Lecture Materials on EEE-423 Electrical and Electronic Engineering, JKKNIU

◼ Optical Absorption
▪ Wave–particle duality principle of light

▪ Light waves could be treated as particles, which are referred to as photons

▪ The energy of a photon is

𝑬 = 𝒉𝝂

▪ h: Plank’s constant and 𝝂: frequency

▪ Wavelength and energy relation

▪ Several possible photon–semiconductor interaction mechanisms

▪ Example: photons can interact with the semiconductor lattice whereby the photon energy is

converted into heat

▪ Photons can also interact with impurity atoms, either donors or acceptors, or they can interact with

defects within the semiconductor

-1-
E-mail: asgar_eee@jkkniu.edu.bd
Lecture Materials on EEE-423 Electrical and Electronic Engineering, JKKNIU

◼ Optical Absorption
▪ However, basic photon interaction process of greatest interest is the interaction with valence

electrons

▪ A photon collides with a valence electron, enough energy may be imparted to elevate the electron into

the conduction band

▪ This process generates electron–hole pairs and creates excess carrier concentrations

Figure: Optically generated electron–hole pair formation in a semiconductor

-2-
E-mail: asgar_eee@jkkniu.edu.bd
Lecture Materials on EEE-423 Electrical and Electronic Engineering, JKKNIU

◼ Photon Absorption Coefficient

Intensity of photon flux

Optical absorption in a differential length Photon intensity versus distance for two
absorption coefficients

-3-
E-mail: asgar_eee@jkkniu.edu.bd
Lecture Materials on EEE-423 Electrical and Electronic Engineering, JKKNIU

◼ Photon Absorption Coefficient

▪ The relation between the

bandgap energies of

some of the common

semiconductor materials

and the light spectrum is

shows that silicon and

gallium arsenide will

absorb all of the visible

spectrum The visible spectrum covers a range of photon energies. Since the band gap of silicon (around 1.1 eV) and gallium arsenide
(around 1.4 eV) is less than the energy of most visible light photons (around 1.8 - 3.1 eV),
▪ Whereas gallium

phosphide, for example,

will be transparent to the

red spectrum Light spectrum versus wavelength and energy

-4-
E-mail: asgar_eee@jkkniu.edu.bd
Lecture Materials on EEE-423 Electrical and Electronic Engineering, JKKNIU

◼ Solar PV Cells

▪ A solar cell (also known as a photovoltaic cell or PV cell) is defined as an electrical device that

converts light energy into electrical energy through the photovoltaic effect

▪ A solar cell is basically a p-n junction diode. Solar cells are a form of photoelectric cell, defined

as a device whose electrical characteristics – such as current, voltage, or resistance – vary

when exposed to light

▪ Individual solar cells can be combined to form modules commonly known as solar panels

▪ The common single junction silicon solar cell can produce a maximum open-circuit voltage of

approximately 0.5 to 0.6 volts

▪ Solar cells are tiny, however, when combined into a large solar panel, considerable amounts of

renewable energy can be generated

-5-
E-mail: asgar_eee@jkkniu.edu.bd
Lecture Materials on EEE-423 Electrical and Electronic Engineering, JKKNIU

◼ Construction of Solar Cell

▪ A solar cell is basically a junction diode although its construction it is little bit different from

conventional p-n junction diodes

▪ A very thick layer of p-type semiconductor is grown on a relatively thin n-type semiconductor

▪ We then apply a few finer electrodes on the top of the p-type semiconductor layer

▪ These electrodes do not obstruct light to reach the thin p-type layer
n
▪ A current collecting electrode at the bottom of the n-type layer is placed

▪ Entire assembly is encapsulated by thin glass to protect the solar cell from any mechanical

shock

-6-
E-mail: asgar_eee@jkkniu.edu.bd
Lecture Materials on EEE-423 Electrical and Electronic Engineering, JKKNIU

◼ Construction of Solar Cell

-7-
E-mail: asgar_eee@jkkniu.edu.bd
Lecture Materials on EEE-423 Electrical and Electronic Engineering, JKKNIU

◼ Current-Voltage relation in Solar Cell

▪ With zero bias applied to junction, an electric field

exists in the space charge region as

▪ Incident photon illumination can create electron–hole

pairs in space charge region that will be swept out

producing the photocurrent 𝐼𝐿 in reverse-biased

direction

▪ Photocurrent 𝐼𝐿 produces a voltage drop across resistive load which forward biases the pn

junction

▪ Forward-bias voltage produces a forward-bias current 𝐼𝐹

▪ Net pn junction current, in the reverse-biased direction

-8-
E-mail: asgar_eee@jkkniu.edu.bd
Lecture Materials on EEE-423 Electrical and Electronic Engineering, JKKNIU

