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34 CHAPTER 1 Solar Cells and Arrays: Principles, Analysis, and Design

4.5 MANUFACTURING SOLAR CELLS
We shall confine ourselves to the manufacturing of the wafer-based, single-crystal
and multi-crystal Si solar cells [22]. The major manufacturing steps are as follows:
1. p-type wafers of 1e10 U cm resistivity and about 0.2 mm thickness are first
þ
diffused with phosphorus to produce an n -layer with a thickness of
0.1e0.2 mm. A diffusion furnace is used to carry out this process. The protec-
tion gas is nitrogen. The diffusion sources are either solid in the form of discs or
liquid spin-on sources. The process may run under oxygen atmosphere.
2. Then the diffused wafers are etched by hydrofluoric acid to remove the diffusion
source layers,
3. The wafers are then metallized either by
a. Evaporation of nickelepalladiumesilver using thermal or electron-beam
evaporators. The evaporation is carried out through metal masks to save
the lithographic step or
b. Screen printing a thick film of silver (w25 mm).
4. Back-side metallization using evaporation.
5. The front side is antireflection coated either by spin-on techniques or by evap-
oration of TaO or TiO and SiO x . Electron beam evaporator is used.
6. In case of solar cell assembly using solder reflow, the contact areas are coated
with solder (Tin þ lead þ silver).
7. Now the solar cells are ready and are normally stored in cassettes.

4.6 TESTING THE SOLAR CELL AND SOLAR PANELS
To characterize the solar cells, one has to measure its IeV characteristics under
different illumination levels [23] and operating temperatures [24]. One may need
also to measure the small signal impedance of the solar cell operating at a specified
DC operating condition as a function of the small signal frequency [24]. The latter
impedance measurements are termed impedance spectroscopy. To get information
about the junction in the solar cell, one may measure the capacitance and conduc-
tance [25] as a function of the voltage in case of reverse bias. In the development
phase, one may need to measure the spectral response of the solar cells represented
by external quantum efficiency as a function of the incident radiation wavelength
[26]. Within the scope of this book with emphasis on the application side of solar
cells, the most important characteristics of interest are the IeV characteristics of
modules. So, we will outline such measurements based on a simple electronic circuit
for testing the PV modules.
The IeV and PeV characteristics of PV modules are traced using the circuit
shown in Fig. 1.30. The circuit is based on MOSFET IRFP260N as a varying elec-
tronic load with a heat sink to dissipate the power.
As V GS is less than the threshold voltage V th , the MOSFET will be OFF. When
V GS is increased above V th , the MOSFET will operate in the active region and the
drain current rises linearly with V GS . The gate voltage is controlled using DAQ