Content
1.
Introduction
2
2.
Theory
2
2.1
Photovoltaic Energy Conversion
2
2.2
Solar spectrum
3
2.3
Fill factor
3
2.4
Solar cell conversion efficiency
3
3.
4.
5.
Procedures
4
3.1
Equipment
4
3.2
Fabrication
6
3.3
Test
7
Data
8
4.1
I-V curve for g1
8
4.2
I-V curve for g2
9
4.3
I-V curve for g3
10
Analysis
11
5.1
Fill factor
11
5.2
Solar cell conversion efficiency
11
5.3
Findings
11
6.
Conclusion
12
7.
Reference
12
1. Introduction
A solar cell is a vital device that turns light energy into electrical energy via the photovoltaic process,
primarily utilizing semiconductor materials. The process involves photon absorption, electron-hole pair
creation, and charge carrier separation. Photons with energy greater than the bandgap (Eg) contribute to the
solar cell output, while excess energy is lost as heat. The p-n junction is commonly used for charge carrier
separation. Understanding the basics of semiconductors and p-n junction solar cells is crucial for both
conventional and new solar cell types, as it helps improve efficiency, lower manufacturing costs, and
minimize energy consumption during fabrication.
Solar cells can be classified into four generations based on time and material categories. First-generation
solar cells, made of single and multi-crystalline silicon, are the most common in the market. Secondgeneration solar cells emerged to address the high material usage and cost of silicon solar cells by reducing
the maximum film thickness. Third-generation solar cells explore light management concepts, including
dye-sensitized, perovskite, and organic solar cells. Fourth-generation solar cells, which are still speculative,
involve the use of composites. Each generation aims to enhance efficiency, decrease costs, and optimize
material usage.
Figure 1. Cross-section of a typical solar cell.
2. Theory
2.1 Photovoltaic Energy Conversion
Photovoltaic energy conversion refers to the direct generation of electrical energy as current and voltage
from electromagnetic energy, including infrared, visible, and ultraviolet light. The fundamental four steps
required for photovoltaic energy conversion include:
A. Absorbing light and transitioning from a ground state to an excited state.
B. Converting the excited state into a minimum of one free negative-charge carrier and one free positivecharge carrier pair.
C. Employing a selective transport mechanism that facilitates the collection of the resulting free carriers
by contacts.
2.2 Solar spectrum
The solar spectrum varies during the day and across locations, spanning from UV to deep infrared. No
single material can serve as the active component that is sensitive to the entire spectrum. Standard solar
spectra are essential in solar cell research, development, and marketing, as the actual spectrum affecting a
cell in use can differ based on weather, season, time of day, and location. The use of standard solar spectra
enables a fair comparison of experimental solar cell performance between different devices.
There are 3 kinds of spectrum: AM0, AM1.5 Global and AM1.5 Direct.
2.3 Fill factor
The electrical power per area Pout generated by the cell shown in Figure 1, operating at voltage V and
delivering current I (current density J) due to the incoming solar power, is calculated as the product of
current I and voltage V, divided by the cell area:
Pout  JV
An experimental J-V curve for the fabricated cell can be observed in Figure 2.
Ideally, the J-V characteristic would be rectangular, delivering a constant current density Jsc until
the open-circuit voltage Voc is reached. A metric called the fill factor (FF) is used to determine how
closely a specific characteristic aligns with the ideal rectangular J-V shape. The fill factor is calculated as
follows:
FF 
I mpVmp
I scVoc
Figure 2. The current density-voltage (J-V) characteristic of the solar cell.
2.4 Solar cell conversion efficiency
For a p-n junction under a voltage, without light, can produce a diode current:
I  I 0 (e qV / kT  1)
The I0 is the reverse saturation current for the p-n junction. For a solar cell I-V curve, which is the
superposition of the I-V curves of the cell in dark and in the presence of light.
I  I 0 (e qV / kT  1)  I sc
By definition,
I 0 (e qVoc / kT  1)  I sc  0
Then, the relationship between Isc and Voc is given by
I sc  I 0 (e qVoc / kT  1)
or
Voc 
 kT  I sc 
kT  I sc

