Unformatted text preview: June 16, 2009 Seminar Nano-Solar cells: Solar Cells of the Future with Nanotechnology Outline
1. Energy Research: Forefront and Challenges Introduction the energy challenge Energy alternatives and the materials challenge Think big, go small 2. Our Energy Future: Nano-Solar Cells Jeongwon Park, PhD. Front End Product Group Applied Materials June 16, 2009 Humanity's Top Ten Problems for next 100 years
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. ENERGY WATER FOOD ENVIRONMENT POVERTY TERRORISM & WAR DISEASE EDUCATION DEMOCRACY POPULATION 2007 2050 6.6 8-10 Billion People Billion People http://energysos.org/ricksmalley/top10problems/
Source: Richard Smalley, Energy & Nanotechnology Conference, Houston. Energy: A National Initiative
"Tonight I'm proposing $1.2 billion in research funding so that America can lead the world in developing clean, hydrogen-powered automobiles... With a new national commitment, our scientists and engineers will overcome obstacles to taking these cars from laboratory to showroom, so that the first car driven by a child born today could be powered by hydrogen, and pollution-free." President Bush, State-of the-Union Address, January 28, 2003 "To finally spark the creation of a clean energy economy, we will double the production of alternative energy in the next three years," President Obama, George Mason University in Virginia, January 8, 2009 3 The World Energy Demand Challenge
2100: 40-50 TW 2050: 25-30 TW 2000: 13 TW 25.00 20.00 15.00 10.00 5.00 0.00 1970 1990 2010 2030
industrial developing US ee/fsu World Energy Demand energy gap ~ 14 TW by 2050 ~ 33 TW by 2100 total 50 40 30 % 20 10 0 TW oil World Fuel Mix 2001 gas coal nucl renew EIA Intl Energy Outlook 2004 http://www.eia.doe.gov/oiaf/ieo/index.html 85% fossil Hoffert et al Nature 395, 883,1998 4 New Materials and Nanoscience will play a role
manipulation of photons, electrons, and molecules
TiO2 nanocrystals adsorbed quantum dots artificial photosynthesis N liquid electrolyte natural photosynthesis quantum dot solar cells nanostructured thermoelectrics nanoscale architectures top-down lithography scanning probes multi-node computer clusters bottom-up self-assembly electrons, neutrons, x-rays density functional theory multi-scale integration smaller length and time scales 10 000 atom assemblies Solar energy requires interdisciplinary nanoscience research
5 characterization theory and modeling Solar Energy Utilization
H2O eh+ H2O O2 CO2 N C3 H O2 CO2 N NH H N N sugar
H NC O H2, CH4 CH3OH natural photosynthesis 50 - 200 C space, water heating Solar Electric
.0002 TW PV (world) .00003 TW PV (US) $0.30/kWh w/o storage artificial photosynthesis 500 - 3000 C heat engines electricity generation process heat Solar Fuel
1.4 TW biomass (world) 0.2 TW biomass sustainable (world) Solar Thermal 0.006 TW (world) 1.5 TW electricity (world) $0.03-$0.06/kWh (fossil) 11 TW fossil fuel (present use) ~ 14 TW additional energy by 2050 2 TW space and water heating (world) 6 What is a Solar Cell?
It is also known as Photovoltaic cell (PV cell) A device that converts light energy (solar energy) directly to electricity. The term solar cell is designated to capture energy from sunlight, whereas PV cell is referred to an unspecified light source. It is like a battery because it supplies DC power. It is not like a battery because the voltage supplied by the cell changes with changes in the resistance of the load. Applications of Solar Cells
Renewable energy Can be powered for remote locations It's free, limitless, and environmentally friendly... Toys, watches, calculators Electric fences Remote lighting systems Water pumping Water treatment Emergency power Portable power supplies Satellites Moore's Law for semiconductor electronics soon, all microchips will be nanoscale devices
DRAM 1/2 pitch, 3-yr cycle DRAM 1/2 pitch, 2-yr cycle MPU gate length 100 nm 10 nm nm1999 2003 2007 2011 2015 2019 2023 2027 2031 2035 2039 2043 2047
CONCLUSION: Moore's law continues for this decade regarding future size, device performance and cost for semiconductor electronics industry. 1 We now need to apply Moore's law to set goals for the energy industry.
