Untersuchung von Grenzflächen in Chalkopyrit‐Dünnschichtsolarzellen

Examination of interfaces in chalcopyrite thin film solar cells using synchrotron radiation Two examples from current research at the division of solar energy research of the Helmholtz‐Zentrum Berlin für Materialien und Energie (HZB) are described, showing the use of synchrotron radiation for the analysis of interface reactions in chalcopyrite thin film solar cells. Deeper knowledge of interface reactions leads to better understanding of the functionality of these solar cells and thus to possibilities to further improve them in terms of efficiency and stability. We show how x‐ray emission spectroscopy can elucidate the oxidation of sulfide at the interface between chalcopyrite solar cell absorbers and zinc oxide window layers. In a second example we demonstrate how high energy photo electron spectroscopy can be used to follow in‐situ the diffusion of copper ions from a chalcopyrite absorber into an indium sulfide buffer layer.     

Surface and Interface Properties in Thin‐Film Solar Cells: Using Soft X‐rays and Electrons to Unravel the Electronic and Chemical Structure

Thin‐film solar cells have great potential to overtake the currently dominant silicon‐based solar cell technologies in a strongly growing market. Such thin‐film devices consist of a multilayer structure, for which charge‐carrier transport across interfaces plays a crucial role in minimizing the associated recombination losses and achieving high solar conversion efficiencies. Further development can strongly profit from a high‐level characterization that gives a local, electronic, and chemical picture of the interface properties, which allows for an insight‐driven optimization. Herein, the authors' recent progress of applying a “toolbox” of high‐level laboratory‐ and synchrotron‐based electron and soft X‐ray spectroscopies to characterize the chemical and electronic properties of such applied interfaces is provided. With this toolbox in hand, the activities are paired with those of experts in thin‐film solar cell preparation at the cutting edge of current developments to obtain a deeper understanding of the recent improvements in the field, e.g., by studying the influence of so‐called “post‐deposition treatments”, as well as characterizing the properties of interfaces with alternative buffer layer materials that give superior efficiencies on large, module‐sized areas.     

Dünnschichtsolarzellen

Thin Film Solar Cells The direct conversion of sunlight into electricity — photovoltaics (PV) — has emerged as a strongly growing market during the past years. Today, more than 80 % of the world market is supplied by solar modules based on mono‐ or polycrystalline silicon wafers. Thin‐film solar cells promise significantly lower costs for photovoltaic energy conversion, and thus will probably dominate the PV‐market in the future. Consequently, the production of thin film solar cells will lead to key technologies of the 21^st century. This article addresses the three most advanced types of thin‐film cells describing the status of these technologies at the laboratory level, in pilot production, and in first production lines. The challenges along the way from laboratory developments towards mass production are discussed, fo cusing on the central role of the vacuum‐based technologies applied for thin film deposition.     

One‐Step Fabrication of Unique Mesoporous NiO Hollow Sphere Film on FTO for High‐Performance P‐Type Dye‐Sensitized Solar Cells

P‐type dye sensitized solar cells (p‐DSCs) deliver much lower overall efficiency than their inverse model, n‐DSCs. However, they have fundamental and practical significance, in particular, their tandem structured solar cells with both p‐ and n‐photoelectrodes could offer great potential to significantly improve the efficiency of existing solar cells. A facile and environmentally friendly method is developed to directly one‐step grow hollow NiO spherical structures on fluorine‐doped tin oxide (FTO) substrate, in which a Ni²⁺ and polymer complex spherical structure is self‐constructed through a controlled solvent evaporation process, followed by calcination‐converting to a unique NiO hollow sphere film. The prepared material is further used as a photocathode in p‐type dye sensitized solar cells, resulting in 41% increase of an open‐circuit voltage and 18% enhancement of power conversion efficiency than NiO nanoparticles photocathode. The improved performance can be ascribed to suppressed charge recombination at the photocathode/electrolyte interface. This template‐free approach could be universally used to fabricate other nanostructured hollow spheres for a wide range of energy conversion applications such as electrochemical capacitors, chemical sensors, and electrochromic devices.     

