All ink-jet printed polyfluorene photodiode for high illuminance applications

Abstract

High intensity light sensors are needed in order to monitor working environmental conditions and help with diagnosis of blast-related injuries which include traumatic brain injury (TBI). We have been developing a monitoring system that could be worn by soldiers to collect daily information that can be used as part of their medical record. This device is designed to be disposable, inexpensive and adhered directly to objects including non-planar surfaces. This requires that the development of light sensors with transparent top electrodes, high light intensity response and process compatibility with flexible substrates. All layers of the light sensor were fabricated by ink-jet printing. The all-printed light sensor is based on a blend of hole-accepting and electron-accepting derivatives of poly(9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine) (PFB) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), respectively. The surface of the PFB-F8BT organic photoactive layer is hydrophobic and represents a challenge when fabricating top light-absorbing devices from solution. We solved this fabrication challenge by using poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) as the top electrode and modifying the PEDOT:PSS ink in order to print directly onto the hydrophobic surface of the PFB:F8BT blend layer, placing the transparent electrode at the top of the light sensor structure. The bottom electrode was printed from a Ag nanoparticle solution. The printed high light intensity sensors were tested under illumination provided by a Newport 1600W solar simulator. I-V characteristics were measured as well as testing of the all-printed PFB:F8BT photosensors under short illumination pulses (0.5 to 0.75 seconds) as it occurs in light exposure from a explosion or blast. The photosensors show good linearity over multiple measurements at high illuminance values of (100 to 400) klux. External quantum efficiency and atomic force microscopy results will also be presented.