The Effect of Gradual Fluorination on the Properties of F_nZnPc Thin Films and F_nZnPc/C₆₀ Bilayer Photovoltaic Cells

Abstract

Motivated by the possibility of modifying energy levels of a molecule without substantially changing its band gap, the impact of gradual fl uorination on the optical and structural properties of zinc phthalocyanine (F n ZnPc) thin fi lms and the electronic characteristics of F n ZnPc/C 60 ( n = 0, 4, 8, 16) bilayer cells is investigated. UV-vis measurements reveal similar Q-and B-band absorption of F n ZnPc thin fi lms with n = 0, 4, 8, whereas for F 16 ZnPc a different absorption pattern is detected. A correlation between structure and electronic transport is deduced. For F 4 ZnPc/C 60 cells, the enhanced long range order supports fi ll factors of 55% and an increase of the short circuit current density by 18%, compared to ZnPc/C 60 . As a parameter being sensitive to the organic/organic interface energetics, the open circuit voltage is analyzed. An enhancement of this quantity by 27% and 50% is detected for F 4 ZnPc-and F 8 ZnPc-based devices, respectively, and is attributed to an increase of the quasi-Fermi level splitting at the donor/acceptor interface. In contrast, for F 16 ZnPc/C 60 a decrease of the open circuit voltage is observed. Complementary photoelectron spectroscopy, external quantum effi ciency, and photoluminescence measurements reveal a different working principle, which is ascribed to the particular energy level alignment at the interface of the photoactive materials.

Importance of Substrate Work Function Homogeneity for Reliable Ionization Energy Determination by Photoelectron Spectroscopy

“…The shoulder at high kinetic energy in the SECO spectrum is again well discernable. The conventional determination of IE from the low kinetic energy cutoff and the low binding energy E HOMO results in an IE for C 60 of 6.06 eV, falling way short of the established literature values of 6.45 ± 0.1 eV . In contrast, taking the higher kinetic energy onset of the SECO spectrum in Figure (c) in combination with the low binding energy E HOMO (i.e., both parameters corresponding to signals from C 60 molecules atop the high local work function FSH‐SAM areas) results in an IE of 6.44 eV, now in agreement with pertinent literature.…”

Charge Separation at Nanostructured Molecular Donor–Acceptor Interfaces

“…This quenching can appear either by a complete exciton transfer, e.g. by Förster or Dexter mechanism as in the case of DIP/CuPc [65], or by converting the Frenkel-type exciton into a long-lived charge transfer (CT) state or free charge carriers as for DIP/C60 or ZnPc/C60 [8,69]. As a figure of merit, the relative quenching Q defined by the PL intensity ratio with and without quencher…”

Charge transport and prototypical optical absorptions in functionalized zinc phthalocyanine compounds: A density functional study

“…These are found to be in close agreement with the values reported by others. () Similarly, other XZnPcs also exhibited both Soret band and Q‐band in both gas phase and DMSO medium, respectively. For example, as reported in Table and shown in Figure , in Br 16 ZnPc, we observe two prominent peaks in B‐band, a B 1 ‐band at 350 nm ( ), and a B 2 ‐band at 327 nm ( ), in DMSO, respectively, and a Q‐band at 688 nm, which corresponds to 11 A 1 u (| H >) (| L >) transitions.…”