- Email: [email protected]
Application of rare-earth complexes for photovoltaic precursors
Solar Energy Materials and Solar Cells
ELSEVIER
Solar Energy Materials and Solar Cells 48 (1997) 35 4l
Application of rare-earth complexes for photovoltaic precursors Katsuyasu Kawano*, Kiyotaka Arai, Haruo Yamada, Naoaki Hashimoto, Ryouhei Nakata Department of Electronic Engineering, The University of Electro-Communications 1-5-1 ChoJugaoka, Chofit, Tokyo 182, Japan
Abstract By applying rare-earth complexes as photovoltaic precursors, the conversion efficiency of solar cells were improved. Rare-earth complexes absorb light at shorter wavelengths and subsequently emit light at longer wavelengths. The high-energy region of the solar spectrum is shifted to longer wavelengths, hence the cell-output power is higher because the emitted light matches with a higher sensitivity region of the basic Si solar cell. For a CaF 2 : Eu single crystal placed on top of an a-Si solar cell experiments showed a maximum relative conversion efficiency of 1.5. Eu ions in CaF 2 SrF 2 mixed crystals, whose emission is even more red-shifted compared with CaF 2 crystals, showed slight improvements. The ion implantation of rare-earth ions into CaF 2 evaluated potential application for antireflection (AR) coatings. Furthermore, porous glass doped with Eu-chelete complexes have been investigated. Keywords: Conversion efficiency; Solar cell; Rare earth; Europium; CaF2; Ion implantation; Porous glass
1. Introduction W i d e s p r e a d solar energy utilization as an alternative to the use of fossil energy is earnestly required in o u r lives in the 21st century. O n e of the key issues is the i m p r o v e m e n t of the c o n v e r s i o n efficiency of solar cells. A l m o s t all research projects on
* Corresponding author. E-mail: [email protected]. 0927-0248/97/$17.00 (~) 1997 Elsevier Science B.V. All rights reserved Pll S 0 9 2 7 - 0 2 4 8 ( 9 7 ) 0 0 0 6 6 - 4
36
K. Kawano et al. 'Solar Energy Materials and Solar" Cells 48 (I997) 35 41
efficiency improvements performed so far have aimed to modify the solar-cell structure itself for fitting the spectral response to the solar spectrum, which is absolutely unchangeable. But our studies point out that it is possible to modify the solar spectrum to better match the sensitivity of the solar cell by applying wavelength shifting layers of rare-earth ions. Rare-earth complexes absorb light at shorter wavelengths and, subsequently, emit light at longer wavelengths. The high-energy region of the solar spectrum which cannot be utilized by conventional Si solar cells can be shifted to a longer-wavelength region, hence the cells' output can be increased. The purpose of this study is to achieve high-conversion efficiency solar cells by applying optically active rare earth complexes as photovoltaic precursors [1]. For practical use, the ion-implantation technique should be used to dope the antireflection coating with Eu 2 +. The antireflection (ARt coating is made of inorganic, insulating crystals which have low refractive indices for light confinement and are perfectly transparent throughout the solar spectrum. In the present study, the Bridgman-grown CaFz single crystals doped with Eu were placed on top of the solar cell, replacing the standard AR film. Fig. 1 demonstrates the red-shift of the fluorescence of a CaF2 thin disc crystal doped with Eu (0.1 mol%) compared to the excitation spectrum of a short-passfiltered Xe-light source. The peak of the spectrum shifts to longer wavelengths by about 70nm. A C a F 2 : E u single crystal has two absorption bands at 214.3 and 334.0 nm at room temperature due to f-d transitions in the divalent Eu (4f7) ion [2, 3] and one fluorescence band at 424.6 nm. Now, the Eu 2 + ion absorbs the sunlight in the 334 nm band, and emits fluorescence light above 400 nm, where the solar cells have higher sensitivity. In this case, a-Si solar cell will be best suited for efficiency improvements.
- -- p u r e ......
