Photovoltaic solar panel windows could soon be on the horizon, thanks to recent quantum-dot research by Los Alamos National Laboratory in the United States and scientists from University of Milano-Bicocca (UNIMIB) in Italy. The project demonstrates that the superior light-emitting properties of quantum dots can be applied in solar energy by helping to harvest sunlight more efficiently.
Quantum dots are ultra-small bits of semiconductor matter that can be synthesised with nearly atomic precision via modern methods of colloidal chemistry. Their emission colour can be tuned by simply varying their dimensions. Colour tunability is combined with high emission efficiencies approaching 100 per cent. These properties have recently become the basis of a new technology – quantum dot displays – employed, for example, in the newest generation of Amazon's Kindle Fire e-reader.
A luminescent solar concentrator (LSC) is a photon management device, representing a slab of transparent material that contains highly efficient emitters such as quantum dots. Sunlight absorbed in the slab is re-radiated at longer wavelengths and guided towards the slab edge equipped with a solar cell.
‘The LSC serves as a light-harvesting antenna which concentrates solar radiation collected from a large area onto a much smaller solar cell, and this increases its power output,’ explained lead researcher Victor Klimov from the Center for Advanced Solar Photophysics (CASP) at Los Alamos.
‘LSCs are especially attractive because in addition to gains in efficiency, they can enable new interesting concepts such as photovoltaic windows that can transform house facades into large-area energy generation units,’ said Sergio Brovelli, who worked at Los Alamos until 2012 and is now a faculty member at UNIMIB.
Because of highly efficient, colour-tunable emission and solution process ability, quantum dots are attractive materials for use in inexpensive, large-area LSCs. One challenge, however, is an overlap between emission and absorption bands in the dots, which leads to significant light losses due to the dots re-absorbing some of the light they produce.
To overcome this problem, the Los Alamos and UNIMIB researchers have developed LSCs based on quantum dots with artificially induced large separation between emission and absorption bands (called a large Stokes shift).
These ‘Stokes-shift’ engineered quantum dots represent cadmium selenide/cadmium sulphide (CdSe/CdS) structures in which light absorption is dominated by an ultra-thick outer shell of CdS, while emission occurs from the inner core of a narrower-gap CdSe. The separation of light-absorption and light-emission functions between the two different parts of the nanostructure results in a large spectral shift of emission with respect to absorption, which greatly reduces losses to re-absorption.
To implement this concept, Los Alamos researchers created a series of thick-shell (so-called ‘giant’) CdSe/CdS quantum dots, which were incorporated by their Italian partners into large slabs (sized in tens of centimetres) of polymethylmethacrylate (PMMA) (see figure 1). While being large by quantum dot standards, the active particles are still tiny - only about hundred angstroms across. For comparison, a human hair is about 500,000 angstroms wide.
‘A key to the success of this project was the use of a modified industrial method of cell-casting, we developed at UNIMIB Materials Science Department,’ said Francesco Meinardi, professor of Physics at UNIMIB.
Figure 1: Shows how the quantum dots are embedded in the plastic matrix and capture sunlight to improve solar panel efficiency
Spectroscopic measurements indicated virtually no losses to re-absorption on distances of tens of centimetres. And, tests using simulated solar radiation demonstrated high photon harvesting efficiencies of approximately 10 per cent per absorbed photon achievable in nearly transparent samples, perfectly suited for use as photovoltaic windows.
Despite their high transparency, the fabricated structures showed significant enhancement of solar flux with the concentration factor of more than four. These exciting results indicate that ‘Stokes-shift-engineered’ quantum dots represent a promising materials platform. It may enable the creation of solution processable large-area LSCs with independently tunable emission and absorption spectra.