Single coaxial silicon nanowire Single coaxial silicon nanowire photovoltaic cellphotovoltaic cell

By: By:

Aparajita MallickAparajita Mallick

Nano wire solar cellsNano wire solar cells Solar cells are attractive candidates for Solar cells are attractive candidates for

clean and renewable powerclean and renewable power With miniaturization, they might serve as With miniaturization, they might serve as

integrated power sources for integrated power sources for nanoelectronic systems. nanoelectronic systems.

In the earlier experiments, nanowires were In the earlier experiments, nanowires were mostly used simply as a conducting mostly used simply as a conducting channel for enhancing the electron channel for enhancing the electron transport transport

It has been shown that silicon nanowires have It has been shown that silicon nanowires have the potential to give an improved optical the potential to give an improved optical absorption.absorption.

Thus silicon nanowires can be used as Thus silicon nanowires can be used as photovoltaic devices where photogeneration photovoltaic devices where photogeneration occurs. occurs. 

Even if the nanowire solar cell never replace Even if the nanowire solar cell never replace large solar cells, they could be useful in large solar cells, they could be useful in experimental nanometer-scale devices, which experimental nanometer-scale devices, which today lack a convenient power source.today lack a convenient power source.

Working PrincipleWorking Principle

The following processes take place in a The following processes take place in a general photovoltaic cell: general photovoltaic cell: 

Presence of a built in electric field.Presence of a built in electric field. On exposure to light, electron-hole pair is On exposure to light, electron-hole pair is

generated.generated. The electron-hole thus move in opposite direction The electron-hole thus move in opposite direction

under the influence of build-in electric field.under the influence of build-in electric field. If both the ends are connected, a current flows in If both the ends are connected, a current flows in

the outer circuit.the outer circuit.

Photovoltaic device requires a built-in Photovoltaic device requires a built-in electric field for electron-hole charge electric field for electron-hole charge separation.separation.

A built in electric field can be achieved by A built in electric field can be achieved by either of the following ways:either of the following ways:

a p-n junction must be presenta p-n junction must be present a Schottky barrier must be presenta Schottky barrier must be present

Here a coaxial p-i-n silicon nanowire Here a coaxial p-i-n silicon nanowire configuration is being used. configuration is being used.

Advantages of the coaxial p-i-n Advantages of the coaxial p-i-n configurationconfiguration

Charge separation can occur efficiently Charge separation can occur efficiently throughout the nanowire lengthsthroughout the nanowire lengths

Charge separation occurs in the radial Charge separation occurs in the radial direction, direction,

Dimension in radial direction is smaller Dimension in radial direction is smaller than the minority carrier diffusion lengththan the minority carrier diffusion length

Bulk recombination decreases significantlyBulk recombination decreases significantly Thus, Improves the efficiency.Thus, Improves the efficiency.

Experimental Background:Experimental Background:

FabricationFabrication FeaturesFeatures Demonstrated ApplicationDemonstrated Application

FabricationFabrication The core p-type nanowire is synthesized by The core p-type nanowire is synthesized by

means of a vapour-liquid-solid (VLS) method. means of a vapour-liquid-solid (VLS) method. It is coated by a chemical vapour deposition of a It is coated by a chemical vapour deposition of a

layer of pure silicon layer of pure silicon The outer n type layer is formed by chemical The outer n type layer is formed by chemical

vapour deposition, phosphine is used as n dopant.vapour deposition, phosphine is used as n dopant.

To get current out of the solar cell, To get current out of the solar cell, researchers first etched away its outer two researchers first etched away its outer two layers at one end. Then they formed metal layers at one end. Then they formed metal contacts at both ends.contacts at both ends.

Features of the fabricated solar Features of the fabricated solar cellcell

The silicon used is lesser pure as compared the The silicon used is lesser pure as compared the quality usually used for solar cell.quality usually used for solar cell.

Due to lesser pure form of silicon, cost is reduced.Due to lesser pure form of silicon, cost is reduced. The efficiency is 3.4%.The efficiency is 3.4%. Over a year of fabrication and use, the quality of Over a year of fabrication and use, the quality of

the solar cell has not been degraded.the solar cell has not been degraded. It is proposed that the efficiency can be further It is proposed that the efficiency can be further

improved up to 20-25%improved up to 20-25%

Demonstrated ApplicationDemonstrated Application

A silicon nanowire pH sensor was A silicon nanowire pH sensor was powered by a single coaxial silicon powered by a single coaxial silicon nanowire photovoltaic device.nanowire photovoltaic device.

The operation of AND logic gates The operation of AND logic gates fabricated from nanowires and powered by fabricated from nanowires and powered by two coaxial silicon nanowires photovoltaic two coaxial silicon nanowires photovoltaic devices.devices.

Modeling of coaxial nanowire Modeling of coaxial nanowire solar cellsolar cell

Diameter of the core p-type is 100 nm. Diameter of the core p-type is 100 nm. Thickness of the i-layer is 80nmThickness of the i-layer is 80nm Thickness of n layer is 100 nm Thickness of n layer is 100 nm Total diameter is 360 nm.Total diameter is 360 nm. Length of the nanowire, is set to one micron.Length of the nanowire, is set to one micron. The doping of p-type and n-type are set to The doping of p-type and n-type are set to

7.4X107.4X101616..

SimulationSimulation

TheThe device has been simulated using Silvaco device has been simulated using Silvaco ATLAS, a 2D/3D semiconductor device ATLAS, a 2D/3D semiconductor device simulator. simulator.

The following steps were taken for simulation:The following steps were taken for simulation: DeckBuild run-time environment received the input files.DeckBuild run-time environment received the input files. Within the input files, Silvaco Atlas was called to Within the input files, Silvaco Atlas was called to

execute the code. execute the code. TonyPlot was used to view the output of the simulation.TonyPlot was used to view the output of the simulation.

Minor ProjectMinor Project

Studying various characteristics of a Studying various characteristics of a conventional solar cell using simulation conventional solar cell using simulation tooltool

Formulating a program to define the Formulating a program to define the structure of a coaxial solar of mentioned structure of a coaxial solar of mentioned dimensions and to get its potential profiledimensions and to get its potential profile

Characteristics studied:Characteristics studied:

Photogeneration RatePhotogeneration Rate Dark CurrentDark Current Light CurrentLight Current Spectral ResponseSpectral Response

I-V characteristicsI-V characteristics

Spectral responseSpectral response

Coaxial Si nanowire solar cellCoaxial Si nanowire solar cell StructureStructure

Potential profile of the solar cell under Potential profile of the solar cell under zero biaszero bias

The profile shows The profile shows potential gradient in the potential gradient in the radial directionradial direction

Due to potential gradient, Due to potential gradient, the electron-hole pair the electron-hole pair move in opposite move in opposite direction to generate direction to generate current in the outer current in the outer circuit.circuit.

Anode->top leftAnode->top left Cathode is at the right Cathode is at the right

bottom.bottom. left: p-type, centre: i-layer, left: p-type, centre: i-layer,

right: n-type.right: n-type.

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