SUNNY DAYS FOR SILICON

A The old saw that "the devil is in the details" characterizes the kind of needling obstacles that prevent an innovative concept from becoming a working technology. It also often describes the type of problems that must be overcome to shave cost from the resulting product so that people will buy it. Emanuel Sachs of the Massachusetts Institute of Technology has struggled with many such little devils in his career-tong endeavor to develop low-cost, high-efficiency solar cells. In his latest effort, Sachs has found incremental ways to boost the amount of electricity that common photovoltaics (PVs) generate from sunlight without increasing the costs. Specifically, he has raised the conversion efficiency of test cells made from multi-crystalline silicon from the typical 15.5 percent to nearly 20 percent—on par with pricier single-crystal silicon cells. Such improvements could bring the cost of PV power down from the current $1.90 to $2.10 per watt to $1.65 per watt. With additional tweaks, Sachs anticipates creating within four years’ solar cells that can produce juice at a dollar per watt, a feat that would make electricity (rum the sun competitive with that from coal-burning power plants.

B Most PV cells, such as those on home rooftops, rely on silicon to convert sunlight into electric current. Metal interconnects then funnel the electricity out from the silicon to power devices or to feed an electrical grid. Since solar cells became practical and affordable three decades ago, engineers have mostly favored using single-crystal silicon as the active material, says Michael Rogol, managing director of Germany- based Photon Consulting. Wafers of the substance are typically sawed from an ingot consisting of one large crystal that has been pulled like taffy out of a vat of molten silicon. Especially at first, the high-purity ingots were left over from integrated- circuit manufacture, but later the process was used to make PV cells themselves, Rogol recounts. Although single-crystal cells offer high conversion efficiencies, they are expensive to make. The alternatives- multi-crystalline silicon cells, which factories fabricate from lower-purity, cast ingots composed of many smaller crystals—are cheaper to make, but unfortunately they are less efficient than single-crystal cells.

C Sachs, who has pioneered several novel ways to make silicon solar cells less costly and more effective, recently turned his focus to the details of multi-crystalline silicon cell manufacture. The first small improvement concerns the little silver fingers that gather electric current from the surface of the bulk silicon," he explains. In conventional fabrication processes, cell manufacturers use screen-printing techniques ("like high-accuracy silk-screening of T-shirts," Sachs notes) and inks containing, silver particles to create these bus wires. The trouble is that standard silver wires come out wide and short, about 120 by 10 microns, and include many nonconductive voids. As a result, they block considerable sunlight and do not carry as much current as they should.

D At his start-up company—Lexington, Mass- based 1366 Technologies (the number refers to the flux of sunlight that strikes the earth's outer atmosphere: 1.366 watts per square meter)— Sachs is employing "a proprietary wet process that can produce thinner and taller" wires that are 20 by 20 microns. The slimmer bus wires use less costly silver and can be placed closer together so they can draw more current from the neighboring active material, through which free electrons can travel only so far. At the same time, the wires block less incoming light than their standard counterparts.

E The second innovation alters the wide, flat interconnect wires that collect current from the silver bus wires and electrically link adjacent cells. Interconnect wires at the top can shade as much as 5 percent of the area of a cell. "We place textured mirror surfaces on the faces of these rolled wires. These little mirrors reflect incoming light at a lower angle--around 30 degrees-—so that when the reflected rays hit the glass layer at Lire top, they stay within the silicon wafer by way of total internal reflection,” Sachs explains. (Divers and snorkelers commonly see this optical effect when they view water surfaces from below.) The longer that light remains inside, the more chance it has to be absorbed and transformed into electricity.

F Sachs expects that new antireflection coatings will further raise multi-crystal line cell efficiencies. One of his firm's future goals will be a switch from expensive silver bus wires to cheaper copper ones. And he has a few ideas regarding how to successfully make the substitution. "Unlike silver, copper poisons the performance of silicon PVs," Sachs says, "so it will be crucial to include a low-cost diffusion barrier that stops direct contact between copper and the silicon." In this business, it's always the little devilish details that count.

G The cost of silicon solar cells is likely to fall as bulk silicon prices drop, according to the U.S. Energy information Administration and the industry tracking firm Solarbuzz. A steep rise in solar panel sales in recent years had led to a global shortage of silicon because production capacity for the active material lagged behind, but now new silicon manufacturing plants are coming online. The reduced materials costs and resulting lower system prices will greatly boost demand for solar-electric technology, according to market watcher Michael Rogol of Photon Consulting.

