www.acsprf.org

Introduction

Solar energy utilization requires effective means of capture and conversion of the solar radiation. To reduce the spectral mismatch losses and thus to bring the efficiency of practical Si solar cells close to or beyond the Shockley-Queisser limit, adapting the solar spectrum to better match the solar cell is an alternative approach.

Down-shifting luminescence from luminescent convertors (phosphors) would be particularly realistic and promising to the poor blue response Si solar cells if wavelengths shorter than approximately 550 nm could all be shifted into the optimum red-near infrared (NIR) range. In order for a Si solar cell to benefit from down-shifting luminescence, it is expected that such luminescent convertors can efficiently shift the high-energy solar photons to lower energies, significantly increase the spectral response of Si solar cells at higher energies, and thus substantially increase the cells' overall efficiency and productivity.

In this project, we dedicated to develop a new and novel class of Cr³⁺-doped Zn-Ga-Ge-O (ZGGO:Cr) NIR long-persistent phosphors that exhibit superior capabilities in solar energy harvesting, storage, NIR light conversion and persistent NIR emission, which appear to meet almost all the demands for the luminescent convertors.

We systematically studied the fabrication of the ZGGO:Cr phosphors with the goal to obtain the best materials in terms of afterglow brightness and persistence time. We conducted extensive optical characterizations on the obtained materials, determined the relationship between afterglow performance and excitation energy, and investigated the underlying phosphorescence mechanisms.

Experimental Results

By carefully tuning the fabrication parameters, including the ratios of the constituent oxides and processing conditions, we determined that Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺ is an optimum formula. The material can be efficiently excited by a broad range of wavelengths (300-650 nm) and emits an intense and broadband NIR photoluminescence (650-950 nm) peaking at 696 nm (Figure 1). Besides the intense and broad NIR photoluminescence, the Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺ phosphor also exhibits super-long lasting, NIR persistent luminescence after the removal of the excitation source. After excited by a 365 nm UV light or natural sunlight for 10 seconds to 5 minutes, the NIR afterglow can last for more than 360 h (Figure 2), which is about 2–3 orders of magnitude longer than that of the NIR persistent phosphors reported before.

Significantly, our extensive outdoor experiments showed that the Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺ phosphor appears to be an all-weather NIR material that can be activated in various outdoor environments. Specifically, the phosphor can be rapidly, effectively, and repeatedly charged by sunlight in various weather conditions (e.g., sunny, cloudy, overcast, and rainy days), at any moment between sunrise and sunset, and at various outdoor locations (e.g., open area and shadows of trees and buildings), and the materials exposed to these various outdoor environments exhibit comparable NIR persistent luminescence behaviors. In addition, besides the activation and emission in air, the material can also be effectively activated by sunlight when they are immersed in various aqueous solutions (e.g., tap water, salt water and more corrosive NaCl-NaHCO₃-bleach water) and emit NIR light as intense and long as they are in air.

To understand the effectiveness of different excitation energies to the persistent luminescence in Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺ phosphor, we studied the relationship between afterglow luminescence intensity (I_10s, the afterglow intensity at 10 s after the stoppage of the illumination) and excitation wavelengths over 300-650 nm (Figure 3a). The photoluminescence excitation spectrum is also given in Figure 3a for comparison. Figure 3a clearly shows that the NIR persistent luminescence can be effectively achieved under UV illumination (300-400 nm), but less effectively achieved under visible light (400-630 nm) illumination (inset of Figure 3a), even though the visible light excitation is very effective to NIR photoluminescence.

The physical mechanism behind the excitation-wavelength dependence of afterglow was revealed by thermoluminescence (TL) measurements. Figure 3b shows the TL curves of Zn₃Ga₂Ge₂O₁₀:0.5%Cr³⁺ phosphor irradiated by direct and filtered sunlight (the cutoff wavelengths are 400 nm, 495 nm and 590 nm). For direct sunlight irradiation, the TL curve recorded with delay time of 2 min (red line) shows an asymmetric broad band covering from 30° to ~300 °C. When the delay time increases to 360 h (black solid line), the low-temperature band (30° to ~120 °C) disappears and the high-temperature band (~120° to ~300 °C) still exists, indicating that the low-temperature shallow traps are emptied. As the irradiation moves to long-wavelength solar spectral region, the intensities of the main TL peaks decrease and the peak positions shift to high-temperature region, indicating that different irradiation energies can fill traps with different depths. The detailed persistent luminescence mechanism is under investigation.

Impact of the Research

The ZGGO:Cr NIR persistent phosphors developed under the support of this PRF-DNI project is new and is by far the best NIR persistent phosphor. The very bright NIR afterglow emission, the very long afterglow time, and the ability of being quickly activated by sunlight endow the material to have many technologically important applications. These applications include not only as luminescent convertors in photovoltaics, but also as identification taggants in defense and security, and as optical probes for in vivo deep tissue bio-imaging.

The impact of this research to the PI's career is tremendous. Basing on the results partially supported by this PRF-DNI fund, a paper related to the ZGGO:Cr NIR phosphor is under revision in Nature Materials, the most prestigious journal in material science. Since the ZGGO:Cr phosphor is new for which not much is known, there are many aspects to discover and understand. Therefore, the on-going success of this project is expected to result in several other high-impact journal papers, have significant impacts in the communities of luminescent materials, photovoltaics, defense and security, bio-imaging, etc., and maintain the PI's group as the most prominent group in NIR persistent phosphor research and applications.

The impact of this research to the postdoc and graduate student involved in this project is significant. They have the opportunity to conduct pioneering research and to prepare themselves to be the future leading scientists in the area of NIR persistent materials.