Chapter 5

Introduction

The solar panels installed around the world today are the product of decades of scientific research, engineering and manufacturing improvement. Modern modules generate more electricity, last longer and cost dramatically less than the photovoltaic technologies that came before them. But solar technology is still evolving.

Researchers are developing new photovoltaic materials capable of converting more sunlight into electricity. Manufacturers are redesigning cells and modules to capture energy more effectively. Engineers are finding new places to install solar, from reservoirs and agricultural land to the surfaces of buildings themselves.

Some of these technologies are already entering commercial use. Others remain inside laboratories or experimental projects, and some may never become economically practical. Together, they point towards an important idea. The solar panel we recognise today may not represent the final form of solar energy.

The Limits of Today's Solar Cells

Most solar panels installed today are based on crystalline silicon. There are good reasons for this. Silicon is abundant, well understood and supported by an enormous global manufacturing industry. Decades of development have produced solar modules capable of operating reliably outdoors for many years while generating electricity at increasingly low cost. But no solar cell can convert all incoming sunlight into electricity.

As we explored in Part 3, photons arrive with different energies. Some do not contain enough energy to excite an electron across the semiconductor's band gap and therefore pass through the material without producing useful electrical energy. Other photons contain more energy than required, but much of that excess energy is eventually lost as heat. There are other losses as well. Some sunlight is reflected from the surface, electrical resistance consumes part of the generated power and the module itself becomes less efficient as its temperature increases. These fundamental mechanisms place limits on how efficiently a conventional single-junction silicon solar cell can convert sunlight into electricity.

Efficiency matters because every improvement allows more electricity to be generated from the same area. A more efficient module can produce more power from a constrained rooftop and can reduce the amount of land, mounting equipment, cabling and other infrastructure required for a given generating capacity.

Researchers are therefore continuing to push silicon technology closer to its practical limits while simultaneously investigating entirely new photovoltaic materials and cell architectures.

Innovation is also focused beyond efficiency. Manufacturers are working to reduce the amount of silicon, silver and other materials required, improve recyclability, extend module lifetimes, reduce degradation and make manufacturing less energy and resource intensive.

The future of solar is therefore not simply about creating the world's most efficient cell. It is about producing more electricity, for longer, using fewer resources and at lower overall cost.

Perovskites - A New Kind of Solar Cell

Silicon has dominated the solar industry for decades, but it is not the only material capable of producing the photovoltaic effect. One of the most promising alternatives being researched is a family of materials known as perovskites.

Perovskite solar cells have attracted enormous interest because they can absorb sunlight extremely effectively and have achieved impressive improvements in efficiency over a relatively short period of research. They also offer potentially different manufacturing routes from conventional silicon.

Producing high-quality crystalline silicon wafers requires several energy-intensive processing stages. Perovskite materials can potentially be deposited in thin layers using processes that could eventually be simpler and less energy intensive.

This opens interesting possibilities. Solar cells could potentially become thinner, lighter and more flexible, allowing photovoltaic technology to be incorporated into surfaces where conventional rigid modules are less suitable. 

But laboratory performance is only part of the challenge. A solar module installed outdoors must survive years of sunlight, heat, moisture, wind and changing temperatures while continuing to generate electricity reliably. Silicon has benefited from decades of engineering experience demonstrating that it can do exactly that.

Perovskite cells still face important challenges, particularly around long-term stability, large-scale manufacturing and the use of materials such as lead in many high-performing formulations. Whether perovskites eventually replace silicon is therefore uncertain. But they may not have to. One of their most promising applications involves combining the two technologies together.

Tandem Solar Cells

A conventional silicon solar cell has a single semiconductor band gap. That creates an unavoidable compromise.

Photons with insufficient energy cannot generate an electron-hole pair, while photons carrying far more energy than required lose much of that excess energy as heat.

What if different parts of the solar spectrum could be handled by different materials?

That is the idea behind tandem solar cells. Instead of relying on a single photovoltaic material, tandem cells stack materials with different band gaps on top of one another. Each layer can then capture a different portion of the incoming solar spectrum more effectively.

One promising approach places a perovskite solar cell above a conventional silicon cell. The perovskite layer can be designed to absorb higher-energy photons while allowing other wavelengths to pass through to the silicon cell beneath it. Rather than forcing one material to handle the entire solar spectrum, the two materials divide the task between them. The result is the potential to convert a greater proportion of incoming sunlight into electricity than either material could achieve alone.

This makes tandem technology particularly exciting because it could build upon the enormous silicon manufacturing industry that already exists rather than requiring the entire solar industry to start again. If tandem cells can be manufactured reliably and economically at large scale, future solar modules could generate substantially more electricity from the same rooftop or area of land.

The challenge, once again, is moving from impressive laboratory devices to products capable of surviving decades in the real world at a competitive cost.

Bifacial and Smarter Modules

Not every improvement requires inventing a completely new photovoltaic material. Engineers are also finding better ways to extract electricity from technologies that already exist. One example is the bifacial solar module.

Traditional solar modules are designed primarily to capture sunlight striking their front surface. Bifacial modules can also generate electricity from light reaching the rear of the panel. Some sunlight that reaches the ground is reflected rather than absorbed. If the surface beneath a solar array reflects enough of this light, the rear surface of a bifacial module can capture part of it and generate additional electricity.

The benefit depends heavily on the installation. Light-coloured surfaces, sand, concrete and snow can reflect more sunlight than darker surfaces, while the height and spacing of modules influence how much reflected light reaches the rear cells.

Bifacial technology therefore demonstrates an important principle: increasing solar output does not always require dramatically changing the solar cell itself. Module architecture continues evolving in other ways as well.

