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Making the Transition from Inorganic to Organic Solar Cells

published: 2012-01-13 9:00

The fluctuating price of energy, due to a variety of reasons ranging from geo-political constraints to national and international economic issues, and the fact that these energy resources are finite, can be seen as the main incentives to make a transition to a clean energy society.

Solar energy is just one of many ways to make this transition a reality, and of the raft of clean energy technologies available, solar energy technology does offer appealing prospects. With the fact that solar energy offers an inexhaustible supply that is literally pollution-free, and coupled to the fact that the planet receives more sunlight in one day than what is required to meet the energy demands of the world for one year, utilizing this resource will undeniably be good for the business community and for the public.

While solar energy has been harnessed for centuries, the modern approach has been to construct solar cells and solar module arrays using silicon; with the experience garnered from its use in the semiconductor industry, silicon was a natural choice for use in converting sunlight into usable electricity.

Current Status of Inorganic Solar Energy Technology

However, the high manufacturing cost and the rate of converting sunlight into electricity (known as the electrical energy conversion efficiency rate) are drawbacks that have impeded the widespread use of solar energy technology. Other factors exist as well, such as the time constraint associated with sunlight and the availability of the raw materials, but these are either impossible to change (number of hours of available sunlight) or tied into the main drawbacks of cost and efficiency (raw material availability). Improvements, therefore, are both warranted and necessary.

Thankfully, improvements have been made in order to reduce the manufacturing costs and improve the efficiency rate. From first generation silicon solar cells, which can be considered the technological backbone of the solar energy industry, second and third generations have been developed and deployed. One particular third-generation solar cell, the organic solar cell, is slated to offer a reduction in the cost of manufacture.

Incorporating Organic Material for Generating Electricity

The technology used for the manufacture of solar cells is predominantly silicon based, an inorganic material. The semiconductor industry developed a deep understanding of the properties of this material, allowing for an easy transfer for use in the solar cell as a solar cell practically mimics the operation of a semiconductor wafer, in that it allows for the transport of an electric current.

Initially starting out as organic pigments for use in certain applications, and then with the introduction of semiconducting polymers, these carbon-containing organic compounds were determined to be suitable for solar cell development. Early research indicated that the incorporation of these organic semiconducting polymers had a positive effect on the efficiency rate of the solar cell, so both industry and academia focused on accelerating the development of these novel solar cells.

As there is always a trade-off between efficiency and cost, so it is with the utilization of organic solar cells. The main difference, and somewhat a significant difference, is in the lower efficiency rate compared to an inorganic semiconducting solar cell, such as the traditional silicon-based versions. With organic solar cells, the charge carrier mobility is low and this leads to a lower efficiency rating. In spite of this, and despite possessing an optical band gap of 2 electron volts (eV), organic solar cells are promising due to their affinity for chemical modification (via chemical synthesis techniques), their low manufacturing cost, and the potential for large-scale manufacture. These three reasons have led to the push for organic solar cells.

Versions of Organic Solar Cells

In its simplest sense, an organic solar cell can be broken down into three components arranged as follows: a single (or double) organic layer sandwiched between two conductive electrodes. The organic layer basically acts as a carbon-based conjugated system, where one carbon atom is covalently bonded to other carbon atoms in the system via alternating single or double bonds.  The band gap, between 1 – 4 eV, is created by the difference in the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) in the organic layer.

There are four main categories when discussing organic solar cells and these all focus on the different structure of the organic layer: single-layer; bi-layer; bulk heterojunction; and graded heterojunction. Single layer organic solar cells are simple to fabricate but their drawbacks include a very low electrical efficiency and a dependence on a technique to produce excitons that is erratic. Bi-layer organic solar cells use two organic layers with sufficient differences in electron affinities and electron ionizations. However, as the thickness of the polymer layer needed in order to absorb light is greater than the diffusion length of the generated excitons, electron wastage is quite high.

