Solar MATERIALS used in. Photovoltaic ce

Material used in solar cell

1. Solar
Materials

8. What Do you think some challenges with solar energy
might be?
Storage
They don’t
work at
night or in
cloudy
weather
incoming solar radiation
1000 watts/sqmeter
800 W heat200 W electricity
They don’t use all the
Sun’s energy
the sun isn’t always shining; -the sun changes position in the sky throughout the day
-how to use all the sun’s light; -making durable solar panels

9. Solar energy & Solar cells
protective layer
n-layer
junction
p-layer
usually glass
semiconductors
-Solar cell is made of 3 parts:
1)Protective layer –usually glass, must be
transparent
2)n-layer –semiconductor rich in electrons
3)p-layer –semiconductor deficient in
electrons
Junction represents where n-and p-layers
touch

10. There are 3 ways a material can interact with light:
1)Reflection where light bounces off the material
2)Transmission where light goes through the material
3)Absorption where light interacts with the material
When light of the right energy (the energy between the ground and excited states) hits a
material, it transfers energy to an electron (absorption). We can use this extra energy to
produce electricity. When absorption of light happens in a material, it’s called the
photovoltaic effect

11. Sunlight
(solar energy)
Photovoltaic
effect
Electricity
How do solar cells work?
Solar cells
transform
sunlight into
electricity
reflection
light
absorption
e
-
this electron
now has more
energy
When this
happens inside a
material, it is
called the
photovoltaic
effect
transmission
absorption
e
-
e
-
e
-
e
-
ground state
excited state

12. n-layer
junction
p-layer
e
-
e
-
e
-
+++
How do solar cells work?
n-layer
junction
p-layer
e
-
e
-
e
-
e
-
e
-
1)Sunlight is absorbed and excited an electron from the p-layer to the n-layer
2)The extra electrons in the n-layer move through the circuit to recombine with the
positive charges left in the p-layer, completing the circuit

15. What are solar cells made from?
P-doped Si
junction
B-or Ga-doped Si
protection layer
Most common material = silicon
Amorphous -13.4%Polycrystalline -20%Single crystal -26%
First developed for the space program
Requires thick layers of Si →relatively expensive
Long lifetimes –very stable
Highest efficiencies of any type of solar cell
Rigid & brittle →limits potential applications

16. Key challenge: Improving efficiency
Efficiency =
amount of electricity produced
amount of sunlight received
incoming solar radiation
1000 watts/sqmeter
800 W heat200 W electricity
New solar materials ideally need
to be:
—Efficient
—Inexpensive
—Abundant materials
—Non-polluting / non-toxic

17. New materials to replace Si
Maximum efficiency
Perovskites
Quantum dots
Organic
cells
Dye-sensitized cells
Single crystal Si
26.1%23.7%16.6%15.6%11.9%

18. Perovskites:materials with a specific structure called ABX
3
Methyl ammonium lead triiodide
perovskite. Image from NREL.
Image from Solliance.
Advantages
—Maximum efficiency = 23.7%
—Variable band gaps
→can be designed for specific applications
—Very efficient absorber of high-energy light
→can be combined with other low-energy
absorbers
Disadvantages
—Most use lead = extremelytoxic
—Poor stability
Perovskite refers to materials with a specific structure
-usually made of lead, a halide (Cl, Br, I), and a
positively charged atom or molecule
-poor stability: do not react well to moisture, high heat,
or extended irradiation

19. Quantum Dots
Advantages
—Band gaps change with QD size
→can be designed for specific
applications
—small size means good power to
weight ratio
Disadvantages
—Most use cadmium or lead
= extremelytoxic
—Degrades when exposed to water
and UV light
Image via Wikimedia Commons.
2 nm 3 nm
4 nm 5 nm 6 nm
QDs are tiny particles only a
few nanometers wide
Image via University of Rochester.

20. e
-
TiO
2
dye Pt
Dye-sensitized solar cells
electrolyte
e
-
e
-
e
-
e
-
e
-
e
-
e
-e
-
e
-
Advantages
—Easy to make
—Semi-flexible and semi-transparent
—Work in low-light
→potentially could be used indoors
Disadvantages
—Low efficiencies (so far)
—Requires expensive materials like Pt
—Uses liquids
→makes it difficult to use in all weather
DSSCs are made of three parts:
dye, TiO
2, and liquid electrolyte
light
examples of dyes

21. Dye-sensitized solar cells
-How a DSSC works:
1) Sunlight is absorbed by the dye molecules
2) The sunlight gives energy to electrons which move from the dye to the TiO2
3)The electrons move between TiO2 molecules and through the circuit, reaching
the Pt electrode
4)The electrons are then transferred to the electrolyte which transfers them
back to the dye molecules, completing the circuit
-easy to make
-semi-flexible & semi-transparent
-also fairly robust –performs better at higher temperatures
-uses expensive materials like Pt for counter electrode or Ru for dye
-requires liquid component →makes it difficult to use in all weather

22. Organic PV Cells
Image via BBC.
Advantages
—Flexible!
→can be deposited on different
materials
—Many possible combinations
—Inexpensive to produce
Disadvantages
—Low efficiency (at least so far)
—Not very stable
→no effective protective coatings yet
OPVs can be made of any organic (carbon-
containing) molecule that absorbs light and
can donate/accept electrons
Image via TCI America.

