Organic photovoltaics: realising the potential

organic photovoltaics
Weighing of organic material: The research and development lab in Dresden, Germany, applies the organic materials to produce test cells. Only 1g of organic material is required to produce 1m2 of organic solar cells.Photo courtesy of Heliatek

By Paul Grad, engineering writer.

When conducting polymers were discovered, it was almost immediately realised one of their most promising applications was in photovoltaic cells. Soon smaller organic molecules were seen to be in some cases even better suited for use in photovoltaics.

Among the main advantages of organic photovoltaics (OPVs) in comparison with silicon solar cells, is that OPVs can be mass-produced at a low cost and in an environmentally friendly manner. They also exhibit superior low-light and high-temperature performance. However, they are considerably less efficient at converting light to electricity and they are also less durable than silicon cells.

If those drawbacks are overcome, OPVs will be cost-competitive with the grid and photovoltaic installations, already very popular in several countries, will really come of age as a major renewable energy alternative.

OPVs represent the third generation of solar energy technology.

The first generation, now used in rooftops around the world, is crystalline solar technology, using silicon wafers.

The second generation is thin-film solar technology, using CdTe (cadmium tellurium), CIGS (copper indium gallium diselenide) and a-Si (amorphous silicon) or micromorph PV.

The third generation, organic solar technology, uses three types of PV: OPV oligomers (smaller organic molecules), OPV polymers, and OPV DSSC (dye-sensitized solar cells).

OPVs are not yet a commercial reality on a large scale, but a few companies are developing OPVs and making rapid progress. One such company, Heliatek GmbH, of Dresden, Germany, recently announced a record-breaking 12 per cent cell efficiency for its organic solar cells, beating its own previous 10.7 per cent efficiency record. The company aims to achieve 15 per cent efficiency by 2015.

The current world record for energy conversion efficiency for a silicon solar cell is 25 per cent, held by the University of New South Wales.

Heliatek has built on the success of the oligomer approach in the OLED (organic light emitting diode) industry. It aims to manufacture solar cells on an industrial scale using a roll-to-roll process. This process uses vacuum deposition to apply small molecules to a substrate, in contrast to OPV technology using large molecules (polymers), which require solvents and printing processes.

Heliatek said the advantages of its roll-to-roll process include the non-toxic fabrication of high quality solar films and the high cost-saving potential with mass production.

Vacuum deposition of oligomers allows for extremely thin yet homogeneous layers down to millimetres. This process allows depositing a large number of layers on top of each other, to absorb a broader spectrum of light.

Other companies producing OPVs include Sigma-Aldrich Corporation, of St Louis, Missouri; Plextronics of Pittsburgh, Pennsylvania; and Solarmer Energy of El Monte, California.

In Australia, Dyesol of Queanbeyan, New South Wales, is a global supplier of dye solar cell (DSC) materials, technology and know-how. DSC is a photovoltaic technology enabling metal, glass and polymeric based products in the building, transport and electronics sector to generate energy and improve energy efficiency. The company has a working relationship with the OPV group at the University of New South Wales.

Sigma-Aldrich provides a wide range of organic semiconductor products for use in OPVs and OLEDs.

Plextronics, founded in 2002 as a spin-out from Carnegie-Mellon University, is developing technology enabling broad market commercialisation of organic electronic devices. The company has installed a pilot-production line for OLED displays and organic solar cells.

Solarmer is developing OPV technology targeting portable power, off-grid power, and building integrated PV markets. The company also employs the roll-to-roll process technology.

In Australia there is a strong research effort in the field of OPVs. A now famous centre for work on photovoltaics at the University of New South Wales was developed under the leadership of Prof Martin Green. The university now has a group working on OPVs, headed by Prof Ashraf Uddin.

CSIRO also has a research effort on OPVs, led by Dr Gerry Wilson (theme leader, flexible electronics) and Dr Scott Watkins (stream leader, organic photovoltaics).

CSIRO is a member of the US-Australia Institute for Advanced Photovoltaics, together with five Australian universities, twelve international universities, three US laboratories, and four industry partners.

Within Australia, the Australian Centre for Advanced Photovoltaics (ACAP) is a Strategic Research Initiative established through funding from the Australian Solar Institute. ACAP involves all key Australian photovoltaic research groups.

The Victoria Organic Solar Cell Consortium (VICOSC) is a collaboration of academia and industry, with funding from the Victorian Government’s Department of Primary Industries and the Australian Solar Institute.

Currently many Australian homes have photovoltaic panels installed on the roof, and in some cases they are able to sell electricity back to the grid.

The Australian utilities are generally happy with this, but the introduction of residential generation onto electricity networks can create problems because current networks were designed to enable electricity to flow in one direction – from power station to end user.

Therefore, Australian utilities generally require that customers wanting to install solar PV must complete an application for network connection. Permission to connect PV installations to the grid depends on a number of issues including voltage and capacity constraints on the network servicing the address of the connection and how many other customers on the same network are already connected.

For example, south-east Queensland is served by Energex and the rest of the state is served by Ergon Energy. According to Ergon Energy, if your home is connected to the grid and you use all of the power generated from a 1.5kW system, you will save about $575 each year in electricity you don’t have to buy. The Queensland Government’s Solar Bonus Scheme pays eligible households for any excess energy exported from their solar PV system to the electricity network.

For customers eligible to receive the 8c/kWh Feed-in Tariff Benefit, it is best to use most of their electricity to match the times their PV system is generating.

For customers eligible to receive the 44c/kWh Feed-in Tariff Benefit, they will save money by connecting appliances to an Economy tariff. Economy tariffs are recorded on a meter other than that used for Feed-in Tariff calculations. By connecting to an Economy tariff the customer will pay a discounted rate for the energy used by those appliances outside peak times while taking greater advantage of the Feed-in tariff benefit.

Many PV system suppliers can offer discounts under the Australian Government’s “Small-scale renewable energy scheme”.

Aurora Energy, the utility serving Tasmania, also buys back the excess energy produced by the customer’s PV installation. For PV systems under 10kW the rate is the same as the tariff the customer pays Aurora for the energy he or she uses.

Ausgrid, which delivers electricity to Sydney, the NSW Central Coast and the Hunter regions, also supports customers who wish to install PV systems in their homes and also has Feed-in tariffs.

Citipower and Powercor, with combined networks delivering electricity to customers throughout Victoria, also support customers who wish to connect environmentally friendly sources of generation to their network, including solar, hydro or wind power. The utilities supply detailed guidelines for connecting a generator to their network.

The much lower manufacturing cost of OPVs, as compared with silicon cells, could soon drastically lower our electricity bills.