Phase Segregation in Cs-, Rb- and K-Doped Mixed-Cation (MA)x(FA)1-xPbI3 Hybrid Perovskites from Solid-State NMR

Abstract

Hybrid (organic-inorganic) multication lead halide perovskites hold promise for a new generation of easily processable solar cells. Best performing compositions to date are multiple-cation solid alloys of formamidinium (FA), methylammonium (MA), cesium, and rubidium lead halides which provide power conversion efficiencies up to around 22%. Here, we elucidate the atomic-level nature of Cs and Rb incorporation into the perovskite lattice of FA-based materials. We use ¹³³Cs, ⁸⁷Rb, ³⁹K, ¹³C, and ¹⁴N solid-state MAS NMR to probe microscopic composition of Cs-, Rb-, K-, MA-, and FA-containing phases in double-, triple-, and quadruple-cation lead halides in bulk and in a thin film. Contrary to previous reports, we have found no proof of Rb or K incorporation into the 3D perovskite lattice in these systems. We also show that the structure of bulk mechanochemical perovskites bears close resemblance to that of thin films, making them a good benchmark for structural studies. These findings provide fundamental understanding of previously reported excellent photovoltaic parameters in these systems and their superior stability.

Figure 1

Schematic representation of structural motifs investigated in this study: (a) black single-cation α-FAPbI₃; (b) black double- (CsFA, RbFA), triple- (CsMAFA), or quadruple-cation (RbCsMAFA) compositions (X = I, Br); and (c) yellow nonperovskite δ-FAPbI₃.

Figure 2

Quantitative ¹³³Cs echo-detected MAS spectra of various (Cs/Rb/MA/FA)Pb(Br/I)₃ systems at 298 K and (a) 10 kHz MAS and (b–j) 20 kHz MAS acquired within 1 h after annealing. Asterisks (*) indicate spinning sidebands, and † is a transmitter artifact.

Figure 3

(a) Variable-temperature solid-state ¹³³Cs MAS NMR spectra of Cs_0.20FA_0.80. (b) Temperature dependence of the ¹³³Cs shift (measured at the maximum of the most intense peak). Spinning sidebands are marked with asterisks (*).

Figure 4

A ¹H–¹³³Cs HETCOR of Cs_0.20FA_0.80 at 100 K and 12 kHz MAS.

Figure 5

11.7 T Solid-state ⁸⁷Rb echo-detected MAS (20 kHz, 298 K) spectra of various (Cs/Rb/MA/FA)Pb(Br/I)₃ systems. The corresponding 100 K ¹³C CP MAS spectra of a–c, e, f, and j show only one FA signal corresponding to it being in a 3D perovskite environment (Figure S11).

Figure 6

(a) Low-temperature (100 K) ¹³C CP MAS spectra, (b) echo-detected ¹⁴N MAS spectra at 300 K and 5 kHz MAS of MAPbI₃ (top) and K_0.10MA_0.90PbI₃ (bottom), and (c) echo-detected ³⁹K spectrum of K_0.10MA_0.90PbI₃ at 300 K and 20 kHz MAS (20 s recycle delay, 12 h total acquisition time).

Figure 7

Solid-state MAS NMR spectra of CsMAFA(Br,I) in bulk (blue) and prepared as thin film on glass (red). (a) Echo-detected ¹³³Cs spectra at 100 K and 12 kHz MAS (Figure 2f is the corresponding 298 K spectrum of the bulk material), (b) ¹³C CP at 100 K and 12 kHz MAS, and (c) ¹⁴N echo-detected spectra at 298 K and 20 kHz MAS (acquisition times: bulk 20 h, thin film 60 h). The isotropic signal marked “†” most likely comes from traces of DMF used during spin-coating.