Contenu connexe



  1. Depletion of the energy resources Renewable and clean energy source Environment friendly Low maintenance Cost effective and resilient power Why there is a need of solar energy?
  2. Applications of solar cell,, Solar-powered car Solar-powered satellite Solar-powered buildings Solar-powered street lights Solar-powered water pumps Solar-powered mobile charger
  3. What is a solar cell? • A solar cell is a type of an electronic device which is able to harvest the energy from the sunlight and produce electricity. • It is also widely called as photovoltaic cell as it is based upon the principle of photovoltaic effect. Components of Solar cell Light absorber Carrier collector Metal contact Typical solar cell device architecture
  4. History of solar cell • 1839- Antoine Cesar Becquerel- reported first photovoltaic effect with platinum electrode by using a silver salt • 1873- Willoughby Smith- photoconductivity of selenium • 1883- Charles Edgar Fritts- copper plates with selenium coating and gold thin layer to develop light sensitive junction • 1887- Heinrich Hertz and Wilhelm Hallwachs- independently studied UV light photoconductivity which led to photoelectric effect discovery • 1888- Aleksandr Stoletov- assembled a cell to demonstrate photoelectric effect • 1905- Albert Einstein- postulated the photoelectric effect • 1941- Russell Ohl- single junction Si semiconductor solar cell • 1954- Pearson, Fuller and Chapin- reported first working PV solar cell • 1970- Zhores Alferov- first GaAs heterostructure solar cells • 1972- Hovel and Woodall- 18% efficiency with AlGaAs and GaAs solar cell • 1976- Carlson and Wronski- 2.4% efficiency solar cell with amorphous silicon • 1991- O’ Regan and Gratzel- photovoltaic cell call as dye sensitized solar cells with 12% efficiency • The subsequent years saw the rise of various generation of solar cells at a very rapid pace.
  5. Type/Classification of solar cells First generation cells Monocrystalline and Multi-crystalline Silicon Second generation cells Thin film solar cells (a-Si, CdSe, CdTe CuInSe, CuInGaSe, CZTS ) Third generation cells DSSC Organic solar cells Perovskite solar cells Quantum dot solar cells Alaaeddin et al., Renewable and Sustainable Energy Reviews, 2019, 102, 318-332 omparing-solar-cell-types/
  6. First generation solar cells • These are based upon either monocrystalline silicon or polycrystalline silicon. Advantages: 1. Long lasting solar cell devices (device can last for up to 25 years). 2. Monocrystalline solar cell is the most efficient solar cell material in the market. 3. Stable cell in terms of humidity, heat and environmental atmospheres. 4. No toxic waste production throughout its operational conditions. Disadvantages: 1. Initial cost of solar cell production is high. 2. At higher temperatures, significant reduction in efficiency 3. Difficult to fabricate on flexible substrates 4. High manufacturing cost as well as energy requirement. Polycrystalline silicon solar cell Monocrystalline silicon solar cell
  7. Second generation solar cells • Second generation solar cells are based upon thin film technologies. • Amorphous silicon, CIGS, CdTe are commonly used solar cell materials. Advantages: 1. Less hindrance on output with change in temperature. 2. Very efficient in areas with low sunlight. 3. Can be fabricated on flexible substrates. Disadvantages: 1. Amorphous silicon has low efficiency. 2. The use of toxic Cd element is hazardous to the environment. 3. Proper disposal and recycling of cell is also an issue. Flexible thin film solar cell device
  8. Third generation solar cells • These are currently researched and future solar cell technologies. • Multijunction solar cells, DSSC, organic solar cell, Perovskite solar cell, Quantum dot solar cells typically form the third generation solar cell technologies. Multijunction or tandem solar cell Dye sensitized solar cell Perovskite solar cell
  9. Solar cell device parameters • Open circuit voltage (Voc)- The maximum voltage of solar cell, when current is zero. • Short circuit current (Isc)- It is the current across the solar cell, when voltage is zero. • Fill factor (FF)- It determines the maximum power of a solar cell device. • Efficiency ()- It tells about the percentage of efficiency of a typical solar cell device. • Short circuit current (Isc) For V = 0, I = Isc • Open circuit voltage (Voc) For I = 0, V = Voc • Fill factor (FF) = I𝑚𝑎𝑥 ×V𝑚𝑎𝑥 Isc ×Voc = Pmax Isc ×V𝑜𝑐 • Power conversion efficiency (η): = Pmax Pin = I𝑚𝑎𝑥 ×V𝑚𝑎𝑥 Pin = FF ×Isc × V𝑜𝑐 Pin * Pin = incident light intensity 1000 W/ m2 J-V characteristic of a typical solar cell device Zhou et al., Energy Environ Sci, 2010, 3, 1851-1864
