A method that accelerates production and evaluation of materials for solar cells

A method for obtaining and evaluating organic materials for photovoltaic applications 50 times faster than conventional methods has been developed. The study has been published in the Advanced Electronic Materials journal and performed in the framework of the ERC project FOREMAT, led by Mariano Campoy-Quiles, scientist at the ICMAB-CSIC.

The fabrication platform enables the obtention of thin film sample that has different pixels with different thickness, or composition, or nanostructure. In the picture, samples obtained with the platform.Organic materials have a huge potential as the main ingredient in cost-effective clean energy technologies as they are abundant and non-toxic. These materials are being investigated as the active layer in devices that convert light or waste heat into electricity, i.e. photovoltaic and thermoelectric technologies.

The efficiency of these materials depends on properties related to their structure, solubility and the sidechains, among others. Every material is actually a mixture of different materials, and factors such as the proportion of each ingredient, how the ingredients are mixed forming the photovoltaic layers, among others, will affect the performance.
 
As there are myriad possibilities to modify these parameters, the list of potential materials is virtually infinite. How to know which parameters will give the best results?

“While theory can help to select promising candidates, the final performance often depends on a number of issues difficult to predict a priori, such as solubility, miscibility of compounds, tendency to crystallize…”, say the scientists. In practice, this means that for a given promising backbone, many materials and parameters need to be tested.

And before evaluating the materials, these have to be manufactured. “At lab scale, the fabrication of an organic solar cells takes between days to a few hours (if processed in parallel), while measuring its efficiency takes just minutes”, explains Mariano Campoy-Quiles, from the Institut de Ciència de Materials de Barcelona (ICMAB) of the CSIC. If a whole family of materials has to be evaluated changing each one of the key parameters, the time frame drastically increases.

This explains why in the last ten years, aproximately 5000 organic conjugated materials have been tested in BHJ solar cells, but only a few tenths have been studied and optimized in depth.

The platform combines several processing methods to obtain photovoltaic materials films controlling parameters, such as thickness, composition and nanostructure

In the framework of the ERC project FOREMAT, led by Mariano Campoy-Quiles, the scientists have developed a platform that combine several processing methods to obtain photovoltaic materials films controlling several parameters, such as thickness, composition and nanostructure.

The parameters are controlled and progresivally changed in order to obtain the gradient of  each one. The fabrication platform enables the obtention of thin film sample that has different pixels with different thickness, or composition, or nanostructure. All of the pixels or devices are produced in a single processing step where one or two parameters are monotonically modified.
 
On the right, correlation of composition, thickness of samples and efficiency.Afterwards, the samples are analyzed using photocurrent and Raman imaging in order to correlate one to one device performance and structural information.

In order to verify the strength of the developed technology, the scientists have validated the platform with three different families of materials, obtaining efficiencies approaching 4%, 6% and 10%, respectively, and using less than 100 mg of each polymer in the process.

The results raise many expectations, as it is 50 times faster and needs 90% less material than current evaluation methods. One of the samples produced by the platform is equivalent to more than 100 conventional samples.
 
The method can accelerate the development of photovoltaic organic materials. It is interesting for laboratories and R&D groups of solar cells or other devices based on thin films, from universities or companies.

Until now, no fundamental limits have been discovered suggesting that organic photovoltaic and thermoelectric materials with extremely high efficiency cannot be synthetized. So, theoretically, efficiencies of 50% or more could be reached for tandem geometries.

In the last ten years, aproximately 5000 organic conjugated materials have been tested in BHJ solar cells, but only a few tenths have been studied in depth

Highest efficiencies found until now are up to 17% in the case or organic materials, and  around 25% in the case of inorganic semiconductors. Although higher efficiencies have been obtained with inorganic semiconductors, the production methods of these materials require a lot of energy. “On the contrary, organic materials are obtained often from a solution and at low temperatures, therefore they are energetically more sustanaible”, adds Mariano Campoy-Quiles.

The study was funded by the Ministerio de Economía y Competitividad of Spain through the “Severo Ochoa”Programme for Centres of Excellence in R&D and by the European Research Council (ERC) project "Finding a needle in a haystack: efficient identification of high performing organic energy materials (FOREMAT)".