Red-to-Black Piezochromism in a Compressible Pb−I−SCN Layered Perovskite

M of controlling semiconductor bandgaps are of great importance in materials design. In particular, fabrication of layered heterostructures with controlled thickness and composition allows for the composite’s electronic structures to be modulated. In recent decades, 2D organic−inorganic perovskites have gained attention as self-assembling heterostructures, whose optoelectronic properties can be tuned through crystal design. Indeed, hybrid perovskites have proven to be a versatile platform for optoelectronics. The high bandgaps (Eg) and large exciton binding energies (Eb) in n = 1 lead-halide perovskites (n = number of lead-halide sheets in each inorganic layer) have aided their application as green lightemitting diodes and white-light phosphors. In contrast, the much lower Eg and Eb values in the 3D (n = ∞) perovskite (MA)PbI3 (MA = CH3NH3 ) have been critical for its successful employment as an absorber for high-efficiency solar cells. Both Eg and Eb values of 2D perovskites decrease with increasing inorganic sheet thickness. Accordingly, the n = 3 perovskite (C6H5(CH2)2NH3)2(MA)2[Pb3I10] was shown to act as an absorber in a solar cell with improved moisture resistance compared to (MA)PbI3. 10 However, self-assembly of single-phased n > 1 perovskites becomes more difficult as n increases. We therefore explored routes to n = 1 structures with reduced Eg and Eb values. Herein we show that (MA)2[PbI2(SCN)2] (1) has atypically low Eg and Eb values for an n = 1 perovskite and dramatic pressure response (Figure 1) that further modulates its photophysical properties. Recently, a proposed analog to the 3D perovskite (MA)PbI3 was reported where two iodides were replaced by two thiocyanides. This material was reported to exhibit greater moisture resistance compared to (MA)PbI3 and similar performance as a solar-cell absorber. A recent correspondence reported its single-crystal structure as the 2D perovskite 1 containing bridging iodides and terminal thiocyanides. Solid 1 has been attributed an Eg of 1.5−1.6 eV, similar to that of (MA)PbI3, which would be promising for photovoltaic applications. Several reports of mixed Pb−I−SCN perovskite absorbers have also ascribed increased photoluminescence (PL) intensity and improved device performance and stability to SCN− incorporation. In our hands, however, we find that the red solid 1, which has an Eg of ca. 2.3 eV (Figure 2A), rapidly decomposes to a black solid in ambient humidity. Upon exposing 1 to 66% relative humidity, we see reflections corresponding to (MA)PbI3 and Pb(SCN)2 in the powder Xray diffraction (PXRD) patterns, which dominate the patterns after 7 min for films and 45 min for powders (Figure S1). The absorption, PL, and vibrational spectra of humidity-exposed 1 also show indicators for (MA)PbI3 formation (Figures S2−S5), which may explain the low Eg and favorable photovoltaic properties previously attributed to 1. Considering stoichiometry, the following decomposition mechanism is possible, driven by the hydration of (MA)SCN.