Is Excess PbI₂ Beneficial for Perovskite Solar Cell Performance?

Abstract

Unreacted lead iodide is commonly believed to be beneficial to the efficiency of methylammonium lead iodide perovskite based solar cells, since it has been proposed to passivate the defects in perovskite grain boundaries. However, it is shown here that the presence of unreacted PbI2 results in an intrinsic instability of the film under illumination, leading to the film degradation under inert atmosphere and faster degradation upon exposure to illumination and humidity. The perovskite films without lead iodide have improved stability, but lower efficiency due to inferior film morphology (smaller grain size, the presence of pinholes). Optimization of the deposition process resulted in PbI2‐free perovskite films giving comparable efficiency to those with excess PbI2 (14.2 ± 1.3% compared to 15.1 ± 0.9%) Thus, optimization of the deposition process for PbI2‐free films leads to dense, pinhole‐free, large grain size perovskite films which result in cells with high efficiency without detrimental effects on the film photostability caused by excess PbI2. However, it should be noted that for encapsulated devices illuminated through the substrate (fluorine‐doped tin oxide glass, TiO2 film), film photostability is not a key factor in the device degradation.

The Influence of Water Vapor on the Stability and Processing of Hybrid Perovskite Solar Cells Made from Non‐Stoichiometric Precursor Mixtures

“…The expected (001) PbI2 reflection at the 2 value of 12.6° is obvious for the PbI2-excess film, whereas it is not present in the other two samples. Previous studies report that the excess PbI2 is located near the substrate, [31] however, its distribution strongly depends on the fabrication route. To infer the location of the PbI2 excess within our absorber layer, we performed grazing-incidence XRD measurements at different incidence angles in the range of 0.3°-2.5°, as presented in Figure S3.…”

“…Since the same fabrication method is used for all samples, we expect the excess MAI to be also distributed homogeneously between the preformed MAPbI3 crystals, presumably at the grain boundaries as reported by Son et al [35] Previous studies report the detrimental effect of residual PbI2 material at the perovskite/TiO2 interface on the film stability towards humidity. [31] Here, we investigate the influence of moisture on perovskite films with a more homogeneous distribution of the precursor material excess which is more comparable to the absorber layers used in state-of-the-art devices showing high efficiencies. In order to compare the moisture stability of the three MAPbI3 films with different precursor stoichiometry, we exposed the samples to air with a constant relative humidity (RH) of 90% in a closed chamber at room temperature.…”

“…[24][25][26][27][28] The remnant PbI2 excess within the final absorber layer is believed to improve perovskite crystallization and to reduce non-radiative recombination rates by passivation of crystal grain boundaries, which could explain the additional boost in PCE. [24,[29][30][31][32] While the community agrees on the beneficial effect of a small PbI2 excess in the perovskite layer on device performance, little is known about the moisture-stability of these non-stoichiometric perovskite films. Previously, Liu et al [31] and Zhang et al [33] prepared methylammonium lead iodide films via a twostep sequential deposition method to study the stability of these samples.…”

“…[24,[29][30][31][32] While the community agrees on the beneficial effect of a small PbI2 excess in the perovskite layer on device performance, little is known about the moisture-stability of these non-stoichiometric perovskite films. Previously, Liu et al [31] and Zhang et al [33] prepared methylammonium lead iodide films via a twostep sequential deposition method to study the stability of these samples. In both cases, the incomplete conversion of PbI2 into the perovskite phase leads to a residual PbI2 interlayer between TiO2 and perovskite, which is reported to accelerate the film degradation upon exposure to humidity.…”

Capturing the Sun: A Review of the Challenges and Perspectives of Perovskite Solar Cells

“…In most onestep techniques, the precursors of MAI and PbI 2 or PbCl 2 are simply dissolved in DMF or DMSO in a specific ratio, i.e., stoichiometric or with an excess of one of the components, [97][98][99][100][101] and are then spin-coated onto the substrate and annealed at a particular temperature and time to obtain crystalline layers of perovskite (Figure 3d,e). As mentioned before, simple onestep methods often lead to inhomogeneous coverage of the substrates.…”