A Primer On Photovoltaics in Solar Energy Systems – Part 1

Photovoltaic

Solar photovoltaic panels – the science and technology used to harness the power of the sun. Photo rights: http://www.photovoltaic-production.com.

Part 1: History, Physics and Manufacture

With issues in climate change and energy technologies it’s often useful to have at least a basic understanding of the science in order to frame the story. So amidst the sea of stories on advances in solar energy that feature in the media, here’s a primer on the current landscape of solar photovoltaic energy – a technology that’s very likely to emerge as a principal source of energy for the planet within this century and a fundamental component in our arsenal for combatting climate change.

Photovoltaics (PV) refers to the science and technology of converting solar radiation into electricity. More often than not, when we read about solar power, it is PV-based technology that is being referred to. Although other techniques exist for making use of the Sun’s energy, for instance concentrated solar power, it is PV that lies behind the rapidly increasing number of solar panels we see on houses and vehicles, in cities and in rural areas around the world.

PV technology hinges on the phenomenon that some materials exhibit a photoelectric effect – a property causing them to absorb photons of light, and as a consequence, release electrons. When charged electrons are released from an atom, they are then free to move within a solid material; under the right conditions they can then be directed into an electric current. It is this physical process that is exploited in PV, allowing us to harvest clean, renewable energy from sunlight without any additional fuels, harmful waste, or even moving parts.

The photoelectric effect was actually first demonstrated by French physicist Edmund Becquerel in 1839. Later, in 1905, it was Albert Einstein who described the law of the photoelectric effect; work for which he won the Nobel Prize in Physics in 1921. The first practical PV module was not built until 1954 by Bell Labs – it was a silicon solar cell with efficiency of about 6%. But it was the exploration of space beginning in the 1960s which drove the space industry toward adopting and significantly advancing PV technology – providing power for spacecraft and satellites.

Photo rights: http://betterarchitecture.files.wordpress.com/2013/06/international-space-station-solar-cells.jpg

The International Space Station has around 2500 sq.m of monocrystalline silicon solar cells on its PV arrays; while one side receives light from the Sun, the other absorbs light reflected from Earth. Photo rights: NASA

The advent of PV technologies as a source of energy in more terrestrial contexts arose in part due to the energy crises that gripped early and later parts of the 1970s. While solar technology remained expensive and relatively inefficient around this time, nevertheless a growing awareness of problems associated with reliance on fossil fuels prompted significant interest, investment and research into PV that would ultimately bring about advances in the technology that made it not only vastly more cost-effective, but also a feasible source of energy to begin rivalling traditional carbon-based ones.

Since the beginning of the 21st century, application of solar technology has grown massively: contributing to electrical grids, powering transportation and buildings as well as providing electricity to developing regions unconnected to power grids. Despite any apparent ubiquity however, PV contributes only a marginal amount to the total energy market, around 0.1% (2). Nevertheless, these circumstances are very likely to change as solar technology continues to develop along the exponential trend witnessed over the last decade (1).  We’ll return in more detail to the growth and contribution of PV energy to the global energy market in a second part to this series on solar energy production.

Global cumulative PV capacities have been increasing exponentially over the last decade. Figure taken from Solar Energy Perspectives, 2011, OECD

The total global amount of electricity produced from photovoltaic systems has been increasing exponentially over the last decade. Figure taken from Solar Energy Perspectives, 2011, IEA.

The basic photovoltaic cell
So how is sunlight converted into electricity within a PV cell?  Well, the answer lies in the interaction of photons with the atoms in semiconductor materials – an interaction described by the photoelectric effect we introduced earlier.

Semiconductor materials (such as silicon) are the critical technology in PV – they exhibit excellent photoelectric effects, meaning that as photons enter them, they are absorbed, and dislodge electrons from atoms of the material. In a PV cell, semiconductor layers are paired: one layer (the N-type layer) is designed to hold more negative charge carriers (electrons), while the opposing layer (the P-type layer) has more free positive charge carriers (an absence of electrons, referred to as ‘holes’). This set-up creates an electric field between layers, a region referred to as a ‘junction’. Consequently, with the release of negatively charged electrons an electric potential difference arises in the junction resulting in positive and negative charges flowing to opposite sides of the cell. When the two ends of the cell are connected, electricity can be conducted away as a direct current.

