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Examination of Single Perovskite Crystals Reveals Much Untapped Potential for Solar and LEDs
LIGHTimes News Staff

February 3, 2015...Engineers at the University of Toronto have for the first time demonstrated some of the optoelectronic properties of pure perovskite crystals. This emerging family of solar-absorbing materials and understanding their optoelectronic properties could lead to more efficient and cheaper solar panels and LEDs. Through their examination of the properties of single perovskite materials, the researchers revealed that perovskites still have much untapped potential for use in solar panels and LEDs.

The perovskites, are particularly good at absorbing visible light, but they had never before been thoroughly studied as perfect single crystals. The researchers employed a new technique to grow pure perovskite crystals and then studied electrons move through the material as light is converted to electricity.

Professor Ted Sargent of the University of Toronto's Edward S. Rogers Sr. Department of Electrical & Computer Engineering and Professor Osman Bakr of the King Abdullah University of Science and Technology (KAUST) used a combination of laser-based techniques to measure certain properties of the perovskite crystals. They tracked down the rapid motion of electrons in the material. From this they were able to determine the diffusion length--how far electrons can travel without imperfections in the material trapping them--as well as mobility—how fast the electrons can move through the material. This week, they published their work in the journal Science.

“Our work identifies the bar for the ultimate solar energy-harvesting potential of perovskites,” said Riccardo Comin, a post-doctoral fellow with the Sargent Group. “With these materials it’s been a race to try to get record efficiencies, and our results indicate that progress is slated to continue without slowing down..”

Certified efficiencies of perovskites have reached new heights of just over 20 percent in recent years. Such efficiency starts to approach the performance of the state-of-the-art commercial-grade silicon-based solar panels mounted in deserts in spain and on roofs in California.

“In their efficiency, perovskites are closely approaching conventional materials that have already been commercialized,” said Valerio Adinolfi, a PhD candidate in the Sargent Group and co-first author of the paper. “They have the potential to offer further progress on reducing the cost of solar electricity in light of their convenient manufacturability from a liquid chemical precursor.

In solar panels, light hits the surface of the perovskite material and gets absorbed, thereby exciting electrons. These electrons easily traverse the crystal structure to electrical contacts on the underside, creating electric current. In LEDs the process happens in reverse. The slab is first powered with electricity, which injects electrons and then releases energy as light.

The Sargent Group is conducting parallel work that aims to improve the performance of solar-absorbing particles called colloidal quantum dots. “Perovskites are great visible-light harvesters, and quantum dots are great for infrared,” said Professor Sargent. “The materials are highly complementary in solar energy harvesting in view of the sun’s broad visible and infrared power spectrum.”

“In future, we will explore the opportunities for stacking together complementary absorbent materials,” said Dr. Comin. “There are very promising prospects for combining perovskite work and quantum dot work for further boosting the efficiency.”

2015 Draper Prize for Engineering Goes to LED Lighting Pioneers
LIGHTimes News Staff

January 8, 2015...The US National Academy of Engineering (NAE) has awarded the 2015 Charles Stark Draper Prize for Engineering to Nick Holonyak Jr, Isamu Akasaki, M. George Craford, Russell Dupuis, and Shuji Nakamura for “the invention, development, and commercialization of materials and processes for LEDs. The prize will be presented at a gala dinner in Washington D.C. on 24 February.

In 1988, in honor of the memory of Charles Stark Draper, known as the 'father of inertial navigation', the National Association of Engineering (NAE) established the $500,000 annual Draper Prize at the request of Charles Stark Draper Laboratory Inc.

The Draper Prize, which is NAE's highest honor, is given to engineers for achievements that have significantly benefited society by improving the quality of life, and/or expanding access to information.

“These prize-winning engineers were the pioneers in a technology that has changed the world we live in, from the aesthetics in our homes, to advancements in our visual capabilities, and to environmental stewardship,” stated NAE president C D. Mote Jr.

