Strong 1550 nm to visible luminescence in In/Er/Yb:LiNbO₃ crystal considered as an upconverter for solar cells

Abstract

Enhanced green and red upconversion emissions are observed in Er/Yb:LiNbO₃ codoped with 3 mol% In³⁺ ions under both 1550 nm and 980 nm excitation, which provides more probabilities to increase efficiently the solar spectrum response and further increase the photovoltaic efficiency of solar cells. The energy transfer (ET: ²F_7/2 Yb + ⁴I_11/2 Er → ²F_5/2 Yb + ⁴I_15/2 Er) and the energy back transfer (EBT: ²F_5/2 Yb + ⁴I_13/2 Er → ²F_7/2 Yb + ⁴F_9/2 Er) processes are also responsible for the population of the red emitting ⁴F_9/2 state (Er) under 1550 nm excitation. The increased red emission is attributed to the dissociation of Er³⁺ cluster sites in the In/Er/Yb:LiNbO₃ crystal.

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1. Introduction

Rare-earth ion doped materials (REDMs), used as upconverters for enhancing the photovoltaic efficiency of solar cells, are currently attracting extensive interest.¹ Solar cells, directly using renewable solar energy to generate heat or electricity, are promising devices to make a significant contribution to the improvement of global energy consumption and to meet the long-term worldwide energy demand.^2–4 In order to suppress the transmission of sub-band-gap light which is one of the major loss mechanisms in solar cells, REDMs have been used as upconverters applied to the back of solar cells since they can easily upconvert the absorbed near-infrared (NIR) light into visible upconversion (UC) emissions.^5–9

Among many trivalent rare earth ions, an Er³⁺ ion doped material may be the most promising candidate to be used to efficiently increase the solar spectrum response since its transitions of ⁴I_15/2 → ⁴I_11/2/⁴I_13/2 can convert NIR sunlight at wavelengths of 980 nm and 1550 nm into visible emissions. Absorbing the 980/1550 nm sunlight could improve the amount of sunlight absorbed by solar cells, for example amorphous silicon solar cells (ASSCs) can only absorb sunlight with wavelengths shorter than 850 nm, and dye-sensitized solar cells (DSCs) can lose the NIR solar radiation due to their wide bandgap (1.7–1.8 eV).¹⁰ A. Shalav et al. first reported that Er³⁺:NaYF₄ phosphors attached to the rear of a bifacial silicon solar cell enhance its responsivity in the NIR range.¹¹ It has been reported by J. de Wild et al. that an enhancement of 10 μA cm⁻² was measured under 980 nm excitation in a single junction ASSC with an upconverter of Yb³⁺/Er³⁺:β-NaYF₄.¹² G. B. Shan et al. reported that about 10% enhancement of photocurrent and an overall improvement of DSC efficiency were found in DSCs after the addition of an external Er³⁺/Yb³⁺:β-NaYF₄ hexagonal nanoplatelet.¹³ Recently, many works on the optical characteristics of Er³⁺ ions and Er³⁺–Yb³⁺ ion pairs under 1550 nm excitation have sought to improve the efficiency of solar cells. G. Y. Chen et al. reported intense visible and near-infrared upconversion emissions in colloidal Er³⁺:LiYF₄ nanocrystals under 1490 nm excitation. It is proposed that there is a cross relaxation (CR: ²H_11/2/⁴S_3/2 + ⁴I_11/2 → 2⁴F_9/2) process between two adjacent Er³⁺ ions under 1490 nm excitation.¹⁴ R. M. Rodríguez et al. has converted successfully infrared (IR) emission into NIR upconversion emission, which is a promising option to enhance the efficiency of solar cells.¹⁵

