Balancing Lattice Strain by Embedded Ionic Liquid for the Stabilization of Formamidinium-Based Perovskite Solar Cells

Abstract

Formamidinium (FA)-based perovskites remained state-of-the-art in the field of perovskite solar cells (PSCs) owing to the exceptional absorption and carrier transport properties, while the transition from photoactive (α-) to photoinactive (δ-FAPbI 3 ) phase is the impediment that causes performance degradation and thus limits the deployment of FA-based PSCs. The unfavorable phase transition originates from tensile strain in the FAPbI 3 crystal lattice, which undergoes structural reorganization for lattice strain balancing. In this work, we found that the ionic liquid (IL) could be used as the strain coordinator to balance the lattice strain for stability improvement of FAPbI 3 perovskite. We theoretically studied the electronic coupling between IL and FAPbI 3 and unraveled the originality of the IL-induced compressive strain. The strain-relaxed α-FAPbI 3 by IL showed robust stability against environmental factors, which can withstand ambient aging for 40 days without any phase transition or decomposition. Moreover, the strain-relaxed perovskite films showed a lower trap density and resulted in conversion efficiency improvement from 18.27 to 19.88%. Based on this novel strain engineering strategy, the unencapsulated PSCs maintained 90% of their initial efficiency under ambient-air aging for 50 days.

Trivalent Europium‐Doped CsCl Quantum Dots for MA‐Free Perovskite Solar Cells with Inherent Bandgap through Lattice Strain Compensation

“…This leads to a change in the electronic band‐edge structure, particularly in the valence band edge, and has the ability to modulate the E g . [ 8b ] The location of the conduction band minimum of Cs–FA is similar to that of pure FAPbI 3 . [ 44 ] Considering the valence band maximum (VBM), the declined average Pb─I─Pb angles lead to a stronger PbI 6 octahedral tilting, [ 45 ] leading to the descending location of VBM, wherein the enlarged bandgap is macroscopically exhibited.…”

“…Owing to the large size of FA + ions, the crystal lattice is inclined to release lattice strain and distort to the yellow phase at room temperature. [ 8 ] After the introduction of CsCl, the phase stability is substantially strengthened owing to the optimized tolerance factor; however, the ionic radius mismatch with FA + results in compressive strain owing to the weak Pb─I─Pb bond owing to (PbI 6 ) − tilting, [ 3b,30 ] which leads to lattice contraction and an enlarged E g . In the case of CsCl:Eu 3+ , the presence of Eu 3+ in the interstitial site moderates the Cs‐induced local strain and offsets the Cs + ‐induced lattice contraction.…”

“…[ 3b ] From the formation enthalpy perspective, the formation enthalpy of δ ‐FAPbI 3 is far lower than that of α ‐FAPbI 3 at room temperature owing to the tensile strain in the lattice of α ‐FAPbI 3 induced by the isotropic‐confined FA + in the (PbI 6 ) 4− . [ 8 ] Therefore, α ‐FAPbI 3 is a substable phase that preferentially forms the δ ‐phase at room temperature. [ 9 ] To solve the issue of phase conversion, substituting FA + with cations with a smaller radius, such as Cs + (167 pm) and MA + (217 pm), or substituting I − by smaller anions, such as Cl − and Br − , to reduce the tolerance factor ( t ) can effectively enhance the phase stability.…”

Recent advances in ionic molecules applied in perovskite solar cells

“…Given that the energy levels of the perovskite layers, charge transporting layers and electrodes are mainly Fig. 19 Chemical structures of BMPBr, 135 EMIFAP, 136 choline bromide, 137 HDZWs, 138 DTPT 140 and APSA. 144 determined by the nature of the materials, a well-matched energy level alignment can be achieved by the utilization of suitable materials.…”

“…134 Duan et al introduced an IL additive, BMPBr, into the FAPbI 3 lattice and fabricated PSCs with significantly enhanced stability through a strain balancing strategy. 135 The electrostatic interactions between BMP + and the uncovered iodide anions at the grain boundaries induced compressive strains in the crystal lattice. DFT calculation indicated that the embedded IL led to a decrease in the Pb–I–Pb average bond angle, and thus the Pb–Pb average distance along the out-of-plane direction was reduced from 6.61 to 6.40 Å.…”