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Journal of Alloys and Compounds 661 (2016) 14e19
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Eu3þ-Mn2þ-doped bi-functional glasses with solar photon downshifting: Application to CdS/CdTe solar cells Pei Song*, Chaomin Zhang, Pengfei Zhu School of Fundamental Studies, Shanghai University of Engineering Science, Shanghai 201620, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 November 2015 Accepted 22 November 2015 Available online 2 December 2015
Luminescent downshifting is a viable way for enhancing the short-wavelength spectral-response of photovoltaic device, especially for CdS/CdTe thin-film solar cells. Phosphate glasses codoped with rare earth Mn2þ-Eu3þ ions were prepared by high temperature melting method, and the absorption, excitation, and emission spectra of the glasses were investigated in detail. Under the illumination of sunlight (<520 nm excitation), the emission of the glasses can vary from ultraviolet to visible with emitting strong red emission, which just matched the spectral-response of CdS/CdTe cells, result in harvesting short wavelength photons and transferring them into the photocurrent output towards the photoelectric conversion efficiency improvement. In this work, such rare earth ions codoped phosphate glasses are investigated for their potential as bi-functional superstrate glass for CdS/CdTe solar cells and experimental performances of Luminescent downshifting layers fabricated with actual CdS/CdTe cells obtained. As a result, prepared Mn2þ-Eu3þ glasses could effectively improve the photoelectric conversion efficiency by 7.140% compared to pure CdS/CdTe solar cells. © 2015 Elsevier B.V. All rights reserved.
Keywords: Rare earth Phosphate Superstrate glass CdS/CdTe Downshifting Efficiency
1. Introduction Thin-film cadmium sulphide/cadmium telluride (CdS/CdTe) heterojunction solar cells have been studied extensively, and progress towards higher cell performance has not been rapid partly due to the parasitic absorption in the front cover glass and the transparent conducting oxide (TCO) front contact, but principally because of its poor spectral response (SR) in the short wavelength area (<520 nm) due to absorption or thermalization losses in the CdS buffer layer [1e3]. For the CdS/CdTe cell, the CdS layer has a band gap energy of 2.4 eV (~510 nm) results in strong absorption in ultraviolet (UV) and visible (VIS) blue photons, which ultimately lead to few utilization in UV light in CdS/CdTe cells and significantly decrease in external quantum efficiency (EQE) when going to the UV area, as shown in Fig. 1. Therefore, a large margin of improvement should be achieved towards harvest short wavelength photons and convert them into the photocurrent output. An alternative approach that does not require optimizing the electronic properties of the CdTe cell but still can be shown to enhance the poor SR in the short wavelength area, is the application of a luminescent
* Corresponding author. E-mail address: [email protected] (P. Song). http://dx.doi.org/10.1016/j.jallcom.2015.11.168 0925-8388/© 2015 Elsevier B.V. All rights reserved.
downshifting (LDS) layer [4]. LDS increases short wavelength SR by using luminescent materials which absorb photons at wavelengths where the cell performs poorly, and re-emit longer wavelength photon, to which the device SR more efficiently [5]. A drawback of transparent cover layer used to reduce convection and radiation losses is the reflection of the incident photon at its top surfaces. Therefore, anti-reflection (AR) layers for solar collector covers are widely used. However, in CdS/CdTe cells, the utilized solar photon is mainly in VIS light area, which occupies the largest part of solar spectrum, and UV photons are few to be utilized in CdS/CdTe cells. Antireflection [6] and LDS are two predominant solutions to solve the problem. On one hand, technology of antireflection is so mate that has little promotion space. On the other hand, the LDS layer exists the problem of low transmission [7]. To some extent, antireflection and LDS co-operation could make complementary advantages that may be an effective approach to increase the photoelectric conversion efficiency (PCE) of CdS/CdTe cells. Such so-called superstrate is covering layer on the sunny side of a photovoltaic (PV) cell, providing protection for the PV cells from impact and environmental degradation while allowing maximum transmission of the appropriate wavelengths of the solar spectrum [8]. In our studies, on one hand, compared with other kinds of glasses, the phosphate glasses have some merits of good thermal and chemical stability, high solubility of doping ions, large emission
P. Song et al. / Journal of Alloys and Compounds 661 (2016) 14e19
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Fig. 1. The normalized solar irradiance (red curve) and the normalized SR of the CdTe cell with the 300 nm CdS buffer layer (blue curve). The optimum wavelength of SR of the CdTe module with the 300 nm CdS buffer layer and the CdS buffer layer are around 855 nm (Eg ¼ 1.45 eV) and 516 nm (Eg ¼ 2.4 eV), respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
and absorption cross sections, and high transparency of VIS area [9,10]. On the other hand, the Eu3þ-Mn2þ-codoped system have intense and broad absorption in UV area and could convert it into VIS area with good LDS properties and high internal quantum efficiency (IQE) [11]. In this work, rare earth (RE) Eu3þ-Mn2þ-codoped phosphate glasses are investigated for their potential as LDS superstrate for CdS/CdTe cells. 2. Experimental procedures 2.1. Preparation and characterization measurement of superstrates Eu3þ ions and Mn2þ ions co-doped phosphate glasses with the chemical compositions of 55P2O5e12Li2Oe7Al2O3e10BaOe 4Na2Oe4ZnOe6MnO2e2Eu2O3 (mole percents) were prepared by melting-quenching technique [12]. Thoroughly mixed raw materials including NH4H2PO4, Li2CO3, Al2O3, BaCO3, NaCO3, ZnCO3, Eu2O3 and MnCO3 were fully sintered at 360 C for 480 min to reduce the surface moisture. Raw materials were then melted at 1355 C in high purity quartz crucibles for 210 min with stirrings, and the melts were cast onto a preheated copper mold. Then the glass was subsequently transferred to a temperature controlled annealing furnace kept at 435 C for 180 min in order to relieve thermal stresses. The annealed glasses were cut and polished in the form of rectangular plates (105 mm 105 mm 3.5 mm), as photos shown in Fig. 2. The absorption, photoluminescence (PL), photoluminescence excitation (PLE) and LDS emission spectra were measured on an Omni-l300 fluorescence spectrophotometer (Zolix Instruments, China) under the excitation of an LHX500 xenon arc lamp (Zolix Instruments, China). The VIS light was detected by a photomultiplier tube detector. 2.2. Preparation of the bare thin-film CdS/CdTe cell The CdS/CdTe cell with sides of 100 mm 100 mm were cut from a full-size and commercial module (Calyxo GmbH, Germany), which is prior to rear-side encapsulation, slightly smaller than the superstrate glass with sides of 105 mm 105 mm to ensure that the CdS/CdTe cell is completely covered. The bare CdS/CdTe cell refers to the combination of the real cell (the active material) and
Fig. 2. The photo above shows superstrate sample prepared in this work (samples from the left are Eu3þ-doped, Mn2þ-doped and Eu3þ-Mn2þ codoped one, respectively). The illustration below shows a cross-sectional architecture of the CdS/CdTe cell top covered with a superstrate.