In 1962, Nick Holonyak Jr created the first visible, red LED while working at General Electric. He studied III-V materials including gallium arsenide (GaAs) and found that adding phosphorus (P) to gallium arsenide resulted in a shortened wavelength. He ultimately tuned the GaAsP LED to emit visible red lightIn 1972, George Craford invented the first yellow LED and increased its brightness by adding nitrogen to the GaAsP LED. Craford also participated in developing processes for the first large-scale commercial production of red LEDs. He later led work that resulted in the first high-brightness yellow and red LEDs, available in 1992, and subsequently contributed to the development of high-power white LEDs.

Russell Dupuis developed and refined the metal-organic chemical vapor deposition (MOCVD) process in 1977, which enabled the production of high-brightness LEDs and is now the basis of nearly all production of high-brightness LEDs, and other high-speed optoelectronic devices including laser diodes and solar cells.

In 1987 Isamu Akasaki used MOCVD to grow high-quality gallium nitride crystals on sapphire substrates, creating the first blue LED (which later enabled efficient, bright, white light sources).

In 1992, Shuji Nakamura also made important contributions to InGaN-based high-brightness double-heterostructure blue LEDs, as well as laser diodes that allowed development of the high-density digital video disk (Blu Ray DVD). Nakamura, who is a professor of materials and of electrical & computer engineering at University of California Santa Barbara (UCSB), received the 2014 Nobel Prize in Physics (shared with professors Isamu Akasaki and Hiroshi Amano) for helping develop the first high-brightness blue LED.

British Reseachers Create Simulations that More Accurately Predict Properties of GaN
LIGHTimes News Staff

January 8, 2015...Researchers from University College London (UCL) worked with teams at Daresbury Laboratory and the University of Bath to reveal the complex properties of gallium nitride using computer simulations. Accurate predictions of these properties can help make better blue LEDs and predict their output before actual fabrication.

LEDs employ two layers of semiconductors: a conduction layer with electrons available and a layer with positive charges or holes. When an electron and a hole meet, they emit a photon (light particle). A cristalline film of a particular material--GaN for blue LEDs--is grown and then doped. Dopants donate an extra positive or negative charge to the material.

GaN, the key material for blue LEDs, has a large energy gap between electrons and holes (known as a wide bandgap). The wide bandgap is essential for tuning the emitted photons to produce blue light. Doping to donate mobile negative charges in the material proved to be easy. However, donating positive charges from GaN failed completely. The innovation, which won the inventors of the blue LED the Nobel Prize for physics last year, required doping GaN with unexpectedly large amounts of magnesium.

"While blue LEDs have now been manufactured for over a decade," said John Buckeridge (UCL Chemistry), the study's lead author, "there has always been a gap in our understanding of how they actually work, and this is where our study comes in. Based on what is seen in other semiconductors such as silicon, you would expect each magnesium atom added to the crystal to donate one hole. But in fact, to donate a single mobile hole in GaN, at least a hundred atoms of magnesium have to be added. It's technically extremely difficult to manufacture GaN crystals with so much magnesium in them, not to mention that it's been frustrating for scientists not to understand what the problem was."

The team published details of their findings in the journal Physical Review Letters. The team used highly sophisticated computer simulations to accurately predict the unusual behavior of doped GaN at the atomic level. While a quantum mechanical model can make accurate predictions about perfect crystals with repeating patterns of atoms, such a model has difficulty dealing with defects which do not fit the repeating pattern of atoms. The computer simulations for accurate prediction of GaN crystals with some defects requires use of supercomputers because of the large numbers of atoms and their interactions.

"To make an accurate simulation of a defect in a semiconductor such as an impurity, we need the accuracy you get from a quantum mechanical model," said David Scanlon (UCL Chemistry), a co-author of the article.

"Such models have been widely applied to the study of perfect crystals, where a small group of atoms form a repeating pattern. Introducing a defect that breaks the pattern presents a conundrum, which required the UK's largest supercomputer to solve. Indeed, calculations on very large numbers of atoms were therefore necessary but would be prohibitively expensive to treat the system on a purely quantum-mechanical level."

