Semitransparent Organic Light Emitting Diodes with Bidirectionally Controlled Emission

Semitransparent OLEDs are a candidate for large-area eco-friendly light sources that can be integrated into building facades, suggesting colorful windows that become luminescent if the OLED is switched on. However, since the light is emitted in two directions, smart light engineering has to be implemented to direct the light into a preferred direction and to prevent for instance huge energetic losses to the outside of a building. We introduce an unprecedented device architecture, composed of a dielectric mirror attached to a semitransparent OLED. Such a system features a dual functionality that depends on the viewing direction: changing the color perception and/or enhancing the light directionality while still preserving a high overall device transparency. First, we motivate the potential of this concept with a theoretical study, showing that broad modifications in the color range can be realized via device optimization and that the maximum possible emission enhancement of the OLED is limited only by the transparency of the interfacial layers and the electrodes. Then, experimental investigations with a semitransparent yellow OLED (transparency = 58.2%) in combination with six different dielectric mirrors validate the theoretical results. Retaining the same color perception, up to 80% of the total emitted light can be directed toward one side while the color is modified at the other side of the device stack. Here, modifications from yellow to purple to dark or light blue can be realized

Performance Evaluation of Semitransparent Perovskite Solar Cells for Application in Four-Terminal Tandem Cells

The efficiency of perovskite-based tandem solar cells and the respective efficiency gain over the single-junction operation of the bottom cell strongly depend on the performance of the component cells. Thus, a fair comparison of reported top cells is difficult. We therefore compute the tandem cell efficiency for the combination of several semitransparent perovskite top solar cells and crystalline silicon or chalcopyrite bottom cells from the literature. We focus on four-terminal configurations but also estimate and discuss the differences between four- and two-terminal configurations. For each top cell, we thereby determine the tandem cell performance as a function of the bottom cell efficiency, which results in a linear relationship. From these data, we extract two parameters to quantify the suitability of the top cell: (i) the slope of the tandem vs. bottom cell efficiency, which is the effective transparency of the top cell, and (ii) the tandem cell efficiency for a targeted bottom cell. These two figures of merit were calculated for a representative set of bottom cells and may serve for comparison of semitransparent perovskite top cells in the future

Performance Evaluation of Semitransparent Perovskite Solar Cells for Application in Four-Terminal Tandem Cells

The efficiency of perovskite-based tandem solar cells and the respective efficiency gain over the single-junction operation of the bottom cell strongly depend on the performance of the component cells. Thus, a fair comparison of reported top cells is difficult. We therefore compute the tandem cell efficiency for the combination of several semitransparent perovskite top solar cells and crystalline silicon or chalcopyrite bottom cells from the literature. We focus on four-terminal configurations but also estimate and discuss the differences between four- and two-terminal configurations. For each top cell, we thereby determine the tandem cell performance as a function of the bottom cell efficiency, which results in a linear relationship. From these data, we extract two parameters to quantify the suitability of the top cell: (i) the slope of the tandem vs. bottom cell efficiency, which is the effective transparency of the top cell, and (ii) the tandem cell efficiency for a targeted bottom cell. These two figures of merit were calculated for a representative set of bottom cells and may serve for comparison of semitransparent perovskite top cells in the future

Assessing Temperature Dependence of Drift Mobility in Methylammonium Lead Iodide Perovskite Single Crystals

Hybrid organic–inorganic perovskites have emerged as cost-effective and high-performance semiconductors for optoelectronic applications. Precise knowledge of charge carrier mobility and especially the temperature dependence of mobility is therefore of utmost relevance for advancing high-performance materials. Here, the charge carrier mobility in methylammonium lead iodide single crystals is investigated with time of flight technique from 290 to 100 K. A nondispersive transport with an electron mobility of 135 (±20) cm²/V s and a hole mobility of 90 (±20) cm²/V s is obtained at room temperature. A power-law temperature dependence of mobility, μ ∝ <i>T</i>^<i>m</i>, with an exponent <i>m</i> = −2.8 and −2.0, is measured for electrons and holes in the tetragonal phase. The highest electron and hole mobilities measured are 635 (±70) and 415 (±20) cm²/V s, respectively. Our results indicate that the scattering of charge carriers with phonons is the limiting factor for carrier mobilities at room temperature

A Bayesian Approach to Predict Solubility Parameters

Solubility is a ubiquitous phenomenon in many aspects of material science. While solubility can be determined by considering the cohesive forces in a liquid via the Hansen solubility parameters (HSP), quantitative structure-property relationship models are often used for prediction, notably due to their low computational cost. Herein, we report gpHSP, an interpretable and versatile probabilistic approach to determining HSP. Our model is based on Gaussian processes (GP), a Bayesian machine learning approach that provides uncertainty bounds to prediction. gpHSP achieves its flexibility by leveraging a variety of input data, such as SMILES strings, COSMOtherm simulations, and quantum chemistry calculations. gpHSP is built on experimentally determined HSP, including a general solvents set aggregated from literature, and a polymer set experimentally characterized by this group of authors. In all sets, we obtained a high degree of agreement, surpassing well-established machine learning methods. We demonstrate the general applicability of gpHSP to miscibility of organic semiconductors, drug compounds and in general solvents, which can be further extended to other domains. gpHSP is a fast and accurate toolbox, which could be applied to molecular design for solution processing technologies.<br