transparent conductive oxide (TCO) as the top electric contact. 2.3. Architecture of superstrates fabricated with bare PV devices and effect measurement The superstrate is attached to the bare CdS/CdTe cell using a matching-glycerine (Nikon, MXA20233), which has a refractive index of 1.47 at 25 C similar to the glass and the CdS/CdTe cell to ensure good optical coupling [13], as shown in Fig. 2. The four edges of such sandwiched layers were encapsulated in commercially available non-formulated ethylene vinyl acetate (EVA) by using the vacuum-laminating process, which occurred at 150 C for a cycle of 3 min under vacuum followed by 5 min under atmospheric pressure [14]. Eu3þ ions and Mn2þ ions were respectively doped into phosphate glasses to form glassy luminescent superstrates with antireflection properties. PL properties can be found with LDS from UV range to a VIS-red range. The superstrate could be a prospective material having antireflection and LDS bi-function of sunlight modification for improving the PCE of the CdS/CdTe cell. By covering the superstrate onto a CdS/CdTe cell, as shown in Fig. 2, effects on PCE enhancement were measured. Current densityevoltage (JeV) curves were measured using a solar simulator (Newport, Class-AAA-94043A, 100 mW/cm2, Xenon-1000 W). 3. Results and discussion 3.1. Analytical experimental measurement The absorption and emission spectra of the Eu3þ ion singly doped phosphate glass is shown in Fig. 3. The Eu3þ ion related absorption peaks are attributed to transitions from the ground state level 7F0 to the excited states 5H4, 5D4, 5G2, 5L6, 5D2, and 5D1 in 319e532 nm wavelength range, as shown in Fig. 3. The position of absorption bands for the Eu3þ ion is relatively concentrated, thus we mainly discuss the absorption in near 400 nm area. Under 395 nm excitation, Eu3þ ions are excited from the ground state 7F0
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P. Song et al. / Journal of Alloys and Compounds 661 (2016) 14e19
Fig. 3. Normalized absorption and emission spectra of the Eu3þ singly doped LDS sample in the 200e700 nm wavelength area.
to a higher excited state 5D2 and the excited Eu3þ ions in 5D2 state can be relaxed to 5D1 and 5D0 states by phonon-assisted nonradiative transition. Because the energy differences between the each division level in the ground state is small, then Eu3þ ions radiatively relax to the ground states 5Fi (i ¼ 0e5), resulting in characteristic 560e700 nm VIS-red emission, in which the strongest emission band is around 615 nm [15]. Mn2þ ions have intense absorption peaks in the broad bands cover the range from 350 nm to 500 nm, with strong excited wavelengths of 410 nm. Under 350 nm, 370 nm, 410 nm, 456 nm and 468 nm excitation, which belong to 6A1(6S) / 4E(4D), 6 A1(6S) / 4T2(4D), 6A1(6S) / [4A1(4G), 4E1(4G)], 6A1(6S) / 4T2(4G) and 6A1(6S) / 4T1(4G) transitions, respectively, the emission of Mn2þ ion are monitored at 615 nm that belongs to 4T1(G) / 6A1(G) [15]. The absorption and emission spectra of the Mn2þ ion singly doped phosphate glass is shown in Fig. 4. The absorption and emission spectra of the Eu3þ-Mn2þ-codoped phosphate glass is shown in Fig. 5. Comparing the Eu3þ and Mn2þ ion singly-doped glass samples with the Eu3þ-Mn2þ-codoped glass sample, the absorption band of the 615 nm emission is broader and stronger, which are helpful to transfer the UV sunlight into the VISred radiation corresponding to the optimum SR of CdS/CdTe cells. The decay curves of the prepared phosphate glasses are shown in Fig. 6. The luminescence decay lifetime (LDT) are calculated as
Fig. 5. Normalized absorption and emission spectra of the Eu3þ-Mn2þ-codoped LDS sample, respectively in the 300e550 nm and 550e750 nm wavelength area.
Fig. 6. Decay curves of the prepared phosphate glasses. The Eu3þ singly doped sample is at 615 nm emission under 395 nm excitation (blue curve), the Mn2þ singly doped sample is at 615 nm emission under 415 nm excitation (red curve) and The Eu3þ-Mn2þcodoped sample is at 615 nm emission under 392 nm excitation (green curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
follows [16].
Z 1 IðtÞdt t ¼ I0
(1)
where I(t) is the luminescence intensity as a function of time t and I0 is the maximum of I(t) that occurs at the initial time t0. The energy shifting efficiency (ESE) hESE is calculated as follows
hEST ¼ 1 tx =t0
Fig. 4. Normalized absorption and emission spectra of the Mn2þ singly doped LDS sample in the 300e800 nm wavelength area.
