DARWIN Digitale Dissertationen German Version Strich

FU Berlin
Digitale Dissertation

Steffen Schuler :
Transport properties and defects in polycrystalline CuGaSe2 thin films and heterostructures
Transporteigenschaften und Defekte in polykristallinen CuGaSe2-Schichten und Heterostrukturen

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Abstract

In this work polycrystalline and epitaxial CuGaSe2 thin films were prepared by PVD for their electrical characterisation by means of Hall effect and conductivity measurements and for application in Mo/CuGaSe2/CdS/ZnO solar cells. The structural properties of the polycrystalline Glass/CuGaSe2 layers and the epitaxial (001)GaAs/CuGaSe2 films were studied by SEM, TEM, XRD and ECP. The Mo/CuGaSe2/CdS/ZnO solar cells reach efficiencies of 7.9%. For the diode characteristics important values like the diffusion length of the minority carriers, space-charge region width, doping concentration and built-in voltage were determined using quantum efficiency measurements. The solar cell characteristics were improved by modifying the CdS buffer preparation, whereby the density of occupied surface defects at the heterjunction could be reduced. By Hall effect and conductivity measurements a grain boundary model for polycrystalline chalcopyrites could be provided for the first time. Hereby it is shown that the charge carrier transport is limited by potential barriers at the grain boundaries. The density of charged defect states and the band bending at the grain boundaries, as well as the doping in the volume of the crystallites were quantified. Concerning the dominating acceptor in CuGaSe2 it is demonstrated that the corresponding thermal activation energy is identical in the infinite dillution limit, but decreases with rising defect concentration. The defect density of differently prepared layers varies by several orders of magnitude. Direct experimental evidence could be provided that semiconductors characterised exclusively by intrinsic defects exhibit the effect of the self compensation.

Table of Contents

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Titelblatt 1
Inhaltsverzeichnis 3
1. Einleitung 9
2. Grundlagen 13
2.1. Materialeigenschaften 13
2.2. Solarzellen auf Chalkopyritbasis 17
3. Stromtransport - Theorie und Methoden 29
3.1 Leitfähigkeit und Hall-Effekt 29
3.2. Ladungsträgertransport in den Bändern 31
3.3. Streuung an Korngrenzen 33
3.4. Hopping -Transport 36
4. Entwicklung von Heterosolarzellen mit CuGaSe2-Absorbern 39
4.1. Präparation und Optimierung des CuGaSe2-Absorbers 39
4.2. Präparation und Modifikation des CdS-Puffers 43
4.3. Solarzellenkennlinien 45
5. Struktur der CuGaSe2-Schichten -- CuGaSe2-Wachstum 47
5.1. Polykristallines CuGaSe2 47
5.2. Epitaktisches CuGaSe2 51
6. Elektrische Volumeneigenschaften und Grenzflächendefekte in CuGaSe2-Solarzellen 55
6.1. Experimenteller Aufbau 55
6.2. Einfluss der Puffermodifikation auf die Stromsammlung 55
6.3. Quanteneffizienz unter Vorspannung 56
6.4. Bestimmung der effektiven Sammlungslänge 57
6.5. Auswirkung der Puffermodifikation auf die Quanteneffizienz 58
6.6. Einfluss der Grenzflächendefekte auf den pn-Übergang 60
7. Transportmechanismen und Defekte in CuGaSe2 63
7.1. Experimenteller Aufbau und Messverfahren 64
7.2. Vorbetrachtung anhand der Schichtleitfähigkeit 65
7.3. Ladungsträgertransport in epitaktischen Schichten 66
7.4. Ladungsträgertransport in polykristallinem CuGaSe2 69
7.5. Korngrenzen-Modell 73
7.6. Transport bei tiefen Temperaturen 74
7.7. Diskussion der Akzeptoraktivierungsenergie 75
7.8. Selbstkompensation und die Rolle des Na in CuGaSe2 79
8. Zusammenfassung und Ausblick 81
Anhang A 85
Anhang B 87
Symbolverzeichnis 89
Literaturverzeichnis 95
Lebenslauf, Publikationen, Danksagung 101

More Information:

Online available: http://www.diss.fu-berlin.de/2002/294/indexe.html
Language of PhDThesis: german
Keywords: CuGaSe2, solar cells, transport properties, grain boundaries, self compensation
DNB-Sachgruppe: 29 Physik, Astronomie
Classification PACS: 84.60.Jt, 72.20.My, 73.50.Jt, 61.72.Vv, 61.82.Fk, 71.20.Nr, 73.40.Lq, 81.15.Ef, 81.15.Hi, 68.55.Ln, 73., 73.50.-h, 73.50.Gr, 72.20.Jv, 74.78.Fk, 79.60.Jv
Date of disputation: 04-Dec-2002
PhDThesis from: Fachbereich Physik, Freie Universität Berlin
First Referee: Prof. Dr. Martha Ch. Lux-Steiner
Second Referee: Prof. Dr. William D. Brewer
Third Referee: Prof. Dr. J. Luther
Contact (Author): steffen.schuler@web.de
Contact (Advisor): lux-steiner@hmi.de
Date created:16-Dec-2002
Date available:20-Dec-2002

 

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