(M): No Class (Memorial Day) 5.27 (W): Energy and Nanotechnology 5.28 (Th): LAB: Solar Cell (M): Project Presentations 6.03 (W): LAB: Antimicrobial Properties of NPs 6.04 (Th): Project Presentations / complete lab (M): Battery/LED/MEMS 6.10 (W): BioNanoTechnology 6.11 (Th): Guest Lecture (M): Safety and Ethics / Lab Check-out 6.17 (W): Final Exam (10:30 – 12:30)

Energy and Nanotechnology Nano 101

Scope of Energy and Nanotechnology Huge Scope Focus on: –Photovoltaics –Energy Storage –LEDs, Photonics

Photovoltaic Semiconductor device that can convert sunlight to electricity 1. Free electron from VB 2. Separate charges to create current

P-N Junction N- type has extra CB electrons P-type is missing VB electrons (has extra holes) Difference of potential drives separation of charges

Photovoltaic Efficiency Electrical energy out / Photon energy in Theoretical limit for 1 PN Junction, solar spectrum = 33.7% –Shockley-Queisser Limit –Blackbody Radiation –Radiative Recombination –Spectrum Losses Si – E g = 1.1 eV GaA – E g = 1.4 eV

Landscape of Photovoltaics

Multi-Junction/Concentrated Solar Cells The Top Performer

Why do we need new PV materials? Cost –Expensive manufacturing –Decreases w/ polysilicon but still high Aesthetics Flexibility

Thin Film PVs CIGS flexible solar cell CdTe E g = 1.5 eV

Dye-Sensitized Solar Cells No P-N junction Colored dye absorbs light, separates charges Electron injected into TiO 2, goes to back contact Redox couple re-oxidizes dye First published 1991, ~ 7% efficient

Maximizing Performance TiO 2 –Nanostructures –Improvement w/ 1-D vs 2-D or 0-D –Oriented, crystalline structures –Hydrothermal synthesis Scientific Reports 3, Article number: 3004 doi: /srep03004

Maximizing Performance Dye –Absorb energy in the solar spectrum –Attach to TiO2 –Inject electrons into TiO2 –Favorable electronic states Nature Chemistry 6, 242– 247 (2014) Advances in Materials Science and Engineering Volume 2014 (2014), Article ID Zn Prophyrin Ru Complex Anthrocyanin

Maximizing Performance Electrolyte –Minimal light absorption –Proper energy levels –Stability mainz.mpg.de/groups/laquai/Research/Organic%20Electronics/DSSC/print_html?lang=en Advances in Materials Science and Engineering Volume 2014 (2014), Article ID Dalton Trans., 2012,41,

Easy to make

Commercial Products GCell (UK) Sony Dyesol (Australia)

Perovskites Methylammonium lead tri-iodide Originally sensitizer for DSSC black-in-the-solar-world

Landscape of Photovoltaics

Perovskite Development Found to be ambipolar semiconductor –Good hole and electron mobility Nature Materials 13, 838–842 (2014)

Pervoskite Synthesis Spin-coat Pb precursur onto TiO 2 Soak in CH 3 NH 3 I precursor Crystallinity tuned by choice of solvents

Concerns Stability – water soluble, does not hold up to direct sunlight Toxicity False positive in efficiency measurements Commercialization – –Oxford Photovoltaics

Quantum Dot Solar Cells Good at absorbing light and separating charge Light absorption can be tuned QDs can be easily mass produced Problems: –Stability –Air-sensitive –High temp synthesis J. Phys. Chem. Lett., 2013, 4 (6), pp 908–918

QD-Sensitized Solar Cell Like DSSC, QD replaces dye J. Phys. Chem. Lett., 2013, 4 (6), pp 908–918

Multiple Exciton Generation 1 photon -> 2 carriers Potential for > 100% efficiency Can you extract the charges before they recombine? J. Phys. Chem. Lett., 2013, 4 (6), pp 908–918

Tandem Solar Cell Quantum dots of different sizes/compositions absorb more of the solar spectrum

Record QD Solar Cell PbS QDs Pyramid shaped electrodes Tuning angles tunes performance of the cell tech/59810

Products Solterra (Arizona) –QD Tetrapod PVs Nanoco (UK) –CIGS and CIS QDs