Fig. 5: Energetic analysis of NP-SA:C60 and MPTA:C60 blends.
From: Light-intensity-dependent photoresponse time of organic photodetectors and its molecular origin
a The potential energy scan as a function of alkene bond dihedral angle between donor and acceptor units (e.g., as indicated for NP-SA in the inset), with highlighted the lower energy configuration at 0 and 180° dihedral angles for NP-SA and MPTA. b HOMO and LUMO energy levels for optimized geometries of NP-SA and MPTA and their isomeric forms, respectively with 0 and 180° D-A dihedral angle, calculated by DFT. c Photoemission spectra of neat and blended donors measured by APS, with the linear fits for the location of emission edge attributed to HOMO (dotted lines). The inset shows the density of tail states extracted from the integrated area below the HOMO onset, indicative of shallow trap density, with error bars as standard deviation across multiple measurements. d Distribution of deep and shallow trap states. MPTA:C60 has a narrower shallow trap distribution and the presence of a deeper trap DOS. The trap states are filled by photogenerated holes from low energy states to high energy states gradually with the increase of light intensity. In low light, hole extraction from deep states of MPTA:C60 is harder than the extraction from shallow states, due to the higher energy barrier, leading to hole trapping in deep states.
