Optimizing the thickness of SnO2, CdS and antireflection layers on CdTe
solar cells in terms of optical loss
Abstract
In this paper thickness of CdS, SnO2 and antireflection (AR) layers are optimized for
CdTe/CdS/SnO2/Glass solar cells by spotting both losses due to reflection and photon
absorbed at the layers. Since a portion of photons absorbed in some materials which are
used as an AR coating (ARC) does not contribute to the current production, it is suggested
that these photons absorption losses are accompanied by solar weighted reflection (SWR)
account for total AR losses. Therefore, it is proposed to use solar weighted reflection and
absorption (SWRA) as a strict criterion for assessment of the ARC performance.
Simulation results show that the SnO2 layer has local minimums of SWRA, which occur at
the thicknesses close to 100, 300, 520, 780 and 1000 nm. Furthermore, Model calculations
show that reducing the CdS thickness from 116 to 50 nm decreases SWRA loss
significantly due to reduced reflectance as well as absorption. Then, by considering the
range of radiant energy absorbed to the cell, single layer of MgF2 with 101 nm thickness as
an AR on glass is designed. Comparison with a previous method in which the aim was
reducing reflection at wavelength that contains the most radiant energy, the results prove
not only the losses are reduced but also less thickness of MgF2 is needed. Furthermore, the
current loss was reduced to 0.34 mA/cm2 by using double layer ARCs containing MgF2
and Al2O3 with thickness of 96 and 149 nm, respectively, in a structure of
Air/MgF2/Al2O3/Glass.
Key words: CdTe/CdS; Solar cell loss; SWR; SWRA; Antireflection
1. Introduction
Solar energy is one of the rapidly growing industries in the energy sector. World
photovoltaic (PV) industry has an average growth rate of 49.5% over the past years, from
2004 to 2009 [1] and worldwide PV market installations reached a record high of 18.2 GW
in 2010, representing growth of 139% Y/Y (GW) in 2010 [2] showing a significant
intention to this mass marketing.
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Thin-film PV technologies for flat-plate modules are attractive due to the fact that they
consume much lower amounts of expensive semiconductors and have the great advantage
of being much easier to product in comparison to crystalline silicon solar cells, giving a
very high throughput [3]. So they are more amenable to much higher levels of production
automation than wafered silicon. However, Thin-film solar cell to become economically
comparable with fossil fuels, this requires optimization of a different set of parameters.
The CdTe bandgap of about 1.45 eV is very close to the theoretical maximum conversion
efficiency (31%), while a short circuit current and open circuit voltage is maximized [4].
These considerable advantages have been proven by the impressive increase of CdTe PV
modules production from a few MW in 2003 up to over 1 GW in 2008 [4], with constantly
decreasing production costs. First Solar has emerged as the largest solar cell manufacturer
in the world with an annual capacity of 1.4 GW in 2010, which is expected to expand more
than 2 GW in 2011 [5]. First Solar's manufacturing costs were 0.77$/W and module
efficiencies were 11.3%, according to their third quarter report for 2010 [6].
The front contact, which is the first layer deposited directly on the glass substrate,
consisting of a transparent conducting oxide (TCO), typically tin oxide doped with fluorine
(FTO) ; the CdS (also known as a buffer layer), which is the n-type semiconductor of the
junction and has to be optically transparent due to permit the absorber to convert most of
the light spectrum; the CdTe, which is the p-type semiconductor of the junction and also
has the function of producing electron-hole pairs.
Materials used for the manufacture of CdTe solar cells result in more than 10% of incident
sunlight is lost by reflection without ARC. Figure 1 shows a measured reflectance for
CdTe/CdS solar cell in which SWR is equal to 13.14%. In this paper at the first section, it
is proposed to use SWRA as a strict criterion for assessment of the ARCs performance or
films such as CdS or TCO in terms of optical loss. Then, by considering the criterion,
optimum thickness for CdS and SnO2 has been simulated. In section 4, by considering the
range of radiant energy absorbed to the cell, single layer of MgF2 as an anti-reflection on a
glass is designed. After that it is compared with previous work, in which the aim was
reducing reflection at a wavelength that contains the most radiant energy. Furthermore, in
this section effect of double layer ARCs is simulated. We used AM1.5G solar spectra for
all the simulations [7].
