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Solar Cells: An Overview

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Presentation on theme: "Solar Cells: An Overview"— Presentation transcript:

1 Solar Cells: An Overview
Onkar S. Game Senior Research Fellow, National Chemical Laboratory, Pune.

2 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

3 Sun: An ultimate source of energy
* Present : 12.8 TW 2050 : 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!!!




7 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)

8 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. 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.

9 Silicon Solar Cell Schematic
Why thickness of p type and n type semiconductor layers are different?

10 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.

11 Equivalent Circuit Rseries Junction IL Rshunt External Load

12 Parameters that characterize solar cell IV curve
Voc: Open Circuit Voltage Isc : Short Circuit Current Pmax: Maximum Power Delivered Vm: Voltage corresponding to Pmax Im: Current corresponding to Pmax FF (Fill Factor): Efficiency = Series Resistance: (dI/dv)-1 at Voc Shunt Resistance: (dI/dv)-1 at Isc

13 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.

14 Solar Cell IV Measurement in Lab
Solar Simulator

15 Quantum Efficiency Set up

16 Current Status of Si Solar Cells
Factors Limiting Efficiencies:

17 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

18 Cost Comparison of Various Photovoltaics

19 Nanotechnology: Towards low cost solar cells

20 Pre-requisite concepts
Transparent Conducting Oxide: Eg ≥ 3 eV e.g. ZnO, TiO2, SnO2 etc. Molecular Levels: HOMO: Highest Occupied Molecular Orbital LUMO: Lowest Unoccupied Molecular Orbital

21 Dye Sensitized Solar Cells (DSSC)
Iodide/tri-iodide electrolyte e - LOAD Dye/QD TiO2 (~ 20 nm) Prof. Michael Gratzel 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

22 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

23 Development of Dyes with broad visible light absorption is current area of research !!!

24 Iodide/tri-iodide electrolyte
LOAD Dye/QD TiO2 (~ 20 nm) ….continued 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 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.



27 Nanostructured Metal Oxides For DSSC
Cu2O Nanoneedles ZnO Flowers ZnO Nanorods Rutile TiO2 Needles TiO2-Nanotubes TiO2-Nanoleaves TiO2-Nanofibers Cu2O nano Spheres Cu2O nano Cubes TiO2 Spheres TiO2-Nanowires ZnO CNT composite

28 Sensitizers Dyes: Quantum Dots: Ruthenium based synthetic dyes
Dyes extracted from natural resources: (e.g. Anthocyanidins extracted from grapes) Quantum Dots: Inorganic Quantum Dots viz. CdS, CdSe, PbS, PbSe etc.

29 DSSC Fabrication protocol
Name Voc (V) Jsc (mA/cm2) FF (%) η (%) Sol-Gel TiO2 0.76 12.5 60 5.7

30 Transparent coatings for DSSC
Transparency a critical issue to avoid loss of incident radiation due to reflection at nanoparticle/TCO interface. Without Dye With Dye

31 Carbon based Nano-Materials for DSSCs
ZnO CNT composite TiO2-MWCNT TiO2-Graphene Eff. 7.4% Eff. 6%

32 Some Results: Efficiency Over 7% Name Voc (V) Isc (A) FF (%) η (%) 1st
η (%) 1st 0.76 0.0044 54.51 7.26 2nd 0.74 0.0043 56.51 7.23

33 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

34 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.

35 Organic Solar Cells

36 New Types of Solar Cells
LUMO LUMO e– e– e– LUMO LUMO ECB h+ h+ HOMO h+ HOMO EVB HOMO h+ HOMO n-type semiconductor p-type semiconductor Anode n-type semiconductor P-type materials Cathode Anode Electron acceptor Hole acceptor Cathode Inorganic cells Hybrid solar cells Organic cells Fast carriers mobility Long life time High production cost Brittle Low Production Cost Flexible Tunable color Light weight Slow carrier mobility Short life time Inorganic n + Organic p ETA Cell Dye-sensitized Solar Cells

37 Example of a organic-inorganic hybrid solar cell

38 Nano p-n junction solar cells

39 Coaxial silicon nanowires as solar cells and nanoelectronic power sources NATURE, 449, 885, 2007

40 Thank You!!!

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