AQ3.1.1 MANUFACTURING OF POLYSILICON
(1 point possible)
Which of the following processes is used in order to fabricate polysilicon?
Note that more than one answer is possible.
The Czochralski and float-zone processes produce single-crystal silicon, so the two correct answers are the Fluid-bed reactor and Seimens processes.
Top of Form
The Czochralski process
Fluid bed reactor process
The float-zone process
The Siemens process
AQ3.2.1 FINGER'S RESISTANCE
(1 point possible)
Consider a c-Si solar cell whose fingers have a resistanceR=0.1Ω. What would be the finger's resistance (in Ω) if the finger's width is doubled and the finger's height is one third of its initial value?
If A is the original cross-sectional area of a finger, the new cross-section is (2/3)A. Since resistance is inversely proportional to cross-sectional area, the new resistance is 3/2 the original resistance, or 0.15 Ω.
Answer: 0.15 Ω.
AQ3.3.1 SURFACE RECOMBINATION AT THE AIR/SILICON INTERFACE
(1 point possible)
We have discussed that a bare crystalline silicon surface contains many defects which act as SRH recombination centers. How can the surface recombination at the air/n-silicon interface be reduced? Note that you can mark more than one answer.
Lowering the high top surface recombination is typically accomplished by reducing the number of dangling silicon bonds at the top surface by using "passivating" layer on the exterior surface. The passivating layer for silicon solar cells is usually an insulator.
Top of Form
By decreasing the doping of the n-layer.
By increasing the doping of the n-layer.
By depositing a thin insulating layer on top of the n-layer.
By depositing a thin conductive layer on top of the n-layer.
AQ3.4.1 CHOOSING THE SILICON LAYER THICKNESS
(1 point possible)
Imagine that you are in the lab and you can decide the thickness of the Si layer of your solar cell. You want to optimize the solar cell performance for a wavelength ofλ = 1000 nm, for which the absorption coefficient isα(1000 nm) = 102 cm−1. Which of the following thicknessesdSiwould give a better performance? Take into account that you already know two things:
(1)Beer-Lambert's law.
(2)For silicon, the minority carrier diffusivity is aroundD = 27 cm2/sand the minority carrier lifetime is aroundτ = 15 μs.
From the Beer-Lambert law, thicker layers absorb more light and so generate more carriers. This would indicate we should use as thick a layer as possible. However, the Beer-Lambert law also predicts that most photons get absorbed within 1/a = 100 mm of the surface, with fewer and fewer penetrating deeper into the Si, so much thicker layers give little added benefit from this perspective. Furthermore, carriers, created where the photons are absorbed, need to traverse the whole layer. They recombine after some period of time and don’t live long enough to traverse an extremely thick layer. So we want a layer thickness somewhat less than the average diffusion length, so most carriers will make it. The diffusion length is given by
.
Thus, from the available choices, L = 180 mm is the most appropriate.
Top of Form
· dSi=100μm
· dSi=180μm
· dSi=300μm
Bottom of Form
AQ3.5.1 BACK SURFACE FIELD
(1 point possible)
In state-of-the-art crystalline silicon solar cells technology, based on p-type silicon, a back surface field (BSF) is implemented. This BSF is implemented, because in this way:
· The barrier for hole collection at the back contact is reduced, increasing the surface recombination velocity at the back contact.
· The barrier for electron collection at the back contact is reduced, decreasing the surface recombination velocity at the back contact.
· A barrier is created for hole collection at the back contact, increasing the surface recombination velocity at the back contact.
· A barrier is created for electron collection at the back contact, decreasing the surface recombination velocity at the back contact.
Bottom of Form
AQ3.6.1 EFFECT OF BYPASS DIODES
(1 point possible)
A given monocrystalline silicon solar cell has the following specifications:
Isc = 5 A, Voc = 0.6 V.
72 identical cells with the above specifications are to be interconnected to create a PV module.
What is the open-circuit voltage (inV) of the PV module if all the solar cells are connected in a series configuration?
When combining N cells in series and M cells in parallel, the short circuit current and open circuit voltages are
,
respectively. With N = 72 and M = 1, we get
AQ3.6.2 EFFECT OF BYPASS DIODES
(1 point possible)
What is the short-circuit current (inA) of the PV module if all the solar cells are connected in a series configuration?
AQ3.6.3 EFFECT OF BYPASS DIODES
(1 point possible)
Now suppose that the PV module mentioned above is set up using a series connection of solar cells with the above-mentioned specifications. Two of the solar cells have gone faulty (completely stop generating power), but fortunately you have bypass diodes connected across the faulty solar cells. Assume that the bypass diodes are ideal (they have a zero voltage drop when conducting).
What is the measured open-circuit voltage (inV) of the above PV module with the faulty solar cells?