Nanostructured and molecular materials for solar energy conversion - - PowerPoint PPT Presentation

Nanostructured and molecular materials for solar energy conversion

Jenny Nelson Department of Physics, Centre for Plastic Electronics and Grantham Institute for Climate Change, Imperial College London THE ROYAL SOCIETY New Fellows Seminar 9-10 July 2014

Printable photovoltaics

contacts active layer Current and voltage output flexible substrate barrier coating Light contacts active layer Current and voltage output flexible substrate barrier coating Light contacts active layer Current and voltage output flexible substrate barrier coating Light

• Variety of materials
• Process from solution
• “One pot, one shot” active layer
• Large area
• High throughput
• Printing or coating
• Conformal
• Lightweight
• Cheap

Why print photovoltaics?

• Minimise production costs
• New product forms
• Potential for innovation in

manufacturing

• Reduce carbon embedded in

manufacture

Chris Emmott et al. Environ. Sci. Tech. (2014)

• 30
• 20
• 10

10 2000 2005 2010 2015 Cumulative CO2 Emissions (MtCO2eq)

Year c-Si, manufactired in China c-Si, if manufactured in Europe CdTe OPV

Model of German PV deployment

5 000 10 000 15 000 20 000 25 000 30 000 35 000 40 000 45 000 2009 2020 2030 2040 2050 Other Wind Solar Hydro Nuclear Biomass and was Oil Gas with CCS Gas Coal with CCS Coal

IEA Energy Technology Perspectives 2012: Global electricity generation in the 2DS

Solution processable photovoltaic materials

2001- Organic (polymer:C60) η ~ 10% Perovskite 2012- η ~ 16% Other new materials and new processes ... 1990- Dye sensitised η ~ 12%

e

• e
• e
• e
• e
• TiO2

Particle slurry CZTS 2010- η ~ 12% 2007- Organic tandem η ~ 11%

UCLA

conjugated polymer

1 nm

Molecular electronic materials

conjugated molecule

• Electronic properties:
• Excited states and charged states are localised
• Electronic states are disordered
• Low relative permittivity εr
• Charge transport is slow
• Charge pairs hard to separate

Photovoltaic energy conversion in molecular materials

donor acceptor HOMO HOMO LUMO LUMO

e- Photovoltage limited by electrical gap ECS(< optical gap Eg)

2

Evac Wa Wc

contacts active layer (~100 nm) flexible substrate barrier coating Light

Separate charges by adding a strong electron acceptor Distributed heterojunction charge separation over a large optical depth Photocurrent direction provided by asymmetric contacts

Materials development

2.5% (2001) 4.4% (2005) 5.5% (2007) 6% (2009) 9.2% (2011)

Schaarber et al., Adv. Mater 2006

2.00 3.00 4.00 5.00 6.00 7.00 8.00 2.00 1.00 9.00 10.00 11.00

2.8 2.4 2.0 1.6 1.2

• 3.0
• 3.2
• 3.4
• 3.6
• 3.8
• 4.0

T

Band Gap [ eV ] LUMO Level Donor [ eV ]

Theoretical limit ?

• ECS

∆

eVoc

Sources of loss in organic photovoltaic heterojunctions

How much do we pay for charge separation? How much do we pay for charge recombination?

+ -

Exciton Charge pair Current

FLUX

• +

POTENTIAL

Absorber Interface Circuit S1

D

ECS eVOC ECT ∆ ∆ ∆ ∆EC ∆ ∆ ∆ ∆ER

Probing charge separation

• Probe the energy of intermediate

state using electroluminescence

Polymer, Fullerene

ECT +

• S1

D

ECS eVOC ECT ∆ ∆ ∆ ∆EC ∆ ∆ ∆ ∆ER

• Probe the yield of charge pairs

using transient spectroscopy

pump pulse Probe light Sample Detector t I(l, t)

Probing charge separation

• Influenced by

– Specific chemical structure and alignment – Molecular packing close to interface – Competition with other excited states

S1

D

ECS eVOC ECT ∆ ∆ ∆ ∆EC ∆ ∆ ∆ ∆ER

Normally > 0.3 eV Limiting efficiency < 20%

Sources of loss in organic photovoltaic heterojunctions

How much do we pay for charge separation? How much do we pay for charge recombination?

