Oxygen complexes in solar silicon

According to the recent report by International Energy Agency (IEA), power generation from solar cells, i.e., photovoltaics (PV) is estimated to have increased by 22% in 2019, to 720 TWh. With this increase, the solar PV share in global electricity generation is now almost 3%. In 2019, PV generation overtook bioenergy and is now the third-largest renewable electricity technology after hydropower and onshore wind. Over 90% of all PV installations are based on silicon solar cells. It is clear that understanding and controlling properties of silicon is of a great importance. Several decades of research and development resulted in technologies that provide very clean material with unintended impurities less then 1 atom per 10 000 - 100 000 silicon atoms. Nevertheless, even impurities with such a low concentration can lead to significant degradation of solar cell. One of the most usual unintended impurities in silicon for solar cells, so-called solar silicon, is oxygen. Experiments show that oxygen can, at certain conditions, cause a significant degradation in solar cell performance. In one of the studies, it was demonstrated that this degradation can results in up to 20% loss in the energy output. In this project, we have investigated how oxygen can affect the electronic properties of solar silicon. At elevated temperatures, the oxygen impurity atoms can start to migrate within the silicon crystal lattice. During this migration, oxygen atoms can meet and bind with other impurities and/or structural defects. We put a particular focus on interaction of oxygen with (i) carbon, another abundant impurity, and (ii) vacancies, the most fundamental structural defect in the crystal. We have detected and characterized such a complex as carbon-dioxygen (C-O2) and tentatively identified divacancy-dioxygen complex (V2-O2) in silicon. We have studied the conditions at which these complexes are formed, as well as conditions at which they can be eliminated. Our findings have been met with interest from scientists and technologists. We believe the results will contribute to improvement of silicon-based solar cell technology.

The project has resulted in a considerable number of publications in form of presentations at meetings and conferences, and in form of some academic articles. In particular, the results have been made known to the industrial partners at FME SUSOLTECH via presentations at The Norwegian Solar Cell Conference (NSCC) organized by the FME. Among specific scientific results that were pioneered in OxSil one can mention: first tentative identification of V2O2 and evaluation of its electronic properties; first studies of thermal stability of CiO2i and determination of the activation energy for dissociation as 2.55 eV; first comprehensive theoretical study of different reaction paths for Ci and oxygen in solar Si.

The project is aimed at improving our fundamental understanding of silicon as a material in solar cells. The proposal is based on our track record and expertise in silicon accumulated over decades. OxSil will address one of most crucial and fundamental challenges in silicon technology for solar cells. This challenge is related to understanding of electronic properties and interaction kinetics of oxygen-related complexes. Oxygen is the most abundant impurity in solar silicon materials and has a decisive impact on the solar cell performance. In this project we will utilize: our background knowledge in defect engineering; our international collaborations on DFT simulations; silicon from established and well-known suppliers; and state-of-the-art experimental equipment located at MiNaLab, UiO and at our international partners. OxSil is divided into three Work Packages led by Principal Investigators: WP1: Formation and annealing kinetics of On and VOn (B.G. Svensson) WP2: Electronic properties and stability of V2O2 and V2O3 (E. Monakhov) WP3: Complexes of interstitial defects with O2i (A. Galeckas) Within the project we will enroll one 3-year postdoc (starting 01.03.2016) and one PhD student (starting 01.07.2016). OxSil will combine collaborative efforts from researchers at three European Universities: -University of Manchester -Technical University in Dresden -University of Aveiro Specific challenges that will be addressed within the project are the following: - kinetics of On and VOn complexes - electronic levels and kinetics of V2O2 and V2O3 - kinetics of IO2 and CiO2 - electronic levels and kinetics of BiO2