Controlling the Formation Process of Methylammonium‐Free Halide Perovskite Films for a Homogeneous Incorporation of Alkali Metal Cations Beneficial to Solar Cell Performance

Abstract

efficiency (PCE) for a perovskite solar cell (PSC) is 25.7%. [12] However, PSCs still need further progress to overcome instability issues caused by moisture, oxygen, and heat, while maintaining or improving their PCE. The composition of ABX 3 based 3D halide perovskite greatly affects the overall performance of perovskite solar cells. Different components can bring desired properties, such as improved optical properties or structural stability. To achieve better performance of PSC, perovskite materials composition has been complexified by combining several A + monovalent elemental and organic cations, from double-cation (e.g., MA-FA, [13,14] Cs/FA, [15,16] Rb/FA, [17] MA + being methylammonium and FA + being formamidinium) to triple-cation (e.g., Cs/MA/ FA, [8,18,19] Rb/MA/FA, [20] K/Cs/FA [21] ), and quadruple-cation (Rb/Cs/MA/FA [18,22] ). The incorporation of other cations, namely MA + , Cs + , Rb + and K + , aims at alleviating the formation of photo inactive δ-FAPbI 3 phase and/or suppress defects in perovskite film. Although MAbased perovskite solar cells can achieve very high PCE, [23][24][25] the presence of MA can favor PVK degradation since MA is easily released and decomposed under heating and humid atmosphere. A lot of works have been dedicated to enhancing the stability of the black α-FAPbI 3 phase, notably to its entropic stabilization by incorporating Cs. [18,19] The structural engineering by introducing elemental monovalent cations can shed light on the long-term stability of PSCs. For instance, introducing another alkali metal, Rb (same column as Cs and K), into FAPbI 3 perovskite has been shown to enhance both the photovoltaic performance and the moisture stability. [17] Moreover, the nonradiative loss and photoinduced ion migration in perovskite film are significantly reduced by using potassium halide to passivate the defects at near surface and grain boundaries. [16,26] All these considerations led us to develop multialkali metal cations (m-AMCs) KRbCsFAPbI 3 3D perovskites. However, phase segregation might happen on m-AMCs perovskite films and the distribution of each element is worth noting. Especially, the homogeneous distribution of Rb + is sought for high stability and performances. [27] Incorporating multiple cations of the 1A alkali metal column of the periodic table (K + /Rb + /Cs + ) to prepare perovskite films is promising for boosting photovoltaic properties but requires a uniform distribution. The effects of NH 4 Cl additives and alkali metal cations (K + /Rb + /Cs + ) on the one-step formation process of methylammonium-free, formamidinium-based, iodide perovskite films are analyzed in a step-by-step manner. NH 4 Cl improves the solubility of PbI 2 in solution by forming an intermediate and then favors the perovskite phase formation. Moreover, during the annealing process, this additive is shown to increase grain size, to improve crystallinity and to suppress PbI 2 formation. K at low concentration is always homogeneously distributed across the film thickness. On the other hand, C...

Stable Yellow Light-Emitting Diodes Based on Quasi-Two-Dimensional Perovskites

“…We noticed that the incorporation of a higher molar ratio of Rb + leads to an increased exciton peak at 509 nm and a decreased exciton absorption peak at 546 nm, which originate from the phases of n = 1 and 2, respectively . This change results from the increase in the formation energy of Rb + occupying the Cs-site with varying the molar ratios from 0 to 40%. − The XRD spectrum of (BA) 2 Cs 1– x Rb x Pb 2 I 7 films is shown in Figure d. It exhibits a strong diffraction peak at 6.4°, indicating the significant increase of the n = 1 phase, which is also consistent with the absorption spectrum .…”

Methylammonium Chloride Additive in Lead Iodide Optimizing the Crystallization Process for Efficient Perovskite Solar Cells

“…The composition of the spindle-shaped grains may be MACl or an intermediate phase formed by the reaction of MACl and PbI 2 . The intermediate phase is distributed on the surface of the PbI 2 layer and may serve as the nucleation site of perovskite grains during subsequent spin-coating of the organic layer, promoting the formation of the perovskite phase [29][30][31][32]. The MACl component in it, sublimed in the subsequent annealing process, may optimize the perovskite film formation process and increase the grain size, which was confirmed in the subsequent experiments.…”

What are Methylammonium and Solvent Fates upon Halide Perovskite Thin‐Film Preparation and Thermal Aging?

“…The variation of the DMSO content with the TAP time in Figure 1d reveals that this solvent is rapidly eliminated from the layer and that a low level is found after only 30s. inner and an upper part, as described in our previous work [32] and in the Supporting Information (Figure S6 and related comment, Supporting Information). We then quantified the solvent content in these layers by integrating the curves.…”