Organic-inorganic hybrid perovskite materials have excellent optoelectronic properties such as high optical absorption coefficient, suitable tunable band gap, low exciton binding energy, high carrier mobility, and long diffusion length[1]. Therefore, it has important potential applications in the fields of photovoltaic devices such as solar cells, photodetectors, light-emitting diodes, and lasers, and has been widely studied and reported.
In the field of photovoltaics, since the advent of the first perovskite photovoltaic device (PSC) in 2009 [2], the device structure of perovskite has been continuously improved, and the photoelectric conversion efficiency (PCE) has steadily increased. The performance of perovskite can be effectively improved by regulating the crystallization process of perovskite through stable solvent engineering, or improving the energy level arrangement of the device through structural design [3~5], but its PCE is still lower than the thermodynamics of perovskite device. The limit or Shockley limit, there is considerable room for improvement [6]. In terms of stability, traditional silicon-based solar cells can work stably for more than 25 years. However, due to the decomposition of perovskite, the high-efficiency PSC stability record so far is usually only a few months, and it can be used in light, heat, water, Under comprehensive conditions such as oxygen, its stability performance may be even worse. Therefore, it is also necessary to suppress the decomposition of perovskite and stabilize the device performance by means of interface engineering [7].
Among the many factors that affect the performance of perovskite, PbI2, as one of the most common by-products in the perovskite preparation process, has been controversial for its influence on perovskite devices. It is generally believed that an appropriate amount of PbI2 can passivate the perovskite to a certain extent, and promote the extraction of interface carriers and improve the device efficiency [8]. On the other hand, the unreacted PbI2 will gradually decompose into Pb and I2 under illumination, and the Pb left in the perovskite will act as the quenching center of the carrier, which will gradually reduce the device performance [9]. In addition, excessive PbI2 may also cause severe hysteresis [10]. Therefore, regulating the presence of PbI2 in perovskites is crucial for the fabrication of efficient and stable PSC devices.
Rb element has good compatibility with perovskite, and its application in perovskite has gradually attracted attention in recent years. On the periodic table of elements, Rb and Cs are adjacent to each other. Due to the small radius of the element, it is difficult to form perovskite itself, but a small amount can be doped into perovskite lattices such as (FAPbI3)0.85(MAPbBr3)0.15, Cs2AgBiBr6, etc. Perovskite materials with excellent properties have been produced [11–13]. Recently, You Jingbi's group from the Institute of Semiconductors, Chinese Academy of Sciences [14] used a two-step method to prepare FAPbI3 perovskite devices, and by introducing RbCl into the precursor solution, the excess PbI2 existing in the perovskite was converted into a new kind of perovskite. The secondary phase (PbI2)2RbCl (hereafter abbreviated as PIRC for convenience), effectively improves the optoelectronic properties of FAPbI3 perovskite and reduces its band gap. When the doping molar ratio of RbCl reaches 5%, an optoelectronic device with a certified efficiency of 25.6% and good stability can be obtained. The research results were published in Science.
The researchers first used scanning electron microscopy (SEM) to characterize the crystallization of the film, and the results are shown in Figure 1(a). The grain size of the PIRC thin-film perovskite reaches about 2 μm, which is much higher than that of the control group (~1 μm). A large number of white needle-like PbI2 particles were uniformly distributed in the control film, which were replaced by sporadic white irregular flakes in the PIRC film. The energy spectrum information shows that the flakes contain I, Pb, Rb, Cl and other elements, but not N, indicating that the flakes may be a mixed product of PbI2/RbCl. Further, the researchers measured the X-ray diffraction spectrum (XRD) of the film. As shown in Figure 1(b), in the control film, diffraction peaks representing the yellow FAPbI3 phase and PbI2 phase can be observed at 11.7° and 12.6°, respectively, while in the PIRC film, the diffraction peak of PbI2 is significantly weakened, Instead, a new peak appears at 11.3°. In addition, the researchers also measured the XRD data of different ratios of PbI2/RbCl precursor solution after annealing. When the molar ratio of PbI2/RbCl reaches 2, the diffraction peak of PbI2 in the film almost completely disappears, while the diffraction peak at 11.3° has been significantly enhanced. Therefore, it can be considered that the diffraction peak at 11.3° of PIRC comes from (PbI2)2RbCl, and the calculation of density functional theory and the results of powder diffraction experiments also confirm this inference.
Effects of RbCl Treatment on Perovskite
In perovskites, excess PbI2 induces the decomposition of the perovskite layer, and when RbCl converts PbI2 into the (PbI2)2RbCl phase, the perovskite stability can be significantly improved. The researchers placed the perovskite film at 85 °C for 48 h and then performed XRD characterization. It was found that the peak intensity ratio of PbI2/perovskite in the control film after aging increased from 0.42 to 0.98, PbI2 increased significantly, while PIRC This ratio in the film has been stable at around 0.1. Due to the existence of Schottky defects in the perovskite, PbI2 in the film also leads to the generation of ion migration. By measuring the electrical conductivity of the control film and PIRC film at different temperatures, the activation energy required for the corresponding ion migration can be calculated, and the ion mobility of the control film and PIRC film can be estimated to be 4.2 × 108 s, respectively. –1 and 3.3×10–3 s–1, confirming that the PIRC film effectively suppresses ion migration by reducing the content of PbI2. This is because PbI2 can absorb adjacent FA and I plasmas under external conditions such as light, heat and electric field, while (PbI2)2RbCl is relatively more stable; on the other hand, it may also be because (PbI2)2RbCl acts as an intercalation compound , lacking channels for ion migration, while layered PbI2 has enough interlayer space for ion migration. In addition, the researchers investigated atomic orbital information in PIRC films using X-ray photoelectron spectroscopy (XPS). In addition to the Rb 3d orbitals, about 5.3% Cl was present in the PIRC films, while in the control group, this proportion was about 2.7% (derived from the addition of MACl). The introduction of RbCl not only provides excess Cl, which can stabilize the perovskite phase through strong Pb–Cl bonds, but also inhibits the formation of the Pb0 state in the perovskite, further stabilizing the black phase of FAPbI3.
Fluorescence spectroscopic characterization showed that the non-radiative recombination of the PIRC-treated films was significantly inhibited, the fluorescence intensity was significantly enhanced, and the carrier lifetime was greatly prolonged. Due to the inhibition of carrier recombination, the perovskite device prepared based on the optimal molar ratio of RbCl doping of 5% has significantly improved short-circuit current and open-circuit voltage, and the device efficiency has increased from 24.6% to 25.6%, the best Device efficiency is 26.1% (certified efficiency is 25.6%). The researchers also measured the device as a light-emitting diode under short-circuit current conditions, and achieved a high electroluminescence efficiency (EQE) of 21.6%, indicating that defect-induced nonradiative recombination in the device is significantly suppressed.
In this study, in view of the problems caused by excess PbI2 in perovskite films, including promoting ion migration, accelerating perovskite decomposition, increasing band gap, and generating non-radiative recombination, an appropriate molar ratio of RbCl to combine with PbI2 was introduced, and a series of characterizations were carried out. The means confirmed the reduction of PbI2 phase and the formation of stable inactive phase (PbI2)2RbCl in the film, and finally successfully fabricated perovskite devices with both excellent performance and excellent stability. This study uses a scientific experimental design to deeply reveal the adverse effects of excess PbI2 on perovskite films and devices from multiple dimensions, which opens up new ideas for future research on the optoelectronic properties and stability of perovskite.
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