Lecture 3
Engineering of Solar PV cells and new
technology developments
Motion: Electrons and Holes
Photovoltaic Device Fundamentals
(1) Charge Generation: Light
excites electrons, freeing them
to move around the crystal.
(2) Charge Separation: An electric
field engineered into the material
(pn junction) sweeps out electrons.
(3) Charge Collection: Electrons
deposit their energy in an
external load, complete the
circuit.
http://www.pveducation.org/
Solar spectrum and Si absorption
Charge Generation by Light
7OKPQRST
e- (electron) e-
7JK = ℎM
7N
h+ (hole) h+
• 7JK < 7N : transparent to the incident light.
• 7JK ≥ 7N : photons are absorbed and electron-hole pairs are generated.
• 7JK ≫ 7N : electron-hole pairs are generated and some energy is lost as heat.
Absorption coefficient
Material Symbol Band gap (eV) @ 302 K
Diamond C 5.5
Silicon Si 1.11
Germanium Ge 0.67
Gallium nitride GaN 3.4
Gallium phosphide GaP 2.26
Gallium arsenide GaAs 1.43
Silicon nitride Si3N4 5
Lead sulfide PbS 0.37
Silicon dioxide SiO2 9
Copper oxide Cu2O 2.1
Solar Cell: PN Junction Diode
Diode is a two-terminal electronic
component that conducts
primarily in one direction
(asymmetric conductance); it has
low resistance to the current in
one direction, and high
resistance in the other.
Doping
Doping
Band diagram
P N
Fermi level
7N
PN Junction
! = ./
e- energy
x
Band diagram – Equilibrium
P space charge
N
Surface chargeregion
region
7N
Fermi level
PN Junction – with Bias
Forward Bias Reverse Bias
PN Junction Diode
No Bias Forward Bias Reverse Bias
Circuit - +
P - + N
- +
E
Band
Diagram
x
e- diffusion:
e- drift:
I
I-V Curve
V
PN Junction – under illumination
e-
PN Junction – under illumination
I-V Curve of solar cell I-V Curve of solar cell
Questions: What is the relationship between ISC and VOC with incident light intensity?
https://www.pveducation.org/
Fill Factor (FF)
The Fill Factor (FF) is essentially a measure of quality of the
solar cell. It is calculated by comparing the maximum power
to the theoretical power (PT) that would be output at both the
open circuit voltage and short circuit current together.
Power Conversion
/ 2
5# 5#Efficiency
33 =
/67 287
06:; /5# 1 25# 33 1 /67 1 287
Power Conversion Efficiency 9= = =
0<= 0<= 0<=
Questions: FF is typically below 100% - what is the implication of that? What is the cause
06:; for/imperfect
5# 1 25# 33 1 Is
FF? /67it1possible
287 to get FF of 100%?
9= = =
0<= 0<= 0<=
Ideal p-n junction PV cell: The Shockley-Queisser limit
Ideal solar PV cell: The Shockley-Queisser limit
Key assumptions in SQ limit:
Note: c-Si has indirect bandgap, shallow absorption edge
In contrast, direct gap materials (GaAs, InP) have step-like abs
In practise, two key physical limitations lower PV efficiency compared to (ideal) SQ limit
1. Imperfect light absorption for photon energies at/below bandgap (last slide)
2. Non-radiative recombination process of charge carriers
Non-rad Non-rad
When charges (e- and h+) recombine, the charges loss their electrical energy. This energy loss
(recombination) process is either:
1) Radiative (electrical energy converted to photons)
2) Non-radiative (electrical energy converted to heat through phonons)
Question: if charge energy is lost during radiative recombination as well, shouldn’t a perfect solar cell
have NEITHER radiative/non-rad recombination?
Instead, SQ limit says that all recombination should be radiative.
Principle of detailed balance:
At thermodynamic equilibrium each elementary process is in equilibrium with its reverse process.
Corresponding to every individual process there is a reverse process, and in a state of equilibrium the average
rate of every process is equal to the average rate of its reverse process
PV cell therefore exchanges energy with the Sun (using photons), so it must be absorbing and emitting photons
at the same time (radiative recombination). At the same time, useful electrical energy is delivered to the external
circuit for work.
Ideal PV cell (SQ limit), only requires the current source and the
diode (the red photon represents radiative recombination with
the current Jem).
Real solar cells are typically described by the addition of a
second diode, representing non-radiative recombination with
current Jnonr (indicated by the blue springs, representing heat
dissipation), a parallel or shunt resistance Rp and a series
resistance Rs.
This means that, rather confusingly, an efficient PV cell must behave as an efficient LED!
