Solar Cells

• Early development of solar tech. starts in the 1960s• Conversion of sunlight to electricity – by photovoltaic effect• In 1974 only 6 private homes in North America were entirely heated by solar power (increasing rapidly since 1979 energy crisis)• Photovoltaic production growth ~ 40% per year since 2000

Nellis Solar Power Plant, Nevada

• The second larger solar power plant in North America • Located in Nellis Air Force Base, Nevada, northeast of Las Vegas • Generate 25 M Kw-h, supply 25% of the power used in the base, more than 12,000 military and civilians at Nellis• Contain 70,000 solar panels,

Theory

• Photovoltaics (PV): a method of generating electrical powere by converting solar radiation into direct current electricity using semiconductor exhibiting photovoltaic effect• Photovoltaic effect: creation of a voltage (or corresponding electric current) in a material upon exposure of light. Generated electrons are transferred from valence to conduction bands, resulting in the buildup of a voltage between two electrodes

CH2 Basic Physics of Semiconductors 10

PN Junction (Diode)

• When N-type and P-type dopants are introduced side-by-side in a semiconductor, a PN junction or a diode is formed.

CH2 Basic Physics of Semiconductors 11CH2 Basic Physics of Semiconductors 11

Depletion Region

• As free electrons and holes diffuse across the junction, a region of fixed ions is left behind. This region is known as the “depletion region.”

CH2 Basic Physics of Semiconductors 12CH2 Basic Physics of Semiconductors 12

Current Flow Across Junction: Drift

• The fixed ions in depletion region create an electric field that results in a drift current.

• At equilibrium, the drift current flowing in one direction cancels out the diffusion current flowing in the opposite direction, creating a net current of zero.

1313

Diode in Reverse Bias

• When the N-type region of a diode is connected to a higher potential than the P-type region, the diode is under reverse bias, which results in wider depletion region and larger built-in electric field across the junction.

CH2 Basic Physics of Semiconductors 14CH2 Basic Physics of Semiconductors 14

Diode in Forward Bias

• When the N-type region of a diode is at a lower potential than the P-type region, the diode is in forward bias.

• The depletion width is shortened and the built-in electric field decreased.

1515

Diode in Forward Bias

• Minority charge profile should not be constant along the x-axis; otherwise, there is no concentration gradient and no diffusion current.

• Recombination of the minority carriers with the majority carriers accounts for the dropping of minority carriers as they go deep into the P or N region.

CH2 Basic Physics of Semiconductors 16CH2 Basic Physics of Semiconductors 16

I-V (current-voltage) characteristic of a p-n junction

• The current and voltage relationship of a PN junction is exponential in forward bias region, and relatively constant in reverse bias region.

)1(exp T

DSD V

VII

CH2 Basic Physics of Semiconductors 18

2020

Light-emitting diode (LED)• A semiconductor light source • Introduced as a practical electronic component in 1962• Earlier LED with red light, modern LED available across visible, UV and infrared wavelengths• The first blue LED were made in 1971 by Jacques Pankove (GaN LED) at RCA Lab.

CH2 Basic Physics of Semiconductors 21CH2 Basic Physics of Semiconductors 21

Applications of LED

• Advantages over incandescent light: lower energy consumption, longer life time, improved robustness, small size, faster switching

• Compared to fluorescent lamp for room lighting: relatively expensive, require more precise current and heat management

• Used for used as indicator lamps, aviation lighting, traffic signals, lighting, automotive lighting

2222

Applications of LED

2323

Operations of LED• Electroluminescence effect electron-hole recombination release photons, the color is determined by the energy gap

2424

Operations of LED• Electroluminescence effect electron-hole recombination release photons, the color is determined by the energy gap