Decoupled luminance decay and voltage drift in polymer light-emitting electrochemical cells: Forward bias vs. reverse bias operation

Abstract

Sandwich polymer light-emitting electrochemical cells (LECs) with low electrolyte loading have been investigated. The LECs can be turned on to emit light at low voltages by applying either a forward- or reverse-bias current. The reverse operation, however, displays significantly higher luminance, frozen-junction behavior, and voltage instability. Moreover, the LECs display de-coupled luminance decay and voltage drift. Under forward-current operation, the luminance decreases for most of the test duration, while the operating voltage remains nearly unchanged. Under reverse-current operation, the luminance remains constant for over 200 h while the operating voltage more than doubles. The highly asymmetric device characteristics between forward and reverse operation can be explained by the different doping profiles created when the initial charge injection barriers are different. Under forward-current operation, the LEC film is predominantly p-doped. This leads to heavy luminescence quenching but a stable operating voltage. Under reverse operation, a predominately n-doped film is responsible for the higher luminance, frozen-junction behavior, and voltage drift. The anomalous device characteristics offer additional insights into the complex operating mechanism of LECs and the behaviours of a doped luminescent polymer.

Recent Progress in Ionic Iridium(III) Complexes for Organic Electronic Devices

“…In contrast, in the ECD model, anions accumulate at the anode and cations accumulate at the cathode, respectively, forming highly conductive p ‐ (positive) and n ‐ (negative) doped regions, which broaden over time until a p–i–n junction is formed in between (i refers to “intrinsic”, i.e., an undoped region), followed by charge recombination and light emission . However, no final conclusion has yet been reached on the definite working mechanism of LECs, in spite of many related efforts …”

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

“…Indeed, cell destruction is often preceded by a rapidly increasing driving voltage even while the cell luminance is fairly constant or even increasing in magnitude. One extreme example is an LEC that exhibits relatively high, nearly constant luminance (between 451 and 486 cd m −2 ) over a period of more than 200 h, while in fact, during an identical period, the cell was close to failure when the driving voltage increased from 4 to 8.7 V . Another indicator of cell degradation is the appearance of nonemissive black spots that reduce the effective emitting area of the cell .…”

Design, fabrication and application of organic power converters: Driving light-emitting electrochemical cells from the AC mains

“…We also find that the stability of LECs driven by the pulsed bias from the half-wave converter is rather limited, presumably since the negative voltage transients will produce electrochemical side reactions at the reverse biased electrodes. [50][51][52] The AC mains in most of Europe, Africa, Asia, Australia, and South America operate at V AC = 230 V and not at V AC = 115 V, but we find that the stability of the half-wave rectifier converter circuit itself is limited during long-term operation at V AC = 230 V (see figure S3(a)-(c)); presumably since a single DC-OFET device has to carry the entire power load during operation (see figure 1(a) and 2(a)), which can lead to severe self-heating in its active material. With that in mind, we shift our attention to the full-wave converter, as it splits the dissipated power during operation between two serially connected DC-OFETs (see figure 1(a) and 2(c)) and self-heating as a consequence should be less of an issue.…”