Advanced Solar Cell Technologies (The Science of Electricity)

Elena Sofronova at Federal Research Center "Computer Science and Control" of Also, methods of conversion of solar energy into electricity, working principles and est advanced Photovoltaic technology are defined. Also.
Table of contents

• This is the most efficient solar panel ever made | World Economic Forum
• The future of solar power technology is bright
• Advanced silicon solar cells
• What is a solar cell?
• Accelerating climate action

Simple passive solar water heaters, little more than a black-painted barrel, were sold commercially in the United States in the late 19 th century. The hot water is stored in an insulated tank until needed. These days, a variety of sophisticated commercial systems are available for water and space heating in the home.

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• This is the most efficient solar panel ever made.

Solar thermal systems are deployed throughout the world, with the largest installed base per capita found in Austria, Cyprus, and Israel. But modern solar truly starts in with the discovery of a practical way to make electricity from light: Bell Labs uncovered the fact that silicon could make a photovoltaic material. The most common type of solar cell is a semiconductor device made from silicon—a cousin of the solid-state diode. The familiar solar panels are made from a number of solar cells wired together to create the desired output voltage and current.

Those cells are surrounded by a protective package and topped with a glass window. Solar cells generate electrical power using the photovoltaic effect, a fact that didn't come from Bell Labs.

In the language of solid state physics, a solar cell is formed from a p-n junction in a silicon crystal. When a photon enters the crystal, if it has enough energy, it may dislodge an electron from an atom, creating a new electron-hole pair. However, if a pathway is provided through an external circuit, the electrons can travel through it and light our homes along the way. When they reach the other side, they recombine with the holes. It is an incredible invention — the only way we make electricity on Earth at scale, without any moving parts.

Solar cells are the only way we make electricity on Earth at scale, without any moving parts. Alongside their price, what we often care about is their efficiency — the amount of electrical-energy-out divided by sunlight-energy-in. When sunlight shines on a solar panel, the DC electricity that it produces is transported by conducting wires into an inverter, which transforms the electricity into the V AC supply that we use to power our telly and fridge.

Researchers have achieved 39 per cent efficiency for normal sunlight and an impressive 46 per cent for concentrated light the equivalent of suns when we focus sunlight from a larger area with a bunch of mirrors in the lab, but these cells are smaller than a CD, designed for space, and made with very expensive materials. The improvements are thanks to the use of new material combinations, advanced anti-reflection coatings, clever semiconductor processing that avoid electrical losses, and a hundred other smart engineering ideas.

The world's mass-producers of solar cells are continually incorporating these advances into their own solar cells, meaning that the panels on rooftops are steadily getting more efficient too. The techniques that made the world's first 20 per cent silicon solar panels 30 years ago are just now being translated into the production lines of the world's largest solar panel manufacturers. Thirty years from now, we should be able to expect the same from research advances happening in the lab today.

In 20 years' time the solar panels on your roof will probably look the same as now with their aluminium frame and glass front, but they will likely be a whole lot cheaper and at least half again as efficient thanks to smart engineering. A range of new technologies being researched now in labs across Australia and the world may overcome the efficiency limits of silicon-only solar cells.

One of the exciting developments in the field is a new semiconductor called "methyl ammonium lead iodide perovskite". Solar cells made out of this cheap and easy to produce material have already achieved 20 per cent efficiency in the lab — matching the efficiency of today's silicon cells. In the future, perovskites may either replace silicon solar cells or be used as a companion material to help them move beyond 26 per cent efficiency — the upper limit of silicon-only cells. The research team I am part of at Monash University and CSIRO is experimenting using perovskites as the top layer in double-decker "tandem" solar cells that absorb different colours of sunlight in each layer.

Newer types of solar cells with layer of perskovite may boost the efficiency of silicon solar cells. In a tandem solar cell, high-energy photons green, blue and UV are absorbed in the top layer, and low-energy photons red, orange and yellow are absorbed in the bottom layer. This allows the solar cell to squeeze more energy out of sunlight — we are aiming for double the efficiency of rooftop solar cells at super low cost.

