57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

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 this project, we dedicated to develop new and novel NIR long-persistent phosphors that exhibit superior capabilities in solar energy harvesting, storage, NIR light conversion and persistent NIR emission. Specially, we developed two new series of Cr³⁺-activated gallates: Cr³⁺-activated Zn-Ga-Ge-O (ZGGO:Cr) and Cr³⁺-activated Li-Ga-O (LGO:Cr).

Experimental Results

Both ZGGO:Cr and LGO:Cr phosphors can be efficiently excited by a broad range of wavelengths (250-650 nm) and emits an intense and broadband NIR photoluminescence (650-950 nm) (Figure 1). Besides the intense and broad NIR photoluminescence, the ZGGO:Cr and LGO:Cr phosphors also exhibit super-long lasting, NIR persistent luminescence after the removal of the excitation source. After excited by a UV light for 10 seconds to 5 minutes, the NIR afterglow can last for several hundred hours (Figure 2).

Significantly, our extensive outdoor experiments showed that the ZGGO:Cr and LGO:Cr phosphors appear to be an all-weather NIR material that can be activated in various outdoor environments. Specifically, the phosphors 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.

Figure 1. Normalized excitation and emission spectra of (a) ZGGO:Cr phosphor and (b) LGO:Cr phosphor at room temperature. The emission spectra are acquired under 400 nm light excitation and the excitation spectra are obtained by monitoring at 713 nm emission for ZGGO:Cr and 716 nm emission for LGO:Cr.

Figure 2. (a-f) NIR images of four ZGGO:Cr phosphor discs taken at different afterglow times (5 min to 360 h) after irradiated by a 365 nm lamp for 10 s to 5 min. (g-k) NIR images of four LGO:Cr phosphor discs taken at different afterglow times (10 min to 480 h) after irradiated by a 300 nm light for 10 s to 5 min. The images were taken using a digital SLR camera via a PVS-14 Generation III night vision monocular in a dark room. The imaging parameters are: a-c, auto/ISO 200/0.3-4 s, d, manual/ISO 200/30 s, e-f, manual/ISO 400/30 s, g-i, manual/ISO 400/10 s, and j-k, manual/ISO 800/30 s.

Besides the NIR photoluminescence and NIR persistent luminescence, the LGO:Cr phosphor also exhibits NIR photostimulated persistent luminescence (PSPL) phenomenon when a UV pre-irradiated LGO:Cr sample is stimulated by a visible light or a NIR light. The NIR PSPL phenomenon is a new optical read-out form, which is analogous to the visible photostimulated luminescence (PSL) phenomenon observed in conventional photostimulable storage phosphors, such as the commercial Al₂O₃:C used in dosimetry and BaFBr:Eu²⁺ used in computed radiograph. Significantly, the PSPL emission can be obtained tens of times within a period of more than 1,000 h, as the NIR images shown in Fig. 3. This result indicates that LGO:Cr is a superb photostimulable storage phosphor which can be used as optical memory media for optical information write-in and read-out.

Figure 3. NIR images for PSPL emission from a LGO:Cr phosphor plate. The size of the LGO:Cr plate is 15 x 15 mm². The center of the plate was covered by an 8 x 8 mm² black paper during the irradiation by a 254 nm UV lamp. Due to the block of the black paper, the center of the LGO:Cr plate remains un-activated. Under a night vision monocular, the UV- irradiated LGO:Cr plate appeared to be a bright square ring. The NIR images in row a show the natural decay of the NIR persistent luminescence of the square ring to 120 h. From 120 to 720 h (row b), the right half of the plate was stimulated by a white LED for 20 s at every 120 h. Due to the photostimulation of the white LED, the right half of the plate lit up, exhibiting enhanced NIR persistent luminescence after each stimulation. From 744 to 1,008 hr (row c), the entire plate was stimulated by a white LED for 20 s at every 24 h. As a result, the entire UV pre-irradiated region on the plate emitted enhanced NIR persistent luminescence. The imaging parameter for the NIR images is manual/ISO 400/10 s.

Impact of the Research

The ZGGO:Cr and LGO:Cr NIR persistent phosphors developed under the support of this PRF-DNI project are new and are the best NIR persistent phosphors. They are expected to find promising applications in photovoltaics, defense and security, optical information storage, and in vivo 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 was published in Nature Materials (11, 58-63(2012)). One paper related to LGO:Cr phosphor was finished and will be submitted. Since the ZGGO:Cr and LGO:Cr phosphors are 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, information storage, 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.