Unreacted PbI₂ as a Double-Edged Sword for Enhancing the Performance of Perovskite Solar Cells

Abstract

Lead halide perovskites have over the past few years attracted considerable interest as photo absorbers in PV applications with record efficiencies now reaching 22%. It has recently been found that not only the composition but also the precise stoichiometry is important for the device performance. Recent reports have, for example, demonstrated small amount of PbI2 in the perovskite films to be beneficial for the overall performance of both the standard perovskite, CH3NH3PbI3, as well as for the mixed perovskites (CH3NH3) x (CH­(NH2)2)(1–x)PbBr y I(3–y). In this work a broad range of characterization techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photo electron spectroscopy (PES), transient absorption spectroscopy (TAS), UV–vis, electroluminescence (EL), photoluminescence (PL), and confocal PL mapping have been used to further understand the importance of remnant PbI2 in perovskite solar cells. Our best devices were over 18% efficient, and had in line with previous results a small amount of excess PbI2. For the PbI2-deficient samples, the photocurrent dropped, which could be attributed to accumulation of organic species at the grain boundaries, low charge carrier mobility, and decreased electron injection into the TiO2. The PbI2-deficient compositions did, however, also have advantages. The record V oc was as high as 1.20 V and was found in PbI2-deficient samples. This was correlated with high crystal quality, longer charge carrier lifetimes, and high PL yields and was rationalized as a consequence of the dynamics of the perovskite formation. We further found the ion migration to be obstructed in the PbI2-deficient samples, which decreased the JV hysteresis and increased the photostability. PbI2-deficient synthesis conditions can thus be used to deposit perovskites with excellent crystal quality but with the downside of grain boundaries enriched in organic species, which act as a barrier toward current transport. Exploring ways to tune the synthesis conditions to give the high crystal quality obtained under PbI2-poor condition while maintaining the favorable grain boundary characteristics obtained under PbI2-rich conditions would thus be a strategy toward more efficiency devices.

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

“…Our findings are, in principle, also applicable to mixed cation/halide systems as these films are also deposited from non-stoichiometric mixtures. [34] In this case we expect a similar effect upon exposure to moisture as the typical cations used, formamidinium and cesium, are also hygroscopic and will likely undergo a similar enhanced mobility in the presence of water.…”

Triple cation mixed-halide perovskites for tunable lasers

“…The better gain performance in these perovskites with a deficiency of lead is in contrast to perovskite solar cells, where a lead excess was found to improve the efficiency [37,38]. Jacobsson et al recently suggested that a lead excess is improving charge carrier transport through grain boundaries while the overall crystal quality suffers, however, with a net positive effect for solar cells [38]. Since charge carrier transport is less important in our optically pumped devices, the better crystal quality together with the smoother surface can explain the lower ASE thresholds.…”

“…Next, we investigated the role of the stoichiometric ratio between lead to the sum of all cations, since recent investigations have revealed a large influence of this parameter on the efficiency of solar cells [37,38]. The lead to cation ratio (Pb/A) is defined as the ratio of the number of lead (Pb) anions divided by the sum of the three A cations (number of MA molecules + number of FA molecules + number of Cs cations) in the precursor solution.…”

“…For optical gain in thin films, a smooth layer is preferable since the waveguide mode exhibits less scattering leading to a higher modal gain [39]. The better gain performance in these perovskites with a deficiency of lead is in contrast to perovskite solar cells, where a lead excess was found to improve the efficiency [37,38]. Jacobsson et al recently suggested that a lead excess is improving charge carrier transport through grain boundaries while the overall crystal quality suffers, however, with a net positive effect for solar cells [38].…”

Reduction in the Interfacial Trap Density of Mechanochemically Synthesized MAPbI3

“…17,18 However, it is challenging to obtain highly crystalline perovskite films with full surface coverage using this procedure. 19 Thus, several modifications including solvent engineering, 20 fast deposition-crystallization 21 and the addition of different additives [22][23][24][25][26] have been developed. These complex preparation methods are sensitive to small changes in the precursors composition and thus reproducibility remains an issue for improving MAPbI 3 films 24 towards the highest performing multication perovskites.…”