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

A stack of hydrogenated amorphous silicon (a-Si) and PECVD-silicon oxide (SiOx) has been used as surface passivation layer for silicon wafer surfaces. Very good surface passivation could be reached leading to a surface recombination velocity (SRV) below 10 cm/s on 1 Ω cm p-type Si wafers. By using the passivation layer system at a solar cell's rear side and applying the laser-fired contacts (LFC) process, pointwise local rear contacts have been formed and an energy conversion efficiency of 21·7% has been obtained on p-type FZ substrates (0·5 Ω cm). Simulations show that the effective rear SRV is in the range of 180 cm/s for the combination of metallised and passivated areas, 120 ± 30 cm/s were calculated for the passivated areas. Rear reflectivity is comparable to thermally grown silicon dioxide (SiO2). a-Si rear passivation appears more stable under different bias light intensities compared to thermally grown SiO2. Copyright © 2008 John Wiley & Sons, Ltd.

Stack system of PECVD amorphous silicon and PECVD silicon oxide for silicon solar cell rear side passivation

BICON is a two-stage concentrator system developed at Fraunhofer ISE which is one-axis tracked. The innovation of this one-axis tracked system is that it enables a high geometrical concentration of 300 × in combination with a high optical efficiency (upto 78%) and a large acceptance angle of ± 23·5° all year through. For this, the system uses a parabolic mirror (40·4 ×) and a three dimensional second stage consisting of compound parabolic concentrators (CPCs, 7·7 ×). For the concentrator concept and particularly for an easy cell integration, rear-line-contacted concentrator (RLCC) cells with a maximum efficiency of 25% were developed and a hybrid mounting concept for the RLCC cells is presented. The optical performance of different CPC materials was tested and is analysed in this paper. Finally, small modules consisting of six series interconnected RLCC cells and six CPCs were integrated into the concentrator system and tested outdoor. A BICON system efficiency of 16·2% was reached at around 800 W/m2 direct irradiance under realistic outdoor conditions. Copyright © 2006 John Wiley & Sons, Ltd.

BICON: high concentration PV using one‐axis tracking and silicon concentrator cells

In order to utilize the full potential of solar cells fabricated on n-type silicon, it is necessary to achieve an excellent passivation on boron-doped emitters. As SiO 2 , the most effective passivation for highly doped n-type surfaces, does not show a sufficient performance on highly boron doped surfaces some effects that possibly lead to this gap in performance are investigated. Especially the question of boron redistribution during oxidation is the focus of this work. The field effect induced either by corona charge or fixed charge in the surface layer is known to strongly affect the surface passivation quality on silicon solar cells. Typical passivation layers used for high-efficiency solar cells as SiO 2 and SiN x feature a built-in positive charge. For highly doped p-type surfaces however, it is shown experimentally that the passivation quality is strongly enhanced by a high density of negative charge. Thus, the negative-charge dielectric Al 2 O 3 is applied as surface passivation layer on high efficiency n-type c-Si solar cells. An independently certified solar cell efficiency of 23.2 % confirms the excellent passivation quality of this negatively charged dielectric on a boron emitter.

Surface passivation of boron diffused emitters for high efficiency solar cells

This study deals with some specific characteristics that the light-induced plating (LIP) process, which is used for the metallisation of solar cells, exhibits compared to classical electroplating processes. We contribute to the general understanding of LIP (multiple electrodes, influence of light) with some basic experiments and propose a simplified equivalent circuit scheme. We also address the challenge of process control through potential and current without external connection of the working electrode (front side grid). In this article, we show a possibility to determine the absolute potential of the front side grid for relevant process parameters. Furthermore, we present a method that allows the measurement of the mean current density at the front side grid during the process, which has a great influence on the plating result.

Electrochemical methods to analyse the light-induced plating process

Thick thermal oxides of more than 100 nm are commonly used for the production of high-efficiency silicon solar cells from mono- and multicrystalline silicon and have led to the highest conversion efficiencies reported so far. This superior performance of oxides is due to the very good surface passivation by the reduction of the density of interface states. The process to achieve such thick oxides are usually performed at high temperatures for a long time. In this paper we investigate different rear stack systems of a thin thermally grown silicon oxide and PECVD silicon nitride and PECVD silicon oxide layers for rear surface passivation. In a comparatively easy high-efficiency process with laser fired rear contacts (LFC) efficiencies above 20% for FZ-Si and 18.2% for multicrystalline silicon were achieved.

Stefan W. Glunz

Papers

Stack system of PECVD amorphous silicon and PECVD silicon oxide for silicon solar cell rear side passivation

BICON: high concentration PV using one‐axis tracking and silicon concentrator cells

Surface passivation of boron diffused emitters for high efficiency solar cells

Electrochemical methods to analyse the light-induced plating process

Silicon oxide/silicon nitride stack system for 20% efficient silicon solar cells