◼ Current-Voltage relation in Solar Cell

▪ As diode becomes forward biased, magnitude of the electric field in space charge region

decreases, but does not go to zero or change direction

▪ Photocurrent is always in reverse-biased direction and the net solar cell current is also always in

reverse-biased direction

▪ Two limiting cases:

(i) short-circuit condition: 𝑅 = 0 so that 𝑉 = 0

𝐼 = 𝐼𝑠𝑐 = 𝐼𝐿

(ii) open-circuit condition R = ∞ so that 𝑰 = 𝟎

Solving for 𝑽𝒐𝒄

-9-
E-mail: asgar_eee@jkkniu.edu.bd
Lecture Materials on EEE-423 Electrical and Electronic Engineering, JKKNIU

◼ Power Delivered to the Load

▪ The power delivered to the load is

▪ For finding current and voltage which will deliver the maximum power to load by setting the

𝑑𝑃
derivative equal to zero, i.e., =0
𝑑𝑉

▪ Therefore,

▪ where 𝑉𝑚 is the voltage that produces the maximum power

▪ Thus,

▪ Value of 𝑉𝑚 may be determined by trial and error

- 10 -
E-mail: asgar_eee@jkkniu.edu.bd
Lecture Materials on EEE-423 Electrical and Electronic Engineering, JKKNIU

◼ Power Delivered to the Load

The short-circuit current is due to the generation and collection

of light-generated carriers. For an ideal solar cell at most
moderate resistive loss mechanisms, the short-circuit current
and the light-generated current are identical

Maximum power rectangle of the solar cell I–V characteristics

- 11 -
E-mail: asgar_eee@jkkniu.edu.bd
Lecture Materials on EEE-423 Electrical and Electronic Engineering, JKKNIU

◼ Conversion Efficiency

▪ Definition: Conversion efficiency of a solar cell is defined as the ratio of output electrical power

to incident optical power

▪ For maximum power output,

▪ Maximum possible current and maximum possible voltage in the solar cell are 𝐼𝑠𝑐 and 𝑉𝑜𝑐 ,

respectively

▪ Fill Factor: The ratio 𝐼𝑚 𝑉𝑚 to 𝐼𝑠𝑐 𝑉𝑜𝑐 is called the fill factor and is a measure of the realizable

power from a solar cell

▪ Typically, fill factor is between 0.7 and 0.8

- 12 -
E-mail: asgar_eee@jkkniu.edu.bd
Lecture Materials on EEE-423 Electrical and Electronic Engineering, JKKNIU

◼ Solar concentrator

▪ Conventional pn junction solar cell has a single semiconductor bandgap energy

▪ When cell is exposed to the solar spectrum, a photon with energy less than 𝐸𝑔 will have no

effect on the electrical output power of the solar cell

▪ A photon with energy greater than 𝐸𝑔 will contribute to solar cell output power, but fraction of

photon energy that is greater than 𝐸𝑔 will eventually only be dissipated as heat

▪ Solar spectral irradiance (power per unit area per unit wavelength) where air mass zero

represents the solar spectrum outside the earth’s atmosphere and air mass one is the solar

spectrum at the earth’s surface at noon

▪ The maximum efficiency of a silicon pn junction solar cell is approximately 28%

▪ Nonideal factors, such as series resistance and reflection from the semiconductor surface, will

lower the conversion efficiency typically to the range of 10 to 15 percent

- 13 -
E-mail: asgar_eee@jkkniu.edu.bd
Lecture Materials on EEE-423 Electrical and Electronic Engineering, JKKNIU

◼ Solar concentrator

▪ A large optical lens can be used to concentrate sunlight onto a solar cell so that light intensity

can be increased up to several hundred times

▪ The short-circuit current increases linearly with light concentration while open-circuit voltage

increases only slightly with concentration

▪ Ideal solar cell efficiency at 300 K for two values of solar concentration

▪ Conversion efficiency increases only slightly with optical concentration

▪ Primary advantage of using concentration techniques is to reduce overall system cost since an

optical lens is less expensive than an equivalent area of solar cells efficiency of a solar cell is

defined as the ratio of output electrical power to incident optical power

- 14 -
E-mail: asgar_eee@jkkniu.edu.bd