The evolution of Materials Acceleration Platforms: toward the laboratory of the future with AMANDA

The development of complex functional materials poses a multi-objective optimization problem in a large multi-dimensional parameter space. Solving it requires reproducible, user-independent laboratory work and intelligent preselection of experiments. However, experimental materials science is a field where manual routines are still predominant, although other domains like pharmacy or chemistry have long used robotics and automation. As the number of publications on Materials Acceleration Platforms (MAPs) increases steadily, we review selected systems and fit them into the stages of a general material development process to examine the evolution of MAPs. Subsequently, we present our approach to laboratory automation in materials science. We introduce AMANDA (Autonomous Materials and Device Application Platform - www.amanda-platform.com), a generic platform for distributed materials research comprising a self-developed software backbone and several MAPs. One of them, LineOne (L1), is specifically designed to produce and characterize solution-processed thin-film devices like organic solar cells (OSC). It is designed to perform precise closed-loop screenings of up to 272 device variations per day yet allows further upscaling. Each individual solar cell is fully characterized, and all process steps are comprehensively documented. We want to demonstrate the capabilities of AMANDA L1 with OSCs based on PM6:Y6 with 13.7% efficiency when processed in air. Further, we discuss challenges and opportunities of highly automated research platforms and elaborate on the future integration of additional techniques, methods and algorithms in order to advance to fully autonomous self-optimizing systems—a paradigm shift in functional materials development leading to the laboratory of the future

Steuerung der elektronischen Kontakteigenschaften in organischen Polymer-Fulleren Solarzellen

