DARWIN Digitale Dissertationen German Version Strich

FU Berlin
Digitale Dissertation

Katja Ernst :
The eta-solar cell: a new solar cell concept with extremely thin absorber based on a structured TiO2/CdTe-junction
Die eta-Solarzelle: Ein neues Solarzellenkonzept mit extrem dünnem Absorber auf der Basis einer strukturierten TiO2/CdTe-Grenzfläche

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Abstract

The concept of a solar cell with an extremely thin absorber (eta) is based on the reduction of transport distances in a solar cell in order to use absorber materials of low electronic quality, i.e. small diffusion lengths. This work presents for the first time an eta-solar cell consisting of a structured pn-heterojunction between a n-type window material and a p-type absorber. The local absorber thickness that determines the maximum transport path is around 150nm. The thesis comprises the screening of different materials such as TiO2, ZnO and ZnTe and different morphologies regarding scattering properties, surface enlargement, structure and absorption behavior. For the solar cell microporous TiO2 has been used as a substrate combined with CdTe as absorber material. The conformal coverage of the structured TiO2-substrates prepared by spraypyrolysis has been achieved by electrodeposition of CdTe. After a post-deposition treatment of the CdTe in CdCl2-environment, which leads to a recrystallization of the material, the TiO2 is covered by small crystallites with a size in the order of the local absorber thickness, so that the light generated minority charge carriers do not have to pass any grain boundaries inside the CdTe for collection. The solar cell shows an open circuit voltage of Voc=680mV and a short circuit current of jsc=8.9mA/cm2. The eta-solar cell is compared to a planar configuration of the same materials: it is shown that the photocurrent is increased drastically when using an eta-configuration. The reason can be found in a low minority carrier diffusion length in the electrodeposited CdTe. In an eta-solar cell the space charge region penetrates most of the CdTe-layer, so that despite of the short diffusion length efficient collection of the photogenerated charge carriers can be achieved. The optical comparison via simulating transmission- and reflection-data shows a possible enhancement of the absorption in the CdTe of about a factor of 6 particularly for very thin CdTe-layers due to multiple scattering processes. A detailed analysis of the TiO2/CdTe interface by XPS and UPS reveals a valence band offset of 2.5eV leading to a conduction band offset of 0.6eV between TiO2 and CdTe. The conduction band offset leads to a diminished electric field inside the CdTe-absorber and, in combination with a short diffusion length of minority charge carriers, to a strong voltage dependent photocurrent collection. The resulting low fill factor of the solar cell turns out to be the limiting factor for the efficiency of this new solar cell concept at the moment.

Table of Contents

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Titelblatt
1 Einleitung 1
2 Konzept der eta-Solarzelle und Kriterien zur Materialauswahl 4
2.1 Prinzip des eta-Konzeptes 4
2.2 Materialauswahl 5
3 Grundlagen 8
3.1 Strahlungseinkopplung in absorbierende Materialien 8
3.2 Parameter des elektronischen Transportes 12
3.3 Halbleiter-Heterokontaktbildung 14
4 Experimentelle Methoden 17
4.1 Charakterisierung 17
4.1.1 Struktur und Morphologie 17
4.1.2 Optische Eigenschaften 19
4.1.3 Elektrische Eigenschaften 21
4.1.4 Bauelement-Charakterisierung 24
4.2 Präparationsprozesse 30
4.2.1 Sprühpyrolyse von TiO2 30
4.2.2 Elektrodeposition von ZnO 33
4.2.3 Elektrochemisches Ätzen von ZnTe 34
4.2.4 Elektrodeposition von ZnTe 36
4.2.5 Elektrodeposition von CdTe 38
5 Bekannte Materialparameter der untersuchten Halbleiter 41
5.1 Substratmaterial: TiO2 41
5.2 Absorbermaterial: CdTe 43
6 Die eta-Solarzelle 45
6.1 Konkrete Substratauswahl aus verschiedenen Materialien und Strukturen 45
6.1.1 Morphologie 45
6.1.2 Streuvermögen 47
6.2 Charakterisierung der porösen TiO2-Schichten 50
6.3 Schichtwachstum des Absorbers 55
6.3.1 Aspekte der Elektrodeposition auf porösen TiO2-Substraten 45
6.3.2 Verbesserung des Absorbermaterials durch Nachbehandlung 58
6.4 TiO2/CdTe-Kontakt 64
6.4.1 Lichteinkopplung 64
6.4.2 Bandanpassung 67
6.5 Realisierung des eta-Solarzelle 73
6.5.1 Vergleich zwischen planarer und poröser TiO2/CdTe-Heterostruktur 74
6.5.2 Diskussion des Einflusses des Rückkontaktes 81
6.5.3 Untersuchung des spannungsabhängigen Photostromes in der eta-Solarzelle 86
7 Zusammenfassung 92
Anhang I
A Abkürzungen und Symbole I
B Prozeßabläufe VI
C Meßaufbau zur Bestimmung des Quantenausbeute VIII
D Präparative Arbeiten am Rückkontakt XI
Literaturverzeichnis XI
Veröffentlichungen und Konferenzbeiträge XIX
Lebenslauf XXI
Danksagung XXIII

More Information:

Online available: http://www.diss.fu-berlin.de/2001/102/indexe.html
Language of PhDThesis: german
Keywords: eta-solar cell, CdTe, porous TiO2, electrical transport
DNB-Sachgruppe: 29 Physik, Astronomie
Classification PACS: 84.60Jt
Date of disputation: 09-May-2001
PhDThesis from: Fachbereich Physik, Freie Universität Berlin
First Referee: Prof. Martha-Christina Lux-Steiner
Second Referee: Prof. Hans-Joachim Lewerenz
Contact (Author): ernst@hmi.de
Contact (Advisor): koenenkamp@hmi.de
Date created:26-Jun-2001
Date available:28-Jun-2001

 

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