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455003437890690007378065370000455003437890350003742690Solar cells
Task 2
15/6/2018
3600028000Solar cells
Task 2
15/6/2018

29377551737341Delft University of Technology
Photovoltaic Lab Course
4000020000Delft University of Technology
Photovoltaic Lab Course
344031766633501.Sameep Karki – 47382922. Santhosh Ramesh – 4742664
3600001.Sameep Karki – 47382922. Santhosh Ramesh – 4742664

5054600-16510000
Solar Cells
Group 3 – Task 2
Sameep Karki, 4738292
Santhosh Ramesh,4742664
ET4379 Photovoltaic Extensive Practical Course
TU DELFT – EEMCS/ESE/PVMD
MEKELWEG 4
2628CD DELFT
THE NETHERLANDS
Measurements date: 15.06.2018
Contents
TOC o “1-3” 1Introduction PAGEREF _Toc517428971 h 1
2Experimental SETUP PAGEREF _Toc517428972 h 1
2.1J-V MEASUREMENTS OF SOLAR CELLS PAGEREF _Toc517428973 h 1
2.2EQE CHARACTERISTICS OF SOLAR CELLS PAGEREF _Toc517428974 h 1
2.3ASA simulations PAGEREF _Toc517428975 h 1
3RESultS & Discussion PAGEREF _Toc517428976 h 2
3.1performance MEASUREMENTS OF thin film a-si SOLAR CELLS PAGEREF _Toc517428977 h 2
3.1.1perfromance parameters PAGEREF _Toc517428978 h 2
3.1.2LED Light source calibration PAGEREF _Toc517428979 h 3
3.1.3error induced by ambient light PAGEREF _Toc517428980 h 4
3.2EQE CHARACTERISTICS OF SOLAR CELLS. PAGEREF _Toc517428981 h 4
3.3solar cell simulations PAGEREF _Toc517428982 h 6
3.3.1PIN junction solar cell PAGEREF _Toc517428983 h 6
3.3.2significance of p-layer thickness in PIN junction solar cell PAGEREF _Toc517428984 h 7
3.3.3significance of i-layer thickness in PIN junction solar cell PAGEREF _Toc517428985 h 8
3.3.4significance of N-layer thickness in PIN junction solar cell PAGEREF _Toc517428986 h 8
3.3.5significance of bACK REFLECTOR in PIN junction solar cell PAGEREF _Toc517428987 h 9
4Conclusions PAGEREF _Toc517428988 h 10
5References PAGEREF _Toc517428989 h 10

IntroductionThe solar power can be a solution to the climate change. Increasing the power producing capacity per unit area is the major challenge for the solar scientists around the world. Solar cells produced at the lab scale has better performance than one implemented in the real time. Bridging the gap is also a major focus in the PV industry.

Various parameters must be taken care to measure the cell performance accurately. PV standards quote the cell performance under the standard test conditions of AM1.5 spectrum, 1000 W/m2 and a temperature of 25 C. Light source is an important factor in measuring the solar cell performance. To use a light source that cam mimic AM1.5 spectrum and provide 1000 W/m2 power is difficult option. So, light source that is close to AM1.5 spectrum can be calibrated and used to our need.

This task focusses on measuring the performance of solar cell and comparing it with the simulations run in ASA simulation tool.

This report is sub divided into following sections:
1. Experiment Setup, explaining the test setup used to measure parameters.

2. Results and discussion, tabulating the measurements and drawing conclusions on the different experiments performed.

3. Conclusion, concluding with final statements
Experimental SETUPIn this section the experimental setup used to do the measurements are explained in detail.

J-V MEASUREMENTS OF SOLAR CELLSIn this first subtask, performance measurements of the given solar cells are done. The white led is used as the light source.

The white LED is calibrated to provide an irradiation of 561 W/m2 which is the equivalent the response of AM1.5 spectrum below 1.6eV. Photodiode is used to calibrate the LED light source. Distance between the light source and photodiode was adjusted such that diode produced a current of 1.141 mA which is equivalent to 561 W.m2. The circuit used to measure the diode current is shown in the below figure.

Figure SEQ Figure * ARABIC 1: Circuit to measure diode current.Once the white LED is calibrated and fixed in position the diode is replaced with the solar cell and experimental setup is set as shown in below figure.

Figure SEQ Figure * ARABIC 2: Circuit to measure the JV characteristics of solar cell
The bias voltage is varied from 0 to 1 V in small step size and the corresponding photocurrent is measured. Multi-meters are used to measure the voltage and current in the circuit. Laboratory power supply is used to power the light source and apply bias voltage. Experiment is repeated for three solar cells.

