Nanotechnology for Next Generation Solar Cells Group 1: Amy Cornforth, Tony Grupp, Ana D’Almeida February 5, 2010

Presentation Overview 1.Solar cell introduction 2.Quantum dot solar cells 3.Dye-sensitized solar cells (DSSC) 4.Hybrid organic solar cells

Solar Cells Units that have the ability of converting sunlight into electricity Made of semiconducting material Can be used for varied purposes, e.g. to power watches, to light houses, and to provide power to the electrical grids Image found at: How Solar Cells Work.

Solar Cells How do they work? –Light is absorbed by semiconductor –Energy of the electrons increases –Electrons move in the material –Charge carriers have to be present Limitations –Band gap of the semi-conducting material –Maximum efficiency of a solar cell (single material) is about 30 % How Solar Cells Work.

Solar Cell Development Three Generations of solar cell technology: 1.Single-crystal silicon based photovoltaic devices Good efficiency High Cost  Higher than traditionally-produced electricity 2.CuInGaSe 2 (CIGS) polycrystalline semiconductor thin films Low Cost Less Efficiency 3.Nanotechnology-enhanced solar cells Low Cost Medium Efficiency

Quantum Dots Advantages Adjustable band-gap Moldable Facilitate collection and transport of carriers Increase efficiency of solar cells by extending the band gap of solar cells by generating more charges from a single photon Quantum Dots and Ultra-Efficient Solar Cells?

Quantum Dots Quantum dot sensitized solar cells (QDSCs) are third-generation photovoltaic devices Semiconductor sensitizers –Very tunable –Theoretically increase efficiency of solar cells up to 44% Published in:Ivn Mora-Ser, Sixto Gimnez, Francisco Fabregat-Santiago, Roberto Gmez, Qing Shen, Taro Toyoda, Juan Bisquert;Recombination in Quantum Dot Sensitized Solar Cells.Accounts of Chemical Research (11), DOI: /ar900134d, Copyright © 2009 ASC Image found at:: html

Quantum Dots How to improve the performance and stability of QDSCs? Deposit CdSe quantum dots on nanostructured mesoporous TiO 2 electrodes Image found at: Published in:Ivn Mora-Ser, Sixto Gimnez, Francisco Fabregat-Santiago, Roberto Gmez, Qing Shen, Taro Toyoda, Juan Bisquert;Recombination in Quantum Dot Sensitized Solar Cells.Accounts of Chemical Research (11), DOI: /ar900134d, Copyright © 2009 ASC

Dye-Sensitized Solar Cells Published in: Hiroshi Imahori and Tomokazu Umeyama; J. Phys. Chem. C 2009, 113 (21). DOI: /jp Copyright © 2009 American Chemical Society

DSSC Basics Thin-film solar cell –Think sandwich Electrons for movement are provided by the photosensitive dye –Electrons provided by silicon base in other cells –Compare with previously demonstrated cell Nanomaterials used to create 3-D structure for dye –Greater number of dye molecules due to greater internal surface area

Image found at cited Secondary Article 2 Basic DSSC Layers: 1.Glass coated with fluorine-doped tin oxide 2.Titanium dioxide layer (n-type semiconductor) 3.Ruthenium dye 4.Electrolyte solution 5.Glass coated with platinum

a)Demonstration of DSSC cell b)TiO Nanostructure c)Electron energy levels 1. Electron injection from dye to conduction band 2. Electron recombination with dye cation 3. Dye regeneration from electrolyte 4. Electron recombination with electrolyte 5. Electron trapping in nanostructure Published in: Qifeng Zhang, Christopher S. Dandeneau, Xiaoyuan Zhou, and Guozhong Cao; Advanced Materials. 2009, 21, 4087–4108. DOI: /adma Copyright © 2009 Wiley-VCH

DSSC Nanostructure Porous interconnected structure Surface area increased 1000 times when compared to bulk materials Crystals cause light-scattering and increase efficiency, but also cause electron trapping Thickness, shape, material all effect cell efficiency Published in: Qifeng Zhang, Christopher S. Dandeneau, Xiaoyuan Zhou, and Guozhong Cao; Advanced Materials. 2009, 21, 4087–4108. DOI: /adma Copyright © 2009 Wiley-VCH

ZnO Nanostructures Published in: Qifeng Zhang, Christopher S. Dandeneau, Xiaoyuan Zhou, and Guozhong Cao; Advanced Materials. 2009, 21, 4087–4108. DOI: /adma Copyright © 2009 Wiley-VCH

a)Diagram of cell with nanowires b)Image of nanowires c)Comparison of cell performance for various shapes and types of nanostructures Published in: Qifeng Zhang, Christopher S. Dandeneau, Xiaoyuan Zhou, and Guozhong Cao; Advanced Materials. 2009, 21, 4087–4108. DOI: /adma Copyright © 2009 Wiley-VCH

