Scientists at EMPA in Switzerland have invented multi-layer organic solar cells with improved charge-collection and power conversion efficiency. The improvement is achieved by using a thin organic or inorganic salt layer to adjust the electronic levels of adjacent layers.
Organic solar cells are promising devices for inexpensive, large-scale solar energy conversion and recent work has demonstrated that using soluble small organic molecules, multi-layer devices with high power conversion efficiencies can be fabricated.
Semiconducting materials with matched electronic and optical functionalities are one obvious requirement for high light-to-electrical-charge conversion. In addition, efficient charge collection requires the proper matching of the electrode work functions to the charge-transporting states in the organic material.
The invention describes a simple process to match the electronic structures at interfaces, thereby facilitating this critical charge transfer.
A great variety of salts may be used to adjust the intermediate matching layer. In addition, the principle can be used to tune the energy mismatch between any two interfaces in a multilayer device.
The technique allows the fabrication of solar cells with long lifetimes using a fast, cheap and simple manufacturing method.
Large scale production of cyanine dyes is already occurring, which promises competitive materials prices for emerging organic solar cell technology. Applying thin salt layers in cyanine dye / C60 bilayer photovoltaic devices between the conductive anode layer and the cyanine layer reduces the energy offset.
The method is not restricted to the material system used in EMPA’s work, but can in principle be applied to any organic or inorganic multi-component system where easy charge transfer across selected interfaces has to be assured.