Semiconductor Research Corporation
9 Example of Moore's law
World PV Cell Production (MW) Annual Growth > 30% For the Last Decade Source: Paul Maycock, PV News, March 2006 10 Global Solar markets Reduction of environmental impact Choice of solar technologies:
Crystalline silicon Amorphous silicon Cadmium telluride Copper indium diselenide CIS family, notably copper indium gallium diselenide CIGS Dye sensitised solar cells DSSC Organic polymer or small molecule Nano solar cell: silicon nanoparticle ink, carbon nanotube CNT and quantum dots, nanowires Inorganic nanorods embedded in semiconducting polymer, sandwiched between two electrodes 200 nm How Solar Cells Work: Photovoltaic Effect Entire spectrum of sunlight : 0.5 eV ~ 2.9 eV (Red light : 1.7 eV , Blue light : 2.7 eV) We need to consider: 1. Energy source Photon excites valence electron Electron-hole pair created Electrons & holes "separate" Band gap determines what is absorbed 2. Absorption 3. Transport 4. Collection
14 How it works To free an electron, the energy of a photon must be at least as great as the bandgap energy. Ephoton Eband gap : absorb to create free electrons. Ephoton < Eband gap : pass through the material. Eband gap of other effective PV semiconductors ranges from 1.0 to 1.6 eV. In this range, electrons can be freed without creating extra heat.
15 Typical P-N Junction Silicon Solar Cell Structure REFLECTION Sunlight Top side metallization grid Anti-reflection coating
0.5 m Average phosphorus~1018cm-3 n+ emitter p base ~100-300 m Boron~1016cm-3 Optional p+ back surface diffusion Back side metallization Characterization of a Solar Cell Device
Photovoltaic power conversion efficiency of a solar cell Current-voltage (I-V) curves of an organic solar cell (dark, - - -; illuminated, -). The characteristic intersections with the abscissa and ordinate are the open circuit voltage (Voc) and the short circuit current (Isc), respectively. The largest power output (Pmax) is determined by the point where the product of voltage and current is maximized. Division of Pmax by the product of Isc and Voc yields the fill factor FF. Pin is the incident light power density.
Harald Hoppea, et al, J. Mater. Res., 19, 1924-1945, (2004) Generation photovoltaic
1st: Consists of a large-area, single layer p-n junction diode, which is capable of generating usable electrical energy from light sources with the wavelengths of sunlight. These cells are typically made using a silicon wafer. 2nd: Based on the use of thin-film deposits of semiconductors. These devices were initially designed to be high-efficiency, multiple junction photovoltaic cells. 3rd: Very different from the previous semiconductor devices as they do not rely on a traditional p-n junction to separate photogenerated charge carriers. These new devices include photoelectrochemical cells, polymer solar cells, and nanocrystal solar cells. Dye-sensitized solar cells are now in production. Examples include Amorphous silicon, Polycrystalline silicon, micro-crystalline silicon, Cadmium telluride, copper indium selenide/sulfide. 4th: Composite photovoltaic technology with the use of polymers with nano particles can be mixed together to make a single multispectrum layer. Then the thin multi spectrum layers can be stacked to make multispectrum solar cells more efficient and cheaper based on polymer solar cell and multi junction technology used by NASA on Mars missions. Comparison of thin film solar cells
