pv solar cells i-v characteristic

8/11/2019 Pv Solar Cells I-V Characteristic.

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1

University of Technology

Laser and Optoelectronics Engineering Department

Optoelectronics Engineering Branch

Detector Lab 2010-2011

7Exp

Solar Cells I-V characteristic.

فخري عد د وتنفيذ لمدرس مكرم عبد لمطل

Object.

Find the current and voltage response under illumination depending on the magnitude of the

variable resistance.

Tools1- Source

2- Variable resistance

3- Slid board

4- Voltmeters

How a Solar Cell Works?

A solar photovoltaic (PV) cell converts sunlight to electricity. In the photoelectric effect at a

metal surface, electrons are freed once the

energy exceeds the bond energy. In a solar cell, an asymmetry is established by contacting twosemiconductors of opposite polarity which drives electrons that are freed by the incident light in

a circuitsolar cell consists of two layers of semiconductor, one positive (p-type) and the other negative(n-type), sandwiched together to form a p/n junction.

When the semiconductor is exposed to light, the energy hµ of incident photons exceeding the

threshold band gap is absorbed by the semiconductors electrons that access the conduction bandstarting to conduct electricity. Electrons in semiconductors, in fact, are weakly bonded to the

atomic nucleus and occupy the valence energy band.

for each negatively charged electron, a corresponding mobile positive charge, a hole, is created.The electrons and holes near the p/n junction are swept across in opposite directions by the

action of the electric field, where a contact drives such electrons to an external circuit where they

lose energy doing work such as powering a

light source and then return to the materials valence band through a second selective contactclosing the circuit .

Only photons whose energy is greater than the energy band gap (EG) are able to create an

electron–hole pair and thus contribute to the energy conversion process. Therefore, the spectralnature of sunlight is a fundamental aspect affecting the design of efficient solar cells.

The solar cell is the photovoltaic’s building block. Usually, it is made of a 100 cm2 silicon wafer

whose surface has been treated to maximize light absorption and thus appears dark blue or black.

Such a cell hit by radiation from the sun generates tens of milliamps per cm2 current caused by a

0.5–1 V potential, which is too low for most applications.

EXP.NO. (7)

Solar Cells I-V characteristic



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The Solar Cell: A Current Generator.

In practice, when a load is present a potential difference develops between the terminals of the

cell. This potential generates a current which acts in the opposite direction to the photocurrent,and the net current is reduced from its short circuit value. This reverse current is called the dark

current, in analogy with the current I dark (V) which flows across the device under an applied

voltage, or bias, V, in the dark.

Most solar cells behave as a diode in the dark, admitting a much larger current under forward bias (V > 0) than under reverse bias (V < 0). For an ideal diode, the dark current density varies

like

Jdark = Jo(eqV/kBT

_1)

Where kB is Boltzmann.s constant, T is the temperature and Jo is a constant. Thusthe net current

flowing in a circuit powered by a solar cell is

J(V) = Jsc _Jdark

J(V) = Jsc _Jo(eqV/kBT

_1)

.

Current–voltage curve for an ideal diode in the dark

and under light.

J–V curve for solar cell shows a strongly

field-dependent photocurrent

EXP.NO. (7)




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Procedure.In this measurement we want to record with high accuracy the voltage versus current

dependence on our device. While the previous measurement allowed us to see the response of the

device to different colors of light we did not get any accurate information as to the magnitude ofthe response. It’s important when reporting the performance of a solar cell to report the

efficiency of the response to a solar light source we will now allow bright white light to excite

the solar cell .we will record the current and voltage response under this illumination depending

on the magnitude of variable resistance.

To determine the Ideality factor of the Solar cell. (Optional)

Fig. 1: Experimental set-up for determining characteristic curves.

Fig. 2: Circuit for measuring the current-voltage characteristic.

EXP.NO. (7)




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typical I-V curve plot.

On an I-V plot, the ordinate refers to current, and the abscissa to voltage. The I-V

curve passes through two significant points, the short-circuit current (Isc

) and the open-circuit

voltage (Voc

). The Isc

refers to the current when the output terminals of the cell are short-circuited.

In the plot this point is the intercept of the

curve with vertical axis. The Voc

is the voltage measured at open circuit conditions and is represented

as the intercept of the curve with the horizontal axis in the plot

.

Figure 3: Typical I-V Curve

Maximum Power (Pmp

)

The solar cell may be operated over a wide range of voltages and currents by varyingthe load resistance from zero to infinity. The Maximum Power (P

mp) point occurs when the

product of the current and voltage is maximum. The current and voltage at the maximum power

point are denoted by Imp

and Vmp

, respectively. (See Figure 3).

The maximum power that a cell can deliver to a load is obtained when . VI =VmIm

Fill Factor (FF)

The fill factor (FF) percentage measures the "squareness" of the I-V curve. It statesthe degree to which the voltage at the maximum power point (V

mp) matches the open-circuit

voltage (Voc

) and that the current at the maximum power point (Imp

) matches the short-circuit

Imax

Voc

Voc

EXP.NO. (7)




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current (Isc

). Therefore, a more “squared” I-V curve will have a higher fill factor.

This relation is given by.

The fill factor (FF) is defined by the ratio

FF =VscIsc

Vm Im

Quantum Efficiency (QE)

Quantum efficiency (QE) is the ratio of the number of charge carriers collected by

the solar cell to the number of photons of a given energy incident on the PV device. QE thereforeis related to the response of a solar cell to the various wavelengths in the spectrum of incident

light on the cell. The QE is given as a function of either wavelength or energy.

QE = pin

pout =

Ps

Pm

Where Pm = VmImPs is incident light power

Variable ResistanceΩ Voltage Vvolt Current I m amp Power Pwatt = V.I0

1050

100

500

1000100000

15000

25000

50000

100000500000

1000000

Discussion.

1-Describe the J-V plot that you made and tell me what the physical effect is responsible for each

of the features that you observe.

2-what is different between photoelectric properties of any type and iso type of heterojunctions

EXP.NO. (7)


pv solar cells i-v characteristic

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