ln 
 1 
ln 
q
q
 I0

 I0 
Hence, for a given Isc, Voc increases logarithmically with decreasing saturation current I0. The output
power is given by
P  IV  I 0V (e qV / kT  1)  I scV
The condition for maximum power is obtained by dP/dV=0, or
Vmp 
1  ( I sc / I 0 )
kT
kT
ln[
]  Voc 
ln( 1  qVmp / kT )
q
1  (qVmp / kT )
q
Iteration can be used to calculate the above equation for Vmp,
The efficiency (η) of the solar cell is determined as the fraction of incident power which is converted to
electrically and is defined as:
Pmax  Voc I sc FF

Pmax Voc I sc FF

pin
Pin
3. Procedures
3.1 Equipment
Spin Coater: A spin coater is a device used for applying thin, uniform films of liquid material onto flat
substrates, such as glass or silicon wafers, through a process called spin coating. During this process, the
substrate is placed on a spinning platform, and a small amount of liquid material (often a solution or
suspension) is dispensed onto its center. As the platform spins at a high speed, the centrifugal force spreads
the liquid evenly across the substrate, forming a thin and uniform layer.
Figure 3: Spin Coater
Furnace: A furnace, in the context of spin coating, is typically used for the annealing or curing process that
follows spin coating. After a thin film is applied onto a substrate using a spin coater, the coated substrate is
placed in a furnace or oven. The purpose of the furnace is to heat the substrate at a controlled temperature
for a specific period of time. This heating process, also known as annealing or curing, helps solidify the
film, remove solvents, enhance material properties, and improve the adhesion of the film to the substrate.
Figure 4: Furnace
Thermal evaporator: In a thermal evaporator, Ag is heated inside a vacuum chamber, typically using a
resistive heating element or an electron beam. As the source material reaches its evaporation point, it
transitions from a solid or liquid state into a gaseous state. The evaporated material then travels in a straight
line within the vacuum chamber, eventually condensing onto the surface of the substrate, which is
positioned above the source material. As the vapor particles accumulate on the substrate, they form a thin,
uniform film.
Figure 5: Thermal evaporator
Solar simulator: A solar simulator is a device that replicates the natural sunlight's spectral distribution,
intensity, and other characteristics in a controlled environment.
Figure 6: Solar simulator
Materials:
P-doped silicon wafers, Spin-on-glass dopant.
3.2 Fabrication
①Begin with a 3-inch p-doped silicon wafer, which should be cut from the wafer to measure 2.5 cm x 2.5
cm for the fabrication process.
②Clean the silicon wafer.
③Apply spin-on-glass (SOG) dopants (phosphorus doping) to the silicon wafer using a spin coater to
achieve an n-doped thin film.
④Place the wafer on a hot plate at 80°C for 10 minutes.
⑤Perform thermal treatment of the wafer in a furnace at 600°C for 50 minutes to allow phosphorus
diffusion, ensuring sufficient time for heating up and cooling down to room temperature.
⑥Remove any dopant residues from the wafer after thermal treatment.
⑦Use a thermal evaporator to deposit silver (Ag) on both sides of the wafer, creating metal-semiconductor
contacts. But unfortunately, we do not use it in this experiment due to the time limitation. We will use it
in next experiment LED.
3.3 Test
⑧Anneal the wafer and plate indium to enhance the electrical contact between the metal and semiconductor.
⑨Measure the J-V characteristics of the fabricated solar cell using a solar simulator. We test 3 groups of
solar cell. In the first week, we use a finish product as follows (group 1, g1). And in the second week, we
use another solar cell which is rightfully plated indium (group 2, g2) and solar cell fabricated by ourselves,
which has a poor contact due to bad indium plating(group 3, g3).
4. Data
We take the output power of solar simulator as 250W, 300W and 350W. For each group we test three
times to compare and ensure accuracy.
4.1 I-V curve for g1
Figure 7, 8, 9: 250W, 300W, 350W g1 by excel
4.2 I-V curve for g2
Figure 10,11,12: 250W, 300W, 350W g2
4.3 I-V curve for g3
Figure 13,14,15: 250W, 300W, 350W g3
5. Analysis
5.1 Fill factor
FF 
I mpVmp
I scVoc
We get a Table 1:
FF
G1
G2
G3
250W
0.734
0.028
0.053
300W
0.731
0.047
0.022
350W
0.709
0.061
0.041
5.2 Solar cell conversion efficiency
Pmax  Voc I sc FF

