High-performance semitransparent perovskite solar cells with solution-processed silver nanowires as top electrodes

In this work, we report efficient semitransparent perovskite solar cells using solution-processed silver nanowires (AgNWs) as top electrodes. A thin layer of zinc oxide nanoparticles is introduced beneath the AgNWs, which fulfills two essential functionalities: it ensures ohmic contact between the PC60BM and the AgNWs and it serves as a physical foundation that enables the solution-deposition of AgNWs without causing damage to the underlying perovskite. The as-fabricated semitransparent perovskite cells show a high fill factor of 66.8%, Voc = 0.964 V, Jsc = 13.18 mA cm−2, yielding an overall efficiency of 8.49% which corresponds to 80% of the reference devices with reflective opaque electrodes

Gedruckte dielektrische Spiegel und deren Anwendung in organischer Elektronik

Organic electronics are one of the future technologies of these days. It offers many advantages in comparison to heavy metal based inorganic electronics. For example, organic electronics are available in various colors, are often semitransparent, they can be fully solution processed and thus allow printing on top of rigid as well as on flexible substrates. All these characteristics enable a complete new area of applications for electronics. For example, the integration of semitransparent photovoltaic or light emitting diodes into windows, printed batteries, sensors directly pasted on the skin, flexible displays and much more. With the utilization of light management within such products, the device efficiency and / or its functionality can be further improved. The aim of this work was to realize fully-printed dielectric mirrors and subsequently to integrate them into printed organic electronics. Dielectric mirrors are also known as one dimensional photonic crystals, Bragg reflectors, dichroic filters, or interference filters and are mainly processed via vacuum based coating technologies. Such mirrors reflect light only in a certain wavelength regime, while in the other regions high transmittance is obtained. Their working principle is based on thin film interferences at dielectric layers. Depending on the position and the width of the maximum reflectance signal with respect to the visible region, such mirrors can be either transparent, semitransparent or non-transparent. Then, a short introduction about the different types of photonic crystals, their stage of development and their application in organic photovoltaic is given. After that, the optical fundamentals of dielectric mirrors as well as the working principle of organic photovoltaic and organic light emitting diodes is discussed in the theoretical part of this thesis. A much more detailed description about the state of the art of dielectric mirrors and their application in photovoltaic and organic light emitting diodes is provided in chapter 3. Next, within a short conclusion, the necessary steps that need to be accomplished to further establish such optical items on the market is described. In chapter 4, all used materials, experimental setups, and applied physical formulas are listed. Detailed information about optical characteristics of own-fabricated fully printed dielectric mirrors are presented in Chapter 5. The color, the transparency, as well as the width of the optical bandwidth can be adjusted via layer thickness variations, the material characteristics, and the number of double layers. Especially, the use of polymer-nanocomposite inks is highly promising for this approach. Very smooth and homogenous layers in the nanometer regime with various refractive indices can be accomplished by systematic ink variations with respect to nanoparticles, polymers or additives. Optical simulations based on the transfer matrix agree well with the experimental results. This further validates, that high quality dielectric mirrors are printable. The substrate is not limited to rigid glass but also printing on flexible foil is possible, due to low processing temperatures, the use of non-corrosively solvents such as alcohols or water, and the fact that the nanoparticles are at least partially embedded in a polymer matrix. Degradation studies for over 1000 h verify high stability of the respective dielectric mirrors against ambient conditions, light, heat, and humidity. Encapsulants, such as additional foils or printed polymeric protection layers, can help to further enhance their mechanical stability. The additional fabrication of solar cells and organic light emitting devices was necessary in order to investigate the impact of dielectric mirrors on semitransparent organic electronics. Silver nanowires are an outstanding material for the fabrication of printed semitransparent electrodes since they are easy to process (deposition technology used within the scope of this work: spin coating, doctor blading, and spray coating), and they show high values of transparencies at low values of sheet resistance. Experimental results according to those characteristics are introduced in chapter 6. High concentrations of a polymer matrix and the admixing of semiconducting colorful polymers to the ink, can negatively influence the average transmittance of the resulting layers and even lead to the formation of holes on the surface of such layers. Silver nanowires are quite sensitive against light irradiation. Here, the silver nanowires break into smaller pieces and agglomerates are formed. Both observations lead to an increase in sheet resistance and a decrease in the average transmittance. Encapsulation helps to protect the silver nanowire layers against degradation that is caused by light. Moreover, it turns out that the sheet resistance of silver nanowire layers is not the only critical characteristics that determines the applicability of such electrodes in organic electronics. Organic solar cells with different types of silver