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)

High resolution scanning optical imaging of a frozen planar polymer light-emitting electrochemical cell: an experimental and modelling study

“…This is in marked contrast to the PL profile of a real p-i-n junction. 58 The PL intensity of the p-doped region to the right is lower, as expected, but still at a significant level despite its dark appearance under UV illumination. The sensitivity of the PL scan is evident in the detection of a PL "transition zone" on the p side of the junction, between 0 and +30 µm.…”

“…Recently, we performed several OBIC studies of planar LECs with an interelectrode spacing ranging from 700 µm to 4.6 mm. [56][57][58] The extremely large gap size made it possible to perform OBIC scans using a variety of optical/cryogenic setups. The large planar cells were also easier to fabricate using shadow masking (vs. photolithographic) techniques.…”

“…A 700 µm frozen planar cell was scanned to reveal a p-i-n junction structure with a depletion width of about 18 µm between the quasi-intrinsic region and the p region. 58 In the present study, we devised a new scanning setup with a 1/e 2 beam size of only 1.9…”

The Dynamic Emission Zone in Sandwich Polymer Light‐Emitting Electrochemical Cells

“…[14,15] The developing p-i-n junction and the position of the zone where light is emitted (emission zone, EZ) can conveniently be followed by optical probing and photoluminescence experiments on so-called planar LECs that have a wide horizontal gap between the two electrodes. [6,[15][16][17][18][19][20][21][22][23][24] The summary of numerous studies is that the EZ in many cases is offcentred. In addition, it has been observed that the EZ is dynamic and shifts across the device until steady state is reached.…”

Stress Testing Polymer Light‐Emitting Electrochemical Cells: Suppression of Voltage Drift and Black Spot Formation

“…Time‐lapsed imaging of planar LECs establishes the fundamental operating mechanism of LECs: in situ electrochemical p‐ and n‐doping of the luminescent semiconductor, followed by light emission in the form of junction EL when the propagating doping fronts make contact to form a p‐n junction . It has been shown that the dynamic doping process involves the entire interelectrode gap, but the LEC junction occupies only a very narrow region between the p‐ and n‐doped neutral regions …”