Investigation of binary and vanadium-doped In2S3 for intermediate band solar cells

Autor: Jawinski, Tanja
Jazyk: angličtina
Rok vydání: 2023
Předmět:
Druh dokumentu: Text<br />Doctoral Thesis
Popis: Im ersten Teil der vorliegenden Arbeit wird der Einfluss der Abscheideparamter von In2S3 Dünnfilmen, die mittels thermischem Verdampfen hergestellt wurden, auf ihre physikalischen Eigenschaften untersucht. Es zeigte sich, dass die Abscheideparameter einen starken Einfluss auf die Oberflächenmorphologie und die strukturellen Eigenschaften haben. Durch eine Optimierung der Herstellungsparameter konnten β-In2S3 Dünnfilme in (103) Orientierung hergestellt werden. Epitaktisches Wachstum von In2S3 Schichten mit jeweils zwei bzw. vier Rotationsdomainen wurden auf c- und a-Saphir erreicht. Die fundamentale optische Bandlücke wurde für alle Dünnfilme zu 2.1 eV bestimmt. Eine starke persistente Photoleitung, welche auf tiefe Defekte innerhalb der Bandlücke zurückgeführt werden konnte, wurde unabhängig von den Abscheideparametern und dem gewählten Substrat beobachtet. Prototypen für Solarzellen wurden aus n-In2S3 und p-Zinkkobaltoxid (ZCO) hergestellt und zeigen ein hohes Sperrverhältniss und photovoltaische Aktivität, welche jedoch durch Absorption im ZCO limitiert wird. Im zweiten Teil der Arbeit wurden In2S3:V Dünnfilme ohne bzw. mit Saat- und Pufferschichten hergestellt, um deren physikalische Eigenschaften zu untersuchen bzw. um Zwischenbandsolarzellen herzustellen. Ein großer Dotierbereich von bis zu 11.4 at-% V, wurde durch einen kombinatorischer Ansatz erziehlt. Für Dünnfilme ohne Saatschicht wurde die Löslichkeitsgrenze von Vanadium in In2S3 zu 3.2 at-% V (auf Saphirsubstraten) bzw. 5.4 at-% V (auf Glassubstraten) bestimmt. Durch die Verwendung einer Saatschicht konnte die In2S3 β-Phase stabilisiert und darüber hinaus die Ausbildung von Fremdphasen unterdrückt werden. In2S3:V Dünnfilme mit über 5.8 at-% V auf Saphirsubstraten zeigten bei Raumtemperatur p-Typ Leitfähigkeit. Für Temperaturen unterhalb einer kritischen Temperatur ergab sich ein Wechsel von p- zu n-Leitung. Darüber hinaus sank die Mobilität dieser Schichten unterhalb der kritischen Temperatur signifikant ab. Die Ladungsträgerdichte blieb jedoch über den gesamte Temperaturbereich hinweg konstant und war mit Werten im Bereich von 1022 cm−3 zudem sehr hoch. Diese elektrischen Eigenschaften sind sehr untypisch für einen gewöhnlichen Halbleiter. Sie konnten jedoch im Rahmen dieser Arbeit durch das Modell der Zwischenbandsolarzelle beschieben werden. Als Schlussfolgerung dessen, wurde die Vanadiumkonzentration, bei der sich das Zwischenband ausbildet zu 3.2 at-% V bestimmt. Da sich herausstellte, das In2S3:V bei Raumtemperatur p-Typ ist, konnten keine Zwischenbandsolarzellen mit p-ZCO hergestellt werden.:1 Introduction 1 2 Theoretical background 3 2.1 Indium sulfide ... ... ... ... ... ... ... ... ... .... . 3 2.2 The physics of solar cells ... ... ... ... ... ... ... .... . 7 2.3 The concept of intermediate band solar cells ... ... ... ... ... 8 2.4 Indium sulfide as intermediate band material ... ... ... ... ... 11 2.5 Charge transport ... ... ... ... ... ... ... ... ... ... 13 2.6 Electronic defect states ... ... ... ... ... ... ... ... ... 14 3 Methods 17 3.1 Growth and structuring techniques ... ... ... ... ... .... . 17 3.1.1 Thermal evaporation ... ... ... ... ... ... ... ... 17 3.1.2 Pulsed laser deposition ... ... ... ... ... ... .... . 19 3.1.3 Sputter deposition ... ... ... ... ... ... ... .... 20 3.1.4 Photolithography ... ... ... ... ... ... ... .... . 21 3.2 characterization techniques ... ... ... ... ... ... ... .... 22 3.2.1 X-ray diffraction measurement ... ... ... ... ... .... 22 3.2.2 Hall effect measurement ... ... ... ... ... ... .... 23 3.2.3 Current-voltage measurement ... ... ... ... ... .... 25 3.2.4 Temperature-dependent current-voltage measurement ... ... 26 3.2.5 Resistance measurement ... ... ... ... ... ... .... 26 3.2.6 Spectroscopic ellipsometry ... ... ... ... ... ... ... 26 3.2.7 Energy dispersive X-ray spectroscopy ... ... ... ... ... 