Research Highlights

With growing demand for renewable energy sources, devising new methods to improve the efficiency of photovoltaic materials is a key challenge in the production of next-generation solar cells based on organic materials rather than silicon. A team led by Tienan Jin from the WPI-AIMR in collaboration with researchers from China have now developed a novel strategy to produce organic solar cells using functionalized carbon ‘fullerenes’ with high yield and at low cost¹.

Photovoltaic cells generate electricity from light based on a system of charge transfer between electron donor and acceptor molecules. Fullerenes — soccer ball-like spheres of carbon atoms — are widely used as an acceptor molecule in organic solar cells in the form of phenyl-C₆₁-butyric acid methyl ester (PCBM), but this complex molecule is expensive to make and attempts to reduce production costs have so far met with limited success. Jin and his co-workers turned to the naturally occurring fullerene C₆₀ as a lower-cost alternative to the C₆₁ derivative. Functionalizing C₆₀ to achieve the same level of electron acceptor performance, however, required the development of a new synthesis strategy.

The researchers functionalized the C₆₀ fullerene (pictured) by grafting on alkyl chains. This type of modification has been performed in the past using magnesium- or lithium-based reagents, but these reactions often result in over-alkylation. Milder, more selective conditions are needed to produce the mono-functionalized fullerenes that are most suitable for use in organic solar cells. “We have previously developed a number of transition metal-catalyzed molecular transformations at carbon–carbon multiple bonds, so we investigated whether we could functionalize C₆₀ in the same way,” explains Jin.

After an exhaustive search of possible catalysts and reaction conditions, the researchers found that a cobalt-based compound was the most efficient catalyst for this grafting reaction. Adding the C₆₀ and an alkyl halide compound to a solution of the cobalt catalyst and a manganese reducing agent gave the mono-alkylated C₆₀ fullerenes in 88% yield in 2 days. Using this efficient procedure, Jin and his colleagues were able to produce fullerenes bearing a zinc porphyrin, a branch-like dendrimer, and even another fullerene to afford a fullerene dimer or ‘dumbbell’ — all of which are difficult to synthesize by previous methods.

A simple organic solar cell with a bulk heterojunction structure constructed using the new fullerene derivatives achieved higher solar-to-electricity conversion efficiency than a comparable device prepared using the conventional PCBM electron acceptor. “Based on this promising result, we plan to design and synthesize other new fullerene derivatives with possible photovoltaic applications,” says Jin.