nanowire electrodes show significant variations in their power conversion efficiency, although all electrodes have comparable sheet resistance but only differ in their ink composition. Eventually, in cooperation with other scientists, high efficient fully solution processed semitransparent small molecule solar cells were established and a new record efficiency for semitransparent perovskite based solar cells could be set via spray coating of silver nanowires. The impact of dielectric mirrors on organic and inorganic solar cells with respect to transparency, photocurrent enhancement and possible modifications in color appearance is discussed in chapter 7. In contrast to previous studies, the solar cell and the dielectric mirror were processed separately, and eventually combined. According to this new and simplified developed device architecture, it was possible to exactly determine the impact of the dielectric mirror on the device performance. Optical simulations and experimental results verify that the percentage increase in photocurrent depends on the angle of light incidence, the stack architecture, the absorption profile of the photoactive layer, the quantum efficiency and the averaged transmittance of the solar cell. Enhancement in photocurrent in the range of about 5% to 45% could be obtained with experiments. To conclude, the use of dielectric mirrors is not suitable for every kind of solar cell. In some cases, an increase in photoactive layer thickness is more beneficial in terms of increasing the photocurrent while almost retaining the overall device transparency. The integration of a dielectric mirror into a solar cell is recommended for photoactive layer materials that can be processed very thin only, or for photoactive layer materials that show a broad absorption spectrum (all over the visible region until the near infrared regime). However, for the case, that the aim is not to enhance the photocurrent particularly, but to modify the device color, dielectric mirrors can be a helpful tool. For example, the color of pristine brown-orange colored perovskite solar cells can be modified to red, blue or even green. Finally, in chapter 8, the impact of dielectric mirrors on organic light emitting diodes (OLEDs) is investigated. With such an unprecedented device architecture, it is possible to direct the emitted light into a preferred direction. For example, with optimized layer sequences, a yellow OLED can emit up to 80% of its total light power in one direction while it still retains a device transparency of 10%. Moreover, at least at one side of the OLED-dielectric mirror stack, color variations occur when the OLED is switched on compared to the case when it is switched off. This effect allows the fabrication of building facades that show color variations during the day. Experimental results can be estimated via optical simulations. Moreover, an additional general code was developed that enables the theoretical optimization of also other device architectures prior performing time- and material-consuming experimental studies.Organische Elektronik ist eine der Zukunftstechnologien unserer Zeit. Sie bietet gegenüber der auf Schwermetall basierenden anorganischen Elektronik einige Vorteile. So ist es beispielsweise möglich, die in den verschiedensten Farben erhältlichen halbleitenden organischen Materialien großflächig sowohl auf feste als auch auf flexible Trägermaterialien zu drucken. Die resultierenden, zum Teil semitransparenten, elektronischen Bauteile erlauben demnach den Einsatz von Elektronik in völlig neuen Anwendungsbereichen wie beispielsweise, in Fenster integrierte semitransparente Solarmodule oder Leuchtdioden, gedruckte Batterien, flexible Sensoren und Displays. Mithilfe von kontrolliertem Lichtmanagment innerhalb dieser Bauteile, kann deren Effizienz weiter gesteigert werden und / oder deren Funktionalität noch weiter optimiert werden. Ziel dieser Arbeit war es daher dielektrische Spiegel großflächig druckbar zu machen und diese dann in ebenfalls gedruckter organische Elektronik zu integrieren. Dielektrische Spiegel sind auch bekannt unter den Namen eindimensionale photonische Kristalle, Bragg Reflektoren, dichroitische Filter oder Interferenzfilter und werden hauptsächlich mittels vakuumbasierten Beschichtungsverfahren hergestellt. Sie bestehen aus optischen Mehrschichtsystemen, die aufgrund Interferenzerscheinungen nur wellenlängenselektiv Licht reflektieren, während sie im restlichen Wellenlängenbereich eine hohe Transmission aufweisen. Abhängig von der Position und der Breite des Reflektionshauptmaximas können somit transparente, semitransparente oder für das menschliche Auge vollständig undurchsichtige Spiegel hergestellt werden. Nach einer kurzen Einführung über die Positionierung von photonischen Kristallen unterschiedlichster Dimensionen in der heutigen Wissenschaft, wird kurz auf deren Anwendungen in der organischen Photovoltaik eingegangen. Der darauffolgende theoretische Teil dieser Arbeit beschreibt detailliert die optischen Grundlagen dielektrischer Spiegel, sowie die Funktionsweise organischer Solarzellen und Leuchtdioden, die für das Verständnis der in den folgenden Kapiteln dargestellten Ergebnisse notwendig ist. Eine ausführlichere Beschreibung in Bezug auf den aktuellen Forschungsstand im Bereich dielektrischer Spiegel, deren gängigen Herstellungsverfahren sowie deren Einsatzmöglichkeiten in Photovoltaik und Leuchtdioden, und die noch nötigen Schritte zur Etablierung derartiger Strukturen auf den Markt, sind in Kapitel 3 zu finden. In Kapitel 4 sind alle experimentellen Aufbauten und Materialien gelistet, die im Zuge dieser Arbeit verwendet wurden. Zudem wird detailliert auf die Methodik der Datenauswertung eingegangen. Im Kapitel 5 werden eigens hergestellte vollständig gedruckte dielektrische Spiegel in Bezug auf ihre optischen Merkmale genauestens diskutiert. Mittels Variation der Schichtdicken, der