Printed Solar Cells

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Solar cells can be mass produced with printing presses just like newspapers and banknotes. The very latest photovoltaic materials can be fabricated using solution-based processing methods, making them highly amenable to printing on thin and flexible substrates. This means a hopeful future for the availability of mass-producible and highly affordable photovoltaic technology.

The advancement of photovoltaic technology has the potential to positively impact global energy generation, decrease pollution and mitigate climate change. Progress so far has been stalled by a number of limitations:

• Power Conversion Efficiency (PCE)
• Stability
• Upscaling
• Module Design

The advancement of solar cell research requires “lab to fab” techniques that provide a solution to issues such as bulky, inflexible materials and complicated, highly expensive production costs.

Why Do We Need Printed Solar Cells?

The mass production of photovoltaic technology at low cost is desperately needed in the solar industry. The power that a PV panel generates is proportional to the surface area exposed to sunlight. According to Cheng et al. (2016) “the world consumes approximately 20,000 terawatt-hours of electricity each year. Meeting this need would require enough PV devices to cover around 100,000 square kilometres, an area about the size of Iceland.”

Printed and flexible solar cells are cheaper to fabricate and produce far less waste. They are lightweight, flexible and translucent in comparison with other materials. They use little material and can generate electricity even in low light conditions.

Printed solar cells can be utilized in a range of applications. They can easily be rolled up and are thus transportable, so can be used for outdoor activities such as camping. They can also be used in walls and windows, wearable devices and even clothing.

That said, according to Cheng et al. (2016) “printing layers that are nanometres to micrometres thick — uniformly and without pinholes, and over many square metres — is difficult.” A traditional method used in the fabrication of solar cells is that of spin-coating. But this technique is unable to meet the demands of mass production. Two important applications drawing the attention of researchers concerns printing technologies of organic solar cells and the use of 3D printing.

Evolution of PV Technology

Here is a brief rundown of how PV technologies have evolved over the generations:

1. First generation solar cells are solely silicon-based
2. Second generation
• Copper indium gallium selenide (CIGS)
• Cadmium telluride (CdTe)
• Gallium arsenide (GaAs)
• Amorphous silicon (a-Si:H)
3. Third generation
• Dye-sensitized solar cells
• Organic solar cells
• Perovskite Solar Cells
4. Fourth generation
• Hybrid materials
• Low-cost polymer films
• Nanomaterials, metal oxides, graphene and carbon nanotubes

At present over 90% of photovoltaics comprise the use of silicon. Silicon currently offers the highest levels of PCE and stability, but it is also rigid and bulky. This material alone cannot offer a sustainable, cost-effective solar energy for use in every application. Solar researchers have sought innovative ways to integrate silicon with newer materials. This way they hope to harness the benefits of each kind.

Third and fourth generation solar cells are highly flexible compared to those of earlier generations. They also work well with solution-based processing methods. This goes hand in hand with a capacity to print materials onto a variety of substrates. These substrates can be thin and flexible, such as flexible plastics, which can be used in conjunction with many other materials.

Printing 3rd and 4th Generation Solar Materials

The fabrication of solar cells requires printing technologies to accommodate the requirement for large surface areas. This would prove beneficial to the optimisation of power generation. Mass production and vast scalability necessitate the use of printing methods. Some available printing methods include the following techniques:

Slot Die Coating

• Precursors deposited through a narrow, adjustable slot die.
• Offers high material usage efficiency, minimizing waste and reducing production costs.
• Suitable for large surface areas due to its continuous and scalable fabrication process.

Blade-Coating

• Also known as “doctor-blading”
• Suitable for large surface areas
• It offers the advantage of a continuous fabrication process

Inkjet Printing

• Widely used to fabricate the functional layer in OSCs
• Can be digitally controlled and has the advantage of producing patterned devices
• Offers a low-material waste solution

Gravure Printing

• Patterns are printed through a perforated screen
• Versatile technique, which can make patternable solar cells
• Require turning materials into a paste for extrusion that can alter precursor chemistry

Screen Printing

• Traditional printing method based on engraving
• Involves passing the substrate over a rotating cylinder
• Produces high-resolution patterns
• Widely used in graphic and package printing

There are disadvantages to printed solar cells compared to using non-printed materials. The printing process itself can be difficult to control and can result in damage to the material layers of solar cells. Additionally material wastage can be high depending on which technique you use. All of this impacts the overall efficiency of the solar technology produced.

The Rise of 3D Printing in Solar Research

The use of 3D printing technologies means we are able to address several of the existing problems of fabrication in solar research. 3D printing involves the layered production of 3D objects directly from Computer Aided Design (CAD) models. In the solar industry the technique is used for the direct deposition of components during the fabrication process.

Some of the advantages of 3D printing are:

• Uniform coating over large areas
• Flexibility by using roll-to-roll (R2R) and sheet-to sheet (S2S) systems
• Digital control enhances efficiency and design freedom
• Waste reduction

3D printing can be combined with other printing methods to harness the best features of both techniques. Emerging techniques involve the use of micro- and nanoscale printing technologies.