Rare Earth Doped Zinc Oxide Nanophosphor Powder: A Future

Oct 20, 2017 - A comprehensive review of up-conversion (UP) and down-conversion (DC) or down shifting of rare earth (RE) doped zinc oxide (ZnO) nanoph...
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Rare Earth Doped Zinc Oxide Nanophosphor Powder: A Future Material for Solid State Lighting and Solar Cells Vinod Kumar,*,† O. M. Ntwaeaborwa,‡ T. Soga,§ Viresh Dutta,† and H. C. Swart∥ †

Centre for Energy Studies, Indian Institute of Technology Delhi, New Delhi 110016, India School of Physics, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa § Department of Frontier Materials, Nagoya Institute of Technology, Nagoya 466-8555, Japan ∥ Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa ‡

ABSTRACT: A comprehensive review of up-conversion (UP) and down-conversion (DC) or down shifting of rare earth (RE) doped zinc oxide (ZnO) nanophosphors is presented. Research interest in the development of RE3+ doped ZnO for UP and DC nanophosphors has been encouraged by the potential application of these materials in light emitting diodes and different types of photovoltaic cells. A range of remarkable characteristics, which are organized into different sections describing the structure, optical, and luminescence properties of these materials, are discussed in detail. Undoped ZnO has two characteristic emissions in the ultraviolet and visible regions related, respectively, to excitonic recombination and intrinsic defects. X-ray photoelectron spectroscopy (XPS) data demonstrated a correlation between the visible emission and intrinsic defects. In the case of the DC or shifting process, there was simultaneous emissions related to f → f transitions of RE ions and defects in ZnO host. These emissions were dependent on the synthesis method, annealing temperature, and RE ion concentration, among other things; only f → f transitions of RE ions were observed in the case of the UC process. These down and up conversion RE doped ZnO phosphors were evaluated for a possible application in solid state lighting and photovoltaic cells. KEYWORDS: ZnO, Rare earth, Solid state lighting, Solar cell, Up conversion, Down shifting band gap of ZnO to ∼3.63 eV (342 nm).10 Soosen et al.11 observed red-shifting of the maximum wavelength of the absorbance peak, and they attributed it to the decreasing quantum confinement effect due to increasing crystallite size. They were able to shift the absorbance edge from 362 to 381 nm by annealing 5 nm ZnO particles at different temperatures and effectively increasing the particle size up to 8 nm. Their UV absorption results also showed that quantum confinement resulted in an increase in the energy value of the optical band gap. A chemical reaction can lead to a substitutional replacement of the lattice atoms with doping ions into the crystal structure of the compound. Kamarulzaman et al.12 have shown that band gap widening occurs in all undoped and doped ZnO nanostructures, compared to the band gap of bulk materials. However, doping ZnO nanostructured materials with transition metals such as Cu and Mn caused the band gap to become narrower. The size of the crystallites (micro- or nanosized) and the type and concentration of the doping involved play the most important roles in the band gap narrowing and widening process. The widening in the band gap