  10. Efficiency trend of all solar cell technologies NREL,
  11. • Piezoelectricity or piezoelectric effect is ability of materials to generate voltage in response to some mechanical stress applied on material. • The origin of phenomenon is specific distribution of electric charges in crystal. Schematic of Piezoelectric effect • Piezoelectric effect can be either direct or reverse piezoelectric effect Kokkinopoulos et al. Energy Procedia50 (2014) Lee, Hyuck et al. Journal of Materials Chemistry A 4.21 (2016) Piezoelectricity
  12. History of piezoelectric materials Zhang et al., Journal of Korean Chemical Society, 55 (5), 2018, 419-439
  13. Types of piezoelectric materials Piezoelectric materials Piezoelectric polymers Piezoelectric ceramics PVDF PET PVDF-TrFE Quartz PZT BaTiO3, ZnO
  14. How piezoelectric effect occurs? (A) (B) (C) • In the initial stage, without any stress applied, molecule is in a neutral state (Fig. (a)). • Under the application of stress, there is some change in crystal and negative and positive charges are separated and electric dipoles are created (Fig. (b)). • The opposite facing poles inside the material cancel each other and fixed charges appear on the surface. The material becomes polarized (Fig. (c)). Dahiya and Valle., Robotic tactile sensing: technologies and system, Springer Science & Business media, 2012
  15. Why piezoelectric material are poled? • In a macroscopic crystalline structure that comprises many unit cells, the dipoles are randomly oriented. • When the material is subjected to a mechanical stress, each dipole rotates from its original orientation toward a direction that minimizes the overall electrical and mechanical energy stored in the dipole. • If all the dipoles are initially randomly oriented, the piezoelectric effect exhibited will be negligible. • So, it is important to align the dipoles more or less in an oriented manner. This is done by process called poling. • The direction along which the dipoles align is called as the poling direction. • The application of electric field to material imparts poling and dipoles orient in the direction of field. • After poling is removed, most of the dipoles are arranged in similar kind of orientation. • By applying reverse electric field to the poling material or temperature change more than Curie temperature, material can be de- poled.
  16. Application of piezoelectric materials • Piezoelectric nanogenerators- It is a type of energy harvesting device in which with the application of mechanical energy on a nanostructured piezoelectric material, we can generate electrical energy. • Piezoelectric sensor- It is a type of device, in which we use piezoelectric phenomenon to measure change in temperature, strain, pressure or force by converting them to electricity. • Piezoelectric actuator- It is a type of device which contracts or expands when electrical charge is applied, these in turn, generate motion and force. • Piezoelectric medical devices- measure pulse rate, lung pressure, eye and brain pressure, heartbeat and respiration detection, voice detection, hand and walk motion. • Other applications- Ultrasonic cleaning device, sonar technology, transmitter and receiver, microphones and sound transducers.
  17. Application of piezoelectric materials as nanogenerators (B) (D) (A) (C) (A) Schematic of types of PZT based nanogenerator (a) crystalline PZT ribbons on flexible PDMS substrate (b) optical microscopy image of nanogenerator (c) Wavy/buckled PZT nanogenerator (d) SEM images of wavy/buckled piezoelectric PZT ribbons via a prestretched method. (e) Fabrication process of flexible PZT thin film based nanogenerator using the laser lift-off method. (f) Image of flexible PZT Nanogenerator. (Qi et al., Nano Letters, 11(3), 2011, 1331-1336). (B) Fabrication of BaTiO3 - carbon based nanogenerator on plastic substrate. (Park et al., Advanced Materials, 24(22), 2012, 2999- 3004). (C) Flexible PENGs using piezoelectric monolayer MoS2 (a-c) Operation scheme of piezoelectric NG based on MoS2 monolayer.(b-d) Morphological, performance characterization of flexible single-layer PENG (e-g) Probing piezoelectric property of free-standing monolayer MoS2. (Wu et al., Nature, 514(7523), 2014, 470-474). (D) Piezoelectric PVDF nanogenerator (a) Near-field electrospinning (NFES) combining direct-write, mechanical stretching, and in-situ electrical poling to create and place piezoelectric nanogenerator comprised of single PVDF nanofiber, two electrodes and plastic substrate (c) Output voltage measured with time under 2Hz strain (d) Output current measured with respect to time under 2Hz strain. (Chang et al., Nano Letters, 10(2), 2010, 726-731).
