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

Abstract

Semiconductor homojunctions such as p-n or p-in junctions are the building blocks of many semiconductor devices such as diodes, photodetectors, transistors or solar cells. The determination of junction properties including depletion width and doping concentration is crucial for the design and realization of high-performance devices. The polymer analogue of a conventional p-n or p-in junction can be created by in situ electrochemical doping in a polymer light-emitting electrochemical cell (LEC). As a result of doping and junction formation, the LECs possess some highly desirable device characteristics. The LEC junction, however, is still poorly understood due to the difficulties of characterizing a dynamic-junction device. Here we report concerted optical-beam-induced-current (OBIC) and scanning photoluminescence (PL) imaging studies of planar LECs that have been frozen to preserve the doping profile. By optimizing the cell composition, electrode work function and the turn-on conditions, we realize a long, straight and highly emissive p-n junction with an interelectrode spacing of 700 µm. The extremely broad planar cell allows for time-lapse fluorescence imaging of the in situ electrochemical doping process and detailed scanning of the entire cell. A total of eighteen scans at seven locations along the junction have been performed using a versatile, custom cryogenic laser scanning apparatus. The Gaussian OBIC profiles yield an average 1/e 2 junction width of only 1.5 µm, which is the smallest ever reported in a planar LEC. The controlled dedoping of the frozen device via warming cycles leads to an unexpectedly narrower OBIC profile, suggesting the presence and disappearance of fine structures at the edges of the frozen p-n junction. The results reported in this work provide new insight into the nature and structure of the LEC p-n junction. Since only about 0.2% of the entire device area is photoactive in response to an incident

Polymer light-emitting electrochemical cells—Recent advances and future trends

“…These conditions have been met in a recent study employing a focused laser beam and a large, frozen-junction planar PLEC. [50,51] Moreover, the scan can simultaneously record the photoluminescence (PL) intensity across the scan path. μm.…”

Visualizing electroluminescence process in light-emitting electrochemical cells.

“…For practical use, however, further improvements are required in their device performance such as operational lifetime and e ciency, for which their operation mechanism must be closely examined. The operating processes and dynamics of LECs have been studied by several experimental methods such as scanning Kelvin probe microscopy [30][31][32][33] , light beam induced current techniques [34][35][36][37][38][39] , and impedance measurements 40 , some of which enabled direct observation of the growth and spatial shift of doped regions for planar type LECs 32,41−44 . However, the rate of the EL turn-on process in LECs depends almost entirely on the slow ion conduction 28,45−48 , where much faster electronic charge injection processes are masked since the ion conduction limits the response rate.…”

Revealing the Impact of Heat Generation Using Nanographene-Based Light-Emitting Electrochemical Cells

“…[3][4][5] However, their wide implementation must also take into account not only a simple device architecture and a low-cost solvent-based fabrication process, 5,6 but also an excellent self-stability upon storage and mechanical stress 7 and the right operating conditions to meet the performance criteria for each application. 3,[7][8][9][10][11][12][13][14][15] In this context, the effect of the temperature in the LEC performance is a critical aspect that has barely been considered in the literature up to date. [16][17][18] In brief, Friend et al reported on the reversible color changes in LECs based on copper(I) complexes upon externally applying temperature, 16 while Slinker and co-workers nicely studied the relationship between the thermal stability of LECs with iridium(III) and ruthenium(II) complexes and the presence of their degradative metal centered excited states.…”

“…This also allows us to easily fabricate devices with a good tolerance toward the active layer thickness, the type of electrodes/substrates, and the type of deposition techniques. Indeed, LECs could be applied to a myriad of lighting applications, including labeling, decorative, and medical applications, among others. − However, their wide implementation must also take into account not only a simple device architecture and a low-cost solvent-based fabrication process , but also an excellent self-stability upon storage and mechanical stress and the right operating conditions to meet the performance criteria for each application. ,− …”