27 3.2.8 Transmittance and reflection spectroscopy ... ... ... ... 27 4 Physical properties of undoped In2S3 ... ... ... .29 4.1 Impact of the growth parameters on the composition ... ... .... 31 4.2 Desorption mechanisms and their influence on the growth rates .... . 33 4.3 Surface morphological properties ... ... ... ... ... ... ... 35 4.4 Structural properties ... ... ... ... ... ... ... ... .... 37 4.5 Optical properties ... ... ... ... ... ... ... ... ... ... 43 4.5.1 Dielectric function and absorption coefficient of In2S3 ... ... 43 4.5.2 Impact of the growth parameter ... ... ... ... ... ... 48 4.5.3 Impact of the composition ... ... ... ... ... ... ... 49 4.5.4 Impact of the substrate crystallinity ... ... ... ... .... 51 4.6 Electrical properties ... ... ... ... ... ... ... ... .... 52 4.6.1 Persistent photoconductivity ... ... ... ... ... .... . 52 4.6.2 Temperature dependent resistivity and Hall effect measurements 63 4.7 Device characterization ... ... ... ... ... ... ... ... ... 69 4.7.1 Impact of the growth parameter ... ... ... ... ... ... 70 4.7.2 Impact of the substrate crystallinity ... ... ... ... .... 79 4.8 Solar cell performance ... ... ... ... ... ... ... ... ... 83 4.8.1 Impact of the growth parameter ... ... ... ... ... ... 83 4.8.2 Impact of the substrate crystallinity ... ... ... ... .... 88 5 Physical properties of vanadium-doped In2S3... ... ... .91 5.1 Vanadium incorporation into the In2S3 thin films ... ... ... ... 93 5.2 Surface morphological properties ... ... ... ... ... ... ... 95 5.3 Structural properties ... ... ... ... ... ... ... ... .... 96 5.4 Optical properties ... ... ... ... ... ... ... ... ... ... 107 5.5 Electrical properties ... ... ... ... ... ... ... ... .... 109 5.6 Device characterization ... ... ... ... ... ... ... ... ... 120 6 Summary and Outlook ... ... ... .125 List of Abbreviations... ... .... 131 List of Symbols... ... .... 133 Bibliography ... ... ... .137 List of Own and Contributed Articles ... ... ... .149 Appendix ... ... ... .151 Publikationsliste nach Promotionsordnung § 11(3)... ... .... 161 Zusammenfassung nach Promotionsordnung § 11(4) ... ... ... .163
In the first part of the presented work the influence of the growth parameter of In2S3 thin films, grown by physical vapor deposition, on their physical properties is investigated. The deposition parameters were found to have a strong influence on the surface morphology and the structural properties. By choosing appropriate deposition parameters β-phase In2S3 with a pure (103) orientation was achieved. Epitaxial growth with 2 and 4 rotational domains could be induced using c- and a-plane sapphire, respectively. The fundamental optical bandgap was determined to be direct with an energy of 2.1 eV for all In2S3 thin films. A strong persistent photoconductivity, which was attributed to deep defects within the bandgap, was observed for all In2S3 thin films independent of the preparation conditions and independent of the kind of substrate. Solar cells of n-In2S3/p-zinc-cobalt-oxide (ZCO) exhibit high current rectifications and photovoltaic activity but suffer from absorption in the ZCO layer. To study the physical properties of In2S3:V thin films and to implement intermediate band solar cells (IBSC) In2S3:V thin films without and with seed and buffer layers were fabricated, respectively. Using a combinatorial material synthesis approach doping concentrations of up to 11.4 at-% V were achieved. Thin films without seed layers exhibit a solubility limit of vanadium of 3.2 at-% V and 5.4 at-% V for thin films on sapphire and glass substrates, respectively. The In2S3:V β-phase could be stabilized and the formation of secondary phases suppresed by inserting a seed layer. A change of the type of the charge carriers from p-type at room temperature to n-type at low temperatures was observed for thin films with doping concentrations above 5.8 at-% V on sapphire substrates. Furthermore, the mobility decreases significantly below the critical temperature. Contrarily, a