A solution to this apparent dilemma is to retain thin films (and the accompanying favourable electronic properties) but incorporate mechanisms to ensure that once photons enter the device, the probability that they escape again (via reflection) is very low. In other words: ensure that light is trapped inside the device.

Patterning technique

Scientists at the Ludwig-Maximilians University in Munich, Germany, have developed a simple, yet effective technique to trap light in solar cells. It relies on a self-assembly process called anodization. In this process, aluminium is oxidized under controlled conditions to form an aluminium oxide membrane featuring ordered nanovoids. This membrane can then be pressed into an organic semiconductor film, where the void structure is then imparted into the film. When a metallic electrode is deposited onto the nanostructured film, it conforms to the topography of the organic layer. The result is an organic-metal interface that features an array of hemispherical nanovoids. In a conventional organic solar cell, this interface is planar and light simply specularly reflects from it. However, in a modified device, the nanovoid interface can interact with incident light, by enabling scattering and allowing surface modes to be excited.

Measurement and analysis

Angle-dependent reflectometry measurements performed at the University of Cambridge, UK, confirm that the nanostructured interfaces ensure that significantly more light is trapped in the sample. Samples with metal-organic semiconductor and metal-transparent polymer interfaces were compared. For each sample type, planar and structured versions were measured. Here, the presence of polarization-independent, resonant absorption features indicate that localized void modes are primarily responsible for the enhancement. The structured interfaces with void periodicities of 500 nm were found to be significantly more effective for trapping light.

Next steps

The technique is scalable and inexpensive. What’s more, the geometry of the nanovoid interface can be modified by adjusting the anodization conditions. Further optimization of the imprinting process is underway.

Additional information can be found in the journal Nanotechnology.