Preventing crystallization of small donor molecules in organic photovoltaic cells

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Organic photovoltaic cells contain a bulk heterojunction composed of two materials: an electron donor and an electron acceptor. Fullerene derivatives are the most commonly used class of electron acceptors because of their high efficiency. In most cases, electron donors are polymers, because they can be easily processed into high-quality thin films. Small molecules are easier to purify and characterize, and give more consistent results because of their discrete nature, but tend to crystallize easily, leading to grainy, poor-quality thin films. The objective of the project consists in developing strategies to use small molecules for both donor and acceptor materials in organic PV cells. To achieve this, it is crucial to develop materials that can be processed into thin films without crystallizing. To this end, an acceptor material (which will be kept constant) capable of forming amorphous thin films and of acting as an amorphous host for various small-molecule donor materials will be designed and synthesized. The Lebel group has shown that some mexylaminotriazine derivatives are capable of forming extremely stable glassy phases without crystallizing over extended periods of time. These compounds can also be chemically bonded to various chromophores to generate adducts that can readily form amorphous thin films, and it is also possible to blend these materials with "regular" small molecules to generate amorphous blends where both components are homogeneously dispersed. A glass-forming fullerene acceptor incorporating a mexylaminotriazine unit will thus be synthesized. In a first step, C60 will be functionalized according to published procedures with a moiety containing a hydroxy, amino or carboxy group, to which a mexylaminotriazine derivative will then be bonded. The target compounds will first be blended to various small-molecule donors to ensure that high-quality thin films are obtained where both components are properly dispersed. Promising blends will then be incorporated into photovoltaic cells to assess their performance.