The world consumes several tens of terawatts (TW) of electricity. If solar energy should have a notable share in the energy generation of the future, the fabrication of solar modules has to be changed from nowadays batch-to-batch processes that operate in the gigawatt regime to a reliable production that allows TW`s. Large area roll-to-roll (R2R) printing enables solar cell manufacturing to proceed to TW production. Organic photovoltaics (OPV) are one of the very promising technologies for printing production. The 10 % hurdle has been overcome after huge progress in increasing the efficiency of OPV over the past years. Now, mainly large scale production and stability issues are in the research focus. Developing the interface layers, situated between active layer and electrodes, of organic solar cells (OSCs) is one of the central parts to solve these challenges. The interface layers are used to maximize efficiency, dene the polarity and increase the lifetime of the devices. Apart from their functionality, the interface layers should fulfill the requirements for a large scale printing production. Metal oxides are a very promising option to provide functionality at the required processing conditions. On the n-type side, this thesis concentrates on zinc oxide (ZnO) and doped zinc oxide as electron transport layer (ETL). The low conductivity (typically 10E-6 S/cm) of intrinsic ZnO restricts the layer thickness of such an ETL to a few tens of nanometers. A conductivity in the order of 10E-3 S/cm was derived from electrical simulations as sufficient to increase the interface layer thickness to over 1 µm which would provide all the desired freedom for this parameter. This conductivity can be achieved via doping with Aluminum (Al). In this thesis, Al-doped ZnO (AZO) is introduced as interface layer from precursor solution with comparable performance to ZnO in thin films. AZO maintains the performance also in thick films, while ZnO devices suffer from electrical losses. Further work was performed to improve the deposition parameters, especially the annealing temperature. The annealing temperature could be improved to a technologically relevant regime of below 150 °C via the engineering of the AZO precursor solution. For the precursor approach, fully functional interface layers with up to 680 nm are experimentally realized to verify the relevance of the simulated results. Thick interface layers may improve the lifetime of the devices and this ETL fulfills the requirements to start lifetime tests. The surface of the ETL plays a deciding role in the contact formation to the active layer. The surface of an AZO ETL was systematically manipulated using self-assembled monolayers (SAMs) to study its impact on device performance. The performance of the resulting devices varied between 13 % and 115 % of the unmodied reference device depending on the used SAM. This demonstrates the crucial role of the surface of metal oxide interlayers. Furthermore, the developed ETL`s were tested with a solution processed electrode. Nowadays preferred transparent electrode indium-tin-oxide (ITO) is expensive and brittle. Both make it unattractive for future R2R production on flexible substrates. Solution processed silver nanowires are a candidate to replace ITO. Silver nanowires are solution processable at low temperatures and flexible. The AgNW`s form a network with a quite rough morphology that needs to be filled and smoothed by an interface layer. The developed ETL`s (ZnO and AZO) are found to be fitting solutions and fully functional devices with 2.7 % efficiency and over 60 % fill factor (FF) are demonstrated. Especially the high FF expresses the excellent electrical functionality of the low temperature, solution processed transparent electrode. This work was an essential step towards fully solution processed and semitransparent devices that have been realized afterwards. Normal architecture solar cells usually employ rather unstable, low work function metals as cathode that limits device lifetime. One approach to make more stable devices is to use metal oxides with high workfunction metals as cathode instead. The previously investigated precursors showed no functionality when applied on top of active layers in the normal architecture. Therefore an AZO nanoparticle (NP) dispersion is employed that shows comparable electrical parameters to the previously used precursors at only 80 °C annealing temperature. The nanoparticle dispersion enables the use of an AZO ETL even on top of previously deposited organic active layers without affecting the morphology or harming the sensitive materials. AZO NP are a replacement with comparable performance to unstable Ca as cathode and can be employed together with stable Ag for normal architecture solar cells. On the hole transport side, the commonly employed organic poly(3,4-ethylendioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was found to be a major source of degradation through its hygroscopic and acidic nature. Transition metal oxides are investigated in this thesis as promising candidates with better intrinsic stability. At first, MoOX deposited from a nanoparticle suspension is introduced as HTL in organic solar cells. The performance is found to match PEDOT: PSS with improved parallel resistance, but an oxygen plasma treatment is necessary for functional films. The experience gained with MoOX was applied to develop an improved approachwith WOX. WOX was chosen because of the possibility to get very small and mono disperse particles that can be stabilized without organic ligands in an alcoholic solvent system. This enables the deposition smooth films that only need a very low temperature (80 °C) treatment without oxygen plasma for functional films and devices. This WOX dispersion can even be used in the inverted architecture where the layer is deposited on top of the organic active layer. Hereby "metal oxide interface only" inverted solar cell with a commercially available active material with high efficiency of around 6 % are made possible. This layer stack contains only electrode materials that are considered as intrinsically stable and that fulfill the previously discussed large area production requirements to a large extent. Promising lifetime studies with devices containing metal oxides that were developed in this thesis, can be started now.Weltweit werden mehrere Dutzend Terawatt Elektrizität konsumiert. Wenn Solarenergie einen bedeutenden Teil zur Energieerzeugung der Zukunft beitragen soll, muss die Fabrikation von Solarmodulen von der heutigen Einzelfertigung im Gigawatt-Bereich auf Prozesse umgestellt werden, die Terawatt erlauben. Großflächiger, Rolle-zu-Rolle (R2R) Druck ermöglicht Solarzellenfertigung im Terawatt-Bereich. Organische Photovoltaik ist eine der sehr vielversprechenden Technologien für die Produktion in Druckverfahren. Die 10 % Hürde wurde nach großen Fortschritten in der Effizienzsteigerung in den letzen Jahren genommen. Jetzt stehen hauptsächlich die Lösung von großtechnischen Produktions- und Haltbarkeitsproblemen im Fokus der Wissenschaft. Die Entwicklung der Zwischenschichten organischer Solarzellen, die sich zwischen aktiver Schicht und Elektrode befinden, ist von zentraler Bedeutung um diese Herausforderungen zu lösen. Die Zwischenschichten werden dazu genutzt die Effizienz zu maximieren, die Polarität der Bauteile zu definieren und um die Lebensdauer zu erhöhen. Abgesehen von ihrer Funktionalität, sollen die Zwischenschichten auch die Anforderung für die großflächige Produktion im Druckverfahren erfüllen. Metalloxide sind sehr vielversprechende Kandidaten um die Funktionalitäten bei den geforderten Prozessierungsbedingungen zu gewährleisten