Voltage loss due to non-radiative charge recombination:
where Qe,lum is the external electroluminescence (EL) quantum efficiency
• Defects in the PV device leads to drop in
electroluminescence (EL) intensity
• where EL is a measure of the probability of
charge recombination events leading to
light emission
• This is the reason why electroluminescence
(EL) imaging is a common technique used in
industry to check PV panel quality – e.g.
image microcracks
• The more microcracks (dark regions, low
EL), the poorer the PV efficiency
EL microcrack imaging
https://www.youtube.com/watch?v=alQFVKYLwT0
Detailed thermodynamic analysis of PV energy conversion SQ limit assumptions
Ø No absorption below bandgap
Ø Perfect absorption above bandgap,
but energy drop down to Egap via
thermalization
Question: Is it possible to design PV cells with efficiencies beyond the SQ limit?
Lecture 3.2
Emerging and future solar PV technologies
Future PV technologies
Current commercial PV cells are limited by:
• Rigid, opaque, heavy
• Require high temperature processing (~600-1000oC)
• High cost?
• Not eco-friendly
Some desirable properties for future PV cells:
Building-integrated PV
1. Tunable optical gap, semitransparent (e.g. for BIPV)
2. Flexible and lightweight (e.g. for VIPV/BIPV)
3. Low fabrication cost, from solution (printing)
4. No negative environmental impacts (free of toxic
materials, no CO2 emission)
5. Can integrate with existing silicon technology
(including manufacturing)
Vehicle-integrated PV
Energies. 12. 1080. 10.3390/en12061080
Vehicle-integrated PV
BIPV market analysis
New PV materials à New opportunities!
Tandem PV
Close to SQ limit
(GaAS PV)
Perovskite PV
~25%
OPV ~18%
DSSC ~13%
Organic PV
Perovskite PV
Carbon is nature’s material choice
Can organic (carbon-based) materials conduct electricity? Usually
insulating materials (plastics, rubber)
à Yes! Thanks to alternative sigma (C-C) and pi (C=C) bonds,
electrons are able to delocalize and conduct electricity
(although low mobility).
à Strong electron-vibration couplings lead to splitting of energy
levels, forming semiconductors (OLED is perfect example)
à the splitting (energy gap) is easily tuned by modifying chemical
structures (endless possibilities)
Important concept: excitons
In inorganic semiconductors (such as Si or GaAs), electrons and holes can separate spontaneously (free carriers).
In organic, however, due to strong vibronic interactions and low dielectric screening, electrons/holes are bound as excitons
Overcoming the exciton binding energy is needed to extract photocurrent.
In OPV, a mix of electron-donating
(donor; p-type) and accepting (acceptor;
n-type) materials are used to separate
excitons into free charges.
à analogous to p-n junction
In the past (before 2015), poor efficiency typically below 12%
But major improvement recently à now ~18%
Donor/acceptor blend
~100nm thick
Active layer ~100nm, but can already absorb same light of a few µm-thick silicon
A successful story: Organic LED
If we reverse the operation of PV, we get an LED.
Organic materials are very good light emitters thanks to the excitonic properties (although it seems to be hindering PV
performance)
Many people didn’t believe OPV can achieve above ~10%, now it is ~18% and still rising… very surprising.
But remember from detailed balance, an efficient LED material should behave also as an efficient PV material… so perhaps
not so surprising after all. But the exact operation mechanism of ~18% OPV is still under intense research.
Energy gap tunability of organic materials is a huge advantage for BIPV
• Tune absorption via chemical design
• Selectively absorb UV and NIR light,
but letting Vis light through
Nature Energy 2, 849–860 (2017)
Metal Halide Perovskites
Perovskite is a crystal structure with the formula ABX3
• Named after Russian mineralogist L. A. Perovski (1792–1856)
• Many are insulating oxides, found in minerals (e.g. CaTiO3)
In 2009, researchers first discovered that methylammonium lead halide perovskites
show promising solar cell efficiency (~4%). Since then rapid progress has been made
and now perovskite solar cells can achieve ~25% efficiency.
Attractive:
• Solution processable (printed)
• Thin-film technology (~500-1000 nm)
• Flexible, lightweight
Downside:
• Contains toxic lead
• Not stable in air and humid conditions
New Tandem strategies to improve PV efficiency
Science 2019
Luminescent solar concentrator (LSC) for colourful BIPV windows
Key advantages:
• No need to make large area/transparent
PV panels, which is challenging
• Instead, use existing c-Si panels on the
edge of the window, and use LSC
materials to guide some of the sunlight
to the edges for energy harvesting
• Highly scalable technology
The future is bright for PV technology
Driven by new scientific and engineering discoveries
• New functionalities (e.g. flexible, transparent, printed and lightweight PV)
• Novel sunlight energy harvesting strategies for better efficiency