The team solved the the issue with an approach pioneered in another Nobel Prize winning research: hybrid quantum and molecular modeling, the subject of 2013's Nobel prize in Chemistry. The new models simulate different parts of a complex chemical system with different levels of theory. Some previously unexplained experimental results about the behavior of GaN now fit with the new simulations

Richard Catlow (UCL Chemistry), one of the study's co-authors said, "Our simulation shows that the behavior of the semiconductor is much more complex than previously imagined, and finally explains why we need so much magnesium to make blue LEDs successfully."

"The simulation tells us that when you add a magnesium atom, it replaces a gallium atom but does not donate the positive charge to the material, instead keeping it to itself," Catlow said. "In fact, to provide enough energy to release the charge will require heating the material beyond its melting point. Even if it were released, it would knock an atom of nitrogen out of the crystal, and get trapped anyway in the resulting vacancy."

"In fact, to provide enough energy to release the charge will require heating the material beyond its melting point. Even if it were released, it would knock an atom of nitrogen out of the crystal, and get trapped anyway in the resulting vacancy," Catlow added.

Aron Walsh of Bath Chemistry noted, that the team is looking forward to using the new simulations to investigate the properties of heavily defective GaN and help develop alternative doping strategies to improve the efficiency of solid-state lighting.

Epistar Qualifies Veeco EPIK700 MOCVD System for High-volume LED Production
LIGHTimes News Staff

September 30, 2014...Veeco Instruments Inc. based in Plainview, New York USA, reported that Epistar Corporation has successfully evaluated and accepted the new TurboDisc® EPIK700™ Gallium Nitride (GaN) Metal Organic Chemical Vapor Deposition (MOCVD) system for LED production.

“As the leader in LED technology and commercialization, it is vital that we continue to push our roadmap to reduce solid state lighting costs with the most innovative and efficient production solutions available,” said Dr. MJ Jou, president, Epistar Corporation. “EPIK’s performance, reliability and production readiness, as well as the support we received from Veeco during the beta testing phase, fully met our high manufacturing standards. The seamless recipe transfer from our installed base of Veeco K465i™ and MaxBright® systems to the EPIK700 is allowing us to quickly produce production-quality LED devices. In addition, the EPIK700’s cost of ownership advantage will help reduce our cost per wafer, making it a highly attractive platform for our future capacity expansions.”

Veeco claims that its newly launched EPIK700 MOCVD system, which is based on its TurboDisc technology, enables customers to achieve a cost per wafer savings of up to 20 percent compared to previous MOCVD systems through increased productivity, reduced operating expenses, and improved wafer uniformity.

Ammono Creates More Economical p-type Bulk GaN
LIGHTimes News Staff

August 27, 2014...Ammono S.A., a producer of bulk gallium nitride (GaN) using ammonothermal technology based in Warsaw, Poland, has added p-type bulk AMMONO-GaN substrates to its portfolio. The company points out that providing electronics majority charge carriers (n-type) though dedicated donor doping can increase GaN conductivity. However, Ammono says that successful and efficient p-type doping of GaN was always difficult technologically because typically acceptors required high activation energy. Previously, epitaxial methods or ion implementation could only obtain thin layers of p-type GaN. The ammonothermal process incorporates the acceptor during the growth, resulting in a greater hole concentration and p-type conductivity, without creating structural defects.

The dislocation density in p-type AMMONO-GaN remains the same as that of n-type AMMONO-GaN substrates, being below 5×104 cm-2. Carrier (free hole) concentration in this material is at the level of 1016 cm-3 while electrical resistivity is 10-100 Ω*cm. According to Ammono, these new p-type GaN substrates should enable the construction of novel devices. The introduction of such a new substrate offers new potential for device architectures. The company expects that LEDs, laser diodes, high-frequency transistors, and high power transistors and high-frequency transistors may gain many performance benefits using the new material.

Dr. Marcin Zajac will present details about the new material during the International Workshop on Nitride Semiconductors (IWN2014) in Wroclaw, Poland on August 25th (Growth 1 session at 15:45).

