Since the discovery of organic semiconductors, material and device processing has been enhanced by improvements in carrier injection, carrier mobility, and device stability. However, these improvements are insufficient in the case of organic devices employing organic semiconductor materials. To improve the performance of organic devices, we need better understanding of the carrier injection property at the electrode-organic layer interface. In this paper, we studied carrier injection and accumulation in organic semiconductor by using electrical and optical measurements, i.e., I-V, C-V, AFM, and FTIR measurements. We used 6,13-Bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) as the organic semiconductor layer. In order to enhance the carrier injection into TIPS-pentacene, we focused on the binding effect of the self-assembled monolayer, i.e., (3-Mercaptopropyl) trimethoxysilane (MPTMS) and pentafluorobenzenethiol (PFBT), deposited on the surface of the substrate.