The most prevalent semiconductor material used in commercial PV manufacture is crystalline silicon, largely due to its ubiquity as the second most abundant element in the Earth’s crust. These silicon cells dominate about 85% of the PV market (1). There are two forms of silicon PV: crystalline silicon (sc-Si) and multi-crystalline silicon (mc-Si) cells. Industrial crystalline silicon cells produce decent levels of efficiency, between 14% and 22% for sc-Si, and between 12-19% for mc-Si (1). Other semiconductor materials are also used, but we’ll come to these shortly.

OECD_PV effect schematic copy

A schematic illustration of a basic silicon photovoltaic cell. Figure taken from Solar Energy Perspectives, 2011, IEA.

The manufacture of silicon cells begins with forging ingots of highly purified silicon, which are sliced into very thin wafers about 0.5mm thick; wafers are treated to optimise their physical properties for use in a PV cell (for instance to produce n- and p-type layers, see here). Wafers are often re-cut into hexagons or rectangles to ensure they can fit together neatly, making maximum use of the surface area of the solar panel – a process that gives rise to their characteristic appearance.  Pure silicon is highly reflective, which is far from ideal on account of the fact that reflected light cannot be absorbed; so anti-reflective coatings are applied to the surface of the wafers. Cells are connected using strips of conductive metal (often palladium, nickel, silver, or simply copper) and contacts are added to receive the current. Cells are coated in an acetate, and protective glass is fitted to secure the cell environment. The manufacture of PV cells in this way has not changed in some time, hence their labelling as first generation PV cell technology.

The manufacturing process of crytalline silicon PV energy systems. Photo rights: http://www.neutek-energy.com.au/Yingli-Solar

A simplified illustration of the manufacturing process of crytalline silicon photovoltaic systems. Photo rights: http://www.neutek-energy.com.au/Yingli-Solar.

Several solar cells connected to one another and mounted together in a support structure is referred to as a module. In turn, several modules may be fitted together to form a solar array that is designed to supply electricity at a particular voltage. An advantage to this modular characteristic of PV is that it may be configured to supply power suited to end-users with particular needs, ranging from residential systems (supplying ~20kW), larger commercial settings (up to 1 MW), as well as utility scale systems (including and beyond 1MW). The modular nature of PV also makes it quick and easy to adapt to a vast range of contexts.

 

Solar resources & its challenges
To grasp why solar energy is such a significant resource, we need to recognise the massive abundance of solar power. It’s often stated that more solar energy reaches the Earth in a day than the total global energy used in a year – this is true: to be precise, around 885 million terawatthours (TWh) worth of power reach the Earth’s surface a year; that’s almost 6200 times more than the total energy demand in 2008; and still several thousand times that of predicted energy demands over the coming decades (1). Consider also that while proven fossil fuel reserves (combining coal, oil and natural gas) equate to a total of about 254 years worth of energy at our current annual rate, the total amount of energy reaching us from the sun in a single year, captured and stored, would provide enough energy for 6000 years (1). Such numbers speak for themselves, but there are significant challenges we must overcome if we’re to come anywhere close to making effective use of the sun’s energy. For the remainder of this article we’ll present some of these challenges.

Total energy resources. Annual solar energy is overwhelmingly abundant relative to all other sources of energy. Acknowledging this fact underlines the significance of developing solar energy systems that are as efficient as possible. Figure taken from Solar Energy Perspectives, 2011, OECD.

The amount of solar energy reaching Earth is overwhelmingly abundant relative to all other sources of energy. Acknowledging this fact underlines the significance of developing solar energy systems that are as efficient as possible. Figure taken from Solar Energy Perspectives, 2011, IEA.

