Solar Cells: An Overview Onkar S. Game Senior Research Fellow, National Chemical Laboratory, Pune. Outline • Introduction: Need for harnessing solar energy • Historical development of modern photovoltaic effect: Example of p-n junction • Thin Film Solar Cells: Examples • Modern Solar Cells: Nanotechnology and Polymers • Current Status and Future Prospective Sun: An ultimate source of energy * Present : 12.8 TW 2050 : 28-35 TW * Needs at least 16 TW Bio : 2 TW Wind : 2 TW Atomic : 8 TW (8000 power plant) Fossil : 2 TW * Solar: 160,000 TW If you want money and fame (and if you are not excellent at acting or sports) develop an efficient Solar Cell!!! Task: Creating free electrons using photons Semiconductors offer solution: Converting incoming photons into electron-hole pairs but creation of electron hole pair competes with electron-hole recombination!!! (which takes place within microseconds) Modern Solar Cell Technology: 1954 • In the early 1950s R.S. Ohl discovered that sunlight striking a wafer of silicon would produce unexpectedly large numbers of free electrons. • 1954 - The multidisciplinary research team at Bell Labs of Gerald Pearson, Calvin Fuller and Daryl Chapin, physicist, chemist and electrical engineer, respectively, announce the creation of the first practical solar cell made of silicon, known as the Bell Solar Battery. These cells had about 6% efficiency. This revolution may mark the beginning of a new era, leading eventually to the realization of one of mankind’s most cherished dreams—the harnessing of the almost limitless energy of the sun for the uses of civilization.- New York Times 1954. Silicon Solar Cell Schematic Why thickness of p type and n type semiconductor layers are different? Working of Si p-n junction solar cell Processes: • Absorption of incoming photons (Ephoton ≥ Band Gap) and creation of free electron-hole pair. (Note: The absorption process has to dominant near junction) • Separation of electron hole pairs in presence of internal potential (junction potential). • Vectorial transport of electrons and holes in opposite direction. Equivalent Circuit Rseries Junction IL Rshunt External Load Parameters that characterize solar cell IV curve 0.0025 • Isc : Short Circuit Current 0.0020 • Pmax: Maximum Power Delivered • Vm: Voltage corresponding to Pmax • Im: Current corresponding to Pmax Current (A) • Voc: Open Circuit Voltage Isc Im 0.0015 0.0010 Pmax 0.0005 • FF (Fill Factor): • Efficiency = FF Vm Im 100 % Voc Isc P max Voc Jsc FF % Pin Pin • Series Resistance: (dI/dv)-1 at Voc • Shunt Resistance: (dI/dv)-1 at Isc 0.0000 0.0 Vm 0.2 0.4 Voltage (V) 0.6 Voc 0.8 Factors Affecting Various Parameters in Solar Cell IV curve • Voc: Depends on difference between the fermi energy of p and n type semiconductor or semiconductor band gap. Ideal limit = Egap/q • Jsc or Isc : Absorption properties of semiconductor i.e. band gap and recombination rate of electron-hole pairs. • Series Resistance: Depends on ohmic losses at front contact (n type semiconductor and metal). Ideally = 0 • Shunt Resistance: Depends on leakage current within solar cell. Ideally = ∞ • FF (Fill Factor): Depends on values of series and shunt resistance. Ideally = 100. i.e. The IV loop should look as ‘rectangular’ as possible. • Efficiency: Depends on Voc, Isc and Fill Factor. Solar Cell IV Measurement in Lab Solar Simulator Current (A) 0.0025 0.0020 Isc Im 0.0015 0.0010 Pmax 0.0005 0.0000 0.0 Vm 0.2 0.4 Voltage (V) 0.6 Voc 0.8 Quantum Efficiency Set up Current Status of Si Solar Cells Factors Limiting Efficiencies: Alternative Thin Film Technologies Disadvantages of Thin Film Solar Cell Technology: • Large scale production is difficult because of sophisticated fabrication techniques. Hence Expensive • Presence of rare elements viz. Indium, Gallium further adds to cost. •Presence of some toxic elements viz. Cadmium may create environmental hazards Cost Comparison of Various Photovoltaics Nanotechnology: Towards low cost solar cells Pre-requisite concepts • Transparent Conducting Oxide: Eg ≥ 3 eV e.g. ZnO, TiO2, SnO2 etc. • Molecular Levels: a) HOMO: Highest Occupied Molecular Orbital b) LUMO: Lowest Unoccupied Molecular Orbital Dye Sensitized Solar Cells (DSSC) Dye/QD