Tuning the electronic band structure of PCBM by electron irradiation
© Yoo et al; licensee Springer. 2011
Received: 5 August 2011
Accepted: 4 October 2011
Published: 4 October 2011
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.
Organic semiconductors such as small molecules [1, 2] and conjugated polymers [3, 4] are widely used in organic photovoltaic cells [4–6], dye-sensitized solar cells [2, 7], organic field-effect transistors [8–10], and organic light-emitting diodes [3, 11]. In particular, [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM) is a small molecule that is most widely used as an electron acceptor in organic photovoltaic (OPV) cells . To improve the power conversion efficiency of OPV cells, open-circuit voltage (V oc) of the cells should be increased. The upper limit of the V oc is determined by the energy difference between the highest occupied molecular orbital (HOMO) level of the electron donor and the lowest unoccupied molecular orbital (LUMO) level of the electron acceptor . Thus, 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 [12–14]. However, these approaches generally require complicated synthetic procedures and result in a low yield of the products . As an alternative, radiation chemistry can be a good strategy to modify the chemical structures of particularly organic materials [15–18]. As a result of the chemical structural modification, the optical properties of the organic materials can be changed [19–21]. Here, we present a simple and novel approach to tune the HOMO and LUMO levels of PCBM based on electron irradiation. Only by irradiating an electron beam onto PCBM, the bandgap as well as the HOMO and LUMO levels of PCBM can be changed, and furthermore the electronic band structures of PCBM can be controlled by adjusting the electron fluence.
Results and discussion
HOMO and LUMO levels of PCBM changed by electron irradiation as a function of electron fluence
E on, red (1)
E p, ox
E on, ox
PCBM (3.6 × 1016 cm-2)
PCBM (7.2 × 1016 cm-2)
PCBM (1.44 × 1017 cm-2)
Consequently, from these analyses, we can conclude that the change in the band structure of electron-irradiated PCBM is attributed to the modification of the molecular structure of PCBM by electron irradiation (Figure 3). Formation of methoxy-substituted phenyl ring on PCBM up-shifts the LUMO level at low electron fluencies; further electron irradiation deforms the C60 cage and gradually converts it to hydrogenated amorphous carbon, resulting in the increase of the HOMO-LUMO gap. This also indicates that the band structure of PCBM can be tuned by adjusting the electron dose.
We have found that the electronic band structure of PCBM is changed by electron irradiation. The LUMO level of PCBM gradually up-shifts toward the vacuum energy level, while the HOMO level slightly down-shifts against the vacuum energy level as the electron fluence increase. Consequently, the bandgap of PCBM can be controlled by adjusting the electron fluence. The variation of the band structure is attributed to the change in the molecular structure of PCBM by electron irradiation. The electron irradiation technique can also be used to control the electronic band structures of other organic semiconductors and thus this irradiation technique can provide a useful strategy to improve the performances of organic photovoltaic and organic optoelectronic devices.
PCBM solution was prepared by dissolving PCBM (99.5% purity, Nano-C, Inc., Westwood, MA, USA) powder into chlorobenzene (≥ 99.5% purity, Sigma-Aldrich, St. Luois, Mo, USA). PCBM films were fabricated on glassy carbon electrodes by spin-coating a chlorobenzene solution containing 24 mM PCBM at 2,000 rpm for 60 s. The irradiation of an electron beam on PCBM films were carried out at room temperature and in vacuum lower than 2 × 10-5 Torr. An electron beam was generated from a thermionic electron gun with electron energy of 50 keV and current density of the electron beam was 1.6 μA cm-2 [16, 32]. The electron fluence was varied by adjusting the irradiation time. PCBM films were irradiated by 1, 2, and 4 h, which corresponds to electron fluence of 3.6 × 1016, 7.2 × 1016, and 1.44 × 1017 cm-2, respectively.
After electron irradiation of PCBM, the reduction and oxidation properties of PCBM were characterized by CV. The CV measurements were carried out using a three-electrode system consisting of the glassy carbon electrode as a working electrode, a platinum (Pt) wire as a counter electrode, and a Ag/Ag+ electrode as a reference electrode in an acetonitrile solution of 0.1 M Bu4NPF6. Potentials were quoted with reference to the internal ferrocene standard (E 1/2 = 0.120 V vs. Ag/Ag+) that was measured in the same electrolyte. The scan rate was 100 mV s-1 for all measurements. The changes in molecular structure of PCBM due to electron irradiation were investigated by 1H NMR (Bruker Biospin AvanceII 900, Bruker, Billerica, MA, USA), FTIR and high-resolution dispersive Raman spectroscopy (Jasco FT/IR-4100 (JASCO, Easton, MD, USA) and LabRAM HR UV/Vis/NIR (HORIBA Jobin Yvon, Edison, NJ, USA), respectively).
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (no. 2011-0020764).
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