Semilab ThinFilmPV Applications[1]

Semiconductor Physics Laboratory Co. Ltd. 1

SEMILAB Semiconductor Physics Laboratory Co. Ltd.


Metrology for Thin Film Applications

Semiconductor Physics Laboratory Co. Ltd. 3 3

Absorber layers

CIS/CIGS

a-Si, !c-Si

CdTe

TCO layers

Minority carriers lifetime Surface photovoltage

ITO AZO FTO etc

Layer Thickness Sheet Resistance

Haze Layer Roughness

Optical Transmission

Solar cells CIS/CIGS a-Si, !c-Si

CdTe

Epi !-PCD SPV

Epi !-PCD

Epi !-PCD

Ellipsometer Eddy Current

Ellipsometer Ellipsometer

Haze Reflection

Quantum efficiency Reflectance

LBIC LBIC

Diffusion Length LBIC

Minority carriers lifetime

Minority carriers lifetime

Electro-luminescence IR Camera

Surface Photovoltage SPV

SPV Surface photovoltage

Layer Thickness Ellipsometer


•! Tools for different purposes: –!Laboratory

tools –!Process

control tools •! At line •! In-line •! Roll to roll

WT-2000 PVN off-line and lab tool

Production control for large panels

GES5E ellipsometer for R&D

Roll to roll ellipsometer


•! Minority charge carrier lifetime: effective parameter to characterize the purity of semiconductor material

•! µ-PCD method: simple, robust, powerful technique for lifetime monitoring

•! Different laser wavelengths can be used to accomodate different layer thickness and material composition

!!bulk Thermal

equilibrium

Excitation (generatio

n of excess charge

carriers)

Redistribution of carriers

diffusion of carriers to the

surface

surface recombination

bulk recombination

!!surface !!diff

D: diffusion constant of minority carriers d: wafer thickness S: Surface recombination velocity !!meas: measured lifetime

!!surface: surface recombination lifetime !!diff: characteristic time for diffusion to the surface from the bulk""!!bulk: bulk recombination lifetime


•! 355nm UV light for charge carrier generation: penetration depth is smaller than conventional IR

•! Measurements on thin layers, SOI, even on SiC

•! Special MW antenna •! With new, fast transient

recorder card, short lifetimes can be measured precisely


•! LLBBIICC aanndd RReefflleeccttaannccee •! Multiple wavelength laser capability: 405 -

1015nm •! Up to 200x200mm area mapping •! High resolution: 100!m •! Adjustable light intensity •! Reflectance measurement for Internal Quantum

Efficiency determination •! Separated direct and scattered reflection

measurement •! Reflected light is detected up to an angle of 60o IIQQEE MMeeaassuurreemmeennttss mmaaddee oonn tthhee ssaammee CCIISS

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Textured Zno


•! A pulsed LED or laser (wavelength 470 nm) creates electron-hole pairs

•! Carriers are diffusing to surface and depletion region is charged

•! Surface potential change is sensed at the transparent electrode by lock-in technique

•! Different frequencies can be used for mapping surface photovoltage (default 1kHz)

•! Change in surface photovoltage reflects change in charge state of surface or at layer interface.


•! SPV frequency scan allows determination of –! Shunt Resistance RSH –! Junction capacitance

CD –! junction lifetime "S=

RSH *CD •! SPV Spectroscopy-

based composition indicatior (due to change in bandgap)

Rshunt

!

!

4'**High frequency Vspv ~ 1/CD CD

2 ~ NA in absorber Low frequency Vspv ~ RSH


•! Principle of non-contact I-V (suns-Voc); Voc is measured as function of light intensity.

•! Measurements of Voc done with SPV or CPD (Contact Potential Difference) probe also known as Kelvin probe.

•! Measurements of Voc as a function of light intensity are converted to I-V curves.

•! Current is normalized to Isc. •! Values of n, FF and Voc are obtained. •! To get absolute values of Isc, calibration to final cell

required.


