The future of solar energy depends on the fusion of old and new technologies. If photovoltaic (PV) devices that convert light into electricity could be mass-produced in printing presses like newspapers and banknotes, they would be affordable and ubiquitous.
Traditional silicon-based solar panels are stiff and bulky. Small, thin and flexible PV devices have already been fabricated on lightweight, translucent films. They use very few materials and can generate electricity even in dark rooms. Incorporating them into phones, clocks, walls and windows will transform the world’s energy production, reduce pollution and mitigate climate change.
But flexible solar panels have some hurdles. Some are based on contaminants such as heavy metals and some use hazardous solvents in their manufacture. Others degrade quickly and are less efficient at converting light into electricity. I’m having trouble printing. For all these reasons, printable solar cells have yet to gain a foothold in the power market.
Most R&D investment is in conventional silicon solar cells, which account for over 90% of global production. However, manufacturing requires a lot of power. This corresponds to approximately 10% of a cell’s lifespan.
Printed solar cells will not become widespread until they are cheap and safe to manufacture. Researchers and companies must work together to improve the efficiency, environmental sustainability and stability of these cells, scale up their manufacturing and plan their market penetration.
PV Primer
Mass production at low cost is what the solar industry desperately needs. The power generated by a PV module is proportional to the area exposed to sunlight. The world uses about 20,000 terawatt hours of electricity each year. Meeting this need would require enough PV devices to cover approximately 100,000 square kilometres, roughly the size of Iceland.
Such production speeds have not yet been achieved. Printed PV devices are typically made from many layers of material on a conductive glass or plastic substrate. Each layer has a function. Semiconductors or sensitizers absorb visible light, while other materials carry charge to the electrodes.
Many types of printed PV devices have been developed. Some contain organic semiconductors such as polythiophenes. Some use light-absorbing dyes containing ruthenium-based polypyridine. And nanoparticles absorb light in quantum dot solar cells.
Other examples include semiconductors containing the chalcogen element (sulphur, selenium, or tellurium), and organic-inorganic light absorbers with structures similar to the mineral perovskite. These are all classified as thin-film solar cells.
The most efficient are perovskite-based cells. These latest ones convert 22% of incident solar energy into electrical energy after just a few years of research (see ‘Catch-up’). This is more efficient than solar cells made from polycrystalline silicon.
However, perovskite batteries are not yet commercially viable as they decompose under high humidity and temperature.
In Detail,
Nanometres to micrometres thick printing layers — uniformly and without pinholes, and over many square metres — is difficult. Screen printing is commonly used by the electronic industry in order to feed a paste through a perforated screen. However, the printed circuit board layers are 100 times thickened than those of PV devices.
In addition, turning materials into viscous pastes alters their physical and electrical properties. Other methods for printing PV devices have been demonstrated in the lab over areas of about 10 square centimetres. A slot-die printing method involves feeding ink through a slit (slot-die printing), spray coating the substrate, gravure printing, and moving a blade over the substrate via an ink supply.
There are downsides to each technique. It is difficult to control the spacing of patterns in slot die printing, for instance, and gaps reduce a panel’s active area. Contact between the printer head and the subtract can damage the underlying layers in gravure printing. Printed solar cells are less than half as efficient as their non-printed counterparts because of these drawbacks.
More precise equipment and laser processing will be required to print thin, pinhole-free layers over more than one square metre.
A PLAN B approach would be to develop PV materials that work with existing industrial printing methods. In order to print, the materials must be able to be formed into liquids, solutions, or pastes. For PV devices, this means using either solution of chemicals (polymers, dyes or hybrid perovskite, for instance) or dispersions of nanoparticles (such as quantum dots).
If not sealed properly, many of these can degrade over days to weeks, and more-stable alternatives, such as silicon, are more difficult to print. The efficiency of a device and the environmental impact of its manufacture have to be maintained a balance.
The most efficient thin-film solar cells use toxic or rare materials, such as cadmium, ruthenium, and lead, as well as hazardous organic solvents. It is also a common ingredient in transparent conductive films for PV devices, and its use is expected to increase. Indium stocks could run out before the end of the century due to depleted mineral deposits and low recycling rates. In order to make efficient devices with little toxic waste, researchers are looking for abundant materials that can be processed in solution. For eg., Lesser consumption of the metal is achieved when carbon-based electrodes and layers are used. In the current scenario, such designs are often less efficient than others.
What The Future Awaits?
Emerging technologies have been hindered by the huge success of silicon panels.
Silicon-based PV device manufacturers share materials, equipment, and processes with related industries such as the computer industry. Developers of printable devices are isolated.
Also, the maturity of the silicon industry means there is little urgency to develop alternatives. As printable PV devices are still under development, capital investment and product commercialization are considered high risk.
Government funding is needed to push such devices from emerging status to competitive status, as happened to the silicon PV industry in China.
Market penetration should be developed in stages. The first printable PV devices should address weaknesses in silicon-based technologies, such as their poor performance in low light and lack of portability. The next wave should complement silicon solar cells and, ideally, be integrated into them.
Perovskite-silicon devices, for example, would harvest more sunlight than silicon devices alone. If printed technologies can capture 5% of the PV market, their advantages should ensure that they play an ever-increasing part in meeting growing demands for renewable energy.