  18. Thermoelectricity • A thermoelectric device is a type of an electronic device which is able to harvest the heat energy and produce electricity. • It is based upon two key effects: 1. Seeback effect- When conductive material is subjected to thermal gradient, charge carrier migrate along the gradient and move from hot to cold, resulting in electric potential difference in the material. A thermocouple measures the changes in potential. 2. Peltier effect- When electric current is passed through thermocouple, heat is released at one junction and absorbed at the other junction. Seeback effect Peltier effect
  19. History of Thermoelectric • 1821- Seeback effect was first observed. • 1834- Peltier effect was observed. • 1851- Magnus stated that potential difference at two junctions is proportional to applied temperature difference. • 1856- Thomson effect was observed. • 1909 and 1911- Altenkirch introduced the term thermoelectric figure of merit. • 1949- Ioffe proposed that thermal conductivity of semiconductors is function of atomic weight of material. • 1954- Goldsmid studied variation of electrical conductivity with crystal structure and mobility of electron. • 1993- Dresselhaus and Hicks proposed that superlattice structure can improve figure of merit. • 2008- Bulusu and Walker proposed non-equilibrium green function method to find thermoelectric operformance of silicon nanowires and its superlattices.
  20. Thermoelectric materials • Bismuth chalcogenides and their nanostructures-Bi2Te3, Bi2Se3 are best room temperature thermoelectric materials with zT between 0.8-1. • Lead telluride-PbTe achieves zT around 1.5 at 773K. • Inorganic clathrates-these achieve zT~0.5 at room temperature and ~1.7 at 800K. • Mg based compound and group 14 element- MgSn, MgSi alloys doped with Sn, Ga, Ag or Li can also be used as thermoelectric material with zT~1.4. • Oxide thermoelectric-Layered Ca3Co4O9 exhibit zT in range 1.4 to 2.7 at 900K. • Conductive polymers such as Poly (3,4-ethylenedioxythiophene) (PEDOT), polyaniline (PANI), polyacetylene, polypyrrole, polycarbazole can also be used as thermoelectric material. • 2D materials such as graphene oxide, transition metal dichalcogenides (MoS2, WSe2, MoSe2), phosphorene, silicene, boron nitride, MXene, layered materials (SnSe, BiCuSeO), superlattice structures (Bi2Te3/Sb2Te3), black phosphorous, silicene and germanene can also be used as thermoelectric materials.
  21. Application of Thermoelectric materials Fitriani et al., Renewable and Sustainable Energy Reviews, 64, 2016, 635-659
  22. Parameters affecting performance of a thermoelectric device
  23. Figure of merit and Power conversion efficiency 1. Figure of merit (zT)- It decides the overall heat to electricity conversion of a material. It is a dimensionless quantity expressed as: 𝑍𝑇 = 𝑆2.𝜎 κ 𝑇 , where σ denotes electrical conductivity, S denotes the Seebeck coefficient, T denotes absolute temperature and κ denotes thermal conductivity, respectively. • To achieve maximum value of ZT, a large Seebeck coefficient (S), high electrical conductivity (σ), and low thermal conductivity (к) are essential for a material. 2. Power conversion efficiency- The power conversion efficiency is denoted as ∆𝑇 𝑇ℎ𝑜𝑡 1−𝑧𝑇 −1 1−𝑧𝑇+ 𝑇ℎ𝑜𝑡 𝑇𝑐𝑜𝑙𝑑 where T/Thot and zT dependent quantity give efficiency of device, Thot and Tcold denote the hot- end and cold-end temperatures. • Figure depicts the plot of thermoelectric efficiency as function of temperature and zT • 10-15% efficiency for zT=1 • 20-30% efficiency for zT=2 • Ideal case- zT  1 for achieving efficiency of more than 10% Fleurial, JOM, 2009, 61, 79-85
  24. Variation in electrical properties of a thermoelectric material as function of carrier concentration 3. Charge carrier concentration (n) • The number of charge carriers determine electronic conductivity and Seebeck coefficient. • Carrier concentration increases with temperature, which improves electrical conductivity. • Carrier concentration is affected by doping, alloying, grain boundaries, impurity, density, addition of dispersed secondary phases. Effect of carrier concentration on thermoelectric performance • From plot, we infer, charge carrier concentration effects both Seebeck coefficient and electrical conductivity. • Increased carrier concentration, increases electrical conductivity and decreases Seebeck coefficient. Gayner and Kar, Progress in Materials Science 2016, 83, 330-382
