Making the transition to a clean energy society is in the best interests of consumers, industry, and the environment. This transition, however, is not trouble-free. The current technology has not yet reached the point where such a transition could be effectively implemented, but discoveries and advancements made in academia and in industry are reducing the distance between the present and that transition point.

Solar energy offers a frustrating example into issues affecting the complete harnessing of a renewable energy resource. Solar energy is one of the cleanest and most abundant natural energy resources. An indication of its abundance is illustrated in that more sunlight strikes the surface of the earth in a single day than what is required to sustain the energy requirements of the world.

However, the technology available to utilize this powerful energy resource is limited by two major interdependent constraints. The first constraint is the low electrical energy conversion efficiency rate (shortened to the efficiency rate) and the second constraint is the high cost. Advancements usually come to one factor, which sometimes has the knock-on effect of altering the other. Rare is the occasion when a development could considerably affect both at the same time. By changing the face of the solar cell unit, and also by using carbon nanotubes to build the next generation of solar cells, these two developments could well be the ones to provide a solution to the full and complete deployment of solar energy.

Carbon Nanotubes for a Cleaner Solar Cell
The third form of carbon has shown great promise in significantly altering the landscape of many industries. This form, known as carbon-60 (C-60) or buckminsterfullerene, is packed with many exciting properties both electrical and mechanical and has the potential to completely change numerous technologies and applications.

Carbon nanotubes, elongated versions of C-60, possess an immense tensile strength that is fifty times greater than steel while being one-ten thousandth the thickness of a single strand of human hair. In terms of their electrical properties, carbon nanotubes are extremely efficient electron transporters and thermal conductors. These properties make for interesting applications based on integrating carbon nanotubes, especially for solar cells.

Carbon is the base material for all organic life. Adopting this material for the next generation of solar cells, replacing the current crop of silicon-based first and second-generation solar cells, could achieve much more than just increasing the efficiency rate and lowering the costs associated with manufacture. Using carbon as the raw material for solar cells could also reduce / replace other elements in the solar cell supply chain and manufacturing process chain that might not be environmentally-friendly. This will make the carbon-based solar cell truly ‘clean’ and ‘renewable’. By using one particular form of carbon, the third form, this radical change is indeed achievable.

The Limitation of 2D Solar Cells
Traditionally, solar cells are flat and uniformly shaped. These two-dimensional structures have been the mainstay of solar energy technology. From this first-generation technology, progress was made in making the solar cells thinner and using different elements, such as cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) thin films. These thin film second-generation solar cells reduced the manufacturing costs but were not comparable to the efficiency of first-generation solar cells. Progress has been made to reduce this efficiency difference but the gap still remains.

Evolving from 2D Solar Cells to 3D Solar Cells
By redesigning the concept of the solar cell and of the actual physical structure, researchers have developed novel, three-dimensional solar cells that are practically changing the face of the solar industry. In 2007, researchers at the Georgia Institute of Technology (USA) released their model 3D solar cell. The research group, led by Dr Jud Ready, changed the dimensionality of the solar cell from the 2D planar version to a 3D one, and in so doing, boosted the efficiency rate of the solar cell.

When sunlight strikes the solar cell unit, three outcomes are possible: the first is that the incident sunlight is reflected; a second possibility is that a photon from the incident sunlight is absorbed, kick-starting the process of generating electricity; and the final possibility is that the incident sunlight passes right through the solar cell. The basis of the 3D solar cells designed by Dr Jud Ready’s research team focused on the fact that for planar solar cells, the majority of incident sunlight is reflected. The expectation was that by changing the structure one might produce a solar cell with a better absorption rate.

Fabrication of 3D Solar Cells
In terms of fabrication, the research team used a silicon wafer to form the base of the solar cell, and incidentally, this base also doubled as the back contact of the solar cell. A thin patterned layer of iron is deposited onto the silicon wafer by photolithography, and then placed in a furnace with hydrocarbon gases flowing through the furnace. These hydrocarbon gases, through the process of chemical vapor deposition, allowed for the growth of carbon nanotubes on the surface of the patterned iron forming arrays of carbon nanotube ‘towers’. Figure 1 shows electron micrographs of the carbon nanotube arrays deposited on a silicon wafer.

The carbon nanotube array was then coated with n-type and p-type semiconducting layers, such as CdTe and cadmium selenide (CdSe). To this was added a thin coating of a transparent conductive layer (TCO) like indium tin oxide (ITO), completing the solar cell. Figure 2 shows an electron micrograph of the carbon nanotube complete with a semiconducting layer of CdTe.

The Benefits of 3D Solar Cells
The solar cell made use of thousands of carbon nanotube ‘towers’ or ‘cones’ to trap incident sunlight for longer with the expectation that trapped sunlight would eventually start the electron-ejection process to generate electricity without being reflected. This light, unable to escape, would then lead to a boost in the efficiency rate as more electron-ejecting processes were started.

The novel and unique 3D solar cells designed by Dr Ready’s team captured nearly all of the incoming light. This significant improvement allows for the possibility of higher efficiency rates, but due to the use of carbon nanotubes, other benefits make 3D solar cells attractive to the solar energy industry.

With the carbon nanotube array forming the integral part of the solar cell, Dr Ready’s research group at Georgia Institute of Technology were also able to reduce the size and weight of the solar cell. Figure 3 shows a single, complete 3D solar cell designed by Ready’s team.

Due to the unique mechanical and electrical properties of carbon nanotubes, the team were able to devise a solar cell that operated just like a 2D planar solar cell but at a fraction of the size. This feature should be particularly interesting to solar cell manufacturers, as this reduction in size will reduce manufacturing costs.