(2)
where tx and t0 are defined as the lifetimes for the samples dopedfree and doped with RE ions, respectively. The lifetimes and ESEs of the prepared phosphate glasses are listed in Table 1. The transmission spectra of Eu3þ-Mn2þ-codoped phosphate glass sample (labeled with #2) with thickness of 3.5 mm, and the transmittance of RE-free glass (labeled with #1) are both shown for comparison, as shown in Fig. 7. Due to the high transmittance (>500 nm) of almost all VIS emission bands of Eu3þ-Mn2þ-codoped
P. Song et al. / Journal of Alloys and Compounds 661 (2016) 14e19
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cone can be expressed as follows [19].
Table 1 Measured LED, ESE & IQE for each sample. Dopant
LDT (ms)
ESE (%)
IQE (%)
RE-free Eu3þ Mn2þ Eu3þ-Mn2þ
35.6 12.1 10.5 4.7
e 66.1 70.5 86.8
e 69.3 74.8 89.7
Fig. 7. Transmittance spectra of the Eu3þ-Mn2þ-codoped glass in the 200e700 nm wavelength area (red curve). The transmittance of RE-free glass is shown for comparison (blue curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
glass samples, the VIS-red photon (>500 nm) transferred from the UV photon in solar radiation can be perfectly transmitted through the CdS layer, and ultimately harvested within the active CdTe cell layer. 3.2. Analytical theoretical model
. L ¼ 2n1 ,n2 ð1 cos qc Þ ðn1 þ n2 Þ2
(4)
where sin qc ¼ n1 =n2 . The n1 and n2 is the refractive index of the air and superstrate, respectively, when applied to the air-supertrate interface, while n1 and n2 is the refractive index of the superstrate and cell, respectively, when applied to the superstrateecell interface. Eq. (4) can calculate back reflection of light into the superstrate for angles less than the critical angle of total reflection. For a refractive index of the superstrate of n2 ¼ 1.56 and air of n1 ¼ 1.00, the critical angle of total reflection amounts to qc ¼ 39.8 . Approximate 85% of the incident photons can be coupled into the CdS/CdTe cell based on good optical coupling between superstrate and the CdS/CdTe cell. The output irradiance E(v) incident at the cell can be calculated from the emission spectrum of Eu3þ-Mn2þ ions when the Eu3þ-Mn2þ ions reemit photons at a reddish-shifted waveband. Ideally, the IQE of LDS process is 1.00, while it actually is average range of 0.75e0.88 based on the empirical values. Based on rudimentary treatment, we consider emission is isotropic including the directions of the emitted fluxes that means the half can go upwards and have the possibility of escaping the LDS glass, only the half that is emitted downwards can be directly collected without considering internal multiple-reflection. In this case, the IQE (g) of 0.8 is calculated, as well as 0.85 (near 0.75 from our measurements) of the emitted photons are directed towards the underlying cell owing to internal multiple-reflection with considering scattering and escape cone losses in the supertrate based on the Eqs. (3) and (4). The function of normalized incident irradiance, absorbed irradiance of Eu3þ-Mn2þ ions and emitted irradiance of Eu3þ-Mn2þ ions are labeled with EI(v), EII(v) and EIII(v), respectively, as shown in Figs. 8 and 5. The output irradiance E(v) incident at the CdS/CdTe cell, as shown in Fig. 8 (see orange curves), can be expressed as follows [12]:
We measured the photon flux contributing on the PV effects of the CdS/CdTe cell when doping the Eu3þ-Mn2þ in the superstrate for the LDS of solar radiation. We focus on relative increase of PCE of the CdS/CdTe cell with Eu3þ-Mn2þ-codoped superstrate based on the assumption that increase of PCE due to LDS belongs to relative wavelength positions of incident sunlight, and excitation/emission peaks of Eu3þ-Mn2þ-codoped superstrate. Here scattering and reflection ascribed to superstrates are common loss factors toward doped and doped-free superstrates [17,18]. The superstrate for the LDS process has 3.5 mm thickness, which is highly transparent and has a relatively low refractive index (n ¼ 1.56), which reduces front surface reflection across all wavelengths compared with front surface of the superstrate. In theoretical calculation, on one hand, we assumed reflectivities are approximately to be independent of wavelength that means the reflection of the superstrate is nearly constant over the whole wavelength area, and we use the Fresnel equation for normally incident light as follows
. R ¼ ðn1 n2 Þ2 ðn1 þ n2 Þ2
(3)
where R is reflectivity and n1 and n2 are refractive indices, of air (n1 ¼ 1), superstrate glass (n2 ¼ 1.56) and matching-glycerine (n ¼ 1.47). We would expect the front surface reflection to be reduced from approximately 0.035 for the superstrate glass. On the other hand, the LDS photons are emitted isotropically so that a part of the transferred photons do not reach the active layer of the cell, but leaves the cell in direction of the incident irradiance. The escape
Fig. 8. Calculated incident solar spectra. Normalized solar irradiance (orange curve) is compared with modified solar irradiance (orange curve) through superstrate glasses. Inset 1 shows corresponding experimental curves. Inset 2 shows solar radiation of which the energy percentages of the UV, VIS and IR bands are 7%, 50% and 43% [20], respectively, and shows the optimum SR of UVL and VIS-red bands. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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P. Song et al. / Journal of Alloys and Compounds 661 (2016) 14e19
EðvÞ ¼ EI ðvÞ þ EIII ðvÞ REI ðvÞ EII ðvÞ
(5)
EII ðvÞ ¼ aðvÞEI ðvÞ RaðvÞEI ðvÞ
(6)
EIII ðvÞ ¼ ΚbðvÞEII ðvÞ
(7)
where a(v) is the absorption coefficient, b(v) is the emission coefficient, and the K is the scaling-factor between absorption and emission. The reflectivity R is 0.48 based on the Eq. (3). The relative PCE (Dhr) of the CdS/CdTe cell is thus defined as the fraction of the hG of the CdS/CdTe cell with a RE-glass to the hB of the same bare cell. While the h of the CdS/CdTe cell with a RE-free glass was not considered due to they were nearly same to those of the case with the bare cell. The Dhr is given by as follows [12]:
Z SðvÞEðvÞdv Dhr ¼ Z
v
(8)
ð1 RÞSðvÞEI ðvÞdv
v
where SR(v) is the SR of the CdS/CdTe cell. We can evaluate the hr by theoretical calculation [14] over the concerned wavelengths from the spectral functions including the EI(v) of the solar simulator, EII(v), EIII(v) of Eu3þ/Mn2þ ions and the S(v) of the CdS/CdTe cell, as shown in Figs. 1, 5 and 8. 3.3. Theoretical modeling compared with measurement We used Atlas-SILVACO software to simulate the performance of CdS/CdTe cell, which the related parameters used in our simulation were chosen from literature [21,22]. The theoretically calculated performance is compared to the measured performance of the CdS/ CdTe cell in the Table 2, and the simulated and measured JeV characteristic curves of CdS/CdTe cells with the superstrate and superstrate-free is shown in Fig. 9. We evaluate the performance of the LDS superstrate by calculating the relative increase in the short circuit current density (Jsc) and PCE as follows
. GþC C C Jsc DJsc ¼ Jsc Jsc
(9)
. JC Dhr ¼ hGþC hC
(10)
GþC and J C are of the superstrate with the cell and the where the Jsc sc bare cell, respectively. The hGþC and hC are their respective PCEs.