Pmax Voc I sc FF

pin
Pin
We get a Table 2:
𝜂%
G1
G2
G3
250W
0.073
2.256 × 10−6
6.455 × 10−5
300W
0.070
1.373 × 10−6
5.367 × 10−5
350W
0.080
9.714 × 10−7
4.571 × 10−5
5.3 Findings
From the data presented in Table 1 and Table 2, we can observe the following findings:
Fill Factor (FF):
For Generation 1 (G1) solar cells, the fill factor remains relatively consistent across different wattages,
with only minor variations.
Generation 2 (G2) and Generation 3 (G3) solar cells exhibit fluctuations in fill factor values across the
tested wattages.
Solar Cell Conversion Efficiency (η%):
Generation 1 (G1) solar cells display a slight decrease in conversion efficiency from 250W to 300W,
but then experience an increase at 350W.
Generation 2 (G2) solar cells demonstrate a general trend of decreasing conversion efficiency as the
wattage increases.
Similarly, Generation 3 (G3) solar cells also show a decline in conversion efficiency with increasing
wattage.
It is worth noting that the efficiency values for G2 and G3 solar cells are significantly smaller compared
to G1 solar cells, which might indicate a need for further optimization in these newer generation
technologies.
Several factors could contribute to the observed data for Generation 2 (G2) and Generation 3 (G3) solar
cells:
Material properties: G2 and G3 solar cells use different materials than G1 solar cells. These materials
might have lower absorption coefficients, bandgap mismatches, or different electronic properties that affect
their performance and efficiency.
Fabrication methods: The manufacturing processes for G2 and G3 solar cells may not be as refined as
those for G1 solar cells. Possible issues could include uneven layer deposition, impurities, or defects in the
materials, which could negatively impact the fill factor and conversion efficiency.
Light management strategies: G2 and G3 solar cells often rely on innovative light management
techniques, such as multi-junction designs or light-trapping mechanisms. If these strategies are not
optimized or properly integrated, they might not provide the expected benefits, resulting in lower
performance.
Thickness limitations: Some G2 and G3 solar cells use thin-film technologies to reduce material usage
and cost. However, thinner layers can sometimes lead to incomplete light absorption or increased
recombination losses, affecting the solar cell's performance.
External factors: The testing conditions or environment could also play a role in the observed data.
Factors such as temperature, incident light angle, or spectral distribution might influence the performance
of G2 and G3 solar cells differently compared to G1 solar cells.
6.Conclusion
In this study, we explored the photovoltaic process in solar cells, focusing on their fabrication and
performance. Solar cells can be classified into four generations, with each generation aiming to enhance
efficiency, decrease costs, and optimize material usage.
We conducted tests on three groups of solar cells and analyzed their fill factor and conversion
efficiency. The results showed that G1 solar cells had relatively consistent performance, while G2 and
G3 solar cells demonstrated fluctuations in both fill factor and conversion efficiency.
Several factors might contribute to the observed data for G2 and G3 solar cells, including material
properties, fabrication methods, light management strategies, thickness limitations, and external factors.
Further research and development could help identify and address these issues, leading to improved
performance and efficiency for G2 and G3 solar cells.
7. References
[1] ASTM Standard E490.
[2] ASTM Standard G173-03.
[3] http://dx.doi.org/10.1016/j.solmat.2014.10.021
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