Ryohei Kobayashi

Wavelength-Selectivity of Organic Photoconductive Devices by Solution Process

Organic photoconductive devices sensitive to blue, green, and red lights were achieved using poly(9,9-dioctylfluorenyl-2,7-diyl)-co-1,4-benzo-(2,1,3)-thiadiazole (F8BT), rhodamine 6G-doped poly(9,9-dioctylfluorene) (PFO), nickel tetrakis(tert-butyl)phthalocyanine-doped PFO, respectively. These organic materials can be coated by a solution process, which leads us to realize the low fabrication cost and the large device area. Selectivity of spectral responses of these organic films were good enough to divide the incident light into three color components (blue, green, and red), indicating the possibility of a color separation without a prism for high resolution cameras. Quantum efficiencies of devices were estimated by absorption coefficients of photoconductive materials and photo-induced current densities while irradiating a visible light. The quantum efficiency of a blue sensitive device was better than those of green and red sensitive devices due to the suitable energy level of the blue sensitive material, F8BT.

Improved Photoconductive Characteristics of Solution-Processed Organic Device by Doping Silole Derivative

By doping 1,1-dimethyl-2,5-bis(N,N-dimethylaminophenyl)-3,4-diphenylsilole (silole-A) in poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), an improved photoconductive characteristics was observed for a single layer organic device fabricated by a spin-coating process. A maximum external quantum efficiency (EQE) was 8.9% at −20 MV/m when the ratio of silole-A:F8BT was 75 mol%. The EQE of the reference device with F8BT only was 0.06%, and the highest EQE was approximately 160 times higher than that of the reference device. In addition, the photoluminescence (PL) quantum efficiency of the silole-A:F8BT neat film was lower than those of silole-A and F8BT neat films. The lower PL quantum efficiency indicates that the photo-induced carriers efficiently dislocate in the organic layer, resulting in the high EQE of organic photoconductive device.

Improvements in Photoconductive Characteristics of Organic Device Using Silole Derivative

Poly[9,9-dioctylfluorenyl-2,7-diyl]-co-1,4-benzo-(2,1,3)-thiadiazole (F8BT) is one of the suitable materials for color-selective organic photoconductive devices owing to its high carrier mobility and absorption coefficient against only the blue light. We investigated a mixing method using a silole derivative, 1,1-dimethyl-2,3,4,5-tetraphenylsilole (DMTPS), in F8BT to improve the ratio between photocurrent and dark current (ON/OFF ratio), which is an important parameter for practical organic photoconductive devices. These organic materials can be coated by a solution process, which leads us to realize a low fabrication cost and a large device area in the future. By adding DMTPS into F8BT, the maximum improvement in ON/OFF ratio of 5 times was achieved compared with the reference device with F8BT only; however, the external quantum efficiency was independent of the concentration of DMTPS less than 50 wt %. In addition, the wavelength selectivity of DMTPS:F8BT in the visible wavelength region was almost the same as that of F8BT only. This result indicates that the DMTPS:F8BT layer can absorb only the blue light, indicating the possibility of a color separation without a prism for high-resolution cameras by combining the green- and red-sensitive devices.

Wavelength-Selectivity of Organic Photoconductive Devices by Wet Process

Organic photoconductive devices sensitive to blue, green, and red lights were achieved using poly[9,9-dioctylfluorenyl-2,7-diyl]-co-1,4-benzo-(2,1,3)-thiadiazole [F8BT], rhodamine6G-doped poly(9,9-dioctylfluorene) [PFO], nickel tetrakisi-(tert-butyl)phthalocyanine-doped PFO, respectively. These organic materials can be coated by a solution process, which leads us to realize the low fabrication cost and the large device area. Selectivity of spectral responses of these organic films were good enough to divide the incident light into three color components (blue, green, and red), indicating the possibility of a color separation without a prism for high resolution cameras. Quantum efficiencies of devices were estimated by absorption coefficients of photoconductive materials and photo-induced current densities while irradiating a visible light. The quantum efficiency of a blue sensitive device was better than those of green and red sensitive devices due to the suitable energy level of the blue sensitive material, F8BT.