Type a-Si Benefit or intended benefit Thin Film, Flexible substrates Potential for roll to roll processing unavailable for mono- or poly-Si. As above with high efficiency Fairly high efficiency Well proven Over $1 billion of orders Lowest cost/watt over life High efficiency at low cost long life transparent no disposal problems, printable Tolerant of polarised/ low level light-can use heat extreme angle of incoming light Transparent/colors, tightly rollable No disposal problems, printable Potential for lowest cost ? Large area possible Tightly rollable No disposal problems, Spray directly onto things? Efficiency 8-10% Challenges Constant degradation low efficiency Not tightly rollable Cd is toxic Controlled disposal only Not tightly rollable Price of indium. New process/ stability Companies Innovalight United Solar Mitsubishi Kovio First Solar Calyxo NanoSi CdTe High? 9-11% CIGS 10-14% HONDA Global Solar Wurth Nanosolar G24i UK Dyesol Sony DSSC 11% Liquids handling Price of ruthenium? 5 year life? Organic 2-6% Cost Efficiency 1 year life? Narrow spectrum Konarka Plextronics Heliatek Thin Film PV Technologies
nanoparticle Si CIGS/CIS CdTe DSSC's Organic PV Best Cell Efficiencies
TYPE OF CELL/ TECHNOLOGY Single crystalline silicon Multi-crystalline Multisilicon Amorphous silicon CIS, CIGS DSSC Multi-Junctions Multi(e.g. GaAs, InGaP) GaAs, InGaP) Organic & hybrid solar cells 2-5 11 ~42 13 ~20 BEST POWER CONVERSION EFFICIENCY (%) 24 18 Source: NREL Current Obstacles
Efficiency vs. cost Solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 42.8% with multiple-junction research lab cells. Solar cell energy conversion efficiencies for commercially available multicrystalline Si solar cells are around 14-16%. The highest efficiency cells have not always been the most economical -- for example a 30% efficient multijunction cell based on exotic materials such as gallium arsenide or indium selenide and produced in low volume might well cost one hundred times as much as an 8% efficient amorphous silicon cell in mass production, while only delivering about four times the electrical power. To Make an Efficient Solar Cell,
Illustration of how absorption, reflection, recombination and conduction work within a PV cell. Tune the p-layer to the properties of incoming photons to absorb as many as possible, and thus, to free up as many electrons as possible. Keep the electrons from meeting up with holes and recombining with them before they can escape from the PV cell. One designs the material to free the electrons as close to the junction as possible, so that the electric field can help send the free electrons through the conduction layer (the Antireflective Coating layer n-layer) and out into the electrical circuit. Antireflective Coating Silicon reflects more than 30% of the light that shines on it. To improve the conversion efficiency of a solar cell: 1. Coat the top surface with a thin layer of silicon monoxide (SiO). a single layer : 10% ; a second layer : less than 4% 2. Texture the top surface: cones and pyramids which capture light rays
J. Vac. Sci. Technol. B 20.6., Nov. Dec 2002 Revolutionary Photovoltaics: 50% Efficient Solar Cells
present technology: 32% limit for single junction one exciton per photon relaxation to band edge
lost to heat Eg 3I
nanoscale formats 3V multiple junctions multiple gaps multiple excitons per photon hot carriers rich variety of new physical phenomena challenge: understand and implement 24 Thin Film Solar Cell Junction Structures Homo junction : p-n junction of crystalline silicon Hetero junction : formed by contacting two different semiconductors-- CdS and CuInSe2 p-i-n / n-i-p : typically, amorphous silicon thin-film cells use a p-i-n structure, whereas CdTe cells use an n-i-p structure. Multi junction : also called a cascade or tandem cell, can achieve a higher total conversion efficiency by capturing a larger portion of the solar spectrum A multijunction device is a stack of individual single-junction cells in descending order of bandgap (Eg). The top cell captures the high-energy photons and passes the rest of the photons on to be absorbed by lower-bandgap cells.
25 Prospects for Nano-enhanced Solar Cells (Ying Guo)
Basic research underway with the technology developments required to achieve the desired applications. Solar Cells based on Nanotechnology:
Organic and Hybrid cells Nano-crystalline TiO2 Film: Dye-Sensitized Solar Cell (DSSC) Quantum dot solar cells Nanowire solar cells Organic and hybrid solar cells
Why organic solar cells ?