0.05mo1%
r-
c" m
5 ~g ©
nc
26o'''46o" 66o" 86o''' ooo Wavelength
[nm]
Fig. 1. The transmission spectrum treal line)observedthrough CaF~ : Eu-thin disc crystal under illumination by a short-pass-filteredXe light (dotted line}which is transmitted through C a F 2 pure crystal.The peak of the spectrum shifts to longer wavelengthsby about 70 nm
K. Kawano et al./Solar Energy Materials and Solar Cells 48 (1997) 35-41
37
2. E x p e r i m e n t a l procedure
The CaF 2 single crystals doped with Eu with concentrations ranging from 0.01 to 2.0 mol% were prepared using a Bridgman apparatus [2, 3]. A crystal, well polished to optical flatness, is placed in close contact on top of a solar cell, and the improvement of the relative conversion efficiency compared with nondoped CaF2 has been verified. For the assessment of the effect of rare-earth ions on the conversion efficiency, the following solar cells were chosen: a-Si solar cell (Sanyo Elec. AM-1411, Osaka) and p-Si solar cell (Solarex MSX-64, USA). As solar simulator, a 150 W xenon lamp (Ushio Elec. DSB150, Tokyo) with appropriate filters was used. Results were confirmed by the authorized solar simulator (Yamashita Denso, YSS-50, Tokyo). The relative change in the conversion efficiency was evaluated by comparing measurements of crystals with different Eu concentrations to the undoped crystal. All the crystals were arranged in the thickness of 1 ram. Fig. 2 shows the dependence of a-Si solar cell conversion efficiency on the Eu concentration for different levels of illumination. The samples with a dopant concentration of 0.05 mol% give the highest increase of efficiency within the investigated concentration range 0.0014).2 mol%. This impurity concentration that gives the highest conversion efficiency is the same as that which gives maximum fluorescence intensity. At this dopant concentration under low illumination, the maximum relative efficiency is 1.5. This means that doping with Eu gives a conversion efficiency of 15% as compared to the efficiency of 10% for a conventional a-Si solar cell. These results show the effectiveness of rare-earth ion doping to obtain higher conversion efficiencies. In a previous paper [2], we showed that Eu 2 + ions doped into CaF2-SrF2 mixed crystals have an even larger red shift as compared with CaF2 crystals. Hence, even
"u
. . . . . . . .
I
. . . . . . .
I
1.6 . . . . . . . . _
[
,,,, :o
1.4-
pure
I
'
'
1.0 . . . . 2.0 . . . . . 4.0 9.0
-
"
-~ 1.2I
IiId
. . . . . . . .
/' / I~',\~ ,".,// \,~~-~
. . . . . . . .
i
i
I
iflllll
. . . . . . . .
",, ',,
I
0.01 0.1 1 Eu Concentration [tool%]
,
Fig. 2. The relative conversion efficiencyagainst Eu concentration in CaF2 : Eu crystals when the illuminance of incident light on the crystal surface was kept fixed.
38
K. Kawam) el al. Solar Enerxv Materials and Solar ('ells 4~' (1997) 35 41
i
=
1.5f
i
E
i
i
i
i
i
Cal_xSrxF2:Eu on a-Si Cell
d3
5 LLI
,7
.~_ 5 cr
0.~
~_~
L
0,.5
.....
,
,
X Fig. 3. The relative conversion clticiency against the lnixlurc ratio x in CaVe SrF2 : EL1 mixed crystals.
