Since the Industrial Revolution, our energy needs have been met primarily through the burning of polluting fossil fuels, with each century producing a more power-hungry generation than the previous. While this underpinned the rapid development of society through the ages, bringing unparalleled economic gains, burning fossil fuels came at an unacceptable environmental cost.
Transitioning to a clean energy society is a sensible evolutionary step to secure our long-term energy supply. While there is a case to be made for using better technology to extract and utilize fossil fuels in the interim, the undeniable fact is that carbon-based fuels are finite. Therefore, harnessing the power of renewable energy resources to meet the demands of our energy-intensive societies is a long overdue step.
To make this transition a reality, assessments must be made of the power and potential of the natural energy resources we have at out disposal. One of the most attractive options is to develop solar power. This clean energy source has the potential to power society alone; such is the abundance of sunlight received. Using the trinity of solar photovoltaic (PV), solar thermal, and concentrator technologies, sunlight is being converted into useful electricity.
The Limitations with Current Solar Energy Technology
Solar energy technology, for all its promise, suffers from two major issues preventing the widespread adoption of this technology. These two issues are the cost of manufacture and the electrical energy efficiency conversion rate. Only by addressing these issues can solar energy technology move forward and realise its true potential, especially when discussing solar PV and solar concentrators.
The chief workhorse of solar energy technology is the silicon-based solar cell, the first generation of solar PV technology. With an average electrical efficiency rate of around 20 %, this solar cell has been widely used for numerous rooftop and ground applications. With an efficiency rate approaching the theoretical limit of solar cells using a p-n junction (known as the Shockley–Queisser limit), and with manufacturing costs stubbornly high, attention was then focused on producing solar cells that could produce similar efficiency rates but at a fraction of the cost.
Thin film solar cells, a group that collectively makes up the second generation of solar cells, provided the manufacturing gains asked by industry while still providing acceptable efficiency rates (currently, between 15 % and 17 %). The family of thin film solar cells can be viewed as being either silicon-based or non-silicon based. Non- silicon-based thin film solar cell technology concerns the use of copper, indium, gallium, and selenide to form CIGS.
This remarkable version holds a lot of promise as the manufacturing costs are low and the efficiency rates are climbing. However, a rather different and innovative thin film solar cell (which actually is a member of the third generation of solar cell technologies) is promising to completely alter the solar industry.
Introducing DSSC in Advancing Solar Energy Technology
Dye-Sensitized Solar Cells (DSSC) first gained attention more than thirty years ago, but those cells were concerned with using a chlorophyll-sensitized zinc oxide electrode. Based on the premise that an illuminated organic dye could generate electricity at the oxide electrode, dye-sensitized cells were investigated and expanded upon to prove the principle that photons could be converted into an electric current in dye molecules.
With an efficiency rate of roughly 1 %, this chlorophyll-based DSSC was poor and this was due to the structure of the cell. The dye molecule layer was only able to absorb about 1% of the light, severely hindering the commercialization of this technology. Progress came in the form of a DSSC with an efficiency rating of 11 %, created in 1991 by Micheal Grätzel and Brian O'Regan of the École Polytechnique Fédérale de Lausanne (Switzerland).
Grätzel, who incidentally was awarded the Millennium Technology Prize in 2010 for his work, and O'Regan overcame this limiting issue by introducing a porous titanium dioxide nanoparticle layer. This titanium dioxide nanoparticle layer (which acts as the anode), covered in an organic molecular dye and immersed in an electrolyte solution (photosensitive ruthenium-polypyridine dye), is placed next to a platinum catalyst (which acts as a cathode) in a schematic analogous to a modern alkaline battery. Figure 1 is an illustration of the DSSC created by Grätzel.
Figure 1: Illustration of the dye-sensitized solar cell by Grätzel and O’Regan in 1991 showing the electron pathway. Credit: Laboratory of Photonics and Interfaces, École Polytechnique Fédérale de Lausanne.
