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A new PV wave making more efficient use of the solar spectrum (FULLSPECTRUM)

Final Report Summary - FULLSPECTRUM (A new PV wave making more efficient use of the solar spectrum)

FULLSPECTRUM was a project sponsored by the European Commission whose primary objective was to make use of the full solar spectrum to produce electricity. The necessity for this research was easily understood, for example, from the fact that present commercial solar cells used for terrestrial applications were based on single gap semiconductor solar cells. These cells could by no means make use of the energy of below bandgap energy photons since these simply cannot be absorbed by the material.

Research, technology, development and innovation within the project were structured along five activities whose content and objectives are briefly described below:

- Multijunction activity.
It aimed to progress as much as possible towards the 40 % efficiency goal using multijunction solar cells. The highest efficiencies were achieved by monolithical triple-junction solar cell fabricated at Fraunhofer Institute for Solar Energy Systems. It was grown by metal organic vapour phase epitaxy, with GaInAs, GaInP and Ge as base materials. The research of suitable solar cells characterised by a 1 eV bandgap, the modelling and characterisation of the cells and the development of suitable characterisation tools were also tasks being developed within the activity.

- Thermophotovoltaics (TPV)
This activity aimed to make a better use of the solar spectrum by using the sun to heat an appropriate emitter whose radiation was indeed used to illuminate solar cells. Cells for TPV are mainly based on GaSb. The research within the activity involved the production of quality GaSb wafers, the manufacturing of the cells and their assembly into modules for their integration in the TPV systems, the research on efficient and adapted to the solar cell gaps emitters and the design of the system itself.

- Intermediate band cells
The intermediate concept pursued the improvement in the utilisation of the solar spectrum by using materials that inherently exhibit an intermediate band located within the otherwise conventional gap. Part of the activity was devoted to the early identification of these materials by means of quantum mechanical calculations. The other part was devoted to experimentally prove the principles of operation of the intermediate band concept by artificially engineering the intermediate band using quantum dots. This also involved a strong activity in specific characterisation and modeling.

- Molecular based concepts
It mainly focused in the development of flat concentrators, dye cells and up and down converters for solar cells. Flat concentrators collect the sunlight reemitting it at a given wavelength. Within the project, ways for obtaining more efficient concentrators were being investigated. In particular, concentrators also capable of emitting at more than one wavelength, suitable for use with multijunction cells were being researched.

- Manufacturing
This activity aimed to implement at industrial level those concepts more promising for making a better use of the solar spectrum. Among these concepts, the one closest to commercialisation was the multijunction solar cell. Within the activity, compact concentrators have been developed. Industrial prototypes consisting of over 200 MJCs with geometrical concentration of 1 000 suns have been qualified. The development of the appropriate setups required to test the optical and overall efficiency of the single compact concentrator during development stages, self calibrating high precision trackers as well as the analysis of the longevity of the systems were also tasks being developed within this activity.

The advances produced within the project can be illustrated by saying that:
- five world record efficiencies have been achieved: concentrator GaAs cell (28.6 % at 293), dual-junction cell (32.6 % at 500-1000), triple-junction cell at high concentration (37.6 % at 1700), fuel-fired TPV system (3.96%), luminescent solar concentrator (7.1%).
- the principles of operation of the IBSC have been experimentally demonstrated using QD prototypes and a bulk IB material based on a transition-metal-doped sulphide has been synthesised. The first hybrid solar / fuel-fired TPV system had been built.
- industrial products developed: triple-junction cells with efficiencies ~35 %, compact high concentration modules, concentrator characterisation tool for mass production.