Half-cut cells can reduce electrical losses and improve performance under some shading conditions. Multi-busbar and advanced interconnection designs can collect current more efficiently. Glass-glass modules can improve durability while protecting cells from the environment, and manufacturers continue reducing the amount of inactive space between cells.

Individually, many of these improvements appear relatively small.

Across millions of modules and decades of electricity generation, however, incremental improvements can produce enormous amounts of additional energy.

Solar in New Places

Improving solar technology is only one way to generate more solar electricity. Another is finding better places to install it. Traditional solar installations are generally associated with rooftops or large areas of open land. But engineers are increasingly designing photovoltaic systems that share space with other activities rather than requiring land exclusively for electricity generation.

Floating solar systems install photovoltaic arrays on bodies of water such as reservoirs. This can reduce competition for land while making use of existing infrastructure and, in some locations, reducing evaporation from the water surface. Designing these systems introduces additional challenges, including anchoring, corrosion, changing water levels and access for maintenance.

Agrivoltaics combine agriculture and solar generation on the same land. Panels can be positioned above crops or grazing areas, allowing electricity and food to be produced from the same site. In some climates, partial shading from the panels may even benefit certain crops by reducing heat stress or water loss, although the optimal arrangement depends heavily on the crop and local conditions.

Buildings themselves offer another enormous opportunity. Building-integrated photovoltaics, or BIPV, incorporate photovoltaic materials directly into parts of the building rather than attaching conventional modules afterwards. Roof tiles, façades and other building surfaces can potentially become electricity-generating components. Researchers and manufacturers are also developing semi-transparent photovoltaic technologies that could allow windows and glazed façades to generate electricity while still allowing some visible light to pass through.

These technologies blur the distinction between a solar panel and the structure supporting it. A roof does not simply have solar panels installed on it. The roof itself can become the solar generator. A façade does not simply protect a building from the environment. It can produce electricity as well.

The future of solar may therefore involve photovoltaics becoming less visually distinct as they are increasingly incorporated into the surfaces already surrounding us.

Beyond the Horizon

Some ideas for the future of solar go far beyond improving the panels and installations we use today. Perhaps the most ambitious is space-based solar power.

A solar array in orbit would experience very different conditions from one installed on Earth. Above much of the atmosphere, sunlight can be collected without interference from clouds and, depending on the orbit and system design, generation could potentially continue for much longer periods than terrestrial solar.

The basic concept involves constructing enormous solar arrays in space, converting sunlight into electricity and then transmitting that energy wirelessly to receiving stations on Earth, potentially using microwave or laser transmission.

The idea is extraordinary. So are the engineering challenges.

Launching enormous quantities of equipment into space is expensive. Structures would need to be assembled and maintained in orbit. Energy would have to be transmitted safely and efficiently across vast distances, while the entire system would need to compete economically with rapidly improving renewable technologies on Earth.

Space-based solar therefore remains far from becoming an everyday source of electricity. But ambitious concepts have value even when their eventual future is uncertain.

Photovoltaic technology itself once powered little more than specialist equipment and spacecraft. Few people looking at those early cells could have imagined that decades later millions of homes would have solar panels installed on their roofs.

Other emerging ideas may prove equally surprising.

Ultra-lightweight solar cells, flexible photovoltaic films, new semiconductor materials, automated manufacturing and entirely new cell architectures could all change what a solar installation looks like in the decades ahead. Not every idea will succeed.

But engineering progress has always depended on exploring possibilities before knowing exactly which ones will become practical.

How Far Can Solar Go?

It is tempting to imagine that the future of solar depends primarily on building a more efficient panel. Efficiency will certainly matter. But the ultimate role of solar energy will depend on much more than the photovoltaic cell itself.

A solar module can only generate electricity when sunlight is available. Storage determines how much of that energy can be shifted into the evening or across longer periods. Transmission determines whether electricity generated in sunny regions can reach distant cities and industries. Demand flexibility determines whether electric vehicles, heating systems and factories can adjust their consumption towards periods when solar electricity is abundant. Electrification determines how much of transport, heating and industry can ultimately make use of that clean electricity. Manufacturing determines how quickly enough panels can be produced. Economics determines whether households, businesses and utilities continue investing in them. Policy and planning influence where projects can be built, how quickly grids can expand and how effectively new technologies can be connected.

Solar's future is therefore inseparable from the energy system surrounding it.

A revolutionary solar cell is far less useful if there is nowhere to send its electricity. A highly renewable grid is far harder to build without storage, transmission and flexible demand. Electrification creates enormous new opportunities for renewable electricity, but also increases the amount of generation and infrastructure that must be constructed.

The technologies explored throughout Part 8 are therefore deeply connected. Better solar technology can provide more electricity. Better storage can make that electricity available for longer. Stronger transmission networks can move it further.

Smarter grids can manage it more effectively. Electrification can allow it to replace fossil fuels in more areas of everyday life.

Solar may never provide every unit of energy humanity consumes, nor does it need to. Its significance comes from being one of several technologies capable of transforming how civilisation is powered.

Looking Ahead

Throughout Solar Fundamentals, we have followed an extraordinary chain of events.

We began billions of years ago, before the Sun itself existed. We followed the formation of a star, the creation of sunlight, the behaviour of photons inside semiconductors and the electrical systems that transform that energy into useful power.

We then moved beyond the panel, exploring how solar systems are designed, installed and maintained, why their economics work and how millions of individual installations are becoming part of a much larger transformation in the world's energy system.

In the final chapter, we'll bring that entire journey together. From the birth of the Sun to the future of energy.