《新能源材料与技术》课程教学课件（讲稿）L2 Solar energy and conversion technologies（1/2）

新能源材料与技术Renewable Energy: Materials and TechnologiesL2: Solar Energy and Conversion Technologies (I)- Photovoltaic (PV)- Dye Sensitized Solar Cells (DSSC) (1)Dr Yimin Chaoy.chao@uea.ac.ukwww.uea.ac.uk/chemistry/people/faculty/chao

新能源材料与技术 Renewable Energy: Materials and Technologies L2: Solar Energy and Conversion Technologies (I) - Photovoltaic (PV) - Dye Sensitized Solar Cells (DSSC) (1) Dr Yimin Chao y.chao@uea.ac.uk www.uea.ac.uk/chemistry/people/faculty/chao

Whydowewanttousesolarenergy?7.2 TW32TW包-86.000 TWHydroGeothermal870TW15 TW印GlobalSolarWindConsumptionCover0.15%oftheearth'ssurfacewith15%efficientsolardevices-problemsolved!

Why do we want to use solar energy? Cover 0.15% of the earth’s surface with 15% efficient solar devices – problem solved! 2

Solar RadiationEnergy from the sun in the form of ultra-violet, visible and infra-redelectromagnetic radiation is known as solar radiation.Annual solar radiation on ahorizontalsurfaceattheequator赤道isover2000kWh/m?In Northern Europe this fallsto about 1000 kWh/m? (perannum)The tilt between the sun andthe land reduces the intensityUltraviolet0.20 -0.39 μmVisibleof the midday sun0.39- 0.78 μmNear-Infrared0.78 - 4.00 μmInfrared4.00-100.00 μm3

Solar Radiation • Annual solar radiation on a horizontal surface at the equator 赤道is over 2000 kWh/m2 • In Northern Europe this falls to about 1000 kWh/m2 (per annum) • The tilt between the sun and the land reduces the intensity of the midday sun Ultraviolet 0.20 - 0.39 µm Visible 0.39 - 0.78 µm Near-Infrared 0.78 - 4.00 µm Infrared 4.00 - 100.00 µm Energy from the sun in the form of ultra-violet, visible and infra-red electromagnetic radiation is known as solar radiation. 3

Question l:Direct sunlight of an average intensity of 200 W m-2 is incidentnormally on a solar cell. The area of the cell is O.1 m?.(i) What is the total incident energy in 1 day?(i) How is this total energy altered if the sunlight falls at anangle of30o to thenormal of the surface of the cell?

4 Question 1: Direct sunlight of an average intensity of 200 W m-2 is incident normally on a solar cell. The area of the cell is 0.1 m2. (i) What is the total incident energy in 1 day? (ii) How is this total energy altered if the sunlight falls at an angle of 30o to the normal of the surface of the cell?

(1)Photovoltaic conversion:> conversion of radiation energy(light) to electrical energy, canbe achieved usingSOLARPANELsemiconductormaterialsLOADIAGRAM> p-n junction is required inn-typesilicororder to produce a gradient toJunctiontransport chargesp-type siliconPHOTOHSELECTROHFLOWPor"HOLEFLOW5

5 (1) Photovoltaic conversion: Ø conversion of radiation energy (light) to electrical energy, can be achieved using semiconductor materials; Ø p-n junction is required in order to produce a gradient to transport charges

Solar Cell Band Diagramconduction band(empty)negative charge(electron)1forbiddenpue snoetiepueg aa/elaoaenergy band(gap)eOFermipositiveenergycharge(hole)Valence band(fullb)metalmetaln-dopedp-dopedcontactcontactsemicondsemicond6

6 Solar Cell Band Diagram

Question2:The bandgap of GaAs is 1.4 eV. Calculate the optimumwavelength of light for photovoltaic generation in a GaAs solalcell.7

7 Question 2: The bandgap of GaAs is 1.4 eV. Calculate the optimum wavelength of light for photovoltaic generation in a GaAs solar cell

Typical Embodiment of a p-n JunctionNeutralARCLayerE.Neutral p-regionn-regionDriftDiffusionLongaShort入2Medium入LhBackelectrode.Finger electrodeW.W.In1Depletion Region0048http://electrons.wikidot.com/solar-cellsARC:anti-reflectivecoating

Typical Embodiment of a p-n Junction 8 Eo ARC Layer Neutral p-region Long λ Medium λ Short λ Depletion Region + _ + - Le Lh Voc Wp Neutral n-region Finger electrode lp ln + + - - Drift Diffusion Back electrode Wn ARC: anti-reflective coating http://electrons.wikidot.com/solar-cells

Description: Cell is illuminated through thin n-region.ARC reduces reflections from surface: Space charge / depletion region extends primarily into p-side (N.W, = NaW) - more heavily doped n-region: As usual there is a built-in electric field across thedepletion region, E。Na -concentration of acceptor atoms = p in p-type semiconductor (concentration of holes)Nd- concentration of donor atoms = n in n-type conductor (concentration of electrons)Wp-depletion width on the p sideWn - depletion width on the n side9

Description • Cell is illuminated through thin n-region • ARC reduces reflections from surface • Space charge / depletion region extends primarily into p￾side (NaWp = NdWn) – more heavily doped n-region • As usual there is a built-in electric field across the depletion region, Eo Na – concentration of acceptor atoms = p in p-type semiconductor (concentration of holes) Nd – concentration of donor atoms = n in n-type conductor (concentration of electrons) Wp – depletion width on the p side Wn - depletion width on the n side 9

Operation. Photons are not all absorbed / reflected from the surface -- butcan penetrate a certain depth into the material (depends onwavelength).As n-region is very thin, (typically < 0.2 μm), most of thephotons are absorbed in the depletion region and the p-regionAbsorption of a photon leads to the creation of an electron-hole-pair (EHP)Within depletion region, EHP drift apart due to the forceexerted by E.Electrons → n-side, holes p-sideThis leads to an open-circuit voltage developing across thedevice, with p-side positive the n-side negative10

Operation • Photons are not all absorbed / reflected from the surface – but can penetrate a certain depth into the material (depends on wavelength) • As n-region is very thin, (typically < 0.2 µm), most of the photons are absorbed in the depletion region and the p-region • Absorption of a photon leads to the creation of an electron￾hole-pair (EHP) • Within depletion region, EHP drift apart due to the force exerted by Eo. Electrons à n-side, holes à p-side • This leads to an open-circuit voltage developing across the device, with p-side positive the n-side negative 10