•! e- and hole generation by chopped LED light

•! This causes change in junction voltage

•! Change spreads laterally, and the attenuation depends on sheet resistance

•! Change of the potential is picked up by capacitive sensors

•! Rs, Cd and Jleak are calculated by fitting the theoretical JPV signal

•! Evaluation of p/n junctions in Thin Films

•! Evaluation of laser-separation quality of active layers (laser 2)

Junction Photo-Voltage Sheet Resistance Measurement

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Eddy current resistivity measurement

Capacitive thickness measurement

Resistivity measurement with Eddy current

An AC current flows in a coil in close proximity to a conducting material. The magnetic field of the coil induces circulating (eddy) currents in the sample. The eddy current measurement is actually the measurement of the electrical loss in the material, which depends on the resistivity.

Capacitive thickness measurement

The capacitance depends on the distance between the probe and the sample:

where d is the probe-sample distance. From the measured capacitance the distance can be calculated, and the distances from both sides of the sample makes the thickness determination possible.


•! Si CCD camera cooled to -75 °C •! 1024 x 1024 array of 13 !m square pixels •! Captures photons emitted during radiative

recombination in the NIR range (900-1100nm ) •! Distribution is determined by local excitation level •! Imaging capabilities for

–! Minority carrier lifetime (diffusion length) –! Micro cracks –! Lateral Series Resistance –! Shunt Resistance –! Defects and impurities in the semiconductor –! Relative efficiency

•! Calibration is needed to absolute methods


•! Spectroscopic Ellipsometers to meet requirements of emerging technologies / materials, R&D and process control

•! Measures complex reflectance ratio

•! Parameters: •! Spectral range:

–! from 190 nm to 2.5 !m high resolution and/or fast measurement mode

•! Unique combination with further techniques:

–! Grazing X-Ray Reflectance –! FT Infra-Red Spectroscopic Ellipsometry up to 33

!m –! Adsorption, EPA: Ellipsometric Porosimeter (EP) at

atmospheric pressure

Wavelength


Metrology for Thin Film Applications


•! a_Si, !c-Si, TCO: Thickness measurement by SE

•! Crystallinity of !c_Si by Raman

•! !c_Si, a_Si Resistivity by 4PP

•! TCO Sheet Resistance by Eddy Current

•! Haze control of Textured TCO

•! Carrier lifetime by !PCD


•! CIGS , CdS, TCO & Dielectric thickness measurement by Optical techniques & SE

•! TCO sheet resistance by Eddy Current

•! CIGS crystallinity and composition measurement

•! Carrier lifeTime by !PCD •! Sheet Resistance of CIGS by

JPV •! LBIC , IQE •! PhotoLuminescence and

Electroluminescence imaging


•! CdTe , CdS and TCO thickness measurement by SE

•! TCO sheet resistance by Eddy current

•! CdTe Carrier lifetime by !PCD

4/22/10


•! Measurements for panels from Gen 1 to Gen 10

•! Ellipsometry, capacitive gauging and sheet resistance measurement can be integrated

•! Manual or automatic loading •! Compatible with conveyor •! Offline or inline application

20


Glass

!Si 774.6 ± 0.5 nm

Roughness 10.4 ± 0.3 nm

Total thickness : 785 nm

!Si is a mixture of 89.2 % of Polysilicon and 10.8 % of a_Si.

Measured and fitted curves


•! 195 points in less than 10 minutes •! Panel size: 1.1 * 1.3 m


Glass

Main ZnO 955.0 ± 1 nm

Top ZnO 188.4 ± 1 nm

Roughness 22.0 ± 0.5 nm

Total thickness: 1165nm

Measured and fitted curves


textured

non textured


Mo

CIGS 2.1 ± 0.02 !m

Top CIGS 0.346 ± 0.008 !m

Total thickness = 2.446 !m

•! Measurement and analysis from 1.6 to 17 !m (580 – 6000 cm-1)

•! CIGS is becoming quite fully transparent.

