Fluorene-Type Polymer Photodetectors Doped with Iridium and Platinum Complexes as Opto-Electrical Conversion Devices
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Fluorene-Type Polymer Photodetectors Doped with Iridium and Platinum Complexes as Opto-Electrical Conversion Devices Hirotake Kajii, Hiroki Ohmori, Yusuke Sato, Akihiro Katsura and Yutaka Ohmori Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan ABSTRACT To improve the conversion efficiency of polymer photodetectors (PDs) fabricated by solution process, the properties of fluorene-type polymer photodetectors doped with iridium (Ir) and platinum (Pt) complexes were investigated. The devices based on poly(dioctylfluorene) and poly(dioctylfluorene-co-benzothiadiazole) (F8BT) had violet and blue sensitivity, respectively. Triplet materials can enhance the incident-photon-to-current conversion efficiency of the devices utilizing the fluorene-type polymers when their triplet levels are lower than the lowest excited singlet states of the host and higher than the lowest excited triplet states of the host. The transmission of a moving picture was successfully demonstrated using the bilayer F8BT device with green Ir complex as an opto-electrical conversion device. We demonstrate that the polymer PDs fabricated by solution process can be applied to short-range optical communication fields, such as opto-electrical conversion devices for optical links. INTRODUCTION Organic photodetectors (OPDs) have high potential for use in the future information technology system, especially for signal processing and optical sensing systems with flexible devices [1]. Photodetectors using organic materials have advantages in lightweight, flexibility, and easy fabrication process, in particular, for the materials which are soluble in solvent. Furthermore, we can select absorption wavelength of the OPDs by choosing organic materials with suitable absorption wavelength. Therefore, it is easy to realize color sensitive photodetectors. Fluorene-type polymers are one of soluble conducting polymers for polymeric devices because they exhibit high efficient emission, high stability and relatively high mobility [2]. Fluorene-type polymers with various absorption wavelengths are one of the promising materials for large-area optoelectronic devices such as organic photodetectors (OPDs). Employing phosphorescent materials in emissive layers leads to high efficiency because breaking the spin conservation rule indicates that both the singlet and triplet excitons contribute to emission.[3] Phosphorescent materials such as iridium (Ir) and platinum (Pt) complexes have heavy atom effects and induce intersystem crossing (ISC), which transforms excitons from singlet into triplet. Triplet excitons have a longer lifetime than singlet excitons. This long lifetime can be expected to reduce exciton recombination and improve conversion efficiency [4,5]. In this study, we investigated the properties of OPDs with Ir and Pt complexes doped in fluorene-type polymers to enhance the photocurrent and the conversion efficiency. EXPERIMENT Organic layers were fabricated by spin-coating onto a glass substrate coated with a patterned indiu
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