QXU7027 (Renewable Energy Materials)
Transcript of QXU7027 (Renewable Energy Materials)
QXU7027 (Renewable Energy
Materials)
CZTS, CIGS & CdTe - simplified
Glass
Mo
p-CZTS
n-CdSZnOTCO (AZO)
Al
Illumination
0.5-1 μm
2 μm
50-70 nm
100 nm300 nm
Glass
Mo
p-CuInGaSe2
n-CdSZnOTCO (AZO)
Ni/Al
Illumination
0.5-1 μm
2 μm
50-70 nm
100 nm300 nm
Glass
p-CdTe
n-CdS
Metal
Illumination
3-5 μm
100 nm – 1 μm
50 nm
TCO (FTO) 300 nm
Tandem solar cell
Advantages
• Efficiency up to 47% (world record)
• Very few losses
• Harvest large portion of spectrum
• Disadvantages
• Very complex
• III-V materials: high purity
requirements
• High-precision growth – molecular
beam epitaxy (MBE) or metal-
organic chemical-vapour deposition
(MOCVD)
Thin Film PVs
Market data
World’s total PV production:>50GWp
Materials limitations
Materials limitations
Low-cost solar cells
Low-cost solar cells
Organic semiconductors
Polymers are traditionally
considered as insulators
Organic semiconductors
CH4-Methane (甲烷) SP3 hybridisation
Organic semiconductors
sp3
Ethane (乙烷)
σ bond
SP3 hybridisation
Organic semiconductors
Pi-bond: electron delocalisation
Ethene (乙烯)SP2Pz hybridisation
Organic semiconductors
What happens when you start to build up π bonds?
→ delocalisation
Benzene ring
Organic semiconductors
What happens when you start to build up π bonds?
→ delocalisation (conjugation = overlapping π orbitals)
Requirement: alternating single and double bonds-π bonds overlap across the σ bond
Polymers with alternating single and double bonds-delocalisation over long distance
conjugated polymers
Organic semiconductors
Delocalisation across polymer chains-Pi stacking
Organic semiconductors
• Conjugated polymers-alternating single and double bonds
• Highly versatile-easy to modify optical and electrical property by changing their
molecular structure
Organic semiconductors
Organic semiconductors
Highest Occupied
Molecular Orbital (HOMO)
Lowest Unoccupied
Molecular Orbital (LUMO)
σ energy gap 6-12 eV
π energy gap 1-3 eV
π delocalised bands
conjugation
Photoexcitation and Excitons
• The electron-hole pair in organic semiconductor is tightly bound
• Binding energy of ~ 0.4-0.5eV, called an “Exciton”
• An exciton cannot dissociate itself
• Excitons are highly localised, with a diffusion length of only ~10nm
BUT…
Excited
state
Organic Solar Cells-History
• Made by sandwiching a layer of
organic electronic materials
between two metallic conductors
• Difference of work function
between the two conductors sets
up an electric field in the organic
layer
• Didn’t work well-Efficiency well
below 1%-why?
• Internal field insufficient to
overcome the exciton binding
energy
• Most excitons get recombined
before being split and collected
at electrodes
• We need a better strategy to split
the excitons!
Organic Solar Cells-History
• Bilayer structure of two
organic materials with
different electron affinity and
ionization energies
• Sufficiently large driving
force to split excitons
• Donor/acceptor-
corresponding to p-type and
n-type semiconductor
• Efficiency slightly increased,
to ~1%
• Why still not working well?
• Excitons only have a
diffusion length of ~10nm
• Only excitons generated
within this range can be split
Electron acceptor
Electron donor
Organic Solar Cells-History
Bilayer structure
How do we solve this?
Organic Solar Cells-History
• Bulk heterojunction
structure of donor and
acceptor components with
nanoscale phase separation
• Excitons can reach an
donor:acceptor interface
and get split
• With careful materials and
device design this structure
can reach an efficiency as
high as 18%
Organic PVs
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.
