Spectroscopy Probes the Energy-Level Landscape of Solar Cells

A new spectroscopic method developed by researchers at Heidelberg University could enable scientists to study the physical principles of organic photovoltaics with extreme precision to better understand how energetic losses occur. The new method combines ultraviolet photoemission spectroscopy and argon cluster etching to allow scientists to directly visualize the vertical energetic landscape in the donor-acceptor blend.

The researchers investigated both model and high-performance photovoltaic systems using their new technique and demonstrated that the resulting photovoltaic gaps were in close agreement with the measured charge transfer energies and open-circuit voltages. They used the new spectroscopic technique to study the evolution of the energetic landscape upon environmental degradation, obtaining information that will help them understand degradation mechanisms and develop mitigation strategies. The spectroscopic method developed by the Heidelberg researchers can map the energetic landscape on a nanometer scale and can be applied at any point during the solar cell’s lifetime.

Artistic representation of an energetic landscape that determines the movement of photo-induced positive (h+) and negative (e-) charges in photovoltaic devices. Courtesy of Yana Vaynzof.

According to professor Yana Vaynzof, who led the research, the new technique solves a key problem in the field of organic photovoltaics. “Without mapping the energetic landscapes, it is difficult to understand how and why devices lose energy in the process of converting light into electricity,” she said. “Now we have a spectroscopic method that allows us to develop new generations of solar cells with reduced energy losses and improved performance.”

Until now, the standard approach has been to estimate the photovoltaic gap of the donor-acceptor blend using the energy values measured on the individual blend components. The researchers said that in the past, mapping energetic landscapes has been so challenging that only rough estimates could be used to study the fundamental processes in organic photovoltaic devices.

The research was published in Joule (https://doi.org/10.1016/j.joule.2019.06.018).