本研究證實成本低廉、簡易可行的網印技術可被使用於將下轉換螢光體
粉能夠更均勻地塗佈於太陽能電池表面，且有效地降低光散射的損失，因此 建議未來頇以下列工作為重點，以期進一步提升轉換效率：
1. 網印塗佈用無機螢光粉粒徑最佳化之研究。
2. 不同稀土離子(寬譜帶激發與寬譜帶發光)摻雜，高量子效率螢光體對電池 光轉換效率的影響。
3. 可應用於太陽能電池之螢光粉主體的篩選與最佳化製程。
4. 製作光波頻譜轉換型太陽電池適當漿料之篩選。
5. 光譜轉換型太陽電池結構(back/front-side contacted and bifacial)與螢光粉層 厚度最佳化之研究。
6. 整合並製作上/下轉換螢光粉塗佈之太陽能電池，並探討其光電轉換效率。
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Yen-Chi Chen, Woan-Yu Huang and Teng-Ming Chen
*Phosphors Research Laboratory and Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan
Abstract：The goal of this work is aimed to improve the power conversion efficiency of single
crystalline silicon-based photovoltaic (PV) cells by using the solar spectral conversion principle, which employs a down-converting phosphor to convert a high-energy ultraviolet photon to the less energetic red-emitting photons to improve the spectral response of Si solar cells. In this study, the surface of silicon solar cells was coated with a red-emitting KCaGd(PO4)2:Eu3+ phosphor by using the screen-printing technique. In addition to the investigation on the microstructure using SEM, we have measured the short circuit current (Isc), open circuit voltage (Voc), and power conversion efficiency (η) of spectral-conversion cells and compared with those of bare solar cells as a reference. Preliminary experimental results revealed that in an optimized PV cell, an enhancement of 0.64+0.01% (from 16.03% to 16.67%) in of a Si-based PV cell has been achieved.Keywords: Solar cells; Down-converting phosphor; KCaGd(PO
4)2:Eu3+; Screen-printing To face the challenge of global warming,the development of green energy materials has been an important issue in materials research.
The photovoltaic (PV) cell is one of the devices that can be used to generate sustainable energy;
therefore, many research attempts have been made to explore materials that are able to enhance the power conversion efficiency () of solar cells. The conversion efficiency from light to electricity in a PV cell is highly spectrum into electricity, with ultraviolet (UV) and infrared (IR) spectral domains wasted.
Attempts to improve the conversion efficiency of PV cells using spectral conversion technique by employing up- or down-conversion mercury-free lamps; mainly, because the host of ABM(PO4)2 exhibits low phonon energy and potential quantum-cutting property.[14].
In this research, the effect of coating a down-conversion phosphor that is expected to form a radiation-sensitive surface on the silicon-based PV cells in attempt to increase the power conversion efficiency was investigated.
Essentially, this deposited phosphor layer is suitable for absorption and emission in the portion of solar spectrum and further benefits utilization of sun light, thus improving the
value of the PV cells. Furthermore, the
coated phosphor layer exhibits lower refractive index, which may serve as an antireflection layer in addition to solar spectral conversion.We have screened and selected phosphors such as KCaGd(PO4)2:Eu3+ (KCGP:Eu3+), with low phonon energy and lower refractive index than Si or Si3N4 that may be capable of converting more UV photons into photons with longer wavelengths to induce a greater spectral response for a Si-based PV cell. This work is attempted to evaluate and examine the potential applications of the down-converting KCaGd(PO4)2:Eu3+ in an attempt to improve
Foundation item: Supported by National Science Council of Taiwan under contract No NSC98-2113-M-009-005-MY3. Corresponding author: Teng-Ming Chen (E-mail: tmchen@mail.nctu.edu.tw; Tel: +886-3-5731695)
form a phosphor layers directly onto a commercial Si PV cell to convert the UV photons into those with wavelengths longer than 500 nm. The dependence of photovoltaic efficiency on the phosphor compositions, photoluminescence (PL) and PL excitation (PLE) spectra, and microstructure of the phosphor layer were investigated and discussed.
1 Experimental
Stoichiometric starting materials of (NH4)2HPO4, K2CO3, Eu2O3, SrCO3, CaCO3 (all analytic grade), and Gd2O3 (99.99% pure) were mixed together with NH4Cl as a flux and transferred to an alumina crucible; the materials were then heat treated at 800°C for 6 h and at 1200°C for 6 h. In comparison with the process described by Zhang et al.[7], our synthesis process takes only one-third of the time needed to prepare KMGd(PO4)2 (M = Ca, Sr). The phase purity of KMGd(PO4)2:Eu3+ phosphors was checked by powder X-ray diffraction with a Bruker AXS D8 advanced automatic diffractometer with Cu Kα radiation and all of reflections between 2θ = 10° and 80° were collected at room temperature.
Photoluminescence (PL) and PL excitation (PLE) spectra were obtained using a Jobin Yvon-Spex FluoroLog-3 fluorophotometer equipped with a 450 W Xe lamp as a light source.
