Scanning photocurrent and PL imaging of a frozen polymer p-i-n junction

Abstract

Polymer light-emitting electrochemical cells (LECs) are two-terminal, solid state devices with a mixed ionic/electronic conductor as the active layer. Once activated by a DC voltage or current, a doping-induced homojunction dictates the electrical and optical response of the LEC, making it highly unique and attractive among organic devices. However, the depletion width, a fundamental parameter of any semiconductor homojunction, has never been determined experimentally for a static LEC junction. In this study, we apply spatially resolved photocurrent and photoluminescence (PL) scanning to an extremely large planar LEC that had been turned on to emit strongly then subsequently frozen. These concerted scanning and imaging studies depict a p–i–n junction structure in which the peak built-in electric field lies at the interface between the intrinsic region and the p-doped region. The corresponding 18 μm depletion width is very small compared to the 700 μm interelectrode spacing. A 700 μm planar polymer light-emitting electrochemical cell has been turned on, frozen and probed optically. Scanning PL and photocurrent imaging reveals a p–i–n doping profile. Photocurrent is only detected over a region of 18 μm centered at the i/p junction. Peak PL occurs near the center of the intrinsic region. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)