Another benefit of employing 3D solar cells over conventional 2D solar cells comes with a reduction in mechanical parts. Planar solar cells, from first-generation to second-generation, need to face the sun. These 2D solar cells perform best when the sun is at ‘high noon’ (roughly at noon), when the sun is at the right angle and sunlight strikes the surface directly. When the angle changes, there will be a drop in the efficiency rate of the solar cell. One solution was to add trackers to the solar cell unit, allowing the unit to track the movement and thus increase the chance of sunlight absorption. However, this added feature increased manufacturing costs and added weight to the overall solar cell unit.

By adopting 3D solar cells, trackers on the solar cell unit could be eliminated as the carbon nanotube-based solar cell will, due to its inherent structure, trap sunlight within its array and reduce the amount of incident sunlight reflected back into the atmosphere. In addition, the carbon nanotube array actually works best at angles not associated with high noon. For example, at high noon, the 3D solar cell had an efficiency rating of 3.5 % but when the angle changed to 45 degrees, the efficiency rating doubled to 7 %. This opens up the prospect of 3D solar cells being used when the light is diffuse, opening up the prospect of being deployed in areas outside the Sunbelt (+/- 35 degrees of the equator). Furthermore, by not requiring trackers, the manufacturing costs will be reduced; a feature that will appeal to solar cell manufacturers.

Non-Carbon-Nanotube Based 3D Solar Cells
Further research is required to make the carbon nanotube-based 3D solar cells comparable to first-generation 2D solar cells, but the work of Dr Ready and his team at Georgia institute of Technology has shown the promise of using 3D solar cells. Since 2007, other research teams have also focused on 3D solar cells and have researched alternatives to using carbon nanotubes. Of the different research groups investigating 3D solar cells, the work done by Professor Jeffrey Grossman of the Massachusetts Institute of Technology (USA) and the work carried out by scientists at the Department of Energy’s Oak Ridge National Laboratory (USA) will be further discussed.

[i] Innovative 3D Structures
Professor J Grossman, at the Department of Materials Science and Engineering, was inspired to delve into this area by his groundbreaking work on energy storage devices based on nature. With Marco Bernardi, a graduate student in Professor Grossman’s research group, a computer program was designed to mimic biological evolution and illustrate the results, as shown by the 3D models in Figure 4.

The images generated were found to be better at harvesting sunlight for longer periods of time. Additionally, these designs were found to generate power at a constant rate and would produce more power for the same space occupied by traditional planar solar cells.

Further work is required before these innovative designs become a reality in the manufacture of solar cell, but they do offer a promising glimpse into the future evolution of solar cells. If solar cells of the future were manufactured and distributed in a flat-pack style, this would reduce the associated costs for solar cell manufacturers. And if Professor Grossman’s concept of a flat-pack solar cell that required only unfolding to become a fully operation solar cell became reality, this would further reduce the associated costs for manufacturers by reducing the costs of installation.

[ii] 3D Solar Nanocones
The researchers at the Oak Ridge Laboratory, led by Dr Jun Xu, designed a 3D solar cell without the use of carbon nanotubes. The solar cell employed n-type ‘nanocones’ of zinc oxide and a p-type layer of polycrystalline CdTe covered this n-type layer. A transparent conductive oxide surrounded the n-type and p-type components and a glass base was used to complete the solar cell. Figure 5 shows the novel 3D solar cell.

This carbon nanotube-free solar cell had a stated light-to-power efficiency boost of 80 % when compared to 2D models using the same materials, from 1.8 % for 2D to 3.2 % for 3D. The solar cell was designed to promote efficient charge transport and a benefit of this was that the solar cell is able to tolerate any defects in the composition of the solar cell; therefore, any defects on the solar cell would not affect the efficiency of the solar cell.

This unique solar cell will be of interest to manufacturers as the raw materials are inexpensive and the manufacturing techniques are proprietary-based, but reasonably priced. This might help in reducing the operating costs of producing such cells. However, further research is required to have a comparable energy efficiency rate with conventional solar cells before these 3D solar cells are ready for the market.

Current Commercial Use of 3D Solar Cells
The move to 3D solar cells from the current planar versions is inevitable, primarily due to the potentially lower manufacturing costs and comparable – if not higher - efficiency rates. Utilizing inexpensive raw materials such as carbon (in the form of carbon nanotubes) and zinc oxide, coupled with a manufacturing process chain that requires no modification to produce these three dimensional solar cells, and the solar energy industry has the perfect development offering low-cost high-efficiency solar cells.

One example of an attempt to commercialize this solar cell advancement is Solar3D, a solar cell manufacturer based in Santa Barbara, California (USA). Solar3D has developed, and is about to mass-produce, their 3D solar cell utilizing micro photovoltaic structures on the surface of the solar cell to capture more of the light. Solar3D has stated the efficiency of its design will be comparable to the theoretical maximum of 29 %, and by using standard semiconducting manufacturing processes, will be able to further reduce costs associated with the manufacture of these 3D solar cells.

Another example of 3D technology being explored is the partnership between SUSS Microtec (a supplier to the semiconductor industry) and Rolith (developer of advanced nanostructured products for the green energy industry). This partnership will explore the development of 3D solar cells and also further developments in building materials for building-integrated photovoltaic (BIPV) projects; one interesting route might be to see if 3D solar cells could be mixed with BIPV materials.

Future Prospects of 3D Solar Cells
Regardless if the area of interest is in solar cell manufacturing or in nanostructured products, be it Solar3D, Rolith, SUSS Microtec or global industrial entities such as Trina Solar, Motech, JA Solar, Kyocera and Suntech, the prospect of reducing costs while still offering highly efficient solar cells should be a tantalizing one. Either by working directly with the research groups mentioned or by licensing the technology for 3D solar cells, these solar cell manufacturers could be in a win-win situation: providing clean and renewable energy while turning a profit.