Illustration of the generation of charge pairs (excitons), the precursors of free charge carriers in the active layer of an organic solar cell. Source: M. Panhans, Technische Universitaet DresdenPhotovoltaic (PV) technology is evolving with increased interest in organic photovoltaic (OPV) solar cells. OPVs have shown improving cell efficiency (currently around 13.2%), a promising initial lifetime of more than 5,000 hours unencapsulated and potential for roll-to-roll manufacturing processes, according to NREL (National Renewable Energy Lab). OPVs use carbon-based materials and require only a small amount to produce the thin films, compared to silicon-based PV, making them an appealing option in the development of the renewable energy mix. A diverse array of organic materials is available for the components, however, improvements still need to be made in device efficiency and lifetime.

A recent study discovered that molecular vibrations cause the voltage loss responsible for low efficiencies in OPVs. Scientists at TU Dresden in Germany and Hasselt University in Belgium demonstrated a direct relationship between molecular properties and macroscopic device properties and found that zero-point vibrations can significantly influence voltage losses. Zero-point vibration, an effect of quantum physics, characterizes the motion at absolute temperature zero.

The study examined the microscopic origin of absorption bands in molecular blend systems and the role they play in organic solar cells. The research focused on the temperature dependence of the absorption characteristics using simulations that considered the molecular vibrations. The simulations closely matched the experimentally measured absorption spectra.

The zero-point vibrations cause a considerable absorption bandwidth and reemission of part of the unused energy. This reduces the open-circuit voltage, which can now be predicted from electronic and vibronic molecular parameters. Even at room temperature, the effect is strong and greatly reduces the efficiency of the organic solar cell.

The results open the door to further development and improvement of OPVs. The paper appears in Nature Communications.