Anion-ion co-passivation strategy achieves efficient and stable perovskite solar cells

On April 27, 2022, the team of Professor Qin Tianshi of Nanjing University of Technology published a new research report entitled “An ammonium-pseudohalide ion pair for synergistic passivating surfaces in FAPbI3 perovskite solar cells” in the journal Matter.

The research work reports that through the co-passivation strategy of anion cations, a low-dimensional perovskite layer and a disulfide chelating passivation layer are constructed on the surface of perovskite at the same time, thereby effectively improving the efficiency and stability of perovskite solar cells. The development of anionic synergistic effects provides more opportunities to promote the development and commercialization of perovskite solar cells (PSCs).

Research highlights

1 Yin (halogen-mimic)-yang (alkylammonium) ions synergistically passivate the positive and negative charge defects of metamidyl perovskites.

2 This pair of ions converts defects in formamidyl perovskites into two waterproof barriers.

3 Passivated pure metamidyl perovskite solar cells achieve 23.21% efficiency and excellent stability.

Brief introduction of results

Recently, Professor Qin Tianshi’s team of Nanjing University of Technology made important progress in pure phase methylamidyl perovskite solar cells (FAPbI3 PSC), which effectively reduced the uncoordinated lead iodide defect on the surface of perovskite through the synergistic passivation of ammonium-quasi-halogen ion pairs on the surface of perovskite, and at the same time had a strong hydrogen bond effect on the methylgamidine vacancy, synergistically passivated the anion defect of perovskite anion, and demonstrated the versatility and strong interaction force of the synergistic passivation effect. It provides new research ideas and strategies for exploring and developing perovskite defect passivation.

Key point 1: Ammonium-quasi-halogen ion pair reacts synergistically with lead iodide

The growth of single crystal crystals was achieved by the strong coordination of alkylammonium cations and halogen-mitisol anions alone with lead iodide (Figures 1C and 1G), and the defect passivation effect of anions on perovskites was explored. Low-dimensional perovskites with ammonium cations, SUCHPbI3, which has a Pb-I distance (3.03 to 3.47 Å) comparable to FAPbI3, manifests itself as a regular arrangement of (100) and (110) lattice spaces of DEAPbI3 and epitaxial growth on the surface of FAPbI3 crystals (Figure 1D); a bi-chelated structure Pb (DEDTC)2 having a Pb-S bond distance (2.72~) having a Pb-I distance shorter than the Pb-I distance in FAPbI3 perovskites 2.91Å) (Figure 1H), indicating that it has a stronger bonding capacity than iodolinate clusters. In situ reactions occurring during post-treatment (Figure 1E) are indicated by MRI characterization: 2DEA-DEDTC + PbI2 → 2DEA-I + Pb (DEDTC)2. This reaction demonstrates that both ammonium cations and quasi-halide anions in DEA-DEDTC have multi-bond advantages and can synergistically passivate defects on iodolinated clusters in FAPbI3 perovskites.

Figure 1: Alkylammonium-halogen-mimetic ion binding to lead iodide

Key point 2: Alkylammonium-quasi-halogen ion pair reacts synergistically with the surface of perovskite

The nanoscale interface structure changes were studied by the ammonium-quasi-halogen ion pair posttreatment perovskite surface, the two-dimensional wide-angle X-ray scattering (2DWAXS) demonstrated the formation of low-dimensional perovskites on the surface of the perovskite (Figures 2A and 2B), and the high-resolution transmission electron microscopy (HR-TEM) demonstrated that the ammonium-halogen-like ion pair-induced low-dimensional crystal domains remained epitaxial growth at the surface of the FAPbI3 crystal. At the same time, the distribution of the ammonium-quasi-halogen-induced double waterproof isolation layer on the surface of the perovskite has also been demonstrated (Figures 2D and 2E).

Figure 2: Nanoscale interface structure of a perovskite film

Key point 3: Alkylammonium-quasi-halogen ions on the surface of the perovskite synergistic passivation

Due to the synergistic reaction of ammonium-quasi-halogen ions with the surface of perovskite, it is manifested in the synergistic passivation of the surface of perovskite. Through the characterization of a series of elements on the surface of perovskite ultraviolet photoelectron spectroscopy, it is shown that: (1) ammonium- halogen-like ions on the surface of perovskite passivation (Figure 3A) ;(2) Ammonium – halogen-like ions effectively inhibit the oxidative degradation of perovskites (Figure 3B) ;(3) Ammonium-quasi-halogen ions and perovskite energy transfer, effectively inhibit the production of perovskite lead elements (Pb0), and achieve synergistic passivation defect effect (Figures 3C and 3D). And to achieve a defect density reduction of nearly half (Figure 3E), effectively improve the perovskite surface energy level (figure 3F), is conducive to increasing the open circuit voltage of the device.

Figure 3: Surface defects and energy levels characterization of perovskite films

Point 4: Optical physics of synergistically passivated perovskite films

Further, the synergistic passivation effect on the perovskite thin film grain size has a significant improvement effect, the grain size is nearly 2μm, while effectively inhibiting the formation of lead iodide (Figures 4A and 4B); in addition, the passivation strategy effectively increases the radiation composite life, nearly doubles, effectively inhibits non-radiative composite, and improves the performance of the device.

Figure 4: XRD and optical characterization of perovskite films.

Key point 5: Study on the performance improvement of alkylammonium-quasi-halogen ion pairs on the device

The anion-ion co-passivation strategy is manifested in the improvement of the performance of perovskite solar cells, and the device based on the optimal passivation conditions has a significant efficiency increase, from 20.60% to 23.21% (Figure 5A), and the efficiency improvement is mainly due to the increase of the open circuit voltage and the filling factor, because the passivation strategy of the ammonium-halogen-quasi-halogen pair to the surface defects of perovskite improves the carrier transport and interface stability in the device (Figure 5D-5F). More importantly, the reduction of defects is conducive to improving the service life of perovskite solar cells, which is manifested in the good output stability of the device at the maximum output power point (Figure 5C), and the stability of more than 1,000 hours under continuous light and room temperature storage, while the film shows amazing stability at 85 °C thermal stress.

Figure 5: Photovoltaic performance of perovskite solar cells

brief summary

In this paper, two waterproof isolation layers are constructed through synergistic passivation of anions and anions, which effectively provide physical and chemical passivation of metamidyl perovskite defects, achieve efficient and stable perovskite solar cells, and provide new research ideas and strategies for exploring and developing perovskite defect passivation. The research work has been funded and supported by the National Key R&D Program of China (2017YFE0131900), the National Natural Science Foundation of China (91833306, 62075094, 52003118) and the Natural Science Foundation of Jiangsu Province (BK20211537, BK20180339). (Source: Science Network)

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