Fig. 9. Measured and simulated JeV characteristic curves of CdS/CdTe cells with the glass and the glass-free. In JeV relations, the blue curve, red curve stand for the bare cell and the glass with the cell, respectively, under simulations, and the gray curve, blue curve stand for the bare cell and the glass with the cell, respectively, under measurements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
When the open circuit voltage (Voc) is slightly changed after the superstrate covered on the CdS/CdTe cell, while the Jsc is significantly improved. It is consistent with the general law that increased absorption of incident photons results in the improvement of the Jsc value, while the Voc and fill factor (FF) are not significantly enhanced [23]. On one hand, it shows that Eu3þ-Mn2þ-codoped phosphate glasses absorb solar UV photons and emit VS-red photons, which is matched with the SR area of the CdS/CdTe cell. On the other hand, incident photons harvesting is key for raising the PCE of the CdS/CdTe cell. Fig. 10 illustrates that the PV glass increases the l-VIS EQE of relative CdS/CdTe cell. It is shows that the PV glass has a positive impact for CdS/CdTe cell. Some studies reported relevant work on relative enhancement in performances of CdS/CdTe cells, such as Dhr of 1.32% in Tb3þ-Eu3þ borate glass [24], and DJsc of 4.3% in dye material [25], our experimental result gives maximum DJsc of 5.636% with Dhr of 7.140% for the CdS/CdTe cell, which shows better performance and can be expected as more promising candidates for application to CdS/CdTe cells. Some adverse factors affect the PCE of the CdS/CdTe cell equipped with the LDS glass: the QE of the LDS process, the
Table 2 LDS effect of Eu/Mn-glasses on CdS/CdTe cells compared between simulations and measurements. Type of values
JeV parameters
Glass-free case
RE-glass þ cell
Measurement
Voc (V) Pmax (mW cm2) FF (%) Jsc (mA cm2) DJsc h (%)
0.815 16.361 77.65 25.853 5.636 16.361 7.140 0.900 19.022 78. 148 27.046 6.966 19.022 8.948
0.814 17.530 78.860 27.310
Dhr Simulation
Voc (V) Pmax (mW cm2) FF (%) Jsc (mA cm2) DJsc h (%)
Dhr
17.530 0.902 20.724 79.418 28.930 20.724 Fig. 10. Comparison of EQE curves for CdS/CdTe before and after their PV glass.
P. Song et al. / Journal of Alloys and Compounds 661 (2016) 14e19
absorbance factor (GAB), and the light loss from reflection or scattering in interfaces. The GAB of the LDS glass is directly influenced by the transmittance related to the thickness and morphology. The Dhr and DJsc are enhanced with the increasing thickness of the LDS glass due to the efficient LDS process and high transmittance. However, as the thickness of the LDS glass increases, the light especially in the VIS area will be reflected or absorbed by the LDS glass. Therefore, when the LDS photons from the LDS glass will not compensate for the loss of photon absorption of sunlight, the Jsc will not be increased. During the process after incident and before absorption, scattering and escape cone losses weaken the strength of the incident photon firstly and the scattering and reflection occur on the surface or interface further weakens the effective harvest. If the absorption of the useful photon within the LDS glass is low, only a small amount of photons will be emitted by the LDS glass even with a high QE, and this will lead to a marginal improvement of the CdS/CdTe cell. 4. Conclusion The inclusion of Eu3þ-Mn2þ-codoped phosphate glasses as the bi-functional superstrate allow for a better use of solar energy. Experiments verified that the emission of the LSD glasses could vary from UV band to VIS band with emitting intense VIS-red photons under the sunlight radiation, which just matched the SR of CdS/CdTe cells, result in harvesting short wavelength photons and transferring them into the photocurrent output towards the PCE raise. The PCE measurements of glass-free case was 16.361% compared with the CdS/CdTe cell equipped with RE-glass, which was about 17.530% and an increase of 1.169% in PCE. Prospects of RE-doped glasses based on bi-functional superstrates for CdS/CdTe cells are foreseen in the development of new days. Acknowledgment This work was supported by Shanghai University Scientific
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Selection and Cultivation for Outstanding Young Teachers in Special Fund (No. ZZgcd14020).
