Hybrid solar cells combine the advantages of inorganic semiconductor photovoltaic materials with their extremely high carrier mobilities, and organic polymers solar cells that strongly absorb sunlight. However, it can be difficult to control the inorganic-organic interface and in particular the electronic and physical properties of the polymers grown on semiconducting nanostructures.

A team of researchers, led by Diana Huffaker and Richard Kraner, may now have found a way to overcome this problem by developing a new method to coat semiconducting nanopillars with PEDOT (a conducting polymer). The nanopillars used in the experiments are densely packed arrays of electro-optically active GaAs semiconductors. Such nanostructures show much promise as solar cells thanks to light-trapping effects between the pillars that dramatically reduce the amount of photons reflected from a device, which ultimately enhances optical absorption. The high surface-to-volume ratio of these materials also increases the all-important photoactive junction area so that more photons are harnessed, leading to enhanced power-conversion efficiency.

Tailoring and tuning properties

The researchers, who have published their work in Nano Letters, employed routine bulk electropolymerization techniques to tailor properties like energy levels, conductivities, carrier mobilities and chemical doping levels in the polymer component. These techniques allow for a much better control of the morphology and properties of a finished device compared with standard spin-coat methods, explains team member Giacomo Mariani. They are also better than common processing methods, such as inkjet printing and spin/drop casting, routinely used to apply an organic layer to inorganic nanostructures and which result in coatings with non-uniform thicknesses.

The 3D hybrid solar cells developed by the UCLA researchers have tunable properties thanks to the fact that the amount and type of dopant in the organic layer can be controlled during deposition. “The possibility of incorporating several dopants at once during this time also allows us to electronically tune the hybrid interface in terms of band-to-band re-alignments between the organic and inorganic layers,” Mariani told nanotechweb.org. “This technique results in an improved short-circuit current density, JSC, of 13.6 mA cm–2, an open-circuit voltage, VOC, of 0.63 V and a peak external quantum efficiency of 58%.”

Monomers and better nanopillars

The team now plans to look at conjugated monomers suitable for electrodeposition. These monomers would form the organic p-side of a p-n hybrid junction in contrast to the Schottky hybrid junction demonstrated in the current work. The researchers also hope to investigate other better performing, high-mobility III-V materials as the nanopillars.

“We are convinced that by intimately controlling both the inorganic and organic sides during solar-cell fabrication, we shall be able to thoroughly characterize the properties of the hybrid junction,” added Mariani. “Hybrid solar cells will only reveal their true potential if we are able to understand the fundamental charge transport properties between the two classes of material.”