10/15/12
1
ECE304
4) Optoelectronic Devices:
Photodetectors
Solar cells
Optoelectronics
Optoelectronics is the study and application of electronic devices that serve to
source, detect, and manipulate light; it is considered a sub-field of photonics.
Optoelectronics is based on quantum mechanical effects of light and its interactions
with semiconductors, sometimes in the presence of electric fields.
Optoelectronic devices are electrical-to-optical (E-to-O) or optical-to-electrical
(O-to-E) transducers.
If the purpose is to detect a light signal, the device is called a photodetector.
If the purpose is to generate electrical power, the device is called a solar cell.
O-to-E: A photodiode is simply a pn junction that permits light to be absorbed to
generate photo-carriers
E-to-O: A diode which creates photons from injected charge carriers.
Spontaneous emission comes from LED.
Stimulated emission comes from laser diode.
10/15/12
2
Light absorption at p-n junction (review)
A photodiode is simply a pn junction that permits light to be absorbed to
generate a photo-current. Often this is a p-i-n junction, where the intrinsic region is
inserted to increase the number of photons absorbed. (Why?)
A solar cell is simply a broad area pn junction the absorbs light to generate photo-
voltaic potential.
A photon can be absorbed at a reversed biased junction:
E
C
E
V
E
i
E
Fn
E
Fp
reverse bias
create an electron and a hole
each of which is swept out of the depletion region
Photodiodes
Incident light hits the active area of the device, and some fraction is absorbed.
The fraction of the light that is absorbed is given by
where ! is the absorption coefficient (recall from past!), d is the detector thickness,
and R is the reflection coefficient of the semiconductor (recall from past!).
photodiode
The photons create EHPs that become a current which is proportional to the
incident light intensity.
!
abs
=
P
abs
P
incident
= 1" R
( )
1" e
"#d
( )
10/15/12
3
Detector photocurrent
Absorbed photons create EHPs, which are swept to the terminals;
we usually only consider current from electrons (holes have much
lower mobility). The photocurrent i
ph
is given by:
i
ph
= P
incident
!
q
h"
The diode equation is modified with an extra term to account for photo-generated
current, i
ph
i = i
o
exp
eV
k
B
T
!
"
#
$
%
&
' 1
(
)
*
*
+
,
-
-
' i
ph
where " is the efficiency with which incident photons are converted into electrons.
Hence we can define the detector responsivity:
R !
i
ph
P
incident
"
q#
abs
h$
=
q#
abs
hc
%
! " !
abs
(if we assume no absorbed photons heat up the detector)
Responsivity
Highly Sensitive Optical Receivers, (The Netherlands: Springer-Verlag Berlin Heidelberg, 2006).
The responsivity tells us the conversion of input absorbed power to photocurrent.
Different semiconductors have different responsivity.
What limits the longer wavelength?
Bandgap energy!
What limits the shorter wavelength?
Occupation of electrons in bands
10/15/12
4
pin example
direction
direction
Photodiode modes of operation
Let light intensity # (0 < #
1
< #
2
)
photodiode
Short circuit
(photoconductive)
Open circuit
(photovoltaic)
change in photocurrent change in photovoltage
10/15/12
5
Impact ionization
Carriers can also be added to the conduction and valence band from:
impact ionization (reverse bias breakdown)
impact ionization in a large E-field:
electron gains kinetic energy in field;
collides with atom in lattice,
dislodges another electron (and creates a hole)
Impact ionization creates a new EHP
now 3 carriers are accelerated
and process can avalanche
E
C
E
V
Avalanche photodiodes
Avalanche breakdown
Apply reverse bias near to breakdown
voltage, V
BR
: absorption of one photon
creates many electrons
E
C
E
V
E
i
E
Fn
E
Fp
The limitation of p-i-n photodetectors is the lack of gain; that is, only one electron
hole pair is generated per absorbed photon.
Low-light applications require detectors with high photoconductive gain to boost the
signal above the noise floor: avalanche photo-detectors
Gain:
G =
I
I
o
10/15/12
6
Total world power demand/year ~ 15 TW
Sun output/year ~ 120,000 TW
Covering 0.125 % of earths surface with 10% efficient solar cells would produce
enough energy to supply the annual global demand.