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Eu3þ-Mn2þ-doped bi-functional glasses with solar photon downshifting: Application to CdS/CdTe solar cells Pei Song*, Chaomin Zhang, Pengfei Zhu School of Fundamental Studies, Shanghai University of Engineering Science, Shanghai 201620, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 November 2015 Accepted 22 November 2015 Available online 2 December 2015
Luminescent downshifting is a viable way for enhancing the short-wavelength spectral-response of photovoltaic device, especially for CdS/CdTe thin-film solar cells. Phosphate glasses codoped with rare earth Mn2þ-Eu3þ ions were prepared by high temperature melting method, and the absorption, excitation, and emission spectra of the glasses were investigated in detail. Under the illumination of sunlight (<520 nm excitation), the emission of the glasses can vary from ultraviolet to visible with emitting strong red emission, which just matched the spectral-response of CdS/CdTe cells, result in harvesting short wavelength photons and transferring them into the photocurrent output towards the photoelectric conversion efficiency improvement. In this work, such rare earth ions codoped phosphate glasses are investigated for their potential as bi-functional superstrate glass for CdS/CdTe solar cells and experimental performances of Luminescent downshifting layers fabricated with actual CdS/CdTe cells obtained. As a result, prepared Mn2þ-Eu3þ glasses could effectively improve the photoelectric conversion efficiency by 7.140% compared to pure CdS/CdTe solar cells. © 2015 Elsevier B.V. All rights reserved.
Keywords: Rare earth Phosphate Superstrate glass CdS/CdTe Downshifting Efficiency
1. Introduction Thin-film cadmium sulphide/cadmium telluride (CdS/CdTe) heterojunction solar cells have been studied extensively, and progress towards higher cell performance has not been rapid partly due to the parasitic absorption in the front cover glass and the transparent conducting oxide (TCO) front contact, but principally because of its poor spectral response (SR) in the short wavelength area (<520 nm) due to absorption or thermalization losses in the CdS buffer layer [1e3]. For the CdS/CdTe cell, the CdS layer has a band gap energy of 2.4 eV (~510 nm) results in strong absorption in ultraviolet (UV) and visible (VIS) blue photons, which ultimately lead to few utilization in UV light in CdS/CdTe cells and significantly decrease in external quantum efficiency (EQE) when going to the UV area, as shown in Fig. 1. Therefore, a large margin of improvement should be achieved towards harvest short wavelength photons and convert them into the photocurrent output. An alternative approach that does not require optimizing the electronic properties of the CdTe cell but still can be shown to enhance the poor SR in the short wavelength area, is the application of a luminescent
* Corresponding author. E-mail address: [email protected] (P. Song). http://dx.doi.org/10.1016/j.jallcom.2015.11.168 0925-8388/© 2015 Elsevier B.V. All rights reserved.
downshifting (LDS) layer [4]. LDS increases short wavelength SR by using luminescent materials which absorb photons at wavelengths where the cell performs poorly, and re-emit longer wavelength photon, to which the device SR more efficiently [5]. A drawback of transparent cover layer used to reduce convection and radiation losses is the reflection of the incident photon at its top surfaces. Therefore, anti-reflection (AR) layers for solar collector covers are widely used. However, in CdS/CdTe cells, the utilized solar photon is mainly in VIS light area, which occupies the largest part of solar spectrum, and UV photons are few to be utilized in CdS/CdTe cells. Antireflection [6] and LDS are two predominant solutions to solve the problem. On one hand, technology of antireflection is so mate that has little promotion space. On the other hand, the LDS layer exists the problem of low transmission [7]. To some extent, antireflection and LDS co-operation could make complementary advantages that may be an effective approach to increase the photoelectric conversion efficiency (PCE) of CdS/CdTe cells. Such so-called superstrate is covering layer on the sunny side of a photovoltaic (PV) cell, providing protection for the PV cells from impact and environmental degradation while allowing maximum transmission of the appropriate wavelengths of the solar spectrum [8]. In our studies, on one hand, compared with other kinds of glasses, the phosphate glasses have some merits of good thermal and chemical stability, high solubility of doping ions, large emission
P. Song et al. / Journal of Alloys and Compounds 661 (2016) 14e19
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Fig. 1. The normalized solar irradiance (red curve) and the normalized SR of the CdTe cell with the 300 nm CdS buffer layer (blue curve). The optimum wavelength of SR of the CdTe module with the 300 nm CdS buffer layer and the CdS buffer layer are around 855 nm (Eg ¼ 1.45 eV) and 516 nm (Eg ¼ 2.4 eV), respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
and absorption cross sections, and high transparency of VIS area [9,10]. On the other hand, the Eu3þ-Mn2þ-codoped system have intense and broad absorption in UV area and could convert it into VIS area with good LDS properties and high internal quantum efficiency (IQE) [11]. In this work, rare earth (RE) Eu3þ-Mn2þ-codoped phosphate glasses are investigated for their potential as LDS superstrate for CdS/CdTe cells. 2. Experimental procedures 2.1. Preparation and characterization measurement of superstrates Eu3þ ions and Mn2þ ions co-doped phosphate glasses with the chemical compositions of 55P2O5e12Li2Oe7Al2O3e10BaOe 4Na2Oe4ZnOe6MnO2e2Eu2O3 (mole percents) were prepared by melting-quenching technique [12]. Thoroughly mixed raw materials including NH4H2PO4, Li2CO3, Al2O3, BaCO3, NaCO3, ZnCO3, Eu2O3 and MnCO3 were fully sintered at 360 C for 480 min to reduce the surface moisture. Raw materials were then melted at 1355 C in high purity quartz crucibles for 210 min with stirrings, and the melts were cast onto a preheated copper mold. Then the glass was subsequently transferred to a temperature controlled annealing furnace kept at 435 C for 180 min in order to relieve thermal stresses. The annealed glasses were cut and polished in the form of rectangular plates (105 mm 105 mm 3.5 mm), as photos shown in Fig. 2. The absorption, photoluminescence (PL), photoluminescence excitation (PLE) and LDS emission spectra were measured on an Omni-l300 fluorescence spectrophotometer (Zolix Instruments, China) under the excitation of an LHX500 xenon arc lamp (Zolix Instruments, China). The VIS light was detected by a photomultiplier tube detector. 2.2. Preparation of the bare thin-film CdS/CdTe cell The CdS/CdTe cell with sides of 100 mm 100 mm were cut from a full-size and commercial module (Calyxo GmbH, Germany), which is prior to rear-side encapsulation, slightly smaller than the superstrate glass with sides of 105 mm 105 mm to ensure that the CdS/CdTe cell is completely covered. The bare CdS/CdTe cell refers to the combination of the real cell (the active material) and
Fig. 2. The photo above shows superstrate sample prepared in this work (samples from the left are Eu3þ-doped, Mn2þ-doped and Eu3þ-Mn2þ codoped one, respectively). The illustration below shows a cross-sectional architecture of the CdS/CdTe cell top covered with a superstrate.
