Phosphorescence from purely organic molecules has been considered to be a rare feature with little practical use. In the last decade however, material research has shown that room temperature phosphorescence (RTP) is found in a very large set of different organic materials and can be brought to respectable photoluminescence efficiencies. Still, one of the key quantities for RTP, namely the excited state lifetime, remains orders of magnitude longer compared to conventional fluorophores or organometallic phosphors. This long lifetime renders RTP unsuited for OLED-display technology.
In this presentation, I will discuss some recent developments towards two application concepts which function only because of the special features of RTP. First, I will introduce programmable luminescent tags that function as an electronics-free, thin-film, flexible and fully transparent information storage device. Second, RTP is used at the heart of a wavelength tracking sensor system. Here, a thin-film comprising an RTP emitter and an additional secondary emitter is effectively used as a spin-mixing layer that allows simple wavelength-discrimination.
For bright area light sources, such as OLED lighting tiles, the electric resistivity of transparent electrodes induces a non-uniform current distribution within the device. At high driving currents, the interaction between conductivity, heat, and power dissipation results in a positive electrothermal feedback loop, which drastically exacerbates inhomogenities in local device temperature and luminance.
Such nonlinear behavior induces unprecedented electrothermal effects that compromise the performance of bright area light sources. In this contribution, we discuss how Joule self-heating sqeezes temperature and current into a tiny device region, while the remainder of the active area decreases in luminance (Switch-back effect). We further introduce how the dimension of the active area governs the current-voltage characteristics of the OLED and how two and even three stable operating branches develop (tristability) that can promote destructive incidences.
Organic light-emitting diodes (OLEDs) suffer from strong electrothermal feedback when operated with high currents. The interaction between conductivity, heat and power dissipation results in a positive feedback loop. When running an IV scan, former modeling revealed a so-called “switched-back” region where the local current density and brightness decreases although the total device current still increases.
Here, we prove the existence of a switched-back region. We demonstrate that its appearance agrees with the simulation that solely uses electrothermal modeling. Our study aims to improve the long-term stability of high brightness OLED lighting tiles e.g. as applied in the automotive sector.
KEYWORDS: External quantum efficiency, Solar cells, Tandem solar cells, Electronic circuits, Silicon solar cells, Solar concentrators, Clouds, Organic semiconductors, Semiconductors, Atomic, molecular, and optical physics
Current matching limits the commercialization of tandem solar cells due to their instability over spectral changes, leading to the need of using solar concentrators and trackers to keep the spectrum stable. We demonstrate that voltage-matched systems show far higher performance over spectral changes; caused by clouds, dust and other variations in atmospheric conditions.
Singlet fission is a process in organic semiconductors which has shown very efficient, 200%, down-conversion yield and the generated excitations are long-lived, ideal for solar cells. As a result, the number of publications has grown exponentially in the past 5 years. Yet, so far no one has achieved to combine singlet fission with most low bandgap semiconductors, including crystalline silicon, the dominating solar cell material with a 90% share of the PV Market.
Here we show that singlet fission can facilitate the fabrication of voltage-matched systems, opening a simple design route for the effective implementation of down-conversion in commercially available photovoltaic technologies, with no modification of the electronic circuitry of such.
The implemention of singlet fission is achieved simply by decoupling the fabrication of the individual subcells. For this demonstration we used an ITO/PEDOT/P3HT/Pentacene/C60/Ag wide-bandgap subcell, and a commercial silicon solar cell as the low-bandgap component. We show that the combination of the two leads to the first tandem silicon solar cell which exceeds 100% external quantum efficiency.
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