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Liquid Solar Cells: a New Phase in Solar Technology Development

published: 2012-11-13 15:08

Dr Dino R Ponnampalam

The world is made up of matter, and matter can be divided into three phases: solid, liquid, and gas.  Our concepts are based on the world around us being one of those three states.  Now imagine if a specific object, with a particular and commonplace state of matter, could exist in a different phase.  That is what is on offer with the latest development in the field of solar energy technology.

The Issues Surrounding Current Solar Cell Technology

Solar modules are tools that allow society to generate electricity from a natural resource, and as the world continues on this shift to a clean energy future, devising better ways to harness the power of sunlight is going to be of paramount importance.  For these multi-generational solar cells and solar modules, the electrical efficiency conversion rate and the manufacturing costs are the two areas that research is focused on improving.  By improving upon these two areas, the true power of sunlight could be utilized.  

Currently, as it stands, solar energy devices are neither very efficient nor very cheap.  The efficiency rate is about 20% for first-generation solar modules; these first generation solar cells and modules are considered to be the workhorses of the industry and offer the best efficiency rate.  Second-generation solar cells and modules, referred to as thin film solar cells, offer efficiency ratings of around 12%.    

In terms of costs, solar cell devices vary, as the cost will depend on the material selected.  As material costs are only one part of the total cost, any reduction would have to focus on reducing both the manufacturing costs and the installation costs.  In general, the cost of a solar module (from first-generation to second-generation) ranges from US$2.29 - US$0.84 Watt peak.  While this may not seem to be very much, this price range could be brought down further, which will only help the solar industry in becoming more widespread. 

Pros & Cons of Using Nanocrystals in Solar Cell Technology

In essence, nanocrystal solar cells are solar cells with a layer of substrate coated with selected nanocrystals: cadmium telluride (CdTe); copper indium gallium seleneide (CIGS); or silicon.  Nanocrystal solar cells are classed as third generation solar cells, cells that are theoretically expected to overcome the Shockley–Queisser limit of between 31% and 41%.  This limit concerns the maximum theoretical limit for solar cells using a single p-n junction (as typically found in a silicon solar cell).  Current solar cell technology ranges from 22% for first-generation solar cells to about 15% for second-generation solar cells.

As solar nanocrystals are classed in the same generational group as dye-sensitized solar cells and quantum dot solar cells, this innovative and rapidly growing class of solar cells offers the promise of lower costs and a higher efficiency rating (in excess of 60%) in future solar cell devices.  However, the current status suffers from an electrical conductivity issue that requires attention in order for these solar cells to achieve their expected potential. 

Researchers at the University of Southern California (California, USA) have recently devised a solution to this particular drawback to using solar nanocrystals, with an additional benefit that affects the manufacture of these solar nanocrystals.

A Promising Candidate for Next-Generation Solar Cells: Liquid Solar Nanocrystals

In April 2012, Professor Richard Brutchey, of the Dornsife College of Letters, Arts and Science at the University of Southern California (USC), together with his postdoctoral researcher David Webber, developed an innovative method to counter the issue affecting the use of liquid solar cells.  Furthermore, this method allows for the production of these solar cells on either glass or plastic; this significant breakthrough will allow for manufacturers to control costs as the solar cells could be, in the future, roll-to-roll printed (like a newspaper) or just simply painted on glass.  Figure 1 is a photograph of a glass slide coated in a film of solar material.

Figure 1: Photograph of a glass slide coated with the semiconducting layer of cadmium selenide.  Credit: Dietmar Quistorf, University of Southern California.

Using cadmium selenide, a semiconducting material, Professor Brutchey and Dr Webber formed a stable liquid suspension in which the 4 nanometer (nm) crystals of cadmium selenide stuck together.  Figure 2 is a photograph of cadmium selenide in solution. 

Figure 2: Photograph showing a suspension of cadmium selenide.  Credit: Dietmar Quistorf, University of Southern California.

At this particular size, more than 250,000,000,000 (250 billion) nanocrystals could be fitted into a space no bigger than the head of a pin.  By suspending the 250 billion nanocrystals in a stable solution, roll-to-roll printing becomes a distinct reality. 

However, the concept of using paint to spray-on solar material is not new.  While many academic institutes (and industrial institutes) have toyed with the idea, progress has been limited resulting in solar paint being nothing more than a niche idea.  The biggest drawbacks to solar paint were the efficiency rating and how to direct the electricity out of the device.  While solar nanocrystals are cheaper to manufacture than traditional silicon-based solar cells, the primary purpose is to generate electricity.  By not being able to transfer the electricity out of the device, solar nanocrystals failed making their cost effectiveness moot.

