It is thought that the next generation of photovoltaics – which form a central plank of Germany’s renewable energy mix – will be based on organic solar cells. As a result, research has been ramping up in recent years into the physics of carbon-based, semi-conducting materials. The latest study to be published by the Center for Advancing Electronics Dresden (CFAED) at the Technical University of Dresden throws some light on the underlying physical causes which have limited the efficiency of organic solar cells to date.
In theory, solar cells made from carbon-based materials are preferable to silicon-based modules because they cost far less to produce (they can be printed onto thin films), they have high throughput and are flexible, resulting in a better ecological footprint overall. The problem is their low performance, which is due to insufficient open-circuit voltage.
To get to the bottom of this problem, the team at CFAED, led by Dr. Frank Ortmann, joined forces with researchers from Hasselt University in Belgium. The authors of the study, which was published in the journal in March, found that the quantum vibrations of molecules in the thin organic films – so-called “zero point oscillations” – can have a significant influence on voltage losses.
Critically, the researchers found that these zero-point oscillations (an effect of quantum physics that characterises the motion at absolute temperature zero) result in a considerable absorption bandwidth which disperses the unused energy, thereby reducing the open-circuit voltage. But they can now predict these voltage losses from electronic and vibronic molecular parameters and know more about how temperature effects charging. In the study, the authors explain strategies to reduce these vibration-induced voltage losses and thereby increase the performance of organic photovoltaic (OPV) systems.