ADVANTAGES Easier and cheaper fabrication processes: non-vacuum, low temperature, available at industrial scale. Low cost materials (?), low quantity (g/m2), flexible and cheap substrates (no glass). High EQE over the whole sun spectrum. High light absorption (100nm are enough to absorb most of the light) DISADVANTAGES Lower carrier mobility than in inorganic semiconductors. New devices, a lot of work for optimising morphology and composition. Examples of organic semiconductors used in organic solar cells Harald Hoppea, et al, J. Mater. Res., 19, 1924-1945, (2004) Organic Hetero-Junction Cell The first cells where built in the 50' (organic dyes with inorganic semiconductors). First organic cells had very low efficiency: 10-2/10-3 %. Due to the very short diffusion length for excitons (about 10nm), only the charges very close to the junction could be efficiently separated and collected by the electrodes.
100/200 nm Illustration of the photoinduced charge transfer with a sketch of the energy level. After excitation in the PPV polymer, the electron is transferred to the C60 The Bulk Hetero-Junction
A composite of two organic semiconductor (n and p) sandwiched between two electrodes with work function matching the energy bands of the composite. It can be seen as nanoscale p-n junctions. The density of p-n junctions is much higher than in bilayer cells. Much larger interfacial area, where charges can be separated. Efficiency increased of 2-3 order of magnitude reaching 1%. Separate conduction path for electron and holes (like in DSC). Organic cells can be a cheap alternative to inorganic semiconductor cells: low cost material, cheap fabrication process, good spectral response. What has to be done? Optimising the morphology is the key for better efficiencies. Two competing goals: good mixing, partial ordering. Hybrid cells
BHJ with organic/inorganic semiconductors ex. P3OT:CdTe, P3HT:CdSe, CuPC:Si, P3HT:TiO2, P3HT:CIS Maximum reported eff. is 2% ADVANTAGES Inorganic semiconductors have much higher carrier mobility. Inorganic semiconductor can be produced in the form of nanocrystal in order to control the ordering of the microstructure Ideal structure of a bulk heterojunction solar cell
Interspaced with an average length scale: 10-20 nm Exciton diffusion length The two phases have to be interdigitated in percolated highways to ensure high mobility charge carrier transport with reduced recombination. Buried Nanoelectrodes
Diffraction gratings and buried nano-electrodes--architectures for organic solar cells M.Niggemann*, M.Glatthaar, A.Gombert, A.Hinsch, V.Wittwer Thin Solid Films 451 452 (2004) 619623 Separate the absorption process from the charge transport in case of different mobility for holes and e- in the blend Hybrid cells
Hybrid Nanorod-Polymer Solar Cells
Science 295 p.2425 (2002) Wendy U. Huynh, Janke J. Dittmer, A. Paul Alivisatos Hybrid solar cells with vertically aligned CdTe nanorods and a conjugated polymer
Yoonmook Kang, Nam-Gyu Park, and Donghwan Kimb
Applied Physics Letters 86, 113101 (2005) The adsorbed dye molecule absorbs a photon forming an excited state. [dye*] The excited state of the dye can be thought of as an electron-hole pair (exciton). TiO
2 Nano-crystalline TiO2 Film Dye-Sensitized Solar Cell (DSSC) h
Mesoporous TiO2 Surface area X1000 Anode Cathode eeThe excited dye transfers an electron to the semiconducting TiO2 (electron injection). This separates the electron-hole pair leaving the hole on the dye. [dye*+] The hole is filled by an electron from an iodide ion. [2dye*+ + 3I- 2dye + I3-] Nano-crystalline TiO2 Film Dye-Sensitized Solar Cell (DSSC) Electrons are collected from the TiO2 at the cathode. Anode is covered with carbon catalyst and injects electrons into the cell regenerating the iodide. Redox mediator is iodide/triiodide (I-/I3-) The dashed line shows that some electrons are transferred from the TiO2 to the triiodide and generate iodide. This reaction is an internal short circuit that decreases the efficiency of the cell. Solid-state dye-sensitized solar cell* H3C O O CH3 Light absorption in dye, electron transfer to TiO2, O hole transfer to Spiro-MeOTAD. Additives in HTM: tbp and Lithium TFSI
*Bach, N H3C H3C O O N N O O N CH3 CH3 CF3 S O N Li CF3 U. et al. Nature 395, 583585 (1998) S O O Spiro
H3C O O CH3 Application of Carbon Nanotubes to Counter Electrodes of Dye-sensitized Solar Cells Kazuharu Suzuki, et al, Chemistry Letters, 32, 28 (2003) Key Step Charge Separation
Charge must be rapidly separated to prevent back reaction. Dye sensitized solar cell, the excited dye transfers an electron to the TiO2 and a hole to the electrolyte. In the PN junction in Si solar cell has a built-in electric field that tears apart the electron-hole pair formed when a photon is absorbed in the junction. Basic problems needed to overcome: 1. 2. the high recombination rate at the TiO2 interface the low conductivity of the hole conductor itself. Solar Cells based on Nanotechnology:
Quantum dot solar cells Nanowire solar cells Strategies for Improvement
(with nanostructures- QDs)
Capture more sunlight - Tune energy band gaps of materials "Stacked Architecture" Potential Advantages: Cheaper Materials Capture more of Solar Spectrum Generate more electrons per photon: "Multiple Exciton Generation" Potential Advantages: Dramatic Efficiency Improvements Greater Solar Spectrum MEG- Multiple Exciton Generation
Basic Solar Cell: 1 photon = 1 exciton (1 electron/ 1 hole + excess energy) MEG Solar Cell: 1 photon = 2+ excitons (2+ electron/holes reduce heat loss!)
