Impact of Ground‐State Charge Transfer and Polarization Energy Change on Energy Band Offsets at Donor/Acceptor Interface in Organic Photovoltaics

Abstract

The fullerene (C60)/copper phthalocyanine (CuPc) interface is one of the widely used donor/acceptor (DA) interfaces for organic photovoltaics (OPVs), and information on the electronic structure at the interface is essential for fully understanding the energetics of excitons and carriers in OPVs. Here, an investigation into the energy levels at the C60/CuPc interface is made using UV photoelectron, X‐ray photoelectron, and inverse photoemission spectroscopies. The vacuum level and core levels rise with C60 deposition on the CuPc film, which indicates that the interfacial dipole is formed with the negative charge on the C60 side. The interfacial dipole can be formed by the electron transfer from CuPc to C60 in the ground state at the interface, which is indicated by the analysis of the UV–vis–NIR absorption spectrum of the CuPc/C60 blended film. On the other hand, the highest occupied and lowest unoccupied molecular orbitals of CuPc and C60 shift in opposite directions at the interface. This is attributed to the changes of the polarization energies of CuPc and C60 at the interface. The formation of the interfacial dipole and the change of the polarization energy result in the anomalous energy band offsets at the C60/CuPc interface, which are entirely different from those in inorganic p–n junctions.

Optical evidence for intermolecular coupling in mixed films of pentacene and perfluoropentacene

“…2 Fig. 2(a) it can be clearly seen that the mixed film spectra are not a linear combination of the single film spectra, which suggests strong intermolecular coupling, 20,21 an assumption further supported by testing also nonlinear mixing models, see Sec. III B.…”

“…Since there are only a few publications concerned with optical spectra of organic blends, [20][21][22][23] no "standard procedure" for a data analysis of coupling effects has emerged. As a first approach, we perform a decomposition into single-component subbands.…”

Correlation between interface energetics and open circuit voltage in organic photovoltaic cells

“…Note that this result, obtained by careful fitting of the spectra, is in contradiction with the result of another UPS/IPES study of the D/A copper-phthalocyanine (CuPc)/C 60 interface, where significant changes in polarization energy (and thus in E t ) was suggested for the interface compared to the neat materials. 15 Knowledge of the acceptor E t at the interface now allows the determination of E PVG of other D/A interfaces by measurement of the energy separation between the D and A HOMO levels at the heterojunction (illustrated in Fig. 1), which we present below.…”

“…10,11 For the time being, experimental determination of interface energetics is indispensable to understand the processes inside an OPVC based on reliable values of E PVG , but only few pertinent studies have been conducted to date. [12][13][14][15] The present study focuses on E PVG values at prototypical organic D/A pairs formed between four organic semiconductors [sexithiophene (6T), fullerene (C 60 ), diindenoperylene (DIP, chemical structure shown in Fig. 1), and poly(3-hexylthiophene) (P3HT)] determined using the combination of ultraviolet and inverse photoemission spectroscopy (UPS and IPES).…”

Charges and Excited States in Organic Semiconductors

“…Figure 2.62 illustrates in an exemplary way the kind of energy shifts that may occur. For more detail, the reader is referred to the original literature [190,[199][200][201].…”