Inorganic-Organic Hybrid Phosphor Layer for Si Solar Cells
Authors: Okura Hiroshi , Sakata Ryosuke , Doi Naoki , Ishigaki Tadashi , Komiyama Ryohei , Miyashita Hidetoshi , Lee Sang-Seok , Ohmi Koutoku 
Published: February 2020

The wavelength conversion layer (WCL) is attracting more attention from many companies because it is expected to increase the conversion efficiency of silicon solar cells by 5% or more. We have attempted to prepare the WCLs using our synthesized nano-phosphor particles.
Nano-phosphor particles were prepared by several solution synthesis methods. To achieve high quantum efficiency, it is important to activate luminescent centers as many as possible without generating defects in the host materials. We pay attention to the microreaction (MR)[1], which is one of a solution synthesis method and has advantages that proper pH level and temperature can be maintained during the synthesis. A Bi3+ and Eu3+ co-activated YVO4 red nano-phosphor was synthesized by MR method with in-situ pH monitoring[2]. The surface morphology of the YVO4:Bi3+,Eu3+ nanophosphor sample was observed with a field emission scanning electron microscope, and spherical grains that are less than 50 nm in diameter have been obtained. Figure 1 shows the PL and PLE spectra of the YVO4:Bi3+,Eu3+ nanophosphor. The six sharp PL peaks due to the 4f6-4f6 electron transition of the Eu3+ centers, the broad PLE band as the YVO4 host (250-300 nm) and the charge transfer state of the Bi3+ centers (300-400 nm) were confirmed [3].
The ZnS:Mn and (Zn,Cd):Mn red nanophosphors were synthesized by controlling the condition of the 3-mercaptopropionic acid (3-MPA) in the reaction solution. Figure 2 shows the PLE spectra of the ZnS:Mn, and (Zn,Cd)S:Mn phosphors. The peak of the ZnS:Mn nanophosphor’s emission is 600 nm, and its excitation spectra range is located at less than 380 nm. The high quantum efficiency (QE) of about 50% has been achieved by changing the amount of 3-MPA quantity and by controlling the proper pH level. Meanwhile, we have attempted to control the excitation wavelength by substituting Zn with Cd. The peak emission wavelength of the synthesized (Zn,Cd)S:Mn nano-phosphor is 600 nm, which is the same as the ZnS:Mn. Contrary, the absorption edge of the PL excitation spectra is varied from 380 nm to 420 nm by changing the content ratio of Cd/Zn. The maximum QE of (Zn,Cd)S:Mn of 40% has been achieved under near-UV excitation at 370 nm.
Nanophosphor particles, ethylene vinyl acetate (EVA) as a binder resin, and toluene as a solvent were used as the WCL slurry. They were mixed in a closed container by a planetary centrifugal mixer after heating at 90 ºC for 30 minutes. The WCLs were prepared by the doctor blade method using the bar coater (Kodaira YOA-B type). The thickness was 400 μm for all WCL samples. The I-V characteristics were measured using a solar simulator. The I-V curves of the WCL-mounted Si solar cells with and without fluorescent materials. The power generation efficiency was increased by 0.2%, 0.3%, and 0.5% by mounting the YVO4:Bi,Eu, ZnS:Mn, and (Zn,Cd)S:Mn WCLs, respectively under simulated AM 1.5 solar irradiation of 100 mW/cm2. Moreover, the inorganic-organic hybrid WCLs were also investigated. Figure 3 shows the inorganic-organic hybrid WCL with fluorescent materials including (Zn,Cd)S:Mn nanophosphors. The red emission based on the fluorescent materials under the 365 nm UV light was confirmed. The hybrid WCLs have a higher light resistance compared to WCLs containing only organic dye materials. It is considered that the inorganic phosphor absorbs a UV light and convert to visible light, resulting in prevention of the organic dye from UV damage. Figure 4 shows the I-V curves of the Si solar cells that had a WCL with and without fluorescent materials. The power generation efficiency was increased by 2.7% by mounting the (Zn,Cd)S:Mn / Eu complex dye WCL.
In the future, the light conversion efficiency is expected to be furthermore improved by optimizing the process parameters of the MR system, and the phosphor dispersion technique into the EVA sheet.
[1] 1) ECS transaction, 33, 95 (2011), 2) Jpn. J. Appl. Phys. 51 (2012) 062602
[2] 1) Phosphor Safari, 5 (2013), 2) The 20thIDW, 797 (2013)
[3] J. Lumin., 129, 9 (2009) 1067

Figure 1