Pseudohalide (SCN^–)-Doped MAPbI₃ Perovskites: A Few Surprises

Abstract

Pseudohalide thiocyanate anion (SCN(-)) has been used as a dopant in a methylammonium lead tri-iodide (MAPbI3) framework, aiming for its use as an absorber layer for photovoltaic applications. The substitution of SCN(-) pseudohalide anion, as verified using Fourier transform infrared (FT-IR) spectroscopy, results in a comprehensive effect on the optical properties of the original material. Photoluminescence measurements at room temperature reveal a significant enhancement in the emission quantum yield of MAPbI3-x(SCN)x as compared to MAPbI3, suggestive of suppression of nonradiative channels. This increased intensity is attributed to a highly edge specific emission from MAPbI3-x(SCN)x microcrystals as revealed by photoluminescence microscopy. Fluoresence lifetime imaging measurements further established contrasting carrier recombination dynamics for grain boundaries and the bulk of the doped material. Spatially resolved emission spectroscopy on individual microcrystals of MAPbI3-x(SCN)x reveals that the optical bandgap and density of states at various (local) nanodomains are also nonuniform. Surprisingly, several (local) emissive regions within MAPbI3-x(SCN)x microcrystals are found to be optically unstable under photoirradiation, and display unambiguous temporal intermittency in emission (blinking), which is extremely unusual and intriguing. We find diverse blinking behaviors for the undoped MAPbI3 crystals as well, which leads us to speculate that blinking may be a common phenomenon for most hybrid perovskite materials.

Low‐Pressure Vapor‐Assisted Solution Process for Thiocyanate‐Based Pseudohalide Perovskite Solar Cells

“…With such high content of S, their XRD diffraction patterns are different from the conventional MAPbX 3 structure and many peaks are denoted as planesf or MAPb(SCN) 2 I. [15,17] Figure 3A shows the UV/Vis spectra for Pb(SCN) 2 ,P bI 2, MAPb(SCN) x I 3Àx ,a nd MAPbI 3 .T he absorption onset for the yellow film of PbI 2 is at 520 nm, which is well matchedw ith previousr eports. [13,14] Whereas ar ecent high efficiency SCN pseudohalide perovskite solar cell using DMF/DMSO as co-solvent in one-step spin-coating showed that only trace amount of Sc an be observed by SIMS.…”

“…[9,10] Noh et al reported that mixed halidesw ith high Br/I ratios (0.20-0.29) can improves tability significantly at high relativelyh umidity (> 55 %). [15] The impact of thiocyanate on structure stabilityr esults from the favorable thermodynamic properties of large formation constant between Pb and SCN. [11] Therefore, some alternative perovskite structures to replace MAPbI 3 shouldb ed evelopedt oe nhance the stability of the solar cells.…”

The Emergence of the Mixed Perovskites and Their Applications as Solar Cells

“…Halder et al claimed that the doped SCN − has brought about an optical band gap increase as well higher PL emission quantum yield. [192] 5. Dual/Triple-Site Mixed PVSK Motivated by the need to improve stability and PCE of PSCs, more complex combinations of ABX 3 hybrid PVSK have been explored, namely, mixed-A/mixed-X, mixed-A/mixed-B, mixed-A/mixed-B/mixed-X PVSK.…”

Research Update: Strategies for improving the stability of perovskite solar cells

“…Due to its similar ionic radius of 0.215-0.220 nm for SCN − compared to 0.22nm for I − , this pseudohalogen can replace a "true halide" such as the I − halogen ion. 111,112 The partial halide substitution, forming a MAPbI 3−x (SCN) x perovskite structure, have had a beneficial impact on grain sizes and trap density, 111 optoelectronic properties, 113 and most importantly on its material stability. [114][115][116] In the case of MAPb(SCN) 2 I, the interaction between Pb 2+ and SCN − is much stronger than in the case of neat MAPbI 3 .…”