Hybrid Quantum Dot Light Emitting Electrochemical Cells

Several new light generation technologies that are overshadowed by LEDs and OLEDs are investigated. Most of these technologies are in a very early stage of research. One such technology, the LEC technology, was presented at LpS 2017. Light emitting electrochemical cells can be compared to OLEDs, but they are based on a much simpler inorganic architecture. The innovation in the presented approach lies in the combination with quantum dots, resulting in a hybrid solution. This novel attempt with all its consequences and future prospects was the reason that the LpS Scientific Award jury voted to bestow the award on Dr. Ekaterina Nannen, Group Leader of the Research Group „Solid State Lighting“ at the Nano-Energie-Technik- Zentrum (NETZ) of the University Duisburg-Essen and her research team, Julia Frohleiks and Svenja Wepfer.

The solution based processing makes light emitting electrochemical cells (LECs) a promising device alternative for large area flexible lighting solutions. Compared to organic light emitting diodes (OLEDs) and Quantum Dot (QD) LEDs, LECs exhibit a much more simple device architecture comprising ionic components, which facilitate charge carrier injection into the device under an applied electrical field. Despite the potential benefits of LECs as well as recent advances, the market entry of these types of devices is challenging mainly due to the lack of coinciding efficient, bright and long-term, stable emitter materials of different colors. We present an all solution based hybrid device concept implementing colloidal QDs as an additional active light emitting layer in Ir-iTMC based LECs. The hybrid devices show light emission from both QDs and LEC emitter. Electro- and photoluminescence measurements indicate that in the chosen device architecture, charge carriers can be injected directly into both light emitting species, which is beneficial for the implementation of QDs with various band gaps, and thus creation of white light emitting hybrid devices. The additional QD layer furthermore improves the electron injection into the active LEC layer which leads to a faster device turn-on and an improved charge carrier balance, resulting in an increased luminance and device efficiency. Hybrid devices containing yellow emitting iTMC and blue QDs emit white light at a maximum CRI of 78 with an external quantum efficiency of 0.03 %.

Future lighting applications exceed the pure creation of light and get more and more inspired by innovative design possibilities due to large-area and at the same time flexible lighting technologies. The prototypes of light-emitting stickers, wallpapers and windows as well as animated rear lights are becoming reality [1-4]. Along with cost-effective and easy solution-based fabrication techniques, the next generation of lighting technologies can overcome challenges of epitaxial LEDs in flexible and shapeable design applications, leading to space-saving installation of light-sources in multiple applications. Light-emitting electrochemical cells (LECs) [4-7] represent a promising alternative for large-area device concepts, besides the more common organic light-emitting diodes (OLED) [8,9]. In contrast to OLEDs, the active layer of the LECs comprises ionic components in addition to the light emitting species (polymers or transition metal complexes). The incorporated ions start moving due to the electrical field at applied voltage and thereby facilitate charge injection into the light-emitting component, so that the multiple charge injection and transport layers, which need to be carefully controlled, specifically optimized and are thus mandatory for OLEDs, can be omitted. The whole layer stack can be simplified (even down to a single layer), only by the addition of mobile ionic species. Furthermore, air-stable electrode materials can be implemented and thickness variations of the active layer can be tolerated, so that easy solution-based fabrication procedures such as spin-coating, spraying, printing or slot die and dip coating can be established even in ambient conditions [4-7,10,11].

Since the charge carrier injection and transport in this type of device is initiated by the movement of the ionic species, the response and turn-on time of LECs is comparatively long, making LECs more applicable in lighting technology rather than high-end displays [5,12]. Based on the active light-emitting material, two different classes of LECs can be defined as it is also the case for OLEDs: polymer LECs (p-LECs) or ionic transition metal complex LECs (iTMC-LECs). iTMC-LECs are able to harvest both singlet and triplet excitons and thus allow for intrinsically higher quantum efficiencies than fluorescent p-LECs, resulting in typically higher brightness above 1000 cd/m² [10,13-16]. In case of yellow iTMC-based LECs, intramolecular π-π stacking interactions led to a stable emitter complex and, together with the introduction of a pulsed operation mode, yielded yellow LEC devices with lifetimes exceeding 4000 hours at maximum luminance over 650 cd/m² and sub-second turn-on times [15,17].

Despite the potential benefits of the iTMC-LECs as well as recent advances [5], the market entry of these types of devices is challenging mainly due to the lack of at the same time efficient, bright, and long-term stable emitter materials of different colors, resulting in challenges in the realization of white emission [18,19]. Combining the stability of semiconducting materials with their solution-based properties, colloidal quantum dots (QDs) offer an alternative approach for large-area lighting concepts due to their high color purity which is tunable over a broad spectral range by tailoring their size, shape and composition [20]. The expansion of the iTMC-LEC device by incorporation of colloidal QDs of different colors opens an alternative path not only for the variation of the LEC emission color. Even white light-emitting or color tunable hybrid devices can be realized depending on the chosen QD type and device geometry.

The device concept of a Quantum Dot LEC Hybrid Devices (QLEC) was presented to potentially improve two critical issues of the iTMC-LEC, the performance and the emission color, simultaneously. Therefore, the typical LEC device design, comprising the transparent ITO, coated by the poly(3,4-ethylenedioxy-thiophene): poly(styrenesulfonate) (PEDOT:PSS) as an anode, and the active LEC layer (iTMC + ionic liquid, IL), is complemented by an additional QD layer prior to the Aluminum cathode (Figure 1). In this case, the iTMC layer can be expected to act as a hole injection and transport layer for the QDs [2], on the other hand the QD layer can also improve the electron injection into the iTMC. Additionally, the device design gives the opportunity to combine the emission of two active light-emitting species, which enables the color tuning of the QLEC device.