In the past few decades, research interest in zinc oxide (ZnO) semiconductor materials has grown exponentially as observed from a growing number of publications in these materials. ZnO is nontoxic, and it can be produced cost effectively on a small or large scale. It has three types of unit cell structures, namely, hexagonal wurtzite, zinc blende, and rock salt as shown in Figure 1.1 The hexagonal wurtzite phase is the most popular structure due to its stability at room temperature and normal atmospheric pressure. The atomic arrangement of the wurtzite structure is comprised of four zinc ions (Zn2+) occupying the corner of a tetrahedral coordinate with one oxygen ion (O2−) located at the center and vice versa. The growth of zinc blende structure of ZnO is challenging because the zinc blende ZnO is only stable in the cubic structure. In the rock salt structure, each Zn or O atom is surrounded by its alternative six nearest neighbors. It only exists at the higher pressures (15%) silicon solar cells conclusively showed an enhancement in shortwavelength spectral response. It was mainly due to the DS effects of the phosphor NPs.207 Wu et al. explored the development of a ZnO:Eu3+ bifunctional layer for use in the inorganic solar cells.210 The bifunctional layer is used as a spectral conversion layer as well as an electron transport layer. To detect the influence of Eu3+ doping on photoactive performance, ZnO:Eu3+ was used as an electron buffer layer in layered organic solar cells with layer configuration ITO/ZnO:Eu 3+ /P3HT:PC61BM/MoO 3 /Al, shown in Figure 26a. The J−V curves of the devices are shown in Figure 26b. A slight increase is observed in the short circuit current density (JSC) from 8.94 to 9.11 mA/cm2 when using ZnO and ZnO:Eu3+ as a buffer layers. This is attributed to a spectral DC process.202 Yao et al. used RE doped ZnO UC/DC materials to enhance the PCE of DSSCs.211 The J−V curve of the DSSCs based on UC/DC phosphors are shown in Figure 27. The optimal efficiency was ∼5.13% with the incorporation of UC and DC materials, which is higher than that of DSSCs based on TiO2. This enhancement is attributed to increase of light harvesting via converting UV and NIR into visible emission using DS and UC luminescence processes, respectively.

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APPLICATION OF RARE EARTH DOPED ZnO Solar Cells. There is an existing mismatch between the conventional single-junction solar cell spectral response (SR) and the solar emission spectrum. It is due to the limited efficiency absorption of the semiconducting material using for the active layer of the solar cell. So, this is the origin of the major amount of losses in commercially available solar cells. The efficiency of the solar cell has a limitation due to these spectral losses. The theoretical maximum efficiency of a singlejunction solar cell consisting of a semiconducting material having a bandgap of ∼1.1 eV (Si) is ∼31%. This is called the Shockley−Queisser limit of the device.201 A large part of sunlight is absorbed in the approximately first micrometer of a semiconducting material. Much research has been done to optimize the top surface of solar cells. The probable increase in the solar cell’s SR passes through making better use of the short-wavelength photon. There are three kinds of losses in a solar cell, which can be reduced by the modification in the spectrum. The first two are thermalization and transmission. Thermalization losses are prominent in smaller band gap material based solar cells, while transmission losses are common in the wider band gap material based solar cells. An electron−hole pair with higher energy is created when a semiconductor absorbs a photon of higher energy than the band gap, and the excess energy is dissipated in the form of heat. The third loss mechanism is insufficient collection of photon-generated carriers due to recombination close to or at the surface. Since photons of high energy are absorbed in this region, the result is a reduced SR at shorter wavelengths. This loss can be reduced using the process of photon conversion, whereby photons are shifted into an energy range where the cell has shown a higher SR. The photon flux as a function of wavelength is shown in Figure 25. The wavelength conversions

Figure 25. Photon flux as a function of wavelength (AM 1.5G Solar Spectrum). Reproduced from ref 202 with permission. Copyright 2012 Royal Society of Chemistry.

of photons are a promising route to decrease the spectral mismatch losses. Therefore, it is considered to be the major part of the efficiency losses in PV devices. Wavelength dependent spectral converters have been explored for enhancing the energy conversion in single junction solar cells in recent years. There are three luminescence processes (QC, DC, and UC). These three approach have potential to enhance the efficiency of solar cells. QC through Q

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Figure 26. (a) The schematic structure of ZnO:Eu3+ based inverted organic solar cells. (b) J−V curves of the inverted organic solar cells using ZnO and ZnO:Eu3+ as a buffer layer. Reproduced from ref 210 with permission. Copyright 2015 IOP Publishing.

