Quantum cutting converting a ultraviolet photon into two near-infrared photons has been demonstrated by spectroscopic measurements in NaYF4:Ho3+,Yb3+ synthesized by hydrothermal method. Evidence is provided to confirm the occurrence of quantum cutting. Upon excitation of Ho3+ 5G4 level, near-infrared quantum cutting could occur through a two-step resonance energy transfer from Ho3+ to Yb3+ by cross relaxation, with a maximum quantum efficiency of 155.2%. This result reveals the possibility of violet to near-infrared quantum cutting with a quantum efficiency larger than 100% in Ho3+/Yb3+ codoped fluorides, suggesting the possible application in modifying the solar spectrum to enhance the efficiency of silicon solar cells.
©2011 Optical Society of America
The mismatch between the solar spectrum and the band gap energy of silicon semiconductor limits the energy efficiency of crystalline silicon based solar cells, which occupy a majority of the market share in the field of solar cells. Photons with energy lower than the band gap cannot be absorbed, while for photons with energy larger than the band gap, the excess energy is lost by thermalization of hot charge carriers . To achieve a higher energy efficiency, tandem solar cells consisted of multiple semiconductor layers have been developed and each semiconductor layer has a characteristic band gap converting a different part of the solar spectrum . However, the complicated fabrication and high cost limit their practical use. Another feasible method allowing the solar cells to capture more energy from the solar spectrum is by spectral modification through near-infrared quantum cutting [3,4]. Cutting a ultraviolet or blue photon into two near-infrared photons, which can be well absorbed by solar cells, can reduce the energy loss related to thermalization of hot charge carriers and then boost the efficiency of solar cells. Because of this potential application in solar cells near-infrared quantum cutting has attracted a lot of recent research attention [5–14].
Effective spectral modification can be obtained by combination of different rare earth ions thanks to the unique and rich energy levels of lanthanide rare earth ions covering a wide spectral range from ultraviolet to infrared. Near-infrared quantum cutting was first achieved in YPO4:Tb3+,Yb3+  where a visible photon was converted into two near infrared photons through cooperative energy transfer from Tb3+ to Yb3+. Yb3+ ion possesses only two manifolds: the 2F7/2 ground state and the 2F5/2 excited state, which are separated by about 10000cm−1 in the energy level scheme. Photons emitted by Yb3+ ions is around 1000nm, just above the band gap of crystalline silicon, where the silicon solar cells show an excellent spectral response. Similar phenomena have also been observed in RE3+/Yb3+ (RE = Bi, Tm, and Ce) codoped phosphors and glasses [10–12]. Nevertheless, due to the second order nature of the transfer process, such a cooperative energy transfer is not as efficient as resonance energy transfer, and a high transfer efficiency will only be possible at relatively heavy Yb3+ doping where Yb3+ emission is largely depressed by concentration quenching . Therefore resonance energy transfer between rare earth ions is more favorable in order to get a higher quantum efficiency. However, until now, there have been few reports of near-infrared quantum cutting through resonance energy transfer [13,14].
Ho3+/Yb3+ codoped NaYF4, a host with phonon energy about 400cm−1 , is well-known as a high-efficiency upconversion phosphor due to efficient energy transfer from Yb3+ to Ho3+. Figure 1 presents a schematic diagram of the energy levels of Ho3+ and Yb3+ that are of interest. Close inspection shows that the energy gap of 5G4 and 5F5, 5F5 and 5I7 are resonant with the energy for the transition from 2F5/2 to 2F7/2 on Yb3+. As a result, resonance energy transfer by cross relaxation can be expected to realize quantum cutting. In this paper we investigate the spectroscopic properties of Ho3+ single doped and Ho3+/Yb3+ codoped NaYF4, and demonstrate the existence of near infrared quantum cutting as well as the quantum cutting mechanism for the first time.