High‐Performance Single‐Crystalline Perovskite Thin‐Film Photodetector

The best performing modern optoelectronic devices rely on single‐crystalline thin‐film (SC‐TF) semiconductors grown epitaxially. The emerging halide perovskites, which can be synthesized via low‐cost solution‐based methods, have achieved substantial success in various optoelectronic devices including solar cells, lasers, light‐emitting diodes, and photodetectors. However, to date, the performance of these perovskite devices based on polycrystalline thin‐film active layers lags behind the epitaxially grown semiconductor devices. Here, a photodetector based on SC‐TF perovskite active layer is reported with a record performance of a 50 million gain, 70 GHz gain‐bandwidth product, and a 100‐photon level detection limit at 180 Hz modulation bandwidth, which as far as we know are the highest values among all the reported perovskite photodetectors. The superior performance of the device originates from replacing polycrystalline thin film by a thickness‐optimized SC‐TF with much higher mobility and longer recombination time. The results indicate that high‐performance perovskite devices based on SC‐TF may become competitive in modern optoelectronics.     

Leistungsfähige Dünnschicht‐Beschichtung für die Photovoltaik

High‐performance thin‐film coating for photovoltaic applications The thin‐film photovoltaics market is large and is expected to grow continuously. Due to its huge potential regarding cost reduction and cell efficiency improvement, sputter‐etched ZnO:Al‐based TCO can make a sustainable contribution to achieve grid parity. One of the most promising approaches here is the large scale volume manufacturing of sputter‐etched TCO glass for thin film silicon solar cells. VON ARDENNE's leading process knowhow, applied to the highly productive and reliable PIA|nova coating system, allows for cost effective production and flexible adaptation of customer specific TCO requirements.     

A Layer‐by‐Layer ZnO Nanoparticle–PbS Quantum Dot Self‐Assembly Platform for Ultrafast Interfacial Electron Injection

Absorbent layers of semiconductor quantum dots (QDs) are now used as material platforms for low‐cost, high‐performance solar cells. The semiconductor metal oxide nanoparticles as an acceptor layer have become an integral part of the next generation solar cell. To achieve sufficient electron transfer and subsequently high conversion efficiency in these solar cells, however, energy‐level alignment and interfacial contact between the donor and the acceptor units are needed. Here, the layer‐by‐layer (LbL) technique is used to assemble ZnO nanoparticles (NPs), providing adequate PbS QD uptake to achieve greater interfacial contact compared with traditional sputtering methods. Electron injection at the PbS QD and ZnO NP interface is investigated using broadband transient absorption spectroscopy with 120 femtosecond temporal resolution. The results indicate that electron injection from photoexcited PbS QDs to ZnO NPs occurs on a time scale of a few hundred femtoseconds. This observation is supported by the interfacial electronic‐energy alignment between the donor and acceptor moieties. Finally, due to the combination of large interfacial contact and ultrafast electron injection, this proposed platform of assembled thin films holds promise for a variety of solar cell architectures and other settings that principally rely on interfacial contact, such as photocatalysis.     

Anlagen für die Solarzellen‐Schichterzeugung benötigen hervorragend ausgelegte Vakuumsysteme

Vacuum pumps are an enabling technology for solar power because all modules require vacuum processing at various stages of production. Specially engineered products that are helping to make solar modules more affordable by reducing equipment downtime and improving process performance are essential to become a successful partner for the solar industry. The processes used for thin film technology are extremely demanding. Pump performance for high flows of light gases must be achieved and in parallel safety aspects must be considered carefully. The pump solutions must be extreme reliable in order to guarantee maximum tool up‐time which is mandatory for a production line in order to be cost competitive. Design and performance of such solutions must be qualified in close collaboration between pump supplier and the equipment producer together wi^th the user of the equipment. Experience and know how from industries using similar processes like semiconductor, large area coating or FPD production can be applied for solar production lines which can help to reduce time to market. The article will outline the specific needs for thin film solar cells based on amorphous silicon and CdTe.     