higher efficiencies were expected for such mixed crystals on top of a-Si (sensitivity peak at 600 nm) as well as p-Si (peak; 700 nm) solar cells. Actually, a slight improvement of efficiencies of a-Si cells were reached as shown in Fig. 3. Thermal-quenching studies of the fluorescence of the investigated doped fluorite crystals showed a gradually decreasing intensity by increasing the temperature which was detectable up to 180 C. This indicates that the proposed solar cell can be used even at high temperatures because standard a-Si solar cells are guaranteed up to 70:C. As a method for doping rare-earth ions into crystals, the ion implantation technique has been applied under the following conditions: disc-shaped targets of undoped CaF2 crystals, acceleration voltage of 110 kV and Eu density of 2.6 × 10~S/cm 2. The spectral analysis showed that Eu 3 + states (exc: 390, 410, 445 nm; ems: 590, 615. 670 nm) were formed, but some thermal annealing treatments are needed for practical use. For industrial production, the ion-implantation techniques can be used to dope the AR coatings of conventional solar cells: this would be a further step towards the development of a new-type solar cell. Another material which was doped with rare-earth ions is porous Bycor glass (Coming 7930) permiated with Eu-(C~ 5H 12()e).~ (Eu-dibenzoylmethide henceforth, Eu-D) chelate complexes. Conversion efficiencies monitored with a p-Si solar cell were measured for various Eu-D concentrations. The porous glass with Eu-D showed fluorescence of the s a m e E u 3+ states (exc: 270, 355 nm: ems: 580, 590, 610 nm) as that of the Eu-implanted CaF2 crystal. For the system of the Eu-chelate p o r o u s glass on top of a p-Si solar cell, the results of the relative conversion efficiencies for Eu-chelate concentrations are given in Fig. 4. The figures are a little lower than 1.0 because the porous glass shows lower transmittance for sunlight and lower brightness of trivalent Eu as compared to CaF2 crystals. The positions of fluorescence, however, are useful to
K. Kawano et al./Solar Energy Materials and Solar Cells 48 (1997) 35 41
39
c
uJ c o
>
0.8
cO
Eu-Chelate Glass on p-Si Cell
O
0.6 ['~
. . . . . . .
t
10-4
. . . . . . . .
I
10 - 3
. . . . . . . .
I
. . . . . .
10 - 2
Concentration [mol/I] Fig. 4. The results of relative conversion efficiencies using the porous Bycor glass (Corning 7930) permiated with Eu-(C1 _sH1202)3 (Eu-dibenzoylmethide; Eu-D) chelate complexes against various Eu~chelate concentrations.
a-Si and p-Si solar cells if its weak brightness were enhanced by any method of light confinement.
3. Discussion
In order to confirm the experimental results, we estimated theoretically the relative conversion efficiency based on the assumption that the achievement of higher conversion efficiency due to wavelength shifting results from relative spectral positions of absorption, emission bands of Eu z + ions and the solar spectrum. Reflection and scattering due to CaF 2 crystal are not significant for the calculation of the relative conversion efficiency because they are common factors for both doped and undoped crystals. If the normalized spectral function of the incident solar light and fluorescence of Eu are denoted by Pi(2) and Pf(2), respectively, the output spectrum Po(2) incident at the solar cell is expressed by using the normalized absorption coefficient ~(2) and the emission rate Em of fluorescence, as follows: Po(&) = (1 - ~(A))P~(£) + E'mPf(,~).
(1)
The power relation between sunlight and fluorescence over the considered wavelength region is Pf(2) d2 = ft. 7fg(2)P~(2) d2,
(2)
K. Kawano et al./Solar Energy Materials"and Solar Cells 48 (1997) 35-41
40
where/~ is the energy correction factor due to the difference in wavelength, and 7 is the fluorescence quantum efficiency. If the spectral sensitivity function is given by S(2) in a normalized form, the conversion efficiency q of a solar cell is s(2)Po d2
q
-
(3)
fs(x)P,
d).
The relative conversion efficiency is derived from the ratio of the conversion efficiency r/r of a solar cell with a rare-earth-doped crystal to r/o of the same cell with a nondoped crystal as follows:
~1~ --=
~1,,
EmfS()jPf d)" - f S()d~()t)Pi d)~ 1 +
(4)
fs().tPi d).