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2. Theoretical reflection and absorption losses
For terrestrial based CdTe solar cells, the concept of a solar weighted reflectance (SWR) is
useful. This accounts for the spectral distribution of solar energy where photons with
energy less than the band gap of CdTe (1.45 eV ~855 nm) do not contribute to the photo
generated current and photons with energy greater than 1.45 eV only contribute 1.45 eV of
energy. Therefore, the SWR gives a more suitable indication of the available energy for
conversion into a photocurrent and is given by [8]
𝑅𝜆 𝑆𝜆 𝜆⁄ℎ𝑐
𝜒
𝑆𝑊𝑅(𝜆) =
(1)
Where Sλ and Rλ are the energy distribution and reflectance, respectively, as a function of
wavelength and χ is a normalizing factor equal to the maximum value of 𝑆𝜆 𝜆⁄ℎ𝑐 . The
total SWR is also given by [8]
𝜆
𝑆𝑊𝑅 =
2
∫𝜆 𝑅𝜆 𝑆𝜆 𝜆𝑑𝜆
1
(2)
𝜆
2
∫𝜆1 𝑆𝜆 𝜆𝑑𝜆
Where λ1 = 295 nm, below which negligible solar radiation is incident on the earth and λ2
is the band gap of CdTe.
Since a portion of photons with high energy absorbed in some material such as tin dioxide,
which be used as a TCO for CdTe solar cells, does not contribute to the current production,
It is suggested that these photon absorption losses are accompanied by SWR, account for
total losses. Therefore, it is proposed to use SWRA as a strict criterion for assessment of
the ARC performance or layers such as CdS or TCO in terms of optical losses for solar
cells. Creating a stack of thin film layers L1…Lm between two infinite layers of glass L0
and CdTe Lm+1; SWRA is given by
1
𝐵
[ ] = 𝑀1 𝑀2 … 𝑀𝑚 [
]
𝑁𝑚+1
𝐶
(3)
4𝑁0 𝑅𝑒(𝐵𝐶 ∗ − 𝑁𝑚+1 )
𝐴𝜆 = 1 − 𝑅𝜆 − 𝑇𝜆 =
(𝑁0 𝐵 + 𝐶)(𝑁0 𝐵 + 𝐶)∗
(4)
3
𝑆𝑊𝑅𝐴(𝜆) =
(𝑅𝜆 +𝐴𝜆 )𝑆𝜆 𝜆⁄ℎ𝑐
𝜒
(5)
Where Mj is the thin film characteristic matrix, N0 and Nm+1 are the effective complex
refractive index of the glass and CdTe respectively, Tλ is transmittance and absorptance is
Aλ [9]. Therefore, the total SWRA is also given by
𝜆
𝑆𝑊𝑅𝐴 =
2
∫𝜆 (𝑅𝜆 + 𝐴𝜆 )𝑆𝜆 𝜆𝑑𝜆
1
(6)
𝜆
2
∫𝜆1 𝑆𝜆 𝜆𝑑𝜆
Furthermore, the loss that is caused by the optical absorption in thin film layers which is
dubbed solar weighted absorptance (SWA), similarly is given by
𝐴𝜆 𝑆𝜆 𝜆⁄ℎ𝑐
𝜒
𝑆𝑊𝐴(𝜆) =
(7)
Therefore the total SWA is also given by
𝜆
𝑆𝑊𝐴 =
2
∫𝜆 𝐴𝜆 𝑆𝜆 𝜆𝑑𝜆
1
(8)
𝜆
2
∫𝜆1 𝑆𝜆 𝜆𝑑𝜆
3. Optimizing the thickness of CdS and SnO2 in terms of optical loss
Table 1 shows refractive indices that are used for simulation [10]. At short wavelengths the
extinction coefficient for both of SnO2 and CdS as shown in Fig. 3, is high and result in
absorption of photons with high energy. Therefore, in terms of optical losses not only
decreasing the SWR should be considered but also minimizing the SWA at the layers
ought to be regarded in which both of loss are entirely satisfied by the SWRA.
For these particular simulations, we are assuming a CdS/SnO2 stack (Fig. 3) where the
solar energy is incident normal to the surface. Reflection from the external environment
(the air/glass) is not considered in simulations. The air mass 1.5 global standard terrestrial
solar spectrum (AM1.5G) is used for simulation; in which the maximum J sc of a CdTe
solar cell could be about 32.1mA/cm2. First, the general trends in the amount of SWR,
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SWA and SWRA by changing thickness of the layers are shown in Fig. 4, Fig. 5 and Fig. 6
respectively. The minimum reflection shown in Fig. 4 is 0.64%, which occurs with 47 nm
of CdS and 69 nm of SnO2. The maximum reflectance is 9%, which occurs without the
layers. The other hand, SWA value increases with increasing thickness and due to that
extinction coefficient of CdS is much more than SnO2, SWR rises more rapidly with
increasing the CdS thickness than the SnO2. At the beginning, as is observed more clearly
in Fig. 8, SWA distortion is due to the fact that the reflection is high and thus the light
entering into the layer is reduced and consequently the absorption is decreased. Fig. 6
shows the total losses of solar irradiance, SWRA, comprising reflectance as well as
absorptance.