GaSe Formation at the Cu(In,Ga)Se2/Mo Interface–A Novel Approach for Flexible Solar Cells by Easy Mechanical Lift‐Off

Implementing photovoltaic devices based on high efficiency thin‐film technologies on cheap, light‐weight and flexible polymeric substrates is highly appealing to cut down costs in industrial production and to accelerate very large scale deployment of photovoltaics in the upcoming years. Lift‐off processes, which allow separating active layers from primary substrates and subsequent transfer onto an alternative substrate without modifying the upstream production process and without performance losses, are an emerging alternative to direct growth on polymeric substrates. This study concerns the feasibily of direct mechanical lift‐off process for high efficiency Cu(In,Ga)Se₂ (CIGS) thin film solar cells grown by coevaporation on glass/molybdenum substrates without performance losses. The study presents an in depth characterization (SEM,AFM,GIXRD,XPS) of samples leading to excellent lift‐off properties. They are explained by a specific gallium rich CIGS graded interface structure according to the interfacial sequence glass/Mo/MoSe₂/Ga_xSe_y/Ga‐rich‐CIGS. The interfacial layer, attributed to GaSe, has a layered structure and out performs the molybdenum diselenide layered layer which forms spontaneously at the interface Mo/CIGS. It allows a very easy lift‐off process at the interface GaSe/CIGS thanks to Van‐der‐Waals adhesion mechanism in GaSe. Key physical‐chemical parameters are identified and analyzed. After lift‐off, an efficiency of 14.3%, higher than the initial reference CIGS solar cell efficiency (13.8%) is measured.     

Highly Efficient (>10%) Flexible Organic Solar Cells on PEDOT‐Free and ITO‐Free Transparent Electrodes

A novel approach to fabricate flexible organic solar cells is proposed without indium tin oxide (ITO) and poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) using junction‐free metal nanonetworks (NNs) as transparent electrodes. The metal NNs are monolithically etched using nanoscale shadow masks, and they exhibit excellent optoelectronic performance. Furthermore, the optoelectrical properties of the NNs can be controlled by both the initial metal layer thickness and NN density. Hence, with an extremely thin silver layer, the appropriate density control of the networks can lead to high transmittance and low sheet resistance. Such NNs can be utilized for thin‐film devices without planarization by conductive materials such as PEDOT:PSS. A highly efficient flexible organic solar cell with a power conversion efficiency (PCE) of 10.6% and high device yield (93.8%) is fabricated on PEDOT‐free and ITO‐free transparent electrodes. Furthermore, the flexible solar cell retains 94.3% of the initial PCE even after 3000 bending stress tests (strain: 3.13%).     

Controlled Growth of High‐Quality ZnO‐Based Films and Fabrication of Visible‐Blind and Solar‐Blind Ultra‐Violet Detectors

ZnO is a wide‐bandgap (3.37 eV at room temperature) oxide semiconductor that is attractive for its great potential in short‐wavelength optoelectronic devices, in which high quality films and heterostructures are essential for high performance. In this study, controlled growth of ZnO‐based thin films and heterostructures by molecular beam epitaxy (MBE) is demonstrated on different substrates with emphasis on interface engineering. It is revealed that ultrathin AlN or MgO interfacial layers play a key role in establishing structural and chemical compatibility between ZnO and substrates. Furthermore, a quasi‐homo buffer is introduced prior to growth of a wurtzite MgZnO epilayer to suppress the phase segregation of rock‐salt MgO, achieving wide‐range bandgap tuning from 3.3 to 4.55 eV. Finally, a visible‐blind UV detector exploiting a double heterojunction of n‐ZnO/insulator‐MgO/p‐Si and a solar‐blind UV detector using MgZnO as an active layer are fabricated by using the growth techniques discussed here.     