If we put fm = 0.5 derived from empirical data, we can estimate the relative conversion efficiency ~/r/r/o with/4 = 0.84 from the numerical calculation over the concerned wavelengths from the spectral functions such as the incident light of the solar simulator, absorption, fluorescence of Eu 2 + and the spectral sensitivity of the a-Si solar cell. This gives a relative conversion efficiency of 1.58 at I' = 1 and 1.29 at 7 = 0.5, where 1' is the fluorescence quantum efficiency of Eu z + ions. These estimated values are comparable with the results obtained in the experiment. The optical properties of the materials used in the antireflection films of solar cells including CaF2 are given in Table 1. The refractive indexes of the inorganic materials cited here are all very similar and, hence, it is expected that the results obtained for CaF2 can be transferred to other materials. Many studies [4] of ion implantation have been carried out using inorganic compounds. Aono et al. [5] studied the luminescence during Eu implantation into CaF2, and indicated that there is little radiative damage in Eu-implanted layers: Rutherford backscattering measurements suggested that Eu atoms occupy
Table I Comparison of the refractive index of CaF 2 with those of other insulating materials used in the antireflection films under AM-I.5 sunlight Material
Refractive index
CaFz MgF2 SiO2 SiO SiO 2 TiO 2
1.41-1.45 1.44 1.40-1.46 1.80- 1.90 1.80-1.96
K. Kawano et al./Solar Energy Materials and Solar Cells 48 (1997) 35-41
41
substitutional sites. However, neither absorption spectra nor annealing effects were reported. Therefore, further experiments are necessary to find the o p t i m u m conditions for creating sufficient diffused impurity states. Moreover, it is mentioned I-5] that the penetration depth of implanted Eu has a peak at 33 nm in the region below 100 nm, in which the real concentration of Eu is four to five orders of magnitude less than that of the 0.05 m o l % Eu crystal grown by the Bridgman method. To obtain the desired efficiencies, other methods such as the multilayer-doping method [6] involving alternate implantation and evaporation or the CVD method using CaF2 powders with Eu dopants may be appropriate.
Acknowledgements The present work was supported by a Grant-in-Aid for Scientific Research on Priority Areas "New Development of Rare Earth Complexes" No. 06241227 and by a Grant-in-Aid for General Research Division No.06452317 from The Ministry of Education, Science, Sports and Culture.
References [1] [2] [3] [4]
R. Nakata, N. Hashimoto, K. Kawano, Jpn. J. Appl. Phys. 35 (1996) L90-L93. K. Kawano, J. Tominaga, H. Satoh, R. Nakata, M. Sumita, Jpn. J. Appl. Phys. 29 (1990) L319-L321. R. Nakata, H. Satoh, J. Tominaga, K. Kawano, M. Sumita, J. Phys. C 3 (1991) 5903-5913. P.D. Townsend, P.J. Chandler, L. Zhang, Optical Effectsof Ion Implantation, Cambridge, New York, 1994, pp. 115-150. ['5] K. Aono, M. Iwaki, S. Namba, Nucl. Instr. and Meth. B 32 (1988) 231-233. ['6] K. Kawano, R. Nakata, Japan Open Patent No. 8-204222, 9 August, 1996.
ELSEVIER
Solar Energy Materials and Solar Cells 48 (1997) 35 4l
Application of rare-earth complexes for photovoltaic precursors Katsuyasu Kawano*, Kiyotaka Arai, Haruo Yamada, Naoaki Hashimoto, Ryouhei Nakata Department of Electronic Engineering, The University of Electro-Communications 1-5-1 ChoJugaoka, Chofit, Tokyo 182, Japan
Abstract By applying rare-earth complexes as photovoltaic precursors, the conversion efficiency of solar cells were improved. Rare-earth complexes absorb light at shorter wavelengths and subsequently emit light at longer wavelengths. The high-energy region of the solar spectrum is shifted to longer wavelengths, hence the cell-output power is higher because the emitted light matches with a higher sensitivity region of the basic Si solar cell. For a CaF 2 : Eu single crystal placed on top of an a-Si solar cell experiments showed a maximum relative conversion efficiency of 1.5. Eu ions in CaF 2 SrF 2 mixed crystals, whose emission is even more red-shifted compared with CaF 2 crystals, showed slight improvements. The ion implantation of rare-earth ions into CaF 2 evaluated potential application for antireflection (AR) coatings. Furthermore, porous glass doped with Eu-chelete complexes have been investigated. Keywords: Conversion efficiency; Solar cell; Rare earth; Europium; CaF2; Ion implantation; Porous glass
1. Introduction W i d e s p r e a d solar energy utilization as an alternative to the use of fossil energy is earnestly required in o u r lives in the 21st century. O n e of the key issues is the i m p r o v e m e n t of the c o n v e r s i o n efficiency of solar cells. A l m o s t all research projects on