Bulk heterojunction organic solar cells, and graded heterojunction organic solar cells, the electron donor and electron acceptor layers are mixed together to form an organic blend. This technique is promising as it allows for control of the organic architecture as well as offering a better environment for electron movement to take place.

Integrating More Carbon into the Organic Solar Cell   

While all four have their pros and cons and are directly advancing the field of organic solar cells, research has also focused on either improving or replacing the two electrodes with better or more suitable material. One such example is the use of carbon, a fairly inexpensive and ubiquitous raw material.

Carbon, in either its form of graphene or as carbon nanowires, has recently pushed organic solar cells from a novel and niche idea to one that could realistically make the switch to mainstream applications. The switch will not be on electrical efficiency rates but more on the manufacturing costs. By reducing the costs even further by using carbon, organic solar cells should find widespread interest for low-level applications and (low-cost) niche applications.

One area of interest for solar cell manufacturers and for academia is in tackling the use of indium for the electrode. Indium tin oxide (ITO) has been the electrode of choice for a long time. This brittle, inflexible, relatively low-transparency, limited (Indium is a raw material that is in scant supply), and expensive choice for an electrode has been targeted for replacement.

[a] Using Graphene

Late 2010, researchers at the Massachusetts Institute of Technology (MIT) managed to successfully offer an alternative. Professor Jing Kong and Professor Vladimir Bulović, from the Department of Electrical Engineering, used graphene (a sheet of carbon one atom thick and arranged like chicken wire) instead of ITO and found that the graphene acted in a similar way to ITO with no difference in function.

The unique properties of graphene allowed for an improvement over ITO with respect to transparency. Graphene is a transparent material and using this material would result in the solar cell capturing more light, improving the efficiency rate. In addition, graphene is flexible: the inference from this is that a graphene-based organic solar cell could be also manufactured using the roll-to-roll printing technique, keeping manufacturing costs low.

[b] Using Carbon Nanowires

Using carbon nanowires for solar cells has been a train of thought for some time. Due to their wide range of exceptional properties both mechanical and electrical, and due to the abundance of the raw material, carbon nanowires in solar cells have been touted as the future of solar cells.

In November 2011, Professor Yang Yang and Professor Paul Weiss from the Department of Materials Science and Engineering at University of California, Los Angels (UCLA) created an alternative to ITO using silver nanowires. The notion of using silver nanowires was put forward before but issues blighted the use of these nanowires. Yang and Weiss overcame these issues by using an innovative technique to offer silver nanowires as a replacement to ITO with good optical and electrical properties.

Previous research involving silver nanowires had resistance issues requiring a complex chemical treatment step. This in turn led to adhesion problems, resulting in the silver nanowire being an unsuitable replacement. Such drastic action usually results in the nanomaterial being unsuitable and the manufacturing costs increasing. The aim was to find a way to use silver nanowires but without the extreme temperature (200 degrees Celsius) and high pressure.

Professors Yand and Weiss found a novel solution that involved spraying on the silver nanowires and then treating the mesh with a titanium dioxide nanoparticle solution. On drying, the silver nanowires are pulled together and this has a positive effect on the nanowires: this pulling together, due to capillary forces, increases the conductivity of the silver nanowires. The next step involved coating the tightened silver nanowires with a conductive polymer mixture, allowing for surface adhesion.

The Organic Solar Cell Future

The method allowed for silver nanowires to be used as a replacement for ITO. What is more, research results indicated that the silver nanowire-based organic solar cell performed as well as the ITO-based solar cell. This is in addition to the positive results obtained using graphene as the electrode.

With no discernible difference in operation, the use of carbon nanowires or graphene offers a significant difference in associated costs: a reduction of manufacturing costs by between 100 and 1000 times can be expected. Carbon-electrode organic solar cells have a very promising future in applications ranging from coating windows to harvest sunlight to making good use of their flexibility and coat nooks and crannies in the architecture of a building. Carbon, the basic unit of life, may soon be a significant part of technology that provides electricity and thus ‘life’ as we know it.

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