Impact of Interlayer Optimization on the Performance of Flexible, Self‐Powered Organic Photodetectors for Ambient Light‐Based PPG Sensing

Abstract This study explores optimizing interlayers in flexible, self‐powered organic photodetectors (OPDs). Integration of conductive PEDOT:PSS (poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate)), PH1000 flexible anodes on PET substrates demonstrates that PH1000 outperforms traditional ITO (Indium tin oxide) anodes, offering higher optical transmittance, minimal surface roughness, and superior mechanical stability, with less than 10% resistance change after 2000 bending cycles. Various hole transport layers (HTLs) and electron transport layers (ETLs) are studied, showing that their incorporation reduces dark current and increases photocurrent at zero bias. Mott‐Schottky analysis reveals that HTLs and ETLs improve exciton dissociation. CuSCN (Copper(I) thiocyanate) HTL and PFN‐Br (Poly(9,9‐bis(3′‐(N,N‐dimethyl)‐N‐ethylammoinium‐propyl‐2,7‐fluorene)‐alt‐2,7 (9,9‐dioctylfluorene))dibromide) ETL provide the highest performance enhancements, reducing trap density and improving exciton dissociation. Capacitance‐frequency measurements and photoluminescence spectroscopy confirm enhanced exciton dissociation due to reduced binding energy, especially for CuSCN. Charge collection efficiency (CCE) is significantly improved, with CuSCN ETL and PFN‐Br ETL achieving a CCE of over 22.93% at zero bias. PH1000‐based, zero‐biased OPDs with CuSCN and PFN‐Br show a maximum responsivity of 0.23 A/W, specific detectivity of 3.89 × 10 ^{1 2} Jones, rapid response times, and excellent mechanical stability. These OPDs, integrated into an ambient light‐based photoplethysmography sensing system, provide accurate heart rate monitoring, demonstrating their potential for next‐generation low power wearables.