The Science of Photovoltaics

Dr. Heinz A. Ossenbrink

Whenever there is a debate on energy and on renewable sources of energy in particular, the word “Photovoltaic” will appear sooner or later. Experts will usually mention it in the context of renewable energy technologies, and how they could provide a solution to reduce emissions which create the greenhouse effect in the atmosphere. There are other. renewable energies generated from wind, biomass, geothermal heat, solar heat and water. All those have been invented and exploited already hundreds of years ago.

Photovoltaic energy is different, already because it is a child of the space age and only about 60 years old. It also can supply modern energy services to the poorest of the world, but very different from the traditional burning of biomass.

What makes Photovoltaics so special? The following will highlight some of the particular features of Photovoltaic energy at a time where its cost is competing with the cheapest energy source: coal.

Photovoltaics – The meaning

Some 30 years ago, there was a poll in one European country asking the meaning of “Photovoltaic”. One of the answers one could cross said it would have something to do with photographic cameras. And this was also the answer, 93% of the population was believing to be the right one.

However, the wording “Solar Panel” was already much better known, as the solar water heaters have been already quite common since the 1970s. The author, when displaying a photovoltaic module during many visits of non-experts, — amongst them also decision-makers for the research programme — was often asked where the water connection would be attached. It was always a perfect occasion to explain that such a module produces electricity, and is warming up nothing else than itself. It is still important to point this out as often, in particular when “Solar Panels” are mentioned.

Meanwhile, Photovoltaics has been established also in a wider audience as the conversion of sunlight directly into electricity. It seems that it is not seen as something very special, even though most people believe that there must be some “magic” involved.

The name is a combination of the Greek “photos” for light, and “Volta”, given tribute to the Italian scientist who made the discovery of how to get electricity out of what today we simply call “battery”. Today, the Japanese term for Solar Cells is still “Sun Battery”. For a long time, the wording “Solar Cell” was the engineering term, and it is still in use today.

Photovoltaics – The secrets of physics

Whilst the functioning of a photovoltaic cell can be described at all levels of complex physics and mathematics, its principle of operation is simply to convert sunlight into electricity. What makes the photovoltaic cell special becomes obvious when another possibility of converting sunlight into electricity is looked at, that of concentrating sunlight to heat water until it evaporates. Connected to relatively conventional steam conversion, like turbines, electricity can then be generated by generators. In a certain sense a very conventional technology, just replacing the combustion of fuel (including nuclear) by another heat source.

Photovoltaics is entirely different: The “Photovoltaic Effect” means the absorptions of photons from light in a solid material, and converting the photon-energy into that of electrons. These electrons are extracted and can then be used in electric circuits. The conversion was first described by Alexandre Andre Becquerel in 1839, but it was only 1905 when Albert Einstein received the Nobel Price for the complete theoretical description of the photovoltaic effect.

The photovoltaic effect as it is used today for electricity generation relies on semiconductor material, the same which is used to make electronic devices such as Diodes, Transistors, Integrated circuits or microprocessors. The physics which describes the functioning is dealing with a three-step process from sunlight to electricity:
(1) charge generation, where the incoming photons generate free positive and negative charged particles. In doing so, photons transfer a part of their energy to carriers which are electrically charged. (2) Without charge separation the positive and negative charge carriers would soon recombine again, making the photovoltaic effect useless.

This is why they need to be separated by an internal electric field, created by two different faces of the solar cells: one attracts positive carriers, the other the negative ones. The negative carriers are known well as electrons, for the positive no equivalent name was given other than just “holes”, basically the place the electron had before it was leaving. This internal electric field determines already the positive and negative contact sides of the cell. The contacts serve for the (3) charge collection, where the electrical carriers of charge are leaving the solar cell, collected by fine grids on the surfaces. The collected charge carriers can now be used for an electric current.