(This is about the size of Texas)
Solar power
solar cell
Solar cells
Photo-generated carriers generated within 1 diffusion length from the edge of the
depletion region, will be accerated by the junction electric fields toward the majority
carrier regions, thus creating photocurrent.
E
C
E
V
E
i
E
Fn
E
Fp
Often it is not convenient to shine light on pn junctions. So instead we absorb
light from the top:
p-type
n-type
10/15/12
7
Solar cells (cont.)
A good design for solar cell*:
1) choose top layer thickness to be 1/!
(which is probably >> depletion width)
2) top semiconductor should have long
minority carrier diffusion length:
minority carrier
diffusion coefficient
p-type
n-type
minority carrier
lifetime
why do we care about
the minority carriers?
How can we have long minority carrier lifetimes?
*this design invented by Gerald Pearson at Bell Labs
Solar cells (cont.)
A solar cell has large area to absorb as
many photons (at as many wavelengths)
as possible.
Large area amorphous Si solar cells
typically have < 20% efficiency.
Laboratory record is 40% and uses multi-
junction compound semiconductors.
Why would having multiple bandgaps
produce higher efficiency?
10/15/12
8
Solar cell absorption
The efficiency of a solar cell is
related to its electrical properties
and its absorption spectrum.
To absorb a sun photon, we need to overlap the solar spectrum.
What limits GaAs absorption at long wavelength?
What limits GaAs absorption at short wavelength?
How can we improve the absorption of solar radiation?
Bandgap energy
Reflectivity and absorption increases
Solar cell absorption (cont.)
Use a material with bandgap wavelength of 0.7 !"# %&' ()*+
Use a material with bandgap wavelength of 1.6 !"# %&' ()*+
Or how about using 3 bandgaps instead?
10/15/12
9
Diode J vs V
Light intensity 0 < #
1
< #
2
J
V
Solar cell photocurrent
The photocurrent of a solar cell is proportional to the area; so we naturally consider
current density, J (units of A/area).
We need the short circuit current (J
SC
) and open circuit voltage (V
OC
) (more on this in
HW5). Recall what we know about diode:
J V
( )
= J
SC
! J
dark
V
( )
= J
SC
! J
o
e
qV
kT
! 1
"
#
$
%
&
'
The short-circuit photocurrent of a
solar cell is defined to be positive
(more light means more photocurrent)
so for an ideal diode:
J
SC
V
OC
J
V
J
SC
V
OC
Solar cell photocurrent (cont.)
Lets focus on the lower right hand quadrant of our J vs V curve,and flip over :
Diode J vs V
Light intensity 0 < #
1
< #
2
J
V
Which for an optimized solar cell becomes:
10/15/12
10
Solar cell photocurrent (cont.)
Solar cell J vs V
Diode J vs V
Light intensity 0 < #
1
< #
2
J
SC
V
OC
The operating bias range of solar cell is
0 volts (short circuit) to V
OC
.
Solar cell efficiency
Solar cell efficiency:
! "
P
max
P
incident
=
J
max
V
max
P
incident
#
J
sc
V
oc
P
incident
Note that J
max
< J
SC
and V
max
< V
oc
.
So the squareness of the J-V curve of a solar cell is a key performance characteristic.
The cell power density is
and the maximum power is
P
max
= J
max
V
max
P = JV
10/15/12
11
What is future of solar cells?
Gut Erlasee solar farm in Germany
Replace corn and soybeans?
Solar mustang of the future?
What happens if a cloud comes along?
Detector summary
Photodiodes: p/n junction in reverse bias that absorbs light
photoconductive mode (short circuit)
photovoltaic mode (open circuit)
Photodetectorsused to measure amount of light incident
Solar cellsused to generate current or voltage from sunlight
PiN photodiodes have a linear responsivity (depends on material)
Avalanche photodiodes have gain (more electrons generated/incident photon)
Efficient solar cells require: large area
long minority carrier lifetimes
high efficiency (square J
max
V
max
)
maximum spectral overlap with solar radiation
