Organic Thermoelectric Device Utilizing Charge Transfer Interface as the Charge Generation by Harvesting Thermal Energy

Currently, environmental power generation technologies that convert not only sunlight but also heat, vibration, and other micro-energies around us into electric power have been attracting attention around the world. Thermoelectric devices using waste heat have been put to practical use, but their use is limited due to the problems of using the high cost of materials and the limited installation space required to form a temperature difference. In this study, we propose organic thermoelectric devices based on a new mechanism by utilizing the charge-transfer (CT) complexes that cause carrier separation with thermal energy as low as room temperature and the charge diffusion ability in organic thin films.<br/>As CT forming materials, CuPc (Copper (II) phthalocyanine) and F₁₆CuPc (Copper (II) 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadecafluoro-29H,31H-phthalocyanine) were used as a donor and an acceptor, respectively. In addition, C₆₀ (fullerene) and BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) were inserted as the electron transport layers. In Device A, which utilizes CuPc (30 nm)/F₁₆CuPc (20 nm)/C₆₀ (40 nm)/BCP (20 nm), we confirmed power generation with the value of V_oc=180 mV, J_SC=20 nA cm^-2, and P_max=0.9 nWcm^-2 in a completely dark environment at room temperature. Further, when the F₁₆CuPc film thickness was changed from 100 nm to 200 nm (Device B), the power generation efficiency improved with increasing F₁₆CuPc film thickness. This can be ascribed to the improvement of diffusion force in the thicker F₁₆CuPc film, resulting in suppressed carrier recombination. When the C₆₀ film thickness was changed from 40 nm to 20 nm (Device C), we observed the thinner C₆₀ film leads to higher efficiency. Further, when the BCP film thickness was changed from 20 nm to 5 nm (Device D), the efficiency increased with decreasing BCP film thickness, while the devices without BCP showed lower efficiency. Based on these results, we fabricated an optimized CuPc (300 nm)/F₁₆CuPc (200 nm)/C₆₀ (3 nm)/BCP (5 nm) device and obtained the value of V_oc=462 mV, J_SC=3.94 μA cm^-2, and P_max=394 nWcm^-2, demonstrating the proposed devices have high potential as a new power generation mechanism.