Crystalline Silicon Thin‐Film Solar Cells on Silicon Nitride Ceramic Substrates

Photovoltaics—the generation of electricity from sunlight—is a technically challenging but environmentally benign technology to generate electricity with a large economical potential. However, the major hurdle for its widespread usage is its present high cost. Various thin‐film solar cell technologies are investigated to bring down the total cost to an economic value. One of them, the crystalline silicon thin‐film (CSiTF) solar cell combines the advantages of conventional wafer‐based silicon solar cells such as high efficiency and non‐toxicity with the benefits of thin‐film technologies such as serial interconnection and large area deposition. This paper reports for the first time the preparation of CSiTF solar cells on specially developed Si₃N₄ ceramic substrates. Three different types of Si₃N₄ ceramic wafers were single‐sided coated with 10μm of microcrystalline silicon, which was recrystallized by a zone melting step and subsequently thickened to approx. 30 μm. Optical analysis of the layer surface and cross sections was done to determine the crystallographic properties of the silicon layers, as well as mass spectroscopy to measure the concentration of transition metal impurities. A one‐side contacted solar cell process was applied on non‐conducting Si₃N₄ substrates. The best 1 cm² cells achieved an efficiency of 8.0 % with an excellent fill factor of 74 % and an open circuit voltage of 554 mV. The solar cell characterization was complemented by measurements of dark current–voltage characteristics, spectrally resolved light beam induced current mapping, and external quantum efficiency.     

Advances in Solution‐Processed Multijunction Organic Solar Cells

The efficiency of organic solar cells can benefit from multijunction device architectures, in which energy losses are substantially reduced. Herein, recent developments in the field of solution‐processed multijunction organic solar cells are described. Recently, various strategies have been investigated and implemented to improve the performance of these devices. Next to developing new materials and processing methods for the photoactive and interconnecting layers, specific layers or stacks are designed to increase light absorption and improve the photocurrent by utilizing optical interference effects. These activities have resulted in power conversion efficiencies that approach those of modern thin film photovoltaic technologies. Multijunction cells require more elaborate and intricate characterization procedures to establish their efficiency correctly and a critical view on the results and new insights in this matter are discussed. Application of multijunction cells in photoelectrochemical water splitting and upscaling toward a commercial technology is briefly addressed.     

Rückseitenmetallisierung Wafer‐basierter Si‐Solarzellen mittels EB‐PVD

Back Side Metallization of Wafer Based Silicon Solar Cells by Means of Electron Beam Evaporation Electron beam evaporation is an innovative vacuum deposition technology regarding the wafer backside metallization of crystalline silicon solar cells. The motivation for the consideration of electron beam evaporation as cell finishing step is based on the one hand on the competition with thin film photovoltaic modules and on the other on the remarkable cost reduction potential by applying EB‐PVD (Electron Beam Physical Vapor Deposition). This article presents a highly productive coater concept and gives an explanation of important aspects for the adaption of the coater concept to typical solar cell features. Various PVD technologies are compared concerning their possible use as wafer backside metallization method. Challenges and chances of the introduction of EB‐PVD in the wafer based solar cell production are considered.     

Charge Transfer from Carbon Nanotubes to Silicon in Flexible Carbon Nanotube/Silicon Solar Cells

Mechanical fragility and insufficient light absorption are two major challenges for thin flexible crystalline Si‐based solar cells. Flexible hybrid single‐walled carbon nanotube (SWNT)/Si solar cells are demonstrated by applying scalable room‐temperature processes for the fabrication of solar‐cell components (e.g., preparation of SWNT thin films and SWNT/Si p–n junctions). The flexible SWNT/Si solar cells present an intrinsic efficiency ≈7.5% without any additional light‐trapping structures. By using these solar cells as model systems, the charge transport mechanisms at the SWNT/Si interface are investigated using femtosecond transient absorption. Although primary photon absorption occurs in Si, transient absorption measurements show that SWNTs also generate and inject excited charge carriers to Si. Such effects can be tuned by controlling the thickness of the SWNTs. Findings from this study could open a new pathway for designing and improving the efficiency of photocarrier generation and absorption for high‐performance ultrathin hybrid SWNT/Si solar cells.     

