Boosting solar cell efficiency

21st February 2013

In collaboration with Satoshi Uchida at the University of Tokyo, Michael Grätzel and his research group at the Swiss Federal Institute of Technology in Lausanne have now developed new sensitisers that should help an inexpensive type of solar cell to be more efficient. As they report in the journal Angewandte Chemie (2008, 47, No 10), the sensitisers are based on the dye indoline. 

Some years ago, Grätzel developed photoelectrochemical solar cells that are inexpensive, easy to produce, and able to withstand long exposure to light and heat. These ‘Grätzel cells’ contain a mesoscopic layer of titanium oxide (TiO2) particles coated with a sensitising dye. 

Upon irradiation with light, electrons are injected from the dye adsorbed on the TiO2, which are then transferred to the conducting band of the TiO2 and collected at the back contact, and carried away by an external circuit. In order for the cell to work, the electrons that are injected into the TiO2 must not recombine with the oxidised dye. 

To prevent this, the cell contains an electrolyte solution with negatively charged iodide and triiodide ions as a redox couple dissolved in a solvent, which immediately reduce the holes created in the dye. The main disadvantage of using volatile organic solvents in the electrolyte is the need for encapsulation of the electrolyte. 

Ionic liquids are an alternative to the use of these volatile solvents. These salts exist as liquids at low temperatures and do not evaporate. However, the high viscosity of these electrolytes is detrimental to the mass transport and consequently a problem for obtaining high efficiency. 

Grätzel and his team compensated for this loss of efficiency by optimising the sensitiser. In place of the usual ruthenium dyes, they used tailor-made organic dyes based on indoline, which have a higher molar extinction coefficient. This allows the TiO2 films to be thinner, in turn reducing the electron path length. The combination thus attained an energy conversion yield of 7.2percent. This is a record for this type of cell (organic dye, ionic liquid, titanium oxide). 

In this case the efficiency of the dye as a sensitiser is not only dependent on its chromophore, but also on its interfacial properties. So using a dye with an additional hydrocarbon chain has improved the performance by retarding the back electron reaction.

A US coating solution

The energy from sunlight falling on only nine per cent of California’s Mojave Desert could power all US electricity needs if the energy could be efficiently harvested, according to some estimates. Unfortunately, current-generation solar cell technologies are too expensive and inefficient for wide-scale commercial applications. 

A team of Northwestern University researchers has developed a new anode coating strategy that significantly enhances the efficiency of solar energy power conversion. A paper about the work, which focuses on ‘engineering’ organic material-electrode interfaces in bulk-heterojunction organic solar cells, has just been published online in the Proceedings of the National Academy of Sciences (PNAS). 

This breakthrough in solar energy conversion promises to bring researchers and developers worldwide closer to the goal of producing cheaper, more manufacturable and more easily implemented solar cells. Such technology would greatly reduce dependence on burning fossil fuels for electricity production as well as reduce the combustion product: carbon dioxide, a global warming greenhouse gas. 

Tobin J Marks, the Vladimir N Ipatieff research professor in chemistry in the Weinberg College of Arts and Sciences and professor of materials science and engineering, and Robert Chang, professor of materials science and engineering in the McCormick School of Engineering and Applied Science, led the research team. Other Northwestern team members were researcher Bruce Buchholz and graduate students Michael D Irwin and Alexander W Hains. 

Of the new solar energy conversion technologies on the horizon, solar cells fabricated from plastic-like organic materials are attractive because they could be printed cheaply and quickly by a process similar to printing a newspaper (roll-to-roll processing). 

To date, the most successful type of plastic photovoltaic cell is called a ‘bulk-heterojunction cell’. This cell utilises a layer consisting of a mixture of a semiconducting polymer (an electron donor) and a fullerene (an electron acceptor) sandwiched between two electrodes – one a transparent electrically conducting electrode (the anode, which is usually a tin-doped indium oxide) and a metal (the cathode), such as aluminum. 

When light enters through the transparent conducting electrode and strikes the light-absorbing polymer layer, electricity flows due to formation of pairs of electrons and holes that separate and move to the cathode and anode, respectively. These moving charges are the electrical current (photocurrent) generated by the cell and are collected by the two electrodes, assuming that each type of charge can readily traverse the interface between the polymer-fullerene active layer and the correct electrode to carry away the charge – a significant challenge.

The Northwestern researchers employed a laser deposition technique that coats the anode with a very thin (5–10nanometres thick) and smooth layer of nickel oxide. This material is an excellent conductor for extracting holes from the irradiated cell but, equally important, is an efficient 'blocker' which prevents misdirected electrons from straying to the ‘wrong’ electrode (the anode), which would compromise the cell energy conversion efficiency (Fig.1). 

In contrast to earlier approaches for anode coating, the Northwestern nickel oxide coating is cheap, electrically homogeneous and non-corrosive. In the case of model bulk-heterojunction cells, the Northwestern team has increased the cell voltage by approximately 40percent and the power conversion efficiency from approximately 3 to 4percent to 5.2 to 5.6percent. 

The researchers currently are working on further tuning the anode coating technique for increased hole extraction and electron blocking efficiency and moving to production-scaling experiments on flexible substrates.

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