In this study, we present a simple method to tune and take advantage of microcavity effects for an increased fraction of outcoupled light in solution-processed organic light emitting diodes. This is achieved by incorporating nonscattering polymer–nanoparticle composite layers. These tunable layers allow the optimization of the device architecture even for high film thicknesses on a single substrate by gradually altering the film thickness using a horizontal dipping technique. Moreover, it is shown that the optoelectronic device parameters are in good agreement with transfer matrix simulations of the corresponding layer stack, which offers the possibility to numerically design devices based on such composite layers. Lastly, it could be shown that the introduction of nanoparticles leads to an improved charge injection, which combined with an optimized microcavity resulted in a maximum luminous efficacy increase of 85% compared to a nanoparticle-free reference device.Keywords: conductive polymer; light outcoupling; microcavity; nanoparticle composites; organic light emitting diodes; silica nanoparticles; solution processing

Charge-transfer (CT) states across organic heterojunctions play an important role in determining the efficiency of organic solar cells. These states can be the precursors of free charges or lead to geminate recombination losses. Here, we use time-resolved photoluminescence measurements to study charge-transfer states in a sequence of polythiophene:fullerene derivative blends with different mixing ratios, over the temperature range from 10 to 290 K, and after excitation with various photon energies. Our results show that (1) excess fullerene leads to a higher probability of CT state formation per absorbed photon and (2) the CT emission intensity is temperature-dependent whereas its emission lifetime is temperature-independent. Observation 1 cannot be explained solely by the increased fraction of excitations formed on fullerene-derivatives in the fullerene-rich blends, suggesting that disruption of the polymer packing at high fullerene loadings can negatively influence charge separation. Observation 2 suggests that relaxed, emissive CT states are formed in this system through a short-lived intermediate state whose separation into free-charges or relaxation into a bound CT state is temperature-dependent. Analysis of the temperature dependence suggests that there is no single activation energy for the CT*-to-CS transition, but rather a wide variety of different sites ranging in activation energy.

A decisive factor for the performance of organic bulk heterojunction solar cells is the competition between charge separation and geminate electron–hole recombination through a charge-transfer (CT) state after exciton separation across the heterointerface. By time-resolving the near-infrared emission of the high-performance low-bandgap PTB7:PC71BM material system, we selectively study the properties of CT states formed after optical excitation with a femtosecond laser pulse. We observe that the CT emission yield is higher after 705 nm excitation of the polymer rather than after 400 nm excitation of the fullerene, which can be attributed to better charge separation for excitons generated in fullerene aggregates. Additionally, the CT states are weakly bound with their emission not far red-shifted from that of the exciton. Examining the time-resolved CT emission from room temperature to 10 K, we observe changes in CT state lifetime with energy and temperature, indicative of CT state separation effectively competing with recombination, especially for the higher-energy CT states. Our findings suggest that this weak binding of CT states in the polymer–fullerene mixed phase is a key factor for the highly efficient charge separation in this material system.

Organic semiconductor distributed feedback (DFB) lasers are of interest as external or chip-integrated excitation sources in the visible spectral range for miniaturized Raman-on-chip biomolecular detection systems. However, the inherently limited excitation power of such lasers as well as oftentimes low analyte concentrations requires efficient Raman detection schemes. We present an approach using surface-enhanced Raman scattering (SERS) substrates, which has the potential to significantly improve the sensitivity of on-chip Raman detection systems. Instead of lithographically fabricated Au/Ag-coated periodic nanostructures on Si/SiO2 wafers, which can provide large SERS enhancements but are expensive and time-consuming to fabricate, we use low-cost and large-area SERS substrates made via laser-assisted nanoreplication. These substrates comprise gold-coated cyclic olefin copolymer (COC) nanopillar arrays, which show an estimated SERS enhancement factor of up to ∼107. The effect of the nanopillar diameter (60–260 nm) and interpillar spacing (10–190 nm) on the local electromagnetic field enhancement is studied by finite-difference-time-domain (FDTD) modeling. The favorable SERS detection capability of this setup is verified by using rhodamine 6G and adenosine as analytes and an organic semiconductor DFB laser with an emission wavelength of 631.4 nm as the external fiber-coupled excitation source.Keywords: laser material processing; nanoimprint lithography; nanostructure fabrication; organic semiconductor distributed-feedback lasers; Raman spectroscopy; surface-enhanced Raman scattering;

