European Inventor Award

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Jörg Horzel, inventor of a simplified process for producing selective-emitter solar cells

Just two decades ago, solar power was at best on the fringe of the renewable energy sector. The process of harnessing energy directly from sunlight was simply too expensive and cumbersome to compete with other forms of alternative energy, like hydro and wind power. Jörg Horzel has made great strides in changing this.

The German scientist and his team simplified the way an existing technology - the selective-emitter silicon-based photovoltaic cell - was made, patenting a more efficient production process that reduced manufacturing costs and increased efficiency. The invention has had a direct effect on around half of the companies in the photovoltaic industry and significantly contributes to making solar power a viable and competitive energy resource.

Jörg Horzel, inventor of a simplified process for producing selective-emitter solar cellsThe heat is on

In the mid-1990s, photovoltaic solar energy was just emerging as a promising source of renewable energy, but there were still a number of challenges in its way. Jörg Horzel, then a researcher at IMEC, a nanotechnology research centre based in Heverlee near Leuven, Belgium, was determined to tackle one of the largest: The production process that allowed crystalline silicon cells to generate energy was cumbersome, expensive and needed to be streamlined.

The problem lay in a production step where phosphorous paste was applied to the top layer of one of two layers in a silicon cell - a process called ‘doping', which is required to create an electric current that can transmit energy from sunlight. Invariably, when the layers were connected, some of the phosphorous paste would end up contacting the lower layer of the cell. If this paste wasn't removed, it would partially short-circuit the cell and reduce output. However, the existing phosphorous removal techniques were expensive and time-consuming.

Streamlining silicon solar cells

Horzel and his team members, Jozef Szlufcik, Mia Honore and Johan Nijs, circumvented this problem and developed an ingenious way to apply the phosphorous paste only to certain parts of the top portion of the cell, minimising the amount of phosphorous that wound up on the lower portion.

Before their invention, solar power experts had already experimented with varying the levels of phosphorous and other chemicals throughout solar cells, something that became known as a selective-emitter solar cell. These selective-emitter cells help reduce the amount of energy that is lost to heat.

However, before Horzel's team developed their production process, no one had been able to completely eliminate the phosphorous-removal step, nor could they allow for varying amounts of phosphorous to be applied to different regions of the cell via one convenient method. These two aspects greatly simplified the production of selective-emitter solar cells.

Wide-scale adoption

It took Horzel only a few months to develop the proof of concept for his invention, but he spent nearly five years optimising it into a world-class process. Among other things, he needed to determine the right temperature for drying the phosphorus paste and the best way to arrange the silicon cells in a diffusion furnace.

Horzel's selective-emitter silicon-based photovoltaic cells set efficiency records for large-area solar cells with screen-printed contacts on mono-crystalline silicon (1998) and multi-crystalline silicon (2000). In 2002, Photovoltec, IMEC's former solar energy spin-off, gained exclusive rights to the patent, which received final approval in 2004.

Photovoltec didn't apply the process itself because it had difficulty acquiring the necessary equipment. While Hozel's exact technique isn't currently being used in solar-cell production,

over the past five years about half of the world's solar-cell manufacturers have begun using selective-emitter solar cells related to the one invented by Horzel.

An industry at a crossroads

The patented production techniques Horzel and his team developed and their indirect adoption by the solar industry are one of the reasons why the sector has been seeing rapid growth over the past decade.

However, while solar-energy production has increased by an average of 40% per year worldwide since 2000, the industry still exhibits many features of a nascent technology sector: overproduction, erratic profit streams for many companies, and far-reaching consolidation.

Many industry experts point to the relatively high cost of solar power compared to other traditional and renewable energy resources as the primary hurdle the industry will have to overcome to become competitive over the long term.

In countries that receive lots of sunlight, such as Nicaragua and Costa Rica, prices for harnessing solar energy are already competitive with electricity generated from fossil fuels. But in other countries, such as Germany, direct investments and government subsidies that once propped up the sector have dried up due to the global economic downturn.

Future work

Making solar power more affordable across the globe will be key to its success in the future, and nobody knows that better than Jörg Horzel. The native of Karlsruhe in southwestern Germany followed his initial invention (the selective emitter process) with about 10 more - all related to the development of solar cells.

The second patent that helped earn Horzel a 2013 European Inventor Award nomination was for a new furnace used in the heating of solar cells that drastically improved their throughput potential, or the number of cells that can be produced in a certain time.

During his ten-year stint with the international technology group Schott Solar, based in Mainz, Germany, Horzel developed a new method for doping solar cells that improved the quality and throughput of the cells while reducing production cost. Schott Solar applied the process for several years in the high-volume production of solar cells. The technology is expected to become even more important in the coming years, as solar cell manufacturers continue to look for more efficient production techniques.

Horzel's many contributions to the solar energy have helped the industry secure a livelihood for tens of thousands of workers, reduce energy-related CO2 emissions and limit our effects on the environment. Continued advances like his can ensure a bright future for solar energy.

How it works

Solar cells are made from the semi-conductive materials: silicon, phosphorus and boron. The cells consist of two plates, one made of silicon doped with phosphorous, the other of silicon doped with boron. The plates are pressed together and connected with wires, which link with metal contacts on the top and bottom of the cells to channel the energy created.

How do the cells generate energy? Silicon, the base material of solar cells, contains four electrons - particles with a negative charge - in its outer shell. Phosphorus, in contrast, has five electrons in its outer shell. When a phosphorus paste is applied to a silicon cell, four of the silicon's electrons pair up with four of the phosphorus' electrons, but the fifth phosphorus electron is left free to serve as a carrier for electric current.

In contrast, boron has only three electrons in its outer shell, leaving a space open for an electron. When sunlight hits a solar panel, the absorbed energy excites the atoms within the cell, jarring loose the extra electron from the phosphorus atom. This electron moves to the open space created by the silicon-boron combination, creating an electrical charge.

Jörg Horzel improved the production process for selective-emitter silicon solar cells, which are formed by applying phosphorus paste to only certain sections of the cell's front side. Using this method, a margin of 1-2 millimetres is left free of phosphorus between phosphorus-doped lines. This prevents phosphorus from diffusing onto the cell's rear side, which would cause an unwanted short circuit. It also allows for specific areas - for example the region around metal contacts - to receive more phosphorus and the cell to work more efficiently. Previously, phosphorus that diffused onto the rear side of the cell had to be removed, a step eliminated by Horzel's method.

Taking in some sun

Despite receiving a relatively small amount of sunlight, Germany is the world's leader in production of solar power (not coincidentally, it's also the country Jörg Horzel calls home).

Germany had installed about 30 gigawatts of solar capacity by the end of 2012 - more than the amount produced by the next three top solar-power producers combined - and solar energy accounted for about 6% of the country's electricity.

Germany's solar industry is a reflection of its overall energy approach - the central European country plans to generate all of its energy from renewable sources by 2050, and some analysts project that solar power could meet 25% of Germany's electricity demand by that year.

Neighbouring Italy is the world's second-largest generator of solar power, producing about half of Germany's total. China, the United States and Japan round out the top five.

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