High Resolution Image Download MS PowerPoint Slide Spectral conversion of sunlight can enhance solar cell efficiencies by shifting short-wavelength photons to longer wavelengths, where the photovoltaic response is stronger. This can be realized with near 100% energy efficiency by materials capable of quantum cutting . These emit two longer-wavelength photons, following the absorption of one shorter-wavelength photon. Although various quantum-cutting materials have been reported, follow-up research has often disproved initial claims. Here, we demonstrate that photon correlation analysis is a universal strategy to unambiguously identify quantum cutting. We investigate two materials, YPO 4 codoped with Tb 3+ and Yb 3+ and yttrium aluminum garnet (YAG) codoped with Ce 3+ and Yb 3+ . Both are reported to perform quantum cutting via blue light absorption followed by cooperative energy transfer to near-infrared-emitting Yb 3+, but some studies have cast doubts for YAG:Ce 3+,Yb 3+ . Our correlation analysis demonstrates bunched emission for YPO 4:Tb 3+,Yb 3+, confirming quantum cutting. In contrast, YAG:Ce 3+,Yb 3+ shows Poissonian emission statistics, disproving its quantum-cutting capability, despite previous claims.
The efficiency of single-junction solar cells is limited to about 30% (the Shockley-Queisser limit). Spectral mismatch losses (transparency to low-energy photons, thermalization of high-energy photons) strongly contribute to lowering the maximum efficiency. To reduce thermalization losses, photon splitting is proposed and observed for a variety of lanthanide-doped materials. For
Er3+, even a one-to-three photon-splitting process has been reported, yielding three IR photons at around 1530 nm following absorption of one blue-green photon. This is especially beneficial for narrow band gap solar cells, such as crystalline
Ge. Here, we report on photon splitting for
Er3+ in
YVO4. Following absorption in the
2H11/2 and
4S3/2 levels (520--550 nm), efficient cross-relaxation (CR) yields two excited
Er3+ ions: one in the
4I9/2 state and one in the
4I13/2 state (CR1). A second CR step from the
4I9/2 state, leaving both
Er3+ ions in the
4I13/2 excited state (CR2), is crucial in realizing efficient three IR photon splitting. It is demonstrated here that the second step has a low efficiency, as a result of competing fast multiphonon relaxation, ${}^{4}{\mathrm{I}}_{9/2}{\ensuremath{\rightarrow}}^{4}{\mathrm{I}}_{11/2}$, and a large energy mismatch, which makes the CR2 step thermally activated. Based on experiments and theory, a maximum quantum efficiency of 170% is calculated for IR emission, following blue-green excitation in
YVO4:Er3+. An outlook is presented for three-photon splitting in low-phonon-energy hosts, where nonradiative multiphonon relaxation is suppressed. The anti-Stokes nature of the second CR step makes three-photon splitting unlikely and prevents the realization of IR quantum yields above 200%.
The deposition of a thin layer of a quantum-cutter material on top of silicon-based solar cells seems to be a promising solution to reduce the thermalization losses. This mechanism has been reported in materials codoped with Pr3+-Yb3+, where Pr3+ can sensitize two Yb3+ ions for one absorbed blue photon. In the present Letter, we analyze precisely energy transfers between Pr3+ and Yb3+ in CaYAlO4, and we measure a quantum-cutting rate of 145%. We show that a very efficient back transfer from Yb3+ toward the (1)G4 level of Pr3+ ion leads to a strong reduction of the quantum yield.
The codoping of CaF2:Pr3+ with Yb3+ or Lu3+ ions is shown to avoid the clustering of Pr3+ ions, which otherwise prevent Pr3+ doped CaF2 to be used for various photonic applications. The breaking of Pr3+ clusters by Lu3+ ions paves the way towards the development of a Pr3+ doped CaF2 visible laser. On the other hand, the formation of Pr3+-Yb3+ clusters in place of Pr3+ clusters leads to extremely efficient energy transfers between Pr3+ and Yb3+ which could be used for quantum cutting applications. Two types of clusters are observed for both types of codopings. Pr3+ to Yb3+ energy transfer analysis shows that for one of the clusters an ultrafast energy transfer takes place with a rate of 5×107 s−1, which is likely due to exchange interaction.
Recently the infrared quantum cutting has achieved an exciting development in enhancing the efficiency of solar cell. The infrared quantum cutting of Er0.3Gd0.7VO4 crystalline is studied in the present paper. It is found that the approximate quantum cutting efficiencies of 1532.5 nm 4I13/2→4I15/2 fluorescence, when the 2H11/2 and 4G11/2 levels are excited, are about 178.55% and 177.61% respectively. Especially, effective three-photon and four-photon infrared quantum cuttings of Er0.3Gd0.7VO4 crystalline excited by visible and near-violet light are found for the first time in the present paper. An interesting "peak - valley" conversion phenomenon in the excitation spectrum is also observed. It is also the first time for us to find the first-order infrared quantum cutting based single species Er3+ ion for Er0.3Gd0.7VO4 crystalline.
Abstract In optical materials energy is usually extracted only from the lowest excited state, resulting in fundamental energy-efficiency limits such as the Shockley–Queisser limit for single-junction solar cells. Photon-cutting materials provide a way around such limits by absorbing high-energy photons and ‘cutting’ them into multiple low-energy excitations that can subsequently be extracted. The occurrence of photon cutting or quantum cutting has been demonstrated in a variety of materials, including semiconductor quantum dots, lanthanides and organic dyes. Here we show that photon cutting results in bunched photon emission on the timescale of the excited-state lifetime, even when observing a macroscopic number of optical centres. Our theoretical derivation matches well with experimental data on NaLaF4:Pr3+, a material that can cut deep-ultraviolet photons into two visible photons. This signature of photon cutting can be used to identify and characterize new photon-cutting materials unambiguously.