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

This article presents a successful laser-powered co-firing process for highly efficient Si solar cells as a more compact and energy-efficient alternative to the conventional firing process in an infrared (IR) lamp-powered heat chamber. The best cell group reaches with laser firing only 0.1%abs lower cell efficiency compared to the best group with conventional firing, demonstrating the industrial potential of this laser firing technology. Adding the laser enhanced contact optimization (LECO) process after firing improves the cell efficiency for laser firing to the level of conventional firing, demonstrating the potential of the combination of the laser firing and the LECO process.

Laser-Powered Co-Firing Process for Highly Efficient Si Solar Cells

The rheology of a low viscous aerosol ink was modified into a hot melt ink by substituting the original solvent system by a thermo plastic alcohol. Additionally the aerosol printer was adapted to hot melt printing by heating all parts which are in contact with the ink, including the atomizer, virtual impactor, printing head and atomization gas. Front side contacts on silicon solar cells were printed with a metal aerosol jet printer using this hotmelt ink. Improved line resolution, higher aspect ratio and higher solid content compared to conventional aerosol inks could be achieved. To demonstrate the industrial feasibility, a multi-nozzle printing system was used. Line widths of 40 μm could be achieved. The aspect ratio of the fingers is increased from 1:40 to 1:6. Large-area multicrystalline silicon solar cells were, printed at room temperature at different printing speeds, fired in an in-line belt furnace and finally plated in an industrial plating machine. Energy conversion rates as high as 16.1% were achieved. The short-circuit current and the fill factor of these cells is around 34 mA/cm and 78%, respectively, showing the excellent contact geometry of the fabricated fingers.

Improved Front Side Metallization by Aerosol Jet Printing of Hotmelt Inks

Printed contact on emitter with low dopant surface concentration

In this work, three industry-related metallization approaches for silicon heterojunction (SHJ) solar cells are presented which are aiming for a reduction of silver consumption compared to conventional screen-printing of low-temperature silver pastes. The presented results are achieved on large-area cells (area of 244.3 cm2). Firstly, the cell results reveal that with silver-coated copper pastes for screen-printing comparable results as with pure silver pastes can be achieved but with a potentially reduced silver consumption of 30%. Median efficiencies of 21.6% are achieved in both cases utilizing a five busbar cell design. A second approach to reduce the silver consumption is the use of inkjet-printing. The influence of the inkjet-printed layer number per contact finger on the cell performance of busbarless cells is investigated. A maximum conversion efficiency of 23.3% of an inkjet-printed solar cell is achieved within this study. Thirdly, a novel printing technology established at Fraunhofer ISE, called FlexTrail-printing, is introduced. By utilizing the same Ag nanoparticle ink like in the case of inkjet-printing, the finger width is reduced from 75 ± 1 µm down to 16 ± 1 µm on alkaline textured SHJ solar cells. To the authors’ knowledge, this is the smallest feature size ever published on ITO-coated, textured silicon surfaces. A maximum conversion efficiency of 23.7% is measured (busbarless cell). Only 0.3 ± 0.1 mg Ag nanoparticle ink is consumed during FlexTrail-printing on a large-area wafer with a front grid of 80 fingers.

Advanced metallization with low silver consumption for silicon heterojunction solar cells

Low‐cost and high‐efficiency tandem solar cells are promising candidates for a future industrial mass production. Nowadays, the passivated emitter and rear cell (PERC) technology makes up the major market share; therefore, it is an attractive option to use the PERC technology as bottom cell concept for a perovskite–silicon tandem device. Long‐term optimization of the PERC technology led to highly efficient, low‐cost, and mature devices. For PERC‐like bottom cells, mainly an adapted front‐side design is needed: Design constrains of a PERC single junction are relaxed to some extent, because such an updated PERC bottom cell only needs to absorb long wavelength photons, transports about half the current and, in a monolithic tandem, lateral transport, and the use of firing‐through local silver contacts is not a mandatory prerequisite. Consequently, to make PERC suitable for tandem application, a systematic reevaluation of the current PERC technology is performed here considering five different front‐side concepts. We investigate locally contacted and full‐area transparent conducting oxide (TCO)‐based interconnection concepts for PERC, as well as the full‐area tunnel oxide passivating contact (TOPCon). Our simulation work elaborates on the advantages and physical constrains of each concept and gives guidelines for the optimization of the phosphorus diffused emitter and front interconnection layers. We conclude that both full‐area and locally contacted front‐side concepts are promising candidates for achieving tandem cell efficiencies of about 30%.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/pip.3524

Stefan W. Glunz

Papers

Laser-Powered Co-Firing Process for Highly Efficient Si Solar Cells

Improved Front Side Metallization by Aerosol Jet Printing of Hotmelt Inks

Printed contact on emitter with low dopant surface concentration

Advanced metallization with low silver consumption for silicon heterojunction solar cells

How to make PERC suitable for perovskite–silicon tandem solar cells: A simulation study