Imagine you want to buy a new car, so you travel out to the closest dealer to see what they offer. When you arrive, you notice that, strangely, all the cars in the lot look exactly the same. The car salesman approaches and begins selling you on only one model, the one you’ve seen covering the lot (let’s say it’s some horribly colored, marsh-green hatchback). When you ask to see more, the salesman says only this one model exists. It works well enough for its main job – to move you around town – and it’s cheap, but it’s not very fuel efficient. When you ask to see more, the salesman say this is it! You leave in disgust (unless you love marsh green hatchbacks!).
This is basically the current situation in organic solar cell research – just replace car with donor-acceptor active layer. Organic, polymer-based solar cells (OSCs) are a great next-generation alternative to inorganic cells because they can be cheaply manufactured using roll printing that allows high throughput of materials (similar to how newspapers are printed). In most cases, no toxic materials are necessary and high temperature processing are not required – more positives! But we only know of a few materials that work as electron donor materials – fluorinated polymers – and only one respectable acceptor, PC71BM, a specific fullerene molecule, that combine to give around 9% efficiency (the best yet):
OSC research could use a boost from either some new materials or processing methods! Luckily, some help may have arrived. A new article in Nature Communications describes a novel method to develop donor-acceptor interfaces that yield efficiencies of almost 11%, matching or exceeding previous cells using the PC71BM molecule, but allows for the use of over 10 different types of fullerene molecules. This is a great opening to widen organic solar cell research to find new combinations of acceptors and donors that might pair well together and provide a large jump in overall efficiency.
The key to the new processing method is the development of a ‘near-perfect’ morphology of the donor-acceptor layer. The authors did this by using a slightly higher temperature when casting the donor and acceptor materials together. Normally, the casting process is done at room temperature, and the ideal PC71BM fullerene molecule must be used because it combines withe donor material in just the right way (known as aggregation) to allow efficient electron-hole separation. Here, the authors instead found an ideal donor material, with the opaque name PffBT4T-2OD, that allowed for advantageous morphology with a variety of fullerenes:Using this donor molecule, the authors then casted the active layer with many different types of fullerene molecules at an elevated temperature slightly above room temperature. In each case, inherent properties of PffBT4T-2OD forced the fullerene molecules to aggregate in a nearly crystalline form. This is especially important because it improves hole transport mobility and allows better electron-hole separation, leading to high efficiencies, even for active layers on the order of 100 nm. Thicker active layers mean more light absorption, more excited carriers, and thus more output power – an added benefit!
Below is a sampling of the different fullerene derivatives using as the accepting material in the OSCs with efficiencies around 10%:They may all look the same – the fullerene is the big C60 buckyball in the middle of each molecule – but the functional groups attached all produce very different chemical environments affecting carrier transport and efficiency. These effects will definitely be explored further, but this is a great first step in finding a processing method that allows for more freedom in choosing acceptor molecules. The key is in choosing a donor molecule that guides Nature to form clean, crystalline active layers that support fast transport and separation. OSCs may be our best bet in mass-producing solar cells if we can get efficiencies up – we just have to get past the marsh-green hatchback!
Liu Y., Zhao J., Li Z., Mu C., Ma W., Hu H., Jiang K., Lin H., Ade H. & Yan H. & (2014). Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells., Nature communications, PMID: http://www.ncbi.nlm.nih.gov/pubmed/25382026