* Corresponding author. E-mail: [email protected]. 0927-0248/97/$17.00 (~) 1997 Elsevier Science B.V. All rights reserved Pll S 0 9 2 7 - 0 2 4 8 ( 9 7 ) 0 0 0 6 6 - 4
36
K. Kawano et al. 'Solar Energy Materials and Solar" Cells 48 (I997) 35 41
efficiency improvements performed so far have aimed to modify the solar-cell structure itself for fitting the spectral response to the solar spectrum, which is absolutely unchangeable. But our studies point out that it is possible to modify the solar spectrum to better match the sensitivity of the solar cell by applying wavelength shifting layers of rare-earth ions. Rare-earth complexes absorb light at shorter wavelengths and, subsequently, emit light at longer wavelengths. The high-energy region of the solar spectrum which cannot be utilized by conventional Si solar cells can be shifted to a longer-wavelength region, hence the cells' output can be increased. The purpose of this study is to achieve high-conversion efficiency solar cells by applying optically active rare earth complexes as photovoltaic precursors [1]. For practical use, the ion-implantation technique should be used to dope the antireflection coating with Eu 2 +. The antireflection (ARt coating is made of inorganic, insulating crystals which have low refractive indices for light confinement and are perfectly transparent throughout the solar spectrum. In the present study, the Bridgman-grown CaFz single crystals doped with Eu were placed on top of the solar cell, replacing the standard AR film. Fig. 1 demonstrates the red-shift of the fluorescence of a CaF2 thin disc crystal doped with Eu (0.1 mol%) compared to the excitation spectrum of a short-passfiltered Xe-light source. The peak of the spectrum shifts to longer wavelengths by about 70nm. A C a F 2 : E u single crystal has two absorption bands at 214.3 and 334.0 nm at room temperature due to f-d transitions in the divalent Eu (4f7) ion [2, 3] and one fluorescence band at 424.6 nm. Now, the Eu 2 + ion absorbs the sunlight in the 334 nm band, and emits fluorescence light above 400 nm, where the solar cells have higher sensitivity. In this case, a-Si solar cell will be best suited for efficiency improvements.
- -- p u r e ......
0.05mo1%
r-
c" m
5 ~g ©
nc
26o'''46o" 66o" 86o''' ooo Wavelength
[nm]
Fig. 1. The transmission spectrum treal line)observedthrough CaF~ : Eu-thin disc crystal under illumination by a short-pass-filteredXe light (dotted line}which is transmitted through C a F 2 pure crystal.The peak of the spectrum shifts to longer wavelengthsby about 70 nm
K. Kawano et al./Solar Energy Materials and Solar Cells 48 (1997) 35-41
37
2. E x p e r i m e n t a l procedure
The CaF 2 single crystals doped with Eu with concentrations ranging from 0.01 to 2.0 mol% were prepared using a Bridgman apparatus [2, 3]. A crystal, well polished to optical flatness, is placed in close contact on top of a solar cell, and the improvement of the relative conversion efficiency compared with nondoped CaF2 has been verified. For the assessment of the effect of rare-earth ions on the conversion efficiency, the following solar cells were chosen: a-Si solar cell (Sanyo Elec. AM-1411, Osaka) and p-Si solar cell (Solarex MSX-64, USA). As solar simulator, a 150 W xenon lamp (Ushio Elec. DSB150, Tokyo) with appropriate filters was used. Results were confirmed by the authorized solar simulator (Yamashita Denso, YSS-50, Tokyo). The relative change in the conversion efficiency was evaluated by comparing measurements of crystals with different Eu concentrations to the undoped crystal. All the crystals were arranged in the thickness of 1 ram. Fig. 2 shows the dependence of a-Si solar cell conversion efficiency on the Eu concentration for different levels of illumination. The samples with a dopant concentration of 0.05 mol% give the highest increase of efficiency within the investigated concentration range 0.0014).2 mol%. This impurity concentration that gives the highest conversion efficiency is the same as that which gives maximum fluorescence intensity. At this dopant concentration under low illumination, the maximum relative efficiency is 1.5. This means that doping with Eu gives a conversion efficiency of 15% as compared to the efficiency of 10% for a conventional a-Si solar cell. These results show the effectiveness of rare-earth ion doping to obtain higher conversion efficiencies. In a previous paper [2], we showed that Eu 2 + ions doped into CaF2-SrF2 mixed crystals have an even larger red shift as compared with CaF2 crystals. Hence, even
"u
. . . . . . . .
I
. . . . . . .
I
1.6 . . . . . . . . _
[
,,,, :o
1.4-
pure
I
'
'
1.0 . . . . 2.0 . . . . . 4.0 9.0
-
"
-~ 1.2I
IiId
. . . . . . . .