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Other ideas being pursued around Australia and the world include reflected-tandems double-decker solar cells placed side by side , quantum-dot solar cells using tiny nanocrystals as the energy absorber material , up-conversion of light converting two low-energy photons, that would otherwise be wasted, to make one high-energy photon and hot-carrier cells collecting charge from solar cells before they have the chance to lose any voltage. It is not too hard to imagine a future with thin, efficient, lightweight and flexible solar cells on mobile phone cases, laptop bags, backpacks, suitcases, hats, tents, you name it….

It takes about two to five years for a solar panel to "pay back" the energy that went into making them depending on how sunny it is where you live. This includes the energy needed to mine the silicon and process it into a solar cell, and also make the aluminium frame and glass in the panel module housing. Solar panels usually come with a guarantee of 80 per cent output for 25 years and there is no reason why they should not last longer , which means energy-wise solar panels are a good thing, by a factor of at least four.

Silicon is the second most abundant element in the Earth's crust second to oxygen in the silicon dioxide that makes up sand and quartz , so there will not be any material shortages in the foreseeable future. Solar panel recycling stations are starting to be set up all over the world — like aluminium recycling, silicon is an excellent candidate for cradle-to-grave-to-cradle material management. First they calculated the growth rate of solar required to achieve 10 TW by and the minimum sustainable price that would elicit that growth without help from subsidies.

Current technology is clearly not up to the task. So what needs to change? Getting all of that to happen quickly enough — within five years — will require near-term policies to incentivize deployment plus a major push on technological innovation to reduce costs so that government support can decrease over time. While costs must be brought down, the technology promises to bring a 7 percent increase in efficiency, and many experts predict its widespread adoption.

In field tests, some modules containing PERC solar cells have degraded in the sun, with conversion efficiency dropping by fully 10 percent in the first three months. That behavior is perplexing because manufacturers thoroughly test the efficiency of their products before releasing them. In addition, not all modules exhibit the problem, and not all companies encounter it.

Interestingly, it took up to a few years before individual companies realized that other companies were having the same problem. Manufacturers came up with a variety of engineering solutions to deal with it, but its exact cause remained unknown, prompting concern that it could recur at any time and could affect next-generation cell architectures. To Buonassisi, it seemed like an opportunity. His lab generally focuses on basic materials problems at the wafer and cell level, but the researchers could equally well apply their equipment and expertise to modules and systems.

By defining the problem, they could support the adoption of this energy-efficient technology, helping to bring down materials and labor costs for each watt of power generated. The company had come to them for help with the unexpected degradation of their PERC modules and reported some odd trends. PERC modules stored in sunlight for 60 days with their wires connected into a closed loop lost no more efficiency than conventional solar cells typically do during their break-in period.

But modules stored in sunlight with open circuits degraded significantly more. In addition, modules made from different silicon ingots displayed different power-loss behavior. And, as shown in Figure 1 in the slideshow above, the drop in efficiency was markedly higher in modules made with cells that had been fabricated at a peak temperature of degrees Celsius than in those containing cells fired at C.

Understanding how defects can affect conversion efficiency requires understanding how solar cells work at a fundamental level. Within a photoreactive material such as silicon, electrons exist at two distinct energy levels. When solar radiation shines onto the material, electrons can absorb enough energy to jump from the valance band to the conduction band, leaving behind vacancies called holes.

If all is well, before the electrons lose that extra energy and drop back to the valence band, they travel through an external circuit as electric current. Generally, an electron or hole has to gain or lose a set amount of energy to move from one band to the other. If an electron and hole both make the move, they can recombine, and the electron is no longer available to pass through the external circuit.

Power output is lost.

What is a solar cell?

The PVLab researchers quantify that behavior using a measure called lifetime — the average time an electron remains in an excited state before it recombines with a hole. To measure lifetime, the team — led by Morishige and mechanical engineering graduate student Mallory Jensen — uses a technique called lifetime spectroscopy. It involves shining light on a sample or heating it up and monitoring electrical conductivity during and immediately afterward.

When current flow goes up, electrons excited by the added energy have jumped into the conduction band.

Accelerating climate action

Changes in conductivity over time thus indicate the average lifetime of electrons in the sample. To address the performance problems with PERC solar cells, the researchers first needed to figure out where in the modules the primary defects were located. Possibilities included the silicon surface, the aluminum backing, and various interfaces between materials.