Although the host materials are confined mainly to nanosystems, glass-systems, polymer-systems and others, it is worthwhile to study the optical characteristics in rare-earth ion doped LiNbO₃ crystals. This is because the well-known versatile LiNbO₃ crystal, exhibiting electro-optic, piezo-electric and nonlinear optical physical properties, can successfully combine with the amplifying and lasing characteristics of Er³⁺ ions.¹⁶ Previous work on Er:LiNbO₃ applied in integrated optics may offer many more options for the miniaturization of the solar cell.^17,18 And LiNbO₃ crystals exhibit high mechanical capabilities and excellent chemical stability. Indium (In³⁺) ions, which have a low threshold concentration of 3 mol%, are used as optical damage resistant ions to suppress the birefringence change and the deformation of the laser beams in Er:LiNbO₃ and Er/Yb:LiNbO₃ crystals.¹⁹

In this paper, we primarily found that the green and red UC emissions could be increased in Er/Yb:LiNbO₃ crystals through codoping with In³⁺ ions under 1550 nm/980 nm excitation. The UC mechanisms of the Er³⁺ ion and Er³⁺–Yb³⁺ ion pairs in LiNbO₃ host materials are studied.

2. Experimental section

Congruent LiNbO₃ crystals codoped with (a) 0.5 mol% Er³⁺ ions, (b) 0.5 mol% Er³⁺, 0.5 mol% Yb³⁺ and 0, 1, 2 and 3 mol% In³⁺ ions were grown using the Czochralski technique. The growth and polarization procedures are described in detail in our previous paper.²⁰ These grown crystals are named Er-0.5, In-0, In-1, In-2 and In-3, respectively.

The XRD spectra were measured using a powder diffractometer with Cu-Kα radiation (D-MAX 2200 VPC, RIGAKU Inc, Japan). The UC emission spectra were measured using a 1550 nm distributed feed back (DFB) laser (ADFB-PU-1550-33-R-FA, Amonics Inc., Hong Kong), and the signal was produced by a Function signal generator (Agilent, America). A combined fluorescence lifetime and steady state spectrometer (FLS920, Edinburgh Inc., England) was used to record the fluorescence spectra. The UC emission spectra, which were excited by a power-controllable 980 nm diode laser, were recorded using a spectrometer (Bruker optics 500IS/SM) equipped with a semiconductor cooled charge coupled device detector (DV440, Andor).

3. Results and discussion

The powder X-ray diffraction (XRD) pattern of the In-3 crystal is shown in Fig. 1, which shows that all of the diffraction peaks can be indexed to the known phase of LiNbO₃ based on the standard XRD pattern (JCPDS). No new peaks appear in the In-3 crystal XRD pattern, indicating the phase purity of the LiNbO₃ product. As illustrated in Fig. 1, compared with the pure hexagonal phase (space group: R3c) of the LiNbO₃ structure, the doping In³⁺, Er³⁺ and Yb³⁺ ions have no influence on the structure of the In-3 crystal, suggesting that this crystal still has a trigonal system. Therefore, the In³⁺, Er³⁺ and Yb³⁺ ions may occupy the normal Li-site or Nb-site rather than interstitial sites within the lattice.

Fig. 1 XRD patterns of the In-3 crystal.

Fig. 2 shows the visible UC emission spectra of the Er-0.5 and In/Er/Yb:LiNbO₃ crystals under 1550 nm and 980 nm excitation. Fig. 2(a) presents two weak green bands centered at 530 nm and 558 nm and a strong red band at 672 nm. The green bands correspond to the ²H_11/2/⁴S_3/2 → ⁴I_15/2 transitions of the Er³⁺ ion, respectively, and the red UC emission is attributed to the ⁴F_9/2 → ⁴I_15/2 transition.²¹ As illustrated in Fig. 2(a), the green and red UC emissions increase for In-1 and In-3, whereas they decrease for In-2. The intensity of the red UC emission of In-3 is over four times higher than that of In-0. Compared with Er-0.5, the introduction of Yb³⁺ ions leads to an increased red UC emission. It can be seen from Fig. 2(b) that the increased green and red UC emissions are observed for the In-3 crystal under 980 nm excitation, indicating that the In³⁺ doping favors not only the conversion of 1550 nm NIR sunlight into visible green and red UC emissions, but also the absorbtion of 980 nm wavelength sunlight. Therefore, using In/Er/Yb:LiNbO₃ as an upconverter will make a significant contribution to increasing the solar spectrum response and further improvement of the high photovoltaic efficiency of solar cells.