Schichtanzahlen und der eingesetzten Materialien ist es nicht nur möglich die Farben und die Transparenzen der Spiegel, sondern auch die Breite der optischen Bandlücke einzustellen. Es stellt sich heraus, dass vor allem Polymer-Nanoverbundwerkstoffe als Tinte gut geeignet sind. Dank gezielten Einsatz von Nanopartikeln, Polymeren und Additiven können homogene Schichten im Nanometerbereich mit unterschiedlichen Brechungsindizes hergestellt werden. Optische Simulationen stimmen gut mit den experimentellen Ergebnissen überein und unterstützen daher die Aussage, dass qualitativ hochwertige dielektrische Spiegel nun druckbar sind. Aufgrund der geringen Fertigungstemperaturen, der Verwendung von auf Alkohol oder Wasser basierenden Tinten, und der Einbettung der Nanopartikel in Polymermatritzen, beschränkt sich das Trägermaterial nicht nur auf Glas. Gedruckte dielektrische Spiegel auf flexiblen PET Folien können ebenfalls mit geringem Aufwand gefertigt werden. Degradationsstudien über einen Zeitraum von bis zu 1000 h, zeigen eine gute Stabilität der hochskalierten Spiegel gegen Licht, Wärme und Luftfeuchtigkeit. Mit Hilfe einer zusätzlichen Folien- oder gedruckten Polymerverkapselung kann die mechanische Stabilität weiter erhöht werden. Um den Einfluss von dielektrischen Spiegeln auf semitransparente organische Elektronik untersuchen zu können, mussten im Zuge dieser Arbeit unterschiedliche Solarzellen sowie Leuchtdioden hergestellt werden. Silbernanodrahtschichten eignen sich hervorragend für den Einsatz als druckbare semitransparente Elektroden, da sie einfach zu prozessieren (in dieser Arbeit verwendete Beschichtungstechniken: Rotationsbeschichtung, Rakeln, Sprühbeschichtung) sind sowie hohe Transparenzen bei niedrigen Schichtwiderständen zeigen. Messdaten bezogen auf diese Charakteristika werden in Kapitel 6 vorgestellt. Ein hoher Polymeranteil der Silbernanodrahttinte und die Zugabe von farbigen leitfähigem Polymeren kann die finale Transparenz der Schichten beachtlich mindern und sogar Löcher in der Oberfläche der Schicht verursachen. Silbernanodrahtschichten sind besonders empfindlich gegenüber Licht. Hierbei sorgt das Brechen der Silbernanodrähte und die Bildung von Agglomeraten innerhalb der Schicht zu einer signifikanten Zunahme des Schichtwiderstandes und einer Abnahme der Transparenz. Dank geeigneter Verkapselung kann eine, durch Licht verursachte Degradation, weitestgehend verhindert werden. Zudem stellt sich heraus, dass nicht ausschließlich der Schichtwiderstand der kritische Faktor für den Einsatz derartiger Materialien in organischen Solarzellen ist. Organische Solarzellen in denen Silbernanodrahtelektroden mit gleichem Schichtwiderstand jedoch unterschiedlichen Tintenkompositionen verbaut sind, zeigen deutliche Schwankungen in ihrer Effizienz. In Zusammenarbeit mit anderen Wissenschaftlern gelang es außerdem neuartige semitransparente "Small Molecule Solar Cells" herzustellen und einen Rekord bezüglich der Effizienz und der Transparenz in semitransparenten lösungsprozessierten Perowskit-Solarzellen aufzustellen. Die Auswirkung von dielektrischen Spiegeln auf semitransparente organische und anorganisch-organische Solarzellen mit Hinblick auf Transparenz, Stromsteigerung, sowie Farbmodifikationen wird in Kapitel 7 diskutiert. Aufgrund der neuartigen Probenarchitektur, wobei der dielektrische Spiegel getrennt von der Solarzelle hergestellt wird, vereinfacht sich nicht nur die Prozessierung des gesamten Bauteils, sondern sie erlaubt auch eine präzise Analyse der Solarzelle zunächst ohne und im Anschluss mit dem entsprechenden dielektrischen Spiegel kombiniert. Anhand von Experimenten und Simulationen wird gezeigt, dass die Effizienzsteigerung abhängig vom Bestrahlungswinkel, der Architektur des dielektrischen Spiegels, dem Absorptionsspektrum der photoaktiven Schicht, der Quanteneffizienz und der gemittelten Transmission der Solarzelle enorm schwanken kann (ca. 5% bis 45%). Es stellt sich außerdem heraus, dass der Einsatz eines dielektrischen Spiegels nicht für jegliche Art von semitransparenter Solarzelle geeignet ist. Oftmals ist eine Erhöhung der Schichtdicke des photoakitven Materials die bessere Methodik, um die Effizienz zu steigern ohne die Transparenz der Solarzelle stark zu beeinflussen. Der Einsatz von dielektrischen Spiegeln ist vor allem dann sinnvoll, wenn die photoaktive Schicht nur sehr dünn prozessiert werden kann oder aber wenn die Solarzelle ein breites Absorptionsspektrum (vom sichtbaren Bereich bis in den nahen Infrarot Bereich) vorweist. Besteht das Ziel nicht nur darin die Effizienz zu steigern, sondern vielmehr darin die Farbe der Zelle zu modifizieren, so kann der Einsatz von dielektrischen Spiegel hilfreich sein. Beispielsweise kann man mittels dielektrischer Spiegel eine ursprünglich braun-orange gefärbte Perowskit-Solarzelle rot, blau oder grün erscheinen lassen. Schließlich werden im darauffolgenden Kapitel 8 die Auswirkungen von dielektrischen Spiegel auf semitransparente organische Leuchtdioden erläutert. Durch die neuartige Kombination beider Bauteile ist es möglich das emittierte Licht der Leuchtdiode vorzugsweise in eine bestimmte Richtung zu dirigieren. So konnte beispielsweise mittels optimierter Schichtsysteme 80% der Gesamtlichtleistung einer gelb emittierenden Leuchtdiode in eine Richtung gelenkt werden, wobei die Transparenz des kompletten Bauteils nicht 10% unterschritt. Aufgrund der durch den dielektrischen Spiegel bedingten Farbveränderungen der ausgeschalteten -nicht leuchtenden- im Vergleich zu der angeschalteten -Licht emittierenden- Leuchtdiode, ist eine farbdynamische Gebäudefassade denkbar. Die experimentellen Daten können gut mittels optischer Simulationen vorhergesagt werden. Die Entwicklung eines weitaus generalisierten Programmcodes ermöglicht zusätzlich den optimalen Aufbau auch für andere OLED Strukturen zu eruieren noch bevor kosten- und zeitintensive Experimente durchgeführt werden müssen