DSSC Modifications Replace organic electrolyte solution –Volatile, undergoes expansion and contraction –Gel electrolyte –Polymer electrolyte –Solid organic conductor –Inorganic semiconductor Replace ruthenium dye –Difficult to produce, environmentally dangerous –Organic dyes –Inorganic quantum dots Replace TiO 2 layer –SnO 2 –ZnO Published in: L. M. Peter; J. Phys. Chem. C 2007, 111 (18). DOI: /jp069058b Copyright © 2007 American Chemical Society

DSSC Development History 1991 –Nature paper by O'Regan and Grätzel –First suggestion of workable DSSC 2006 –Use of nanowires and nanoparticles –Demonstrated good chemical and thermal resistance 2007, 2008 –Use of low-cost organic dyes and solvent-free electrolyte solution investigated Dye-Sensitized Solar Cell. Wikipedia.

DSSC Efficiency High chance of proton absorption and high chance of electron movement –90% Quantum Efficiency for green light Quantum Efficiency-chance that one photon will convert one electron Overall efficiency is 11% or less, depending on materials of construction Dye-Sensitized Solar Cell. Wikipedia.

DSSC Summary Medium efficiency Low cost Problems to be addressed: –Liquid electrolyte (freezing, expanding, volatility) –Poor performance in red region of light Dye-Sensitized Solar Cell. Wikipedia.

Organic Hybrid Solar Cells PT (polythiophene) and other oligomers have better morphology and optoelectronic properties for increased efficiency Based on P3HT (poly-3(hexylthiophene)) derivatives Image at

What is an Oligothiophene? Definition: Molecules in which two or more thiophene rings are linked together Gives rise to many optical and electrical properties such as fluorescence, semiconductance, and light emission Both images found at

Scheme 1: Mechanism of Excited State Deactivation of Higher Generation Thiophene Dendrimers Published in: Guda Ramakrishna; Ajit Bhaskar; Peter Bauerle; Theodore Goodson; J. Phys. Chem. A 2008, 112, DOI: /jp076048h Copyright © 2008 American Chemical Society

Why are oligothiophenes important? Highly versatile chemistry Very simple to synthesize basic molecules Used in organic light emitting diodes (LEDs) Field effect transistors –Uses an electric current to control the conductivity of charge Organic photovoltaic and light harvesting devices (solar cells)

P3HT – poly(3-hexylthiophene) One of the major layers in an organic solar cell to increase efficiency In some lower quality solar cells the addition of P3HT increased efficiency from 0.05% up to 0.29% The best organic solar cells can reach up to 4-5% efficiency Current commercial solar cells use highly purified silicon and reach 22% efficiency

PT and P3HT A) PT B) P3HT Both are derived from the basic oligothiophene structure P3HT has a hexane chain added to the C5 position of each thiophene ring Image found at

Figure 1 Molecular structures of the investigated 3D oligothiophene dendrimers. Published in: Guda Ramakrishna; Ajit Bhaskar; Peter Bauerle; Theodore Goodson; J. Phys. Chem. A 2008, 112, DOI: /jp076048h Copyright © 2008 American Chemical Society

Atomic structure in the case of (a) P3HT with 2510 atoms and (b) P3HT with atoms. Hydrogen atoms have been removed for clarity. Main chains are shown in black and side chains in gray. Published in: Nenad Vukmirović; Lin-Wang Wang; J. Phys. Chem. B 2009, 113, DOI: /jp808360y Copyright © 2008 American Chemical Society

What is needed? Organic solar cells have two main objectives: –1. They must have efficient excitation delocalization and energy transfer to best mimic natural systems (such as plants) –2. Must be able to convert solar energy and have large electron mobility properties (P3HT helps considerably with this) Published in: McGehee, M.; Mayer, A.; Parmer, J.; Rowell, M.; Topinka, M.; Burkhardt, G. Improving Organic Solar Cells C 2007 Stanford University

Zinc and Titanium Oxide Nanorods Simple solar cell design where zinc oxide nanorods are grown and a layer of titanium oxide is layered on those rods P3HT is layered overtop the rods as the hole- conducting polymer Significantly increases the voltage difference across the cell, and can be exposed to atmospheric air to increase efficiency Published in: McGehee, M.; Mayer, A.; Parmer, J.; Rowell, M.; Topinka, M.; Burkhardt, G. Improving Organic Solar Cells C 2007 Stanford University

