Heterjunctions of small molecular weight organic semiconductors

Photodiodes made of small molecular weight organic semiconductors have been fabricated. The organic materials under investigation are p-type Copper phthalocyanine (CuPc) and n-type 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI). Current-voltage and capacitance-voltage measurements have been performed to evaluate the electrical properties of the organic photodiodes. Optical measurements including responsivity and temporal response have also been carried out. It has been found that device efficiency of the photodiodes is influenced by the thickness of organic layers as well as the device structures. It is believed that organic layer thickness is a strong function of materials' exciton diffusion lengths. The thickness of the organic layer employed in the fabricated photodiode is 100 A or less. However, the fabricated devices with thin organic layer showed poor performance in terms of saturation current and breakdown voltage. As a result, new device structures have been proposed to overcome the problems. Stacking structure double heterojunction (SDH) and novel comb-like structure single heterojunction (CSH) device configurations have shown a substantial improvement in both electrical and optical parameters. There is at least 3 times enhancement in photocurrent and 10 times increase in external quantum efficiency for photodiodes employing CSH device structure and 100 A organic layer. Improvements in saturation current and breakdown voltage have also been observed. In addition, responsivity measurements in CSH devices have yielded a 7 times improvement. Furthermore, the carrier lifetime of the photodiode is estimated to be around 1 microsecond from the temporal response measurements. The combined effect of thin organic layers and new device structures leads to an overall improvement in device performance