27. Chlorophyll
Organic semiconductors
Tang, Appl. Phys. Lett. 48 (1986) 183.
Cu phthalocyanine
Meiss et al., Adv. Funct. Mater., 22 (2012) 405.
•Earth abundant elements
(cheap, non-toxic)
•Strong absorbers
•Tuneable properties
(optical, electronic, processing)
‘Dial-a -semiconductor’

28. Organic semiconductors -a world of coulombic interactions
Coulomb’s Law: +q -q
r
E
g
LUMO
HOMO
Molecular (VdW) Solid
Hopping transport r
qq
E
r

0
21

Isolated Molecule
Vacuum Level
Molecular Orbital (MO)
Core Levels (AOs)
Nuclei
Energy
Vacuum Level –just outside solid surface where electron is at rest.
HOMO = Highest occupied MO; LUMO = Lowest unoccupied MO

29. •Low charge carrier mobility →large electric field (F) needed for extraction.
• so for low film thickness (d), can easily achieve required F.
•>100×thinner than c-Si
•Advantages associated with ‘thin film’.
•Positive temperature coefficient.
d 200 nm
Organic photovoltaics (OPV) –device architectured
V
F=
*Figure from http://www.easac.eu/fileadmin/docs/Low_Carbon/KVA_workshop/Renewables/2013_09_Easac_Stockholm_Leo.pdf
*

30. It’s good to be flexible
Why is flexibility important?
•Compatibility with roll-to-roll fabrication (rapid fabrication →low cost)
•Compatible with light weight substrates (i.e. plastics) –typically flexible.
•New applications possible (e.g. integration with fabrics).
•Weak intermolecular interactions in molecular solids impart flexibility.

31. +
-
•Mott-Wannier exciton
•
r =10-15
•Binding energy < 25 meV
Excitations in molecular semiconductors Excitons
= lattice site
Crystalline inorganic
semiconductor
+
-
•Frenkelexciton
•
r =2-4
•Binding energy > 0.2 eV
Molecular
semiconductor
Exciton diffusion
length 10 nm
Architecture must: (1) split excitons; (2) Overcome diffusion length limitation.

32. Splitting excitons into free electrons and holes
•Efficient exciton dissociation can be achieved at an organic heterojunction
Electron acceptor
Electron donor
Tang, Appl. Phys. Lett. 48 (1986) 183.
S +H

33. SubPc
C
60
-3.4
-4.2
-5.5
-6.2
Splitting excitons into free charge carriers
•Rapid photo-induced electron transfer (100 fs), long lived charge separated state (ms).
e
-e
-
SubPc C
60
SubPc
F
6
-
SubPc
-3.6
-5.8
F
6-SubPc
e
-
Sullivan, et al., Advanced Energy Materials (2011) 1, 352–355.
•From donor to acceptor, by design:

34. Summary of fundamental processes
1.Light absorption to form an exciton.
2.Exciton diffusion to the heterojunction.
3.Exciton dissociation at the organic heterojunction.
4.Charge carrier transport to electrodes.
5.Charge carrier extraction.

35. •Exciton diffusion length in organic semiconductors 10 nm.
•Photoactive layer must be structured to accommodate this:
Donor
Acceptor
Transparent Electrode
Opaque electrode
The exciton diffusion bottleneck
Bi-layer
Thin film (high electric field)
Organic hetero-junction (split excitons)
Interpenetrating heterojunction (overcome limitation in exciton diffusion length)
Bulk-heterojunction (BHJ)

36. •High purity
•High degree of control over layer thickness (0.1 nm)
•Multi-layer architectures possible
•High capital outlay
•Energy intensive (high vacuum (10
-6
mbar) needed)
Processing (vacuum processing)
Solar
Simulator
Solution
Processing
AreaEvaporation
Chamber

37. Spin coating Spray deposition Printing
Spin-coating
(wasteful)
minimal loss & scalable to large area
Processing (solution processing)
•Low cost equipment.
•Low embodied energy (no vacuum required) / short payback time.
•Difficult to make bilayer architectures.
•Simple to make BHJ (spontaneous donor/acceptor phase separation).
* Picture from Advancing spray deposition for low-cost solar cell production K. Xerxes Steirer, et al., 25March2009, SPIE Newsroom. DOI: 10.1117/2.1200903.1555
*

38. Solution processable organic semiconductors
•Chemical modification:
Alternating co-polymer
E
g
E
g
Homo-polymer
•Semi-conducting polymers:

39. Challenges: Improving efficiency
•Semiconductors that can be processed from non-toxic solvents.
•Materials amenable to rapid processing.
Donor
Acceptor
•Narrow band gap organic semiconductors.
MaximumV
oc

40. 6
5
4
3
2
1
0
Voc/ V,
PCE/%
605550454035302520151050
Time/ d
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Jsc/ mA cm-2,
FF
Burn-in time ~6d
1.01 mA cm-2, 5.16 V, 0.52, 2.67 % Photograph by P. Sullivan, University of Warwick.
Challenges: improving stability
•Photo-stability of organic semiconductors
(Materials for OLEDS > 1 millions hours lifetime)
•Interface stability (delamination at soft contacts)
•Blocking water/oxygen ingress
(particularly challenging on flexible substrates)

41. Challenges: Reducing materials cost
•Need for low cost, transparent, flexible electrode.
* Photograph from Hatton research group, University of Warwick.

42. Concluding remarks
OPVs are an emerging thin film PV technology which is
potentially verylow cost and compatible with flexiblesubstrates.
OPVs are fundamentally different from c-Si PV in the mode of
operation and device architecture.
The challenges in this field of research are multi-faceted and
inherently interdisciplinary.
* Photograph by R. da Campo, Molecular Solar Ltd.