The powder samples of NaYF4 doped with Ho3+(0.5mol%) and Yb3+(0,5,10,20mol%) were synthesized by hydrothermal method. All the chemicals are of analytical grade reagents and used without further purification. The RE(NO3)3 standard solutions(Y/Ho/Yb, 2 mmol in total) with desired stoichiometric ratio were added into 35ml aqueous solution containing 16 mmol NaF by dropwise under magnetic stirring. Then the resulting suspension was transferred into a 50 ml Teflon bottle held in a stainless steel autoclave, sealed, and maintained at 180°C for 24h. After cooling to room temperature, the precipitates were separated by filtration, washed with ethanol for several times and then dried in air. The crystalline phases of the synthesized samples were characterized by XRD(MAC Science Co. Ltd. MXP18AHF), using nickel-filtered Cu Kα radiation in therange from 10° to 70°. Luminescent spectra were measured with a Jobin Yvon Fluorolog-3 system equipped with a 450W Xe-lamp as excitation source and a 50W flash lamp for time resolved measurements, where the visible and infrared emission were detected with a Hamamatsu R928 photomultiplier tube and a liquid nitrogen-cooled DSS-IGA020L InGaAs detector, respectively. To compare the emission intensities of samples with different Yb3+ doping, the emission spectra were recorded at identical experimental conditions. All the measurements were carried out at room temperature.
3. Results and discussion
The XRD patterns for NaYF4 samples with 0.5mol% Ho3+ and different Yb3+ doping concentrations are shown in Fig. 2 . The little change of the lanthanide ions in their atomic radii facilitates the substitution for Ho3+ and Yb3+ within the NaYF4 matrix. Compared with standard data(JCPDS No.16-0334), all the samples exhibit the peaks of pure hexagonal phase. No second phase is detected in the XRD pattern, revealing the successful doping of Ho3+ and Yb3+ ions in NaYF4. Additionally, the fairly narrow full width at half maximum and intense diffraction peaks indicate the well crystallization of the sample. However, due to the preferential growth effect in the hydrothermal process , the relative intensities of the peaks are a little different from those in standard data.
In Fig. 3 the emission spectra for NaYF4 doped with Ho3+(0.5mol%) and Yb3+ (0,5,10,20mol%) are shown for Ho3+ 5G5’ excitation at 359nm. For the Ho3+ single-doped sample, excitation into the Ho3+ 5G5’ level yields the Ho3+ characteristic emission from the 5G4, 5G5, 5F3, 5S2, 5F5 level. The intensities of the 5G4, 5G5 and 5F3 emission are rather weak compared to the strong emission originating from the 5S2→5I8 transition(about two orders of magnitude weaker in integrated intensity). The energy gap between the 5G4, 5G5 and 5F3 level to the next lower level is about 2000cm−1, 1500cm−1 and 2000cm−1, respectively. The energy gap law  predicts that radiative decay and multiphonon relaxation can compete only when the gap is five times the phonon energy. Based on the present observations and the energy gap law, we can assert that multiphonon relaxation dominates over radiative decay for the 5G4, 5G5 and 5F3 level. Apart from visible emission from Ho3+, infrared emission band around 1200nm, corresponding to the 5I6→5I8 transition, as well as intense Yb3+ 2F5/2 emission around 1000nm are also observed. The appearance of Yb3+ infrared emission in Ho3+/Yb3+ codoped sample results from energy transfer from Ho3+ to Yb3+. Furthermore, visible emissions from Ho3+ are significantly weakened in their intensities with the increase of Yb3+ concentration, while the most intense Yb3+ emission at 1000nm is obtained for 5mol% Yb3+ doping as a result of concentration quenching. In addition, Ho3+ emissions in the visible range can hardly been observed in the sample with 20mol% Yb3+ doping, suggesting an efficient energy transfer from Ho3+ to Yb3+.
Further information of energy transfer from Ho3+ to Yb3+ can be obtained from the excitation spectra for Ho3+ emission at 540nm corresponding to the 5S2→5I8 transition, as shown in Fig. 4 . The excitation spectra for all the samples with various Yb3+ concentrations are normalized to the 5G6/5F1 level. A series of bands appear in the excitation spectra as excitation into these energy levels is followed by multiphonon relaxation to the 5S2 level. An obvious decrease in the excitation intensities for the bands corresponding to the transition to the 5G4 level as well as the upper levels (from 325nm to 400nm, denoted as Part A) compared with the bands below the 5G4 level (from 400nm to 500nm, denoted as Part B) is observed as Yb3+ concentration increases, whereas the excitation bands inside Part A and Part B undergo almost no change in their relative intensities. This decrease is ascribed to the cross relaxation from Ho3+(5G4→5F5) to Yb3+(2F7/2→2F5/2), since it depopulates the population of the 5S2 level only when excitation is not lower than the 5G4 level. If no such cross relaxation occurs, however, the relative intensities of the excitation spectra should remain unchanged. Assuming that excitation into levels between 5G2 and 5F3 is followed by multiphonon relaxation to the 5S2 level except for the cross relaxation from Ho3+ 5G4 to Yb3+, the efficiency of the cross relaxation can be expressed asEq. (1), a cross relaxation efficiency of 25.5%, 45.3% and 61.2% can be obtained for the samples with 5, 10 and 20mol% Yb3+ doping, respectively. It is worthwhile to note that this cross relaxation competes mainly with multiphonon relaxation to the next lower energy level of 5G4 and a host lattice with a lower phonon energy will facilitate the cross relaxation rate.
