Elsevier

Journal of Luminescence

Volume 156, December 2014, Pages 262-265
Journal of Luminescence

Quantum tripling in Tm3+ doped La2BaZnO5 phosphors for efficiency enhancement of small band gap solar cells

https://doi.org/10.1016/j.jlumin.2014.08.016Get rights and content

Highlights

  • Tm3+ doped La2BaZnO5 phosphors are prepared.

  • 1700 nm emission is enhanced with increasing Tm concentration.

  • The enhanced IR emission is due to increased 4f–4f cross-relaxations.

  • Cross-relaxations lead to emission of 3 IR photons after 1 blue photon is absorbed.

  • Quantum tripling can be used to increase efficiency of small band gap solar cells.

Abstract

The emission of three infrared photons at 1700 nm by Tm3+ for each absorbed photon at 465 nm is reported as a function of the Tm3+ concentration in La2−2xTm2xBaZnO5 phosphors. It was observed that the intensity of the Tm3+ 3F43H6 emission with respect to emissions from the 1G4 state after 1G4 excitation at 465 nm increases when the Tm3+ concentration is raised from x=0.001 to x=0.032. This increase is the accumulated effect of four different possibilities for cross-relaxation that can take place between neighboring Tm3+ ions, which explains the efficiency of the quantum tripling process. An optimum in the cross-relaxation efficiency was found for the La1.936Tm0.064BaZnO5 phosphor. At higher Tm3+ concentrations the 3F43H6 emission decreases as a result of concentration quenching. Quantum tripling can be used in high power IR phosphor converted LEDs or to increase the efficiency of small band gap solar cells like germanium.

Introduction

The solar cell efficiency is mainly limited by the mismatch between the solar spectrum and the solar cell response. In a single junction photovoltaic device photons with energy lower than the band gap of the solar cell absorber material are lost because they cannot be absorbed, while the excess energy of the photons with energy higher than the band gap is lost by thermalization. By choosing a small band gap semiconductor material like GaSb or Ge, losses due to limited absorption are reduced. However, thermalization losses are greatly enhanced as compared to larger band gap cells like Si, CIGS or CdTe. As a result, the Shockley–Queisser limits [1] of GaSb (band gap of 0.7 eV) and Ge (band gap of 0.67 eV) are 22% and 21% respectively, which is much smaller than the Shockley–Queisser limit of 30% for crystalline silicon (band gap of 1.12 eV).

One approach to reduce the thermalization losses in solar cells is the integration of a luminescent quantum cutting layer on top of the cell, which converts a photon with more than twice the band gap energy into two photons with energy just above the band gap of the cell. This quantum cutting mechanism has been demonstrated in for example Tb3+ and Yb3+ co-doped YPO4 [2]. In this material absorption of a 490 nm photon by Tb3+ is followed by a cooperative energy transfer to two neighboring Yb3+ ions, resulting in the emission of two 1000 nm photons by the Yb3+ ions. These photons can subsequently be absorbed by a Si solar cell.

For solar cells with smaller band gaps it could be beneficial to not just split the high energy photons into two lower energy photons, but to split them into three photons. In a Ge solar cell about 24% of the absorbed photons have energy of at least three times the band gap. Calculations show that an ideal quantum tripling layer on top of a Ge cell would result in a relative efficiency increase of almost 25% [3]. When integrating a combination of a quantum tripling layer and a quantum cutting layer, the relative increase in efficiency could even be 80% [3].

Jaffrès et al. showed that the quantum tripling mechanism takes place in Tm3+ doped La2BaZnO5 phosphors [4]. The Tm3+ ion has several 4f–4f transitions of the same energy. For example, the 3H63F2 absorption and the 1G43F4 emission are both located around 670 nm (see Fig. 1). Because of this, different cross-relaxations can take place between the Tm3+ ions, which can result in the emission of three 1700 nm photons after the absorption of one 465 nm photon. Tm3+ cross-relaxation has also been studied in other materials, like TeO2–CdCl2 glasses [5], Ge30As10S60 glasses [6] and silica [7].

For the research described in this paper, La2BaZnO5 phosphors with different Tm3+ concentrations have been prepared and the spectroscopic properties of phosphors are discussed as a function of the Tm3+ concentration. From these results the efficiency of the quantum tripling mechanism of this phosphor will be discussed, and with it the applicability for the efficiency enhancement of small band gap solar cells.

Section snippets

Synthesis

Tm3+ doped La2BaZnO5 samples have been prepared by solid-state reaction synthesis. BaCO3 (Merck, 99%), La2O3 (Aldrich, 99.99%), ZnO (Aldrich, 99.999%) and Tm2O3 (Aldrich, 99.9%) were used as starting materials. These powders were combined in the appropriate molar ratio and thoroughly mixed with mortar and pestle. The samples were fired twice at 1200 °C in a high temperature tube furnace in air atmosphere with intermediate crunching in between. After firing, the samples were crunched and the

Results and discussion

The crystal structure of La2BaZnO5 has been reported by Michel et al. [9]. The material has a tetragonal crystal structure with space group I4/mcm. The X-ray diffraction pattern of a La2−2xTm2xBaZnO5 sample with x=0.032 is shown in Fig. 2 and compared with the reference pattern (ICSD 87078). The patterns of the samples with other Tm3+ concentrations (up to x=0.128) looked very similar. Note that in the La2BaZnO5 crystal the shortest distance between two La ions is about 3.40 Å (ICSD 87078),

Conclusions

The Tm3+ concentration dependence of doped La2BaZnO5 shows an enhancement of Tm3+ 3F43H6 emission with increasing Tm3+ concentration. This is due to four different cross-relaxation mechanisms that can take place between Tm3+ ions, resulting in the emission of three 1700 nm photons after the absorption of one 465 nm photon. This quantum tripling process makes Tm3+ doped phosphors interesting for increasing the efficiency of small band gap solar cells like germanium. However, the higher Tm3+

Acknowledgments

This work is part of the Joint Solar Programme (JSP) of the Stichting voor Fundamenteel Onderzoek der Materie (FOM) and financially supported by HyET Solar (Grant no. 09JSP33).

References (10)

  • A. Jaffrès et al.

    Chem. Phys. Lett.

    (2012)
  • Y.B. Shin et al.

    J. Non-Cryst. Solids

    (1996)
  • S.D. Jackson

    Opt. Commun.

    (2004)
  • C. Michel et al.

    J. Solid State Chem.

    (1982)
  • W. Shockley et al.

    J. Appl. Phys.

    (1961)
There are more references available in the full text version of this article.

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