Fig. 1: EQE and \({E}_{{\rm{U}}}^{{\rm{app}}}\) for organic D:A blends with different energetic offsets between ECT and Eopt.
From: A universal Urbach rule for disordered organic semiconductors
Schematic illustration of the energetic offset \(({E}_{{\rm{opt}}}-{E}_{{\rm{CT}}})\) decreasing from left to right. Gaussian fits in the spectral range of CT state and trap state absorption are performed where possible. For low offset blends PM6:Y6, PM6:ITIC and PBDB-T:EH-IDTBR, the EQE tail has the form \({e}^{E/{kT}}\), and \({E}_{{\rm{U}}}^{{\rm{app}}}\) roughly equals \({kT}\) in the spectral range that is dominated by the absorption of the acceptor. For large offset material systems, such as BQR:PC70BM, PBDB-T:PC70BM and PCDTBT:PC70BM, \({E}_{{\rm{U}}}^{{\rm{app}}}\) shows a \(2{kT}{\lambda }_{{\rm{CT}}}/({E}_{{\rm{CT}}}+{\lambda }_{{\rm{CT}}}-E)\) dependence in the spectral range of CT absorption.
