Opto-electronics
Solar Cell
Module-5
CONTENTS
• Introduction
• Structure
• Working Principle
• Current and Voltage calculation
• Power calculation
• Conversion Efficiency
• Recent Development
• Applications
Introduction
• A solar cell is a solid state device that converts
incident solar energy directly into electrical
energy.
• Efficiencies from a few percent up to 20-30%.
• No moving parts.
• No noise.
• Lifetimes of 20-30 years or more.
• Solar cell can supply power for the electronic
equipment aboard a satellite over a long period
which is distinct advantage over batteries.
Contd.
• To utilize a maximum amount of available
optical energy, it is necessary to design a solar
cell with a large area junction located near the
surface of the device.
• The planar junction is formed by diffusion or
ion implantation & the surface is coated with
appropriate materials to reduce reflection & to
decrease surface recombination.
Contd.
The junction depth “d” must be less than “Lp” in the
n-type material to allow holes generated near the surface to
diffuse to the junction before they recombine.
Contd.
• similarly the thickness of the “p” region must
be such that the electrons generated in this
region can diffuse to the junction before
recombination takes place.
Contd.
▪ It is desirable to have a large contact potential V0
to obtain a large photo voltage & therefore heavy
doping is indicated.
▪ It is important that the series resistance of the
device be very small so that power is not lost to
heat due to ohmic losses in the device itself.
▪ p-type area is large to make series resistance small.
▪ This requirement implies a proper match between
the electron diffusion length Ln, the thickness of
the p region,& the mean optical penetration depth
1/ alpha.
working
• Consider a pn junction as shown in figure
showing zero bias and photons are incident on
the junction.
Current and voltage
• Due to incident photons EHP generation takes
place in the space charge region that will
swept out producing IL in the reverse bias
direction.
• Due to the photo current a voltage drop in the
resistive load which will forward the diode and
produce forward current IF.
• The net current is given by
Contd.
• So the net current in the solar cell is always
reverse-bias direction.
For R=0 => V=0,the current is short-circuit
current as
• For The net current is zero and the
voltage produced is open-circuit voltage.
Power Calculation
• The power delivered to load
• The value of Vm is determined by trial and error
basis. Vm is the voltage which will produce maximum
power.
• A plot of diode current vs voltage is shown and
maximum value Vm and Im and maximum power Pm
is shown in rectangle where Im is the current when
V=Vm.
Conversion efficiency
• The conversion efficiency of a solar cell is defined as
the ratio of output electrical power to incident
optical power.
• For the maximum power output, we can write
Maximum power is given by
Typically, fill factor is between 0.7-0.8
Contd.
• The maximum efficiency of ‘Si’ solar cell is
25%.
• This is less efficient compared to cost and
area involved.
• Amorphous ‘Si’ thin-film solar cells can be
more cheaply but have only 10% efficiency.
• To increase efficiency, more light can be
focused by mirrors.
• Otherwise GaAs related compounds are use to
make the hetero junction cells for good
efficiency.
Contd.
• In 2016, researchers at Fraunhofer ISE announced
a GaInP/GaAs/Si triple-junction solar cell with two
terminals reaching 30.2% efficiency without
concentration.
• In 2017, a team of researchers at National
Renewable Energy Laboratory (NREL), EPFL and
CSEM (Switzerland) reported record one-sun
efficiencies of 32.8% for dual-junction GaInP/GaAs
solar cell devices.
• In addition, the dual-junction device was
mechanically stacked with a Si solar cell, to
achieve a record one-sun efficiency of 35.9% for
triple-junction solar cells.
Applications
▪ Arrays of p-n junction solar cells are currently used to
supply electrical power for many space satellites.
▪ Solar cells can supply for the electronic equipment
aboard a satellite over a long period of time, which is
a distinct advantage over batteries.
Applications
The roof, bonnet and large Dawn's 10 kW triple-junction
parts of the outer shell of Early solar-po
gallium arsenide solar array
the Sion are equipped with wered
at full extension
highly efficient calculator
monocrystalline silicon cells
Contd..
• Floating solar farms (aka ‘floatovoltaics’)
• Silicon panels are becoming cheaper and more efficient
day-by-day. According to experts, if photovoltaic panels are placed on
reservoirs and other water bodies, they offer even greater efficiency.
