Presentation on theme: "Solar Cells: An Overview"— Presentation transcript:
1Solar Cells: An Overview Onkar S. GameSenior Research Fellow,National Chemical Laboratory, Pune.
2Outline Introduction: Need for harnessing solar energy Historical development of modern photovoltaic effect: Example of p-n junctionThin Film Solar Cells: ExamplesModern Solar Cells: Nanotechnology and PolymersCurrent Status and Future Prospective
3Sun: An ultimate source of energy * Present : 12.8 TW2050 : TW* Needs at least 16 TWBio : 2 TWWind : 2 TWAtomic : 8 TW (8000 power plant)Fossil : 2 TW* Solar: 160,000 TWIf you want money and fame (and if you are not excellent at acting or sports) develop an efficient Solar Cell!!!
7Task: 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)
8Modern 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.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.
9Silicon Solar Cell Schematic Why thickness of p type and n type semiconductor layers are different?
10Working 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.
12Parameters that characterize solar cell IV curve Voc: Open Circuit VoltageIsc : Short Circuit CurrentPmax: Maximum Power DeliveredVm: Voltage corresponding to PmaxIm: Current corresponding to PmaxFF (Fill Factor):Efficiency =Series Resistance: (dI/dv)-1 at VocShunt Resistance: (dI/dv)-1 at Isc
13Factors 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/qJsc 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 = 0Shunt 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.
14Solar Cell IV Measurement in Lab Solar Simulator
16Current Status of Si Solar Cells Factors Limiting Efficiencies:
17Alternative Thin Film Technologies Disadvantages of Thin Film Solar Cell Technology:Large scale production is difficult because of sophisticated fabrication techniques. Hence ExpensivePresence of rare elements viz. Indium, Gallium further adds to cost.Presence of some toxic elements viz. Cadmium may create environmental hazards
20Pre-requisite concepts Transparent Conducting Oxide: Eg ≥ 3 eV e.g. ZnO, TiO2, SnO2 etc.Molecular Levels:HOMO: Highest Occupied Molecular OrbitalLUMO: Lowest Unoccupied Molecular Orbital
21Dye Sensitized Solar Cells (DSSC) Iodide/tri-iodide electrolytee-LOADDye/QDTiO2 (~ 20 nm)Prof. Michael GratzelExcitation of dye molecule or Quantum Dot (QD) by incident sunlightTransfer of electron from dye/QD to TiO2Regeneration of oxidized dye/QD using a hole carrying electrolyteTransport of electron through TiO2 and external loadRegeneration of electrolyte at counter electrode
22Cross-sectional SEM of DSSC (counter-electrode and electrolytemissing)Excitation of dye molecule or Quantum Dot (QD) by incident sunlightTransfer of electron from dye/QD to TiO2Regeneration of oxidized dye/QD using a hole carrying electrolyteTransport of electron through TiO2 and external loadRegeneration of electrolyte at counter electrode
23Development of Dyes with broad visible light absorption is current area of research !!!
24Iodide/tri-iodide electrolyte LOADDye/QDTiO2 (~ 20 nm)….continuedWhy Nanoparticles?: Higher Surface area than what is projected. Higher dye adsorption leads to higher photocurrentWhy 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 filmWhy Platinum nanodot coated Fluorine doped Tin Oxide: To catalyze the I3- reduction 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.
30Transparent coatings for DSSC Transparency a critical issue to avoid loss of incident radiation due to reflection at nanoparticle/TCO interface.Without DyeWith Dye
31Carbon based Nano-Materials for DSSCs ZnO CNT compositeTiO2-MWCNTTiO2-GrapheneEff. 7.4%Eff. 6%
32Some Results: Efficiency Over 7% Name Voc (V) Isc (A) FF (%) η (%) 1st η (%)1st0.760.004454.517.262nd0.740.004356.517.23
33Various Experimental Techniques Used to Characterize DSSC IV measurement under Solar SimulatorWavelength Dependant IV measurement: IPCE Setup or Quantum Efficiency SetupElectrochemical Impedance Spectroscopy: To determine time dynamics in DSSC upto microsecond scaleTransient pump-probe measurement setup: To determine time dynamics in DSSC on nanosecond and picosencond time scale
34Current Status of DSSCHighest 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.
36New Types of Solar Cells LUMOLUMOe–e–e–LUMOLUMOECBh+h+HOMOh+HOMOEVBHOMOh+HOMOn-type semiconductorp-type semiconductorAnoden-type semiconductorP-type materialsCathodeAnodeElectron acceptorHole acceptorCathodeInorganic cellsHybrid solar cellsOrganic cellsFast carriers mobilityLong life timeHigh production costBrittleLow Production CostFlexibleTunable colorLight weightSlow carrier mobilityShort life timeInorganic n + Organic pETA CellDye-sensitized Solar Cells
37Example of a organic-inorganic hybrid solar cell