This document discusses the impact of defects and ambient temperature on the performance of Heterojunction with Intrinsic Thin layer (HIT) solar cells. It describes the structure of a HIT solar cell and simulates its performance using AFORS-HET software. The effects of varying ambient temperature and defect densities in different layers are studied. Increasing the temperature decreases the efficiency while higher defect densities significantly degrade performance for densities over 1013 cm-3. A maximum efficiency of 24.28% is achieved for the optimal structure and conditions.

1. International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS)
Volume VI, Issue IV, April 2017 | ISSN 2278-2540
www.ijltemas.in Page 99
Impact of Defects and Ambient Temperature on the
Performance of HIT Solar Cell
Ambar Khanda1
, Malay Saha2
and Tapas Chakrabarti3
ECE Department, Heritage Institute of Technology, Kolkata, India
Abstract: Heterojunction with intrinsic thin layer or “HIT” solar
cells are considered favorable for large-scale manufacturing of
solar modules, as they combine the high efficiency of crystalline
silicon c-Si solar cells, with the low cost of amorphous silicon
technology. This article is based on the ambient temperature
and the defects density in the Hetero-junction with Intrinsic Thin
layers solar cells (HIT) strongly influences their performances.
In this paper the structure: ITO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-
Si:H(n)/ITO is presented where we study the effect of the
ambient temperature and the defects density in the gap of the
crystalline Silicon layer and amorphous Silicon intrinsic layer on
the performance of the heterojunction solar cell with intrinsic
layer (HIT). The structure is simulated in AFORS-HET
simulation software environment.
Keywords: Defect, Temperature, HIT, AFORS-HET
I. INTRODUCTION
n today's fast growing world, solar energy has become one
of the most focused sources of obtaining „green‟ energy in
last few decades.
The hetero-junction solar cells (HJ) are obtained by joining
two materials with different energy gaps (Eg). Hetero-junction
was first studied in 1974 by Fuhs [1] and in 1983 the first
heterojunction solar cell was fabricated [2-3]. Heterojunction
with intrinsic thin layer or “HIT” solar cells is combined of
the high stable efficiency of crystalline silicon (c-Si) cells
with the low temperature deposition technology of
hydrogenated amorphous silicon (a-Si:H).The resulting cells
can achieve high conversion efficiencies, while using the thin
film silicon reduces the cost of the HIT cell compared to the
c-Si solar cells [4]. In the year 1994, the first HIT solar cell
was developed by „SANYO‟ Ltd [5]. In November 2014, the
Panasonic Corporation (Sanyo) announced a record efficiency
of 25.6% at research level using HIT solar cell.
In this paper the structure: ITO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-
Si:H(n)/ITO is presented and a study on the effects of the
ambient temperature and the defects densities in the gap of the
crystalline Silicon layer on the performance of the HIT solar
cell is performed in AFORS-HET simulation software
environment.
AFORS-HET (Automat FOR Simulation of Hetero-
structures) software has been developed by a group from the
Hahn-Meitner Institute of Berlin and is used for Simulating
the hetero-junction solar cells [6],[7]. The software provides a
convenient way to evaluate the role of the various parameters
(thickness, doping concentration, band gap etc.) present in the
fabrication process of HIT solar cells.
II. STRUCTURE OF THE HIT SOLAR CELL
The HIT solar cell structure is ITO/a-Si:H(p)/a-Si:H(i)/c-
Si(n)/a-Si:H(n)/ITO and the structure is shown in Fig1. In this
HIT solar cell structure, a-Si(p), a-Si(i), c-Si(n), a-Si(n) layers
are used as emitter, buffer, absorber and BSF layers
respectively[5,8]. A study of the performance evolution,
basedon the defects density parameters and ambient
temperature is performed for this structure.
ITO(contact)
a-Si:H(p)(10nm)
a-Si:H(i)(7nm)
c-Si(n)(300um)
a-Si(n)(10nm)
ITO(contact)
Fig1: Schematic Structure of the HIT solar cell
In this structure the thickness of the a-Si(p), a-Si(i), c-Si(n)
and a-Si(n) layers are taken as 10nm, 7nm, 300um and 10 nm
respectively.
Many other standard parameters are taken into consideration
in the present simulation and their values are reported in table
1.
Table1: Parameter values of different layers[9] [10] [11]
[12]
I

