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  2. 2.  Photovoltaic Conventional solar cell  Introduction  Working  Limitations Energy bands in solids Intermediate band solar cell Quantum dot Intermediate band quantum dot solar cell  Introduction  Construction  Working  Advantages  Applications  Limitations
  3. 3.  Generations of voltage from photons Light energy ( photons) are converted into electrical energy ( voltage). This conversion is called “ photovoltaic effect”.
  4. 4.  First generation: silicon wafer- based solar cells Second generation: thin-film deposits of semiconductors Third generation: photo- electrochemical cells Fourth generation: composite photovoltaic technology
  5. 5.  The solar cell (or photovoltaic cell) is a device that converts light energy into electrical energy. Fundamentally, the device needs to fulfill only two functions: 1. Photo-generation of charge carriers (electrons and holes) in a light-absorbing material. 2. Separation of the charge carriers to a conductive contact that will transmit the electricity.
  6. 6. .
  7. 7.  The intermediate band (IB) is an electronic band located within the semiconductor band gap, separated from the conduction and the valence band by a null density of states. Intermediate band solar cells (IBSCs) are photovoltaic devices. Used to exploit the energy of below band gap energy photons.
  8. 8.  Higher photocurrent Higher efficiency arising from absorption of 2 sub-band gap photons to create one electron-hole pair. High voltage V=(EF,CB- EF,VB)/q V~Eg for main semiconductor Essentials for operation 3 quasi-Fermi levels IB “disconnected” from emitters Need IB half-filled with electrons Non-overlapping absorption coefficients
  9. 9. Answer“Introduce Quantum Dots”
  10. 10.  A quantum dot is a nano meter sized particle of a low band gap material surrounded by a material with larger band gap. “Artificial atom” with energy levels depending on the dot size and on the band gap difference. If many quantum dots are placed closed to each other in a lattice one or more intermediate bands can be formed and a new semiconductor with tailored properties has been made.
  11. 11.  A quantum dot is a portion of matter (e.g., semiconductor) whose excitons are confined in all three spatial dimensions. Quantum dots have properties combined between  Those of bulk semiconductors  Those of atoms
  12. 12. The structure is as follow :
  13. 13.  Diluted II-VI oxide semiconductors e.g., Zn1-yMnyOxTe1-x alloys Transition-metal impurities in semiconductors e.g., Ga4P3M and GaxPyM alloys, where M is a transition metal such as Ti Quantum dots, e.g., InGaAs/AlGaAs
  14. 14.  Dot sized shape, composition Dot spacing Dot regularity Materials Doping
  15. 15.  Higher Efficiency. Balance between the two factors : (I) Cost/Watt (II) Efficiency
  16. 16.  Photovoltaic devices: solar cells Biology : biosensors, imaging Light emitting diodes: LEDs Quantum computation Flat-panel displays Memory elements Photodetectors Lasers
  17. 17.  Weak absorption of sub-band gap photons Low open-circuit voltage Low currents Cost
  18. 18.  QD SL cells show photo responses extended to longer wavelengths than GaAs control cells, demonstrating current generation from the absorption of sub-bandgap photons. IBSC theoretically offers a way to significantly increase cell efficiency compared to that of a single-junction solar cell.
  19. 19.  Much more work needs to be done before IBSC can make a major contribution to the PV market. “ Miles To Go Before I sleep”