Rethinking the Role of Excess/Residual Lead Iodide in Perovskite Solar Cells

Abstract

It is widely believed that excess/residual lead iodide (PbI2) can affect the performance of perovskite solar cells . Moderate PbI2 can enhance efficiency by passivating defects, while extremely active PbI2 leads to non‐negligible hysteresis effects and reduces device stability. Although several efforts are made to investigate the role of excess PbI2, its impact is still underestimated. Recent advances further demonstrate the extraordinary potential of modifying excess PbI2; however, a comprehensive study is required to obtain a deeper understanding. Herein, the important breakthroughs regarding excess PbI2 are reviewed and the mechanism of excess PbI2 in terms of efficiency and stability is rethought. In addition, the origins, verification, and regulation of residual PbI2 are summarized.

Chemical Reduction of Iodine Impurities and Defects with Potassium Formate for Efficient and Stable Perovskite Solar Cells

“…[ 12 ] The management of residual/excess PbI 2 is important to balance its effect on efficiency and stability of PSCs, which is not the focus of this work. [ 13 ] The control perovskite films without HCOOK showed an average grain size of 580 nm and improved to 681 and 864 nm when 0.9 and 1.8 mol% (with respect to Pb) HCOOK was added, respectively. Such an increase in the grain size is usually accompanied by a decrease in the grain boundaries, which may contribute to the reduction of the defects in the perovskite films.…”

Pure-Phase, Large-Grained Wide-Band-Gap Perovskite Films for High-Efficiency, Four-Terminal Perovskite/Silicon Tandem Solar Cells

“…Lin et al reduced the nonradiative recombination losses in wide-band-gap perovskite films by a synergetic treatment with Pb­(SCN) 2 and PEABr, bringing a relatively high V oc of 1.29 V and a PCE of 17.32%. On the other hand, it is revealed that the Pb­(SCN) 2 additive usually causes the formation of the PbI 2 impurity phase. − Some works have pointed out that excess PbI 2 is detrimental to the long-term stability of PSC owing to its decomposition into Pb 0 and I 2 under continuous illumination, and the as-generated Pb 0 species would act as nonradiative recombination centers, finally leading to the significant degradation on V oc and fill factor (FF) of final PSC. − …”

Managing Excess Lead Iodide with Ordered Distribution and Reduced Photoactivity via Chelating Ligands for Stable Inverted Perovskite Solar Cells

“…8−10 However, excess PbI 2 can be a double-edged sword for PSCs. 11,12 Numerous publications have reported that excess PbI 2 readily decomposes to gaseous I 2 and metallic Pb 0 under light and thermal stress, acting as catalytic sites to trigger perovskite decomposition. 13−16 It is therefore necessary to manage the unstable PbI 2 in perovskite films to achieve an efficient and stable PSC.…”

“…The degradation of PSCs is mainly attributed to the hybrid ionic nature of perovskite materials, which is intrinsically unstable in the presence of moisture, oxygen, heat and light. − In addition, inhomogeneity of polycrystalline perovskite films due to imperfect stoichiometric ratios, nonequilibrium reactions, and partial evaporation of the organic components also deteriorates the long-term stability of PSCs . For instance, state-of-the-art PSCs have typically utilized excess lead iodide (PbI 2 ) (5–10 mol %) in the precursor solution to prepare polycrystalline perovskite films. , This is due to the fact that a slight excess of PbI 2 at the perovskite grain boundaries (GBs) and surfaces has some benefits for high efficiency PSCs, including defect passivation, reduction of halide vacancy concentration, and enhancement of carrier lifetime. − However, excess PbI 2 can be a double-edged sword for PSCs. , Numerous publications have reported that excess PbI 2 readily decomposes to gaseous I 2 and metallic Pb 0 under light and thermal stress, acting as catalytic sites to trigger perovskite decomposition. − It is therefore necessary to manage the unstable PbI 2 in perovskite films to achieve an efficient and stable PSC.…”

“…The first is the presence of a slight excess or incomplete conversion of PbI 2 in the perovskite precursor solution. The other source is from the loss of volatile organic cations (especially MA + ) during the preparation of the perovskite films . Notably, 2-MNA is a multisite heterocyclic ligand containing a carbonyl, a sulfur, and a pyridinic nitrogen, which have been used to prepare metal coordination complexes. , It has been shown that 2-MNA ligands have two coordination modes of chelating (O, S) and (N, S) .…”