Characterisation of high-performance photovoltaic materials for space applications using a correlative cathodoluminescence/photoluminescence spectrosc
Lead Research Organisation:
University of Cambridge
Department Name: Materials Science & Metallurgy
Abstract
In the context of photovoltaics for space applications, the term high-performance encompasses several material properties. A key requirement for the photovoltaic devices and one that ties together many of the desired material properties is that of high efficiency. The power supply to satellites is the limiting factor in the possible mission objectives and usually also determines their lifetime. It is therefore the need for materials that enable the fabrication of high efficiency devices, not only at the beginning of life, but also maintained despite the harsh conditions of space, that drives the research of this project.
The project will explore two sides of this high efficiency problem, improvement of both efficiency at beginning of life as well as sustainability of on-mission performance. It will aim to do so through characterization of materials using cathodoluminescence and photoluminescence spectroscopy methods, which will improve our understanding of the optical and electrical properties of materials and give us the information to make vital improvements to materials or device structures.
The current industry standard for space power systems is an on-wafer III-V multi-junction technology. In this project we will develop materials for an alternative high efficiency concept: the hot-carrier solar cell. This emerging device concept could enable high efficiency in a thin, comparatively simple and thermodynamically elegant design. We will use luminescence techniques to study hot carriers in semiconductors and quantum confined structures, including their thermalization and extraction. Advances in retardation of thermalization mechanisms coupled with improved efficiency of extraction of hot carriers could lead to a new, fundamentally different class of photovoltaics, with efficiencies beyond the Shockley-Queisser limit.
We will also address the challenge of radiation damage in these materials which is the main cause of performance deterioration for space photovoltaics. Impinging radiation introduces defects in the absorber, which act as non-radiative recombination sites for photogenerated carriers and therefore lead to a decrease in device-efficiency and therefore performance. The experimental techniques identified above can be used to effectively study defect sites. The planned correlative approach, together with capacity for time-resolved measurements in both cathodoluminescence and photoluminescence spectroscopy may provide new insight on defects and their effects in novel device structures.
The project will explore two sides of this high efficiency problem, improvement of both efficiency at beginning of life as well as sustainability of on-mission performance. It will aim to do so through characterization of materials using cathodoluminescence and photoluminescence spectroscopy methods, which will improve our understanding of the optical and electrical properties of materials and give us the information to make vital improvements to materials or device structures.
The current industry standard for space power systems is an on-wafer III-V multi-junction technology. In this project we will develop materials for an alternative high efficiency concept: the hot-carrier solar cell. This emerging device concept could enable high efficiency in a thin, comparatively simple and thermodynamically elegant design. We will use luminescence techniques to study hot carriers in semiconductors and quantum confined structures, including their thermalization and extraction. Advances in retardation of thermalization mechanisms coupled with improved efficiency of extraction of hot carriers could lead to a new, fundamentally different class of photovoltaics, with efficiencies beyond the Shockley-Queisser limit.
We will also address the challenge of radiation damage in these materials which is the main cause of performance deterioration for space photovoltaics. Impinging radiation introduces defects in the absorber, which act as non-radiative recombination sites for photogenerated carriers and therefore lead to a decrease in device-efficiency and therefore performance. The experimental techniques identified above can be used to effectively study defect sites. The planned correlative approach, together with capacity for time-resolved measurements in both cathodoluminescence and photoluminescence spectroscopy may provide new insight on defects and their effects in novel device structures.
Organisations
| Description | The work funded through this award has significantly improved understanding of the operation of ultra-thin GaAs solar cells for space applications and the impact of radiation-induced defects on them. This has been achieved through fabrication of the cells and their subsequent characterisation and subjection to radiation testing. The devices were characterised by cathodoluminescence hyperspectral imaging (CL), including time-resolved (TR) measurements, and electrically, by current-voltage and quantum efficiency measurements. For radiation testing, the devices were exposed to high energy protons (20 keV, 100 keV, 1 MeV, 3 MeV, 68 MeV) and electrons (1 MeV) using either pelletron, Cockroft-Walton or cyclotron accelerators. This work has gone beyond previous studies on the radiation tolerance of ultra-thin cells, by irradiating them to their point of failure. By correlating this point of failure with carrier lifetimes in the absorber layer of the device, extracted from TRCL measurements, a model was developed to explain the high radiation tolerance of these devices. Furthermore, it was demonstrated that this point of failure occurs at a radiation dose so high, that these devices could survive significantly longer in harsh radiation environments in space than conventional space photovoltaics, or would survive the amount of time with significantly thinner cover glass to shield them, which could result in overall lighter solar panels on space power systems, reducing launch costs. Improvements in the understanding of the current-voltage behaviour of the solar cells and the effects of defect introduction and changes in carrier lifetime were also made, once again by the correlation of data acquired by luminescence and electrical measurements. It was demonstrated that Shockley-Read-Hall recombination statistics is sufficient to explain the relationship between carrier lifetime and reverse saturation current, but is unable to account for the complex variations in slope of the dark current-voltage curves, which have an impact on the open-circuit voltage. Work to understand the origin of this behaviour is ongoing. Characterisation of semiconductor material and devices irradiated with electrons and protons of different energies, including those where nuclear reactions are possible and those where they are not is providing new insight into the validity (in a particular energy range) of the current mainstream theory (non-ionising energy loss) that allows direct comparison of radiation damage induced by different particles at different energies. |
| Exploitation Route | The results of this research should feed into continued development of cathodoluminescence as a quantitative technique for evaluation of semiconductor material quality, particularly through characterisation of carrier dynamics. As the results further illustrate the behaviour of ultra-thin solar cells and the dependence of their performance on material parameters and radiation damage, they should be considered in future design and optimisation of similar devices. Explanation of these results can be aided by future studies into the identity and introduction mechanisms of radiation-induced defects and how they interact with carriers as a function of electric field and temperature. The results of this project are suggesting that more experimental and theoretical work is needed to develop the theory allowing direct comparison between radiation damage induced by different sources, including those where nuclear reactions are possible. The outcomes of this work will be put to use in the development of photovoltaics for space based solar power. |
| Sectors | Aerospace, Defence and Marine,Creative Economy,Energy,Environment,Other |