How can we improve? One field of current research is to form a mesh of carbon nanotubes with a P3HT light absorbing film The following slides show one experiment from Stanford University with the current and voltage across a solar cell Published in: McGehee, M.; Mayer, A.; Parmer, J.; Rowell, M.; Topinka, M.; Burkhardt, G. Improving Organic Solar Cells C 2007 Stanford University

Results Efficiency over the system was nearly triple from previous experiments, going up to 3% using a 95% transparent film over the top of the cell An increase in the carbon nanotube density of 20% resulted in a increase of conductivity by 15-fold Increasing the thickness of the P3HT layer aided electron transfer Research should be done to improve the transparency of the top film layer to be above 95%

Sources Main Article: 1.Nanotechnology for Next Generation Solar Cells. Prashant V. Kamat and George C. Schatz. J. Phys. Chem. C, Secondary Articles: 1.Hiroshi Imahori and Tomokazu Umeyama. Donor−Acceptor Nanoarchitecture on Semiconducting Electrodes for Solar Energy Conversion. J. Phys. Chem. C Wikipedia. Dye-sensitized solar cell. sensitized_solar_cellh 3.Qifeng Zhang, Christopher S. Dandeneau, Xiaoyuan Zhou, Guozhong Cao. ZnO Nanostructures for Dye-Sensitized Solar Cells. Advanced Materials. C Peter, L. M. Characterization and modeling of dye-sensitized solar cells. J. Phys. Chem. C Prashant V. Kamat. Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion. J. Phys. Chem. C Yasuhiro Tachibana, Kazuya Umekita, Yasuhide Otsuka, Susumu Kuwabata. Charge Recombination Kinetics at an in Situ Chemical Bath-Deposited CdS/Nanocrystalline TiO2 Interface. J. Phys. Chem. C, 2009, 113 (16), pp 6852–6858

Group S1 Rebuttal Most of the comments were positive, which were appreciated. Of the negative comments, while we agree with most, the ones we don’t agree with was our shortened introduction. We believe that our topic was a continuation of the solar cell discussion Dr. Seminario gave on the first day of class, and therefore a long introduction was not needed. Group S1

Group S2: Review of Solar Technology Chris Heflin Rachael Houk Michael Jones

Positives Group S1 was the first to present, and therefore had a harder time knowing what to expect with the presentation. However, they presented a professional, well organized presentation. Each presenter was knowledgeable on their respective areas of the topic, spoke clearly and fluently.

Negatives The group should make use of the microphones and vocal projection in order to be well heard. Everything was very quiet. Many of the slides contained only words and no pictures, making the presentation less interesting. Some of the material was a bit more technical than most were prepared for. A bit more introduction would be beneficial.

Bradford Lamb Michael Koetting James Kancewick Week 1 Additional Slides Seminar Group S3

 We felt S1 should have had more detailed background slides towards solar technology.  The information that they presented was somewhat lost on the audience because it was too detailed without having a solid background.  Thus, we attached two additional slides that improve background knowledge. Group S3

 Solar powered electrical generation relies on heat engines and photovoltaics  limited only by human ingenuity  most common way is to use solar panels  Passive solar or active solar Group S3

 used to make saline or brackish water potable  Solar energy may be used in a water stabilization pond to treat waste water without chemicals or electricity Group S3

Group S4 Review of Solar Cell Technology Joshua Moreno Scott Marwil Danielle Miller Group S4

Things Done Well The group created a very nice power point that was full of good visuals and rich information The group spoke very clearly and made minimal use of words like “um.” The group presented the material in a fun and interesting way. Group S4

Things That Need Improvement The group needs to try to not fit so much information on every slide. The slides got a bit wordy in some areas. The group needs to develop the introduction a little bit more. We felt like it was too short and did a poor job of leading into the material. Group S4

Group 5 Pradip Rijal Jason Savatsky Trevor Seidel Laura Young Group S5 Group S5 Review of Solar Cell Technology

Presentation Review The group overall did a very good job. They talked about the use of DSSC and Quantum Dots being used in Solar Cells but they did not tell us what they were. Organization was satisfactory. Could work on speaking louder. Group S5

Critiqued by S6 Michael Trevathan Daniel Arnold Michael Tran John Baumhardt Group S6

Summary  Discussed new solar cell efficiencies resulting from nanotechnology  Needed to discuss the feasibility of this technology becoming a substantial source of energy  Needed more analysis on cost – at least some estimated ranges based on the material  They all dressed nicely and spoke clearly  They were knowledgeable and directed their attention toward the audience  Overall – great presentation! Group S6