Comparison of various sol-gel derived metal oxide layers for inverted organic solar cells

Inverted bulk-heterojunction solar cells have recently captured high interest due to their environmental stability as well as compatibility to mass production. This has been enabled by the development of solution processable n-type semiconductors, mainly TiO(2) and ZnO. However, the device performance is strongly correlated to the electronic properties of the interfacial materials, and here specifically to their work function, surface states as well as conductivity and mobility. It is noteworthy to say that these properties are massively determined by the crystallinity and stoichiometry of the metal oxides. In this study, we investigated aluminum-doped zinc oxide (AZO) as charge selective extraction layer for inverted BHJ solar cells. Thin AZO films were characterized with respect to their structural, optical and electrical properties. The performance of organic solar cells with an AZO electron extraction layer (EEL) is compared to the performance of intrinsic ZnO or TiO(x) EELs. We determined the transmittance, absorbance, conductivity and optical band gap of all these different metal oxides. Furthermore, we also built the correlations between doping level of AZO and device performance, and between annealing temperature of AZO and device performance. (C) 2011 Elsevier B.V. All rights reserved

Inverted organic solar cells using a solution processed aluminum-doped zinc oxide buffer layer

In this article, we demonstrate a route to solve one of the big challenges in the large scale printing process of organic solar cells, which is the reliable deposition of very thin layers. Especially materials for electron (EIL) and hole injection layers (HIL) (except poly(3,4-ethylene dioxythiophene):(polystyrene sulfonic acid) (PEDOT:PSS)) have a low conductivity and therefore require thin films with only a few tens of nanometers thickness to keep the serial resistance under control. To overcome this limitation, we investigated inverted polymer solar cells with an active layer comprising a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) with solution processed aluminum-doped zinc oxide (AZO) EIL. Devices with AZO and intrinsic zinc oxide (i-ZnO) EIL show comparable efficiency at low layer thicknesses of around 30 nm. The conductivity of the doped zinc oxide is found to be three orders of magnitude higher than for the i-ZnO reference. Therefore the buffer layer thickness can be enhanced significantly to more than 100 nm without hampering the solar cell performance, while devices with 100 nm i-ZnO films already suffer from increased series resistance and reduced efficiency. (C) 2011 Elsevier B.V. All rights reserved

A universal method to form the equivalent ohmic contact for efficient solution-processed organic tandem solar cells

The highly transparent, conductive and robust intermediate layer (IML) is the primary challenge for constructing efficient organic tandem solar cells. In this work, we demonstrate an easy but generic approach to realize the fully functional, solution-processed IMLs. In detail, solution-processed silver-nanowires are packed at low concentration between hole- and electron-transporting layers to convert an otherwise rectifying interface into an ohmic interface. The IMLs are proven to be of ohmic nature under applied bias, despite the unipolar charge selectivity of the single layers. Ohmic recombination within IMLs is further proven in organic tandem solar cells fabricated by doctor-blading under ambient conditions. The tandem solar cells based on PCDTBT:[70]PCBM as the bottom cell and pDPP5T-2:[60]PCBM as the top cell give a power conversion efficiency of 7.25%, which is among the highest values for solution-processed organic tandem solar cells fabricated by using a roll-to-roll compatible deposition method in air

Photovoltaic properties of benzotriazole containing alternating donor–acceptor copolymers: Effect of alkyl chain length

A series of variable alkyl chain length substituted donor-acceptor (D-A) conjugated polymers with thiophene ring as the donor and benzotriazole moiety as the acceptor has been investigated in bulk heterojunction solar cells. The optical and electrochemical properties showed that the absorption onsets and the energy levels of the copolymers were not affected by alkyl substitution revealing 1.9 eV of optical band gap. The morphologies of the blend film can be fine-tuned by increasing the chain length attached to the benzotriazole unit. Photovoltaic devices were fabricated using (6,6)-phenyl-C-61-butyric acid methyl ester (PC60BM) and (6,6)-phenyl-C-71-butyric acid methyl ester (PC70BM) as the acceptors. The maximum performance was achieved with a V-oc, of 0.75V, J(sc) of 3.70 mA/cm(2), FF of 45% and a PCE of 1.23% for the PTBT-DA10:PC70BM device using 1:4 w/w ratio in chlorobenzene (CB)

