Large-Scale Deposition of Organic Solar Cells


Whilst organic photovoltaic (OPV) efficiencies have exceeded 14% in research,1,2 the majority of proposed systems are small-scale devices manufactured using spin coating - which wastes large amounts of materials, and is a batch processing technique. In order for these cells to be made on a large scale, less wasteful methods that are compatible with continuous processes must be used. These can broadly be divided into coating techniques, which are suitable for lab-scale up to large-scale manufacture, and printing techniques, which are commonly adapted from large-scale commercial processes. This guide will explore the techniques mentioned below.

 

Coating techniques that can be used to make OPVs include:

  • Blade coating (sometimes called "doctor blading")
  • Slot-die coating
  • Spray coating
  • Dip coating
  • Wire-bar coating

Common printing techniques include:

  • Screen printing
  • Flexographic printing
  • Gravure printing
  • Inkjet printing

 

Continuous processes with reduced waste are favourable for large-scale manufacture. This is because they can be incorporated into a ‘roll-to-roll’ system, where a flexible substrate is passed between large rollers and layers are coated in subsequent steps. Roll-to-roll processing is seen as the ideal method for large-scale processing as it can produce large-scale devices very quickly, can combine several different printing/coating techniques, and has already seen success in the large-scale production of OPVs.3

 

OPV roll-to-roll process diagram
Figure 1: Simplified illustration of a roll-to-roll process, where layers are deposited in subsequent steps on a continuous line.

 

In moving away from spin coating to a larger-scale technique, it is important to remember that the kinetics of film formation can fundamentally differ between different techniques. Depending on how quickly the thin-film is deposited and how quickly the solvent evaporates, the layer will form in different lengths of time - and possibly with different morphology.4 As a result, the conditions that are optimal for spin coating are likely not optimal for a different deposition technique.

Most of the techniques detailed here can be classified as either self-metered or pre-metered systems. In self-metered techniques (such as dip coating), the final thickness obtained is dependent on the process - so thickness can be easily tuned using external parameters. In pre-metered techniques, the final thickness obtained is not dependent on the process, but the rate at which the solution passes through the system - meaning it can be pre-determined. Examples of pre-metered techniques include slot-die coating and spray coating.

 

Coating Techniques

Spin coating is the standard deposition technique used in thin-film research. Here, the solution is deposited onto a substrate that is either already spinning (dynamic deposition) or still (static deposition) and then spun after deposition. The rotation of the substrate will distribute the solution across the surface, where the eventual film thickness will depend on the solution viscosity and the speed and time of rotation. It is good for small-scale procedures that require well-defined film thicknesses. However, it is difficult to apply to large-scale manufacture, and wastes large amounts of material. A helpful guide to spin coating theory & techniques can be found here.

 

Thin-film deposition via spin coating
Figure 2: A representation of thin film deposition via spin coating.

 

One of the most common alternatives to spin coating is blade coating. In small-scale production, the film is deposited by dragging the blade of the coater across the substrate. The gap between the blade and the surface will determine the eventual film thickness., which will also depend on coating speed, the wetting of the solution on the substrate, and the solution viscosity and temperature. Volatile solutions can evaporate too quickly to be blade-coated and result in uneven surfaces.

In larger-scale production, the blade is kept static whilst a continuous substrate is passed underneath. Blade coating only requires 20% of the material that spin coating does,6 and hence is significantly less wasteful. It is also very simple and easy to study in terms of kinetics and morphologies. Blade coating's main disadvantage is its difficulty in generating patterned substrates (where only certain parts of the surface are coated).

 

Thin-film deposition via blade coating
Figure 3: An approximation of small-scale thin film deposition via blade coating.

 

Blade-coating has been used with success in both fullerene-based systems7,8 and non-fullerene acceptors,9,10  exceeding efficiencies of 11%. Blade-coating is often used as a prototyping method for slot-die coating (a depiction of which can be seen in Figure 4).

 

Thin-film deposition via slot-die coating
Figure 4: An approximation of small-scale thin film deposition by slot-die coating.

 

In slot-die coating, the solution is deposited through a head containing a reservoir that allows precise dispensing onto the substrate (which is moved underneath the static head). An in-depth discussion of the theory behind this can be found in the Ossila guide: ‘Slot-Die Coating: Theory, Design, & Applications’. Slot-die coating operates via similar principles to blade coating, but is much more precise, and allows for patterned coating and has the advantage of minimal waste. The film thickness achieved with slot-die coating is dependent on the solution flow rate, the coating width and speed and the solution viscosity. Its main disadvantage is that the process can be slow, expensive, and difficult to optimise. Despite this, it is considered one of the most promising alternatives to spin coating and has achieved comparable efficiencies in literature.11,12

 

A further deposition technique used in research is dip coating, which can be seen in Fig. 5, where a thin film is deposited by immersing the substrate in solution, followed by a controlled removal. The solution will wet the film during immersion and dry upon removal and subsequent solvent evaporation. The final dry film thickness is dependent on withdrawal speed, air flow, viscosity of solution and solvent evaporation rate amongst other factors. An in-depth discussion of the mechanism and theory of dip coating can be seen in the Ossila Dip Coating Theory: Film Thickness guide. Dip-coating has seen some, limited applications to organic PV, both for active layer27 and electrode deposition,28 and has achieved efficiencies approaching that of spin coating.