EQE CHARACTERISTICS OF SOLAR CELLSIn the second subtask, the photodiode was illuminated using DS68 Luxeon and the diode current is noted down. The circuit to measure the diode current is same as that shown in REF _Ref517366523 h * MERGEFORMAT Figure 1. The LED was mounted 22 cm away from the target to get homogenous distribution of light. The Current to the LED was maintained at 0.4 A. After measuring the diode current for each of the color LEDs the phot diode is replaced with solar cell and the short circuit current density for each of the color LEDs was measured. Experiment was repeated for the other two given solar cells.

ASA simulationsASA simulation tool developed at TU delft was used simulate the performance of the a-Si PIN junction solar cell. Input file “P-I-N.cas” was edited to change the different parameter of the solar cell such as layer thickness, back reflector material. Simulation output such as JV curve, EQE curve are saved as “.csv” files. Each of the layer’s thickness is varied, simulations are run, and results are recorded. Finally, the back-reflector material is varied, and simulations results are recorded.

RESultS ; Discussion
performance MEASUREMENTS OF thin film a-si SOLAR CELLSIn this subtask, white LED was used as a power source to measure the performance of the a-Si PIN junction solar cell. The intensity of the white LED was calibrated to match the equivalent spectral intensity of the AM1.5 spectrum. Photodiode is used to calibrate the white LED source.

Photodiode vs Solar cell: photodiodes and solar cell are basically PN junction diodes that converts light into electricity. Photodiodes have small area, low capacitance and hence they faster response time. While solar cells have large area, high capacitance and have comparatively low response time. The photodiodes work in reverse bias while solar cells work in forward bias CITATION Res17 l 16393 1.
perfromance parametersThe performance parameters for 3 different solar cells were studied after the calibration of white LED using the photodiode. The input power to the cell is given by equation 3.1.

Pin=I × Acell STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 1
Where I is the incident power which is 1000 W/m2. Acell is the area of the cell which is 16mm2. The bias voltage was varied, and JV characteristics was of the cells were measured. The fill factor was calculated using the equation
FF=VMPP×JMPPVOC×ISC×100% STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 2
The efficiency of the cell is measured from the equation 3.3.
?=FF*VOC×ISCinput power STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 3
The JV characteristics of 3 cells are measured and plotted in REF _Ref517353674 h * MERGEFORMAT Figure 3.

Figure SEQ Figure * ARABIC 3: JV characteristics of a-Si PIN junction cells.

Figure SEQ Figure * ARABIC 4: PV characteristics of a-Si PIN junction cells.

It can be noted that all three cells have similar characteristics. The performance parameters measured for each of the cell is tabulated in REF _Ref517353989 h * MERGEFORMAT Table 1.

Table SEQ Table * ARABIC 1: Performance parameter of a-Si cellsParameters Cell A Cell B Cell C
Voc V0.769 0.773 0.8
Jsc mA/cm218.06 15.93 16.81
Vmpp V0.582 0.585 0.633
Jmpp mA/cm213.68 12.43 13.31
Pmpp mW/cm21.27 1.16 1.34
FF %57.35 59.05 62.65
?Cell %7.97 7.28 8.43
LED Light source calibrationA light source is suitable if the spectrum of the light source is close to the AM1.5 spectrum. The intensity of the irradiation spectrum is 1000 W/m2. White LED was used a light source. The white LED must be calibrated before it can be used as a light source. Assuming the a-Si solar cell’s band gap to be 1.6eV the intensity delivered by the AM1.5 spectrum in the relevant spectral range is around 561 W/m2.So, the white LED must be calibrated such that it delivers 561 W/m2.

Figure SEQ Figure * ARABIC 5: AM1.5 spectrum and Relative spectral density of White LED
A photo diode with a known spectral response is used to calibrate the LED. Current produced by the photodiode is proportional to the incident light.

Ipd=S×Pin STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 4
Where, Ipd is the current detected by the diode. S is the sensitivity in A/W. Pin is the incident power in W. Since the spectral response of the photo diode depends on wavelength, spectral irradiance of the light source must be considered.

Ipd=f(S?,Ie,?)=k×S(?0)×Pin STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 5
Where, k is the correction factor. S(?0) is diode sensitivity at wavelength ?0. The correction factor k can be derived as follows.

