Molten‐Salt‐Assisted CsPbI₃ Perovskite Crystallization for Nearly 20%‐Efficiency Solar Cells

Abstract

efficiency of laboratory-scale all-inorganic solar cells has been improved to 20.37%, [5] approaching ~70% of the efficiency limit based on the Shockley-Queisser (S-Q) theory. [9][10][11] Among all the high-performance all-inorganic perovskite devices, the photocurrent and fill factor have exceeded 95% and 90% of their theoretical S-Q limits, respectively, while the open-circuit voltage (V oc ) falls at ≈80% of the limit. Therefore, there is relatively larger room for improvement in the V oc for yielding higher power conversion efficiency (PCE). [5,[12][13][14][15] The V oc depends on the dynamics of charge carrier recombination, which is connected to the non-radiative recombination processes. [16][17][18][19] Defects usually scatter the carriers as non-radiative recombination reaction centers. [20][21][22][23] Therefore, preparing high-quality perovskite films with low-defect density is a prerequisite for high-performance photovoltaic devices. The quality of the perovskite active layer can be manipulated by the crystallization processes, thus dynamic control of crystallization appears to be very important. In a common solvent crystallization process, the perovskite grain growth is mainly completed in the initial solvent evaporation process because: 1) solvent evaporation leads to the super-saturation of the solute, producing a mass of seed crystals; 2) solvent evaporation drives the migration of the perovskite precursor colloids, which is beneficial to the grain growth. Subsequent extended annealing has faint effect on the grain growth. We assumed that if the "reactive solvent medium" can remain for a longer time, the colloids will react completely and the grains can merge better. Inspired by this idea, we consider molten salt (MS) synthesis, a classical method for preparing functional materials owing to its simplicity, speed, large-scale compatibility, and low cost. [24][25][26][27][28] The MS melts can exhibit homogeneous heat and mass transfer to carry out chemical transformation even at low temperature. [24,29] MS methods have been widely used in industrial production. For example, they are regarded as ideal reactors for the closed Th-U cycle due to their lower fissile inventory [30] and are applied in lignite pyrolysis to affect the products. [31] The MS processes have been applied not only in chemical reactions but also in material morphology manipulation. [32][33][34] So far, MS can be classified into two types: solid high-temperature MS, [27,34,35] and low-temperature MS (ionic liquids [ILs]) [32,36,37] depending on their states within a certain temperature range. The solid MSs show high ionic conductivity only in the liquid state, and thereby should be used at high temperatures above their melting points. ILs are considered Dynamic manipulation of crystallization is pivotal to the quality of polycrystalline films. A molten-salt-assisted crystallization (MSAC) strategy is presented to improve grain growth of the all-inorganic perovskite films. Compared with the traditional solvent annealing, MSAC enables...

Vacuum‐Assisted Thermal Annealing of CsPbI3for Highly Stable and Efficient Inorganic Perovskite Solar Cells

“…Moreover, the inside pinholes could provide the channels for moisture invasion and cause phase transformation from the black perovskite phase to the yellow non‐perovskite phase, affecting the stability of the resultant devices. As such, various additives were employed in the precursor solution to adjust the perovskite crystallization [32–36] . For instance, Liu's group recently reported a molten‐salt‐assisted crystallization (MSAC) strategy for improving grain growth of the CsPbI 3 perovskite films [32] .…”

“…As such, various additives were employed in the precursor solution to adjust the perovskite crystallization. [32][33][34][35][36] For instance, Liu's group recently reported a molten-salt-assisted crystallization (MSAC) strategy for improving grain growth of the CsPbI 3 perovskite films. [32] The MSAC promoted intensive mass transfer in the present of the fluent liquid medium, yielding high-quality CsPbI 3 perovskite films with suppressed pinhole and crack formation.…”

Defect healingviaa gradient cooling strategy for efficient all-inorganic perovskite solar cells

“…For example, Zhang et al used a moltensalt-assisted strategy to improve the grain growth of CsPbI 3 and achieved a PCE of 19.83%. 9 By exploiting methylammonium chloride (MACl) to control the crystallization process and octylammonium iodides for surface passivation, a PCE of 20.37% was reported for the CsPbI 3 perovskite. 10 As compared to the CsPbI 3 perovskite, the CsPbBr 3 perovskite possesses stronger stability.…”

Air‐Processed Carbon‐Based Cs0.5FA0.5PbI3–Cs4PbI6 Heterostructure Perovskite Solar Cells with Efficiency Over 16%

“…[21,22] Therefore, MA-free PSCs have received extensive attention in recent years. [14,[23][24][25][26][27][28] Currently, MA-free perovskites mainly include FAPbI 3 , CsPbI 3 , and Cs x FA 1-x PbI 3 with mixed cations. Because the ionic radius of FA þ (2.53 Å) is too large and Cs þ (1.88 Å) is too small, the lead iodine octahedral [PbI 6 ] can easily distort.…”