In *ab initio* simulations, interactions among particles are treated on the basis of quantum mechanics without introducing any empirical parameter. Some physical approximations are necessary for solving the Schrödinger equation of the system. Among them, the Density Functional Theory (DFT) considers the electrons as independent particles that move on the effective potential created by the rest of the electrons of the system. The key issue in DFT is finding this effective potential, and different alternatives are available: local density approximation, generalized gradient approximation...

We have used ab initio simulations to evaluate the formation energy of small defect clusters in silicon, and to investigate the mid-gap levels introduced by these defects in the gap of c-Si. We have also used this simulation technique to investigate the interactions of carriers with defects in both crystalline and amorphous Si, and at their interface. We have identified intrinsic hole traps in a-Si associated to locally strained regions and studied their interaction with B atoms, and we have characterized the relevant defect states at c-Si/a-Si:H heterojunctions.

Electronic localization (red clouds) obtained with DFT simulations around relevant defects found at a-Si:H/c-Si interfaces: (a) H bridges, (b) dangling bonds, and (c) strained bonds. White and blue spheres represent Si and H atoms, respectively (I. Santos, *et al.*, J. Phys.: Condens. Matter 26, 095001 (2014) - Atomistic study of the structural and electronic properties of a-Si:H/c-Si interfaces).

Ab initio |

Tight-binding |

Classical molecular dynamics |

Binary collision approximation |

Kinetic Monte Carlo |

- Kinetic Monte Carlo simulations: an accurate bridge between ab initio calculations and standard process experimental data
- Insights on the atomistic origin of X and W photoluminescence lines in c-Si from ab initio simulations
- Atomistic study of the structural and electronic properties of a-Si:H/c-Si interfaces