The fabrication of KCaGd(PO4)2:Eu3+
(KGP:Eu3+)-coated solar cells is summarized in the flow diagram shown in Figure 1. Briefly, the KCaGd(PO4)2:Eu3+ phosphor was well-mixed with and dispersed in a composite
Figure 1 Flow diagram for fabrication of phosphor-coated Si solar cells.
then screen-printed on top of the prestructured Si3N4 reflective layer of a 6”x6” Si solar cell to form a transluscent film with 3-4
m in
thickness. The phosphor-coated solar cell was then baked at about 130℃ in the air for 10 minutes. The device structure of down-converting KCaGd(PO4)2:Eu3+ phosphor-coated solar cells is schematically shown in Figure 2.
Figure 2 The device structure of down-converting KCaGd(PO4)2:Eu3+ phosphor-coated solar cells.
Furthermore, the open-circuit voltage (Voc), short-circuit current (Isc), and power conversion efficiency () of the phosphor-coated solar cells were then measured using an h.a.l.m IV curve tracer (cetisPV-CTL1) and a Sun simulator (Xenon-Flasher cetisPV-XF2).
2 Results and discussion
The XRD patterns of KCaGd(PO4)2:Eu3+
and KSrGd(PO4)2:Eu3+ samples shown in Figures 3a and 3b, respectively, were found to match well with those reported in JCPDS cards 34-0125 and 34-0118, respectively. Except for slight differences in the cell parameters of the
Figure 3 Indexed XRD patterns of (a) KCaGd(PO4)2:Eu3+
and (b) KSrGd(PO4)2:Eu3+.
The KCaGd(PO4)2 host wasfound not to absorb in the ultraviolet region. The KGP:Eu3+
phosphor can be excited with 393 nm UV light
domain of 260 to 530 nm and a maximal emission in the wavelength domain of 580 to 700 nm, as indicated in Figure 4. Since the silicon wafer shows poor absorption in the UV spectral range, applying the KGP:Eu3+ or reflectance spectra for bare, soley binder-coated, and KGP:xEu3+ (x = 5%, 10%, 30%, 50%, and 100%)-coated solar cells.
Figure 5 Comparison of reflectance spectra for bare and KGP:Eu3+-coated solar cells. From top: bare cell, binder+cell, and cells with 5%-, 10%-, 30%-, 50%, and 100% KGP:Eu3+-coating.
Comparison of the reflectance spectra for the bare, binder-coated, and phosphor-coated cells indicates that the phosphor coating on the surface of the Si wafer can effectively reduce
coated cell. In addition to the decrease in reflectance, the observed Isc obtained from KCGP:Eu3+-coated solar cells was simultaneously found to increase with phosphor coating.
Figure 6 presents the current-voltage
data for an optimized Si solar cells with and without coating of KCGP:Eu3+, respectively.Data analysis indicates that KCGP:Eu3+-coated solar cell has greater Isc due to the efficient light conversion. That is, the Isc value was found to increase from 7.9664 to 8.3058 A, Voc increases from 0.6231 to 0.6247 V, and
was found to increase from 16.03% to 16.67%.These data revealed that Isc increases significantly with Voc unchanged upon phosphor coating.
Figure 6 Experimental current–voltage curves for a representative Si solar cells: (a) without and (b) with KGP:Eu3+ phosphor coating.
To further verify the experimental results that coating of KCGP:Eu3+ phosphor increases the power conversion efficiency, we measured the Isc, Voc, η,
Isc, Voc and η for forty
solar cells coated with and without KCGP:Eu3+phosphor.
Table 1 summarizes the comparison
on the average values and standard deviations of Isc, Voc, η, Isc, Voc, and η.On the average, we have observed that short-circuit current increases appreciably from 8.10 to 8.35A and the open-circuit voltage varies from 0.63115 to 0.63200V insignificantly, respectively.
We have also observed that
increase for
0.48% from an average value 16.52% to 17.00%. To investigate the microstructure of the phosphor-coated solar cell, we have investigated the SEM micrographs of a screen-printed KGP:Eu3+-coated solar cell after baking.Figures 7(a)
and (b) show the top- and side-view of the cell and the surface exhibits granular feature inherited from phosphor particles, whereas the radiation-sensitive layer is estimated to be 3.4m in thickness, as
revealed in the side-view SEM micrograph.Figure 7 SEM micrographs of a KGP:Eu3+-coated Si solar cell: (a) top view and (b) side view.
3 Conclusions
We have prepared a double phosphate phosphorKCaGd(PO4)2:Eu3+ and demonstrated that the down-converting KCaGd(PO4)2:Eu3+
phosphor coated on the surface of a
polycrystalline silicon solar cell can effectively increase the values of Isc, Voc, andof the cell The increase in
is about 0.48% on the
average, which corresponds to an increment of from 16.52% to 17.00%. However, in an optimized case, we have observed an increase in from 16.03% to 16.67%, which
corresponds to an increase of 0.64%. Coating orange red-emitting down-converting phosphors by screen printing technique on the surface of conventional solar devices is effective in enhancing the value and has been demonstrated in this research. The coated phosphor forms not only a spectral conversion layer for one part of the solar spectrum but also serves a low reflective layer for a different part of the solar spectrum. The PMMA may provide a transparent matrix for phosphor coating.Further work to improve the power conversion efficiency and select more efficient phosphors for solar application is currently in progress.
Acknowledgments
This research was supported by National Science Council of Taiwan under contract No.
NSC98-2113-M-009-005-MY3.
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