PCBM, a fullerene derivative that revolutionised photovoltaics

The chemical properties of fullerene materials such as PCBM make them an exceptional tool with versatile uses. It is fullerene and its derivatives that stand at the forefront of scientific discovery in photovoltaics and organic electronics in general. What’s more, the market speaks for itself. There is a steadily growing stream of customers eager to buy PCBM for the great properties it offers in terms of renewable energy and the yet unforeseen potential it promises for the future. In this article you’ll find all the information you need to know regarding the history, qualities, current use and the future of this material.

 

PCBM, in full form Phenyl C61 butyric acid methyl ester – where did it come from?

This remarkable material, has been discovered and created relatively recently, as late as the 1990s, from buckminsterfullerene, which itself is a rather new phenomenon (though it has apparently existed in nature and space before). Despite the fact that the existence of what was later called buckminsterfullerene has been hypothesised in the 1960s and 1970s already, it was only in 1985 that the material has been (more or less by chance) created by a laser evaporation of graphite. The soccer-ball resembling structure of the resulting substance earned it the name buckminsterfullerene after the architect Buckminster Fuller, famous for his futuristic constructions of geodesic domes. Since then, other so-called fullerenes have come to light and are the subject of intense research up to this day. PCBM derivative is the result of such research.

 

A perfect blend for photovoltaics - PCBM and polymer

It wasn’t long after the discovery of fullerenes that their derivatives, which enhance or modify their basic properties, began to be synthesised. Once it was discovered that photoinduced electronic transfer from a semiconducting polymer to a fullerene is possible, the basic C60 buckminsterfullerene has been for this purpose replaced by its derivative - the more practical PCBM. This meant a significant leap forward in photovoltaics. Now solar panels utilising this material point the way to the future!

 

PCBM’s solubility and other properties greatly increase the photovoltaic efficiency

Among the reasons why C60 fullerene has been replaced by its derivative to achieve a more efficient electron transfer is a better solubility in organic solvents offered by PCBM. The resulting substance makes for an excellent organic semiconductor. Furthermore, the chemical structure of PCBM material allows for the insertion of other functional groups into it that may further enhance both its photophysical and absorptive properties. This makes it an extremely multifaceted acceptor that can work together with plentiful polymers (most typically with P3HT) to achieve the best results. Though other, very efficient derivatives are making their name in the field, PCBM remains the golden standard of effectivity by which everything else is measured to this day.

 

Thanks to PCBM, synthesis of light becomes significantly cheaper

Since PCBM is an organic, carbon-based material related to plastic, with great photovoltaic properties, its mass use in the field of renewable energy, namely in production of solar cells and panels, is just a question of time. There are many benefits that show that organic solar panels may indeed be the future. These are:

  • Cheap production. In contrast with traditional solar panels that are silicon based, the production of carbon-based ones is much simpler, more akin to producing plastic. Since organic materials are, unlike silicon, plentiful, and more easily manufactured, their low cost gives them a huge competitive advantage.
  • Energy efficiency. Though it’s been thought at first that organic solar cells couldn’t match the silicone ones in efficiency, thanks to the continued research, a great deal of progress has been achieved. Organic solar cells’ effectiveness is now getting significantly closer to that of silicone and is geared to overtake it in the upcoming years
  • Flexibility and lighter weight. This might be the ultimate edge over traditional solar cells. Thanks to these, organic solar cells might be easily used indoors, on cars, or on roofs and surfaces that could not support standard solar panels. They also offer a great possibility for flexible personal use – as phone charges, wearables, as part of outdoor equipment.

 

How well does PCBM fare in terms of absorption spectrum?

An often cited disadvantage of this material is its supposed inability to make proper use of and absorb the full solar spectrum. This is the key aspect of solar energy generation – the absorption capacities of the material should match different solar radiation wavelengths as much as possible, otherwise the energy will be wasted as heat. The more photons it can harvest this way, the bigger the potential of generating eventual electricity. Although the compact and symmetric structure of standard fullerene doesn’t make it as optimal for absorbing solar radiation, a great deal of progress has been made in remedying this fact. For example – the absorption potential of PCBM has been significantly enhanced thanks to modifying methyl ester with chromophores.

When considering P3HT: PCBM absorption spectrum, other factors must be also weighed

When we imagine an organic solar cell, it usually has the following structure (from top down):

  • ITO (Indium tin oxide) layer that acts as an electrode collecting holes, usually 180 nm thick. The holes are evenly spaced out.
  • A thin sheet of PEDOT: PSS (35 nm) which functions as a hole transporting material.
  • A 50 nm thin semiconducting blend, typically consisting of PH3T: PCBM
  • An aluminium layer (200 nm). This acts as an electrode that collects electrons.

Depending on the proper patterning of these layers, the absorption efficiency could be further improved irrespectively of the actual P3HT: PCBM absorption qualities. A vast body of research is currently conducted in this sphere with promising results so far.

 

How does PCBM fare in terms of HOMO – LUMO?