Pin=A×?1 ?2 Ie,? . d? STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 6
k= f(S?,Ie,?) S(?0)×Pin STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 7
fS?,Ie,?= A×?1 ?2 Ie,? . S?.d? STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 8
=A×?1 ?2 I(?0).Irel(?) . S?0.Srel?.d? So, the expression for k becomes
k= ?1 ?2 Irel(?) . Srel?.d? ?1 ?2 Irel(?) .d? STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 9
The maximum bandgap is 1.6 eV for an a-Si solar cell which gives maximum wavelength (?2) as 775 nm.

E= hc? STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 10
The datasheet of white LED gave ?1 as 379 nm.

Figure SEQ Figure * ARABIC 6: Relative spectral response of diode
From the data provided for white LED relative spectral emission, a fitted curve was obtained as shown in figure 4. The expression for Srel fitted curve is as follow.

Srel?=p1×?3+p2 ×?2+p3×?+p4 STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 11
Coefficients are
p1= -2 e-8p2= 3e-5p3= -0.016p4=2.6974The correction factor will change if a solar cell of the different bandgap is used because the spectrum range of wavelength will change. For a solar cell with bandgap of 1.2 eV the wavelength range will be 300 – 1035 nm.

error induced by ambient lightThe LED light source was calibrated in open area. There will be an effect of the ambient light in the LED calibration. Diode current was first measured with the ambient light and was found to be 6.6 ?A. The diode current measured with LED turned on is 1141 ?A. The error induced by the ambient light is given by the below equation.
% error=IambILED×100% STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 12
The error induced by the ambient light is calculated to be 0.57 % which is very less and can be neglected.

EQE CHARACTERISTICS OF SOLAR CELLS.In this subtask, the EQE of 3 a-Si solar cells was measured using 7 different light sources in the visible range of the spectrum. The short circuit current density is measured and thereby the EQE of the cell is measured.

The correction factor can be associated with the relative special response taking the spectrum of colored LED as a single Gaussian and the relative spectral response fitted curve given in the previous task with the formula 3.11 can give correction factor for 7 different colored LED lights.

Table SEQ Table * ARABIC 2:Correction factor of colored LED
LED Wavelength(nm) Correlation factor (k)
Royal Blue 447.5 0.13
Blue 470 0.18
Cyan 505 0.26
Green 530 0.32
Amber 590 0.45
Red-orange 617 0.51
Deep Purple 655 0.60
As the LED is p-n junction semiconductor material that emits light when current is passed through it. The REF _Ref517364221 h Table 3 shows the Bias voltage across each colored LED lights.
Table SEQ Table * ARABIC 3: Colored LED’s Bias Voltages
S. No LED Colour Voltage (V)
1 Royal Blue 3.07
2 Blue 3.04
3 Cyan 2.98
4 Green 2.95
5 Amber 3
6 Red-Orange 2.39
7 Deep Red 2.16
It can be inferred that all the colored LED has different bias voltages. The energy required to emit a photon from the blue spectrum is much more than the energy required to emit a photon in red spectrum. By applying a constant current of 0.4A blue led requires more voltage to reach the required energy than a red led. The trend in decreasing voltage as we move along the spectrum from blue to red is clearly seen in REF _Ref517364221 h * MERGEFORMAT Table 3.

The spectral irradiance and photon flux are the important properties of any light source. Spectral irradiance Ie,? is the power received by a given surface for a wavelength of light CITATION Arn l 16393 2. Whereas the photon flux is defined as number of photons incident on the surface per unit time CITATION Arn l 16393 2.

Ipd=k×S?0×A×Ie,? STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 13
Here k is correlation factor, S?0 is the spectral sensitivity at peak wavelength, Ipdis photodiode current, A is the photodiode area and Ie,? is the spectral power density. Spectral power density and photon flux is related by the below equation.

Ie,?= ?ph,?×hc? STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 14
Equation 3.13 and 3.14 can be rearranged to form the below relation.

?ph,?=Ipd×?k×S?0×A×h×c STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 15
The External quantum efficiency (EQE) is defined as the ratio of number of electron hole pair created to the number incident photons. The EQE is dependent on wavelength so, it is computed over a certain range of wavelength as given in equation.

