No abstract

A linear array of aluminum discs is deposited between the driving electrodes of an extremely large planar polymer light-emitting electrochemical cell (PLEC). The planar PLEC is then operated at a constant bias voltage of 100 V. This promotes in situ electrochemical doping of the luminescent polymer from both the driving electrodes and the aluminum discs. These aluminum discs function as discrete bipolar electrodes (BPEs) that can drive redox reactions at their extremities. Time-lapse fluorescence imaging reveals that p- and n-doping that originated from neighboring BPEs can interact to form multiple light-emitting p-n junctions in series. This provides direct evidence of the working principle of bulk homojunction PLECs. The propagation of p-doping is faster from the BPEs than from the positive driving electrode due to electric field enhancement at the extremities of BPEs. The effect of field enhancement and the fact that the doping fronts only need to travel the distance between the neighboring BPEs to form a light-emitting junction greatly reduce the response time for electroluminescence in the region containing the BPE array. The near simultaneous formation of multiple light-emitting p-n junctions in series causes a measurable increase in cell current. This indicates that the region containing a BPE is much more conductive than the rest of the planar cell despite the latter's greater width. The p- and n-doping originating from the BPEs is initially highly confined. Significant expansion and divergence of doping occurred when the region containing the BPE array became more conductive. The shape and direction of expanded doping strongly suggest that the multiple light-emitting p-n junctions, formed between and connected by the array of metal BPEs, have functioned as a single rod-shaped BPE. This represents a new type of BPE that is formed in situ and as a combination of metal, doped polymers, and forward-biased p-n junctions connected in series.

We use a micro-manipulated vacuum probe station to generate and visualize bipolar electrochemical redox reactions in a solid-state polymer light-emitting electrochemical cell (PLEC). In situ electrochemical p-and n-doping of a luminescent polymer is initially induced via a pair of biased metallic probes in direct contact with the luminescent polymer. Subsequently, the biased probes are moved to contact the planar aluminum driving electrodes of the PLEC to activate the device. By analyzing the complex doping patterns generated, we conclude that the doped polymers have functioned as bipolar electrodes (BPEs), from which electrochemical p-or n-doping are induced wirelessly. The potential energy barrier between the polymer BPE and the undoped polymer have played a major role in doping initiation. In a separate planar cell of a smaller gap size, a pair of planar aluminum electrodes was driven in such a way that they functioned as long BPEs to create five coupled and strongly emitting polymer p-n junctions. These results offer vivid visualization of the intriguing bipolar electrochemical phenomena in a solid-state polymer blend. The ability to form a BPE in situ, and in the form of a heavily doped polymer offer innovative ways to modify the doping profiles in molecular devices. The all-polymer BPE also expands the realm of bipolar electrochemistry to beyond that of a conventional liquid cell containing metal or carbon electrodes.

The long-term luminance decay of sandwich polymer light-emitting electrochemical cells has been investigated. The cells have been operated multiple times over a period of four months, all under a constant current density of 167 mA/cm2. In-between the constant-current runs, the cells were stored at room temperature for up to two months. We identify several factors that affect the luminance and its decay. The peak luminance reached during the virgin runs decreases if the cells are stored after the deposition of the top electrode. During operation, the luminance also decreases after reaching a peak value. However, extended storage at room temperature leads to the recovery of the peak luminance. The luminance recovery can be attributed to the relaxation of doping which reverses the effect of luminescence quenching. A long term, irreversible luminance decay is also observed and attributed to the formation of non-emitting, non-conductive black spots which leads to the loss of emitting area and an increased effective current density. The results illustrate the importance both On and Off states in characterizing the stability of polymer LECs.

Semiconductor homojunctions such as p-n or p-i-n junctions are the building blocks of many semiconductor devices such as diodes, photodetectors, transistors, or solar cells. The determination of junction depletion width is crucial for the design and realization of high-performance devices. The polymer analogue of a conventional p-n or p-i-n 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, the 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/e2 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 optical beam, the effective junction width (or volume) must be dramatically increased to realize a more efficient device.