The describing physics is based on quantum mechanics, and partly only developed only in the last 60 years. It is remarkable from a physics viewpoint, that one type of elementary particles (photons) convert to another type (electrons and holes). All this takes place in an atomic lattice of very regular shape, a crystal, without actually consuming anything of this material. This latter fact is another surprise often expressed by newcomers, because it is not common to think of generating energy without consuming any visible fuel.

Photovoltaics – The Engineering

There was silence about the photovoltaic effect for 50 years. Nuclear fission was discovered, and attracted most of the physics R&D budgets. There was no need for producing electricity from sunlight, at least not on earth. In 1954, researchers at Bell Labs (USA) demonstrated the first practical silicon solar cell, based on crystalline Silicon with a conversion efficiency of 6%. Soon it became the power source of choice for the beginning space age, and the first solar-powered satellite Vanguard 1 was launched in 1958. It is today the oldest man-made object in the orbit, and its solar cells where functional at least for 6 years.

Making solar cells for spacecraft applications needed sophisticated manufacturing processes to be developed. Fortunately, many of the processes could be taken from the fast-growing semiconductor industry which was also keen to reduce cost for making crystalline, electronic grade silicon of high purity. Silicon raw material is extracted from sand, but needs to undergo quite some intensive chemical treatment. It needs also to be cleaned from natural impurities which would be detrimental to the electric performance, and this is achieved by melting metallurgical silicon and subsequent recrystallisation. Overall a very energy-intensive and expensive processing, economically viable initially only to produce semiconductor devices such as diodes, transistors and integrated circuits.

End of the 70’s of last century a politically induced oil-crises made solar cells appear as the electricity source for the future also on earth. Much R&D effort has since spent to make the production process ever cheaper, today a solar cells costs about 1000 times less than in 1980. Mass production started, and so came also automatized factories for mass production. One of the engineering successes regards also the packaging of solar cells: the 30 to 150 solar cells which are connected together to form a sensible electricity source, needed to be encapsulated to survive the outdoor exposure for more than 20 years. Today practical lifetime is at least 30 years, and there is no other sophisticated product like solar cells which could survive outdoors this long time.

Manufacturing of solar cells has been growing ever, today it takes less than 10 minutes to manufacture the entire yearly production of 1980. A single manufacturing plant can yield about 80 solar cells per second, a bit more than a module. It is due to this throughput that solar cells cost has been decreasing 1000 times since 1980.

The planets: Solar irradiance and irradiation

The light travels 8 minutes from the sun until it arrives on planet earth, where it is absorbed to a large extent, only a small fraction is reflected back into space by the clouds, snow or land/sea reflection. The remaining energy is about 3000 times more than the whole worlds energy consumption. Even if only 10% of sun’s energy could be successfully converted into daily-use energy, it would need only a third of Algeria’s land area, to feed the whole world with energy.

The oil-crisis in 1978 and the publication of the Club of Rome’s “Limits of Growth” led many scientists proposing to use solar cells for the provision of electricity also on the ground, and not only to power spacecrafts. The United States were initially leading the largest research programmes aiming to drastically reduce the cost of solar cells, whilst making them also fit for prolonged outdoor use. Europe was following only a few years later. It was indeed a fascination to work for a dream to become true: a forever available energy source, never depleting and with neglect able environmental impact.

However, the big promise, namely to provide free energy for all and forever became realistic only very recently. The price of PV systems has decreased so much, that at electricity selling prices comparable to those of conventional energy sources the high investment costs can be recovered in less than 20 years. So far so good, but what is for the following years? Current technology is designed in a way, that one can expect a slow decrease of 1% per year for the performance of the PV System, caused either by module failures or a slowly progressing degradation. If an operator sets a side 1% of the investment every year to replace or reinstall enough modules, the PV system operates always at its initial, 100% performance. And after the financial payback time, this 1% plus another for maintenance, cleaning, contracting etc. are the only operational costs. There electricity is generated almost free at about 10$ per MWh, about 5 times less than any other electricity source today. Forever. This is what now becomes the reality for an energy source which practically costs only initial investment, and has no fuel cost.