EQE?= dne(?)dtAcell×?ph(?) STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 16
Here dne(?)dt is the number of electrons exiting the solar cell, Acell (16 ??2) is the area of the solar cell and ???(?) is the incident photon flux at certain wavelength.
The rate of change of electron movement gives rise to the short circuit current Isc.

dne(?)dt =Isce STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 17
Here, e is elementary charge. The EQE in terms of photocurrent, correlation factor, and spectral sensitivity can be expressed by using equation
EQE?= k×S?0×A×h×c×IscIpd×? ×e × Acell STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 18
Electron flux (?e) can be obtained from short circuit current as follows,
?e=Jsce STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 19
The nominal wavelength of LED, photodiode current, and photon flux is shown in REF _Ref517390045 h * MERGEFORMAT Table 4.

Table SEQ Table * ARABIC 4: Electron flux of different LED sources
LED Colour Wave
length nm Photodiode Current uA ??? Photon flux 1020ph/(m2.nm)Royal Blue 447.5 63.5 1.57
Blue 470 43.1 0.81
Cyan 505 72.1 1.01
Green 530 77 0.92
Amber 590 195.1 1.84
Red-Orange 617 322 2.81
Deep Red 655 213.8 1.68

The electron flux generated in each of the cell is tabulated in REF _Ref517390620 h Table 5.

Table SEQ Table * ARABIC 5 : Electron flux generated in a-Si cell for different light sources.

LED Colour Short-circuit Current uA Electron flux
1020elec/(m2.nm)Cell A Cell B Cell C Cell A Cell B Cell C
Royal Blue 250 241 223.4 0.98 0.94 0.87
Blue 179.9 168 180.5 0.70 0.66 0.70
Cyan 204 188.5 248.2 0.80 0.74 0.97
Green 211.5 196.1 229.6 0.83 0.77 0.90
Amber 452 382.6 332.5 1.76 1.49 1.30
Red-Orange 580 390.5 605 2.26 1.52 2.36
Deep Red 391.2 335.4 296 1.53 1.31 1.15
The calculated EQE for all the three cells for different color LEDs are tabulated in REF _Ref517391078 h * MERGEFORMAT Table 6.

Table SEQ Table * ARABIC 6: EQE of a-Si solar cells for different light sources
LED Colour Cell A Cell B Cell C
Royal Blue 0.62 0.60 0.56
Blue 0.86 0.81 0.87
Cyan 0.79 0.73 0.96
Green 0.90 0.83 0.97
Amber 0.96 0.81 0.71
Red-Orange 0.81 0.54 0.84
Deep Red 0.91 0.78 0.69
The REF _Ref517391536 h * MERGEFORMAT Figure 7 shows EQE of the solar cell A, B and C. We can notice that EQE for all the solar cell show similar trend are close to 1. This shows that the optical and electrical losses in the solar cell is very minimal. Losses might be due to the surface reflection and surface recombination. Even though the top surface is hydrogen passivated, surface recombination might increase due to the scratches in the solar cell surface.

Figure SEQ Figure * ARABIC 7: EQE of a-Si solar cells
Short circuit current density can be obtained using photon flux and EQE of the cell using the below formula.

Jsc=e?1?2EQE(?)?phd? STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 20
The short circuit current density calculated through this method is tabulated in the REF _Ref517427615 h Table 7.

Table SEQ Table * ARABIC 7: Short circuit current density
Solar Cell Jsc A/m2White LED Jsc A/m2Colored LED
Cell A 180.6 141.78
Cell B 159.3 118.10
Cell C 168.1 132.2
The short circuit current density is different measured through the colored LEDs are lower because the of the difference in the spectrum.

solar cell simulationsIn this section ASA simulation tool developed by TU delft was used to study the effect of thickness of each of the layer in a PIN junction solar cell. The effect of different back reflector material has also been studied.
PIN junction solar cellPIN junction solar cell is manufactured in superstrate configuration with TCO layer deposited on glass superstrate. In thin film silicon the holes have lower mobility than electrons p layer is deposited first. Since the generation rate is higher at the top of i-layer holes can reach the reach the p-layer before it recombines CITATION Arn l 16393 2. After the p-layer, i-layer and n-layer are deposited. PIN layers are deposited using RF -PECVD or VHF-PECVD or HW-CVD method. The TCO layer is deposited by sputtering technique. The metal contacts are deposited using thermal evaporation CITATION Ift12 l 16393 3.

Figure SEQ Figure * ARABIC 8: PIN junction solar cell
When the PIN junction is brought together electric field developed reaches far into the intrinsic region. The slope in the diagram below signifies the electric field. Electrons drift towards the n region and holes towards the p region. The defect density of the doped layers is more in case of a-Si. So, the diffusion length of the majority carriers is very less and the photogenerated carriers from the doped layers does not contribute to the photogenerated current. Because of the small diffusion length thickness of p and n layer must be small.