persistence, and possible energy savings.206−208 Normally, three different approaches can be used for generating white light emission based LEDs. The first approach is by the mixing of three LEDs, namely, red, green, and blue (RGB), together, the second one is by using an UV LED to stimulate RGB phosphors, and the third one is by using a blue-emitting diode that excites a yellow-emitting phosphor. Today’s commercial phosphor converted light emitting diodes (pc-WLEDs) normally use a 450−470 nm blue GaN LED chip covered by a yellowish YAG:Ce phosphor coating. However, these pcWLEDs have some weaknesses, such as a poor color rendering index and low stability of the color temperature,217,218 since white light emission is generated by the combination of the blue light emitted by an LED chip and the yellow light emitted by the YAG:Ce phosphors. The RGB LEDs require different driving currents for different color LEDs, complicating their fabrication.219 White UVLEDs are fabricated by using UV-LED chips that are coated with white light-emitting single-phased phosphors or RGB tricolor phosphors.218−220 A great challenge is to produce a single phosphor that can be used to achieve white light emission. Ji et al. have developed a novel two-step method for the synthesis of isocrystalline core−shell nanocrystals of ZnO:[email protected] An efficient energy transfer was observed from ZnO to Tb3+ ions. RE doped isocrystalline core−shell (ICS) nanocrystals have excellent optical properties with possible applications to be used as light emitting devices. RE ions (Tb3+, Dy3+, and Er3+) are incorporated into ZnO nanostructures using ICS. These core−shell spherical nanocrystals lead to an increase of the ZnO band-edge absorption in PLE and to the improvement of the characteristic emissions of the RE ions compared with the core-only. So inner doping and efficient energy transfer from the ZnO host to the doped RE ions are realized. These RE doped core−shell nanocrystals are ideal candidates for light emitting device applications.222 The CIE diagram of undoped and RE doped ZnO NPrs under different conditions is shown in Figure 28. According to our previous reported work, ZnO has broad emission range from red to white light to green to blue. So we can control the emission light with the variation of defects, different dopant ions, and doping concentration. The question may arise: What is the real quantum efficiency of ZnO and RE doped ZnO to be used in SSL and solar cells? The overall efficiency of white light LEDs is clearly dependent on the development of highly efficient, environmentally friendly, and inexpensive phosphor materials. Balestrieri et

Figure 27. J−V curve of UC/DC based DSSC. Reproduced from ref 211 with permission. Copyright 2015 Elsevier.

The PV systems should maintain the initial efficiency over a lifetime of at least 20 years. It must be not only mechanically stable but also resistant to degradation caused by exposure into the environment. To succeed in the implementation of UC/ DC optimized solar cells in commercial systems more work must be directed to improving their stability and robustness in ambient conditions. A significant efficiency enhancement in UC/DS based solar cells is still a major challenge.212,213 Solid State Lighting. The high energy demand adding increasing pressure on fossil fuel reserves increases on a daily basis. The impact on the environment is tremendous.214 Conventional incandescent and fluorescent lamps rely heavily on either heat or the discharge of gases. These phenomena are associated with a huge amount of energy losses due to the high temperatures and involvement of large Stokes shifts.215 A new kind of lighting device was invented by Nichia Chemical Company using a blue InGaN LED chip coated with yttrium aluminum garnet yellow phosphor (Y3Al5O12:Ce, YAG:Ce) in 1996.216 The InGaN chip emits blue light through an electron− hole recombination in the p−n junctions. Some of the blue light from the LED excites the YAG:Ce phosphor, consequently emitting yellow light and then the rest of the blue light from the InGaN chip is mixed with the yellow light to generate white light. This light emission process by an LED is called solid-state lighting (SSL). The main advantages of SSL are small size, high luminous efficiency, ecofriendly nature, long R

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used to achieve NIR EL. The aforementioned multicolor and NIR EL originating from the impact excitation of the RE ions incorporated into the ZnO host can be activated with threshold voltages as low as ∼5 V. Another possible obstacle in the use of phosphor materials that has to be overcome is the degradation of phosphor materials with time in different environments and conditions.231

