Fabrication of Organic Photovoltaic Devices

Most of today's commercially available solar cells are made from inorganic materials such as highly-purified silicon, which makes them expensive and less competitive with other sources of energy such as coal. The next generation of solar cells will be light, flexible, attractive and most importantly, cheap, because they will be made from organic (plastic) materials. Their flexible lightweight properties will enable them to be deployed over a wide range of new applications for example furnishings, building components etc. enabling energy to be generated where it is used.

Plastic solar cells produced from organic semiconductors offer the potential to deliver efficient solar energy conversion with low-cost fabrication. A major challenge to overcome is to improve efficiency, there is a need to develop light-absorbing materials with efficient charge separation and charge transport properties and fabricate them into active layers of solar cells with a controlled nanomorphology.

Similar to devices made from silicon, the light-absorbing layer of organic photovoltaic (OPV) cells consist of a p-type (electron donor, D) and an n-type (electron acceptor, A) material, Figure 1. Commonly used p-type organic semiconductors include polymers based on thiophene building blocks such as P3HT, PBTTT, PCPDTBT.¹ The best n-type materials so far have been fullerene derivatives such as PC₆₁BM and PDI. This active layer composite is sandwiched between a transparent anode (e.g. indium tin oxide, ITO) and a metallic (e. g. aluminium, Al) cathode.

Figure 1 Energy diagram of an organic solar cell showing the processes involved in generating a photocurrent. Also shown are examples of typical p- and n-type organic semiconductors.

In order to generate a charge, the incident light excites an electron in the donor material from its ground (or highest occupied molecular orbital - HOMO) to the excited state (or lowest unoccupied molecular orbital - LUMO) and leaves behind a hole or a positive charge (Step 1 in Fig. 1). This exciton then travels to the D-A interface, where it undergoes a charge transfer to the LUMO level of the acceptor (Step 2). Transport of the electron to the electrode and recombination with the hole through the external circuit produces a photocurrent (Step 3).

One of the key challenges in organic materials is their inherently low dielectric constant, resulting in relatively short electron diffusion lengths compared to inorganic semiconductors.² In order to achieve an efficient electron transfer between the donor and the acceptor it is necessary that the two materials are within 10 nm proximity (Fig. 2A). However, despite the usual high absorption coefficient of organic dyes, a minimum thickness of 100 nm is required to maximize the light absorption.

This problem can be overcome by optimizing the interface between the donor and the acceptor by arranging the two materials in a dispersed or bulk heterojunction (BHJ) morphology during the fabrication process (Fig. 2B). As a result, an interpenetrating network is formed consisting of donor- and acceptor-rich domains with a high interfacial area while providing channels for charge transport to the electrodes (Fig. 2C).

Figure 2. Scheme of (A) bilayer OPV, structurally similar to traditional inorganic solar cells, (B) ideal and (C) real BHJ. The arrows indicate pathways for the charge transport to the electrodes.

While most prototype solar cells developed in the laboratory are fabricated by either spin-coating or sublimation, the real potential for future low cost power generation comes from the fact that those organic semiconductors are able to be manufactured at high volume through existing commercial reel-to-reel printing process on large area flexible substrates.³ Scientists and engineers around the world have begun to develop processes to achieve this goal.

In Australia, researchers from CSIRO's (Commonwealth Scientific and Industrial Research Organisation) Future Manufacturing Flagship, partnering with national and international groups, have recently reported successful printing trials in collaboration with Securency International, a banknote printing company.⁴ Operating at full speed, the production could be ramped up to print 200 meters per min or a total of 100 kms per day. Assuming a targeted 10% efficiency, enough solar cells could be printed in five months to generate the equivalent electricity output of one gigawatt powerstation (Fig. 3).

Figure 3. Flexible organic solar cells on printing press

Despite all the advantages and enormous potential for low cost power generation, organic solar cells will need continuing investment and further development in order to enter the consumer market. In particular, questions related to morphology and long-term stability of the organic materials have to be answered. Nevertheless, the great prospects offered by organic solar cells will lead us on an exciting journey towards environmentally-friendly and sustainable power generation in the future.

References

1. A.J. Moule and K. Meerholz "Morphology control in solution-processed bulk-heterojunction solar cell mixtures" Advanced Functional Materials 19 (2009) 3028-3036
2. B.A. Gregg and M.C. Hanna "Comparing organic to inorganic photovoltaic cells: Theory, experiment, and simulation" Journal of Applied Physics 93 (2003) 3605-3614
3. F.C. Krebs "Fabrication and processing of polymer solar cells: A review of printing and coating techniques" Solar Energy Materials & Solar Cells 93 (2009) 394-412
4. Media release: https://www.csiro.au/ (last accessed October 2009)