Solar energy, like most renewable energies, is neither as energy dense nor as readily convenient to convert to electricity as fossil fuels are. What we refer to by the term ‘energy dense’ is the amount of energy in a resource (such as a fuel) per unit volume or mass; having a lower density makes it harder to harvest considerable amounts of energy from that resource. Consider that one litre of gasoline provides 35 megajoules of energy; this is the same amount of energy as one square meter of land receives in ten hours from the sun under optimal conditions. Simply put, considerable infrastructure is needed to support a typical PV system that produces any significant amount of electricity. The key challenge for renewable industries, and arguably solar in particular, is to improve efficiency and reduce manufacturing costs to the point where the inherent advantages of the energy (it is clean and inexhaustible) can compensate for the inconveniences of capturing that energy. This is an important point to make before considering how PV efficiencies are being researched and developed.

 

Understanding photovoltaic efficiency
Naturally, the electrical capacity of a PV cell is highly dependent on the amount of sunlight that reaches its surface (quantified as irradiance) – a point that brings into discussion the impact of temporal and spatial solar variation as well as ways to accommodate this unavoidable feature of solar-based systems (but more on this in Part 2). For now it’s important to be aware that ecological solar variance is circumvented in research by using concentrated sources of energy; demonstrating the efficiency of a PV cell under optimal conditions. It’s for this reason that lab-based efficiencies are generally higher than commercial ones.

Okay, leaving aside issues surrounding amounts of sunlight, let’s return to the efficiency of the PV cell itself. The cell’s architecture determines its efficiency by influencing physical properties that include reflectance efficiency, charge carrier separation efficiency, conduction efficiency values and thermodynamic efficiency. Of these, improving thermodynamic efficiency is where most research aimed at improving overall PV efficiency is taking place.

Efficiency is calculated as the ratio of electrical energy that is output from the cell per second (power) relative to the amount of light energy that reaches the cell per second (incident energy).

Thermodynamic efficiency is crucial in the conversion of sunlight to electricity at the atomic level within semiconductors. Remember how photons were described as entering a semiconductor and freeing electrons to produce an electric current? Well, in order to free an electron the photon’s energy must be able to be absorbed in a particular way by the material. How well a semiconductor material can absorb photons and convert them into a potential difference that produces an electric current is determined by its band gap – the energy range (measured in electronvolts (eV)) of a solid within which free electron states cannot exist.

Only photons with energies equal to or greater than the band gap of the material they are absorbed by can free an electron. When photons with an energy below that of the material’s band gap are absorbed they generate heat and don’t contribute towards the efficiency of the cell. Simply put, the band gap of a semiconductor material places an inherent limitation on how much solar energy the PV cell can convert into electricity.

Considering that the spectrum of sunlight ranges from 0.5eV to ~2.9eV, but that crystalline silicon has a energy bandgap of 1.1eV, it’s clear that silicon PV technology cannot capture all of the sun’s energy.  Consequent to the hard physics (and implications) of band gaps, novel technologies have been researched and adopted in pursuit of allowing us to harness much more of the sun’s energy.

 

Variations of PV Technologies
One of the most important areas of PV research has concerned advancing thermodynamic efficiencies through band gap engineering – the manipulation of semiconductor materials to achieve higher levels of solar to electrical conversion by capturing more of the energy spectrum of sunlight. The introduction of new semiconductor materials has been complemented with novel cell designs, altogether leading to several variations in PV cell types.

Significant amongst several new generations of PV cell has been the technique adopted in multi-junction (MJ) solar cells. Here, multiple layers of different semiconductor materials, each with unique band gaps, are stacked on top of one another, with band gaps of the materials getting progressively lower at each level. The result is a solar cell which absorbs photons over a much broader range of wavelengths than single junction PV cells. Inherent to their design, especially in consideration of the materials being used, MJ cells are expensive, and typically their use is confined to aeronautical and space applications (for instance, the Mars Rover and most satellites rely on MJ cells for power). Nevertheless, their efficiency is remarkable, and actually the world record for PV efficiency was reached using a four-junction cell producing 44.7% efficiency in 2013.  In comparison, single-junction silicon PV cells have a theoretical limit of 29.8% efficiency.