Fabrication of Transistors on Flexible Substrates: from Mass‐Printing to High‐Resolution Alternative Lithography Strategies

In this report, the development of conventional, mass‐printing strategies into high‐resolution, alternative patterning techniques is reviewed with the focus on large‐area patterning of flexible thin‐film transistors (TFTs) for display applications. In the first part, conventional and digital printing techniques are introduced and categorized as far as their development is relevant for this application area. The limitations of conventional printing guides the reader to the second part of the progress report: alternative‐lithographic patterning on low‐cost flexible foils for the fabrication of flexible TFTs. Soft and nanoimprint lithography‐based patterning techniques and their limitations are surveyed with respect to patterning on low‐cost flexible foils. These show a shift from fabricating simple microlense structures to more complicated, high‐resolution electronic devices. The development of alternative, low‐temperature processable materials and the introduction of high‐resolution patterning strategies will lead to the low‐cost, self‐aligned fabrication of flexible displays and solar cells from cheaper but better performing organic materials.     

Electrochemical Self‐Assembly of Dye‐Modified Zinc Oxide Thin Films

One‐step electrochemical self‐assembly is an excellent new method for the construction of hybrid inorganic/organic films for dye‐sensitized solar cells (DSSCs). The promising oxide semiconductor zinc oxide can be electrodeposited in the presence of organic dye molecules to give porous thin films with varying morphology suitable for DSSCs (the Figure shows a ZnO film grown in the presence of a tetrasulfophthalocyanine).     

Mixed Valence Perovskite Cs2Au2I6: A Potential Material for Thin‐Film Pb‐Free Photovoltaic Cells with Ultrahigh Efficiency

New light is shed on the previously known perovskite material, Cs₂Au₂I₆, as a potential active material for high‐efficiency thin‐film Pb‐free photovoltaic cells. First‐principles calculations demonstrate that Cs₂Au₂I₆ has an optimal band gap that is close to the Shockley–Queisser value. The band gap size is governed by intermediate band formation. Charge disproportionation on Au makes Cs₂Au₂I₆ a double‐perovskite material, although it is stoichiometrically a single perovskite. In contrast to most previously discussed double perovskites, Cs₂Au₂I₆ has a direct‐band‐gap feature, and optical simulation predicts that a very thin layer of active material is sufficient to achieve a high photoconversion efficiency using a polycrystalline film layer. The already confirmed synthesizability of this material, coupled with the state‐of‐the‐art multiscale simulations connecting from the material to the device, strongly suggests that Cs₂Au₂I₆ will serve as the active material in highly efficient, nontoxic, and thin‐film perovskite solar cells in the very near future.     

Low‐Temperature Soft‐Cover Deposition of Uniform Large‐Scale Perovskite Films for High‐Performance Solar Cells

Large‐scale high‐quality perovskite thin films are crucial to produce high‐performance perovskite solar cells. However, for perovskite films fabricated by solvent‐rich processes, film uniformity can be prevented by convection during thermal evaporation of the solvent. Here, a scalable low‐temperature soft‐cover deposition (LT‐SCD) method is presented, where the thermal convection‐induced defects in perovskite films are eliminated through a strategy of surface tension relaxation. Compact, homogeneous, and convection‐induced‐defects‐free perovskite films are obtained on an area of 12 cm², which enables a power conversion efficiency (PCE) of 15.5% on a solar cell with an area of 5 cm². This is the highest efficiency at this large cell area. A PCE of 15.3% is also obtained on a flexible perovskite solar cell deposited on the polyethylene terephthalate substrate owing to the advantage of presented low‐temperature processing. Hence, the present LT‐SCD technology provides a new non‐spin‐coating route to the deposition of large‐area uniform perovskite films for both rigid and flexible perovskite devices.     

Reaktives Magnetronsputtern von Dünnschichtsolarzellen

Reactive Magnetron Sputtering of Thin Film Solar Cells We show that reactive magnetron sputtering is well suited to deposit CuInS₂‐thin film absorber layers of high electronic quality. Using metallic targets and substrate temperatures below 500 °C, compact films with grain sizes in the micrometer range can be obtained. The structural and electronic properties of these layers are comparable to CuInS₂ thin films prepared by a 2‐step sulfurization process, which is being commercialized at present. In particular, the reactively sputtered films show minority carrier diffusion lengths larger than the layer thickness (≈ 2μm). This results in solar cells with conversion efficiences larger than 10 %, comparable to the best conversion efficiencies for CuInS₂‐solar cells obtained from other deposition processes. These results are promising for the potential application of magnetron sputtering as a large area deposition process for absorber layers in thin film solar cells.