Figure SEQ Figure * ARABIC 9: Energy band diagram of PIN junction solar cell
The reference cell structure used for the simulation is tabulated in the below table.

Table SEQ Table * ARABIC 8: Reference cell design parameters
S. No Layer/Property Value
1 Glass 1 mm
2 TCO 750 nm
3 p-layer 15 nm
4 i-layer 300 nm
5 n-layer 20 nm
6 Back reflector Ag
7 p-layer activation energy 0.48 eV
8 n-layer activation energy 0.2 eV
The JV characteristics of the simulated reference cell is plotted below. The external parameters t

Figure SEQ Figure * ARABIC 10: JV characteristics of simulated solar cell.

Table SEQ Table * ARABIC 9: Performance parameters of reference cell.

Parameter Value
Voc V0.88
Jsc mA/cm215.22
Vmpp V0.74
Jmpp mA/cm212.63
Pmpp mW/cm29.35
FF %68.96
?Cell %9.3
The performance parameters of the simulated cell were comparable to that of the cells measured in the task. The performance of the cell when each of the layers in PIN junction solar cell is varied in thickness is simulated and compared. This would help us understand the significance of using optimum thickness of each of layers.

significance of p-layer thickness in PIN junction solar cellp-layer is the first layer that incident light passes through. For efficient functioning of the solar cell, the p-layer must have following properties.

More transparent.
High bandgap to allow more blue light.

Small thickness to avoid parasitic absorption.

The simulated EQE measurement of PIN solar cell with different p-layer thickness is shown in REF _Ref517220163 h * MERGEFORMAT Figure 11. It can be clearly noted that the p-layer thickness does not affect the absorption in the latter half of the spectrum. This is because the p-layer is at the top and significantly affects the absorption of the blue photons which has smaller wavelengths and are absorbed faster. This is governed by the Beer Lambert’s law of absorption.
It= I0×exp?(-?×d) STYLEREF 1 s 3. SEQ Eq * ARABIC s 1 21
Where It is the intensity of transmitted light through the material, ? is the absorption coefficient, d is the thickness and I0 is the incident light intensity CITATION Arn l 16393 2. The above equation proves that thicker the material, more the absorption.

Figure SEQ Figure * ARABIC 11: Effect of p-layer thickness on EQE.

Increasing the p-layer thickness to 45nm showed poor EQE performance. This is due to the increased parasitic absorption of blue photons in the p-layer. Decreasing the p-layer thickness seems to improve the EQE in blue region slightly. The REF _Ref517220634 h * MERGEFORMAT Figure 12 shows the effect of the p-layer thickness on performance parameter of the cell.

Figure SEQ Figure * ARABIC 12: Effect of p-layer thickness on JV characteristics
Increased p-layer thickness shows reduction in short circuit current density of the cell. This is obvious as the parasitic absorption of blue light increased. FF of the cell reduces due to the increase in the cell resistance. Recombination also increases as the thickness increases.

On the other hand, if the p-layer thickness is reduced drastically to 5nm the Voc of the cell drop. Smaller the p-layer width, smaller the width of the depletion region formed and thus smaller the built-in voltage. We can also notice that Voc does not increase after 15nm thickness. So, thickness of 15nm is optimal.

significance of i-layer thickness in PIN junction solar cellThe i-layer is the photoactive layer in a PIN junction solar cell. For an efficient use of sun light i-layer must have following properties.

Thickness must be lesser than the diffusion length of the charge carriers.

Less series resistance.

Optimal bandgap for more absorption of light.

The effect of i-layer thickness on EQE can be seen in the REF _Ref517280740 h * MERGEFORMAT Figure 13. The EQE does not vary in the blue spectrum as most of the blue light is absorbed at top of the i-layer. So, change in the thickness does affect the blue spectrum absorption. EQE in the middle of the spectrum is affected at smaller thickness because the light in that wavelength range does not get absorbed in the i-layer instead it might get absorbed in the n-layer. Increased thickness does not affect the EQE as the charge carriers produced at lengths below the charge carrier diffusion length recombine before they are collected.

Figure SEQ Figure * ARABIC 13: Effect of i-layer thickness on EQE.

