2, University of Oxford, Oxford, , United Kingdom
Thin-film solar cells provide an alternative to conventional silicon-based photovoltaics, and modules based on thin film cadmium telluride (CdTe) are cost-competitive with silicon. Device efficiency has increased from 16% to 21% in the last 5 years. Key factors enabling the improvements have been alterations in the device structure, including replacement of the traditional cadmium sulphide (CdS) window layer with higher band-gap alternatives like magnesium-doped zinc-oxide (MZO), and grading/alloying of the near-interface region in the CdTe absorber layer with selenium. However, research on devices incorporating these changes is limited. Little high resolution microstructural/chemical/electronic characterisation has been published. Improved characterisation, and hence understanding, of these devices, can lead to improved cell design/processing for enhanced performance, taking record cell efficiencies closer to the thermodynamic limit of ~30%.
In this work, devices with three architectures are characterised by high resolution dynamic SIMS (‘NanoSIMS’) giving 3-Dimensional chemical maps of the cells at nanometre resolution. This is supplemented by correlative high resolution EBSD and cathodoluminescence measurements, to establish the effects of observed microstructural and chemical features on performance. The three absorber/buffer layer architectures are: 1) traditional CdS/CdTe; 2) MZO/CdTe; and 3) high efficiency, MZO/CdSeTe/CdTe, selenium-graded devices. This set spans the evolution from more conventional device structures to the newer, high-efficiency structures.
The advantage of NanoSIMS is that it’s high sensitivity, combined with high resolution, can directly detect the behaviour of chlorine in grain interiors. This means that, where present, variations in chlorine concentrations can be observed, both between grains and within grains. For instance, data shows concentration gradients within grains that indicate ingress of chlorine from grain interiors to the grain bulk. These local variations in grain interior chlorine concentration can then be correlated to the local luminescence intensity, to ascertain whether chlorine effects carrier recombination activity in the grain bulk. Another interesting behaviour observed is the segregation of chlorine to linear or ribbon-shaped features within many of the grain interiors in all three device types. The 3D nature of the data enables behaviour of chlorine at the three different interfaces to be tracked.
In addition to chlorine, the chemical maps show the three-dimensional behaviour of sulphur and selenium alloying in each of the device types.