Non-Fullerene Acceptors for Solution-Processed Small-Molecule Organic Solar Cells
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Friday, November 6, 2015 - 1:30pm to 3:00pm
Dr. Ian G. Hill, Department of Physics and Atmospheric Science, Dalhousie University
The efficiencies of both small-molecule and polymer-based organic solar cells have increased dramatically over the past decade. The record lab-scale cell efficiency now stands at 11.5%, well above the 10% threshold at which a technology is thought to be commercially viable. While these efficiencies are significantly lower than those achieved by mono- and multi-crystalline silicon solar cells, near room temperature solution-processed organic semiconductor materials enable low-cost integration of solar cells into consumer electronics, clothing, windows, building facades and roofing materials, to name a few applications that are difficult to achieve with silicon.
Several challenges remain before ubiquitous, low-cost, organic solar cells can be realized. Perhaps the two most glaring shortcomings of the current state-of-the-art are lifetime and cost. Most organic solar cells have lifetimes measured in months, rather than the 20-25 year industry standard for silicon solar cells (note that this is also the typical lifetime of windows, roofing materials, and most other building systems). On the financial side, some of the organic materials used are orders of magnitude more expensive than the cost point required for a low-cost technology rivaling silicon.
All high-performance organic solar cells use fullerene-based electron acceptors, such as C60, PCBM and ICBA. These fullerenes possess excellent electron affinities, relatively high electron mobilities and they tend to phase separate from the electron donor materials on the correct length scales to promote charge separation. Unfortunately, they possess an enormous embodied energy, making low-cost, sustainable production impossible. In addition they absorb light very weakly, so they contribute very little to the photovoltaic action of the cell, and their electron transporting properties degrade severely and quickly in air, leading to short cell lifetimes, even when encapsulated.
The field of non-fullerene acceptors aims to replace fullerenes with low-cost, scalable and sustainable materials with high optical absorption coefficients, and air stability. By tailoring the optical absorption spectrum, acceptors with absorption complementary to the donor can be used to more efficiently utilize the solar spectrum. Until recently, the efficiencies of devices made with non-fullerene acceptors were significantly lower than their fullerene-based rivals. Over the past year and a half, record efficiencies have increased from <3% to 7%, and the field has attracted much attention. In this talk, I will review the recent progress in this field, our contributions to it, and discuss the molecular and film design methodologies that have resulted in this impressive performance improvement.