very high charge carrier concentration was observed independent of the temperature. This behavior, which is untypical for conventional semiconductors, could be described using the intermediate band (IB) model. According to the results of this work and the IB model, one can conclude, that above a vanadium concentration 3.2 at-% V an IB has formed. Due to the p-type conductivity of In2S3:V thin films at room temperature, rectifying IBSCs could not be implemented using p-type ZCO. Therefore, it should be replaced by an n-type material in future investigations.:1 Introduction 1 2 Theoretical background 3 2.1 Indium sulfide ... ... ... ... ... ... ... ... ... .... . 3 2.2 The physics of solar cells ... ... ... ... ... ... ... .... . 7 2.3 The concept of intermediate band solar cells ... ... ... ... ... 8 2.4 Indium sulfide as intermediate band material ... ... ... ... ... 11 2.5 Charge transport ... ... ... ... ... ... ... ... ... ... 13 2.6 Electronic defect states ... ... ... ... ... ... ... ... ... 14 3 Methods 17 3.1 Growth and structuring techniques ... ... ... ... ... .... . 17 3.1.1 Thermal evaporation ... ... ... ... ... ... ... ... 17 3.1.2 Pulsed laser deposition ... ... ... ... ... ... .... . 19 3.1.3 Sputter deposition ... ... ... ... ... ... ... .... 20 3.1.4 Photolithography ... ... ... ... ... ... ... .... . 21 3.2 characterization techniques ... ... ... ... ... ... ... .... 22 3.2.1 X-ray diffraction measurement ... ... ... ... ... .... 22 3.2.2 Hall effect measurement ... ... ... ... ... ... .... 23 3.2.3 Current-voltage measurement ... ... ... ... ... .... 25 3.2.4 Temperature-dependent current-voltage measurement ... ... 26 3.2.5 Resistance measurement ... ... ... ... ... ... .... 26 3.2.6 Spectroscopic ellipsometry ... ... ... ... ... ... ... 26 3.2.7 Energy dispersive X-ray spectroscopy ... ... ... ... ... 27 3.2.8 Transmittance and reflection spectroscopy ... ... ... ... 27 4 Physical properties of undoped In2S3 ... ... ... .29 4.1 Impact of the growth parameters on the composition ... ... .... 31 4.2 Desorption mechanisms and their influence on the growth rates .... . 33 4.3 Surface morphological properties ... ... ... ... ... ... ... 35 4.4 Structural properties ... ... ... ... ... ... ... ... .... 37 4.5 Optical properties ... ... ... ... ... ... ... ... ... ... 43 4.5.1 Dielectric function and absorption coefficient of In2S3 ... ... 43 4.5.2 Impact of the growth parameter ... ... ... ... ... ... 48 4.5.3 Impact of the composition ... ... ... ... ... ... ... 49 4.5.4 Impact of the substrate crystallinity ... ... ... ... .... 51 4.6 Electrical properties ... ... ... ... ... ... ... ... .... 52 4.6.1 Persistent photoconductivity ... ... ... ... ... .... . 52 4.6.2 Temperature dependent resistivity and Hall effect measurements 63 4.7 Device characterization ... ... ... ... ... ... ... ... ... 69 4.7.1 Impact of the growth parameter ... ... ... ... ... ... 70 4.7.2 Impact of the substrate crystallinity ... ... ... ... .... 79 4.8 Solar cell performance ... ... ... ... ... ... ... ... ... 83 4.8.1 Impact of the growth parameter ... ... ... ... ... ... 83 4.8.2 Impact of the substrate crystallinity ... ... ... ... .... 88 5 Physical properties of vanadium-doped In2S3... ... ... .91 5.1 Vanadium incorporation into the In2S3 thin films ... ... ... ... 93 5.2 Surface morphological properties ... ... ... ... ... ... ... 95 5.3 Structural properties ... ... ... ... ... ... ... ... .... 96 5.4 Optical properties ... ... ... ... ... ... ... ... ... ... 107 5.5 Electrical properties ... ... ... ... ... ... ... ... .... 109 5.6 Device characterization ... ... ... ... ... ... ... ... ... 120 6 Summary and Outlook ... ... ... .125 List of Abbreviations... ... .... 131 List of Symbols... ... .... 133 Bibliography ... ... ... .137 List of Own and Contributed Articles ... ... ... .149 Appendix ... ... ... .151 Publikationsliste nach Promotionsordnung § 11(3)... ... .... 161 Zusammenfassung nach Promotionsordnung § 11(4) ... ... ... .163
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