Abstract
Tuning the electronic band structures such as band-edge position and bandgap of organic semiconductors is crucial to maximize the performance of organic photovoltaic devices. We present a simple yet effective electron irradiation approach to tune the band structure of [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) that is the most widely used organic acceptor material. We have found that the lowest unoccupied molecular orbital (LUMO) level of PCBM up-shifts toward the vacuum energy level, while the highest occupied molecular orbital (HOMO) level down-shifts when PCBM is electron-irradiated. The shift of the HOMO and the LUMO levels increases as the irradiated electron fluence increases. Accordingly, the band-edge position and the bandgap of PCBM can be controlled by adjusting the electron fluence. Characterization of electron-irradiated PCBM reveals that the variation of the band structure is attributed to the molecular structural change of PCBM by electron irradiation.
Highlights
Organic semiconductors such as small molecules [1,2] and conjugated polymers [3,4] are widely used in organic photovoltaic cells [4,5,6], dye-sensitized solar cells [2,7], organic field-effect transistors [8,9,10], and organic light-emitting diodes [3,11]
The first reduction peak of pristine phenylC61-butyric acid methyl ester (PCBM) was located at -1.11 V vs. Ag/Ag +; the peak was negatively shifted to -1.22 V, -1.41 V, and -1.49 V as the electron fluence was increased to 3.6 × 1016, 7.2 × 1016, and 1.44 × 1017 cm-2, respectively
PCBM is a fullerene derivative of which molecular structure comprises a side chain of butyric acid methyl ester and a phenyl ring attached on a C60 cage [1], Fourier transform infrared (FTIR) spectra show that the intensities of all the peaks decreased with increasing the electron fluence (Figure 2A), suggesting that the molecular bonds of PCBM are gradually decomposed by electron irradiation
Summary
Organic semiconductors such as small molecules [1,2] and conjugated polymers [3,4] are widely used in organic photovoltaic cells [4,5,6], dye-sensitized solar cells [2,7], organic field-effect transistors [8,9,10], and organic light-emitting diodes [3,11]. [6,6]-phenylC61-butyric acid methyl ester (PCBM) is a small molecule that is most widely used as an electron acceptor in organic photovoltaic (OPV) cells [1]. Several efforts have been made to increase the LUMO level of PCBM by chemical approach, for instance, placing electron-donating and electron-withdrawing substituents on the phenyl ring or synthesizing bisadduct analogue of PCBM [12,13,14]. These approaches generally require complicated synthetic procedures and result in a low yield of the products [13].
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