1. © Imperial College London 1 Photovoltaics: Research at Imperial College Jenny Nelson Department of Physics Imperial College London Grantham Climate Change Workshop Imperial College London 26 June 2007

2. © Imperial College London 2 Radiant power at Earth’s surface~ 100000 TW Electricity consumption ~2 TW ~ 80% from fossil fuels & nuclear, ~0.05% from PV At 10% power conversion efficiency, solar resource can meet demand with 0.02% of Earth surface area PV is the only technology to convert solar power directly into electricity The solar energy resource

3. © Imperial College London 3 Present PV system costs: –7 - 20 Euro/W p off-grid applications. –4 - 8 Euro/W p grid-connected. –Module 2 – 4 Euro / W p Aim for: –1 - 2 Euro/W p for power generation –4 - 10 Euro/W p for smaller applications. Cost of Si based system falling through economies of scale To accelerate cost reductions, need technological innovations 3.7 GW installed by end 2005 PV market growth

4. © Imperial College London 4 Strategies to cost reduction Use less photovoltaic material? Use cheaper photovoltaic material? More work per photon? Inorganic thin film materials Molecular PV materials Multijunction or “tandem” structures Extracting more work per photon Concentration of sunlight Light trapping

5. © Imperial College London 5 Strategies to cost reduction Use less photovoltaic material? Use cheaper photovoltaic material? More work per photon? Inorganic thin film materials Molecular PV materials Multijunction or “tandem” structures Extracting more work per photon Concentration of sunlight Light trapping Imperial Research: Luminescent concentrators Photovoltaic windows Thin film silicon Imperial Research: III-V semiconductor tandem structures Quantum-well solar cells Imperial Research: Molecular photovoltaic materials

6. © Imperial College London 6 Cheaper photovoltaic materials contactsactive layer Current and voltage output flexible substrate barrier coating Light Molecular photovoltaic materials enable low cost device fabrication Active layer ~100 nm thick Low temperature processing enables use of flexible substrates Highest efficiencies around 5% from blend of conjugated polymer with fullerenes

7. © Imperial College London 7 Molecular photovoltaics Research at Imperial Focus on plastic electronic materials for ultra low cost and flexible solar cells Research into: donor-acceptor composite materials for PV polymer-fullerene, polymer-polymer, polymer-molecule, small molecules, polymer-metal oxide, organic dye-sensitised structures Dye and polymer based solar cells to mimic natural photoconversion fundamental properties of active layers charge transport, charge separation, charge recombination device design and processing Cross disciplinary programme linking Physics to Chemistry, Materials, Elec. Eng. and Env.Sci.Tech. Industrial partners including BP, BP Solar, Sumitomo, Merck, Dupont. Wide network of national and international academic collaborators Supported by: EPSRC, DTI, EU, BP and others Lead researchers: Prof D Bradley, Prof J Nelson, Prof J Durrant, Dr S A Haque, Dr J de Mello C60 polymer

8. © Imperial College London 8 Device structures Donor-acceptor bulk heterojunction devices Al cathode Donor-Acceptor blend ITO anode Glass substrate h+h+ e-e- Active layer can be 100s of nm - limited by charge diffusion length Domain size ca. 10 nm. ~ exciton diffusion length Both components deposited from same solution

9. © Imperial College London 9 Device structures Donor-acceptor bulk heterojunction devices Al cathode Donor-Acceptor blend: e.g. polymer / fullerene, polymer / nanocrystal, polymer / polymer ITO anode Glass substrate h+h+ e-e- Both components deposited from same solution

10. © Imperial College London 10 Key challenges Voltage Current density J Silicon Best  24% Organic solar cell Best  ~4-5% Electron acceptor Electron donor Maximum eV oc

11. © Imperial College London 11 Challenges to address in the Grantham funded Ph.D. project on molecular PV materials Energy levels of donor and acceptor: –What are the conditions for charge separation? –Influence of morphology Morphology –What is the optimum morphology for current generation –How does morphology influence charge transport  Design rules for more efficient molecular solar cells! Interdisciplinary project between Physics (JN) and Chemistry (Dr Saif Haque)