Fig. 2: Organic light-emitting devices.

a, Material combinations for matrix-free Systems A, B and C. b, EL spectra of System A, with photographs of OLED emission shown in the insets. c, EL spectra of System B, with En-Per single-crystal X-ray structure shown in the inset. XRD, X-ray diffraction. d, EL spectra of System C, with photographs of OLED emission shown in the insets.