Maximising light in optoelectronic devices

Colloidal quantum dots (CQDs) are at the centre of a new and rapidly evolving research field, with the promise for applications in efficient and cost effective solar cells. Used as the absorbing photovoltaic material, they have the advantage of having a band gap that can be tuned simply by changing the nanoparticle size. This allows them to easily absorb different parts of the solar spectrum.

However, the thickness of the CQD layer is restricted to retain efficient charge extraction. Thus, novel light-trapping schemes are required to improve light absorption and efficiency. Plasmonic metal nanostructures have the potential to further enhance light trapping in the ultra-thin absorbing CQD layers.

The EU-funded project 'Plasmonically enhanced colloidal quantum dot photodetectors and photovoltaics' (PECQDPV) studied the optical and electrical effects of embedding engineered photonic structures in simple photoconductor and photodiode devices, fabricated from lead–sulphide (PbS) CQD films.

Using photoconductor test devices with embedded arrays of random, self-assembled metal nanoparticles that strongly scatter light, scientists demonstrated a 2.4 increase in photocurrent at wavelengths around the exciton peaks of PbS quantum dots of a given size.

Furthermore, they studied the electrical effects of embedding other metal nanostructures in these devices. Depending on the metal, direct contact with nanoparticles led to photocurrent suppression or enhancement. These findings were important for designing plasmonic CQD optoelectronic devices.

Focus was also placed on exploring the physical mechanisms behind plasmonic enhancement. To this end, scientists performed full-field optical simulations and developed simple analytical models. In simulations with Ag nanoparticles, the angular distribution of the scattered light was found to be relatively narrow, thus reducing its overall light-trapping potential. Experimental studies showed that mode structure of the thin semiconductor film is fundamental in determining the amount of light trapping.

To increase the efficiency of light trapping beyond that provided by random structures, periodically arranged nanostructures were investigated. Scientists developed a conceptual model to provide simple design rules for optimal light trapping in thin films with 2D grating couplers. The grating couplers were integrated into photodiodes as the Au back contact, and achieved photocurrent enhancements of up to a factor of 3 for thin diodes and 1.5 for thick diodes, relative to planar reference devices of similar thickness.

PECQDPV sought to enhance the absorption of CQD devices by incorporating plasmonic nanostructures. The findings enhance understanding of the challenges regarding choice of plasmonic material and methods that enhance light trapping in a CQD device of particular geometry.