Rubicon Technology to Showcase Large-Diameter Patterned Sapphire Substrates (PSS) at LED Korea 2014
LIGHTimes News Staff

February 13, 2014...Rubicon Technology, Inc. of Bensenville, Illinois USA, a provider of sapphire substrates and products to the LED market, announced that it will showcase large-diameter patterned sapphire substrates (PSS) as well as its line of 6”polished sapphire wafers for the LED industry at LED Korea 2014 at COEX, Seoul, Korea, February 12-14, 2014. Rubicon will exhibit its line of sapphire products in Booth #4707.

The company notes that most high-brightness LED manufacturers etch a pattern into the sapphire wafers in order to both improve epitaxial growth and extract more light from each chip. Rubicon says that its patterned sapphire substrates have been available for purchase in smaller diameters from other companies, but claims that it is the first company to offer 6” and 8” patterned sapphire substrates. Rubicon points out that it offers 4", 6", and 8" patterned sapphire substrates for LED chip manufacturers.

“Rubicon Technology continues to pioneer innovations in sapphire substrates,” said Raja M. Parvez, president and CEO, Rubicon Technology. “As the world’s most experienced provider of 6-inch sapphire wafers, Rubicon is uniquely positioned to drive the evolution of substrates – patterning large diameter sapphire substrates. This advance helps LED manufacturers gain the efficiency of larger diameters, combined with the industry’s most precise patterning capability, all from a supplier known for quality and reliability at high volume.”

Rubicon offers fully customizable sub-micron patterning capability with dimensional tolerances, within ±0.1 µm. The company reportedly maximizes usable area with an edge exclusion zone as small as 1 mm. Patterning comes in a range of shapes including: cone, dome and pyramid, and in a variety of orientations.

Aixtron SE and Manz AG to Collaborate on OLED Manufacturing
LIGHTimes News Staff

January 8, 2014...Aixtron SE of Aachen, Germany, and Manz AG, a Reutlingen-based engineering company, have agreed to collaborate on developing further solutions for use in efficient organic light-emitting diode (OLEDs) production. The partners will be developing a new system based upon Aixtron's OVPD process technology to demonstrate efficient organic layer deposition up to a substrate size of Gen8 (2,300 mm x 2,500 mm). The new demonstration system is expected to enable the efficient production of OLEDs for displays and lighting applications on an industrial scale and at a reasonable prize for the first time.

Manz will be contributing its experience in purifying and handling large-scale glass substrates and in developing and manufacturing large vacuum systems. Together the companies hope to manufacture extremely homogenous, easily scalable thin films with high material efficiency and deposition rates. The new system will be assembled in the coming months in a clean room at Aixtron. As well as demonstrating the proprietary OVPD process and its scalability for large substrates, a key focus will be the qualification of new components.

OVPD technology has been exclusively licensed to Aixtron from Universal Display Corporation (UDC), Ewing, New Jersey USA, for equipment manufacture. OVPD technology is based on an invention by Professor Stephen R. Forrest et al. at Princeton University, USA, which was exclusively licensed to UDC. Subsequently, Aixtron and UDC jointly developed and qualified OVPD pre-production equipment.

San’an Opto's Xiamen Subsidiary Receives RMB 80 Million Subsidy
LIGHTimes News Staff

December 18, 2013...San’an Opto, an LED maker based in mainland China, reported that the company will purchase 20 single chamber or five four-chambered GaN MOCVD systems from global companies for its Xiamen subsidiary on Dec. 16, 2013. Xiamen San’an Opto received RMB 24 million (US$ 3.95 million) subsidies for four four-chambered MOCVD systems. This accounts for about 30 percent of total subsidies as of Dec. 13, 2013. This latest subsidy is part of an ongoing effort by the Xiamen government and People’s Government of Siming for subsidizing companies importing MOCVD equipment in the Xiamen Torch Hi-tech Industrial Development Zone to encourage optoelectronic industry development.. So far, San’an Opto has received 70 percent subsidiy for four multi-reactor MOCVDs. The manufacturing equipment subsidies received from People’s Government of Siming are considered as deferred income and are recorded in terms of the company's profit and loss over the equipment life span. 

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