Making inkjet-printed organic electronic devices more efficient

Inkjet printing technology has been used in graphic arts since the 1970s to print barcodes, documents, banners and brochures. Of late, it is also being used to print organic solar cells to help produce electricity from sunlight that can be used to power equipment or recharge a battery.

The existing approaches to printing organic solar cells, however, are mainly focused on improving the efficiency of the cell by controlling the inkjet printing parameters such as drop spacing, drop volume, wave form, print head temperature, substrate temperature, and fluid properties such as viscosity and surface tension. Studies about the interaction of the ink with the base substrate, have been few and far between.

This is what prompted Sivakumar Ganesan, a research scholar with the IITB-Monash Research Academy, to take up a project titled, ‘Inkjet-printed organic electronic devices’ under the supervision of Dr Wei Shen and Dr Dipti Gupta.

“Inkjet printing is a non-impact printing process where minute droplets of ink are sprayed into the surface of the substrate,” explains Sivakumar. “The spreading of ink depends mainly on the surface tension of the ink and the surface energy of the substrate. For proper adhesion, it is critical that the latter is more than the former. Since organic solar cells are made up of a stack of multiple ink layers, the surface tension of each ink should be less than the previous printed layer.”

He adds, “It has become imperative to develop an alternative to fossil fuels such as coal, petrol and natural gas — which cause air pollution. Solar energy is the only energy that is not owned by any country or person!”

At present, silicon is the most widely-used material for the manufacture of solar cells. However, silicon is costly and the steps involved in the production process are cumbersome.

Organic solar cells are cheaper, more flexible, lighter, and can be produced by simple coating and printing techniques. Due to the short life span of organic solar cells, they are mainly being used to satisfy short term needs such as recharging batteries for laptops and smart phones, displays in super markets, toys, etc. It is perhaps time to examine whether they can also be used to manufacture large scale grid-connected power production solar modules to meet our day-to-day energy requirements.

“An organic solar cell is made up of a transparent front electrode, an active layer which converts photons from sunlight into holes and electrons, a back electrode, an electron transport layer and a hole transport layer,” says Sivakumar. “Only a few researchers have been able to print all the layers successfully and fabricate an organic solar cell with high efficiency. Inkjet printing is the ideal technology for the production of organic solar cells. For better performance of inkjet-printed organic solar cells, proper adhesion, morphology of the printed layers, and stability of the active layer polymer in ambient conditions are required. We are also working on the encapsulation of the organic solar cells. Proper encapsulation can reduce the degradation of the solar cell.”

The IITB-Monash Research Academy is a collaboration between India and Australia that endeavors to strengthen scientific relationships between the two countries. Graduate research scholars like Sivakumar study for a dually-badged PhD from both IIT Bombay and Monash University, spending time at both institutions to enrich their research experience.

Fig. 2: (a)Structure of organic solar cell and (b) Fabricated organic solar cell

Says Prof Murali Sastry, CEO of the Academy, “India is a tropical country; the average temperature here is between 30°C and 40°C. Hence, we must explore ways to harness the energy from the sun. Organic solar cells can be produced in any size and can be easily mounted on surfaces such as roofs of buildings, windows, automobiles, gardens, and even on spacecraft. The IITB-Monash Research Academy was conceived as a unique model for how two leading, globally focused academic organisations can come together in the spirit of collaboration to deliver solutions and outcomes to grand challenge research questions facing industry and society. This is, perhaps, the technology of the future, and we wish Sivakumar well in his endeavours.”