  25. 4. Seebeck coefficient () • It depends on band gap, crystal structure, carrier concentration and varies with temperature in a non-linear manner. • Typical values of Seebeck coefficient for metal, semiconductors and insulators are ~10, ~200 and >200µV/K. • The Seebeck coefficient should be measured when temperature and voltage are in a steady state, voltage response to temperature gradient varies in a linear manner. Effect of Seebeck coefficient on thermoelectric performance Effect of carrier concentration on Seebeck effect of doped PbSe and other materials • In Na doped PbSe, Seebeck coefficient reduces with carrier concentration. At 270C, Seebeck coefficient for system is ~17µV/K with carrier concentration of 17x1019 cm-3. • Tl dopants have high Seebeck coefficient that shows less variation with carrier concentration due to tuning of resonant energy levels by Tl atoms in Tl- PbTe system. • K doped dopants have varied Seebeck coefficient. • Nature of dopants and concentration affects the Seebeck coefficient. Gayner and Kar, Progress in Materials Science 2016, 83, 330-382
  26. 5. Effective mass (m*) • It reflects the band curvature and doping is major parameter which affects effective mass. • Parabolic band models are used for evaluation of electronic properties of thermoelectric materials, in which effective mass determines the electronic structure and electronic transport. • For non parabolic infinite-gap semiconductors, energy and temperature dependent mass affects thermoelectric material. • In heavily doped semiconductors, at a given carrier concentration and temperature, thermopower increases with effective mass. • Seebeck coefficient can be expressed using constant scattering time approximation as:  = 4𝜋2𝑘𝑏 2 𝑒ℎ2 𝑚 ∗ 𝑇 4𝜋 3𝑛 2/3 ,for parabolic bands, thermopower  m*. • The For isotropic parabolic bands, conductivity, carrier concentration and m* are also related by equation:  =ne2τ/m*, where n is carrier concentration and τ is the scattering time. • A high Seebeck coefficient can be attained by generating a large DOS effective mass, which decreases the mobility (i.e., conductivity). • A lower effective mass leads to high ZT values and a greater effective mass to low ZT values; doping and temperature can be used to tune the effective mass. Effect of effective mass on thermoelectric performance
  27. Effect of electrical & thermal conductivity on thermoelectric performance 6. Electrical conductivity () • Electrical conductivity of semiconductor depends on carrier mobility and concentration , denoted as:  =e (µe. n + µh. p) where µe, µh , n, p denote the electron mobility, hole mobility, number density of electrons and the number density of holes. • Lattice and impurity scattering determine mobility of electrons and holes. • With increase in temperature, lattice vibrations increase, causing decrease in mobility. • Impurity scattering increases with a decrease in temperature, causing decrease in mobility. • The type of material, doping, impurities, temperature affect electrical conductivity. • Structural parameters such as grain size, strain and lattice constant also affect electrical conductivity. 7. Thermal conductivity (κ) • The total thermal conductivity is sum of lattice thermal conductivity(κlattice) and charge-carrier thermal conductivity(kc). • A lower thermal conductivity can be achieved by maximizing the κc/κlattice ratio, which is achieved by lowering the lattice thermal conductivity. • Measurement of thermal conductivity is a good tool for investigating lattice defects or imperfections. • The charge carrier thermal conductivity is changed by bipolar diffusion, when carrier concentration and mobility of electrons and holes are comparable. • The lattice thermal conductivity depends on the crystal structure and lattice parameters of material. • Factors such as lattice parameter, density of material, anharmonic lattice vibration, temperature, all cause effect on thermal conductivity in one way or other. • For materials with small particle size, thermal conductivity is low as small grains cause increased phonon scattering.