Fabrication and Simulation of Ag Doped ZnO Hetero junction Solar cell

Under the able guidance of Dr. Rinku Sharma

Presented By: Deepak Kumar Baghel

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Zinc oxide is an n-type semiconductor . ZnO is an II– VI group based direct-

transition-type compound semiconductor . Wide energy band gap of 3.3 eV . Hexagonal wurtzite structure. Large exciton binding energy of 60 meV at

room temperature.

Introduction

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The ZnO has wurtzite crystal structure, named after the mineral wurtzite

The wurtzite structure is non-centrosymmetric (i.e., lacks

inversion symmetry).

Structure of ZnO

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Properties Value

Lattice constants (T = 300 K) a0c0 Density Melting point Relative dielectric constant Gap Energy Intrinsic carrier concentration Exciton binding Energy Electron effective mass Electron mobility (T = 300 K) Hole effective mass Hole mobility (T = 300 K)

  0.32469 nm0.52069 nm 5.606 g/cm3 2248 K 8.66 3.4 eV, direct < 106 cm-3 60 meV 0.24 200 cm2/V s 0.59 5-50 cm2/V s

Physical properties Of ZnO

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Why ZnO?:

◦Cheap.◦abundant .◦non-toxic elements.◦ it can be produced for large scale coatings.◦Can be tailored of the ultraviolet absorption.◦high stability in a hydrogen plasma.◦Crystalline growth at low temperature.◦Deposition on various substrates. ◦More resistant to radiation damage.◦  Relatively high mobility .

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ZnO can be grown as a crystalline material at relatively low deposition temperature.

The ZnO can be deposited on various substrates such as silicon or amorphous glasses. So it is possible to realize a total transparent ZnO-TFT by depositing ZnO on the ITO glasses.

Because of wide band gap of ZnO, the characteristics of ZnO-TFT will not degrade in the exposure of visible light.

Advantages of ZnO

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The ZnO thin film used as an active channel layer can achieve a relatively high mobility.So it is possible to reduce the gate voltage for low power goals.

The most unique property of ZnO is its large exciton binding energy of 60 meV, which is much larger than those of GaN (24 meV).

ZnO is also much more resistant to radiation damage than are other common semiconductor materials, such as Silicon, and GaN.

Advantages of ZnO Contd…

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Both the grain and grain boundary resistances of ZnO increase with the addition of Ag.

Ag+ may behave like many other monovalent dopant ions (e.g. Na+ andK+) which have the ability to occupy both the lattice and interstitial sites.

The presence of Ag solutes increases the rates of densification and grain growth of ZnO.

Why Ag doped ZnO ??

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Fabrication Deposition of

◦ ZnO on p-Silicon <100> by thermal evaporation of ZnO .

◦ Ag doped ZnO on p-Silicon <100> by thermal evaporation of sintered (1200oC) Ag2O doped ZnO . p-Silicon <100> ,post deposition of ZnO to form

bilayer.

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Steps of Fabrication of Ag doped ZnO

Appropriate percentage ( ̴ 3%) by weight of Ag2O:ZnO was chosen

Dry (0.5 Hrs) and wet (in acetone – 2.5 Hrs) ball milling was done using agate mortar and pestle.

Removal of acetone by drying at 150oC for 15 Mins.

Sintering was done at 1200oC for 1 Hr with the rate of rise in temperature at 5oC/Hr in air atmosphere.

Pellet was formed by hydraulic pressing with a weight of 7 tons.

Deposition was done on p- Si and annealed at 450oC followed by top and back metallization by aluminium.

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Maximum grain size obtained at ( ̴ 3%) by weight of Ag2O:ZnO.

Ag doping was done using Ag2O because it was observed to have given lower resistance over direct doping of Ag.

450oC was chosen as previous experiments for ZnO as well as literature showed larger grain size with no significant improvement after this temperature.

Reasons behind specifications

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Possible reasons of suppression of spectrum in visible region◦ Interfacial mismatch might be reduced due to the presence of

Ag◦ Ag doped ZnO was annealed in air and ZnO was annealed in

nitrogen. Annealing in air might have reduced oxygen vacancies.

The slight shift towards the lower frequency on Ag doping is due to increase in band gap. Bilayer exhibits the property of the underlying ZnO which was annealed in nitrogen and after deposition of Ag doped ZnO it was annealed in air.

Analysis of Ag doped ZnO

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Details of Simulation using ATLAS

Device dimensions ZnO thickness : 0.2 μm SiO2 thickness : 0.1 μm Si thickness : 40 μm Channel Width: 5 μm Device Length: 20 μm Electrode Length: 7.5 μm

Doping◦ ZnO (n-type) : 1e16 per cc◦ Si (p-type) : 1e14 per cc

Models used◦ Boltzman (Default)

  Method used for solving :

◦ Newton Gummel iteration

Characteristics obtained◦ I-V Characteristics◦ Doping Profile

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Investigating change in efficiency and characteristics with the variation of thickness of ZnO.

Comparing optical and electrical properties of Ag2O doped ZnO with that of ZnO based solar cells.

Investigating Properties of bilayer.

Future Work

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Thank you