Infrared Quantum Cutting In Rare-Earth Doped Transparent Glass-Ceramics

The urgency of solar cell industry is to enhance the conversion efficiency at limited cost.And solar cell is a 'one electron-hole pair for one photon' device, which means waste ofexcess energy in the ultra-violet region. Quantum cutting, converting one high energy photoninto two or more photons, is likely to improve the efficiency limit to 37%, after detailedcalculation.However, most works focus on fluoride phosphors or ceramic materials. Those materialsare of great rigidity and hard to shapen. The high cost of sophiscated manufacture processlimites their application. While, rare earth doped oxy-fluoride glass-ceramics exhibit both thecrystals' excellent optical property and the glasses' easy fabrication property. Fewinvestigations are reported on QC in glass-ceramics.We report on near infrared QC in rare earth doped glass ceramics. Precipitation of BaF₂nano-crystals under appropriate heat treatment condition is confirmed by XRD and HRTEManalysis. We choose RE³⁺ ions (Tb³⁺, Pr³⁺ and Tm³⁺) to co-dope with Yb³⁺ion, since theenergy of ⁵D₄→⁷F₅ transition of Tb³⁺ion, the energy of ³P₀→³H₄ transition of Pr³⁺ion and theenergy of ³H₆→¹G₄ transition of Tm³⁺ion are of twice the energy of Yb³⁺ion's ²F_5/2→²F_7/2transition. After excitation, the energy transfered from sensitive ion to Yb³⁺ ion result in itsNIR emission and this process is the typical quantum cutting phenomenon.We employ PL spectra and decay curve to find how concentration (Yb³⁺ ion), heattreatment condition affect the energy transfer process. We also calculate the lifetime of RE³⁺emission, the energy transfer efficiency (EFE) and the efficiency of QC.We find adding more Yb³⁺ ions can speed up the ET process. For example, within theTb³⁺, Yb³⁺ codoped system, as the concentration of Yb³⁺ ions increases, the lifetime of Tb³⁺emissioncentered at 542nm decreases and the QC efficiency increases to 145%. The QCefficiency of Pr³⁺, Yb³⁺ system and Pr³⁺, Yb³⁺ system rises from 151% to 191% and 101.6%to 135.4%, respectively.The heat treatment condition affects the QC effect within RE³⁺, Yb³⁺ions by changingthe phase of BaF2 crystals. As the temperature rising and time prolonged, the size of crystalgrow and more rare earth are incorporated into nano-crystals. As we know, crystalcircumstance with low phonon energy is beneficial to energy transfer and transition emission.In the Tm³⁺, Yb³⁺ system, as the temperature rises from 640℃to 740℃, the crystal size increases from 25nm to about 70nm, the lifetime of Tm³⁺ ion decrease from 21.41μs to 7.11μs,while the QC efficiency increases from 138.5% to 179.6%.