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.

Nanometres to micrometres thick printing layers

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.