As sunlight strikes the molecular dye, after passing through the transparent electrode, an electron is ejected and makes its way through the titanium dioxide nanoparticle layer. The electrons flow towards the transparent electrode where load collecting takes place before flowing on to an external circuit. After this, the electrons are reintroduced into the cell through the counter electrode where the electrolyte transports the electrons to the dye molecule.
Advantages to Using DSSC Solar Energy Technology
While improving the efficiency rating is one goal of the improvements to a solar cell, the other aim is to lower the manufacturing costs. By using the DSSC created by Grätzel, the efficiency rating was improved dramatically from 1 % to 11 % but this pales in comparison to first-generation silicon solar cells. Through time, the expectation is that this efficiency rating will further rise; this belief was featured in the European Union Photovoltaic Roadmap, indicating the role this pioneering technology has in contributing to the move away from finite sources of fuel.
Unlike traditional silicon-based solar cells, these dye-sensitized cells require no complex manufacturing processes and the raw materials are inexpensive, leading to a lower manufacturing cost. Similar to other thin film solar cells, the DSSC created by Grätzel is mechanically strong yet lightweight. By being mechanically robust, Grätzel’s dye-sensitized cell also experiences higher efficiencies at higher temperatures. Conventional silicon solar cells are sensitive to the elements, resulting in protective measures being forced upon the cell such as encasing the silicon solar cell in a glass box.
At higher temperatures, more electrons are produced into the conduction band of the semiconductor. As silicon solar cells are not able to dissipate the build-up, this leads to an increase in the internal temperature. For silicon solar cells, an increase in the internal temperature leads to a decrease in the efficiency rate.
DSSCs are thin and mechanically robust allowing for heat to be radiated quickly and efficiently, which avoids the problem faced by traditional silicon-based solar cells. These two features make DSSCs suitable for low-density applications, such as rooftop collectors. Being a member of the thin film family of solar cells, Grätzel’s solar cell can also be used in Building Integrated Photovoltaic (BIPV) applications.
One extremely interesting advantage is that DSSCs work in low-light conditions. Due to their very favorable electrochemical kinetics, DSSCs do not share the same cutoff point as other solar cells in terms of charge carrier mobility and recombination. This ensures that DSSCs will work even when the sunlight is diffuse and even when there is no direct sunlight.
Disadvantages to Using DSSC Solar Energy Technology
Even though DSSCs are promising, issues with their handling have slowed their adoption. The liquid electrolyte suffers from stability problems: at low temperatures the liquid electrolyte can free which will then halt the electron production process, and at high temperatures the liquid expands.
Another drawback is with the molecular dye. To fully utilize the potential these cells offer the dye needs to absorb from a broad part of the electromagnetic (EM) spectrum. Currently, there is a lack of organic dyes capable of covering the EM spectrum.
The cost of the organic molecular dye, the platinum catalyst, and the conducting glass are variables that need to be controlled to ensure DSSCs remain cost-effective solar cells. As these three components are not fixed, this could be viewed as a disadvantage.
To develop this solar cell further, research has produced numerous advancements to overcome these problems and make DSSCs suitable for widespread utilization. Progress has been in both improving the efficiency rate as well as finding ways to reduce the manufacturing cost.
Current DSSC Research Progress Contributions
In terms of solar energy technology, the development by Grätzel of a dye-sensitized cell can be considered one of the most important advancements in the field; untreated titanium dioxide absorbs in the ultraviolet region of the EM spectrum, but using titanium dioxide and a dye molecule allows the solar cell to absorb in the visible light region of the EM spectrum as well. Since 1991, a lot of research attention has gone into improving its efficiency from its current high of a fraction over 11 % as well as improving the design.
Professor Arie Zaban of the Department of Chemistry at Bar-Ilan University focused on the design of the solar cell as the energy transfer was not ideal due to the distance between the donor dye molecules and the charge separating dye molecules. Using inorganic semiconductor nanocrystals, effectively quantum dots, Zaban, together with Dr Dan Oron from the Physics department of the Weizmann Institute of Science (Israel),produced a Grätzel cell (another name for this type of dye-sensitized solar cell) with increased light absorption features and a broader spectral absorption.
