Low-Power Organic LED Fabricated by a Novel Solution-Based Process for Photoplethysmography Sensing

Power consumption level of electronic devices has always been considered as an important factor for a wide range of applications. In this study, a novel fabrication step, namely, predrying, applicable to solution-based fabrication processes is proposed to minimize the overall power consumption of organic light-emitting diodes (OLEDs). The predry step allows the solution to dwell on the substrate for a certain short time before the spin coater is turned on. It is experimentally shown that the predrying step for OLEDs with the hole transfer layer (HTL) of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) can reduce the ohmic loss by a factor of 2.41, in addition to reducing the turn-on voltage by 0.9 V and increasing the luminance 1.3 times at 15 V in comparison to the fabricated OLED without predrying step. To further minimize the power consumption of OLEDs, it is proposed to replace PEDOT:PSS with copper(I) thiocyanate (CuSCN) as HTL, which can be deposited by a solution fabrication process including the predrying step. A comparison among the fabricated OLEDs with the predried HTLs made of CuSCN and PEDOT:PSS indicates that the generated luminance increases by a factor of 4.28 from 2.8 to 12 kcd/m2 for an identical voltage of 11 V. Due to the superior performance of the proposed OLED based on the predried HTL of CuSCN, its behavior as a light source for photoplethysmography (PPG) sensing systems is surveyed. In this article, the PPG sensor is implemented in the form of a flexible-hybrid sensor system, in which the fabricated OLED and a commercial photodiode are housed on a flexible substrate. Results confirm that the implemented flexible-hybrid PPG sensor operating in reflectance mode can acquire high-quality PPG signals with relatively low power consumption and accurately estimate heart rate (HR). The competency of the proposed OLED as a low-power solution for PPG sensor technology is highlighted by comparing its performance with the state of the art.