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

These days low-pressure chemical vapor deposition (LPCVD) is commonly used by the photovoltaic industry to deposit Si layers for tunnel oxide passivated contact (TOPCon). This work summarizes the development of an alternative TOPCon deposition process using a tube plasma-enhanced chemical vapor deposition (PECVD) tool. In the first part, the main results of the German federal project “Upgrade Si-PV” are summarized. The goal of this project was the transfer of the TOPCon process to an industrial-proven tube PECVD system. In the second part, recent progress on the implementation of this process into a TOPCon cell is presented. It is demonstrated that the PECVD system is capable of producing screen-printing compatible TOPCon layers yielding recombination currents of ≈ 3 fA/cm2 and ≈ 200 fA/cm2 in the passivated and metallized region, respectively. The best TOPCon cell which was realized with the PECVD process yielded the following current-voltage parameters: Voc = 700 mV, FF = 79.6%, Jsc = 41.2 mA/cm2, and η = 22.95%.

Industrial TOPCon Solar Cells Realized by a PECVD Tube Process

Power rating procedure of hybrid concentrator/flat-plate photovoltaic bifacial modules

ISC-VOC curves measured by the suns-VOC method are widely used for solar cell characterization due to its being unaffected by series resistance effects. A common setup for this measurement system uses a xenon photoflash for illumination purposes, resulting in a fast acquisition of the suns-VOC measurement data during the decaying edge of one flash. However, the use of a xenon photoflash accompanies also several disadvantages. Measurement errors are expected from the imperfect illumination homogeneity on the measurement stage. Also the discrepancy of the flash spectrum compared to the standard AM 1.5G spectrum leads to spectral mismatch between the sample and monitor cells when their spectral response differs. In addition, the divergence of the flash light leads to different illumination densities on the sample and the monitor cell if the height of these two cells differs. In this article these photoflash-caused measurement errors are investigated in detail, analyzing the resulting deviation in illuminatio...

Illumination-induced errors associated with suns-VOC measurements of silicon solar cells

Commercial photovoltaic cells used in energy harvesting applications suffer from low power conversion efficiencies at
low light intensities. Typical efficiencies are in the range of 2 to 5% at 100 Lux. This is because the cells are typically
optimized and specified with respect to the AM 1.5 sun spectrum at 1000 W/m2 light intensity, which is far away from
the required operating range. This paper infers design rules for photovoltaic cells with special attention to the low
intensity range. It discusses the major parameters for the optimization of photovoltaic cell for different material classes
like crystalline silicon, amorphous silicon and III-V materials. Measurement results for an optimized silicon cell with
more than 15% efficiency at 100 Lux are presented.

Photovoltaic energy harvesting for smart sensor systems

The performance of n-type back-contacted back-junction silicon solar cells where the boron-doped emitter diffusion on the rear side is locally overcompensated by a phosphorus-doped base-type back surface field (BSF) diffusion has been analysed theoretically and experimentally. By overcompensating the emitter diffusion the noncollecting base-type region can be reduced significantly allowing electrical shading losses to be minimized. It has been found that for solar cells with a lowly doped BSF diffusion the local external quantum efficiency and the short-circuit current density Jsc could be improved significantly. For reference solar cells with an undiffused gap between emitter and BSF diffusion and a large noncollecting base-type region, a maximum Jsc of 40.9 mA/cm2 could be achieved and for solar cells with a locally overcompensated boron-doped emitter diffusion featuring a small noncollecting base-type region a maximum Jsc of 41.4 mA/cm2 has been measured. The reduction of Jsc losses caused by free carrier absorption (FCA) in highly doped silicon at near-infrared wavelengths is also shown. Furthermore, theoretical investigations are performed by one-dimensional device simulations and the influence of highly doped and lowly doped emitter and BSF diffusions on the open-circuit voltage Voc is presented. For solar cells with a locally overcompensated boron-doped emitter diffusion Voc could be improved from 629 to 652 mV when lowly-doped diffusions and thermally grown SiO2 and antireflection plasma enhanced chemical vapour deposited (PECVD) SiNx passivation stacks are applied. For the reference solar cells with an undiffused gap between the lowly doped emitter and BSF diffusions Voc of 693 mV could be achieved for a plasma enhanced atomic layer deposited (PEALD) Al2O3 passivation layer.

Stefan W. Glunz

Papers

Industrial TOPCon Solar Cells Realized by a PECVD Tube Process

Power rating procedure of hybrid concentrator/flat-plate photovoltaic bifacial modules

Illumination-induced errors associated with suns-VOC measurements of silicon solar cells

Photovoltaic energy harvesting for smart sensor systems

Improved diffusion profiles in back‐contacted back‐junction Si solar cells with an overcompensated boron‐doped emitter