Device Growth and Characterisation Facilities

The solar cell research facilities at Cranfield University cover the complete range from device modelling and fabrication to testing. We focus on cells constructed from thin films of compound semiconductors. These are made using a range of different techniques that include chemical bath deposition, close space sublimation, spray deposition and sputter coating.

The close space sublimation system (left) During the growth of a CdTe layer (right)

Several of these techniques contain multiple sources that can either be aligned to produce a film with a uniform composition or repositioned to produce a film with a non-uniform composition across its surface. This allows us to use combinatorial synthesis to explore a wide range of chemical compositions across a single specimen. For example our sputter coating system has three targets that can each be individually tilted and positioned to achieve a particular compositional gradient across the substrate’s surface.
Device testing can be spilt into the evaluation of the multilayered structures used to make devices and the testing of completed solar cells.

A full understanding of the multilayered structures found in devices is essential to the understanding of how they work and how they are affected by individual growth steps. The main analytical techniques that we use are:

Scanning electron microscopy for the physical structure of devices and individual layers, and chemical composition by X-ray microanalysis
X-ray diffraction for the identification of specific compounds within our devices, and to evaluate grain size, strain and intermixing between layers
Ion beam analysis to measure film thickness and composition and to evaluate intermixing between layers
Spectroscopic ellipsometry to determine the optical properties of individual layers and the structure of multilayers including layer thicknesses, intermixing and roughness

All of these techniques can be used to map properties over the surface of a specimen or library. For example our Woollam M2000UI spectroscopic ellipsometer is equipped with a 15cm by 15cm automatic sample stage.

A specimen on the Woollam M2000U1 spectroscopic ellisometer (left) a refractive index map (right)

Completed cells are tested using a high pressure Xe arc lamp solar simulator. This has a uniform beam diameter of about 5 cm and interchangeable filters to match its spectrum to the AM0, AM1 and AM1.5 solar standards. Full characteristic curves are collected both with and without illumination, to provide standard cell parameters such as open circuit voltage, short circuit current, fill factor and efficiency, as well as shunt and series resistances. We also have a spectral response system to determine how the device efficiency varies with the wavelength of light.