Fig. 2 The UC emission spectra of In/Er/Yb:LiNbO₃ and Er-0.5 (a) under 1550 nm excitation, (b) under 980 nm excitation.

In order to understand the effect of the In³⁺ ions on the optical characteristics of the Er³⁺ ion and Er³⁺–Yb³⁺ ion pair under 1550 nm excitation, the UC emission intensities (I) were measured as a function of the pump power (P). The number of photons (n) required to populate the upper emitting state can be obtained from the equation I ∝ Pⁿ.²² Log–log plots of pump power dependence in In/Er/Yb:LiNbO₃, shown in Fig. 3, illustrate that the slope values of n are about 3.0 for the green UC emissions, confirming that the green emitting ²H_11/2/⁴S_3/2 states are populated via a three-photon process. The red UC emissions yield n = 2.8, 2.8, 2.8 and 2.6 for In-0, In-1, In-2 and In-3, respectively. Considering the energy conservation law, at least three 1550 nm laser photons are needed to populate the red emitting ⁴F_9/2 state. Therefore, the fact that the n values of 2.8 and 2.6 slightly deviate from the expected n = 3 implies that a two-photon process is also involved in populating the ⁴F_9/2 state as well as the three-photon process.

Fig. 3 The pump power dependency of the green and red UC emissions of In/Er/Yb:LiNbO₃ under 1550 nm excitation.

Fig. 4 shows the energy level diagrams of Er³⁺ and Yb³⁺ ions, as well as the UC mechanisms under 1550 nm excitation. Here, laser excitation of the Er³⁺ ion is only considered since the Yb³⁺ ion has no corresponding energy level which matches the 1550 nm pump laser. As illustrated in Fig. 4, three 1550 nm photons excite the Er³⁺ ions from the ground ⁴I_15/2 state to the green emitting ²H_11/2 state via ground state absorption (GSA: ⁴I_15/2 + a 1550 nm photon → ⁴I_13/2), excited state absorption (ESA1: ⁴I_13/2 + a 1550 nm photon → ⁴I_9/2) and ESA3 (⁴I_9/2 + a 1550 nm photon → ²H_11/2) processes. The Er³⁺ ions at the ⁴I_9/2 state nonradiatively relax to the ⁴I_11/2 state, and the red emitting ⁴F_9/2 state is populated via the ESA2 process (⁴I_11/2 + a 1550 nm photon → ⁴F_9/2). The two-photon process for populating the red emitting ⁴F_9/2 state may be attributed to the energy transfer (ET) process, which is depicted as follows: after the two-photon process of GSA and ESA1, ET (²F_7/2 Yb + ⁴I_11/2 Er → ²F_5/2 Yb + ⁴I_15/2 Er) and the energy back transfer (EBT: ²F_5/2 Yb + ⁴I_13/2 Er → ²F_7/2 Yb + ⁴F_9/2 Er) processes are responsible for the population of the ⁴F_9/2 state (Er). The similar EBT process (⁴S_3/2 Er + ²F_7/2 Yb → ⁴I_13/2 Er + ²F_5/2 Yb) also occurs in Er/Yb-codoped ZrO₂ nanocrystals under 980 nm excitation.²³ It is proposed that the increase of the red UC emission observed in In-0 related to Er-0.5 arises from ET and EBT processes under 1550 nm excitation.

Fig. 4 Energy level diagrams of Er³⁺ and Yb³⁺ ions.