Qualitative Analysis of Bulk-Heterojunction Solar Cells without Device Fabrication: An Elegant and Contactless Method

The enormous synthetic efforts on novel solar cell materials require a reliable and fast technique for the rapid screening of novel donor/acceptor combinations in order to quickly and reliably estimate their optimized parameters. Here, we report the applicability of such a versatile and fast evaluation technique for bulk heterojunction (BHJ) organic photovoltaics (OPV) by utilizing a steady-state photoluminescence (PL) method confirmed by electroluminescence (EL) measurements. A strong relation has been observed between the residual singlet emission and the charge transfer state emission in the blend. Using this relation, a figure of merit (FOM) is defined from photoluminescence and also electroluminescence measurements for qualitative analysis and shown to precisely anticipate the optimized blend parameters of bulk heterojunction films. Photoluminescence allows contactless evaluation of the photoactive layer and can be used to predict the optimized conditions for the best polymer–fullerene combination. Most interestingly, the contactless, PL-based FOM method has the potential to be integrated as a fast and reliable inline tool for quality control and material optimization

Improved High-Efficiency Perovskite Planar Heterojunction Solar Cells via Incorporation of a Polyelectrolyte Interlayer

Improved High-Efficiency Perovskite Planar Heterojunction Solar Cells via Incorporation of a Polyelectrolyte Interlaye

Low-Temperature Solution-Processed Kesterite Solar Cell Based on in Situ Deposition of Ultrathin Absorber Layer

The production of high-performance, solution-processed kesterite Cu₂ZnSn­(S_<i>x</i>,Se_1–<i>x</i>)₄ (CZTSSe) solar cells typically relies on high-temperature crystallization processes in chalcogen-containing atmosphere and often on the use of environmentally harmful solvents, which could hinder the widespread adoption of this technology. We report a method for processing selenium free Cu₂ZnSnS₄ (CZTS) solar cells based on a short annealing step at temperatures as low as 350 °C using a molecular based precursor, fully avoiding highly toxic solvents and high-temperature sulfurization. We show that a simple device structure consisting of ITO/CZTS/CdS/Al and comprising an extremely thin absorber layer (∼110 nm) achieves a current density of 8.6 mA/cm². Over the course of 400 days under ambient conditions encapsulated devices retain close to 100% of their original efficiency. Using impedance spectroscopy and photoinduced charge carrier extraction by linearly increasing voltage (photo-CELIV), we demonstrate that reduced charge carrier mobility is one limiting parameter of low-temperature CZTS photovoltaics. These results may inform less energy demanding strategies for the production of CZTS optoelectronic layers compatible with large-scale processing techniques

Fully Solution-Processing Route toward Highly Transparent Polymer Solar Cells

We report highly transparent polymer solar cells using metallic silver nanowires (AgNWs) as both the electron- and hole-collecting electrodes. The entire stack of the devices is processed from solution using a doctor blading technique. A thin layer of zinc oxide nanoparticles is introduced between photoactive layer and top AgNW electrode which plays decisive roles in device functionality: it serves as a mechanical foundation which allows the solution-deposition of top AgNWs, and more importantly it facilitates charge carriers extraction due to the better energy level alignment and the formation of ohmic contacts between the active layer/ZnO and ZnO/AgNWs. The resulting semitransparent polymer:fullerene solar cells showed a power conversion efficiency of 2.9%, which is 72% of the efficiency of an opaque reference device. Moreover, an average transmittance of 41% in the wavelength range of 400–800 nm is achieved, which is of particular interest for applications in transparent architectures

Brightly Luminescent and Color-Tunable Formamidinium Lead Halide Perovskite FAPbX₃ (X = Cl, Br, I) Colloidal Nanocrystals

In the past few years, hybrid organic–inorganic and all-inorganic metal halide perovskite nanocrystals have become one of the most interesting materials for optoelectronic applications. Here, we report a facile and rapid room temperature synthesis of 15–25 nm formamidinium CH­(NH₂)₂PbX₃ (X = Cl, Br, I, or mixed Cl/Br and Br/I) colloidal nanocrystals by ligand-assisted reprecipitation (LARP). The cubic and platelet-like nanocrystals with their emission in the range of 415–740 nm, full width at half-maximum (fwhm) of 20–44 nm, and radiative lifetimes of 5–166 ns enable band gap tuning by halide composition as well as by their thickness tailoring; they have a high photoluminescence quantum yield (up to 85%), colloidal and thermodynamic stability. Combined with surface modification that prevents degradation by water, this nanocrystalline material is an ideal candidate for optoelectronic devices and applications. In addition, optoelectronic measurements verify that the photodetector based on FAPbI₃ nanocrystals paves the way for perovskite quantum dot photovoltaics