables Facile Processing of High‐Efficiency Solar Cells

Abstract

With record power conversion efficiencies (PCE) close to 24%, perovskite solar cells (PSCs) hold great promise to combine high photovoltaic performance with low processing cost. [1][2][3][4] This combination can be ascribed to several factors, such as the remarkable optoelectronic properties of perovskites-evidenced by a sharp absorption onset, a small Urbach energy, and a low exciton binding energy-as well as the fact that perovskites use earth-abundant elements. [5][6][7] The organic-inorganic metal halide perovskites used in such devices feature the characteristic AMX 3 structure, where A is an organic or inorganic cation, most often methylammonium (CH 3 NH 3 + , MA + ), formamidinium (HC(NH 2 ) 2 + , FA + ), or cesium (Cs + ), M is a metal cation, such as Pb 2+ or Sn 2+ , and X is a monovalent anion (halide ion Cl, Br, or I). [8][9][10] Various deposition techniques have been developed to obtain polycrystalline perovskite films, including spin-, dip-, and blade-coating, as well as vacuumbased processes. [11,12] To date, one-step spin coating remains the most popular and facile implementation to fabricate high-quality hybrid perovskite thin films from solution. The device efficiency and stability are strongly dictated by the electronic properties and trap state densities in the perovskite film. These, in turn, are influenced by the film crystallization, its morphology, and the presence of defects and imperfections. [13,14] However, despite the simplicity of the one-step deposition, the exact mechanisms underlying the growth process, from precursor solution to solid-state film, and the crystallization mechanism within the sol-gel state, are still poorly understood. This lack of information is partially caused by the heavy reliance on post-deposition ex situ characterization methods to date. [15,16] Recently, in situ time-resolved grazing-incidence wide-angle X-ray scattering (GIWAXS) has emerged as a powerful technique to study the microstructural evolution from the solution-precursor to the solid state, revealing the crystallization behavior and formation of crystalline intermediates and byproducts. [17,18] Using in situ GIWAXS methods, the classic MAPbI 3 perovskite material has already been investigated; Gong and coworkers identified the Perovskite solar cells increasingly feature mixed-halide mixed-cation compounds (FA 1−x−y MA x Cs y PbI 3−z Br z ) as photovoltaic absorbers, as they enable easier processing and improved stability. Here, the underlying reasons for ease of processing are revealed. It is found that halide and cation engineering leads to a systematic widening of the anti-solvent processing window for the fabrication of high-quality films and efficient solar cells. This window widens from seconds, in the case of single cation/ halide systems (e.g., MAPbI 3 , FAPbI 3 , and FAPbBr 3 ), to several minutes for mixed systems. In situ X-ray diffraction studies reveal that the processing window is closely related to the crystallization of the disordered sol-gel and to the number of crystalline byprodu...

Multifunctional Phosphorus‐Containing Lewis Acid and Base Passivation Enabling Efficient and Moisture‐Stable Perovskite Solar Cells

“…Moreover, a wide antisolvent processing window is beneficial to prepare reproducible MLMP films. [ 8,9 ] Therefore, MLMPs have been established as ideal candidates in PSCs with the remarkable PCE and long‐term stability.…”

CsI‐Antisolvent Adduct Formation in All‐Inorganic Metal Halide Perovskites

“…An antisolvent quench is also incorporated to intentionally control the exact nucleation time through the introduction of an antisolvent that supersaturates the precursor solution and removes excess precursor solvent . The antisolvent quench must happen within a specific nucleation window since quenching too early will not cause supersaturation and quenching after the film has already begun to nucleate will not induce the ideal nucleation site density . When the quenching occurs within this window, heterogeneous nucleation is promoted at the initial crystallization stage and minimizes pinholes in the resulting films .…”

Impact of Cesium/Rubidium Incorporation on the Photophysics of Multiple‐Cation Lead Halide Perovskites

“…Overall, this may result from the improved crystal quality during processing. [15,16] The second-order recombination rate constants, namely k 2 , are on the order of 10 À11 cm 3 s À1 . It should be noted that the photon reabsorption is not taken into account here, and this may cause underestimation of the second-order recombination.…”

“…[12][13][14] This leads not only to increased performance, but also to better thermal stability and improved device reproducibility. [15,16] However, the precise reasons for these improvements are still unclear and fundamental studies of the effect of Cs/Rb-ion incorporation are scarce. In fact, most work to date has mainly focused on how the incorporation contributes to the stability of the perovskite phase and changes the perovskite's entropy, [12,14,17] whereas only few investigations have addressed the impact of incorporation of monovalent cations on the photophysics of lead halide perovskites.…”