Solar energy and solar cells As another renewable energy

Photovoltaic power generation Solar cells utilizes the photovoltaic effect with the related semiconductors. Solar energy comes on the surface of the earth and the maximum flux density per 1 m 2 is kW, which is called the solar constant. Most of the materials to produce solar cells are abundant on the earth.

Solar energy

Disadvantages of photovoltaic power The cost per kilowatt is relatively high. The energy density is low. (It needs a large space.) It depends on hours of sunlight. It requires an extra system to store electricity.

Cost of electricity by sources Nuclear power as the reference to compare Plant typesCost Nuclear1 Coal1.07 Natural gas1.20 Hydropower1.19 Wind turbine Solar (large) Solar (small)

Analysis of sunlight Contents of spectra  Visible rays (47%); Infrared rays (46%); and Ultraviolet rays (7%) Solar cells and sunlight spectra  Crystallized silicon absorbs 0.4  m – 1.2  m.  Amorphous silicon absorbs below 0.8  m.

Hours of sunshine The sunniest places on earth CityCountry Hours of sunshine per year Hours of sunshine per day Yuma, AZUSA Phoenix, AZ USA AsswanEgypt Las Vegas, NV USA DongolaSudan

Payback time Energy Pay-Back Time (EPBT)  EPT is defined as (entire energy spent through the life cycle)  (the energy produced for one year). CO 2 Pay-Back Time (CO 2 PBT)  CO 2 PT is defined as (entire CO 2 produced through the life cycle)  (the CO 2 reduced by introducing a renewable energy system for one year). Most of solar cells have 1.4 to 2.7 years for CO 2 PBT.

Energy payback time Renewable energyPayback time (year) Biomass2 ~ 6 Hydropower0.6 Geothermal1 Wind turbine0.6 ~ 0.8 Solar cell1.0 ~ 1.9 Wave ~ 3.8

Solar cells and semiconductors 1 Solar cells are made of semiconductors.  N-type semiconductor: Extra electrons become carriers to make current flows.  P-type semiconductor: Holes are become carriers for current.  I-type semiconductor: There is no extra electrons or holes. It needs certain heat or light energy to produce extra electrons from the covalent bonds.

Solar cells and semiconductors 2 PN junction (single crystal)  A basic structure of solar cells – The efficiency is about 24.2%. PIN junction (thin film)  I-type is sandwiched by p-type and n-type semiconductors. – The efficiency is about 10%, but it is inexpensive.

Materials for solar cells (semiconductors) Silicon Boron Phosphorus Titanium dioxide Gallium – arsenate Cadmium – tellurium, etc.

The power of solar cells Open-circuit voltage  minimum voltage Short-circuit current  maximum current Power (max. output) = operating voltage  operating current

Why the efficiency cannot be 100% Light is reflected on the surface of the cell. Some of light transmits through the cell. It cannot absorb all of the wavelengths of sun light. Part of carriers occur pair annihilation. Inside solar cells contain electric resistance.

The structure of a single crystal solar cell Higher efficiency 25% Higher cost

The structure of a polycrystalline solar cell Lower efficiency 18% Lower cost

Amorphous silicon (a-Si) solar cell Amorphous means non-crystallized. Rate of absorption is large.  This is because of random configuration of atoms. One can use various substrates and produce thinner solar cells. Photo deterioration of a-Si reduces the output by 10%. However, the output under high temperatures is better than the others.  High temperature improves photodeterioration.

The structure of an amorphous solar cell

Microcrystalline silicon (  -Si) cell Photo deterioration is small. Absorption rate is higher within wavelengths of sun light. The efficiency is about 10%. The thickness is 2 ~ 3  m. The overall properties of  -Si are between crystallized and amorphous Si.

Multi-junction silicon solar cell These are more efficient.  Silicon-type: 20% or more; Compound-type: 35% or more These can absorb more wavelengths due to multiple materials. Multiple structure makes it complicated and increases internal losses of various properties. Multiple junction makes it bulky and heavy.

Spherical silicon solar cells The amount of silicon used is 1/5 to 1/30 compared with the other cells. The spherical silicon is made by free fall and its surface tension. The efficiency is 11% ~ 14%.

Compound solar cells 1 Periodic table Group number Groups for semiconduc tors IIIIIIIVVVI Period 1H (Hydrogen) Period 2___ B (Boron) C (Carbon) N (Nitrogen) O (Oxygen) Period 3___ Al ( Aluminum ) Si (Silicon) P (Phosphorus) S (Sulfur) Period 4___Cu (Copper) Zn (Zinc) Ga (Gallium) Ge (Germanium) As (Arsenic) Se (Selenium) Period 5___Ag (Silver) Cd (Cadmium) In (Indium) Sn (Tin) Sb (Antimony) Te (Tellurium)

Compound solar cells 2 If the total of the number of electrons in the valence band is multiples of 8, it becomes a stable crystal. Si-Si has: = 8. Ga-As has: = 8. Cu(In,Ga)Se 2 has:  2=16. Cu 2 ZnSnS 4 has: 1   4=32

Compound solar cells 3 (Classification) Silicon-diamond structure III-V: Zincblende structure (In, Ga, As, P) II-VI: Zincblende structure (Cd, Te, S) I-III-VI: Chalcopyrite structure CIS (Cu, In, Se) and CIGS (Cu. In, Ga, Se) I-II-IV-VI: (Cu, Zn, Sn, S, Se)

Compound solar cells 4 (Example) Examples of compound semiconductors III-V groupII-VI groupI-III-VI group Binary compound AlAs GaAs InP InAs AlSb GaSb GaP AlP ZnS ZSe CdTe CdS ZnTe Ternary compound AlGaAs GaAsP GaInP AlGaSb ZnSSe CdZnTeCIS: CuInS 2 Quaternary compound InGaAsP InGaAlP InAlGaAs InGaAsSb CIGS: Cu(In,Ga)Se 2

Compound solar cells 5 The most efficient ones are GaAs and InGaAs solar cells. (35.8%) The theoretical efficiency of compound multi-junction solar cells: one layer = 37%; two layers = 50%; three layers = 56%; and 36 layers = 72%

Concentrator Photo Voltaic System When the collection efficiency is n, the area of solar cell required is 1/n. This system is used for more expensive semiconductors. The current efficiency is about 40%.

Cadmium-tellurium solar cell Lower cost This type of solar cell reached grid parity in (Namely, the cost is equal to the electric power generation.) The amount of cadmium is very small and it will not harm the environment and human.

Organic-type solar cell 1 OSC (Organic Solar Cell)  Conductive polymer and fullerene are used for the surface layer. DSC (Dye-sensitized Solar Cell)  The mechanism is based on Grätzel cell.  The pigments in electrolyte are positively ionized to absorb electrons with light.  Fluorine-doped tin oxide (FTO) and Indium tin oxide (ITO) are used.

Organic-type solar cell 2 This type can be printed (OSC)  Evaporation method  Print-on method The efficiency is about 7%. DSC has achieved about 10% of efficiency.

Quantum dot solar cell 1 Confine electrons 3 dimensionally with the diameter of dozens of nanometers. When photons hit the quantum dots, excitons are generated by the energy absorption. Therefore, electrons and holes are emerged.

Quantum dot solar cell 2 The small quantum dots absorb light with shorter wavelengths. The large quantum dots absorb light with longer wavelengths.

Quantum dot solar cell 3 This solar cell can utilizes wide range of light. This absorbs various wavelengths by using different sizes of quantum dots to laminate the cell. In theory, the efficiency can go up as the technique improves.