•Exciton dissociation at polymer–fullerene interfaces is studied theoretically.•The effect from interface dipoles is specially focused on.•Calculations beyond a harmonic approximation are performed.•Interface dipoles may have a larger impact than anticipated until now.Understanding the mechanism of the efficient exciton dissociation in multicomponent systems belongs to the most prominent challenges in applied soft-matter physics. The model with discrete dipoles along the internal interface between the different material components is often considered as promising to account for the exciton dissociation. However, the dissociation efficiency in the framework of this model has been so far calculated only within the harmonic approximation. In the current article we calculate the dissociation efficiency in the dipolar model beyond the harmonic approximation and show that the exciton dissociation probability appears drastically larger than assumed so far. We also consider the effect of a finite spatial extent of the dipole charges and show that if the charges cannot be treated as point-like species, harmonic approximation is not applicable.

•Digitally printed silver grids with 20 μm line widths fabricated on top of inverted organic solar cells.•Highly transparent (>90%) and low resistive (∼10 Ω/□) printed current collecting grids.•Significantly increased PCE by 64% due to low sheet resistance.•Ink formulation was optimized for homogeneous printing results at higher printing speeds.•Devices with printed top electrode grids have been demonstrated with 1D, 2D, and hexagonal grid layout.Aerosol jet deposited metallic grids are very promising as transparent electrodes for large area organic solar cells and organic light emitting diodes. However, the homogeneity and the printing speed remain a challenge. We report homogeneous and rapidly printed metallic lines based on a complex-based metal–organic silver ink using a processing temperature of 140 °C. We show that inhomogeneities, which are present in printed structures at increased printing speeds and mainly caused by drying effects, can be improved by adding high boiling point solvents. We demonstrate solution processed highly conductive and transparent hybrid electrodes on inverted organic solar cells comprising digitally printed top silver grids.Graphical abstract

In this study, we investigate for the first time morphological and compositional changes of silicon quantum dot (SiQD) light-emitting diodes (SiLEDs) upon device operation. By means of advanced transmission electron microscopy (TEM) analysis including energy filtered TEM (EFTEM) and energy dispersive X-ray (EDX) spectroscopy, we observe drastic morphological changes and degradation for SiLEDs operated under high applied voltage ultimately leading to device failure. However, SiLEDs built from size-separated SiQDs operating under normal conditions show no morphological and compositional changes and the biexponential loss in electroluminescence seems to be correlated to chemical and physical degradation of the SiQDs. By contrast, we found that, for SiLEDs fabricated from polydisperse SiQDs, device degradation is more pronounced with three main modes of failure contributing to the reduced overall lifetime compared to those prepared from size-separated SiQDs. With this newfound knowledge, it is possible to devise ways to increase the lifetimes of SiLEDs.

We present fully solution processable hybrid electrodes comprising highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) layers and hydrothermally grown zinc oxide nanorods. These electrodes exhibit conductivities of about 1400 S/cm and a transmittance of more than 80%. By incorporating these electrodes into inverted polymer photovoltaic devices, power conversion efficiencies of 1.6% are achieved. Charge carrier lifetime measurements by means of impedance spectroscopy indicate an improved charge carrier extraction from the device upon the incorporation of the nanorod array.Highlights► A precursor based seed layer for hydrothermally grown ZnO nanorods is presented. ► ZnO nanorods are grown on a highly conductive printable polymer layer. ► Polymer/nanorods composite layers are highly transparent and conductive (1400 S/cm) ► Incorporation of these hybrid electrodes into 1.6%-efficient polymer solar cells is demonstrated. ► Improved charge transport properties due to the ZnO nanorods are verified by means of impedance spectroscopy.

We present highly efficient electroluminescent devices using size-separated silicon nanocrystals (ncSi) as light emitting material. The emission color can be tuned from the deep red down to the yellow-orange spectral region by using very monodisperse size-separated nanoparticles. High external quantum efficiencies up to 1.1% as well as low turn-on voltages are obtained for red emitters. In addition, we demonstrate that size-separation of ncSi leads to drastically improved lifetimes of the devices and much less sensitivity of the emission wavelength to the applied drive voltage.