Basics of solar cells

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Whats a solar cell?

A solar cell is an electronic device which directly converts sunlight into electricity.

Nearly all photovoltaic energy conversion uses semiconductor materials in the form of a p-n junction.

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Whats a solar cell?

Light shining on the solar cell produces both a current and a voltage to generate electric power. Firstly, a material in which the absorption of light

raises an electron to a higher energy state. secondly, the movement of this higher energy

electron from the solar cell into an external circuit. The electron then dissipates its energy in the

external circuit and returns to the solar cell.

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Solar Cell Structure

The basic steps in the operation of a solar cell are:

the generation of light-generated carriers;

the collection of the light-generated carries to generate a current;

the generation of a large voltage across the solar cell; and

the dissipation of power in the load and in parasitic resistances.

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The first process is the absorption of incident photons to create electron-hole pairs.

Minority can recombine A second process, the collection

by the p-n junction, prevents recombination by using a p-njunction to spatially separate the electron and the hole. 5

Collection of carriers

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Minority carriers cannot cross a semiconductor-metal boundary and to prevent reconbimation, they must pass the junction if they are to contribute to current flow.

Recombination of carriers

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Collection Probability

The "collection probability" describes the probability that a light generated carrier absorbed in a certain region of the device will be collected by the p-n junction and therefore contribute to the light-generated current.

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The impact of surface passivation and diffusion length on collection probability

The light-generated current depends on the generation of carriers and the collection probability of these carriers

Sensitive to surface

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Quantum efficiency

The "quantum efficiency" (Q.E.) is the ratio of the number of carriers collected by the solar cell to the number of photons of a given energy incident on the solar cell.

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The "external" quantum efficiency of a silicon solar cell includes the effect of optical losses such as transmission and reflection.

IQE is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy that is absorbed by the cell.

The IQE is always larger than the EQE.

A low IQE indicates that the active layer of the solar cell is unable to make good use of the photons. A low EQE can indicate that, but it can also, instead, indicate that a lot of the light was reflected.

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Internal quantum efficiency

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A Typical Solar cell

How PV Cells Work

The Photovoltaic Effect

The collection of light-generated carriers by the p-njunction causes a movement of electrons to the n-type side and holes to the p-type side of the junction.

Under open circuit conditions, the forward bias of the junction increases to a point where the light-generated current is exactly balanced by the forward bias diffusion current, and the net current is zero. The voltage required to cause these two currents to balance is called the "open-circuit voltage".

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Electrical characteristics of a solar cell

In the dark, a solar cell behaves like a diode

The current that flows across solar cell under a bias voltage in the dark is called the dark current (Idark).

Under illumination, the minority carrier flow creates an opposite current to the dark current.

Imax called light-generated current (Isc). 14

Important parameters of a solar cell

Open circuit voltage (Voc): The voltage @ I = 0 Short circuit current (Isc): The current @ V=0 Max power density (Pmax):

Pmax = Imax V max The Fill Factor (the squareness of the IV curve):

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Short circuit current Isc

2. The thickness of the semiconducting materials.3. The collection probability.4. Light intensity: increasing intensity, increases Isc.5. The area of the solar cell: J (mA/cm2) is used instead of I

(mA)

Factors affect Isc1. Band gap of semiconductor : smaller band-gap, lower energy

photon is required to excite electron, the higher Ic.

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Factors affecting Voc

1. Band gap: smaller band-gap, lower Vbi, lower Voc2. Traps in the band gap induced by impurities, dislocations, etc3. Traps at the semiconductor/metal contact4. Temperature 17

How bandgap affects efficiency

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Filling factor (FF)

Graphically, the FF is a measure of the "squareness" of the solar cell and is also the area of the largest rectangle which will fit in the IV curve.

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Power Conversion Efficiency

Where Ps is the incident light power density.

The standard test condition for solar cell is the Air Mass 1.5 spectrum (AM1.5), where Ps= 1000 W/m2 at T=25oC.

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Tandem Solar Cells

Series connected cells are simpler to fabricate but the current is the same though each cell so this contrains the band gaps that can be used

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Tandem Solar Cells

Efficiency of a an ideal stack of solar cells as a function of the number of bandgaps. The spectrum used here is the direct spectrum where the diffuse radiation is excluded.

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Effect of Parasitic Resistances

The most common parasitic resistances are series resistance and shunt resistance.

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Origins of series resistance

The series resistance is caused by:1. Resistance of p and n layers to majority carriers flow.2. Semiconductor/metal contact resistance.3. The resistance of the top and rear metal contacts.4. The other due to poor insulation, for example on the

edges of the cell.

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Origins of shunt resistance

1. Shunt resistance, RSH, is typically due to manufacturing defects, rather than poor solar cell design.

2. Low shunt resistance causes power losses in solar cells by providing an alternate current path for the light-generated current.

3. Such a diversion reduces the amount of current flowing through the solar cell junction and reduces the voltage from the solar cell.

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Effect of Temperature

T increase equilibrium population of electrons (ni) increases Idark increases lower Voc.

T increase band gap reduction increase Isc. Voc reduction is more prominent than Isc enhancement.

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Basics of solar cellsWhats a solar cell?Whats a solar cell?Solar Cell StructureSlide Number 5Collection of carriersRecombination of carriersCollection Probability Slide Number 9Quantum efficiencyInternal quantum efficiency Slide Number 12The Photovoltaic Effect Electrical characteristics of a solar cellImportant parameters of a solar cellShort circuit current IscFactors affecting VocHow bandgap affects efficiencyFilling factor (FF)Power Conversion EfficiencyTandem Solar Cells Tandem Solar Cells Effect of Parasitic Resistances Origins of series resistance Origins of shunt resistance Effect of Temperature