DENSITY OF STATES MEASUREMENTS OF THIN FILM SEMICONDUCTOR MATERIALS

USING PHOTOCURRENT METHODS

NASTITI PUSPITOSARI

DIRECTEUR DE THÈSE : CHRISTOPHE LONGEAUD

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• Observe semiconductor materials’ sensitivity to light/ photoconductivity through spectroscopic methods

• Perform spectroscopic measurements to obtain semiconductor materials’ spectral dependency of photoconductivity

• Study the semiconductor materials’ electrical properties through their density of states’ models

Goals

•  Establish material characterization techniques essentially for materials applicable to PV devices

•  Support the development of new materials for PV

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Motivation

Introduction

Photocurrent Spectroscopy, e.g : CPM or FTPS

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CPM VS. FTPS

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Both techniques are electrical measurements using the concept of constant photocurrent spectroscopy

CPM FTPS

•  Uses monochromator •  Perform incident flux

intensity variation •  Perform measurements

for each wavenumber •  Measurement time up to

4-6 hours

•  Uses FTIR spectrometer •  No incident flux intensity

variation •  Perform simultaneous

measurements for all wavenumber using FT

•  Measurement time ~5 minutes

FTPS Principles

• Performed using FTIR spectrometer (Michelson interferometer principles)

Samples Photocurrent vs. Wavenumber

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•  FTPS Experimental setup

FTPS Experiment (1)

FTIR spectroscopy range : 4000-15800 cm-1

Low-pass filter : cut off at 12000 cm-1 Detectors for flux measurements : 1.  InGaAs : 600-1800 nm 2.  Si : 400-1100 nm

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voltage source

• Experimental Procedures (1)

FTPS Experiment (2)

Flux Measurement

sample

Incident flux Transmitted flux

Photodiode

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• Experimental Procedures (2)

FTPS Experiment (3)

Sample Measurement

sample

Incident flux

Bias Voltage

Photocurrent

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Data Manipulations (1) • Obtaining absorption coefficient

Fdiode hυ( ) = Sdiode hυ( )Gdiode ⋅SRDIODE ⋅A

Isample hυ( ) = Ssample hυ( )Gsample

α hυ( ) ∝Isample hυ( )Fdiode hυ( )

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• Reproduction of transmission results using Diplot

Data Manipulations (2)

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• Absorption coefficient results using Diplot

Data Manipulations (3)

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Work in progress •  FTPS measurements for thin film silicon carbon alloys • Example : Results of CH4 flux variation

[CH4] Fc (%) [C]/[Si] 1.0 65 0.07 1.2 25 0.10 1.6 0 0.17 1.8 0 0.20

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Accepted for : - Oral presentation in ICANS 26 (Aachen, 13-18 Sept ‘15) - Poster presentation in EUPVSEC ’15 (Hamburg, 13-18 Sept ‘15)

Density of States Measurements (1) • Simulations using Deost

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Density of States Measurements (2) • Simulations using Deost

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Future works •  FTPS measurements using a FTIR spectrometer with

sensitivity up to visible light range (>15800 cm-1) to directly obtain the absolute value of absorption coefficient (without performing transmission spectroscopy)

•  FTPS measurements for PV devices/ solar cells to study the devices’ spectral response/EQE

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THANK YOU!

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