The decreased i-layer thickness reduces the short circuit current density. Increasing the thickness to 600nm increases the recombination rate and resistance thereby reducing the fill factor of the device. I-layer thickness should be less than the diffusion length of the charge carriers. It can be noted from REF _Ref517281771 h * MERGEFORMAT Figure 14 that Voc decreases with increase in thickness. This is because the recombination increases with increase in thickness. From both REF _Ref517280740 h * MERGEFORMAT Figure 13 and REF _Ref517281771 h * MERGEFORMAT Figure 14 it can be clearly noted that the optimal thickness of the i-layer id 300 nm.

Figure SEQ Figure * ARABIC 14: Effect of i-layer thickness on JV characteristicssignificance of N-layer thickness in PIN junction solar cellThe n-layer is the last silicon layer in PIN junction solar cell. n-layer is used to create a low resistance path for the electrons. For efficient energy production n-layer must have following properties.

n-layer must be highly reflective.

Smaller thickness to reduce parasitic absorption.

Good transverse conductivity.

It can be noted from REF _Ref517282690 h * MERGEFORMAT Figure 15 that n-layer thickness does not affect the blue region of the spectrum. As most of the blue light would have been already absorbed in top layers. The middle region of the spectrum clearly shows the effect of parasitic absorption. Higher the thickness, more light of middle region is absorbed in the n-layer and that charge carriers are not collected at the contacts. Smaller the thickness the light passes through the n layer and gets reflected at the back reflector and comes into the i-layer to get absorbed. Increase in EQE in the middle region can be noted in the REF _Ref517282690 h * MERGEFORMAT Figure 15

Figure SEQ Figure * ARABIC 15: Effect of n-layer thickness on EQE
The effect of the parasitic absorption is clearly reflected in the JV curve. Smaller the n-layer thickness better the short circuit current density.

Figure SEQ Figure * ARABIC 16: Effect of n-layer thickness on JV characteristics
significance of bACK REFLECTOR in PIN junction solar cellBack reflectors are an important part of a solar cell. Using the advanced back reflection techniques, the optical path length of the material with refractive index of n can be enhanced up to a factor of 4n2 CITATION Moh17 l 16393 4. To be used as back reflector the metal should have high reflectance for higher wavelengths of light.

When a light travel from a medium of higher refractive index to a medium of lower refractive index reflection occurs. More the difference between the medium higher the reflection. The REF _Ref517287818 h * MERGEFORMAT Figure 17 shows the comparison of the refractive index of different back reflector materials with that of Si. Silver CITATION PBJ1 l 16393 5 seems to have lowest refractive index at higher wavelength regions than aluminum CITATION KMM l 16393 6 and chromium CITATION PBJ l 16393 7.

Figure SEQ Figure * ARABIC 17: Comparison refractive index with that of Si
When these three metals are considered for back reflector and simulations were run. The results showed that the silver has better reflectance than aluminum and chromium. The REF _Ref517288121 h * MERGEFORMAT Figure 18 shows the effect of different reflectors on the EQE of the cell and proves the theory discussed above.

Figure SEQ Figure * ARABIC 18: Effect of back reflector on EQE
As a direct effect of the better reflection cell with the silver as back reflector has better short circuit density thus better efficiency. REF _Ref517288795 h * MERGEFORMAT Figure 19 shows direct variation in Jsc as the back-reflector material is changed.

Figure SEQ Figure * ARABIC 19: Effect of back reflector on JV characteristics
Assuming the measured solar cell to be the reference cell A and when we compare it with cell B, Cell B has slightly higher Voc but smaller Jsc (refer to REF _Ref517353989 h Table 1). As discussed earlier this behavior can be compared to a reduce i-layer thickness (refer to REF _Ref517281771 h Figure 14.). Similarly, when we compare the cell A and cell C we similar trend as cell B. Cell C has higher Voc and lesser Jsc. This also show the behavior where the i-layer thickness is reduced.

ConclusionsIn the first subtask, performance parameter of the a-Si solar cells was studied. Calibrated white LED was used as light source. The analytical expression for the correction factor was derived and the error induced by the ambient light in calibration was studied.

In the second subtask the EQE of the solar cell was measured for different colored LED sources. It was studied that the solar cells under the LED color had different photo conversion efficiency. The EQE of every cell has was similar in the considered wavelength range.
In the final, the ASA simulation with varying parameters of solar cells was studied. Simulation proved that optimum thickness is required for better solar cell performance. And solar cells performed best with silver back reflectors.

References BIBLIOGRAPHY
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4 M. I. Hossain, W. Qarony, M. K. Hossain, M. K. Debnath, M. J. Uddin and Y. H. Tsang, “Effect of back reflectors on photon absorption in thin-film amorphous silicon solar cells,” 2017.

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