Renewable Energy Global Innovations features: A non-fullerene acceptor with a diagnostic morphological handle for streamlined screening of donor materials in organic solar cells

Significance 

Solution-processable organic solar cells are an emerging technology that is capable of providing a cheap route for solar energy conversion. Researchers continue to set new records in power conversion efficiencies every year, yet the organic solar cell technology has remained more of an academic interest. In order to extend the existing lab-scale methodologies to large-scale commercialization, there are a number of inadequacies that will have to be addressed.

Transitioning to high-performance practical materials from currentmaterials with complex and multi-step preparations remains a major issue. This has made the materials expensive and more or less limited to academic settings. To address this issue, low cost and scalable N-annulated perylene diimide building blocks have been incorporated into a wide range of final materials with overall power conversion efficiencies between 2-8% when implemented as a non-fullerene acceptor. While these materials are simple to access and scale-up, their high efficiencies have relied on the use of polymeric donor materials, which are often quite expensive.

In view of the above limitations, solar energy scientists have now shifted their focus to finding simple and scalable donor materials that can sufficiently complement these acceptor materials. Researchers led by Professor Gregory Welch at the University of Calgary streamlined the screening of low cost and scalable donor materials using an N-annulated perylene diimide derivative. The authors established a simple air-processed and air-tested organic photovoltaic device preparation method in order to realize their objective. Their research work is published in Journal of Materials Chemistry A.

The authors took advantage of the diagnostic morphological handle inherent in N-annulated perylene diimide derivative and devised an approach for screening compatible donor materials. Implementing this efficient approach, the authors were able to screen a series of simple donor polymers constructed from low-cost building blocks and settled on PDTT-BOBT as good competitor to the now standard, high performance, yet expensive polymer, PTB7-Th.

The authors observed that optimizing the active layer blend of PDTT-BOBT:PDI-DPP-PDI led to an increase in the performance upon post-deposition chloroform vapor annealing. The best cell power conversion efficiency improved from 1.9 to 4.5% compared to 1.7 to 4.6% for the PTB7-Th. These high efficiencies made the authors recognize PDTT-BOBT as an alternative to PTB7-Th and supported its credibility for screening new acceptor materials.

While performance was impressive, negligible light absorption of PDTT-BOBT beyond 700nm as well as poor photochemical stability in air appear to be the major drawbacks to the polymer design. This polymer has high ionization potential and a high open circuit voltage. It would be therefore prudent to red-shift the onset of absorption with less alteration on the ionization potential. For this reason, any future modifications to the polymer design must be centrally focused on modifying the acceptor component.

Enhancing light stability of this polymer would definitely necessitate substituting the alkoxy side chains on the benzothiadiazole moiety with stable solubilizing substituents, without necessarily minimizing polymer solubility or affecting its self-assembly tendencies.

Addressing these challenges will call for the preparation of a number of new polymeric materials. Seth McAfee and colleagues in this study therefore proposed a simple approach for easy screening of these derivatives to come up with superior polymer designs.

About the author

Seth McAfee is a PhD candidate in Chemistry at the University of Calgary (Canada) working under the supervision of Dr. Gregory Welch.

Seth’s research in the Welch Research Group is focused on practical organic materials development for use in electronic devices, specifically organic solar cells. Motivated to access more sustainable and cost-effective active layer materials, Seth has been designing his organic pi-conjugated compounds to make use of organic dyes, known for their ease of commercial accessibility and excellent light harvesting capabilities.

Current efforts are focused on exploiting a material composed of perylene diimide (structural derivative of Pigment Red 190) and diketopyrrolopyrrole (structural derivative of Pigment Red 254). This compound is easily synthesized in high yields on multi-gram scale and has been able to achieve impressive device efficiencies acting as the electron-accepting material within the bulk heterojunction of solution-processable organic solar cells. A key feature of this material is the solvent vapour annealing induced solid-state re-organization of the compound. This was found to dramatically improve organic solar cell device efficiencies with an array of different electron-donating materials and highlights the versatile compatibility of the compound.

Contact: seth.mcafee@ucalgary.ca

Reference

Seth M. McAfee, Abby-Jo Payne, Sergey V. Dayneko, Gururaj P. Kini, Chang Eun Song, Jong-Cheol Lee and Gregory C. Welch. A non-fullerene acceptor with a diagnostic morphological handle for streamlined screening of donor materials in organic solar cells. Journal of Materials Chemistry A, 2017, 5, 16907.

 

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