University of Calgary scientists improve materials and methods for making efficient, sustainable organic solar cells

Most of us think of solar cells as those rigid, rectangular, silicon-based solar panels atop rooftops or deployed in large solar energy farms.

But a next-generation technology called organic photovoltaics, or organic solar cells, is being developed that promises numerous new applications not possible with silicon solar cells.

Imagine large-area solar cells that can be printed like a newspaper and attached to windows and walls. Or going camping and generating electricity with a tent that is one big, flexible solar cell. Or wearing clothing of lightweight, paper-thin organic solar cells that generate power for your cell phone and laptop.

Organic solar cells use conductive organic polymers (chemical compounds) or small organic molecules to absorb light and generate an electrical charge to produce electricity.

New research, led by two postdoctoral associates in the Department of Chemistry in the Faculty of Science, helps advance toward the commercial production of sustainable, high-performance organic solar cells.

Dr. Richard Pettipas, PhD, and Dr. Anderson Hoff, PhD, have developed new environmentally friendly materials and simpler, more efficient assembly processes for printed electronic devices, with a focus on large-area organic solar cells.

Both are members of the research group led by Dr. Gregory Welch, PhD, professor in the Department of Chemistry.

“One of the major findings is that we could make these organic compounds in a ‘greener’ way, instead of using toxic and expensive solvents,” says Pettipas, whose role was synthesizing molecules to make novel materials for the solar cells.

“I was able to show you can really do this with a bunch of different molecules,” he says. “I really focused on doing this in a way that’s actually going to be industrially scalable.”

Pettipas developed a series of organic materials that can be dissolved in a variety of green solvents, such as ethanol, which allows for multilayer organic film formation.

Hoff then assembled these materials into small, functional organic solar cells. Together, they developed methods to deposit the materials from ethanol onto the solar cell’s active layer film, in “real-world” conditions.

Hoff was able to make rigid and flexible solar cells, or interlayers, within the cells using “slot-die coating.” This is a printing technique compatible with roll-to-roll processing which can create electronic devices on a roll of flexible substrate.

“This shows it’s possible to translate these methods into large-area solar cells that could be printed layer by layer,” Hoff says.

The pair’s research is published in ACS Applied Materials & Interfaces in two studies: “Green Solvent-Processible N–H-Functionalized Perylene Diimide Materials for Scalable Organic Photovoltaics,” and “Tin Oxide Electron Transport Layers for Air-/Solution-Processed Conventional Organic Solar Cells.”

Organic solar cells showed high performance

Both studies by Pettipas and Hoff focused on innovation of new interlayers for large-area printed electronics.

“We developed valuable guidelines to improve the performance of devices using low-cost materials,” Pettipas says.

For example, they also were able to optimize a commercially available electron-transporting ink formed by nanoparticles.

Using tin oxide nanoparticles that were slot-die coated from alcohol, they were able to form a highly efficient interlayer ideally suited for organic solar cells using rigid and flexible substrates.

The pair used a “solar simulator,” which provides a known amount of energy intensity, to measure their organic solar panels’ efficiency, or how much of the incident power is directly converted to electrical power. They were able to achieve efficiencies ranging from six to 11 per cent.

The high end of that efficiency rate is among the best efficiencies reported for organic solar cells made in ambient air, Hoff notes.

While silicon solar cells have an average efficiency of 18 per cent and can be as high as 22 or 23 per cent, the types of applications for these rigid, multi-crystalline cells are limited.

“But there’s a lot more potential when it comes to organic solar cells,” Pettipas says.

Another issue with silicon solar cells is that it takes a lot of energy to manufacture them, and sometimes this energy is carbon-intensive. Many of the world’s solar cells are made in China, for example, using coal-fired power.

In addition to organic solar cells, the printed electronics methods that Pettipas and Hoff developed also could be applied to other devices, such as chemical and temperature sensors for smart packing, or light-emitting diodes for signage and agricultural applications.

“Organic solar cells have tremendous potential to provide cheap and clean electricity for our ever-evolving high-tech lifestyles and smart building initiatives,” along with potential for use in remote regions such as the Arctic, says Welch.

“While academic interest has been vast, there are few commercial sources and no industrial manufacturing presence in Canada,” Welch says. “The work by Anderson and Rick has helped bridged the gap in lab-to-fab (laboratory-to-fabrication) that will accelerate commercialization.”