Trace neodymium ions effectively inhibit ion migration in the perovskite lattice

On November 17, 2022, Yang Yang’s research group at the University of California, Los Angeles and J.W. of Sungkyunkwan University in South Korea Lee’s group and Ilhan Yavuz of Marmara University in Turkey published a paper in the journal Nature Materials entitled “Suppressing ion migration in metal halide perovskite via interstitial doping with a trace amount of multivalent cations” Research results.

This achievement breakthrough found the blocking effect of trace neodymium ions on ion migration in the perovskite lattice, which provided a guiding direction for efficiently improving the working life of perovskite solar cells. The first author of the article is Zhao Yepin.

Starting in 2020, the commercialization of metal halide perovskite solar cells began to heat up around the world, due to impressive progress in power conversion efficiency (PCE) of perovskite solar cells. However, its instability in the working environment has become the main bottleneck, which greatly restricts its rapid development. Ion migration in the perovskite layer of perovskite solar cells is thought to be the main cause of device instability.

In view of this, the method of using cations to occupy the gap sites of perovskite crystals is widely used to inhibit ion migration to improve the performance and stability of perovskite photoelectrons. However, excessive gap doping inevitably introduces microstrain in the crystal lattice, which impairs the long-range order and stability of the crystal. One needs to find a balance between the benefits of gap doping and the micro-strain it introduces, and exploits the strengths and avoids the weaknesses. To this end, the research team led by Professor Yang Yang of the University of California, Los Angeles, deeply analyzed and compared the influence of valence on the doping effect by introducing three gap-doping ions of similar size, namely sodium ion (Na+), calcium ion (Ca2+) and neodymium ion (Nd3+). The researchers found that trivalent neodymium ions (Nd3+) can more effectively alleviate ion migration in the perovskite lattice and reduce the impact of negative electric defects on the efficiency of perovskite solar cells. More importantly, the amount of Nd3+ required for doping is minimal (0.08% molar ratio), which minimizes the adverse effects of microstrain introduced by doping on cell efficiency and stability.

Figure 1: Theoretical model of iodine ion migration to nearby iodine vacancies and proof of the adverse effects of doping producing tensile microstrain

By simulating the migration route of iodine ions to nearby iodine vacancies (Figure 1a), the researchers found that in the presence of Nd3+ in the perovskite lattice, there would be a huge barrier to the migration of iodine ions (Figure 1b). At the same time, the researchers demonstrated that ion doping at the gap site introduces tensile microstrains in the perovskite lattice, potentially disrupting lattice stability (Figure 1c). As shown in Figure 1d, when the dopant concentration reaches 1%, the intensity of the (001) α-FAPbI3 peak decreases, while the intensity of the (001) PbI2 peak increases. When the dopant concentration further increased to 5%, the (001) non-perovskite δ-FAPbI3 peak appeared, indicating the instability of the α-FAPbI3 perovskite phase with the increase of the interstitial dopant concentration.

Figure 2: Photovoltaic performance of Nd3+, Ca2+ or Na+ doped perovskite solar cells and corresponding surface topography of perovskite films

At the same time, the researchers compared the effects of three ions on the photoelectric performance of perovskite solar cells at different doping concentrations (Figure 2). The researchers found that the required doping concentration to achieve the highest PCE value varied according to the valence state of the doped ions. The required doping concentrations for Nd3+, Ca2+, and Na+ are 0.08%, 0.25%, and 0.45% (molar ratio), respectively. That is, ions with higher valence states are the best for improving the efficiency of perovskite solar cells. With only 0.08% Nd3+ doped, the open circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF) of the solar cell are all greatly improved. Moreover, the researchers found that the doping of Nd3+ effectively reduced the difference in forward and reverse sweep efficiency in the device, while the doping of Ca2+ and Na+ only partially reduced this difference.

Figure 3: Comparison of the passivation effect of doped cations on perovskite defects

By comparing the interaction of negatively charged defects by doping cations, the researchers found that Nd3+ can reduce the adverse effects of defects on perovskite solar cells more effectively. According to the density function theory (DFT) calculation of the interaction energy between three cations and negatively charged defects, the researchers found that the interaction energy between Nd3+ and the defect was the highest, followed by Ca2+ and Na+. Given the similar radius size of these cations, the relatively strong interaction energy should result from a higher ionic valence charge, resulting in a longer carrier lifetime and higher device efficiency.

Figure 4: High stability of perovskite solar cells due to inhibition of ion migration

Finally, the researchers directly demonstrated the blocking effect of Nd3+ on ion migration in perovskite by measuring the activation energy (Ea) of ion migration. At the same time, the researchers confirmed that the addition of trace Nd3+ can greatly enhance the stability of perovskite through the in-situ PL measurement of perovskite film. In the measurement of operational stability of solar cell installations, devices incorporated with 0.08% ND3+ show better stability than other devices when heated, continuously illuminated, and continuously operating at the maximum power point. The initial efficiency of 93.4% was maintained after 1008 hours of continuous maximum power point, while the reference device lost more than half of its initial efficiency after 300 hours.

The multivalent gap doping strategy proposed in this work maximizes ion migration inside metal halide perovskites. The extremely small amount of Nd3+ doping effectively avoids the adverse effects of lattice microstrain on perovskite solar cells. Thanks to the stronger interaction between Nd3+ and charged defects and stronger restriction on ion migration, the efficiency and stability of perovskite devices doped with Nd3+ have been significantly improved. This study emphasizes the importance of minimizing the dose of interstitial dopants, provides guidance for maximizing the photovoltaic performance and stability of perovskite solar cells, and promotes the commercialization prospects of perovskite solar cells. (Source: Science Network)

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