Origin of Dark Current Robustness in Photomultiplication Organic Photodetectors Enabled by Ionic Conjugated...

Photomultiplication-type organic photodetectors (PM OPDs), which rely on signal amplification via electron trapping, suffer from a fundamental trade-off: high gain is typically accompanied by a steep increase in dark current under high reverse bias. Here, we overcome this challenge by employing ionic conjugated polyelectrolyte (CPE)-based electron blocking layers (EBLs) with an alternating fluorene-co-triphenylamine backbone. CPE-based EBLs in PM OPDs enable robust dark current stability under reverse bias up to -10 V while maintaining external quantum efficiencies (EQE) exceeding 2000%. Charge-dynamics analysis using Fowler-Nordheim plots reveals that a nanometer-thick CPE layer provides electron-blocking performance comparable to that of a ∼20 nm Al₂O₃ layer. At the same time, we show that the nature of the ionic side chains (cationic vs anionic) in CPEs governs interfacial energy-level alignment, thereby modulating hole selectivity and photocarrier dynamics. The design principles establish a general framework for interfacial and field engineering across a broad range of photodetector platforms-from organic to hybrid systems-guiding the development of next-generation photodetectors.