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

Graphene-Supported Pd Nanoclusters Probed by Carbon Monoxide Adsorption

The adsorption of CO on graphene-supported Pd nanoparticles was studied in situ with high-resolution synchrotron-based X-ray photoelectron spectroscopy. At 150 K, CO adsorbs mainly in bridge and 3-fold-hollow sites. The nanoparticles are considered as a mixture of low-index facets. The variation of the amount of deposited Pd revealed identical CO adsorption behavior for all investigated cases, confirming a similar average cluster size over a wide range of Pd coverages. The desorption characteristics were studied with temperature-programmed XPS. The observed desorption maxima at 230 and 430 K are in good agreement with temperature-programmed desorption data on stepped Pd single crystals. At 500 K, CO is completely desorbed from the Pd clusters. The adsorption and desorption of CO are found to be not fully reversible as the Pd particles undergo restructuring upon heating

High-performance semitransparent perovskite solar cells with solution-processed silver nanowires as top electrodes

In this work, we report efficient semitransparent perovskite solar cells using solution-processed silver nanowires (AgNWs) as top electrodes. A thin layer of zinc oxide nanoparticles is introduced beneath the AgNWs, which fulfills two essential functionalities: it ensures ohmic contact between the PC60BM and the AgNWs and it serves as a physical foundation that enables the solution-deposition of AgNWs without causing damage to the underlying perovskite. The as-fabricated semitransparent perovskite cells show a high fill factor of 66.8%, Voc = 0.964 V, Jsc = 13.18 mA cm−2, yielding an overall efficiency of 8.49% which corresponds to 80% of the reference devices with reflective opaque electrodes