By understanding the Förster resonance energy transfer (FRET) process, which describes the energy transfers that take place between two chromophores, the quantum dots allowed for an increase in the number of photons available to the organic dye for use in the solar cell. By acting as an antenna, the quantum dots become part of the solid electrode and at a fixed distance that maximizes efficient energy transfer. The knock-on effect is that photostability of the solar cell improves.
Reducing the Manufacturing Cost of DSSCs
In 2010, researchers at the École Polytechnique Fédérale de Lausanne and at the Université du Québec à Montréal determined ways in which to reduce some of the drawbacks associated with DSSCs. Professor Benoît Marsan at the Depertment of Chemistry of the Université du Québec à Montréal devised a solar cell based on a DSSC but replaced components that will improve the design and cost-efficiency of the DSSC.
Professor Marsan’s dye-sensitized cell contains a hybrid electrolyte that is both non-corrosive and transparent. This allows for better stability and for an increase in the cell’s efficiency rate. To reduce costs further, Professor Marsan replaced the platinum cathode with cobalt sulfide. In addition to being inexpensive compared to platinum, cobalt sulfide is also stable and easy to manufacture in a laboratory or on an industrial scale.
With an efficiency rate of 6.4 %, this patented electrochemical solar cell is far from competing with first-generation solar cells but thanks to its durability and roll-to-roll printing technique, this innovative solar cell is attractive for manufacturers wishing to exploit low-light applications.
Commercialization of DSSC Applications in the Solar Sector
Many industrial entities have focused on DSSCs due to the promise they hold. Low-cost, manufacturing ease, an acceptable efficiency rate, and a variety of real-world applications have made DSSCs a serious contender in terms of solar energy technology.
Sony Corporation started researching DSSC in 2001 as the company believed in the technology and the advantages over silicon. For example, the ability to use different dyes to produce different colors and their low-light ability meant these solar cells had applications both as a candidate for BIPV applications but also home energy management system sensors.
Using their patented ‘Concerto Effect’, Sony produced a DSSC in 2010 with an efficiency rating of 9.9 %; an efficiency rating of 10 % or above is considered acceptable for manufacture due to the lower costs involved with DSSCs. The Concerto Effect is based on modifying the components in such as way as to deliver the best possible output.
Mixing two dyes of different wavelengths allows for an expansion in the absorption ability of the cell. By doing so, the efficiency rate improved as better absorption will lead to enhanced absorption densities on the titanium dioxide layer. This is the Concerto Effect.
Another company making waves in the field of DSSC is SolarPrint. The company, based in Ireland, is focused on making home-based DSSC applications such as intelligent management systems. By innovating with the constituents of the molecular dye, SolarPrint has managed to produce intelligent sensors for the home that will help reduce the power consumption of a building by up to 50 %.
Utilizing the DSSC characteristic of harvesting diffuse or ambient light, and making use of the fact that manufacturing these cells uses the screen-printing technique, SolarPrint has manufactured these intelligent home management sensors to make the home energy-efficient as 40 % of carbon emissions is due to the built environment.
The Future Outlook for DSSCs
If a solar cell can be manufactured inexpensively and yet still maintain a high degree of mechanical robustness, then the future of that solar cell is secure. Due to its flexible nature and acceptable levels of efficiency, DSSCs can envisage applications ranging from home and car rooftops to window glass panes and even home intelligent sensors.
With manufacturers in Taiwan such as Everlight Chemical Industrial Corporation and Taiwan DSC PV Corporation (of the Jintex Group) active in the field of DSSC, and with a domestic market ripe for DSSC applications to be rolled out, this resourceful and inspired dye-based solar cell could certainly rethink our concept of a solar cell as well as our view of the environment around us.
