# Towards a Glass New World: The Role of Ion-Exchange in Modern Technology
## 2 Glass Strengthening: The Ion-Exchange Contribution
### Chemical Strengthening: Applications
LeoflexTM glass, with its lower sodium concentration at the surface, exhibits better electrical resistivity and higher mechanical strength caused by the chemical toughening process and strongly reduces the risk of potential induced degradation (PID), which represents one of the main reasons for the failure of photovoltaic and space solar cells. All these features made this material an ideal candidate as a cover glass for the development of highly performing photovoltaic application [142]. Additionally, the ion-exchange process also plays a key role in the efficiency of the same cell by introducing noble metal ions/nanoparticles capable of triggering particular energy transfer mechanisms (i.e., down-conversion from ultraviolet (UV) to visible wavelengths or up-conversion from infrared (IR) to visible range), especially when the glasses are doped with elements belonging to the rare earth group. This subject, together with other ones which involve the use of metal aggregates for different application fields, will be addressed in the next section. Noble Metal Ions/Nanoparticles by Ion-Exchange in Glass Substrates: Novel Platform for Photonic & Sensing Applications  
Ion-Exchanged Noble Metal Ions/Nanoparticles in Rare-Earth--Doped Glasses for Enhanced Light Sources and Photovoltaic Cells  
The development of solid-state light sources by using glasses doped with different combinations of rare earths has driven the attempts to further improve their efficiency by using sensitiers added to the glass matrix. Their function is to increase the absorption of the excitation light, also at wavelengths where more efficient, reliable, and less expensive pump sources are available, and efficiently transfer the collected energy to the emitting rare earths. The same materials can also be tested to be used to enhance the efficiency of photovoltaic (PV) cells. In fact, a large portion of solar radiation in the UV spectrum cannot be absorbed or efficiently converted in electricity by silicon-based PV systems. In case the solar light in the spectral windows where PV cells are less efficient could be converted entirely inside the working wavelength interval of the cells, their theoretical efficiency would increase from 29% to 38%. It is therefore apparent that even a more realistic partial result could be a crucial improvement. Many approaches have been pursued, but, as we will describe in the following, usually metallic silver nanoparticles have been used as sensitizer. Their advantage is a strong broadband absorption band, joined to the possibility of introducing them in the glass by means of ion-exchange and subsequent thermal processes, which makes this mature technological approach fairly convenient and economical. [PERSON] et al. reported, in 1985, the enhancement of Eu\({}^{3+}\) luminescence in melted calcium boron oxy-fluoride glass, which they related to the local field enhancement produced by the surface plasmon resonance of the Ag particles [54]. In this case, silver was introduced directly in the glass melting process. This result was confirmed by [PERSON] et al. in solgel glasses where reduction of the silver ions to metallic form was obtained by annealing the glass in H\({}_{2}\)/N\({}_{2}\) atmosphere [55]. Similarly, [PERSON] et al. demonstrated for the first time non resonant energy transfer from Ag\({}^{+}\) ions (hence, non-metallic) to Dy\({}^{3+}\), Sm\({}^{3+}\) and Tb\({}^{3+}\)[143]. These are only few of many examples of rare earth doped glasses where ion-exchange was not used as a means for introducing silver nanoparticles in the glass and that showed the potential of using silver as sensitizer. In the following, we will focus on and limit to examples where ion-exchange was applied. [PERSON] et al. investigated the effects of silver introduced by ion-exchange in sodium borate glasses on Eu\({}^{2+}\) and Eu\({}^{3+}\) ions fluorescence [144]. The authors stressed the different role that few Ag atom/ion complexes, up to full grown nanoparticles, may play in influencing fluorescence emission.