Additive manufacturing (AM) alias 3D printing translates computer-aided design (CAD) virtual 3D models into physical objects. By digital slicing of CAD, 3D scan, or tomography data, AM builds objects layer by layer without the need for molds or machining. AM enables decentralized fabrication of customized objects on demand by exploiting digital information storage and retrieval via the Internet. The ongoing transition from rapid prototyping to rapid manufacturing prompts new challenges for mechanical engineers and materials scientists alike. Because polymers are by far the most utilized class of materials for AM, this Review focuses on polymer processing and the development of polymers and advanced polymer systems specifically for AM. AM techniques covered include vat photopolymerization (stereolithography), powder bed fusion (SLS), material and binder jetting (inkjet and aerosol 3D printing), sheet lamination (LOM), extrusion (FDM, 3D dispensing, 3D fiber deposition, and 3D plotting), and 3D bioprinting....

Polymers for 3D Printing and Customized Additive Manufacturing

The efficiency of silicon solar cells has a large influence on the cost of most photovoltaics panels. Here, researchers from Kaneka present a silicon heterojunction with interdigitated back contacts reaching an efficiency of 26.3% and provide a detailed loss analysis to guide further developments.

Silicon heterojunction solar cell with interdigitated back contacts for a photoconversion efficiency over 26

The photovoltaics of organic–inorganic lead halide perovskite materials have shown rapid improvements in solar cell performance, surpassing the top efficiency of semiconductor compounds such as CdTe and CIGS (copper indium gallium selenide) used in solar cells in just about a decade. Perovskite preparation via simple and inexpensive solution processes demonstrates the immense potential of this thin-film solar cell technology to become a low-cost alternative to the presently commercially available photovoltaic technologies. Significant developments in almost all aspects of perovskite solar cells and discoveries of some fascinating properties of such hybrid perovskites have been made recently. This Review describes the fundamentals, recent research progress, present status, and our views on future prospects of perovskite-based photovoltaics, with discussions focused on strategies to improve both intrinsic and extrinsic (environmental) stabilities of high-efficiency devices. Strategies and challenges regardi...

Halide Perovskite Photovoltaics: Background, Status, and Future Prospects

Perovskite solar cells typically comprise electron- and hole-transport materials deposited on each side of a perovskite active layer. So far, only two organic hole-transport materials have led to state-of-the-art performance in these solar cells1: poly(triarylamine) (PTAA)2–5 and 2,2ʹ,7,7ʹ-tetrakis(N,N-di-p-methoxyphenylamine)-9,9ʹ-spirobifluorene (spiro-OMeTAD)6,7. However, these materials have several drawbacks in terms of commercialization, including high cost8, the need for hygroscopic dopants that trigger degradation of the perovskite layer9 and limitations in their deposition processes10. Poly(3-hexylthiophene) (P3HT) is an alternative hole-transport material with excellent optoelectronic properties11–13, low cost8,14 and ease of fabrication15–18, but so far the efficiencies of perovskite solar cells using P3HT have reached only around 16 per cent19. Here we propose a device architecture for highly efficient perovskite solar cells that use P3HT as a hole-transport material without any dopants. A thin layer of wide-bandgap halide perovskite is formed on top of the narrow-bandgap light-absorbing layer by an in situ reaction of n-hexyl trimethyl ammonium bromide on the perovskite surface. Our device has a certified power conversion efficiency of 22.7 per cent with hysteresis of ±0.51 per cent; exhibits good stability at 85 per cent relative humidity without encapsulation; and upon encapsulation demonstrates long-term operational stability for 1,370 hours under 1-Sun illumination at room temperature, maintaining 95 per cent of the initial efficiency. We extend our platform to large-area modules (24.97 square centimetres)—which are fabricated using a scalable bar-coating method for the deposition of P3HT—and achieve a power conversion efficiency of 16.0 per cent. Realizing the potential of P3HT as a hole-transport material by using a wide-bandgap halide could be a valuable direction for perovskite solar-cell research. A double-layered halide architecture for perovskite solar cells enables the use of dopant-free poly(3-hexylthiophene) as a hole-transport material, forming stable and scalable devices with a certified power conversion efficiency of 22.7 per cent.

Efficient, stable and scalable perovskite solar cells using poly(3-hexylthiophene)

Recent developments in photovoltaic materials have led to continual improvements in their efficiency. We review the electrical characteristics of 16 widely studied geometries of photovoltaic materials with efficiencies of 10 to 29%. Comparison of these characteristics to the fundamental limits based on the Shockley-Queisser detailed-balance model provides a basis for identifying the key limiting factors, related to efficient light management and charge carrier collection, for these materials. Prospects for practical application and large-area fabrication are discussed for each material.

