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Organic Solar Cells Understanding microstructure of PV materials

Researchers at the University of Surrey (UK) have achieved record power conversion efficiencies for large-area organic photovoltaics (OPVs). 

The research was led by the University of Surrey's Advanced Technology Institute (ATI) in collaboration with Oxford University (UK), the Aristotle University of Thessaloniki (Greece), and the University of Stuttgart (Germany). 

The results are unveiled in the paper “Achieving 6.7% Efficiency in P3HT/Indene-CBisadduct Solar Cells through the Control of Vertical Volume Fraction Distribution and Optimized Regio-Isomer Ratios,” published in Advanced Electronic Materials. The article establishes that dependencies between the chemical and physical properties of the photoactive layer's building blocks within organic solar cells determine the efficiency of these solar cells.

Importance of photoactive layer’s microstructure

“We show in our paper the importance of the microstructure and thus the molecular packing of the photoactive layer of the hybrid material, which ultimately dictates the overall device power conversion efficiency,” elaborates study co-author Professor Ravi Silva, who is also director of the Advanced Technology Institute (ATI) and head of the Nano-Electronics Centre at Surrey. “There is a difference in the crystallization transformation of the blend between P3HT and the regio-isomeric mixture of IC70BA, depending on the fabrication process used.”

Based on these findings, Silva and his colleagues have determined the optimal fabrication conditions for the production of organic photovoltaics with power conversion efficiencies above 6.7% for a particular regio-isomeric distribution.

A new approach

The study illustrates the sensitivity of the spatial molecular packing of the bulk heterojunction (BHJ) between donor materials and isomeric fullerene acceptors, with different isomeric fractions of IC70BA. “As such,” the professor interprets, “we demonstrate how the solar cell characteristics can be improved in the future based on an understanding of the formation kinetics of the BHJ phase separation, which allows for the fabrication of highly efficient OPVs.”

Silva explains why dependencies between the chemical and physical properties of the photoactive layer's building blocks determine the efficiency of organic solar cells: “The chemical and physical properties of the photoactive layer’s blocks (the donor and acceptor materials) are in a close interaction with the BHJ phase separation and therefore play an important role for the number of photons that can be efficiently absorbed in the BHJ,” says the professor, further explaining that this photon absorption is only the first step of a complex process, which leads to the generation of free charge carriers that determine the electrical output of a solar cell — and hence its efficiency.

Professor Ravi Silva

Designing the next generation of organic solar cells

The new findings could have profound impact on the design of the next generation of organic solar cells. “The outcome of our study provides a deep understanding of the D/A [donor/acceptor] phase formation and separation, and paves the way towards a fundamental understanding of the spatial molecular packing of numerous BHJ materials systems — including ternary BHJs — in which fullerene derivatives with isomeric properties are used as electron-accepting materials,” Silva confirms. “It also shows that the realization of the full potential of the different organic material systems is an engineering problem rather than a hard physical barrier. The realization of 10% and higher efficiencies with materials such as P3HT should be experimentally possible.”

Different ICBA samples, dissimilar isomeric mixtures

Silva and his colleagues have discovered that different ICBA samples consist of dissimilar isomeric mixtures. The professor explains the significance of this research breakthrough: “ICBA is one of many electron accepting materials with isomeric properties,” he says. “Understanding the dependencies between the isomeric content of the acceptor and the formation of the BHJ between the latter and the electron donating material allows the transfer of knowledge onto various other D/A systems.”

As a result, the expert projects this discovery will furthermore not only lead to an increase of reported device power conversion efficiencies in the future but also add an extra dimension to the parameters that can be further optimized to reach the ultimate potential of the organic material systems.

How the study will help to advance organic solar cells

According to the professor, the nano-morphological network created between the donor and the acceptor materials is crucial for any organic solar cell’s efficiency. “Deepening our understanding of the molecular packing of the BHJ will lead to an increased yield of photogenerated charge carries and higher device outputs, suitable for powering the vast number of devices that will need energy in the burgeoning field of the IoT,” says Silva.

Next steps

Silva and his colleagues are currently working on the fabrication of printed organic solar cell modules with areas above 50 square centimeters. In this respect, they aim to show that high device efficiencies are scalable and therefore possible for large area devices. “With this, it opens up the possibility for us to power small electronic devices that need power requirements of the order of tens of megawatt, even when the device is indoors, to allow for the energy provision for the IoT,” the professor concludes.

The project is part of SMARTONICS, a four-year European Commission FP7 program aimed at developing large-scale pilot lines for the fabrication and printing of organic polymer solar cells. Professor Silva explains his study’s role within this framework: “Within the SMARTONICS program, we led a work package on enhancing the overall organic solar cell performance with the incorporation of plasmonic nano-particles as well as examining the upper limits of the current preferred organic materials to improve its efficiency beyond the current state of the art,” the nano-electronics and renewables expert says. “As a result, we worked on the ultimate performance possible with P3HT/acceptor materials and looked to isomerically purify the IC70BA acceptor to achieve this.”

Written by Sandra Henderson, Research Editor, Solar Novus Today

Labels: University of Surrey,large-area organic photovoltaics,organic solar cells,University of Surrey Advanced Technology Institute,Professor Ravi Silva,Nanoelectronics,SMARTONICS

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