Polymer Light‐Emitting Electrochemical Cells: Doping Concentration, Emission‐Zone Position, and Turn‐On Time

Abstract

Direct optical probing of the doping progression and simultaneous recording of the current–time behavior allows the establishment of the position of the light‐emitting p–n junction, the doping concentrations in the p‐ and n‐type regions, and the turn‐on time for a number of planar light‐emitting electrochemical cells (LECs) with a 1 mm interelectrode gap. The position of the p–n junction in such LECs with Au electrodes contacting an active material mixture of poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐p‐phenylene vinylene) (MEH‐PPV), poly(ethylene oxide), and a XCF3SO3 salt (X = Li, K, Rb) is dependent on the salt selection: for X = Li the p–n junction is positioned very close to the negative electrode, while for X = K, Rb it is significantly more centered in the interelectrode gap. Its is demonstrated that this results from that the p‐type doping concentration is independent of salt selection at ca. 2 × 1020 cm–3 (ca. 0.1 dopants/MEH‐PPV repeat unit), while the n‐type doping concentration exhibits a strong dependence: for X = K it is ca. 5 × 1020 cm–3 (ca. 0.2 dopants/repeat unit), for X = Rb it is ca. 9 × 1020 cm–3 (ca. 0.4 dopants/repeat unit), and for X = Li it is ca. 3 × 1021 cm–3 (ca. 1 dopants/repeat unit). Finally, it is shown that X = K, Rb devices exhibit significantly faster turn‐on times than X = Li devices, which is a consequence of a higher ionic conductivity in the former devices.

Electrochemical doping during light emission in polymer light-emitting electrochemical cells

“…The correspondence between the optical and electronic observation is strong; the charge consumed can be predicted by multiplying the front position by a constant, which includes the thickness and width of the film and the average doping concentration value during front progression. This factor does not vary, even when the applied potential varies between 5 and 20 V, [10][11][12] thus confirming our previous assumption that the doping front progressing indeed takes place at a constant doping concentration. The relatively small size of the n-doped region and side reactions that compete with the initiation of the n-doping process 12 prevents the analogous analysis of the n-doping front.…”

The dynamic organic p–n junction

“…[23][24][25][26][27][28][29][30] These ions rearrange during operation, which in turn allows for a range of attractive device properties, including low-voltage operation with thick active layers and stable electrode materials. [31][32][33][34][35][36] However, the further development of LECs is currently hampered by an inadequate understanding of the device operation. In fact, an active debate regarding the fundamental nature of LEC operation has continued for more than a decade, and two distinct models are competing for acceptance: the electrochemical doping model 18,32,[37][38][39][40] and the electrodynamic model 36,[41][42][43][44] .…”

“…Importantly, we performed SKPM measurements and light emission probing under similar conditions on separate sets of LEC devices (see methods section for details), in which the light emission zone is positioned far away from the electrode/active material interfaces. 35,39 As motivated above, the latter is critical in order to distinguish between the two models. It is made possible in these experiments by the use of an appropriate organic semiconductor-electrolyte active material driven with a relatively high voltage, so that undesired electrochemical side-reactions at the electrode/active material interfaces are effectively suppressed.…”

Operating Organic Electronics via Aqueous Electric Double Layers

“…Classic semiconductor devices such as Light Emitting Diodes (LEDs) [4] , Field Effect Transistors (FETs) [5][6] and solar cells [7][8][9][10] including electronic polymers as the semiconductor have already been developed. Moreover, some of the organic semiconductors (OSC) are also electrochemically active, which enables the development of organic electrochemical transistors (OECTs) [11] , super capacitors [12][13] , electrochromic display cells [14][15] and light emitting electrochemical cells (LECs) [16][17] . In addition, organic materials express several unique features making them suitable for biological and biochemical applications.…”