Solar Cell Semiconductor Physics Basic Fundamentals of Solar Cell Semiconductor Physics

Review Topics

Wavelength and Frequency Period (sec) amplitude time Frequency (n) = 1/Period [cycles/sec or Hertz] Wavelength (l) = length of one Period [meters] For an electromagnetic wave c = nl, where c is the speed of light (2.998 x 108 m/sec)

Spectrum Intensity Frequency (n) Range of frequency (or wavelength, c/n) responses or source emissions. The human eye has a response spectrum ranging from a wavelength of 0.4 microns (0.4 x 10-6 meters) (purple) to 0.8 microns (red)

Solar Spectrum at Earth Surface (noon time) 925 W/m2 E (eV) = hc/l l = hc/E Visable range .75 mm (red) - .4 mm (purple) 1.6 eV - 3.1 eV Solar Spectrum at Earth Surface .5 eV - 3.6 eV 2.6 mm (infrared) - 0.34 mm (ultraviolet) visible inrfared ultraviolet Solar Spectrum at Earth Surface (noon time)

Energy and Power Electromagnetic waves (light, x-rays, heat) transport energy. E = hn or hc/l [Joules or eV (electron-volts)] 1 eV = 1.6 x 10-19 Joules h = Plank’s constant (6.625 x 10-34 Joule-sec or 4.135 x 10-15 eV-sec) n = frequency c = speed of light l = wavelength Power is the amount of energy delivered per unit time. P = E/t [Joules/sec or Watts]

Photons A light particle having energy. Sunlight is a spectrum of photons. X-rays and heat are photons also. Photon Energy E = hn or hc/l [Joules or eV (electron-volts)] (higher frequency = higher energy) (lower energy)

Amount of power over a given area, Watts/m2 Irradiance Amount of power over a given area, Watts/m2 4 red photons every second Area = 2.00 m2 Energy of 1 red photon = hc/l = (6.63 x 10-34 J-s)(2.99 x 108 m/s)/(0.80 x 10-6 meters) = 2.48 x 10-19 J = 1.55 eV Irradiance = Power/Area = (4 photons/sec)(Energy of 1 photon)/2.00 m2 = 4.96 x 10-19 W/m2 Typical sunlight irradiance is 0.093 W/cm2 = 930 W/m2 at l = .55 mm

Transmission, Reflection, and Absorption incident light reflectance (R) air transmittance (T) + absoprtance (A) material Incident light = T + R + A = 100% Non-transparent materials have either very high reflection or very high absorption. Absorption decreases transmission intensity with increasing depth into material.

Polarization Polarizer Unpolarized light (e.g. sunlight) Linearly polarized light Only one plane of vibration passes

Basics of Semiconductor Physics

Semiconductor Crystal Lattice covalent bond atom Simple Cubic Structure Silicon has a more complex lattice structure but a lattice structure exists nevertheless.

Crystalline Silicon Bonds valance electrons Si atom (Group IV) = covalent bond (electron sharing)

Breaking of Covalent Bond Creating Electron-Hole Pair free electron moving through lattice e- created hole (missing electron) + covalent bond Si atom Photon (light, heat) Photon hits valance electron with enough energy to create a free electron and hole

Movement of a Hole in a Semiconductor + + Thermal energy causes a valance electron to jump to an existing hole leaving a hole behind

Valance and Conduction Energy Bands free electron moving in lattice structure Conduction Energy Band e- Ec Band Gap Energy, Eg = Ec - Ev Valance Energy Band Ev + Hole within valance band covalent bonds

Valance and Conduction Energy Bands Thermal Equilibrium free electron combines with hole free electron within lattice structure Conduction Energy Band e- e- Ec Eg Heat enery given up Heat energy absorbed Valance Energy Band Ev + + Hole created within valance band covalent bonds Energy absorbed = Energy given up