Photovoltaics – The Believers

Unlike the other energy source developed last century, there was no military or industrial interest to develop solar Photovoltaics. The drivers for this technology were people which were either convinced that the advantages of solar energy will prevail and ultimately displace other energy sources or good scientists and engineers who turned the vision in a practical technology. But it needed also foresight by some political people to make Photovoltaics a sizeable, public funded research and demonstration programme, and later to conceive an intelligent policy of incentives to give this young energy source the kick into the market. It is a remarkable development of technology which grew by very different driving forces which were so different from those other technologies, like telecommunications, computers or nuclear power. Enthusiasm and vision shared by many people comes probably closest to the cause of the success. However, they always have been, and probably still are, a minority.

Photovoltaics – The Energy for Societies

Different from any other energy source, solar energy can be exploited on any place on this planet. Certainly there are differences between northern and equatorial regions, but the difference is not much more than a factor of two, comparing best solar regions with polar regions, even north of the polar circle. Average rainfall, and thus cloud cover is the next important factor influencing how much energy can be harvested on a yearly basis. But, and this is what makes Photovoltaics so unique, one can still generate electricity at any place, in any country, in any village or on remote islands. It is more the “soft”costs, like financing, infrastructure, project preparation, labour cost and permits which influence the cost of PV electricity more than climatic conditions. Another difference of Photovoltaics compared to other energy sources is that the electricity cost do not depend strongly on the size of one single system. Small systems are not more than double as expensive as large systems, and can be affordable for families even in poverty regions. A new situation is all of a sudden visible: as availability of energy decides about the wealth of a country, suddenly the resource-poor countries in the world have access to a resource for their own use. They do not need to import energy and becoming dependent on energy prices established by rich countries, but can now develop an own energy system, decentralised, smart, adaptable to any required size and available for basic needs like in households, but also for water supply and manufacturing.

It is therefore also an energy source for peace, as its modularity avoids societal conflicts about resources.

But the boom actually developed in very industrialised and rich countries, and concern for the environmental damage conventional power generation can and does cause was the driving force. Subsidy policies in many countries in Europe, the US and Japan have been deployed in order to ease the growth of Photovoltaics, but also of other renewable sources in particular wind energy and bio-energy. It was in accepting that these new form of energy needs a relatively high investment, but would then later payback as no fuel consumption was involved. Again, the modularity of Photovoltaics allowed homeowners to install their small system and get remunerated for the surplus energy they fed in the existing electricity grid. The concept was so successful, that all countries needed to readjust this “feed-in”-price in ever shorter intervals, as the amount of money to be paid for the owners of PV systems skyrocketed. Today, more than 40% of all PV systems are owned by private citizens or farmers. It is now a situation almost as if it was postulated by Karl Marx: important economic resources in the hands of the people.

Architects around the world have taken up Photovoltaics as a new stimulation, and to use it as a material for entire new buildings, which adds energy generation as a new element of functionality. It is there where Photovoltaics becomes visible for everybody, and it will be in our environment for long.

Photovoltaics – The Enemies

Some countries have seen a strongly growing penetration of PV generated electricity. Technically, this created problems with the management of the conventional electricity grid which was designed around large blocks of power plants and not for a decentralized generation. More serious problems developed due to the fluctuating nature of solar energy which needed to be compensated for, in particular during bad weather and at night. One can suspect that this gave also good arguments for utilities to ask for higher prices for the grid management.

