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 m2 is 1.366 kW, which is called the solar constant.

Most of the materials to produce solar cells are abundant on the earth.

Solar energy

Energy reaching the surface of the

earth (100%)

Photosynthesis (0.02%)

Stored in oceans (23%)

Transforming into kinetic energy of waves, winds, and currents

(0.2%)

Surface radiation of sun

(3.86x10 kW)23

Reflection from the surface of the

earth to space (30%)

Energy density1.366 kW/m2

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 types Cost

Nuclear 1

Coal 1.07

Natural gas 1.20

Hydropower 1.19

Wind turbine 1.11-1.94

Solar (large) 3.38-5.14

Solar (small) 3.75-4.30

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

City Country Hours of sunshine per

year

Hours of sunshine per

day

Yuma, AZ USA 4015 11.0

Phoenix, AZ USA 3872 10.6

Asswan Egypt 3863 10.6

Las Vegas, NV USA 3825 10.5

Dongola Sudan 3814 10.4

Payback time

Energy Pay-Back Time (EPBT) EPT is defined as (entire energy spent through

the life cycle) (the energy produced for one year).

CO2 Pay-Back Time (CO2PBT) CO2PT is defined as (entire CO2 produced

through the life cycle) (the CO2 reduced by introducing a renewable energy system for one year).Most of solar cells have 1.4 to 2.7 years for CO2PBT.

Energy payback time

Renewable energy Payback time (year)

Biomass 2 ~ 6

Hydropower 0.6

Geothermal 1

Wind turbine 0.6 ~ 0.8

Solar cell 1.0 ~ 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)

SiliconBoronPhosphorusTitanium dioxideGallium – arsenateCadmium – tellurium, etc.

The power of solar cells

Open-circuit voltage minimum voltage

Short-circuit current maximum current

Power (max. output) = operating voltage operating current

Current

Voltage

Short-circuitcurrent

Open-circuitvoltage

Voltage-current curve

Maximum power

Operatingvoltage

Operatingcurrent

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

Glass substrate

Transparent electrode SnO2

Single silicon crystal (N)

Single silicon crystal (P)

Reflecting electrode (Ag)

150~300m

Higher efficiency 25%Higher cost

The structure of a polycrystalline solar cell

Glass substrate

Transparent electrode SnO2

Polycrystalline (N)

Reflecting electrode (Ag)

100~200m

Polycrystalline (P)

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

Transparent electrode SnO2

Amorphous (N)

Reflecting electrode (Ag)

Amorphous (P)

Less than 1m

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 moreThese 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%.

Reflector

Spherical silicon

P-type

N-type

Positive electrode

Negative electrode

Compound solar cells 1

Periodic tableGroup

number1 11 12 13 14 15 16

Groups for semiconduc

tors

I II III IV V VI

Period 1 H(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: 4 + 4 = 8. Ga-As has: 3 + 5 = 8. Cu(In,Ga)Se2 has: 1+3+62=16. Cu2ZnSnS4 has: 12+2+4+64=32

I II III IV V VIGroup

The number of electrons in the valence band

1 2 3 4 5 6

Compound solar cells 3 (Classification)

Silicon-diamond structureIII-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 group II-VI group I-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 CdZnTe CIS: CuInS2

Quaternary compound

InGaAsP InGaAlP InAlGaAs InGaAsSb

CIGS: Cu(In,Ga)Se2

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%.

lens

Solar cell

Cadmium-tellurium solar cell

Lower cost

This type of solar cell reached grid parity in 2009. (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.

Small quantum dots

N-type Substrate

Reflecting reducing film

Transparent electrode

Surface electrode

Large quantum dots

Visible light Near infrared light

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.