Sep 25, 2012
Imprinted nanovoids trap light in solar cells
A high absorption of solar photons is critical for efficient solar cells. Especially when the semiconductor film in the solar cell is thinner than the absorption depth of incident light across a significant fraction of the solar spectrum. This condition is frequently encountered when scientists insist on thin films to reduce costs or decrease the electrical resistance of the device. Organic solar cells, which are based on conjugated polymers or small molecules, are typically used in a thin-film format (around 100 nm thick). Devices with thicker layers are limited by high series resistances. Conversely, devices with thinner layers are limited by low absorption. In each case, the result is reduced power conversion efficiency.
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.
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.
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.
About the author
The work was performed at the Ludwig-Maximilians University (LMU) Munich in the Hybrid Nanostructures group headed by Prof. Lukas Schmidt-Mende. Recently the group moved to the University of Konstanz, Germany. The current research of the group is concerned with nanostructured materials for energy conversion. Ricky Dunbar recently finished his PhD in which he investigated the use of metallic nanostructures to trap light and enhance absorption in an organic solar cell. Thomas Pfadler is performing his PhD in the group. He investigates the influence of nanostructures in organic solar cells.