Crystallization Kinetics and Morphology Control of Formamidinium–Cesium Mixed‐Cation Lead Mixed‐Halide Perovskite via Tunability of the Colloidal Precursor Solution

Abstract

spin-coating, [2,3] two-step interdiffusion, [4] dip-coating, [4] inkjet printing, [5,6] and spray pyrolysis. [7] These solution-based fabrication methods of perovskite films have been used in a variety of devices ranging from field-effect transistors, [8] light-emitting devices (LED), [9] photodetectors, [10,11] to solar cells. [2][3][4]12] Although most perovskite thin films are fabricated using a saltbased precursor solution, little effort has been dedicated to characterizing the structure and time-dependent evolution of the species present in solution and the impact this has on the quality of the resulting films. Furthermore, it has been shown that additives to the precursor solution can impact the nucleation rate, morphology, grain size, and crystallinity of the perovskite. Additives, such as polymers, [13] small molecules, [14,15] metal ions, [16] water, [17] or acids, [18][19][20][21][22][23][24] can play a vital role in material quality, enabling improvements in material stability, optoelectronic properties, and photovoltaic performance.Work by Yan et al. characterized the 1:1 CH 3 NH 3 I:PbI 2 stoichiometric precursor solution as a colloidal dispersion in a mother solution, rather than a pure solution. [25] When fabricated on planar substrate, this colloidal solution forms films with poor morphology, due to the presence of rodshaped colloids comprised of a soft coordination complex in the form of a lead polyhalide framework. [25] The authors show that the colloid distribution can be tuned by varying the organic to inorganic compound ratio in the precursor solution, using excess CH 3 NH 3 I (MAI) and CH 3 NH 3 Cl (MACl). However, the authors were unable to remove the 1D PbI 2 rod-like structures in the widely used stoichiometric 1:1 precursor solution. Adding excess organic cation to dissolve these colloids may be incompatible with recently published mixed-cation perov skites that contain both organic and inorganic cations, [26][27][28][29] since this will alter the precise [HC(NH 2 ) 2 ] + (formamidinium, FA) to Cs ratio, which is required to form the appropriate perovskite composition. In addition, FAI has a lower effective volatility than MACl, for instance, and hence excess quantities will not be so readily removed. Herein, we address this problem by utilizing acidic additives to trigger the dissolution of the Perovskite Solar CellsIn recent years, metal halide perovskite solar cells have gained tremendous attention, reaching certified efficiencies of 22.1% power conversion efficiency. [1] This rapid success is partly due to a versatile solution-based fabrication process which allows for a wide range of deposition techniques such as:

Elucidating the Role of a Tetrafluoroborate‐Based Ionic Liquid at the n‐Type Oxide/Perovskite Interface

“…[21,22] More recently, we have demonstrated ptype doping of the perovskite surface, which is an additional route to suppressing recombi nation at the perovskite/holetransport material interface. [15,32] Examples of this are: solvent mixtures, and the addition of amines, acids, or excess ammonium halides to the precursor solution. [15,32] Examples of this are: solvent mixtures, and the addition of amines, acids, or excess ammonium halides to the precursor solution.…”

“…[24] In cases where a "passivating" agent is added directly to the precursor solution, the resulting improvement in the optoelec tronic properties of the film is often due to the additive medi ating crystallization such that the resulting film has fewer grain boundaries and surface defects, for example, through altering the size and concentration of colloids in the solution. [15,[32][33][34][35] Improvements in the device performance have also been achieved by changing the electron selective layers in the device stack. [15,[32][33][34][35] Improvements in the device performance have also been achieved by changing the electron selective layers in the device stack.…”

Lead‐Free Solar Cells based on Tin Halide Perovskite Films with High Coverage and Improved Aggregation

“…[17] As olution of FA 0.75 MA 0.25 SnI 3 in DMSO was used as the precursor and the spin coating was performed at 5000 rpm for 100 s. Thec hlorobenzene (Ph-Cl) antisolvent was dropped onto the spinning film at the 63 s mark to promote faster crystallization and the perovskite film was afterwards annealed at 100 8 8C. [28][29][30][31][32][33][34] With the stable Pb intermediate states serving as nucleation centers,c rystal growth is retarded under room temperature,e nabling am ore controllable film formation process. 10 mol %ofSnF 2 was added in the precursor to stabilize the Sn perovskite and to prevent oxidation.…”

“…Thee ntire process was performed under Ar atmosphere.D etails of the method can be found in the supporting information. [29][30][31][32][33][34] Instead, for Sn-based perovskite,w es uggest that the small perovskite crystals serving as nucleation centers form immediately after antisolvent dripping.T he proposed mechanism is shown in Figure 2. [14] Thef ilm color turned from transparent to dark brown immediately after antisolvent dripping, but faded to at ransparent film again before the spinning was stopped at 100 s( Figure S2a).…”

Scalable Deposition Methods for Large‐area Production of Perovskite Thin Films

“…Moore et al [10] , Yan et al [19] , and later extended by Hu et al [20] and McMeekin et al [21] showed that lead polyhalide and lead salt colloidal particles played a role in crystallization growth and morphology for both simple methylammonium lead triiodide as well as mixed-cation varieties in DMF and DMSO systems. Moore et al [10] , Yan et al [19] , and later extended by Hu et al [20] and McMeekin et al [21] showed that lead polyhalide and lead salt colloidal particles played a role in crystallization growth and morphology for both simple methylammonium lead triiodide as well as mixed-cation varieties in DMF and DMSO systems.…”