Intrinsic (pure) Silicon Electron-Hole Pairs Thermal Equalibrium ni = 1.5 x 1010 cm-3 at 300° K Conduction Band e- Ec hole density = electron density number of holes per cubic centimeter = number of free electrons per cubic centimeter pi = ni = 1.5 x 1010 cm-3 Eg = 1.12 eV pi = 1.5 x 1010 cm-3 at 300° K Valance Band Ev + covalent bonds Number of electron-hole pairs increase with increasing temperature

Creating a Semiconductor

Doping or Substitutional Impurities Group V Atom (Donor or N-type Doping) Phospherous (Group V) P atom e- covalent bond Si atom (Group IV) The donor electron is not part of a covalent bond so less energy is required to create a free electron

Energy Band Diagram of Phospherous Doping intrinsic free electron donor free electron Conduction Band e- e- Ec Donor Electron Energy n > p (more electrons in conduction band) A small amount of thermal energy elevates the donor electron to the conduction band Eg Valance Band Ev + intrinsic hole covalent bonds N-type Semiconductor

- Doping or Substitutional Impurities Group III Atom (Acceptor or P-type Doping) Boron (Group III) B atom - covalent bond + created hole covalent bond Si atom Boron atom attacts a momentarily free valance electron creating a hole in the Valance Band

Energy Band Diagram of Boron Doping intrinsic free electron Conduction Band e- Ec p > n (more holes in valance band) A small amount of thermal energy elevates the acceptor electron to the Acceptor band Eg acceptor electron Acceptor Electron Energy e- Valance Band Ev + + created hole intrinsic hole covalent bonds P-type Semicondutor

Formation and Basic Physics of PN Junctions

Intrinsic Silicon Wafer PN Junction Formation Masking Barrier Boron Atom Doping Phophorous Atom Doping Intrinsic Silicon Wafer Doping Atoms are accelerated towards Silicon Wafer Doping Atoms are implanted into Silicon Wafer Wafer is heated to provide necessary energy for Doping Atoms to become part of Silicon lattice structure

PN Junction in Thermal Equilibrium (No Applied Electric Field) Space Charge Region metallurgical junction metallurgical junction - + P-type N-Type p n Initial Condition E field Equilibrium Condition Free electrons from n-region migrate to p-region leaving donor atoms behind. Holes from p-region migrate to n-region leaving acceptor atoms behind. Internal Electric Field is created within Space Charge Region.

Solar Cell Basic Operation

PN Junction Solar Cell Operation Photon hn > Eg Space Charge Region + e- E field p n Photons create hole-electron pairs in space charge region Created hole-electron pairs swepted out by internal E field Excess holes in p-region Excess electrons in n-region

PN Junction Solar Cell Operation Photon hn > Eg Space Charge Region + e- E field p n Icell Resistor - + Vcell Attaching a resistive load with wires to the PN Junction creates current flow from p to n regions Electrons flow from n-region to combine with holes in p-region Photons create new hole-electron pairs to replace combined pairs

Typical Silicon Solar Cell Design Photons Protective High Transmission Layer P-type Doping Wires N-type Silicon Wafer 0.6 mm 4-6 inches To load Photons transmit through thin protective layer and thin P-type doped layer and create hole-electron pairs in space charge region Typical Silicon Single Cell Voltage Output = ~ 0.5 volts

Silicon Solar Cell 6 Volt Panel Series-Parallel Design 12 cells in series = 6 volts p to n connection - 6 volts +

External Factors Influencing Solar Cell Effeciency Photon transmission, reflection, and absorption of protective layer Maximum transmission desired Minimum reflection and absorption desired Polarization of protective layer Minimum polarized transmission desired Photon Intensity Increased intensity (more photons) increases cell current, Icell Cell voltage, Vcell, increases only slightly Larger cell area produces larger current (more incident photons) Theoretical Silicon Solar Cell Maximum Efficiency = 28% Typical Silicon Solar Cell Efficiency = 10-15%