The cross relaxation depopulates the 5G4 level but simultaneously populates the 5F5 level. Thus an increase of the 5F5 emission intensity is expected in NaYF4:Ho3+,Yb3+ compared with that in NaYF4:Ho3+. On the contrary, the 5F5 emission intensity drops with increasing Yb3+ concentration according to Fig. 3. Figure 5 presents the decay curves of Ho3+ 5S2 emission at 540nm as well as 5F5 emission at 650nm. It is noted that both the Ho3+ emissions in all the samples show a nearly exponential decay and the lifetime, fitted by a single exponential, decreases rapidly with the increase of Yb3+ concentration as can be seen in the insets of Fig. 4. Since the Ho3+ concentration was maintained constant in all the samples, the decline of the lifetime should not be attributed to the concentration quenching of Ho3+ but to extra decay pathway introduced by Yb3+ doping: resonance energy transfer from Ho3+ to Yb3+ by cross relaxation from Ho3+(5S2→5I6) to Yb3+(2F7/2→2F5/2) and Ho3+(5F5→5I7) to Yb3+(2F7/2→2F5/2), which accelerates the depopulation of 5F4(5S2) and 5F5 level, respectively, and leads to a shorter lifetime and weaker intensity for both emission. The presence of the former cross relaxation can also be confirmed by the pronounced increase of 5I6 emission in Ho3+/Yb3+ codoped sample. The 5I6 emission around 1200nm, however, is beyond the spectral response of Si solar cells. From the decay curves the energy transfer efficiency can be determined using the following equation :
In the above discussions near-infrared quantum cutting involved two-step sequential cross relaxation: Ho3+(5G4→5F5) to Yb3+(2F7/2→2F5/2) followed by Ho3+(5F5→5I7) to Yb3+ (2F7/2→2F5/2), has been clearly demonstrated as exhibited in Fig. 1. A measurement of excitation spectra of Yb3+ emission in the Ho3+/Yb3+ codoped sample can provide a direct test of the performance of quantum cutting in comparison with excitation spectra of Ho3+ 1190nm emission in Ho3+ single doped sample as shown in Fig. 6 . The infrared excitation spectra were run with a wider monochromator slit width and thus it has a lower spectral resolution compared with the visible excitation spectra. The excitation spectra are normalized to the 5G6/5F1 level as well. If quantum cutting occurs in the 5G4 level, the relative intensity of the excitation bands related to the transition to this level as well as the levels above should increase in the excitation spectra of Yb3+ emission; meanwhile, the excitation bands below the 5G4 level should remain the same, since in the quantum cutting process two near-infrared photons are generated, rather than one in a trivial one-step energy transfer from the 5S2 level . In Fig. 6 an expected increase of excitation intensity at 5G4 level together with the levels above is clearly observed, verifying the occurrence of near infrared quantum cutting. Supposing there is no nonradiatvie energy loss by defects and impurities, the quantum efficiency can be estimated, to a good approximation, according to the formula below:
In summary, an efficient near-infrared quantum cutting through two sequential resonant energy transfer from Ho3+ to Yb3+ has been demonstrated in Ho3+/Yb3+ codoped NaYF4. A maximum quantum efficiency of 155.2% can be determined from excitation spectra and decay curves. The high quantum efficiency and the intense Yb3+ near-infrared emission indicate the potential application in realizing a high energy efficiency of crystalline silicon based solar cells.
This work was supported by National Nature Science Foundation of China (10774140, 11011120083, 11074245, and 10904139), Knowledge Innovation Project of the Chinese Academy of Sciences (KJCX2-YW-M11), and Special Foundation for Talents of Anhui Province, China (2007Z021).
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