+ -

Exciton Charge pair Current

FLUX

• +

POTENTIAL

Absorber Interface Circuit S1

D

ECS eVOC ECT ∆ ∆ ∆ ∆EC ∆ ∆ ∆ ∆ER

50 100 150

• 1.0
• 0.5

0.0 0.5 1.0 energy E [eV] position x [nm]

Nature of charge recombination

• Electronic state energies are disordered
• Recombination occurs between free and

trapped charges

• density dependent mobility and

lifetime

• Intensity dependent PV performance

0.0 0.2 0.4 0.6 0.8 1.0 energy E [eV]

• 

       ∝

ch

E E DOS exp 12

R G e

n

− = ⋅ ∇ − J 1

Understanding disorder is critical

Example: Effect of fullerene structure on charge collection

• M. Lenes et al., Adv. Funct. Mater. (2009); M. Faist et al., J. Pol. Sci.., (2010)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

• 10
• 5

Current density (mA cm

• 2)

Voltage (V)

Fullerene multi-adducts Reduce photocurrent

Mono Bis , Tris 10

• 6

10

• 5

10

• 4

10

• 3

10

• 2

10

• 1

FET electron mobility / cm

2 V

• 1s
• 1

Reduce mobility Energetic disorder? Packing disorder? Why?

Example: Modelling effect of fullerene structure

Coarse grain

• F. Steiner, J. M. Frost et al (2014)

Representative structures Electronic coupling & transport Distinguish effects

Packing disorder Experiment Packing and energetic disorder

Where do we go from here?

• Solar electricity is abundant, sustainable, versatile

and available

• To accelerate its use, cheaper materials or

technologies are needed

• Nanostructured and molecular materials offer

potential for radically different and cheaper solar- electric conversion technologies.

• Challenges remain for physicists, chemists and

materials scientists – but none of them known to be insurmountable

Thanks to:

PhD students: Carol Olson, Dmitry Poplavskyy, Mili Eppler, P. Ravirajan, Felix Braun, Rosie Chandler, Sachetan Tuladhar, James Kirkpatrick, Jessica Benson, Joe Kwiatkowski, Thilini Ishwara, Justin Dane, Toby Ferenczi, Sam Foster, Jarvist Frost, Clare Dyer Smith, Mark Faist, Anne Guilbert, George Dibb, Sheridan Few, Davide Moia, Chris Emmott, Valerie Vaissier, Jizhong Yao, Florian Steiner, Michelle Vezie, Xingyuan Shi, Jason Rohr, Phil Sandwell, Cleaven Chia, Florent Delval Post Docs: Dr Stelios Choulis, Dr. Amanda Chatten, Dr. Youngkyoo Kim, Dr. Roberto Pacios, Dr. Johann, Boucle, Dr. Amy Ballantyne, Dr. Mariano Campoy Quiles, Dr. Pedro Atienzar, Dr. Monika Voigt, Dr. Panagiotis Keivanidis, Dr. Tiziano Agostinelli, Dr Roderick MacKenzie, Dr. Thomas Kirchartz, Ajay Gambhir, Dr. Antonio Urbina, Dr. Andrew Telford, Dr Dorota Niedzialek, Dr Florent Deledalle, Collaborators at Imperial Prof Donal Bradley, Dr Piers Barnes, Dr Ned Ekins-Daukes, Prof. James Durrant, Dr. Saif Haque, Prof Iain McCulloch, Dr Marin Heeney, Dr Brian O’Regan, Dr Ji-Seon Kim, Dr Natalie Stingelin and elsewhere… THE ROYAL SOCIETY