transparent conductive oxide (TCO) as the top electric contact. 2.3. Architecture of superstrates fabricated with bare PV devices and effect measurement The superstrate is attached to the bare CdS/CdTe cell using a matching-glycerine (Nikon, MXA20233), which has a refractive index of 1.47 at 25 C similar to the glass and the CdS/CdTe cell to ensure good optical coupling [13], as shown in Fig. 2. The four edges of such sandwiched layers were encapsulated in commercially available non-formulated ethylene vinyl acetate (EVA) by using the vacuum-laminating process, which occurred at 150 C for a cycle of 3 min under vacuum followed by 5 min under atmospheric pressure [14]. Eu3þ ions and Mn2þ ions were respectively doped into phosphate glasses to form glassy luminescent superstrates with antireflection properties. PL properties can be found with LDS from UV range to a VIS-red range. The superstrate could be a prospective material having antireflection and LDS bi-function of sunlight modification for improving the PCE of the CdS/CdTe cell. By covering the superstrate onto a CdS/CdTe cell, as shown in Fig. 2, effects on PCE enhancement were measured. Current densityevoltage (JeV) curves were measured using a solar simulator (Newport, Class-AAA-94043A, 100 mW/cm2, Xenon-1000 W). 3. Results and discussion 3.1. Analytical experimental measurement The absorption and emission spectra of the Eu3þ ion singly doped phosphate glass is shown in Fig. 3. The Eu3þ ion related absorption peaks are attributed to transitions from the ground state level 7F0 to the excited states 5H4, 5D4, 5G2, 5L6, 5D2, and 5D1 in 319e532 nm wavelength range, as shown in Fig. 3. The position of absorption bands for the Eu3þ ion is relatively concentrated, thus we mainly discuss the absorption in near 400 nm area. Under 395 nm excitation, Eu3þ ions are excited from the ground state 7F0
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P. Song et al. / Journal of Alloys and Compounds 661 (2016) 14e19
Fig. 3. Normalized absorption and emission spectra of the Eu3þ singly doped LDS sample in the 200e700 nm wavelength area.
to a higher excited state 5D2 and the excited Eu3þ ions in 5D2 state can be relaxed to 5D1 and 5D0 states by phonon-assisted nonradiative transition. Because the energy differences between the each division level in the ground state is small, then Eu3þ ions radiatively relax to the ground states 5Fi (i ¼ 0e5), resulting in characteristic 560e700 nm VIS-red emission, in which the strongest emission band is around 615 nm [15]. Mn2þ ions have intense absorption peaks in the broad bands cover the range from 350 nm to 500 nm, with strong excited wavelengths of 410 nm. Under 350 nm, 370 nm, 410 nm, 456 nm and 468 nm excitation, which belong to 6A1(6S) / 4E(4D), 6 A1(6S) / 4T2(4D), 6A1(6S) / [4A1(4G), 4E1(4G)], 6A1(6S) / 4T2(4G) and 6A1(6S) / 4T1(4G) transitions, respectively, the emission of Mn2þ ion are monitored at 615 nm that belongs to 4T1(G) / 6A1(G) [15]. The absorption and emission spectra of the Mn2þ ion singly doped phosphate glass is shown in Fig. 4. The absorption and emission spectra of the Eu3þ-Mn2þ-codoped phosphate glass is shown in Fig. 5. Comparing the Eu3þ and Mn2þ ion singly-doped glass samples with the Eu3þ-Mn2þ-codoped glass sample, the absorption band of the 615 nm emission is broader and stronger, which are helpful to transfer the UV sunlight into the VISred radiation corresponding to the optimum SR of CdS/CdTe cells. The decay curves of the prepared phosphate glasses are shown in Fig. 6. The luminescence decay lifetime (LDT) are calculated as
Fig. 5. Normalized absorption and emission spectra of the Eu3þ-Mn2þ-codoped LDS sample, respectively in the 300e550 nm and 550e750 nm wavelength area.
Fig. 6. Decay curves of the prepared phosphate glasses. The Eu3þ singly doped sample is at 615 nm emission under 395 nm excitation (blue curve), the Mn2þ singly doped sample is at 615 nm emission under 415 nm excitation (red curve) and The Eu3þ-Mn2þcodoped sample is at 615 nm emission under 392 nm excitation (green curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
follows [16].
Z 1 IðtÞdt t ¼ I0
(1)
where I(t) is the luminescence intensity as a function of time t and I0 is the maximum of I(t) that occurs at the initial time t0. The energy shifting efficiency (ESE) hESE is calculated as follows
hEST ¼ 1 tx =t0
Fig. 4. Normalized absorption and emission spectra of the Mn2þ singly doped LDS sample in the 300e800 nm wavelength area.
