Identifying Molecular Orientation in a Bulk Heterojunction Film by Infrared Reflection Absorption Spectroscopy

Abstract

The molecular orientation of organic molecules of zinc phthalocyanine (ZnPc) in single-component films on copper iodide (CuI) substrates can be controlled to achieve a molecular orientation lying flat on the substrate (flat-on) owing to π–d orbital interactions between the ZnPc molecules and the CuI. A 3-fold enhancement in the performance of organic photovoltaic cells has been reported by introducing a CuI interlayer between a ZnPc:fullerene (C60) bulk heterojunction (BHJ) film and the substrate. However, the mechanism underpinning the resultant solar cell performance enhancement was unclear. Herein, we report on the results of using in situ reflection absorption spectroscopy measurements during the vacuum deposition of coevaporated ZnPc:C60 BHJ films on various substrates to investigate the ZnPc molecular orientation. Our results revealed that the flat-on molecular orientation of ZnPc molecules in ZnPc:C60 BHJ films on CuI interlayers and flat-on ZnPc substrates can be successfully identified via the strong π–π interactions between the BHJ film and the substrate. The π–π interactions between individual ZnPc molecules are stronger than the π–d interactions between ZnPc molecules and CuI in coevaporated ZnPc:C60 films, as is evident from the molecular orientation of ZnPc, as determined by in situ reflection absorption spectroscopy. Our findings demonstrate that precisely controlling the molecular orientations of the films could enhance organic photovoltaic (OPV) performance. The present work provides important insights that will enable the design of higher performance OPV cells.

Vibrationally resolved absorption spectra and ultrafast exciton dynamics in α-phase and β-phase zinc phthalocyanine aggregates

“…Owing to its characteristics of good thermal stability, easily adjustable electronic properties, long exciton lifetime, and high absorbance in the visible and near-infrared regions, zinc phthalocyanine (ZnPc) has been widely applied in organic photovoltaics as a donor material. 1–11 The ultimate performance of the devices is closely associated with the microscopic structure features of the materials such as molecular orientation and stacking arrangements.…”

Local Cross-Coupling Activity of Azide-Hexa(ethylene glycol)-Terminated Self-Assembled Monolayers Investigated by Atomic Force Microscopy

“…11 Infrared Reflection-Absorption Spectroscopy (IR-RAS) Analysis. We introduced a facile and efficient approach, IR-RAS measurement technique 58,59 to investigate the chemical composition of the N 3 -EG 6 -SAM surfaces and to confirm the CuAAC reaction. Figure 5 shows the representative IR-RAS spectra of the bare N 3 -EG 6 -SAM and N 3 -EG 6 -SAM exposed to the CuAAC conditions with the TEGME molecules.…”

Temperature-dependent changes in the molecular orientation and visible color of phthalocyanine films

“…IR-RAS is regarded as a useful method to evaluate the molecular orientation of thin lms of phthalocyanines. [34][35][36][37] Although the vibrational modes of unsubstituted phthalocyanine zinc complexes have been well-studied, 38 it was necessary to assign the out-of-plane and in-plane vibrational modes of 1 before analyzing its spectra. For that purpose, we compared the experimental IR spectrum of KBr-dispersed 1 (as a model of the non-oriented state) and the theoretical (gas phase) spectrum of 1 obtained using DFT calculations (Fig.…”