CrySaLID

The development of the photovoltaic (PV) sector requires significant progress in performance and reductions in cost. For crystalline Si solar cells in particular, it is well-known that the grain structure, impurities and defects left after the crystallisation step, have a major impact on the final PV properties. However, the involved fundamental mechanisms are still not fully understood which hinder an efficient and reproducible control of the crystallisation processes. In the last few years, research efforts concerning mono-like silicon crystal growth which is an interesting compromise between single crystalline and multicrystalline silicon both used for PV applications were resumed. Indeed, single crystals yield high PV efficiency but their fabrication requires costly crystal growth processes. Oppositely, multicrystalline silicon results in lower PV efficiency but can be fabricated using cheaper solidification processes close to metallurgical casting processes. Knowing this, the objective is to produce mono-like crystals i.e. with as few grains as possible using casting or directional solidification processes being initiated on seeds or by selecting the crystalline orientation by dendrite growth. For successful mono-like growth, the nucleation of parasitic grains and twins during crystallisation must be minimized and the subsequent growth controlled to favour the desired crystalline orientation. In addition, impurities and defects especially dislocations have major detrimental effects and are closely linked to the grain growth from seeds. The scientific objectives of the CrySaLID project are to deepen the understanding of the mechanisms of crystallisation of silicon grown from seed as parasitic grains nucleation, grain competition and twins with pure silicon or silicon containing light impurities. Moreover, the characterisation of defects and light impurities linked to the crystallisation and grain structure will be addressed to understand the generation of structural defects and the impact of impurities. The correlation with PV property measurement will also be conducted. The first technological objective is to define processing conditions for improved grain structure, impurity segregation and defect control for a growth from a selected crystalline orientation. The second aims at developing a 3D and predictive Si growth simulation tool at industrial scale. Concerning the company EMIX which is part of the consortium, one major objective within the project is to fabricate silicon with lower C levels suitable for subsequent mono-like crystal growth. The objectives of the CrySaLID project will be achieved by: i) complementary investigations of Si crystallisation mechanisms (grain growth and competition) by in situ X-ray imaging of its growth, Kyropoulos and mono-like crystal growth, bi-crystal growth investigation, ii) structural defects, impurity and PV characterisation, iii) multi-scale modelling (thermodynamic environment, impurity segregation, phase field modelling of grain boundary grooves and 3D grain structure industrial scale modelling), iv) Feedback to the industrial process.