The absorption cross section (σ_abs), namely, the optical transition strength, is used to understand the relationship between the optical characteristics and the distribution of Er³⁺ ions. According to the equation given by W. Q. Shi et al.,²⁴ the values of σ_abs of In-3 based on ultraviolet-vis-near infrared (UV-VIS-NIR) absorption spectra²⁵ (Fig. S1, please see the ESI†) are calculated and displayed in Table 1. The σ_abs of In-1 and In-2 can be obtained from our previous work²⁸ since UV-VIS-NIR absorption spectra of In-1 and In-2 are measured under the same conditions as In-3. As displayed in Table 1, the values of σ_abs for In-3 are larger than those of In-1 and In-2, suggesting that there are fewer Er³⁺ cluster sites in In-3. This is because the σ_abs of the Er³⁺ cluster site is smaller than that of the Er³⁺ isolated site.²⁶

Table 1 Absorption cross sections of the In/Er/Yb:LiNbO₃ crystals

Samples	σabs (×10^−20 cm^2)
	^4G9/2	^2H9/2	^4F3/2	^4F7/2	^2H11/2	^4S3/2	^4F9/2	Ref.
In-1	0.167	0.109	0.128	0.384	2.751	0.103	0.842	27
In-2	0.107	0.077	0.078	0.240	1.914	0.072	0.476	27
In-3	0.267	0.161	0.170	0.521	3.007	0.140	1.369	This work

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In general, there are two types of Er³⁺ ion distributions in LiNbO₃ crystals. One is the Er³⁺ isolated site (Er_Li²⁺ or Er_Nb²⁻), which means the Er³⁺ ion occupies the Li or Nb site. The other is the Er³⁺ cluster site (Er_Li²⁺–Er_Nb²⁻), which consists of one Er_Li²⁺ site and one Er_Nb²⁻ site.²⁷ As for the UC mechanisms in Er:LiNbO₃, an inherent ESA process occurs in the Er³⁺ isolated sites, and the ET process happens among the Er³⁺ cluster sites. Obviously, there is a competition between the ESA and ET for an excited state Er³⁺ ion. Therefore, the decreased Er³⁺ cluster sites suppress the ET process and lead to an efficient ESA process in In-3, which is responsible for the enhanced green and red UC emissions in In-3 (shown in Fig. 2(a)). Contrarily, the larger content of Er³⁺ cluster sites in In-2, arising from the minimum values of σ_abs, is deleterious to the ESA process. The inefficient ESA processes mean the decreased populations of the ²H_11/2/⁴F_9/2 states of Er³⁺ ions. Consequently, the reduced green and red UC emissions are observed in In-2, in agreement with the experimental results shown in Fig. 2(a).

4. Conclusions

In summary, enhanced visible green and red UC emissions induced by 980 nm and 1550 nm excitation are gained in Er/Yb:LiNbO₃ through tridoping with 3 mol% In³⁺ ions, which will provide more possibilities to increase the photovoltaic efficiency of solar cells. The stronger red UC emission is attributed to the dissociation of Er³⁺ cluster sites in the In-3 crystal, and the EBT process of ²F_5/2 Yb + ⁴I_13/2 Er → ²F_7/2 Yb + ⁴F_9/2 Er increases the red UC emission in the Er³⁺–Yb³⁺ codoped system under 1550 nm excitation. Additionally, conversion of NIR wavelength of 980 nm/1550 nm into visible UC emission re-absorbed by the solar cells may directly apply to other materials doped with Er³⁺ and Yb³⁺ ions, such as oxide and fluoride crystals, fibers, films and nanoparticles.

Acknowledgements

This work was supported by the National Natural Science Foundation (no. 11304402, 11232015, 11072271 and 10972239), Postdoctoral Science Foundation of China (no. 2012M521640 and 2013T60818), and The National Research Foundation for the Doctoral Program of Higher Education of China (20120171110005).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra45270k

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