Fig. 4: Characterizations of interaction between Tween and cesium copper iodides.

a FTIR spectra. The spectrum of Tween shows peaks at 2866 and 2924 cm⁻¹ ascribed to the C–H stretching vibrations, and a peak at 1122 cm⁻¹ ascribed to C–O–C stretching vibration. After adding Tween to CsI, the relative intensity of 2866 and 2924 cm⁻¹ peaks changes, and the 1122 cm⁻¹ peak shifts to 1107 cm⁻¹. b XPS spectra of CsI, Tween:CsI, CsI:CuI, and Tween:CsI:CuI films. After adding Tween to CsI, the Cs 3d and I 3d peaks of CsI shift to the lower binding energy. c, d Time-resolved GIWAXS profiles of samples without (c) and with (d) Tween. The sample without Tween simultaneously forms CsCu₂I₃ at |q| = 1.84 Å⁻¹ and Cs₃Cu₂I₅ at |q| = 0.93, 1.08, 1.71, 1.82, and 1.88 Å⁻¹ (around 33 s). After the inclusion of Tween, the CsCu₂I₃ appears at 46 s, which is 4 s later than the Cs₃Cu₂I₅.