(2)
where tx and t0 are defined as the lifetimes for the samples dopedfree and doped with RE ions, respectively. The lifetimes and ESEs of the prepared phosphate glasses are listed in Table 1. The transmission spectra of Eu3þ-Mn2þ-codoped phosphate glass sample (labeled with #2) with thickness of 3.5 mm, and the transmittance of RE-free glass (labeled with #1) are both shown for comparison, as shown in Fig. 7. Due to the high transmittance (>500 nm) of almost all VIS emission bands of Eu3þ-Mn2þ-codoped
P. Song et al. / Journal of Alloys and Compounds 661 (2016) 14e19
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cone can be expressed as follows [19].
Table 1 Measured LED, ESE & IQE for each sample. Dopant
LDT (ms)
ESE (%)
IQE (%)
RE-free Eu3þ Mn2þ Eu3þ-Mn2þ
35.6 12.1 10.5 4.7
e 66.1 70.5 86.8
e 69.3 74.8 89.7
Fig. 7. Transmittance spectra of the Eu3þ-Mn2þ-codoped glass in the 200e700 nm wavelength area (red curve). The transmittance of RE-free glass is shown for comparison (blue curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
glass samples, the VIS-red photon (>500 nm) transferred from the UV photon in solar radiation can be perfectly transmitted through the CdS layer, and ultimately harvested within the active CdTe cell layer. 3.2. Analytical theoretical model
. L ¼ 2n1 ,n2 ð1 cos qc Þ ðn1 þ n2 Þ2
(4)
where sin qc ¼ n1 =n2 . The n1 and n2 is the refractive index of the air and superstrate, respectively, when applied to the air-supertrate interface, while n1 and n2 is the refractive index of the superstrate and cell, respectively, when applied to the superstrateecell interface. Eq. (4) can calculate back reflection of light into the superstrate for angles less than the critical angle of total reflection. For a refractive index of the superstrate of n2 ¼ 1.56 and air of n1 ¼ 1.00, the critical angle of total reflection amounts to qc ¼ 39.8 . Approximate 85% of the incident photons can be coupled into the CdS/CdTe cell based on good optical coupling between superstrate and the CdS/CdTe cell. The output irradiance E(v) incident at the cell can be calculated from the emission spectrum of Eu3þ-Mn2þ ions when the Eu3þ-Mn2þ ions reemit photons at a reddish-shifted waveband. Ideally, the IQE of LDS process is 1.00, while it actually is average range of 0.75e0.88 based on the empirical values. Based on rudimentary treatment, we consider emission is isotropic including the directions of the emitted fluxes that means the half can go upwards and have the possibility of escaping the LDS glass, only the half that is emitted downwards can be directly collected without considering internal multiple-reflection. In this case, the IQE (g) of 0.8 is calculated, as well as 0.85 (near 0.75 from our measurements) of the emitted photons are directed towards the underlying cell owing to internal multiple-reflection with considering scattering and escape cone losses in the supertrate based on the Eqs. (3) and (4). The function of normalized incident irradiance, absorbed irradiance of Eu3þ-Mn2þ ions and emitted irradiance of Eu3þ-Mn2þ ions are labeled with EI(v), EII(v) and EIII(v), respectively, as shown in Figs. 8 and 5. The output irradiance E(v) incident at the CdS/CdTe cell, as shown in Fig. 8 (see orange curves), can be expressed as follows [12]:
We measured the photon flux contributing on the PV effects of the CdS/CdTe cell when doping the Eu3þ-Mn2þ in the superstrate for the LDS of solar radiation. We focus on relative increase of PCE of the CdS/CdTe cell with Eu3þ-Mn2þ-codoped superstrate based on the assumption that increase of PCE due to LDS belongs to relative wavelength positions of incident sunlight, and excitation/emission peaks of Eu3þ-Mn2þ-codoped superstrate. Here scattering and reflection ascribed to superstrates are common loss factors toward doped and doped-free superstrates [17,18]. The superstrate for the LDS process has 3.5 mm thickness, which is highly transparent and has a relatively low refractive index (n ¼ 1.56), which reduces front surface reflection across all wavelengths compared with front surface of the superstrate. In theoretical calculation, on one hand, we assumed reflectivities are approximately to be independent of wavelength that means the reflection of the superstrate is nearly constant over the whole wavelength area, and we use the Fresnel equation for normally incident light as follows
. R ¼ ðn1 n2 Þ2 ðn1 þ n2 Þ2
(3)
where R is reflectivity and n1 and n2 are refractive indices, of air (n1 ¼ 1), superstrate glass (n2 ¼ 1.56) and matching-glycerine (n ¼ 1.47). We would expect the front surface reflection to be reduced from approximately 0.035 for the superstrate glass. On the other hand, the LDS photons are emitted isotropically so that a part of the transferred photons do not reach the active layer of the cell, but leaves the cell in direction of the incident irradiance. The escape
Fig. 8. Calculated incident solar spectra. Normalized solar irradiance (orange curve) is compared with modified solar irradiance (orange curve) through superstrate glasses. Inset 1 shows corresponding experimental curves. Inset 2 shows solar radiation of which the energy percentages of the UV, VIS and IR bands are 7%, 50% and 43% [20], respectively, and shows the optimum SR of UVL and VIS-red bands. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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P. Song et al. / Journal of Alloys and Compounds 661 (2016) 14e19
EðvÞ ¼ EI ðvÞ þ EIII ðvÞ REI ðvÞ EII ðvÞ
(5)
EII ðvÞ ¼ aðvÞEI ðvÞ RaðvÞEI ðvÞ
(6)
EIII ðvÞ ¼ ΚbðvÞEII ðvÞ
(7)
where a(v) is the absorption coefficient, b(v) is the emission coefficient, and the K is the scaling-factor between absorption and emission. The reflectivity R is 0.48 based on the Eq. (3). The relative PCE (Dhr) of the CdS/CdTe cell is thus defined as the fraction of the hG of the CdS/CdTe cell with a RE-glass to the hB of the same bare cell. While the h of the CdS/CdTe cell with a RE-free glass was not considered due to they were nearly same to those of the case with the bare cell. The Dhr is given by as follows [12]:
Z SðvÞEðvÞdv Dhr ¼ Z
v
(8)
ð1 RÞSðvÞEI ðvÞdv
v
where SR(v) is the SR of the CdS/CdTe cell. We can evaluate the hr by theoretical calculation [14] over the concerned wavelengths from the spectral functions including the EI(v) of the solar simulator, EII(v), EIII(v) of Eu3þ/Mn2þ ions and the S(v) of the CdS/CdTe cell, as shown in Figs. 1, 5 and 8. 3.3. Theoretical modeling compared with measurement We used Atlas-SILVACO software to simulate the performance of CdS/CdTe cell, which the related parameters used in our simulation were chosen from literature [21,22]. The theoretically calculated performance is compared to the measured performance of the CdS/ CdTe cell in the Table 2, and the simulated and measured JeV characteristic curves of CdS/CdTe cells with the superstrate and superstrate-free is shown in Fig. 9. We evaluate the performance of the LDS superstrate by calculating the relative increase in the short circuit current density (Jsc) and PCE as follows
. GþC C C Jsc DJsc ¼ Jsc Jsc
(9)
. JC Dhr ¼ hGþC hC
(10)
GþC and J C are of the superstrate with the cell and the where the Jsc sc bare cell, respectively. The hGþC and hC are their respective PCEs.