Dip coating thin-film deposition
Figure 5: An approximation of thin-film deposition via dip coating.

 

Another deposition technique used in research is wire-bar coating, an approximation of which can be seen in Fig. 6. Here a metal rod wound with wire is passed over the substrate and solution deposited through the gaps between the wires. The eventual film coating thickness is dependent on bar height and pressure, and deposition speed, as well as solution concentration, viscosity and drying characteristics. This has seen some applications in research to organic thin films,29,30 but remains a rarely-used technique.  

 

Wire-bar coating thin-film deposition
Figure 6: A representation of thin-film deposition via wire-bar coating.

 

The final alternative deposition technique used in research is spray coating. In this, the solution is broken up by a stream of pressurised air/gas or an ultrasonic system, then dispensed in a continuous flow of fine droplets. Here, the eventual film thickness will depend on the solution surface tension and viscosity, along with the properties of the gas flow and nozzle, the wetting of the solution, and the working distance and speed of coating.

The advantage of this is speed, minimal waste, and the possibility for easy multi-layer coating due to the fast-drying small droplets. The main disadvantage of spray coating is the difficulty in patterning, as using a mask can generate large amounts of waste. Spray coating has been used in several publications (mostly in fullerene-based systems) with some success.13,14

 

Thin-film deposition via spray coating
Figure 7: Approximation of thin-film deposition via spray coating.

 

Printing Techniques

Printing techniques that can be used in roll-to-roll processes include screen printing, flexographic printing, gravure printing, and inkjet printing. Screen printing uses a screen mesh pressed against the substrate to produce a pattern. This requires a high-viscosity, low-volatility solution - which can restrict the applications in terms of OPVs - but there has been some moderate success in screen printing interfacial layers or electrodes.15 The deposition of electrodes (e.g. by using silver paste) through printing is particularly favourable to avoid high-temperature evaporation steps under vacuum. 

 

Simplified diagram of roll-to-roll screen printing
Figure 8: A depiction of roll-to-roll screen printing used to produce a patterned film.

 

Less commonly, electrodes can also be printed using flexographic printing. This can also easily produce a patterned film by using a combination of engraved rollers. Despite not being studied as much in OPV research compared to other coating methods, flexographic printing has seen some success16 and is advantageous due to easier patterning and faster coating when compared to slot-die coating.

 

Roll-to-roll flexographic printing
Figure 9: A simplification of a flexographic roll-to-roll system used to produce a patterned film.

 

Flexographic printing can also use less viscous solutions than screen printing, which allows for more applications in the context of OPVs. This also applies to gravure printing, a similar process that uses a combination of engraved rollers (shown in Figure 10), which is able to produce very thin layers. Whilst gravure printing is advantageous due to speed and high resolution, the ability for complex patterning is limited and few systems with high efficiencies have been  published.17,18

 

Roll-to-roll gravure printing
Figure 10: A simplification of a roll-to-roll gravure printing process.

 

The final common method of printing applied to OPVs is inkjet printing. This has seen the most interest in research due to high resolution, ease of patterning, fast printing, and minimal waste. There have been several successful publications using inkjet printing to manufacture OPVs,19,20 including some studies achieving efficiencies similar to those obtained with spin coating,21 but there remains significant complexity related to nozzle clogging, solution additives, drying conditions, and circulation systems that restrict easy optimisation.

Simplified diagram of an inkjet roll-to-roll system
Figure 11: A simplified diagram of printing using an inkjet roll-to-roll system.

 

Final Thoughts

This guide has discussed scalable deposition techniques in the context of OPVs. Similar themes have also emerged in related areas, such as perovskite solar cells.22,23 Several less-used techniques that have not been discussed here include dip coating, knife-over-edge, and wire-bar coating.

Lastly, whilst advances have been made in spin coating alternatives, several areas for improvement and exploration still remain. True scalability will ideally be achieved using non-halogenated solvents, ambient processing24 without vacuum steps, and ITO-free devices25,26 - all of which still need development. Scalability (in terms of synthetic complexity) and stability of the materials themselves is also important. Future advances are likely to be made using non-fullerene acceptors instead of conventional and less stable fullerene-based systems.  



Written by Emma Spooner, a PhD student in Fullerene-Free Photovoltaic Devices at the University of Sheffield  in collaboration with Ossila Ltd.


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