Adjustable white-light emission from a photo-structured micro-OLED array

White organic light-emitting diodes(OLEDs)are promising candidates for future solid-state lighting applications and backplane illumination in large-area displays.One very specific feature of OLEDs,which is currently gaining momentum,is that they can enable tunable white light emission.This feature is conventionally realized either through the vertical stacking of independent OLEDs emitting different colors or in lateral arrangement of OLEDs.The vertical design is optically difficult to optimize and often results in efficiency compromises between the units.In contrast,the lateral concept introduces severe area losses to dark regions between the subunits,which requires a significantly larger overall device area to achieve equal brightness.Here we demonstrate a color-tunable,two-color OLED device realized by side-by-side alignment of yellow and blue p-i-n OLEDs structured down to 20μm by a simple and up-scalable orthogonal photolithography technique.This layout eliminates the problems of conventional lateral approaches by utilizing all area for light emission.The corresponding emission of the photo-patterned two-unit OLED can be tuned over a wide range from yellow to white to blue colors.The independent control of the different units allows the desired overall spectrum to be set at any given brightness level.Operated as a white light source,the microstructured OLED reaches a luminous efficacy of 13 lm W^(−1) at 1000 cd m^(−2) without an additional light outcoupling enhancement and reaches a color rendering index of 68 when operated near the color point E.Finally,we demonstrate an improved device lifetime by means of size variation of the subunits.

Asymmetric photocurrent extraction in semitransparent laminated flexible organic solar cells

Scalable production methods and low-cost materials with low embodied energy are key to success for organic solar cells.PEDOT(PSS)electrodes meet these criteria and allow for low-cost and all solution-processed solar cells.However,such devices are prone to shunting.In this work we introduce a roll-to-roll lamination method to construct semitransparent solar cells with a PEDOT(PSS)anode and an polyethyleneimine(PEI)modified PEDOT(PSS)cathode.We use the polymer:PCBM active layer coated on the electrodes as the lamination adhesive.Our lamination method efficiently eliminates any shunting.Extended exposure to ambient degrades the laminated devices,which manifests in a significantly reduced photocurrent extraction when the device is illuminated through the anode,despite the fact that the PEDOT(PSS)electrodes are optically equivalent.We show that degradation-induced electron traps lead to increased trap-assisted recombination at the anode side of the device.By limiting the exposure time to ambient during production,degradation is significantly reduced.We show that lamination using the active layer as the adhesive can result in device performance equal to that of conventional sequential coating.

Experimental proof of Joule heating-induced switched-back regions in OLEDs

Organic light-emitting diodes(OLEDs)have become a major pixel technology in the display sector,with products spanning the entire range of current panel sizes.The ability to freely scale the active area to large and random surfaces paired with flexible substrates provides additional application scenarios for OLEDs in the general lighting,automotive,and signage sectors.These applications require higher brightness and,thus,current density operation compared to the specifications needed for general displays.As extended transparent electrodes pose a significant ohmic resistance,OLEDs suffering from Joule self-heating exhibit spatial inhomogeneities in electrical potential,current density,and hence luminance.In this article,we provide experimental proof of the theoretical prediction that OLEDs will display regions of decreasing luminance with increasing driving current.With a two-dimensional OLED model,we can conclude that these regions are switched back locally in voltage as well as current due to insufficient lateral thermal coupling.Experimentally,we demonstrate this effect in lab-scale devices and derive that it becomes more severe with increasing pixel size,which implies its significance for large-area,high-brightness use cases of OLEDs.Equally,these non-linear switching effects cannot be ignored with respect to the long-term operation and stability of OLEDs;in particular,they might be important for the understanding of sudden-death scenarios.

Real-time beam shaping without additional optical elements

Providing artificial light and enhancing the quality of the respective light sources is of continued interest in the fields of solid state,condensed matter,and semiconductor physics.Much research has been carried out to increase the luminous efficiency,lifetime and colour stability of such devices.However,the emission characteristics of a given light source do not necessarily comply with today’s often sophisticated applications.Here,beam shaping addresses the transformation of a given light distribution into a customized form.This is typically achieved by secondary optical elements often sporting elaborate designs,where the actual light source takes up only a small fraction of the system’s volume.Such designs limit the final light source to a single permanent operation mode,which can only be overcome by employing mechanically adjustable optical elements.Here we show that organic light-emitting diodes(OLEDs)can enable real-time regulation of a beam shape without relying on secondary optical elements and without using any mechanical adjustment.For a red light-emitting two-unit OLED architecture,we demonstrate the ability to continuously tune between strongly forward and strongly sideward emission,where the device efficiency is maintained at an application-relevant level ranging between 6 and 8%of external quantum efficiency for any chosen setting.In combination with additional optical elements,customizable and tuneable systems are possible,whereby the tuning stems from the light source itself rather than from the use of secondary optics.

Catalyzing n-doping

Electrical doping,i.e.,inducing p-or n-type conductivity by adding a dopant,is of fundamental importance for device physics of semiconductors and is used in about every commercially produced device.For organic semiconductors,the conductivity is intrinsically low(typically 10^(-4)-10^(-8) S/cm)due to the mostly amorphous nature,the weak coupling between molecules,and the low number of free charge carriers in the presence of large energy gaps.