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

Methods for the determination of the emitter saturation current density J0e in high and low injection regime and the influence of different models for the Auger recombination on the evaluation are presented and compared. In this study symmetrical test structures with different emitter profiles on highly doped and lowly doped substrates have been investigated. The samples were analyzed by injection-dependent lifetime spectroscopy (IDLS) with the contactless measurement of the photoconductance. For highly injected substrates, the model for the Auger recombination of Kerr et al. can be used for a wider range of excess carrier densities resulting in a more conservative estimation than with the models of Schmidt et al., Glunz et al. and Sinton et al.. With the Auger parameterisation of Kerr et al. at low injections the same observations as in high injection regime have been made. A good agreement was found between the different methods showing that an accurate determination of J0e is possible for both low and high resistivity materials. The results for samples with different diffused phosphorus emitter profiles show that J0e decreases with increasing sheet resistance which is in good agreement with other emitter studies.

Comparison of Emitter Saturation Current Densities Determined by Quasi-Steady-State Photoconductance Measurements of Effective Carrier Lifetimes at High and Low Injections

Erratum on “Determining the defect parameters of the deep aluminum-related defect center in silicon” [Appl. Phys. Lett. 91, 122109 (2007)]

A valuable nondestructive measurement and analysis method for determining individual excess carrier lifetimes in multilayer systems is presented. This is particularly interesting for the characterization of crystalline silicon thin-film samples consisting of an electrically active epitaxial layer on top of a highly doped crystalline substrate. The analysis principle is based on a comparison between a measured photoluminescence intensity ratio and associated simulated radiative recombination ratios for samples with different epitaxial layer thicknesses. It benefits from the fact that for low excess carrier lifetimes within the epitaxial layer the carrier concentration in the said layer is limited by bulk recombination, while for high carrier lifetimes surface and interface recombination is dominating. In order to verify this measurement and analysis principle, results of a set of crystalline silicon thin-film samples with varying epitaxial layer thickness on a highly doped Czochralski substrate are presented.

Determining the excess carrier lifetime in crystalline silicon thin-films by photoluminescence measurements

The damage to the Si-SiO/sub 2/ interface by electron-beam in comparison to thermal (i.e. resistive heating) evaporation of Al and Ti is investigated for oxide thicknesses ranging from 14 to 105 nm. C-V and PCD measurements on MOS test structures fabricated on 1 to 100 Omega -cm p-type FZ-silicon reveal: (1) severe damage to the interface is caused by e-beam, but not by thermal evaporation, (2) in terms of midgap interface state densities and PCD time constants the electron-beam damage is removed by a postmetallization anneal in forming gas for all oxide thicknesses, (3) a photoresist or Al layer of up to 1.6- mu m yields no effective shielding. Solar cells with thick passivating oxides (105 nm) gave comparably high efficiencies above 20% for both metallization techniques. >

Impact of metallization techniques on 20% efficient silicon solar cells

The influence of the base dopand on the cell performance in a cell type with selective front phosphorus diffusion and an alloyed aluminum rear doping is investigated in this work. First this was done by using two dimensional device simulations to vary the doping species (n- or p-type) and the concentration over a broad range. For n-type base material we found that for a given front side system (diffusion and passivation) the fill factor tends to increase while the short circuit current decreases with increasing base doping concentration. The ideal base doping is therefore a function of the quality of the front side diffusion and passivation. In comparison to the n-type cells the dependence of the p-type cell results on the base doping concentration is much weaker. Based on the simulations a base doping concentration of 8 □cm was chosen to process n-type solar cells, which were then compared with identically processed p-type cells. For our front and rear screen-printed large-area n- as well as for our p-type cells we have achieved efficiencies up to 17.6 %.

Stefan W. Glunz

Papers

Comparison of Emitter Saturation Current Densities Determined by Quasi-Steady-State Photoconductance Measurements of Effective Carrier Lifetimes at High and Low Injections

Erratum on “Determining the defect parameters of the deep aluminum-related defect center in silicon” [Appl. Phys. Lett. 91, 122109 (2007)]

Determining the excess carrier lifetime in crystalline silicon thin-films by photoluminescence measurements

Impact of metallization techniques on 20% efficient silicon solar cells

All screen-printed industrial n-type Czochralski silicon solar cells with aluminium rear emitter and selective front surface field