/' / I~',\~ ,".,// \,~~-~
. . . . . . . .
i
i
I
iflllll
. . . . . . . .
",, ',,
I
0.01 0.1 1 Eu Concentration [tool%]
,
Fig. 2. The relative conversion efficiencyagainst Eu concentration in CaF2 : Eu crystals when the illuminance of incident light on the crystal surface was kept fixed.
38
K. Kawam) el al. Solar Enerxv Materials and Solar ('ells 4~' (1997) 35 41
i
=
1.5f
i
E
i
i
i
i
i
Cal_xSrxF2:Eu on a-Si Cell
d3
5 LLI
,7
.~_ 5 cr
0.~
~_~
L
0,.5
.....
,
,
X Fig. 3. The relative conversion clticiency against the lnixlurc ratio x in CaVe SrF2 : EL1 mixed crystals.
higher efficiencies were expected for such mixed crystals on top of a-Si (sensitivity peak at 600 nm) as well as p-Si (peak; 700 nm) solar cells. Actually, a slight improvement of efficiencies of a-Si cells were reached as shown in Fig. 3. Thermal-quenching studies of the fluorescence of the investigated doped fluorite crystals showed a gradually decreasing intensity by increasing the temperature which was detectable up to 180 C. This indicates that the proposed solar cell can be used even at high temperatures because standard a-Si solar cells are guaranteed up to 70:C. As a method for doping rare-earth ions into crystals, the ion implantation technique has been applied under the following conditions: disc-shaped targets of undoped CaF2 crystals, acceleration voltage of 110 kV and Eu density of 2.6 × 10~S/cm 2. The spectral analysis showed that Eu 3 + states (exc: 390, 410, 445 nm; ems: 590, 615. 670 nm) were formed, but some thermal annealing treatments are needed for practical use. For industrial production, the ion-implantation techniques can be used to dope the AR coatings of conventional solar cells: this would be a further step towards the development of a new-type solar cell. Another material which was doped with rare-earth ions is porous Bycor glass (Coming 7930) permiated with Eu-(C~ 5H 12()e).~ (Eu-dibenzoylmethide henceforth, Eu-D) chelate complexes. Conversion efficiencies monitored with a p-Si solar cell were measured for various Eu-D concentrations. The porous glass with Eu-D showed fluorescence of the s a m e E u 3+ states (exc: 270, 355 nm: ems: 580, 590, 610 nm) as that of the Eu-implanted CaF2 crystal. For the system of the Eu-chelate p o r o u s glass on top of a p-Si solar cell, the results of the relative conversion efficiencies for Eu-chelate concentrations are given in Fig. 4. The figures are a little lower than 1.0 because the porous glass shows lower transmittance for sunlight and lower brightness of trivalent Eu as compared to CaF2 crystals. The positions of fluorescence, however, are useful to
K. Kawano et al./Solar Energy Materials and Solar Cells 48 (1997) 35 41
39
c
uJ c o
>
0.8
cO
Eu-Chelate Glass on p-Si Cell
O
0.6 ['~
. . . . . . .
t
10-4
. . . . . . . .
I
10 - 3
. . . . . . . .
I
. . . . . .
10 - 2
Concentration [mol/I] Fig. 4. The results of relative conversion efficiencies using the porous Bycor glass (Corning 7930) permiated with Eu-(C1 _sH1202)3 (Eu-dibenzoylmethide; Eu-D) chelate complexes against various Eu~chelate concentrations.
a-Si and p-Si solar cells if its weak brightness were enhanced by any method of light confinement.
3. Discussion
In order to confirm the experimental results, we estimated theoretically the relative conversion efficiency based on the assumption that the achievement of higher conversion efficiency due to wavelength shifting results from relative spectral positions of absorption, emission bands of Eu z + ions and the solar spectrum. Reflection and scattering due to CaF 2 crystal are not significant for the calculation of the relative conversion efficiency because they are common factors for both doped and undoped crystals. If the normalized spectral function of the incident solar light and fluorescence of Eu are denoted by Pi(2) and Pf(2), respectively, the output spectrum Po(2) incident at the solar cell is expressed by using the normalized absorption coefficient ~(2) and the emission rate Em of fluorescence, as follows: Po(&) = (1 - ~(A))P~(£) + E'mPf(,~).