What makes PCBM such a sought-after substance in photovoltaics is its low HOMO (Highest Occupied Molecular Orbital) – LUMO (Lowest Unoccupied Molecular Orbital) convergence gap between PCBM and the respective polymer (usually P3HT, but it might be other polymers). This allows for a great electron mobility between the donor and acceptor – in this case PCBM is the acceptor, P3HT (or other polymer) acts as the electron donor. As electrons are knocked from their place by the incoming photons, they leave behind a hole and enter an excited state in which they are loose, but still bound to the hole – they become so-called excitons. The next step is for the exciton to split, so that the now freed electron may move to a different hole. The constant movement of the electrons between holes thus generates electric current which is then transferred to a metal conductor – and beyond. None of it would be possible without a proper HOMO – LUMO energy difference.

 

The levels of PCBM LUMO may be shifted by electron irradiation

As part of maximising the potential of PCBM in terms of organic photovoltaic devices, a concentrated effort could be made to shift the LUMO levels up while shifting HOMO levels down via electron irradiation, therefore narrowing the desired band gap even further. This has been proven to increase electron fluence and makes it is this discovery that marks another important milestone in the field of organic photovoltaics. The head start of traditional silicon-based photovoltaics is being narrowed by a significant margin.

 

Among the varied types of fullerene derivatives, PCBM 60 is the best known

When we talk about PCBM, we always need to further specify which type. Since the discovery of fullerene, many different compounds have been synthesised, each offering a slightly different quality. Indeed the best known remains the PCBM 60. It remains the most widely used electron accepting material in organic photovoltaic devices. PCBM 60 is easily dissolved in common solvents for polymers, therefore creating a perfect fullerene-polymer cast which makes a great bulk heterojunction, thus offering a fast and effective charge transfer and movement of electrons. Typically it finds its use either in solar cells (OPVs), thin film transistors (OFETs) and increasingly also in photodetectors. You may find PCBM 60 of different purity on the market, most likely it’s going to be 99 % purity for use in solar cells and photovoltaics and 99.5 % purity for use in film transistors that require somewhat higher rate of crystallisation and electron mobility. 99.9 % pure PCBM is suitable primarily for cutting-edge research.

 

Do PCBM fullerene derivatives really show the way forward in renewable energy industry?

It is highly likely. Since their discovery just slightly over two decades ago, they have been the subject of intense study and have already made a significant mark in the field – and continue to do so. In the 21st century world of dwindling resources and a growing concern for global warming, there is an increased urgency to look for alternative and renewable energy sources.  Organic solar cells may provide the desired answer as they are an unlimited source of clean energy and the logistics of their production, mobility and maintenance far outstrips that of more traditional silicon solar cells while slowly offering more and more energy output. Commercial silicone cells are typically able to convert 15-22% of sunlight into energy (the absolute record is 27.3%). Until recently, organic cells have been able to convert only about half of that. Recently, however, a 15% mark has been reached by them, with 17% following soon after. Now it’s thought that even 25% may be possible! Indeed, this branch of photovoltaics may prove to be the saving grace for the global energy industry that is tethering on the brink of sustainability.

 

Some further variations on the theme: bis – PCBM and others

Experimentation with C60 fullerene has produced many promising variants of it, including the PC60BM Bisadduct, or bis-PCBM. A result of two simultaneous addition reactions on the fullerene molecule, this material has been created with the intention of increasing open circuit voltage of solar cells roughly by 0.1 V.  This is thanks to the raising of a LUMO level. In addition to that, bis – PCBM is often utilised in scientific research, namely in the study of crystallisation.

To exhaustively list all the possible variations of fullerene that are either already in market circulation or in production would be almost impossible in this given space, so we’ll put forward just a few notable examples.

  • PC70BM with its slightly elongated shape has been created to improve optical absorption of the visible sunlight spectrum in comparison to a simple PC60 Thus the number of collected photons is increased, which potentially allows for greater photocurrent to occur. PC70BM is utilised in some of the most efficient organic solar cells known thus far.
  • PCBB is somewhat more soluble. This fact can be taken advantage of by using selected organic solvents to boost the performance of some OPV devices.
  • Thienyl-C61-Butyric-Acid-Methyl Ester, or [60] ThCBM has been created with the purpose of better blending with semiconducting donor materials like P3HT.

 

Finally, let’s look at PCBM MSDS (Material Safety Data Sheet) – is it safe?

There’s no need to be concerned by a potential hazards of this material. According to EC Regulation no. 1272/2008, it is not classified as posing any hazards and does not need to be labelled as such.

It may cause some irritation if swallowed or inhaled or coming into contact with eyes or sensitive skin. Otherwise there’s no need for special caution while handling it.

 

Now you should have a clearer overview of PCBM, its history, variants, how it works within solar cells and also its potential for the future. What is your take on the issue? Do you see organic solar cells replacing traditional silicone ones any time soon? When do you think this will happen? We’re eager to hear your opinions – share them with us in the comments!

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