CdTe/CdS thin film in Solar Cell

By: Zahra Behboodi “Master Student-Nano Physics”

Advisor :Dr. Zohreh Qurannevis

The presentation for the Course “Analysis of Nano Structure” 22th December 2015

–

Science & Research Branch

Outlines: Introduction to Cadmium telluride photovoltaics

How a CdTe Solar Cell works?

Fabrication of CdS Thin Film by Spray Pyrolysis Technique CdTe layers deposited by Pulsed Plasma Deposition (PPD)

References

Cadmium telluride CdTe

• Structure:

• Crystal Structure

• Direct Band Gap Semiconductor: 1.5 eV

• Can be growth both p-type

and n-type

https://upload.wikimedia.org/wikipedia/commons/thumb/2/22/Sphalerite-unit-cell-depth-fade-3D-balls.png/200px-Sphalerite-unit-cell-depth-fade-3D-balls.png

Cadmium telluride photovoltaics • Cadmium telluride (CdTe) photovoltaics describes a

photovoltaic (PV) technology that is based on the use

of cadmium telluride, a thin semiconductor layer

designed to absorb and convert sunlight into

electricity

• Cadmium telluride PV is the only thin film technology

with lower costs than conventional solar cells made of

crystalline silicon.

• upload.wikimedia.org/wikipedia/commons/thumb/5/5b/N

REL_Array.jpg/300px-NREL_Array.jpg

cadmium telluride (CdTe) solar panel

The Structure of a CdTe/CdS Sollar Cell

How a CdTe Solar Cell works?

http://finestsolar.co.uk/uploads/images/content-images/power-generation-01.jpg

Fabrication of CdS Thin Film by Spray Pyrolysis

Technique There are a number of physical and chemical ways to preparing thin

films:

chemical vapor deposition, chemical bath deposition, electro deposition RF-sputtering , pulsed laser deposition, thermal evaporation, ion-beam sputtering, vacuum deposition, spin coating technique , co-precipitation, sol-gel, successive ionic layer adsorption and reaction method , spray pyrolysis

Comparison Spray Pyrolysis with other Techniques

• Compared with other above mentioned techniques spray pyrolysis is very simple, fast, low-cost method and get uniform deposition of the films

Spray Pyrolysis Setup

• The spray pyrolysis setup consists of a substrate heater, spray gun, air compressor, solution reservoir and a gas exhaust unit. The heating of the substrate was performed using a ceramic heating plate with electrical heating wires.

http://www.hindawi.com/journals/isrn/2012/275872.fig.002.jpg

Solution Preparation

• The precursor solution used for Cadmium sulphide thin films was obtained by dissolving the salts of cadmium chloride and Thioacetamide in the molar ratio of 1:2 in double distilled water, ammonia as complexing agent and N, N-DMF as a solvent were added drop by drop. The amount of solution was made to 50 ml.

Cadmium Chloride [CdCl2.2H2O] and Thioacetamide[CH3CSNH2 ]

Spray Process & Parameters for Spray Pyrolysis

• Spray Process

• The aqueous solution was then sprayed on the preheated glass substrate maintained at different temperatures 250°C, 300°C and 350°C by conventional chemical spray pyrolysis technique to obtain homogeneous thin films

• Parameters for Spray

• (a) Type of salt: water soluble, (b) Air pressure: 15 Kg/cm3, (c) Deposition spray rate: 2.5 Kg/cm3, (d) Substrate temperature: 250°C, 300°C & 350°C, (e) Nozzle to substrate

• distance: 20 Cm, (f) Spray time: 5 Sec, (g) Repeated cycles: 2 minutes,

Movie Showing Spray Pyrolysis

https://www.youtube.com/watch?v=eJbuY4HJFV0

Structural Properties / X-ray diffraction patterns of the CdS thin films • They are polycrystalline

hexagonal structures. Film formations have a single peak represented at 25° with planes oriented in (1 0 0) direction and it has good agreement with standard X-ray diffraction data which was reported in JCPDS card number 02 – 0549.

Figure 1: XRD pattern of CdS thin films at different substrate temperatures

Structural Properties / X-ray diffraction patterns of the CdS thin films …

• The obtained average grain size was 11 nm, 18 nm and 22 nm for 250°C, 300°C and 350°C respectively. The peaks became sharper by increasing the substrate temperature which indicates that as the average grain size decreases.

Temperature increases the quality of the film

The AFM images of CdS thin films

The film obtained at substrate temperature of 250°C shows that it has dense structure with un-even distribution of particles. At substrate temperature of 300°C the film exhibits rough surface and strips like structures. The image at 350°C substrate temperature possess that the film is homogeneous and plain, showing without any cracks.

• Reporting the results of the new deposition technique, Pulsed Plasma Deposition (PPD), for the growth of the CdTe layers on CdS/ZnO/quartz and quartz substrates. The PPD method allows to deposit at low temperature. The optical band gap of deposited layers is 1.50 eV, in perfect accord with the value

What is PPD?

• It is the technology based on the production of the pulsed plasma accompanied by the generation of high energy electron pulses(up to 20 keV) directed to the source of deposited material (target). Electron and plasma beam interaction with target generates the target material plasma which propagates towards the substrate and deposits on its surface.

• This technique, due to simple and compact electron gun construction and high deposition rates, can be easily adapted to large area deposition and represents a new choice for industrial production.

How Dees PPD Works?

The core of the PPD system is an electron gun generating the pulsed beam of plasma and highenergy

electrons (up to 20 keV). The plasma and electron beam is created from working gas (e.g.oxygen, argon or nitrogen) flowing at low pressures (10-6 - 10-2 mbar) through the gun operational

body.

How Dees PPD Works? …

electron packet (impulse) on the target surface causes the energy transfer from the beam into the target material and, consequently, the target ablation. In other words, the explosion of the target surface in the form of the target material plasma (plume) which propagates in the direction of the substrate where it is deposited

Schematic representation of the system for PPD technology

arxiv.org/pdf/1103.4539

XRD spectra of CdTe/quartz and CdTe/CdS/ZnO/quartz samples

arxiv.org/pdf/1103.4539

XRD spectra of CdTe/quartz and CdTe/CdS/ZnO/quartz samples…

• Figure shows that in both systems the CdTe film exhibits a polycrystalline cubic phase (S.G.= F43m (216), a= 6.48 Ǻ, ICSD #: 043712), as determined by the analysis of the Bragg peak positions.

• The grain sizes of the CdTe, estimated by the FWHM of the peaks applying the Debye-Sherrer Equation resulted to be of the same order of magnitude, roughly 30 nm for both samples.

• The crystallinity of the CdTe film is largely affected by the nature of the underneath layer. The degree of the crystallinity is lower for the CdTe film deposited on the CdS substrate than that one for the CdTe film grown on the quartz substrate, as suggested by the lack of numerous reflections in the pattern of the CdTe/CdS spectra otherwise observed for the CdTe/quartz.

References:

• www.ijsr.net/conf/ATOM2014/ATOM2014_07.pdf

• arxiv.org/pdf/1103.4539

• thesis.um.ac.ir/moreinfo-53720-pg-1.html

• https://en.wikipedia.org/wiki/Cadmium_telluride_photovoltaics