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

Low-cost solar cells

Organic semiconductors

Polymers are traditionally

considered as insulators


CH4-Methane (甲烷) SP3 hybridisation


sp3

Ethane （乙烷）

σ bond

SP3 hybridisation


Pi-bond: electron delocalisation

Ethene （乙烯）SP2Pz hybridisation


What happens when you start to build up π bonds?

→ delocalisation

Benzene ring


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


Delocalisation across polymer chains-Pi stacking


• Conjugated polymers-alternating single and double bonds

• Highly versatile-easy to modify optical and electrical property by changing their

molecular structure


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!


• 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


Bilayer structure

How do we solve this?


• 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 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


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

•…

Photochemical (chemical reaction of the photoactive materials in the presence of light and oxygen)


Morphological (changes in the blend morphology under elevated temperatures)


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

Dye-sensitised solar cell

• Dye-sensitized solar cell (DSSC or Grätzel cell)

• Co-invented in 1988 by Brian O'Regan and Michael Grätzel


Based on a semiconductor formed between a photo-sensitized anode

and an electrolyte, a photoelectrochemical system


• Uses metal-organic dye to absorb sunlight


• 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’


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.



• Extremely poor charge transport in dye

• Monolayer needed (nm thickness)

• μm needed for light absorption

• => Add porous TiO2 film

• Extremely high surface area


• 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:

‘Perovskite’ solar cells


• ‘Perovskite’ describes crystal structure

• More precisely ‘hybrid organic inorganic-lead-halide perovskite’


Excellent light absorption → <1 μm thickness


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


• Tunable bandgap by changing composition

MA

FA



• 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



• Could allow all-perovskite tandem device (perovskite on perovskite…)

• These have reached 25% efficiency – theoretically ~32%

• Also could have added value for BIPV


31/01/2020: 29.1%


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: 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)


Vehicle Integrated

PV (VIPV)

1000 mini-solar panels, add 50-65 km of daily mileage, ~ £135,000 in price


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


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

qxu7027 (renewable energy materials)

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