However it becomes visible that it is not the technical problem of integrating PV in the electricity system, but rather the financial/economic integration which does not take place. In first hand, the operators of power plants, were facing losses in sold electricity, as it was increasingly produced by former clients. Probably 5% is the threshold above which the share of privately owned renewable generation triggered an increasing opposition of energy operators against this new “grass-root”competition. Even though the revenue loss was more than compensated by increase in grid-management prices, conventional utilities were slow in adopting proactively the new situation. As electricity is traded, and the trading price goes with supply and demand, renewables needed to have priority access, meaning that all energy is permanently fed into the grid, and conventional power gets traded for the remaining rest, which now depended not only on the fluctuation demand, but more often also on fluctuating whether conditions.

With the wish of allowing ore competition and thus reducing consumer prices the European Union established at the turn into this century an “Unbundling” of power generation, transmission and distribution. Initially thought of helping also renewables, it has now some shortcomings, as the power generation sector looses to renewable electricity, the transport part needs to deal with massive power flows in a topology not designed for renewables, and the distribution part needs to invest in the more difficult management of the grid. Today, across countries there is a rather mixed picture as to how utilities deal with renewables, it ranges from embracing and consequently construction own PV systems to open hostility, including influencing policies and regulations detrimental to the further growth.

Photovoltaics – The Future

The development of photovoltaic solar energy has got a long way since 1955, more than 60 years. Another energy technology discovered and developed last century, nuclear fission, was 60 years after its discovery already in decline. Even the most conservative energy analysts and modellers agree that the second half of this century PV will be the main energy source, for two simple reasons: it will be the cheapest energy source and has no emissions which contribute to global warming. If the societies of this world are determined to abandon fossil fuels, only renewable sources and nuclear power remain as an option.

As optimistic as the forecast for PV’s future might sound, the challenge will be the huge efforts which will be necessary for deploying not only unprecedented amount of PV modules but also storage systems. By 2100, the global energy need requires about 15 Tera-Watt of power kept continuously running. If it would be supplied only by nuclear power, one would need to finish a nuclear power plant every two days until 2100, starting now. If it would be delivered by PV, it would need about 1250 Gigawatt to be installed every year on average. This is about 15 times the yearly production of 2017, which is indeed not unrealistic and reachable, given that there is not really a raw-material (sand!) bottleneck, even fairly distributed throughout the globe.

The depletion of fossil energy sources undoubtedly creates international conflicts, and the world would better not progress into the situation where the battle for the last remaining barrels of oil becomes fierce and cruel.

It might well be that Photovoltaics is an important ingredient for solving many global problems the global community faces in this century. Or, as the New York Times wrote in 1955 the day after the birth of the first solar cell, that it may mark the beginning of a new era, leading eventually to the realization of one of mankind’s most cherished dreams — the harnessing of the almost limitless energy of the sun for the uses of civilization.”

 

About the author:

Dr. Heinz A. OSSENBRINK, born in 1951, has a PhD in Nuclear Physics from Hahn Meitner Institute, Berlin and joined the European Commission’s Joint Research Centre (JRC) in 1982. He built up the JRC’s activity on Photovoltaics when Europe started its research and pilot programme for Photovoltaic systems. 

In 1995 he became Head of the Unit for Renewable Energy, and expanded research and support activities to Energy Efficiency and Bio-Energy, notably Biofuels. His work was dedicated to the scientific support of EU legislation for Renewable Energies and Energy Efficiency. More recently, he was developing the Unit’s portfolio to support Africa’s efforts for a renewable energy supply. He retired 2016 from the European Commission.

His publications cover measurement and testing methods for photovoltaic generators, economic assessment of renewable energy, global environmental impacts of extended bio-fuel and bio-energy use and the economic assessment of Energy Efficiency policy as a means for Climate Change mitigation.

He chaired 12 years the Technical Committee 82 of the International Electrotechnical Commission and guided in this function many international standards on photovoltaic technology and served for 18 years as Programme Chair for the series of European Photovoltaic Solar Energy Conferences.

Living on the shores of Lake Maggiore in northern Italy he practices flying, sailing, and skiing, and is deeply interested in global sustainability issues, in particular the ecological footprint of energy systems. He is still active as a freelance consultant and researcher.