1. Realising the Potential of Solar Power Peter Weightman Physics Department, University of Liverpool, Oxford Street, Liverpool

2. Liverpool Energy Institute The Problem: Climate Change Current2050 Estimates Global power need 13 TW 26 TW Fossil Fuels ~ 11 TW 0 ??? Nuclear Fission 1 TW 5 TW (optimistic) Fusion 0 TW ? Other renewables < 1 TW 4 TW (optimistic) Photovoltaics 3 GW Great potential Artificial Photosynthesis 0 TW Great potential Potential: Power reaching the earth from the sun 100,000 TW (0.3 % of the sunlight reaching the Sarah Desert meets Europe’s power needs)

3. ALICE: A New Tool “Scientific advance is more often driven by the development of a new tool than a new concept” Freeman Dyson In a review of a biography of the mathematician George Green

4. 1µ 10µ 100µ 1m 10m 100m 1 10 100 1000 100 10 1.1.01 I mpatt Gunn III-V's Lead salts Output Power ( Watts ) SLED Frequency (Terahertz ) RTD array RTD HG QC Laser Courtesy: J. Allen. M. Chamberlain Accelerator Sources of Terahertz Radiation Power of laboratory instruments At 1 THz ~ 100  watts Average power ~ 24 mW Peak power ~ 70 kW Short electron bunches When bunch length < wavelength Coherent emission ---> massive output power Daresbury ERLP/ALICE

5. Energy Recovery Linear Accelerator / ALICE Daresbury The most intense broad band source of THz in Europe and only the 3rd in the world. 5 years under construction now commissioning NW Science Fund: Liverpool Liverpool THz beamline

6. .. Liverpool THz Beamline 1st Floor Tissue Culture Facility Lower level hutch for THz energy experiments Beamline funded and built by physics dept.

7. Improving Solar Cells Problem: Solar spectrum is broad, absorbing structures, band gaps, are narrow h       , E g, wasted h  >   , E g, wasted Liverpool Energy Institute h   creates an electron-hole pair (exciton) Bulk semiconductors 1 h     i  Shockly-Queiser limit Photovolatic energy collection < 32% Due to phonon emission Nanocrystals: PbS (Klimov et al 2004) For h  > E g can create many excitons (MEG) To get electron and hole out attach functional organic groups Controversy (Science 322 1784 December 2008) Reproducibility. Exciton lifetime. Do organic groups quench MEG production? Key is to understand dynamics. Exciton energy levels are in the THz Need a high intensity THz source with good time structure: ALICE

8. Artificial Photosynthesis Key elements: A photo receptor, often a metal complex Function: adsorb photons and release excited electrons A transducer, often organic ligands Function: transport electrons from the photo receptor to the catalytic reactor A catalytic reactor, also often a metal complex Function: reduce CO 2 to CO, split H 2 from H 2 O, or convert CO 2 and H 2 O to formic acid HCOOH The goal is to use sunlight to create high-energy molecules which can then be recombined with other molecules to release the stored chemical energy. The principle is applied in living organisms (bacteria, plants). Harnessing it for technological applications has the potential to create cycles of energy production and consumption, which have no negative impact on the environment. So why hasn’t this been done already? Short answer: it turns out to be rather difficult. But the good news is: we know that it works. Courtesy Werner Hofer

9. Scheme of a cell for artificial photosynthesis e- C RED H+H+ ½ H 2 e- Antenna C OX A ½ H 2 O H + +½ O 2 C chromophore A electron acceptor D electron donor C D h C RED H+H+ ½ H 2 C OX ½ H 2 O H + +½ O 2 C RED Surface Photoelectrochemical Synthesis Cell (PES Cell) Courtesy Werner Hofer

10. Recent advances in artificial photosynthesis (Julia Weinstein, University of Sheffield) Generation of very stable charge separated state in d 8 organometallic system Combination of transient bond formation with long distance charge separation Liverpool Energy Institute Concept Light induced S-S bond e transfer via Pt from S-S to N N \ Structural reorganisation in excited state traps the energy and prevents back electron transfer Key issues Chemical synthesis (electrodes?) Fast light sources to monitor transient changes in electronic and geometrical structure in real time ALICE and NLS J.A. Weinstein, M.T. Tierney, E.S. Davies, K. Base, A.A. Robeiro and M.W. Grinstaff Inorg. Chem. 45 4544 (2006)