Charge Transfer States in Dilute Donor–Acceptor Blend Organic Heterojunctions

Abstract

We study the charge transfer (CT) states in small-molecule blend heterojunctions comprising the nonpolar donor, tetraphenyldibenzoperiflanthene (DBP), and the acceptor, C70, using electroluminescence and steady-state and time-resolved photoluminescence spectroscopy along with density functional theory calculations. We find that the CT exciton energy blue shifts as the C70 concentration in the blend is either decreased or increased away from 50 vol %. At 20 K, the increase in CT state lifetime is correlated with the increasing diameter of C70 nanocrystallites in the blends. A quantum confinement model is used to quantitatively describe the dependence of both CT energy and lifetime on the C70 or DBP domain size. Two discrete CT emission peaks are observed for blends whose C70 concentration is >65%, at which point C70 nanocrystallites with diameters >4 nm appear in high-resolution transmission electron micrographs. The presence of two CT states is attributed to coexistence of crystalline C70 and amorphous phases in the blends. Furthermore, analysis of CT dissociation efficiency versus photon energy suggests that the >90% dissociation efficiency of delocalized CT2 states from the crystalline phase significantly contributes to surprisingly efficient photogeneration in highly dilute (>80% C70) DBP/C70 heterojunctions.

Free and trapped hybrid charge transfer excitons at a ZnO/small-molecule heterojunction

“…Electron trapping likely occurs at surface defects due to Zn interstitials and oxygen vacancies, whose energies are reported to lie 0.2 -0.8 eV below the ZnO conduction band edge 19 . We note that the EL spectra peak position, and its width and intensity is dependent on ZnO processing conditions, and as noted above, that spectral peak dependence on voltage and temperature are not found in organic HJ spectra 25,26 . These observations support the conclusion that ZnO surface trap states dominate the behavior of the EL spectra peak at this OI-…”

Outdoor operation of small-molecule organic photovoltaics

“…The EQE increases slightly with temperature, with larger increases near the edges of the absorption features at l ¼ 540 nm, 590 nm, and 700 nm. The longest wavelength corresponds to the DBP:C 70 charge transfer (CT) state absorption [24].…”

The Impact of Local Morphology on Organic Donor/Acceptor Charge Transfer States

“…Take a recent example in which solar cells based on tetraphenyldibenzoperiflanthene (DBP)/C 70 showed two distinct CT features in their emission spectra. In addition, the authors found that IQE dropped off dramatically in the absorption range of the lower energy CT feature . The authors identified the peaks as originating from either ordered or disordered regions of the interface, and given this assignment, it is reasonable that both the high and low energy states are emissive since they occupy distinct volumes in space.…”

“…In addition, the authors found that IQE dropped off dramatically in the absorption range of the lower energy CT feature . The authors identified the peaks as originating from either ordered or disordered regions of the interface, and given this assignment, it is reasonable that both the high and low energy states are emissive since they occupy distinct volumes in space. However, the authors' assignment of the higher energy DBP:C 70 CT state as the more delocalized one stands in contrast to the majority of works, which correlate a higher degree of delocalization with lowering of the CT state energy.…”