•! Determination of thickness and refractive

•! index is much easier •! The roughness becomes small

compare to the wavelength and does not disturb measurement. Measurement and model fit


•! Measurement parameters: –! Excitation laser wavelength: 350 nm –! Spatial resolution 0.5 mm –! Lifetime variation 49ns – 168ns –! Sample size: 1.5*3cm

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0.5!m !c-Si on glass : 87ns 1!m, !c-Si on ceramic: 68ns


•! Measurement parameters: –! Excitation laser wavelength: 535 nm –! Spatial resolution 1 mm –! Lifetime variation 23ns – 50ns

8!m CdTe layer on glass after CdCl2 treatment (10x10 cm)

Sample Transient


•! Different lifetimes after different process steps •! Tendency is in good agreement with theory expectations •! Combining results between different manufacturing stages

enables absorber layer quality & process control

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CIGS solar cell structure

CIGS: 30.3ns CIGS/CdS:59.2 ns CIGS/CdS/TCO:110.8 ns

Luifetime results on different CIGS structure on steel foil (2x3 cm)


•! Carrier lifetime and Internal quantum efficiency map comparison on CIGS/CdS/TCO structure on steel substrate

•! Strong correlation can be seen between minority carrier lifetime and internal quantum efficiency

30

LBIC IQE map of same CIGS Cell (1x2 cm) Lifetime map of CIGS Cell (1x2 cm)


Measured lifetime values of two series 1X2 inch sized CIGS/CdS/TCO samples what are in different manufacturing stages.

On the series A&B different processes were performed.


Transparent layer

Sample structure: Thin ~1!m TCO layer on 4 mm glass substrate

Measurement raster: 0.5mm Edge exclusion: 7mm necessary

Measurement range: 0.02 "/sq - 250 "/sq

On TCO layers a linear correlation between 4pp and Eddy current can be observed

Repeatability <1%

32

TCO DOPANT

SnO2 Sb,F,As,Nb,Ta

ZnO

Al, Ga,B,In,Y,Sc,F,V,Si,Ge,Ti,Zr,Hf,Mg,As,H

In2O3 Sn,Mo,Ta,W,Zr,F,Ge,Nb,Hf,Mg

CdO In,Sn

Ta2O

GaInO3 Sn,Ge

CdSb2O3 Y


•! CIGS Cell with TCO layer on metal sheet (5X2cm)

•! Hamamatsu CCD Camera 512x256 pix

•! < 1sec integration time •! 1-1.5 V and 0.6-0.8A

BIAS •! 20% QE at 1!m

wavelength •! Contrast enhanced by

software


Electroluminescence pictures comparison with Internal Quantum Efficiency map measured with LBIC head 6x3 cm CIGS Cell on steel foil

El. Picture with reverse bias shows shining shunts*

El. Picture* IQE map*

Dark spots like this are shunts*

Ring shaped bright object*

Ring shaped bright object shows higher IQE value*


Bright regions marked with red circles show extra current (shunts) and locally

increased temperature.

Electroluminescence picture: 15V; 6mA; 60sec exp. time

Electroluminescence picture: 19V; 46mA; 3sec exp. time

With smaller current density it can be seen that shunts effect

the whole cells.

10x11 inch sized integrally interconnected CIS panel (25x28cm)

Internal Quantum Efficiency map

The IQE map with same pattern as

electroluminescence picture


1-2 !m thick CdS layer on glass (12x12cm)

1-2 !m thick CIGS/CdS/TCO structure on steel sheet (2.5x8cm)

1x3” sized cm 1-2 !m thick CIGS/CdS structure on steel sheet (2.5x8cm)

CIS solar cell Structure (6x28cm)

Average SPV signal: 20mV

Average SPV signal: 0.65mV



•! Topography correlates to layer interface charge density distribution •! SPV mapping helps in electric field engineering


Internal Quantum Efficiency

Reflectance

405nm

40.2%

980nm

40.3%

11.2% 9.4%


Comparison of 3 CIGS samples

Semilab ThinFilmPV Applications[1]

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