Influence of Cl Incorporation in Perovskite Precursor on the Crystal Growth and Storage Stability of Perovskite Solar Cells

Abstract

Solar cells based on organic−inorganic hybrid lead-halide perovskites are very promising because of their high performance and solution process feasibility. Elemental engineering on perovskite composition is a facile path to obtain high-quality crystals for efficient and stable solar cells. It was found that partially substituting I − with Cl − in the perovskite precursor promoted crystal growth, with the grain size larger than the layer thickness, and facilitated the generation of a selfpassivation layer of PbI 2 . Whereas the residual Cl − ions were suspected to diffuse to the hole-transport layer consisting of ubiquitously spiro-OMeTAD, the formation of highly bounded ionic pairing of Cl − with the oxidized state of spiro-OMeTAD led to insufficient charge extraction and severely reversible performance degradation. This issue was effectively alleviated upon Br − doping owing to the generation of Pb−Br bonds in the lattice that strengthened the phase stability by improving the binding energy between each unit. The binary halide (Br − / Cl − )-doped perovskites resulted in a champion power conversion efficiency of 20.2% with improved long-term storage stability.

Mixed-halide perovskites solar cells through PbICl and PbCl2 precursor films by sequential chemical vapor deposition

“…The respective cross sectional SEM in Fig. 4d-f shows continuous grains across the film thickness, which is an advantage for efficient charge transport as there are less grain boundaries and hence suppressed charge recombination in the device (Zhang et al, 2019). Further, these cross-sectional SEM micrographs also illustrate a perfect contact between the perovskite films and the underlying c-TiO 2 layer.…”

“…Even though doping hybrid perovskites with chlorine (Cl -) ions improves device performance, its role has been largely debated and its incorporation within the perovskite structure has also been doubted (Chae et al, 2015;Ono et al, 2017). It is known that enhanced crystallization of perovskite films with improved grain size occurs when Cl is included in one of the perovskite deposition precursors, such as lead chloride (PbCl 2 ) of MA chloride (MACl) (Chae et al, 2015;Fan et al, 2017;Luo et al, 2015b;Wittich et al, 2018;Zhang et al, 2019). The improved crystallization of Cl-doped perovskites is achieved through slow crystal growth compared to the pure iodine based counterparts; this in return suppresses the defect density at the grain boundaries (Richter et al, 2016;Yang et al, 2017Yang et al, , 2016Yang et al, , 2015.…”

“…The improved crystallization of Cl-doped perovskites is achieved through slow crystal growth compared to the pure iodine based counterparts; this in return suppresses the defect density at the grain boundaries (Richter et al, 2016;Yang et al, 2017Yang et al, , 2016Yang et al, , 2015. These general conclusions mainly relate to perovskite films where the solution method is used at either during single or two step synthesis (Chae et al, 2015;Fan et al, 2017;Luo et al, 2015b;Richter et al, 2016;Wittich et al, 2018;Yang et al, 2017Yang et al, , 2016Yang et al, , 2015Zhang et al, 2019). The uncertainty about the presence of Cl in perovskite films stems from the difficulty in detecting it through elemental composition characterization techniques such as X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDS) (Ng et al, 2015;Williams et al, 2014).…”

“…This difficulty arises because the Cl composition in the films is usually very low and below the detection limit (0.1 at.%) of these characterization techniques (Ono et al, 2017). This low Cl concentration in the films is due to an easy replacement of a smaller Clionic radius (1.67 Å) by a larger Iionic radius (2.07 Å) during the perovskite intercalation reaction (Zhang et al, 2019).…”

Structure, Morphology, and Photovoltaic Implications of Halide Alloying in Lead‐Free Cs3Sb2ClxI9–x 2D‐Layered Perovskites

“…This could be a possible reason behind the formation of larger grains upon chloride incorporation. [ 38 ] Although larger grains may facilitate better carrier transport, the concomitantly increased surface roughness may limit the device performance at the extraction interface. This result implies that too high of a concentration of SbCl 3 in the precursor may be detrimental to device efficiency under these processing conditions, and provided a first indication that a targeted amount of SbCl 3 in the precursor may be required to balance grain size and surface roughness to get the best quality device.…”