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The thinner, the better: From silicon solar cell to photovoltaics of tomorrow

Maciej Sibiński.
Maciej Sibiński. Source: TalTech

Our earth is more than 25 percent silicon. It is cheap, harmless, and as ubiquitous as sand in the desert, and in the form of semiconductor devices, it has contributed significantly to the development of modern electronics. So why is it that this handy companion may be phased out of photovoltaic devices in the future, and why will tomorrow's solar cells probably work entirely without it? Maciej Sibiński, professor at Taltech's institute of materials and environmental technology, writes.

So how can the slim solar cells made of, let's say, unusual compounds effectively win this technology race in the near future?

Well, to answer this question, we need to take a closer look at the design of standard photovoltaic devices and their evolution over the last 70 years.

The beginning of modern photovoltaics dates back to 1954 when three gentlemen from Bell Laboratories in the U.S., D.M. Chapin, C.S. Fuller, and G.L. Pearson, constructed the first solar cell with the reasonable efficiency of 4.5 percent, improving this after just 6 months to 6 percent.

It is important to know that these first solar cells were made of silicon, but also of an atypical semiconductor compound, cadmium sulfide (CdS). Soon after these inventions the era of rapid commercialization began, when another company, Hofman Electronics, produced a set of devices that quickly reached the efficiency of 14 percent in 1960. But why did they use silicon?

Meanwhile, however, modern technology exploded. A few years earlier, John Bardeen, William Shockley, and Walter Brattain developed the first operational semiconductor amplifier, a mysterious device known as the transistor, at the same Bell Laboratories. It was based upon germanium, a semiconductor element that had been in use since the Second World War for radar construction.

Schematic of a typical solar cell structure as the P-N diode. Source: Maciej Sibiński

Germanium was difficult to purify and had a limited operating temperature range and an even more limited blocking voltage range. In this search for a new material candidate, scientists' eyes turned to silicon. It looked much more promising, and in the magical year of 1954, the first working silicon transistor was built, again, at the same Bell Laboratories. It seemed very successful and was copied worldwide. But even schoolchildren know that every transistor consists of at least two diode junctions and that every solar cell is actually... a diode. Back then, silicon was ready-at-hand.

The transition to silicon technology in photovoltaics was rapid and overwhelming. For the next 30 years, solar cell production focused exclusively on crystalline silicon. The material technology was well established and all processes were continuously optimized, leading to the breaking of the 20 percent efficiency barrier in 1985 by the team from the University of New South Wales in Australia. Today, more than 90 percent of all photovoltaic production is crystalline silicon. Unfortunately, there was an unexpected iceberg lurking on the course, and it was only a matter of time before we ran into trouble.

So what is the problem with silicon – it is odd, but the material is not as accessible as it might seem at first glance. If we take away the large deposits of silicon, we have to consider that in order to produce the ideal crystal structure – required in electronics and monocrystalline solar cells – we have to correct it drastically. For this reason, the silicon material must be re-melted several times before the crystallization process can even begin. This re-melting takes place at temperatures above 1,400 degrees Celsius, so it is easy to imagine how energy-intensive this process can be.

Then there is the production of monocrystalline structures. The most popular process of this technology was invented by the Polish scientist Jan Czochralski at the beginning of the 20th century. A monocrystalline seed is placed in a glass filled with molten silicon and slowly lifted, cooled and turned. Of course, the whole process takes place in a special chamber that is heated and filled with neutral gas to prevent oxidation. Only then is it possible to obtain a silicon block suitable for cutting into wafers – the basis for the further production of solar cells.

Schematic of Czochralski production process, used for monocrystalline silicon and obtained mono - silicon ingot Source: Maciej Sibiński

Unfortunately, this is not the only limitation of silicon. As you may recall, 20 percent efficiency in silicon cells was achieved almost 40 years ago. Well, we are still at 27 percent in a single device and around 24 percent in a module. This sad truth is due to one simple fact – silicon is approaching the theoretical limit of possible photoconversion efficiency. The iceberg is coming.

Can we avoid it? It seems so. After intensive Si cell development, several scientists realized there were many more efficient solar cell materials.

Monocrystalline silicon and cheaper (but less efficient) polycrystalline material have been used. Other semiconductor compounds like GaAs, CdS, CdTe, CuInSe2, CuGaSe2, CuO, and organic materials have been studied as well.

Solar cells can be divided into three or even four generations. Let's focus on the second generation, a very promising inorganic thin-film cell. A good example of recent achievements in this field are the antimuonium based Sb2S3 and Sb2S3 structures that have been intensively developed at the Laboratory of Thin Film Energy Materials at Tallinn University of Technology.

Looking at the schematic of these cells, we can see a lot of different layers. However, all together they form the device with a thickness of only a few hundred nanometers to a few micrometers, whereas a monocrystalline silicon structure needs more than 200 micrometers to absorb 90 percent of the incident light.

This results in lower material consumption, but also in high flexibility of the material.

Returning to silicon cells for a moment, their high price and efficiency limitations are unfortunately just the tip of the iceberg. Traditional silicon solar cells are constructed as rigid, separate devices. To make a module, dozens of these fragile devices must be assembled, soldered, and laminated into a frame.

Even when performed by robots, this process is relatively long and cumbersome. The answer to this problem from the thin-film structures such as anti-muonium cells is the so-called monolithic integration.

Thin film Sb2S3 solar cell in reversed (superstate) structure, based on transparent glass sheet and practical and ultra-thin transparent device manufactured at TalTech Source: Maciej Sibiński

In this case, the production can be carried out in the continuous processes, fulfilling the industry dream of roll-to-roll manufacturing in the constant flow of substrate and delivery of final product.

But why anti-muonium in particular, and why should we focus on these devices right now? The answer lies in the technology. Scientists at TalTech have successfully constructed antimuonium-based cells with an efficiency of nearly 5 percent and an optical transmittance of 27 percent.

It is possible to use these devices in regular window glass, for example, as this type of transparency is sufficient. In fact, this material can be mass-produced and, unlike other thin-film counterparts, it is non-toxic and can be long-term durable. These are the demands of today's electronics industry.

Also, in line with new Internet of Things needs, it can be used in several off-grid indoor applications. In the lower insulation the photoconversion of these magic devices can grow even by 300 percent, which is critical not only inside the buildings, but also in the countries like Estonia, where the illumination intensity is low during the most part of the year.

It is also a unique and promising technology that can be manufactured entirely in Europe, where technological progress is more of an issue than just energy prices, unlike silicon modules from the Far East.

These cells are not going to replace the huge production of photovoltaic silicon in the near future, but they complement it in the applications where traditional cells seem to be problematic.

We will soon see the first practical applications of this important invention in our daily lives, helping in the efficient green transformation process.

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Editor: Kristina Kersa

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