Dye/QD TiO2 nm) TiO (~(~2020 nm) 2 e - Iodide/tri-iodide electrolyte Prof. Michael Gratzel LOAD LOAD Excitation of dye molecule or Quantum Dot (QD) by incident sunlight Transfer of electron from dye/QD to TiO2 Regeneration of oxidized dye/QD using a hole carrying electrolyte Transport of electron through TiO2 and external load Regeneration of electrolyte at counter electrode Cross-sectional SEM of DSSC (counter-electrode and electrolyte missing) Excitation of dye molecule or Quantum Dot (QD) by incident sunlight Transfer of electron from dye/QD to TiO2 Regeneration of oxidized dye/QD using a hole carrying electrolyte Transport of electron through TiO2 and external load Regeneration of electrolyte at counter electrode Development of Dyes with broad visible light absorption is current area of research !!! ….continued Dye/QD Dye/QD TiO2 nm) TiO (~(~2020 nm) 2 e - Iodide/tri-iodide electrolyte LOAD LOAD Why Nanoparticles?: Higher Surface area than what is projected. Higher dye adsorption leads to higher photocurrent Why ZnO or TiO2?: Light absorption and electron transport are separated. Why liquid electrolyte: Porous nature of TiO2 Film needs better percolation of hole conducting species throughout the film Why Platinum nanodot coated Fluorine doped Tin Oxide: To catalyze the I3reduction at counter electrode. Why Fluorine doped Tin Oxide as Bottom electrode? FTO is a transparent conducting oxide hence it allows light to pass through it and it is conducting. Nanostructured Metal Oxides For DSSC ZnO Flowers ZnO Nanorods Rutile TiO2 Needles TiO2-Nanotubes Cu2O Nanoneedles TiO2-Nanoleaves TiO2-Nanofibers Cu2O nano Spheres Cu2O nano Cubes TiO2 Spheres TiO2-Nanowires ZnO CNT composite Sensitizers • Dyes: • Quantum Dots: Ruthenium based synthetic dyes Inorganic Quantum Dots viz. CdS, CdSe, PbS, PbSe etc. Dyes extracted from natural resources: (e.g. Anthocyanidins extracted from grapes) DSSC Fabrication protocol 2 Current Density (mA/cm ) 14 12 10 8 6 4 TiCl4 Treated Film 2 Area 0.25cm 0 0.0 0.2 2 0.4 0.6 0.8 Voltage(V) 50 QE ~ 43% 40 Name Sol-Gel TiO2 Voc (V) Jsc (mA/cm2) FF (%) η (%) 0.76 12.5 60 5.7 QE (%) 30 20 10 0 400 500 600 Wavelength (nm) 700 800 Transparent coatings for DSSC Transparency a critical issue to avoid loss of incident radiation due to reflection at nanoparticle/TCO interface. 70 60 %R 50 40 Opaque Film Transparent Film 30 20 10 0 200 Without Dye 300 400 500 600 700 Wavelength (nm) With Dye 800 Carbon based Nano-Materials for DSSCs TiO2-MWCNT ZnO CNT composite 100nm Eff. 7.4% TiO2-Graphene Eff. 6% Some Results: 0.005 Efficiency Over 7% Current (A) 0.004 0.003 0.002 Transparent TiO2 20nm + HS 0.001 1st Film 2nd Film 0.000 0.0 0.2 0.4 0.6 0.8 Voltage (V) Name Voc (V) Isc (A) FF (%) η (%) 1st 0.76 0.0044 54.51 7.26 2nd 0.74 0.0043 56.51 7.23 Various Experimental Techniques Used to Characterize DSSC • IV measurement under Solar Simulator • Wavelength Dependant IV measurement: IPCE Setup or Quantum Efficiency Setup • Electrochemical Impedance Spectroscopy: To determine time dynamics in DSSC upto microsecond scale • Transient pump-probe measurement setup: To determine time dynamics in DSSC on nanosecond and picosencond time scale Current Status of DSSC • Highest Efficiency on small area test cells: 11.3%. Further increase is a challenge. • Highest efficiency on modules: 9.2% • Issues related to use of liquid electrolyte and its evaporation. Development of solid state electrolytes. • Development of dyes with enhanced visible light absorption. Organic Solar Cells New Types of Solar Cells e– e– LUMO LUMO e– e– LUMO e– LUMO ECB HOMO h+ h+ HOMO h+ EVB HOMO n-type semiconductor p-type semiconductor Inorganic cells Fast carriers mobility Long life time High production cost Brittle Anode HOMO h+ n-type P-type Cathode semicond materials uctor Hybrid solar cells Inorganic n + Organic p ETA Cell Dye-sensitized Solar Cells Anode Electron acceptor Hole Cathode acceptor Organic cells Low Production Cost Flexible Tunable color Light weight Slow carrier mobility Short life time Example of a organic-inorganic hybrid solar cell Nano p-n junction solar cells Coaxial silicon nanowires as solar cells and nanoelectronic power sources NATURE, 449, 885, 2007 Thank You!!!