The effects of chlorine and selenium in cadmium telluride solar cells

Abstract

Solar photovoltaics (PV) holds great promise to change the way that electricity is produced and used globally. As it stands, electricity is generated mainly by large coal and gas-fired power stations, which are expensive to build and rely on a fuel supply that becomes more expensive over time. By contrast, the costs of solar PV are falling rapidly, and solar is already producing electricity at lower levelised costs than coal and gas power stations. Moreover, it can do so with a very low environmental impact, and since it is a ‘distributed’ power source that does not require a fuel supply, also improves access to electricity and the overall security of supply.However, if these benefits are to be realised, deployment of solar PV needs to continue to scale significantly. Solar PV currently supplies only ~3% of worldwide electricity demand, and demand for electricity is set to nearly double by 2050 as a result of the electrification of heating and transport and rising living standards. In order to continue its rapid growth and help to meet a significant portion of future electricity demand, solar module efficiencies need to continue to rise and production costs need to decrease further.Fast-deposited thin-film PV technologies like cadmium telluride (CdTe) offer a promising route to achieve the necessary price decreases and industry scale-up because they are intrinsically less expensive to produce than the incumbent silicon PV modules, which require careful crystal growth and individual processing of each wafer, cell and module. The downside of fast thin-film deposition however is that the devices invariably have polycrystalline absorber layers with small crystal ‘grains’, and high defect densities. This not only limits power conversion efficiency compared to single crystal counterparts like silicon, but also makes the devices much more microstructurally and compositionally complicated, and hence more difficult to characterise and control. In particular, device-level characterisation techniques that were developed for homogeneous single crystal absorber layers are not sufficient to resolve the complexities of thin-film cells and high-resolution characterisation techniques have been under-used, slowing device development.Here we use high-resolution correlative characterisation techniques to investigate the effects of two elements that are vital to producing high efficiency cadmium telluride solar cells – chlorine and selenium. Using 3-dimensional NanoSIMS compositional mapping we find that following the essential cadmium chloride heat treatment, chlorine is not just present in grain boundaries – where it is known to have a passivation effect – but permeates every region of the CdTe absorber layer. It is found segregated at the front interface between the CdTe and the buffer layer, at incoherent twin boundaries that span grain interiors, and at dopant concentrations in grain interiors. In selenium alloyed CdTe devices we use high resolution NanoSIMS and SEM-based cathodoluminescence, on the same area of the absorber, to reveal that selenium alloying lowers non-radiative recombination levels in CdSeTe grain interiors, helping to explain the record performance of selenium-graded devices. We then use TEM-based cathodoluminescence to show that selenium also has a passivation effect on grain boundaries in CdSeTe, which is the first time that high-resolution TEM-CL mapping has been achieved on a solar cell.Together, these results help to explain how cadmium telluride devices have achieved efficiencies of over 22%, despite their fast absorber layer deposition and small grain sizes. The results suggest new routes for further efficiency improvement of CdTe solar cells, including by increasing selenium concentrations at grain boundaries and in the bulk material at the back of the absorber layer. This can reduce costs further for what is currently the lowest cost of all solar and fossil fuel electricity generation technologies, and hence help to spread the cost, security, and environmental benefits of solar photovoltaics. It is also intended that the work will encourage more high resolution, correlative characterisation of thin-film PV technologies in general.