Determination of Energy Level Alignment within an Energy Cascade Organic Solar Cell

Abstract

The interfacial band alignment among boron subnaphthalocyanine chloride (SubNc), boron subphthalocyanine chloride (SubPc), and α-sexithiophene (α-6T) is explored using ultraviolet, inverse, and X-ray photoemission spectroscopies (UPS, IPES, and XPS, respectively). With these tools, the ionization energy (IE) and electron affinity (EA) for each material are determined. Layer-by-layer deposition of SubPc and SubNc on α-6T as well as SubPc on SubNc, combined with UPS and IPES, allows for the direct determination of the energy level alignment at the interfaces of interest. A small dipole is found at the α-6T/SubNc/SubPc interface, expanding the donor-LUMO to acceptor-HOMO gap and explaining the large open circuit voltage obtained with these devices. However, there is a small electron barrier between SubNc and SubPc, which may limit the efficiency of electron extraction in the current device configuration. Excess chlorine may be responsible for the high IE and EA found for SubNc and could potentially be remedied with improved synthetic methods or further purification.

Mapping Energy Levels for Organic Heterojunctions

“…To further validate our findings on the interface general energy map, we compared literature data on polymers and small molecules with our current model. By adding the literature data reported by various research groups into our universal energy map for both HOMO binding energies (see Figure S5a, Supporting Information) and interface dipoles (see Figure S5b, Supporting Information), we found that these data points follow the same trend as described in this Communication.…”

Organic Planar Heterojunctions: From Models for Interfaces in Bulk Heterojunctions to High‐Performance Solar Cells

“…In principle, FRET can collect more excitons to the charge‐separation interface if the energy acceptor is close to the interface. This exciton‐cascade strategy has been reported in PHJs . For efficient exciton collection at the D/A interface, the optical energy gaps of the stacked materials must be reduced gradually depending on the distance from the D/A interface, leading to sequential Förster energy transfer to the D/A interface .…”

Boron Subphthalocyanines and Silicon Phthalocyanines for Use as Active Materials in Organic Photovoltaics

“…This device was later replicated with some device optimization, yielding a slightly higher PCE of 3.09 % . Minor photocurrent contribution from tetracene was detected in the EQEs, but it is likely singlets being dissociated given the relatively shallow LUMO level of Cl‐BsubPc . The singlet fission process in tetracene is considered slow compared to materials like pentacene, where singlets are rapidly converted to triplets before dissociation can occur .…”

“…Compared to other electron donors that were initially optimized for fullerenes, α‐6T is better suited for pairing with BsubPc electron acceptors. This due to its broad, complementary absorbance (350 nm–550 nm) with many BsubPc and BsubNc derivatives, and shallower energy levels . The inherent surface roughness of α‐6T also facilitates charge dissociation at the donor/acceptor interface .…”