Highly flexible and solution-processed organic photodiodes and their application to optical luminescent oxygen sensors

We present a solution-processed flexible organic photodiode (f-OPD) with a bulk heterojunction (BHJ) structure based on a blend of poly (3-hexylthiophene-2,5-diyl) and 1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C-61 (P3HT:PCBM). We used Cs2CO3 -doped polyethyleneimine ethoxylated (d-PEIE) as the electron transport layer (ETL) material, which significantly improved the electron injection properties of the f-OPD. Compared with f-OPDs with conventional ETL materials such as Cs2CO3 , the external quantum efficiency (EQE) of the d-PEIE-based f-OPD was highly improved. Analytical results showed that the d-PEIE reduced the work function of the cathode, thereby facilitating the efficiency of electron injection from the active layer (AL) to the cathode of the f-OPD. In addition, after 10,000 cycles of tensile bending at a bending radius of 5 mm, the normalized I-D variation (ID/ID0) in the d-PEIE-based f-OPD remained above 90%, indicating an excellent device bending stability. Finally, f-OPD-based luminescent oxygen (O-2) sensors were successfully fabricated consisting of a photo-luminescent O-2 sensing film, a light source, and an f-OPD. The O-2 sensors based on d-PEIE-based f-OPDs showed the highest photocurrent and O-2 sensitivity in relation to the O-2 concentration compared with O(2 )sensors based on f-OPDs with conventional ETL materials.