A schematic illustrating how materials of three band gaps can be used to absorb a broad amount of the solar spectrum than any one used on its own.  In actuality, three such layers would be stacked on top of one another with highest band gap at the top.  Photo rights: http://energy.gov/sites/prod/files/styles/large/public/cspv_3.gif?itok=pOGISdni

A schematic illustration showing how materials of three band gaps can be used to absorb a broader amount of the solar spectrum, and consequently more energy, than any one material used on its own. In actuality, three such layers would be stacked on top of one another with highest band gap at the top. Photo rights: http://www.energy.gov.

Thin-film (second generation) PV technology uses lower quantities of semiconductor material, such as copper, indium, gallium, selenium as well as amorphous silicon, deposited in thin layers to create PV cells. Thin-film PV cells are not as efficient as traditional silicon cells, usually by several percent, commercial variants reach up to only around 12% efficiency (1).

Third-generation PV cells are using materials other than silicon together with some very exciting novel technologies; multi-exciton generation, thermophotovoltaics, organic PV cells and nanostructured PV all feature in third-generation PV research. These hold some of greatest potential for achieving significantly higher efficiencies than we see at the moment, though it may be some time before we see their transition from laboratories into industrial and commercial settings.

Of this new generation of technology, one avenue of research is quantum dot PV. Quantum dots are semiconductor materials that have the unique capacity to modulate their atom’s energy levels, meaning they can effectively adjust their band gap without changing its basic architecture. Despite estimation that quantum dots could reach up to 65%, efficiency of only 7.0% has been demonstrated to date.

OECD_PV prospects

This graph depicts the improvement in efficiencies of multiple types of photovoltaic technology. While all variations are advancing, some hold more promise than others. Figure taken from Solar Energy Perspectives, 2011, IEA.

Reducing costs
Ultimately the growth of PV into an energy source that can compete with fossil fuels is dependent upon reducing the costs required to produce energy – a challenge faced by all renewable energies. The topic of parity of energy systems and competitiveness will be a presented more fully in Part 2, but it’s useful to be aware of the major strands of research that are seeking to further PV as a mainstream energy.

Three key factors have been identified as contributing reduction in costs: efficiency of cells, manufacturing plant size, and cost of silicon. As efficiencies increase there are several positive consequences that affect the economic viability of PV systems: fewer cells are needed to produce the same amount of energy; this means fewer panels, smaller systems, and fewer materials used in manufacture. Smaller systems also means less land or roof area is required for a given output demand, an important consideration for end-users with limited space, especially in urban environments.

An encouraging indicator for the future of PV can be drawn from assessment of the technology’s ‘learning rate’ – a measure of cost reduction as manufacturers accumulate experience of developing and deploying a particular technology. The historical learning rate for PV modules has been exceptional: on average 19.3% per year between 1976 and 2010 – the highest of any energy industry (1). While this figure will gradually fall, projections still expect it to remain at around 17% for the coming decades; a figure still higher than onshore wind (7%) and offshore wind (8%), geothermal (5%), and concentrated solar power (10%) (1). This is important because such predictions provide great weight to the argument that solar PV can, and will, continue to become more accessible as costs are reduced.

Conclusions
So there we are – a brief review of the physics, development and prominent challenges of PV technology. The avenues of PV technology are at various stages of development but they all share common roots and hold significant potential for improvement given a sustained research approach. The good news is that this research is already taking place. The role of governments to foster innovation and support sustained research and application of PV cannot be understated though – it is here where governments and industry must partner to ensure expedient delivery a cleaner future for us all.

Future PV technologies will be more diverse, widespread, and efficient. Despite the small proportion of the renewable energy market that solar energy currently occupies, it represents a resource what will eventually become a cornerstone of future energy production, and an absolute necessity in combatting climate change. Such a projection is based on the clear advantages of solar energy: it’s clean, limitless and holds technological validity for contributing massively to global energy demands.

 

References & Useful Resources:

1) Solar Energy Perspectives. International Energy Agency (2011)

2) Transforming Global Market for Clean Energy Products: Energy Efficient Equipment, Vehicles and Solar Photovoltaics. International Energy Agency (2010)

You can read more about the manufacturing process of silicon semiconductors here.