Efficient and Stable Perovskite Solar Cells and Modules using Tailored Organic Halide Passivators

A novel organic halide salt ortho-(phenylene)di(ethylammonium) iodide was developed to passivate defects for highly efficient and stable perovskite solar cells and modules, which paves the way for scaling up perovskite photovoltaics to sizes of commercial relevance.

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Organic-inorganic metal halide perovskite solar cells (PSCs) are an emerging photovoltaic technology that can disrupt the mature silicon solar cell market due to their relatively simple and low-cost solution processes. However, the stability and a significant gap in efficiency between laboratory-scale cells and modules inhibit their real-world applications. One of the reasons is the abundant defects existing in the solution-processed large-area perovskite films, particularly at surfaces and grain boundaries. Such defects behave as nonradiative recombination centers to limit the photovoltaic performance, and are responsible for charge accumulation, accelerated ion migration, and the initial invasion of moisture or oxygen, ultimately causing device instability issues. Therefore, organic halide salt passivation is proposed as a common strategy to afford efficient and stable perovskite devices. Typically, an additional two-dimensional (2D) perovskite layer, especially under higher temperatures, is formed on top of the three-dimensional (3D) perovskite absorber after treatment with organic halide salts. However, a stubborn in-plane orientation and high exciton binding energy are usually observed for the surficial 2D perovskite layer, which potentially suppresses charge transport and draws back the defect passivation effect.

To overcome this limitation, we molecularly engineered structural isomers of (phenylene)di(ethylammonium) iodide (PDEAI2) for tailored defect passivation. From an investigation of the ortho-, meta-, and para-isomers for PDEAI2, the ortho-PDEAI2 (o-PDEAI2) effectively increases the energy barrier of the 2D perovskite formation and prevents the bulky organic cations from entering the perovskite lattice even at elevated temperatures. As a result, surficial o-PDEAI2 exhibits a comprehensive passivation effect on both shallow- and deep-level defects, which leads to highly efficient and stable PSCs and modules:

a The structures of the PDEAI2 isomers. b tDOS distribution in PSCs showing fewer trap densities after o-PDEAI2 passivation. c J-V characteristics of the champion device with o-PDEAI2 measured in both reverse (red) and forward (orange) scanning directions (the inset shows its stabilized power output). d Photograph of the fabricated perovskite solar module. e Schematic showing the interconnections of the module. f J-V characteristics of the champion perovskite solar module with an active area of 26.00 cm2.
Fig. 1 Summary of this work. a The structures of the PDEAI2 isomers. b tDOS distribution in PSCs showing fewer trap densities after o-PDEAI2 passivation. c J-V characteristics of the champion device with o-PDEAI2 measured in both reverse (red) and forward (orange) scanning directions (the inset shows its stabilized power output). d Photograph of the fabricated perovskite solar module. e Schematic showing the interconnections of the module. f J-V characteristics of the champion perovskite solar module with an active area of 26.00 cm2.
  1. We synthesized the ortho-, meta-, and para-isomers of PDEAI2 and investigated the energy barrier of their 2D perovskite formation. Experimental and theoretical studies indicate that locating the two ammonium cations in the most sterically hindered ortho position endows o-PDEAI2 with the highest formation energy barrier of surficial in-plane favored 2D perovskite among the PDEAI2 isomers.
  2. The o-PDEAI2 treatment reduces the trap densities of perovskite films over both shallow and deep trap bands, causes upwards band bending, improves the hole extraction, and reduces the recombination losses at the interface. As a result, the o-PDEAI2 passivation improves the power conversion efficiency (PCE) to 23.92%. Importantly, an efficiency as high as 21.4% was achieved for the perovskite module with an active area of 26 cm2. The high module performance originates from the good uniformity of the perovskite layer, the reduced trap density, and suppressed interfacial recombination, confirming the utility of the o-PDEAI2 passivation strategy for scale-up of PSCs.
  3. The o-PDEAI2 passivated devices show enhanced humidity (40-50% relative humidity), thermal (85 °C heating), and light (AM1.5G illumination) stabilities, which maintain 85%, 75%, and 90% of their initial PCEs over 1000 hours, respectively. The robust ambient and operational stability of the o-PDEAI2-based device is attributed to the hydrophobicity of the phenyl group, the mitigated interfacial charge accumulation, and the suppressed ion migration benefiting from the passivation of defects.

This study demonstrates that altering functional groups and chemical structures should be an effective strategy to develop novel organic cation passivation with a continuous and stable passivation effect, which may pave the way for scaling up perovskite photovoltaics to sizes of commercial relevance.

More details can be found in our article published in Nature Communications  (https://www.nature.com/articles/s41467-021-26754-2).

Cheng Liu

Dr., North China Electric Power University & École polytechnique fédérale de Lausanne (EPFL)