The evolution of Materials Acceleration Platforms: toward the laboratory of the future with AMANDA

The development of complex functional materials poses a multi-objective optimization problem in a large multi-dimensional parameter space. Solving it requires reproducible, user-independent laboratory work and intelligent preselection of experiments. However, experimental materials science is a field where manual routines are still predominant, although other domains like pharmacy or chemistry have long used robotics and automation. As the number of publications on Materials Acceleration Platforms (MAPs) increases steadily, we review selected systems and fit them into the stages of a general material development process to examine the evolution of MAPs. Subsequently, we present our approach to laboratory automation in materials science. We introduce AMANDA (Autonomous Materials and Device Application Platform - www.amanda-platform.com), a generic platform for distributed materials research comprising a self-developed software backbone and several MAPs. One of them, LineOne (L1), is specifically designed to produce and characterize solution-processed thin-film devices like organic solar cells (OSC). It is designed to perform precise closed-loop screenings of up to 272 device variations per day yet allows further upscaling. Each individual solar cell is fully characterized, and all process steps are comprehensively documented. We want to demonstrate the capabilities of AMANDA L1 with OSCs based on PM6:Y6 with 13.7% efficiency when processed in air. Further, we discuss challenges and opportunities of highly automated research platforms and elaborate on the future integration of additional techniques, methods and algorithms in order to advance to fully autonomous self-optimizing systems—a paradigm shift in functional materials development leading to the laboratory of the future

A solution-processed barium hydroxide modified aluminum doped zinc oxide layer for highly efficient inverted organic solar cells

Inverted organic solar cells (iOSCs) with air stable interface materials and top electrodes and an efficiency of 6.01% are achieved by inserting a barium hydroxide (Ba(OH)2) layer between the aluminum doped zinc oxide (AZO) electron extraction layer and the active layer. A low bandgap diketopyrrolopyrrole–quinquethiophene alternating copolymer (pDPP5T-2) and phenyl-C61-butyric acid methyl ester (PC61BM) were chosen as the active layer compounds. Compared to the control device without Ba(OH)2, insertion of a few nm thick Ba(OH)2 layer results in an enhanced VOC of 10%, JSC of 28%, FF of 28% and PCE of 80%. Modification of AZO with a solution processed low-cost Ba(OH)2 layer increased the efficiency of the inverted device by dominantly reducing the energy barrier for electron extraction from PC61BM, and consequently, reduced charge recombination is observed. The drastic improvement in device efficiency and the simplicity of fabrication by solution processing suggest Ba(OH)2 as a promising and practical route to reduce interface induced recombination losses at the cathode of organic solar cells

High shunt resistance in polymer solar cells comprising a MoO3 hole extraction layer processed from nanoparticle suspension

In this report, we present solution processed molybdenum trioxide (MoO3) layers incorporated as hole extraction layer (HEL) in polymer solar cells (PSCs) and demonstrate the replacement of the commonly employed poly(3,4-ethylene dioxythiophene):(polystyrene sulfonic acid) (PEDOT:PSS). MoO3 is known to have excellent electronic properties and to yield more stable devices compared to PEDOT:PSS. We demonstrate fully functional solar cells with up to 65 nm thick MoO3 HEL deposited from a nanoparticle suspension at low temperatures. The PSCs with an active layer comprising a blend of poly(3-hexylthiophene) and [6,6]-phenyl-C61 butyric acid methyl ester and a MoO3 HEL show comparable performance to reference devices with a PEDOT:PSS HEL. The best cells with MoO3 reach a fill factor of 66.7% and power conversion efficiency of 2.92%. Moreover, MoO3 containing solar cells exhibit an excellent shunt behavior with a parallel resistance of above 100 kΩ cm2