Organic PVs
Organic PVs
OPV Other coating technologies
OPV R2R
Organic PV fabrication
• Wide range of techniques
• High speed (cm–m/s)
• Flexible, cheap & lightweight substrates
Organic PVs: Roll-to-Roll (R2R) processing
OPV-diversity
OPV-diversity
OPV Solar Park in Denmark
Research ForefrontSynthesis and processing of new materials
RegioRandom P3HT
RegioRegular P3HT
Research ForefrontSynthesis and processing of new materials
Research ForefrontIncrease charge separation: minimize phase separation
Research ForefrontIncrease charge transport: crystallization; interpenetrating
Stability Limitation
Stability Limitation
Typical Operating
Conditions:
•Solar light soaking
(~100mW/cm2 during
noon time in a sunny
day)
•-10oC-85oC
temperature with
heating/cooling cycles
(day and night)
•Oxygen exposure
•Humidity exposure
•Weathering conditions
(rain/snow etc)
•Mechanical stress
•…
Stability Limitation
Photochemical (chemical reaction of the photoactive materials in the presence of light and oxygen)
Stability Limitation
Morphological (changes in the blend morphology under elevated temperatures)
Stability Limitation
Morphological (changes in the blend morphology under elevated temperatures)
Stability Limitation
Organic PVs
• Solution processed: cheap, compatible with roll-to roll
• Very thin: 100-300 nm (very high absorption coefficient)
• Efficiency: 17.4% cell, 11.7% module
• Many different molecules = many different properties, e.g. band gap
• Earth-abundant materials: based on C
• Stability challenge
P3HT PCBM
Low-cost solar cells
Dye-sensitised solar cell
• Dye-sensitized solar cell (DSSC or Grätzel cell)
• Co-invented in 1988 by Brian O'Regan and Michael Grätzel
Dye-sensitised solar cell
Based on a semiconductor formed between a photo-sensitized anode
and an electrolyte, a photoelectrochemical system
Dye-sensitised solar cell
• Uses metal-organic dye to absorb sunlight
Dye-sensitised solar cell
• Photoexcitation in dye
• Excited electrons
transferred to wide-
bandgap ‘window’
material TiO2 (Bandgap
~3.2 eV)
• Electron circulate
through external circuit
• Electrolyte ‘regenerated’
• Dye ‘regenerated’
Dye-sensitised solar cell
I3- + 2e- 3I-
3I- I3-
The following steps convert in a conventional n-type DSSC photons (light)
to current:
• Incident photons absorbed by the photosensitizer (dye) adsorbed on
the TiO2 surface.
• The dye is excited from the ground state to the excited state.
• The excited electrons are injected into the conduction band of the TiO2
electrode. This results in the oxidation of the photosensitizer (S+).
• The injected electrons in the conduction band of TiO2 are transported
between TiO2 nanoparticles with diffusion toward the back contact
(TCO).
• The electrons finally reach the counter electrode through the circuit.
• The oxidized photosensitizer (S+) accepts electrons from the redox
mediator, typically I− ion redox mediator, leading to regeneration of the
ground state (S), and two I−-Ions are oxidized to elementary Iodine
which reacts with I− to the oxidized state, I3−.
• The oxidized redox mediator, I3−, diffuses toward the counter electrode
and then it is reduced to I− ions.
Dye-sensitised solar cell
Dye-sensitised solar cell
• Extremely poor charge transport in dye
• Monolayer needed (nm thickness)
• μm needed for light absorption
• => Add porous TiO2 film
• Extremely high surface area
Dye-sensitised solar cell
• Extremely poor charge transport in dye
• Monolayer needed (nm thickenss)
• μm needed for light absorption
• => Add porous TiO2 film
• Extremely high surface area
DSSC Tandems
• 2003: École Polytechnique Fédérale de Lausanne (EPFL) has reportedly
increased the thermos stability of DSSC by using a novel ruthenium-based
sensitizer in conjunction with quasi-solid-state gel electrolyte. The
stability of the device matches that of a conventional inorganic silicon-
based solar cell. The cell sustained heating for 1,000 h at 80 °C.
• 2006: The first successful solid-hybrid dye-sensitized solar cells were
reported.
• 2007: Massey University, New Zealand has experimented with a wide
variety of organic dyes based on porphyrin, a natural building block found
in nature for plants and animals
• 2011 Dyesol and Tata Steel Europe announced the development of the
world's largest dye sensitized photovoltaic module, printed onto steel in a
continuous line
• 2018 Researchers have investigated the role of surface plasmon
resonances present on gold nanorods in the performance of dye-
sensitized solar cells.
DSSC Recent developments
Dye-sensitised solar cells: summary
Advantages
• Solution processed, very low cost
• Compatible with roll-to-roll
• Efficiency 14% (cell), 8.8% (submodule)
• Many dyes – colour choice – BIPV
Disadvantages
• Stability – liquid leakage
• Pt shortage
• Competition from perovskites!
