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

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Presentation on theme: "Solar Cells: An Overview Onkar S. Game Senior Research Fellow, National Chemical Laboratory, Pune."— 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 * 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 Sun: An ultimate source of energy If you want money and fame (and if you are not excellent at acting or sports) develop an efficient Solar Cell!!!

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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 (E photon ≥ 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 ILIL Junction R shunt R series External Load Equivalent Circuit

12 Parameters that characterize solar cell IV curve V oc : Open Circuit Voltage I sc : Short Circuit Current P max : Maximum Power Delivered V m : Voltage corresponding to P max I m : Current corresponding to P max 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 V oc : Depends on difference between the fermi energy of p and n type semiconductor or semiconductor band gap. Ideal limit = E gap /q J sc or I sc : 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 Simulator Solar Cell IV Measurement in Lab

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: a)HOMO: Highest Occupied Molecular Orbital b)LUMO: Lowest Unoccupied Molecular Orbital

21 Dye Sensitized Solar Cells (DSSC) Iodide/tri-iodide electrolyte e - LOAD Dye/QD TiO2 (~ 20 nm) e - LOAD Dye/QD TiO 2 (~ 20 nm)  Excitation of dye molecule or Quantum Dot (QD) by incident sunlight  Transfer of electron from dye/QD to TiO 2  Regeneration of oxidized dye/QD using a hole carrying electrolyte  Transport of electron through TiO 2 and external load  Regeneration of electrolyte at counter electrode Prof. Michael Gratzel

22  Excitation of dye molecule or Quantum Dot (QD) by incident sunlight  Transfer of electron from dye/QD to TiO 2  Regeneration of oxidized dye/QD using a hole carrying electrolyte  Transport of electron through TiO 2 and external load  Regeneration of electrolyte at counter electrode Cross-sectional SEM of DSSC (counter-electrode and electrolyte missing)

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

24 ….continued Iodide/tri-iodide electrolyte e - LOAD Dye/QD TiO2 (~ 20 nm) e - LOAD Dye/QD TiO 2 (~ 20 nm) Why Nanoparticles?: Higher Surface area than what is projected. Higher dye adsorption leads to higher photocurrent Why ZnO or TiO 2 ?: 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 I 3 - 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.

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27 Nanostructured Metal Oxides For DSSC Cu 2 O Nanoneedles ZnO Flowers ZnO Nanorods Rutile TiO 2 Needles TiO 2 -Nanotubes TiO 2 -NanoleavesTiO 2 -Nanofibers Cu 2 O nano Spheres Cu 2 O nano CubesTiO 2 Spheres TiO 2 -Nanowires ZnO CNT composite

28 Sensitizers Dyes: 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 NameVoc (V) Jsc (mA/cm 2 ) FF (%) η (%) Sol-Gel TiO

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 TiO 2 -MWCNT TiO 2 -Graphene Eff. 7.4%Eff. 6%

32 Some Results: NameVoc (V)Isc (A)FF (%) η (%) 1 st nd Efficiency Over 7%

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 n-type semiconductor p-type semiconductor h+h+ e–e– E CB E VB Inorganic cells Hybrid solar cells Electron acceptor Hole acceptor CathodeAnode HOMO LUMO h+h+ e–e– e–e– Organic cells n-type semicond uctor P-type materials CathodeAnode e–e– HOMO LUMO h+h+ e–e– Fast carriers mobility Long life time High production cost Brittle Low Production Cost Flexible Tunable color Light weight Slow carrier mobility Short life time h+h+ ETA Cell Dye-sensitized Solar Cells Inorganic n + Organic p

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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