Electronic structure of fullerene derivatives in organic photovoltaics

Highlights

• Electronic structure of fullerene derivatives for OPVs was systematically studied.

• The correlation between the molecular structure and the electronic structure is discussed.

• The side chains have great influences on ionization energy and affinity of the compounds.

Abstract

The electronic structures of the fullerene derivatives [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), [6,6]-diphenyl C₆₂ bis (butyric acid methyl ester) (bisPCBM), C₇₀, [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM), [6,6]-phenyl-C₆₁-butyric acid butyl ester (PCBB), [6,6]-phenyl-C₆₁-butyric acid octyl ester (PCBO), [6,6]-thienyl-C₆₁-butyric acid methyl ester (TCBM), and indene-C₆₀ bisadduct (ICBA), which are frequently used as n-type materials in organic photovoltaics, were studied by ultraviolet photoelectron spectroscopy and inverse photoemission spectroscopy. We also performed molecular orbital calculation based on density functional theory to understand the experimental results. The electronic structures near the energy gap of the compounds were found to be governed predominately by the fullerene backbone. The side chains also affected the electronic structures of the compounds. The ionization energy and electron affinity were strongly affected by the number of carbons and functional groups in the side chain.

Introduction

Organic photovoltaics (OPV), particularly OPV devices containing a polymer/fullerene-based bulk heterojunction (BHJ), have attracted much interest because of their potential for low-cost, large-area, lightweight, and flexible devices with simple structures [1], [2], [3].

The fullerenes C₆₀ and C₇₀ and their derivatives bearing various functional group side chains have been used as n-type semiconductor materials in OPV devices with high-efficiency photoelectric conversion [4], [5], [6], [7], [8]. C₆₀ and C₇₀ are incompatible with solution processes because of their low solubility in common organic solvents. Soluble derivatives have been synthesized by adding functional groups to these fullerene backbones [9], [10], thus allowing OPV devices to be fabricated using solution processes such as spin-coating [11]. [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) is a well-known soluble derivative and has been frequently used as an acceptor in OPVs [4], [9]. Electronic structure of the fullerene derivatives has been investigated by some groups so far [12], [13], [14], [15], [16]. It has been reported that the side chains of PCBM and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM) affect the solubility and morphology of the film, and its electronic structure, which may improve device performance [14], [15], [16].

OPV performance, particularly optical absorption, carrier injection, and carrier transport, strongly depends on the electronic structure of the donor and acceptor molecules [17], [18]. The electronic structure around the Fermi level (E_F), such as the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), plays an important role in determining the optical and transport properties [19]. For example, the correlation between the electronic structure of the donor or acceptor molecules and the open-circuit voltage (V_OC) of the device, which is an energetic driving force for electron transfer from the donor to the acceptor, is still not fully understood. It is thought that V_OC is related to the difference between the LUMO energy of the acceptor and the HOMO energy of the donor [20], [21]. Furthermore, the excitons, which are created after light absorption and migrate to the donor/acceptor interface, separate into electrons in the LUMO of the acceptor and holes in the HOMO of the donor. Thus, understanding the electronic structures of donor and acceptor molecules is important for elucidating the mechanisms by which OPV devices operate and for optimizing materials for high-performance devices.

Akaike et al. [14], [15] reported the effects of the side chain on the electronic structure of PCBM and [6,6]-diphenyl C₆₂ bis(butyric acid methyl ester) (bisPCBM). They concluded that a subtle charge transfer from the side chain to the C₆₀ backbone destabilizes the electronic states of the molecule. They also suggested that the effects of the side chain on the electronic structures of PCBM and bisPCBM may improve the performance of the OPV devices compared with devices containing C₆₀ [22], [23]. Their work demonstrates that measures of OPV device performance, such as V_OC, J_SC, and fill-factor, can be discussed in terms of electronic structure. There are few other studies of the electronic structure of fullerene derivatives.

The purpose of this study is to systematically investigate the electronic structure of the fullerene derivatives used in OPVs. Fundamental information about the electronic structure of fullerene derivatives can be expected to guide the synthesis of new molecules optimized for high-performance OPVs. The electronic structures of the fullerene derivatives PCBM, bisPCBM, C₇₀, PC₇₀BM, [6,6]-phenyl-C₆₁-butyric acid butyl ester (PCBB), [6,6]-phenyl-C₆₁-butyric acid octyl ester (PCBO), [6,6]-thienyl-C₆₁-butyric acid methyl ester (TCBM), indene-C₆₀ monoadduct (ICMA), and indene-C₆₀ bisadduct (ICBA) (Fig. 1) were examined by ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). To interpret the experimental results, molecular orbital (MO) calculations were performed.

The electronic structures of the fullerene derivatives strongly depended on structural features, including the type of backbone, number of side chains, side chain length, and functional groups. We investigated the effect of side chains by comparing C₇₀ with PC₇₀BM using the same method as Akaike et al. [14], [15] The dependence of the electronic structure on the fullerene backbone and the side chain length are discussed by comparing PCBM, PC₇₀BM, PCBB, and PCBO. The difference in electronic structure caused by replacing a phenyl group with a thienyl group in the side chain is also investigated by comparing PCBM with TCBM. The effect of introducing a different type of side chain on the electronic structure was examined by investigating ICBA.