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Photovoltaic materials: Present efficiencies and future challenges

The presented hybrid bifacial CPV module combines III-V four-junction and bifacial Si solar cells to convert direct, diffuse and rear side radiation into electricity. At standard test conditions the 4-terminal module converts 34.2% of the reference AM1.5g spectrum into electricity and additionally generates 11.2 W/m2 for every 100 W/m2 of rear irradiance. Our analysis shows that nearly the same performance is possible in a 2-terminal configuration. The expected power loss was quantified by scaling the independently measured I-V characteristics of each array into a voltage-matched parallel interconnection. Despite the wide meteorological and spectral variation, the mean power loss remains at 0.1% rel .

Hybrid Bifacial CPV Power Output Beyond 350W/m 2

This study examines concepts to increase the efficiency of fluorescent concentrator systems. Different system sizes and configurations are investigated in detail by external quantum efficiency measurements, light beam induced current maps and by IV-measurements. A photonic structure which serves as a bandstop reflection filter for the light emitted from the dyes in the fluorescent concentrator increases the system efficiency by 20% relative. Such photonic structures are especially beneficial for larger systems. The combination of two fluorescent concentrators made with different dyes in one stack increases system efficiency from 5.1% with only one dye to 6.7% for the stack.

Efficiency Enhancement of Fluorescent Concentrators With Photonic Structures and Material Combinations

For highly efficient silicon solar cells, lowly doped phosphorus or boron emitters are essential prerequisites to achieve sufficient surface passivation. However, contact formation to such lowly doped surfaces remains a challenge for traditional printing technologies. The question arises if such emitters can be contacted reliably applying advanced metallization inks. For this investigation, we manufactured lowly doped boron and phosphorus emitters with ND/A < 1·10 cm onto silicon substrates with high pyramid texture. For metallization we used our optimized aerosol jet printing process and altered the ink deposition density from 6 to 23 g/m2. On textured wafers the seed layer width remained < 40 μm in average with our own developed seed layer ink. For characterization purposes the lines were thickened with electro-plated silver. Contact resistivity measurements were performed to judge the suitability of this metallization scheme. It is shown that contact formation mainly depends on the firing temperature and less on the amount of deposited ink. Contact resistivities of 1-7 mΩ·cm2 for the phosphorus and 612 mΩ·cm2 for the boron emitter could be achieved.

Optimized Aerosol Jet Printed Silver Contacts on Lowly Doped Phosphorus and Boron Emitters

Motivation Silicon quantum dot absorber layers are used as building blocks for all-silicon tandem solar cells. A control of the Si nanocrystal size allows the adjustment of essential material parameters such as bandgap and oscillator strengths due to size quantization effects. This allows to fabricate silicon based top and middle cells with an engineered and optimized band gap for all-silicon tandem cells, increasing the theoretical efficiency limit of a silicon solar cell from 29 % to 42.5 % for two and 47.5 % for three cells (1). In this paper, we present the optical and electrical characterization of Si quantum dots embedded in a dielectric SiC matrix for solar cell applications with special emphasis on the electrical transport through the superstructure and the reduction of recombination via defects at the interface between nanocrystals and the dielectric matrix. In order to build a photovoltaic device, both carrier transport properties and recombination properties have to be optimised by appropriate deposition, annealing and passivation procedures.

Silicon quantum dot superstructures for all-silicon tandem solar cells

This work shows the possibility to use local laser chemical processing (LCP) boron dopings to create local back surface fields (LBSF) in a high efficiency p-type silicon solar cell structure. In order to create boron local back surface fields with LCP an alkaline aqueous boron solution was used. Our recent work shows the potential of this doping source to create local dopings with a surface concentration of about 1020 cm−3 and a doping depth of up to 1.3 μm. For the first time the successful application of boron LCP as LBSF for high efficiency solar cells is shown. The processed LCP-PERL (passivated emitter and rear locally diffused) solar cells show a maximum cell efficiency of η = 20.9% and an absolute cell efficiency benefit of Δη = 0.3–0.4%abs. in comparison to the reference cells with undoped local rear contacts processed with water-LCP. The presented solar cells show the best cell efficiencies ever achieved with LCP dopings. For this study different LCP-parameters were varied such as the contact geometry (line openings and openings formed with a cascade of overlapping LCP dots) and the pitch of the rear side contact structure. The results show that the LCP boron doping process allows the fabrication of industrial feasible local back surface field structures with a line contact design for high efficiency p-type silicon solar cells.

Stefan W. Glunz

Papers

Hybrid Bifacial CPV Power Output Beyond 350W/m 2

Efficiency Enhancement of Fluorescent Concentrators With Photonic Structures and Material Combinations

Optimized Aerosol Jet Printed Silver Contacts on Lowly Doped Phosphorus and Boron Emitters

Silicon quantum dot superstructures for all-silicon tandem solar cells

Boron LCP local back surface fields for high efficiency silicon solar cells