Fig. 9. Measured and simulated JeV characteristic curves of CdS/CdTe cells with the glass and the glass-free. In JeV relations, the blue curve, red curve stand for the bare cell and the glass with the cell, respectively, under simulations, and the gray curve, blue curve stand for the bare cell and the glass with the cell, respectively, under measurements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
When the open circuit voltage (Voc) is slightly changed after the superstrate covered on the CdS/CdTe cell, while the Jsc is significantly improved. It is consistent with the general law that increased absorption of incident photons results in the improvement of the Jsc value, while the Voc and fill factor (FF) are not significantly enhanced [23]. On one hand, it shows that Eu3þ-Mn2þ-codoped phosphate glasses absorb solar UV photons and emit VS-red photons, which is matched with the SR area of the CdS/CdTe cell. On the other hand, incident photons harvesting is key for raising the PCE of the CdS/CdTe cell. Fig. 10 illustrates that the PV glass increases the l-VIS EQE of relative CdS/CdTe cell. It is shows that the PV glass has a positive impact for CdS/CdTe cell. Some studies reported relevant work on relative enhancement in performances of CdS/CdTe cells, such as Dhr of 1.32% in Tb3þ-Eu3þ borate glass [24], and DJsc of 4.3% in dye material [25], our experimental result gives maximum DJsc of 5.636% with Dhr of 7.140% for the CdS/CdTe cell, which shows better performance and can be expected as more promising candidates for application to CdS/CdTe cells. Some adverse factors affect the PCE of the CdS/CdTe cell equipped with the LDS glass: the QE of the LDS process, the
Table 2 LDS effect of Eu/Mn-glasses on CdS/CdTe cells compared between simulations and measurements. Type of values
JeV parameters
Glass-free case
RE-glass þ cell
Measurement
Voc (V) Pmax (mW cm2) FF (%) Jsc (mA cm2) DJsc h (%)
0.815 16.361 77.65 25.853 5.636 16.361 7.140 0.900 19.022 78. 148 27.046 6.966 19.022 8.948
0.814 17.530 78.860 27.310
Dhr Simulation
Voc (V) Pmax (mW cm2) FF (%) Jsc (mA cm2) DJsc h (%)
Dhr
17.530 0.902 20.724 79.418 28.930 20.724 Fig. 10. Comparison of EQE curves for CdS/CdTe before and after their PV glass.
P. Song et al. / Journal of Alloys and Compounds 661 (2016) 14e19
absorbance factor (GAB), and the light loss from reflection or scattering in interfaces. The GAB of the LDS glass is directly influenced by the transmittance related to the thickness and morphology. The Dhr and DJsc are enhanced with the increasing thickness of the LDS glass due to the efficient LDS process and high transmittance. However, as the thickness of the LDS glass increases, the light especially in the VIS area will be reflected or absorbed by the LDS glass. Therefore, when the LDS photons from the LDS glass will not compensate for the loss of photon absorption of sunlight, the Jsc will not be increased. During the process after incident and before absorption, scattering and escape cone losses weaken the strength of the incident photon firstly and the scattering and reflection occur on the surface or interface further weakens the effective harvest. If the absorption of the useful photon within the LDS glass is low, only a small amount of photons will be emitted by the LDS glass even with a high QE, and this will lead to a marginal improvement of the CdS/CdTe cell. 4. Conclusion The inclusion of Eu3þ-Mn2þ-codoped phosphate glasses as the bi-functional superstrate allow for a better use of solar energy. Experiments verified that the emission of the LSD glasses could vary from UV band to VIS band with emitting intense VIS-red photons under the sunlight radiation, which just matched the SR of CdS/CdTe cells, result in harvesting short wavelength photons and transferring them into the photocurrent output towards the PCE raise. The PCE measurements of glass-free case was 16.361% compared with the CdS/CdTe cell equipped with RE-glass, which was about 17.530% and an increase of 1.169% in PCE. Prospects of RE-doped glasses based on bi-functional superstrates for CdS/CdTe cells are foreseen in the development of new days. Acknowledgment This work was supported by Shanghai University Scientific
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Selection and Cultivation for Outstanding Young Teachers in Special Fund (No. ZZgcd14020).
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