ABSTRACT
Solar cells are considered as an important source of renewable energy to solve the shortage of energy today. Various solar cells have been developed and one of the most promising solar cells is Organic Solar Cells (OSCs). In recent years, OSCs have received much attention due to several potential applications, including compatibility with flexible substrates, low-manufacturing cost and large-area fabrication process [1-5]. The power conversion efficiency of OSCs had increased steadily in the recent years due to the improvement of materials and device structure. In organic materials, light absorption leads to the formation of excitons (bound electron-hole pairs) rather than the free electron-hole pairs. That is to say, it needs an efficient exciton dissociation
2 EXPERIMENTAL
The materials in the device were molybdenum oxide (MoO3) (99%, Aldrich), PCDTBT (99%, Lumintech), PC71BM (99.9%, Nano C), and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (99%, Aldrich), which used without further purification. The ITO-glass substrates (with a sheet resistance of 7, 15 Ω/sq) are sequentially cleaned by ultra sonic treatment in acetone, isopropyl alcohol, deionized water for 5 min, and blown by N2 gas before deposition. The MoO3 was deposited at a deposition rate of 0.02∼0.04 nm/s on the ITO substrate. The mixture of PCDTBT: PC71BM at ratio 1:4 dissolved in dichlorobenzene solvent with a concentration 7 mg/ml, and spincast on the top of MoO3 layer at the rotation speed of 1000-3000 rpm. The film was baked for 60 min at 75 °C in glove box. Finally, the BCP layer was deposited on the top of PCDTBT: PC71BM and the 100 nm Al as a cathode was deposited by thermal evaporation through a shadow mask, giving an active area of 6 mm2 at a deposition rate of 2∼3 nm/s. The thickness of thin film was monitored by an oscillating quartz thickness monitor. The structure of device in this study is shown in Figure 1. The current densityvoltage (J-V) characteristics are measured with a Keithley 2400 sourcemeter, under an illumination of 100 m W/cm2 with an AM1.5G sun simulator. The intensity of irradiation was measured by an optical power meter. All measurements were carried out in air with encapsulation.
