Bias-Switchable Broadband–Narrowband Organic Photodetectors Enabled by Vertical Phase Separation in Thick PM6: Y6 Bulk Heterojunctions

Bias-switchable organic photodetectors (OPDs) are promising candidates for multifunctional optical sensing due to their ability to reversibly transition between distinct detection modes within a single device. However, existing dual-mode OPDs typically rely on complex multilayer architectures or bidirectional illumination, which limit device integration and scalability. Here, we report a simplified dual-mode OPD consisting of a single thick PM6: Y6 bulk heterojunction (BHJ) active layer (∼810 nm) in an ITO/PM6: Y6/Au architecture. The device exhibits two distinct and reversible spectral-response modes under a low operation voltage of ±0.5 V. Under forward bias (+0.5 V), the OPD delivers a broadband photoresponse spanning 300-900 nm with an external quantum efficiency (<i>EQE</i>) up to 41.3%, dark current density of 1.9 × 10^-7 A cm^-2, and a detectivity (<i>D</i>_sh*) of 1.1 × 10¹² Jones at 840 nm. Under reverse bias (-0.5 V), the device transitions to a narrowband response centered at 900 nm (FWHM = 47.8 nm), accompanied by a reduced dark current of 1.8 × 10^-8 A cm^-2, linear dynamic range of 112.6 dB, and -3 dB bandwidth of 45.1 kHz. Depth-dependent compositional analysis and optical-field simulations reveal pronounced vertical phase separation, with PM6 enriched near the ITO and Y6 toward the Au electrode, combined with wavelength-dependent absorption in the thick BHJ. This morphology enables charge-collection narrowing (CCN) under reverse bias and broadband photoconductive behavior under forward bias. Moreover, the closely matched work functions of the ITO and Au electrodes minimize the built-in electric field, which is essential for preserving the CCN region and ensuring robust bias-switchable dual-mode operation. The work elucidates key design principles for single-layer dual-mode OPDs and demonstrates a low-cost, high-performance, and integration-ready architecture for advanced optoelectronic applications.