Pushing efficiency limits for semitransparent perovskite solar cells

While perovskite-based semitransparent solar cells deliver competitive levels of transparency and efficiency to be envisioned for urban infrastructures, the complexity and sensitivity of their processing conditions remain challenging. Here, we introduce two robust protocols for the processing of sub-100 nm perovskite films, allowing fine-tuning of the active layer without compromising the crystallinity and quality of the semiconductor. Specifically, we demonstrate that a method based on solvent-induced crystallization with a rapid drying step affords perovskite solar cells with 37% average visible transmittance (AVT) and 7.8% PCE. This process enhances crystallization with a preferential phase orientation presumably at the interface, yielding a high fill factor of 72.3%. The second method is based on a solvent–solvent extraction protocol, enabling active layer films as thin as 40 nm and featuring room-temperature crystallization in an ambient environment on a few second time span. As a result, we demonstrate a maximum AVT of 46% with an efficiency of 3.6%, which is the highest combination of efficiency and transparency for a full device stack to date. By combining the two methods presented here we cover a broad range of thicknesses vs. transparency values and confirm that solvent-induced crystallization represents a powerful processing strategy toward high-efficiency semitransparent solar cells. Optical simulations support our experimental findings and provide a global perspective of the opportunities and limitations of semitransparent perovskite photovoltaic devices

Coloring Semitransparent Perovskite Solar Cells <i>via</i> Dielectric Mirrors

While perovskite-based semitransparent solar cells for window applications show competitive levels of transparency and efficiency compared to organic photovoltaics, the color perception of the perovskite films is highly restricted because band gap engineering results in losses in power conversion efficiencies. To overcome the limitation in visual aesthetics, we combined semitransparent perovskite solar cells with dielectric mirrors. This approach enables one to tailor the device appearance to almost any desired color and simultaneously offers additional light harvesting for the solar cell. In the present work, opto-electrical effects are investigated through quantum efficiency and UV-to-visible spectroscopic measurements. Likewise, a detailed chromaticity analysis, featuring the transmissive and reflective color perception of the device including the mirror, from both sides and in different illumination conditions, is presented and analyzed. Photocurrent density enhancement of up to 21% along with overall device transparency values of up to 31% (4.2% efficiency) is demonstrated for cells showing a colored aesthetic appeal. Finally, a series of simulations emulating the device chromaticity, transparency, and increased photocurrent density as a function of the photoactive layer thickness and the design wavelength of the dielectric mirror are presented. Our simulations and their experimental validation enabled us to establish the design rules that consider the color efficiency/transparency interplay for real applications

Graphene-Templated Growth of Pd Nanoclusters

Graphene grown on Rh(111) was used as a template for the growth of Pd nanoclusters. Using high-resolution synchrotron radiation-based X-ray photoelectron spectroscopy, we studied the deposition of Pd on corrugated graphene in situ. From the XP spectra, we deduce a cluster-by-cluster growth mode. The formation of clusters with 3 nm diameter was confirmed by low-temperature scanning tunneling microscopy measurements. The investigation of the thermal stability of the Pd particles showed three characteristic temperature regimes: Up to 550 K restructuring of the particles takes place, between 550 and 750 K the clusters coalesce into larger agglomerates, and finally between 750 and 900 K Pd intercalates between the graphene layer and the Rh surface

Interface Engineering of Perovskite Hybrid Solar Cells with Solution-Processed Perylene–Diimide Heterojunctions toward High Performance

Perovskite hybrid solar cells (pero-HSCs) were demonstrated to be among the most promising candidates within the emerging photovoltaic materials with respect to their power conversion efficiency (PCE) and inexpensive fabrication. Further PCE enhancement mainly relies on minimizing the interface losses via interface engineering and the quality of the perovskite film. Here, we demonstrate that the PCEs of pero-HSCs are significantly increased to 14.0% by incorporation of a solution-processed perylene–diimide (PDINO) as cathode interface layer between the [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) layer and the top Ag electrode. Notably, for PDINO-based devices, prominent PCEs over 13% are achieved within a wide range of the PDINO thicknesses (5–24 nm). Without the PDINO layer, the best PCE of the reference PCBM/Ag device was only 10.0%. The PCBM/PDINO/Ag devices also outperformed the PCBM/ZnO/Ag devices (11.3%) with the well-established zinc oxide (ZnO) cathode interface layer. This enhanced performance is due to the formation of a highly qualitative contact between PDINO and the top Ag electrode, leading to reduced series resistance (<i>R</i>_s) and enhanced shunt resistance (<i>R</i>_sh) values. This study opens the door for the integration of a new class of easily-accessible, solution-processed high-performance interfacial materials for pero-HSCs