Recent years have seen the development of a new type of nanostructure that combines the strong resonant light-harvesting properties of plasmonic nanoparticles with the unique absorption capabilities of quantum dots. Such structures are the basis for a novel class of intermediate band solar cells that may be capable of approaching the maximum theoretical efficiency (up to 63%) for this type of device. Furthermore, the method to fabricate these solar cells relies on solution-processed nanoparticles that can be integrated using low-cost and high-throughput processes on a broad range of host materials.

One of the major drawbacks of conventional single-bandgap solar cells is their inability to exploit the infrared portion of the solar spectrum, since most infrared photons do not have sufficient energy to pump electrons across the bandgap of the semiconductor material used to make the solar cell. Quantum confinement effects provide a promising route towards bandgap engineering and in the last decade, several research teams around the world have been looking at using quantum nanostructures in PV materials. This is the case for intermediate band solar cells (IBSCs) invented by scientists at the Instituto de Energía Solar (IES) of the Universidad Politécnica de Madrid.

IBSCs are composed of arrays of quantum dots (QDs) that allow photons with energy below their host semiconductor bandgap to produce photocurrent, which allows for higher solar cell efficiency. Although the IBSC concept has already been verified in prototype devices, the impact of the QDs on cell performance is still marginal mainly because these structures only weakly absorb light.

Improving the performance of IBSCs

Manuel João Mendes and his co-workers at the IES have now found a promising route to increase the amount of light absorbed by QDs by coupling them with localized plasmons sustained by metal nanoparticles (MNPs). At their surface plasmon resonance, MNPs act as optical antennas gathering light from their surroundings and strongly focusing it into a nanoscopic region (the near-field) located in their vicinity. In this study, the IES researchers have developed a method to deposit colloidal MNPs and QDs that can self-organize the particles in planar hybrid arrays where the QDs become assembled in the intense scattered near-field coming from the MNPs. This configuration provides a means to trap and concentrate the incoming sunlight in the small nanoscopic volume of the QDs, thereby increasing the amount of light they absorb and ultimately improving the performance of the intermediate band material.

Spurred on by these initial findings, the researchers are now busy fabricating the first prototype plasmon-enhanced intermediate band solar cells composed of hybrid arrays of colloidal quantum dots and metal nanoparticles.

More information can be found in the journal Nanotechnology 24 345402.

Further reading

Silicon quantum dot/crystalline silicon solar cells (May 2008)
Nano-heterojunctions improve solar cells (Jul 2012)
Quantum dots for superior solar cells (Aug 2012)
Quantum dot photonic nanocrystals increase conversion efficiency of solar cells (May 2012)