To ensure uniformity and prevent the formation of pinholes, the CdS should be sufficiently
thick (>60 nm) [11]; and the SnO2 must be sufficiently thickness (>80 nm) [11] to cease
power loss associated with series resistance, which can result in low fill factor or cell
performance. Therefore, with regard to current situation, SWR, SWA and SWRA losses
are shown in Fig. 7, Fig. 8 and Fig. 9 respectively. The minimum reflectance shown in Fig.
7 occurs in about 50 nm thickness of the CdS and increases by raising the CdS thickness
up to 115 nm. Further increase in the thickness of CdS film results in a local minimum at
190 nm. Moreover, increase in the thickness of SnO2 results in a local minimum which
occur at 100, 300, 520, 780 and 1000 nm. Fig.8 shows SWA loss which increases more
quickly by raising CdS thickness than SnO2 one. Simulation Results for both losses in Fig.
9 shows that less CdS is preferred despite the local minimum of SWR at 115 and 190 nm,
due to the high absorption coefficient of CdS.
Simulations were carried out by fixing the SnO2 thickness at 520 and 1000 nm and varying
the CdS thickness in the range of 0 to 400 nm. Figure 10 shows that reducing the CdS
thickness from 116 to 50 nm, reduces SWRA loss due to reduced reflectance and the
absorption; it is noticeable that the range is the same for both mentioned thickness of the
SnO2. However, there is a slight difference in their loss amplitude. It is clear from Fig. 10
that the SWR due to reflectance is more sensitive to the CdS film when the thickness falls
below 116 nm. Nevertheless, further decrease in the CdS thickness below 50 nm
introduces an extra photons loss due to increased reflectance. Moreover, reducing the CdS
thickness below 50 nm in an actual cell may be risky due to the possibility of pinholes in
very thin CdS films, which could lead to low shunt resistance and degrade Voc and fill
factor.
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By using SCAPS (Version 2.8.06), simulations were carried out for different thickness of
CdS and SnO2, and the results were shown in Table 2. These simulations predict that the
Jsc of a CdS/CdTe cell should increase by about 2mA/cm2 and improve efficiency up to
1%.
4. Minimization of the Front Glass Surface Reflectance
Even though the thicknesses of the SnO2 and CdS layers were optimized to decrease the
reflectance, there is still about 4% of incident light that is reflected from the air/glass
interface (n = 1.52 for the glass). This reflectance corresponds to about 1.28 mA/cm2
current loss in the CdTe solar cells under AM 1.5G illuminations. Therefore, appropriate
ARC could decrease the loss. At previous work that has been done, ARC has been
designed for a specific wavelength that corresponds to a maximum incident [11]. However
accurate method is that by considering all photons' energy absorbed by the CdTe solar
cells, the ARC should be designed. Figure 11 shows the reflectance by using 101 nm of
MgF2 as an ARC. Table 3 compares these two methods. In the second method not only
the reflectance loss decreased but also 7 nm less thickness of the MgF2 obtained.
Therefore, the proposed method results in less loss and improved efficiency, even with less
thickness of the ARC.
Fig. 12 shows the reflection by using double layer ARCs containing MgF2 and Al2O3 with
thickness of 96 and 149 nm, respectively, in the structure of Air/MgF2/Al2O3/Glass.
Compared to the single layer ARC, the current loss is reduced to 0.34 mA/cm2, which is a
0.2 mA/cm2 less loss.
5. Conclusion
It was proposed to use SWRA as a strict criterion for assessment of the ARCs performance
for solar cells. Then by considering parameters, including SWRA, the thickness of CdS
and SnO2 were optimized, which could improve the efficiency up to 1%. After that by
considering all photons' energy absorbed by the CdTe solar cells, single layer of MgF2 as
an ARC was designed on the glass which concurrently decreased not only reflection loss
but also required less thickness in comparing with the previous method. At the end, double
layer ARCs containing MgF2 and Al2O3 with thickness of 96 and 149 nm, respectively,
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were designed and in compared with the single layer, resulting in only a 0.2 mA/cm2 less
loss.
Acknowledgements
We would like to thank the research group of Doctor M.Burgelman of ELIS for providing
SCAPS.
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