PAUL NASH

Paul Nash, the elder son of William Nash and his first wife, Caroline Jackson, was born in London on 11th May 1889. His father was a successful lawyer who became the Recorder of Abingdon. According to Ronald Blythe: “In 1901 the family returned to its native Buckinghamshire, where the garden of Wood Lane House at Iver Heath, and the countryside of the Chiltern Hills, with its sculptural beeches and chalky contours, were early influences on the development of the three children. Their lives were overshadowed by their mother’s mental illness and Nash himself was greatly helped by his nurse who, with some elderly neighbors, introduced him to the universe of plants.”

Nash was educated at St. Paul’s School and the Slade School of Art, where he met Dora Carrington. Unlike some of his contemporaries at the Slade School, Nash remained untouched by the two post-impressionist exhibitions organized by Roger Fry in 1910 and 1912. Instead, he was influenced by the work of William Blake. He also became a close friend of Gordon Bottomley, who took a keen interest in his career.

Nash had his first one-man show, of ink and wash drawings, at the Carfax Gallery in 1912. The following year he shared an exhibition at the Dorian Leigh Gallery with his brother, John Nash. Myfanwy Piper has added: “Nash had a noteworthy sense of order and of the niceties of presentation; his pictures were beautifully framed, drawings mounted, his studio precisely and decoratively tidy, and oddments which he collected were worked up into compositions.”

Paul Nash was strongly attracted to Dora Carrington: He later recalled: “Carrington… was the dominating personality, I got an introduction to her and eventually won her regard by lending her my braces for a fancy-dress party. We were on the top of a bus and she wanted them then and there.”

On the outbreak Nash considered the possibility of joining the British Army. He told a friend: “I am not keen to rush off and be a soldier. The whole damnable war is too horrible of course and I am all against killing anybody, speaking off-hand, but besides all that I believe both Jack and I might be more useful as ambulance and red cross men, and to that end we are training. Nash enlisted in the Artists’ Rifles. He told Gordon Bottomley: “I have joined the Artists’ London Regiment of Territorials, the old Corps which started with Rossetti, Leighton, and Millais as members in 1860. Every man must do his bit in this horrible business so I have given up painting. There are many nice creatures in my company and I enjoy the burst of exercise – marching, drilling all day in the open air about the pleasant parts of Regents Park and Hampstead Heath.”

In March 1917 he was sent to the Western Front Nash, who took part in the offensive at Ypres, had reached the rank of lieutenant in the Hampshire Regiment by 1916. Whenever possible, Nash made sketches of life in the trenches. In May 1917 he was invalided home after a non-military accident. While recuperating in London, Nash worked from his sketches to produce a series of war paintings. This work was well-received when exhibited later that year. As a result of this exhibition, Charles Masterman, head of the government’s War Propaganda Bureau (WPB), and the advice of Edward Marsh and William Rothenstein, it was decided to recruit Nash as a war artist. In November 1917 in the immediate aftermath of the battle of Passchendaele Nash returned to France.

Nash was unhappy with his work as a member of the War Propaganda Bureau. He wrote at the time: “I am no longer an artist. I am a messenger who will bring back word from the men who are fighting to those who want the war to go on forever. Feeble, inarticulate will be my message, but it will have a bitter truth and may burn their lousy souls.” However, as Myfanwy Piper has pointed out: “The drawings he made then, of shorn trees in ruined and flooded landscapes, were the works that made Nash’s reputation. They were shown at the Leicester Galleries in 1918 together with his first efforts at oil painting, in which he was self-taught and quickly successful, though his drawings made in the field had a more immediate public impact.

In 1919 Nash moved to Dymchurch in Kent, beginning his well-known series of pictures of the sea, the breakwaters, and the long wall that prevents the sea from flooding Romney Marsh. This included the Winter Sea and Dymchurch Steps. Nash also painted the landscapes of the Chiltern Hills. In 1924 and 1928 he had successful exhibitions at the Leicester Galleries. Despite this popular acclaim in 1929, his work became more abstract. In 1933 Nash founded Unit One, the group of experimental painters, sculptors, and architects.

During the Second World War Nash was employed by the Ministry of Information and the Air Ministry and paintings produced by him during this period include the Battle of Britain and Totes Meer. His biographer, Myfanwy Piper, has argued: “This war disturbed Nash but did not change his art as the last one had. His style and his habits were formed, and in the new war, he treated his new subjects as he had treated those he had been thinking about for so long. His late paintings, both oils, and watercolors are alternately brilliant and somber in color with the light of setting suns and rising moons spreading over wooded and hilly landscapes. Paul Nash died at 35 Boscombe Spa Road, Bournemouth, on 11th July 1946.