2. International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS)
Volume VI, Issue IV, April 2017 | ISSN 2278-2540
www.ijltemas.in Page 100
Properties a-Si (p) a-Si (n) a-Si(i) c-Si (n)
Dielectric constant 11.9 11.9 11.9 11.9
Electron affinity 3.9 3.9 3.9 4.05
Band gap 1.74 1.74 1.72 1.12
Effective
conduction band
density
1E20 1E20 1E20 2.8E19
Effective valance
band density
1E20 1E20 1E20 1.04E19
Electron mobility 20 20 20 1040
Hole mobility 5 5 5 412
Doping
concentration of
acceptors
1E20 0 0 0
Doping
concentration of
donors
0 1E20 0 1E16
III. RESULTS AND DISCUSSION
The structure has been developed in the AFORS-HET
simulation software environment. The open circuit voltage
(Voc), short circuit current (Isc), fill factor (FF) and efficiency
(Eff) has been achieved 726.4mV, 40.14mA/cm2
, 83.26% and
24.28% respectively for this structure. The output J-V curve is
shown in figure 2.
Fig 2: J-V curve of the proposed structure
A. Effect of temperature on the I-V characteristics
The definition of the temperature coefficient for a parameter
relates to the change in that parameter when only temperature
is varied, other factors that might influence the parameter
being held constant. Temperature coefficients for Isc, Ipp,
Voc, Vpp, Pmax, FF, and η can all be determined for given
photovoltaic modules. Regression analysis is used to
determine temperature coefficient parameters for Isc, Ipp,
Voc, Vpp, Pmax and FF[13]. The energy conversion
efficiency η of modules is defined by:
η = Pout/Pin = Ipp*Vpp/Pin = FF*Voc*Isc/Pin
Here Pin is the total radiative input power of all light incident
on the cell/module, and Pout is the electrical power output of
the cell/module. The fill factor, FF is defined by:
FF = (Ipp*Vpp/Isc*Voc)*100%
The fill factor measures how square the I-V curve. The higher
the FF the more power the cell produces. The relation between
short-circuit current and open- circuit voltage is given by.
Isc = I0 (eq*Voc/AkT-1) and,
Voc= (AkT/q) ln (Isc/I0 + 1)
Where Isc= short circuit current (the current at V = 0. Ideally
this is equal to the light generated current (IL). Voc= open
circuit voltage (the voltage at I = 0, Voc depends strongly on
the properties of the semiconductor by virtue of its
dependence on dark current I0. K = Boltzmann constant, T =
temperature of cell, q = electronic charge, A = diode quality
factor of p-n junction.
Fig 3: I-V characteristics of solar cell under different temperature
B. Sensitivity of the solar cell output to the defect densities of
each layer :
Figure 4 shows the distributions of the gap state densities of c-
Si layer in our solar cell. The crystalline silicon n-type is
selected with a doping of 1016
cm-3 and a thickness of 300
μm. We define the defect density in crystalline silicon is
chosen as single defect at 0.56eV with a concentration of
1x10^10 cm-3.
Fig 4: The gap state distribution in c-Si layer

3. International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS)
Volume VI, Issue IV, April 2017 | ISSN 2278-2540
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Now define the parameters for amorphous silicon
hydrogenated thin film. The gap of this material may vary
between 1.55eV and 2.10eV, but the standard value is 1.74eV
at 300K. Electron mobility was taken 20cm 2
V-1
s-1
and that for
holes 5cm 2
V-1
s-1
. For amorphous layers, the density of states
has been assumed to be both acceptor like states (in the upper
half of the gap) and donor like states (in the lower half of the
gap). Both of these acceptor and donor like states consist of
exponential band tail and Gaussian mid-gap states.
Fig5: The gap state distribution in a-Si (p) layer
Fig6: The gap state distribution in a-Si (n) layer
Fig7: The gap state distribution in a-Si (i) layer
From this work, it is observed that the solar cell performance
is degraded after adding these defects to the layers of the
cell. The I-V curve is shown in figure 8 and the Voc, FF and
Eff is acheived724.6mV, 80.08%, 23.05% respectively.
Fig 8: After adding the defects the I-V curve
C. Sensitivity of the solar cell output to vary the defects on c-
Si layer :
Fig 9: I-V curve with varying the c-Si defect State
Fig.9 shows how the cell performance is strongly depends on
the density of defects inthe gap of the absorber. The defect
density is varied from 1010
to 1018
cm-3
[14].
Table 2: Parameters value with varying the c-Si defects
Defects[cm-3
] Eff[%]
Isc[mA/cm2
]
Voc[mV] FF[%]
10^10 22.99 39.2 725.7 80.81
10^11 22.97 39.1 725.5 80.8
10^12 22.46 38.58 721.3 80.71
10^13 19.74 35.77 702 78.6
10^14 16.01 30.43 669.4 78.57
10^15 11.51 24.47 627.3 74.98
10^16 7.512 20.12 559.9 66.68
10^17 3.983 16.02 451.9 55.01
10^18 0.8194 11.75 186.9 37.31

4. International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS)
Volume VI, Issue IV, April 2017 | ISSN 2278-2540
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Fig10: Efficiency vs Defects
Fig11: Fill Factor vs Defects
Fig12: Current density vs Defects
Fig13: Voltage vs Defects
From the above graphs , it is observed that how the
Efficiency, Open circuit voltage , Short circuit current density
and Fill Factor is impacted with the increasing of defect state
densities. From these graphs, it is observed that the solar cell
performance is marginally affected when the defect density is
below 1013
cm-3
and the cell performance degraded drastically
when defect density is more than 1015
cm-3
.
IV. CONCLUSION
In this work, the effects of the ambient temperature and the
defects density in the Hetero-junction with Intrinsic Thin
layers solar cells (HIT) have been studied. It is shown that, the
performances of the solar cell were influenced with variation
in ambient temperature and also in defects density. A record
efficiency of 24.28% could be obtained. The other relevant
parameter values of Voc, Jsc and FF are 726.4mV,
40.14mA/cm2 and 83.26% respectively.
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www.ijltemas.in Page 103
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