(1)
The power relation between sunlight and fluorescence over the considered wavelength region is Pf(2) d2 = ft. 7fg(2)P~(2) d2,
(2)
K. Kawano et al./Solar Energy Materials"and Solar Cells 48 (1997) 35-41
40
where/~ is the energy correction factor due to the difference in wavelength, and 7 is the fluorescence quantum efficiency. If the spectral sensitivity function is given by S(2) in a normalized form, the conversion efficiency q of a solar cell is s(2)Po d2
q
-
(3)
fs(x)P,
d).
The relative conversion efficiency is derived from the ratio of the conversion efficiency r/r of a solar cell with a rare-earth-doped crystal to r/o of the same cell with a nondoped crystal as follows:
~1~ --=
~1,,
EmfS()jPf d)" - f S()d~()t)Pi d)~ 1 +
(4)
fs().tPi d).
If we put fm = 0.5 derived from empirical data, we can estimate the relative conversion efficiency ~/r/r/o with/4 = 0.84 from the numerical calculation over the concerned wavelengths from the spectral functions such as the incident light of the solar simulator, absorption, fluorescence of Eu 2 + and the spectral sensitivity of the a-Si solar cell. This gives a relative conversion efficiency of 1.58 at I' = 1 and 1.29 at 7 = 0.5, where 1' is the fluorescence quantum efficiency of Eu z + ions. These estimated values are comparable with the results obtained in the experiment. The optical properties of the materials used in the antireflection films of solar cells including CaF2 are given in Table 1. The refractive indexes of the inorganic materials cited here are all very similar and, hence, it is expected that the results obtained for CaF2 can be transferred to other materials. Many studies [4] of ion implantation have been carried out using inorganic compounds. Aono et al. [5] studied the luminescence during Eu implantation into CaF2, and indicated that there is little radiative damage in Eu-implanted layers: Rutherford backscattering measurements suggested that Eu atoms occupy
Table I Comparison of the refractive index of CaF 2 with those of other insulating materials used in the antireflection films under AM-I.5 sunlight Material
Refractive index
CaFz MgF2 SiO2 SiO SiO 2 TiO 2
1.41-1.45 1.44 1.40-1.46 1.80- 1.90 1.80-1.96
K. Kawano et al./Solar Energy Materials and Solar Cells 48 (1997) 35-41
41
substitutional sites. However, neither absorption spectra nor annealing effects were reported. Therefore, further experiments are necessary to find the o p t i m u m conditions for creating sufficient diffused impurity states. Moreover, it is mentioned I-5] that the penetration depth of implanted Eu has a peak at 33 nm in the region below 100 nm, in which the real concentration of Eu is four to five orders of magnitude less than that of the 0.05 m o l % Eu crystal grown by the Bridgman method. To obtain the desired efficiencies, other methods such as the multilayer-doping method [6] involving alternate implantation and evaporation or the CVD method using CaF2 powders with Eu dopants may be appropriate.
Acknowledgements The present work was supported by a Grant-in-Aid for Scientific Research on Priority Areas "New Development of Rare Earth Complexes" No. 06241227 and by a Grant-in-Aid for General Research Division No.06452317 from The Ministry of Education, Science, Sports and Culture.
References [1] [2] [3] [4]
R. Nakata, N. Hashimoto, K. Kawano, Jpn. J. Appl. Phys. 35 (1996) L90-L93. K. Kawano, J. Tominaga, H. Satoh, R. Nakata, M. Sumita, Jpn. J. Appl. Phys. 29 (1990) L319-L321. R. Nakata, H. Satoh, J. Tominaga, K. Kawano, M. Sumita, J. Phys. C 3 (1991) 5903-5913. P.D. Townsend, P.J. Chandler, L. Zhang, Optical Effectsof Ion Implantation, Cambridge, New York, 1994, pp. 115-150. ['5] K. Aono, M. Iwaki, S. Namba, Nucl. Instr. and Meth. B 32 (1988) 231-233. ['6] K. Kawano, R. Nakata, Japan Open Patent No. 8-204222, 9 August, 1996.