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

THE ROYAL SOCIETY New Fellows Seminar 9-10 July 2014 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

Printable photovoltaics • Variety of materials • Process from solution • “One pot, one shot” active layer • Large area • High throughput • Printing or coating contacts contacts contacts active layer active layer active layer • Conformal Current and Current and Current and • Lightweight voltage output voltage output voltage output • Cheap flexible substrate flexible substrate flexible substrate Light Light Light barrier coating barrier coating barrier coating

Why print photovoltaics? 45 000 Other • Minimise production costs 40 000 Wind Solar 35 000 Hydro 30 000 Nuclear 25 000 Biomass and was • New product forms 20 000 Oil 15 000 Gas with CCS 10 000 Gas IEA Energy Technology Perspectives 2012: 5 000 Coal with CCS Global electricity generation in the 2DS Coal • Potential for innovation in 0 2009 2020 2030 2040 2050 manufacturing Chris Emmott et al. Environ. Sci. Tech. (2014) 10 Model of German PV deployment Cumulative CO 2 Emissions (MtCO 2eq ) Year • Reduce carbon embedded in 0 2000 2005 2010 2015 manufacture -10 c-Si, manufactired in China -20 c-Si, if manufactured in Europe CdTe -30 OPV

Solution processable photovoltaic materials 1990- Dye sensitised 2001- Organic (polymer:C60) 2007- Organic tandem e TiO 2 - e UCLA - e η ~ 12% - η ~ 10% η ~ 11% e e - - 2010- Particle slurry CZTS 2012- Perovskite Other new materials and new processes ... η ~ 12% η ~ 16%

Molecular electronic materials • Electronic properties: • Excited states and charged states are localised • Electronic states are disordered Low relative permittivity ε r • conjugated molecule conjugated polymer • Charge transport is slow • Charge pairs hard to separate 1 nm

Ph otovoltaic energy conversion in molecular materials Distributed heterojunction � charge separation over a large optical depth e - LUMO contacts active layer (~100 nm) LUMO flexible substrate Light HOMO barrier coating E vac donor HOMO acceptor W a 2 Separate charges by adding W c a strong electron acceptor Photocurrent direction provided by asymmetric contacts Photovoltage limited by electrical gap E CS (< optical gap E g )

Materials development Theoretical limit ? Schaarber et al., Adv. Mater 2006 11.00 -4.0 10.00 8.00 9.00 LUMO Level Donor [ eV ] -3.8 9.2% (2011) 7.00 5.00 -3.6 6.00 -3.4 1.00 T 4.00 6% (2009) -3.2 2.00 3.00 2.00 -3.0 2.8 2.4 2.0 1.6 1.2 Band Gap [ eV ] ���� 5.5% (2007) ∆ � � ���� � � E CS eV oc 4.4% (2005) ���� 2.5% (2001) ����� ���� ��������

Sources of loss in organic photovoltaic heterojunctions + - - + FLUX Exciton Charge pair Current D S 1 ∆ ∆ E C ∆ ∆ E CS E CT ∆ ∆ ∆ ∆ E R eV OC POTENTIAL Absorber Interface Circuit How much do we pay for charge separation? How much do we pay for charge recombination?

Probing charge separation D S 1 ∆ E C ∆ ∆ ∆ E CS E CT ∆ E R ∆ ∆ ∆ eV OC • Probe the yield of charge pairs using transient spectroscopy Detector • Probe the energy of intermediate pump I(l, t) pulse state using electroluminescence t Probe light - Sample E CT + Polymer, Fullerene

Probing charge separation D S 1 ∆ ∆ E C ∆ ∆ Normally > 0.3 eV E CS E CT ∆ E R ∆ ∆ ∆ eV OC Limiting efficiency < 20% • Influenced by – Specific chemical structure and alignment – Molecular packing close to interface – Competition with other excited states

Sources of loss in organic photovoltaic heterojunctions + - - + FLUX Exciton Charge pair Current D S 1 ∆ ∆ E C ∆ ∆ E CS E CT ∆ ∆ ∆ ∆ E R eV OC POTENTIAL Absorber Interface Circuit How much do we pay for charge separation? How much do we pay for charge recombination?

Nature of charge recombination 1.0 1.0 0.8 0.5 � �� 1 energy E [eV] energy E [eV] 0.6 J − ∇ ⋅ = − G R � 0.4 � 0.0 n e � �� 0.2 -0.5 0.0   E -1.0 exp   ∝ 0 50 100 150 DOS   position x [nm] E   ch • Electronic state energies are disordered • Recombination occurs between free and trapped charges � density dependent mobility and • lifetime � Intensity dependent PV performance • Understanding disorder is critical 12

Example: Effect of fullerene structure on charge collection 0 -1 M. Lenes et al., Adv. Funct. Mater. (2009); M. Faist et al., J. Pol. Sci.., (2010) -1 s -2 ) Current density (mA cm 2 V -1 10 FET electron mobility / cm -2 10 -3 10 -5 -4 10 -5 10 -6 10 -10 Mono Bis , Tris 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Fullerene Voltage (V) Reduce mobility multi-adducts Reduce photocurrent Why? Packing disorder? Energetic disorder?

Example: Modelling effect of fullerene structure Coarse grain Representative structures F. Steiner, J. M. Frost et al (2014) Packing Packing and Experiment disorder energetic disorder Electronic coupling & transport Distinguish effects

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