High‐Efficiency Perovskite Solar Cells with Imidazolium‐Based Ionic Liquid for Surface Passivation and Charge Transport

Abstract

Surface defects have been a key constraint for perovskite photovoltaics. Herein, 1,3‐dimethyl‐3‐imidazolium hexafluorophosphate (DMIMPF6) ionic liquid (IL) is adopted to passivate the surface of a formamidinium‐cesium lead iodide perovskite (Cs0.08FA0.92PbI3) and also reduce the energy barrier between the perovskite and hole transport layer. Theoretical simulations and experimental results demonstrate that Pb‐cluster and Pb‐I antisite defects can be effectively passivated by [DMIM]+ bonding with the Pb2+ ion on the perovskite surface, leading to significantly suppressed non‐radiative recombination. As a result, the solar cell efficiency was increased to 23.25 % from 21.09 %. Meanwhile, the DMIMPF6‐treated perovskite device demonstrated long‐term stability because the hydrophobic DMIMPF6 layer blocked moisture permeation.

Zwitterionic Ionic Liquid Confer Defect Tolerance, High Conductivity, and Hydrophobicity toward Efficient Perovskite Solar Cells Exceeding 22% Efficiency

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Perovskite solar cells (PSCs) have had a lasting impression on the scientific community because of their fast progress as high efficient and low-cost technology. [1] Starting from Kojima et al, in 2009, maximum power conversion efficiencies (PCEs) exceeding 25% have already been certified, resulting from crystallinity optimization, [2,3] morphology control, [4] interface engineering, [5,6] and defects passivation. [3,7,8] Several methods have been reported to prepare high-quality perovskite films, including thermal evaporation, twostep sequential deposition, vapor-assisted solution processing, and one-step spincoating method.

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Nonhalide Materials for Efficient and Stable Perovskite Solar Cells

“…The interface modifiers containing molecular anions were thus used to modify perovskite/HTL interface. [ 14,43 ] An heterogeneous interface layer of FA 0.88 Cs 0.12 PbI 3−x (PF 6 ) x was in‐situ formed via the ion exchange reaction between I − and PF 6 − ( Figure a). It was interesting that FA 0.88 Cs 0.12 PbI 3−x (PF 6 ) x passivation layer grew along the GBs of perovskite films.…”

“…A 1,3‐dimethyl‐3‐imidazolium hexafluorophosphate (DMIMPF 6 ) IL was reported to modify perovskite/HTL interface. [ 43 ] Both theoretical and experimental results showed that DMIMPF 6 passivated Pb‐cluster and Pb‐I antisite defects in perovskite films, which improved PCE to 23.25%. Moreover, long term environmental stability was ameliorated after DMIMPF 6 treatment due to the hydrophobic nature of DMIMPF 6 .…”

“…[ 9,10 ] Finally, various interfacial modifiers containing molecular anions have been developed to modify perovskite/ETL interface [ 13,21,39–42 ] and perovskite/HTL interface. [ 14,43 ]…”

Fluoroethylamine Engineering for Effective Passivation to Attain 23.4% Efficiency Perovskite Solar Cells with Superior Stability

“…[ 22 ] In addition, fluorine is super hydrophobic, thus it can synergistically improve the environmental stability. [ 23–26 ] F‐containing Spiro (Spiro‐mF), [ 27 ] 2‐(2,3,4,5,6‐pentafluorophenyl) ethylammonium iodide, [ 17 ] 4‐trifluoromethyl‐pyridine, [ 28 ] and 1,3‐dimethyl‐3‐imidazolium hexafluorophosphate (DMIMPF 6 ) [ 29 ] have been widely employed to simultaneously improve the efficiency and stability. However, these molecules are used as surface passivators of the perovskite film and have negligible effect in the bulk.…”