Potential Efficiency Improvement Why Quantum Dots for solar cells?
Thermal relaxation of excited charge arriers can be significantly slowed down. 1. Enhanced photovoltage = collect charges while their hot. 2. Enhanced photocurrent = get more from the hot ones. Schaller. Nano Letters 6 424 (2006) Nozik. Physica E 14 115 (2002) Detection
Higher photon energy/band gap ratios give higher carrier multiplication efficiencies. Onset at ~3Eg. Schaller. Nano Letters 6 424 (2006) Schaller. PRL 92 186601 (2004) QD Solar Cells
The Trick: Have to be able to extract charge carriers produced in quantum dots. Nozik. Physica E 14 115 (2002) Coaxial silicon nanowires as solar cells and nanoelectronic power sources
Device fabrication and diode characterization 100nm 200nm 1.5um Charles M. Lieber et al, Nature 449, 885 (2007) Characterization of the p-i-n silicon nanowire photovoltaic device The overall apparent efficiency of the p-i-n coaxial silicon nanowire photovoltaic elements--3.4% (upper bound) and 2.3% (lower bound)--exceeds reported nanorod/polymer and nanorod/dye systems, and could be increased substantially with improvements in Voc by means of, for example, surface passivation.
Charles M. Lieber et al, Nature 449, 885 (2007) Application of the p-i-n silicon nanowire photovoltaic device Charles M. Lieber et al, Nature 449, 885 (2007) Strained Si Nanowires: Solar cells
Efficient charge separation for solar cells could be achieved by applying strain to silicon nanowires The added strain in the nanowire changes the structure of energy bands, such that positive and negative charges can be separated into different regions of the nanowire. The main potential benefit of silicon nanowire cells over traditional crystalline silicon cells is that the silicon would not need to be doped with other materials. This means that nanowire solar cells could be made inexpensively from much lower quality silicon, and without labour-intensive processing.
Wu, Z., Neaton, J. B. & Grossman, J. C. Charge separation via strain in silicon nanowires. Nano Lett. doi:10.1021/nl9010854 (2009). Nanowire applications for solar cells Y. Cui, Stanford Choice of solar technologies:
Crystalline silicon Amorphous silicon Cadmium telluride Copper indium diselenide CIS family, notably copper indium gallium diselenide CIGS Dye sensitised solar cells DSSC Organic polymer or small molecule Others such as silicon nanoparticle ink, carbon nanotube CNT and quantum dots Nano solar cell: Inorganic nanorods embedded in semiconducting polymer, sandwiched between two electrodes 200 nm Summary
A mix of future sustainable energy conversion technologies will be needed New materials and nanoscience discoveries are necessary to its development Strong interplay between basic and applied sciences is a key to success Interdisciplinary approaches, and coupling theory/experiment are vital Working with industry at all stages is a key factor The challenges and constraints are global and complementary among different countries International collaboration and networking must be encouraged and supported 53 ...
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