DSSC R2R
DSSCs
• UK company G24Power (former G24i)
• R2R production of DSSCs
Semicond. Sci. Technol. 26 (2011) 045007 doi:10.1088/0268-1242/26/4/045007
DSSC R2R
DSSCs
• UK company G24Power (formerly G24i)
• R2R production of DSSCs
• Commercial products:
Low-cost solar cells
‘Perovskite’ solar cells
‘Perovskite’ solar cells
‘Perovskite’ solar cells
• ‘Perovskite’ describes crystal structure
• More precisely ‘hybrid organic inorganic-lead-halide perovskite’
‘Perovskite’ solar cells
Excellent light absorption → <1 μm thickness
‘Perovskite’ solar cells
2009: 3.8% 2011: 6.5% 2012: 10.9% 2013: 15%
‘Perovskite’ solar cells: evolution
‘Perovskite’ solar cells: fabrication
Spin coating Co-evaporation
R2R coating
‘Perovskite’ solar cells: fabrication
‘Perovskite’ solar cells – key information
orTiO2
Glass
CH3NH3PbI3 (i)
TiO2 (or ZnO) (n)
Metal
Illumination
(superstrate)
300-800 nm
20-100 nm
50 nm
TCO (FTO) 300 nm
spiro-OMeTAD (p) 100 nm
N I P
3.2 eV
1.55 eV3.2 eV
‘Perovskite’ solar cells
• Tunable bandgap by changing composition
MA
FA
‘Perovskite’ solar cells
• Tunable bandgap by changing composition
• Could allow all-perovskite tandem device (perovskite on perovskite…)
• These have reached 25% efficiency – theoretically ~32%
MAPbCl3
2.88
MAPbBr3
2.3
MAPbI3
1.55
FAPbI3
1.5
MAPb0.5Sn0.5I3
1.17
MASnI3
1.3
MA
FA
‘Perovskite’ solar cells
• Tunable bandgap by changing composition
• Could allow all-perovskite tandem device (perovskite on perovskite…)
• These have reached 25% efficiency – theoretically ~32%
• Also could have added value for BIPV
‘Perovskite’ solar cells
31/01/2020: 29.1%
‘Perovskite’ solar cells
Oxford PV's industrial scale perovskite pilot
line, in Brandenburg an der Havel, Germany
• April 2018 secured £8.02 Million funding
‘Perovskite’ solar cells-Stability challenges
‘Perovskite’ solar cells-challenges
‘Perovskite’ solar cells-challenges
‘Perovskite’ solar cells-challenges
‘Perovskite’ solar cells: summary
• Perovskite structure, ABX3
• Standard CH3NH3PbI3
Advantages
• No rare elements → ‘earth abundant’
• Tunable bandgap (1.5-3 eV) by substitution: Br, Cl, Sn, …
• Very high absorption coefficient: <1 μm required
• Excellent transport properties (low recombination)
• 25.2% max efficiency (lab)
• Max module 16%
• Tandem with silicon → 30%?
Disadvantages
• Lead toxicity & control
• Unstable!
• Scale-up
Br content
Future application scenarios
Solar roof, up to 3000 km extra mileage Solar roof+walls, up to 11,000 km extra mileage
Vehicle Integrated
PV (VIPV)
Future application scenarios
Vehicle Integrated
PV (VIPV)
1000 mini-solar panels, add 50-65 km of daily mileage, ~ £135,000 in price
Future application scenarios
The Future • EVs powered by its paints
and windows
• Spray-coated thin film PV
integrated into the vehicle
• Solar powered mileage of
up to 10000 km per year
in UK
• Reduce CO2 emission by
up to 2 tons per year
• Fully 5G/IoTs integrated
• Lifetime guarantee (>10
years)
• Less than $600 of extra
cost per vehicle
Future application scenarios
Future application scenarios
Topic 3: Learning Outcomes
By the end of this topic you will be able to:
• Reproduce the elements of both the physical and band
structures of key PV technologies
• Critically compare the key photovoltaic technologies
including their methods for production and materials used
PV technologies - summary
Thick-film technology (1st gen)
- Indirect bandgap → thick (200-500 μm)
- Monocrystalline silicon
- Multicrystalline silicon
- Efficiency 20-25%
- ~90% commercial PVs
- Relatively high processing cost
Thin-film technology (2nd gen)
- Direct bandgap → thin (2-5 μm)
- CdTe
- CIGS (CuInSe2 with Ga)
- Efficiency 20-22%
- ~10% commercial PVs
- Materials issues
3rd generation – high efficiency
- Tandem cells
- Based on Ge or GaAs
- MBE or MOCVD growth
- 30-46% efficiency
- $50,000/m2!
→ concentrators
3rd generation – low cost (‘emerging’)
- CZTS (Cu2ZnSnS4) – 12.6% max
- Earth-abundant
- Phase-segregation & purity
- Organic PV (OPV) - 8-14% efficiency
- 100-300 nm, printable, cheap
- Dye-sensitised solar cell (DSSC)
- TiO2 ‘scaffold’; 10-14% efficiency
- Printable, but liquid leakage issues
- Perovskite solar cell – 15-23% efficiency
- Possibility of tandems (inc. with Si)
- Issues with Pb, stability
PV technologies - quiz
1 2 3
4 56
7 89 10
PV technologies
Development of Global PV market
China’s leadership of Global PV market
China’s leadership of Global PV market
China’s leadership of Global PV market
China’s leadership of Global PV market