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CONCLUSION ZnO is a low-cost, ecofriendly, and versatile material to be used to generate emission colors all over the rainbow spectrum. The reason for the color tuning is the band edge emission as well as the different defect related emissions. The defect related emission in ZnO is dependent on the synthesis method, annealing temperature, and several other parameters. The O 1s XPS peak was correlated to the defect related emission in ZnO. The emitted color can easily be tuned by doping with the appropriate RE materials. RE doped ZnO nanophosphors can be synthesized by different methods. The emitted light can be tuned throughout the visible range by using different RE dopant materials or different synthesis techniques. ZnO can also be used as the host of dopant ions for up and down conversion. The up and down conversion nanophosphor materials can be used to enhance the efficiency of solar cells. This enhancement in the efficiency is due to the enhancement of light harvesting via converting UV or NIR to visible emission. To succeed in the implementation of up and down conversion ZnO optimized solar cells in commercial systems, more work must be directed to improvement of their stability and robustness in ambient conditions because the solar cells should maintain their initial efficiency over a lifetime of at least 20 years and they must be, therefore, not only mechanically stable but also resistant to degradation caused by exposure to the environment. For the SSL point of view, a stable and high color purity single host based white light emission is under development. This review concludes that RE doped ZnO nanophosphors will be promising up and down conversion materials for future applications in solid state lighting and solar cells.

Figure 28. CIE diagram of undoped and RE doped ZnO NPrs with different synthesis condition.

al.223 showed that although the DC theoretical value of splitting one photon into two photons is 200%, the experimentally obtained results for example Pr doped ZnO and codoped Pr and Yb ZnO thin films is still very low. Sun et al.224 prepared Ln doped ZnO quantum dots (QDs) by a sol−gel method. The doped QDs exhibited enhancement of the luminescent properties and quantum yield. For lightly S doped ZnO powder an efficiency of 55% was observed, while at higher doping concentrations, the efficiency was higher (75%), and further improvement may be possible by optimization of the excitation wavelength.225 Elleuch et al.226 used ZnO;Er thin films for DC photon generation and a possible application as antireflective layer, when placed in front of Si solar cells. A low reflectance and high refractive index of the film were measured at 632 nm. DC was also achieved for this film under visible excitation by 980 nm photons, which might be useful for Si solar cell. In the light of current available literature, it is clear that excellent measured values of efficiency are difficult to find, and the next step therefore will be to obtain the real efficiency and to test these materials in real industrial environments. A limited number of research reports on electroluminescent (EL) thin films for LED applications utilizing RE doped ZnO have been published.227 Most of the investigations on LEDs are based on ZnO:Er films, for achieving green and NIR (∼1.54 μm) EL.227 The visible and ∼1.54 μm EL from an n-ZnO:Er/pSi heterostructure under reverse bias with a threshold voltage of ∼10 V was reported by Harako et al.228 LEDs fabricated using these samples exhibited bright green (536 and 556 nm), red (665 nm), and 1.54 μm emissions. Francesco Pecora et al.229 demonstrated an LED device based on a Si-rich ZnO:Er/p+-Si heterostructure, which exhibited detectable ∼1.54 μm emission under forward bias with a threshold voltage of ∼15 V. By incorporating Si atoms, it was demonstrated that the Si mediated enhancement of PL intensity on the Er-doped ZnO and EL. A low-voltage driven ZnO:Er/p+-Si heterostructured device (∼1.54 μm EL) was developed by Yang et al.230 A novel device structure was developed using ZnO:RE as a lightemitting layer, capped by MgxZn1−xO layer. They have successfully obtained red, green, and blue EL from the devices using ZnO:Eu, ZnO:Er, and ZnO:Tm films as the lightemitting layers, respectively. ZnO:Nd and ZnO:Er films are

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AUTHOR INFORMATION

Corresponding Author

*E-mail addresses: [email protected], [email protected] (Vinod Kumar). ORCID

Vinod Kumar: 0000-0002-6425-5120 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS One of the authors (V.K.) is thankful to DST, New Delhi, India, for support through DST-Inspire faculty award [DST/ INSPIRE/04/2015/001497].

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REFERENCES

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Review

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DOI: 10.1021/acsphotonics.7b00777 ACS Photonics XXXX, XXX, XXX−XXX

ACS Photonics

Review

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DOI: 10.1021/acsphotonics.7b00777 ACS Photonics XXXX, XXX, XXX−XXX