Improving the Conductivity of Liquid Solar Nanocrystals

By clumping together in the liquid solution, the solar nanocrystals were able to form a stable suspension.  However, this in itself was the cause of the poor efficiency.  To reduce the impact of this clumping, an organic ligand molecule would normally have been added.  Unfortunately, this organic molecule further reduced the ability of the material to conduct electricity, making the whole system a good insulator.

To solve this issue, Professor Brutchey and Dr Webber chose a synthetic organic ligand molecule called 1,2,3,4-thiatriazole-5-thiolate.  As an anion in solution, this organic ligand strongly binds to the cadmium selenide nanocrystals and offers a higher degree of stability than previously selected organic molecules.  To their surprise, the synthetic ligand not only stabilized the solution of cadmium selenide but also improved the conductivity of the solar compound.

The use of this synthetic anionic ligand resulted in connections or bridges being made between the molecules of cadmium selenide, allowing the nanocrystals to potentially conduct electricity.  With this breakthrough, a pathway has been created that will allow solar paints to become more than just niche concepts.  A lot of further research will be required in order to make solar paint more mainstream, but with this anionic ligand molecule offering stability and conductivity, progress has been made on this long journey.   

An Alternative Manufacturing Substrate

An additional bonus to this discovery by Professor Brutchey and Dr Webber concerns the manufacturing of future solar panels.  Conventional solar cells make use of silicon, and utilize a multi-step process with some steps conducted in a high-temperature environment.  This might all change as the technique developed by the Brutchey Group at the University of Southern California offers a new manufacturing option.

The use of the organic ligand ensures the process is a low-temperature one.  This opens up the possibility that a different substrate could be used for the solar cell.  Traditionally, glass has been the most common substrate used for manufacture, as glass is able to handle the high temperatures involved during manufacture.  With this new low temperature process, a plastic substrate could be considered. 

Plastic substrates generally suffer from degradation issues as the typically high temperature of the solar module manufacturing process is beyond the tolerance parameters of plastic.  However, with the option of a low-temperature process on offer by using this anionic organic ligand, plastic substrates can now be a suitable choice.  Moreover, this new substrate choice is also less costly than glass, allowing manufacturers a cost-effective solution too. 

Another benefit to having a plastic substrate is that the shape of the finished solar module is flexible.  By using a plastic substrate with a coating of semiconducting nanocrystals, every surface is ripe for solar exploitation.  As some surfaces are irregular, such as those found in residential buildings, plastic-based solar cells and modules could be molded into any shape required.  This will greatly assist in the rollout of solar technology in residential buildings and areas.

Limitations on Using Cadmium Selenide

In spite of the promise offered by this discovery, a problem remains and this is down to the selection of cadmium selenide.  Elemental cadmium, and cadmium compounds, is extremely toxic.  With numerous research studies highlighting the adverse effects of cadmium on the human health system, the use of cadmium has been restricted for many years. 

Thankfully, Professor Brutchey and his research team are investigating suitable alternatives to cadmium selenide.  The hope is that by substituting cadmium selenide with a less toxic compound the technique could be commercialized and brought to market.  With the current advantages of cheaper manufacturing costs when using nanocrystals, flexibility in substrate selection, a breakthrough in conduction technology, and a promise of higher electrical efficiency rates, it is quite clear as to why determining a satisfactory non-cadmium compound is vitally important.

The Promising Future Applications of Nanocrystal Solar Paint

The discovery by researchers at the University of Southern California has provided an opportunity to further expand the use of solar technology.  While the prospect of commercialization is - at the moment - a distant one, the journey has begun and this looks likely to deliver promising results.

The commercialization of this solar paint will find use in a wide range of applications.  For example, the facades of residential and commercial buildings could be exploited for use as solar generators.  In addition, even the rooftops of buildings, commercial and residential, could be included in this plan.  Another application would be to coat the road transport and rail transport infrastructure with this solar paint, allowing for solar-derived electricity to be generated.  In countries that receive a significant amount of solar irradiation, this might be an elegant solution. 

By viewing the concept of matter as being versatile, by treating a solid as a liquid, researchers made an interesting discovery that holds great potential in advancing the field of solar energy technology.  With solar energy offering the greatest potential as an energy resource, such innovative thinking will be required in order to fully harness this unlimited and clean energy resource and make the transition to a clean energy future. 

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