Origin of Dark Current Robustness in Photomultiplication Organic Photodetectors Enabled by Ionic Conjugated Electron‐Blocking Layers

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 Al 2 O 3 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.