Quantum cutting em nanocristais fluoretos co-dopados com terras-raras como proposta de otimização de células solares

Abstract

There has been a growing demand for renewable energy, among them, solar energy is undoubtedly one of the most important. However, silicon-based solar cells have low efficiency in the ultraviolet (UV) region, precisely a region where the sun emits a lot of energy. In the regions where the efficiency of this type of solar cell is high, in the nearinfrared (NIR), solar emissions are not very intense, compared to those in the UV and visible. Thus, a way to increase the efficiency of solar cells is to use systems that can convert high-energy photons (in the UV) into low-energy photons (in the NIR). One mechanism that does this is called quantum cutting (QC). With this goal in mind, we synthesize NaYF4 nanocrystals (NCs) here by the coprecipitation method using ethylene glycol as a solvent. They were co-doped with Yb3+ and several other rare earths (REs = Pr3+, Ho3+, Er3+ and Tm3+). Various concentrations of Yb3+ and REs were used to optimize the QC process. In addition to these NCs, we also use LaF3 (core-shell type) NCs codoped with 10 mol% Yb3+ (in the core) and 10 mol% Nd3+ (in the shell) (LaF3:10Yb@LaF3: 10Nd). All NCs used were subjected to heat treatment at 500 ºC for 3 hours. In all samples, QC was obtained under excitation in the UV with emission around 1000 nm from Yb3+ ions. As an example of the high efficiency obtained in this process, for the NaYF4:0.5Pr3+/10Yb3+ (0.5 mol% of Pr3+ and 10 mol% of Yb3+) we obtained a quantum efficiency of QC of 188%. Individual studies on each ion pair (Yb/RE) were performed. However, the proposal to use several RE ions is to cover as much as possible the solar absorption spectrum in the UV-Visible and for that, after these individualized studies of the ion pairs, the most suitable/efficient samples were identified to make a mixing process of powders and thus obtain a “perfect” system for absorption of the solar spectrum with Yb3+ emission around 1000 nm. Thus, our idea of using REs ions to convert high-energy photons (in the UV, where the efficiency of silicon-based solar cell is low) into low-energy photons (in the NIR, where their efficiency is high) is shown as an extremely interesting proposal to increase the efficiency of this type of solar cell. Mainly, considering the fact that we propose to mix the NCs here in different proportions in weight%, which is a much simpler process to be done than synthesizing a nanoparticle with all these RE ions together.