Institut de Physique de l'Universit´ede Neuch^atel(Suisse) - Advanced LPCVD ZnO - Challenges in Application for Thin Film Solar Cells and Modules These` pr´esent´ee`ala Facult´edes Sciences de l'Universit´ede Neuch^atel pour l'obtention du grade de doctuer `essciences Par Daniel Messerschmidt Soutenue le 23 juillet 2014 En pr´esencedu directeur de th`ese: Et des rapporteurs: Prof . Philipp Aebi Prof. Christophe Ballif Prof. Thomas S¨udmeyer Prof. Edda R¨adlein Dr. Jens Eberhardt Neuch^atel2014 Faculté des sciences Secrétariat-décanat de Faculté Rue Emile-Argand 11 2000 Neuchâtel - Suisse Tél: + 41 (0)32 718 2100 E-mail: [email protected] IMPRIMATUR POUR THESE DE DOCTORAT La Faculté des sciences de l'Université de Neuchâtel autorise l'impression de la présente thèse soutenue par Monsieur Daniel MESSERSCHMIDT Titre: Advanced LPCVD ZnO – Challenges in Application for Thin Film Solar Cells and Modules sur le rapport des membres du jury composé comme suit : • Prof. Philipp Aebi, Université de Fribourg, directeur de thèse • Prof. Thomas Südmeyer, Université de Neuchâtel • Prof. Christophe Ballif, EPF Lausanne • Prof. Edda Rädlein, Technische Universität Ilmenau, D • Dr. Jens Eberhardt, Bosch Solar Energy AG, D Neuchâtel, le 12 août 2014 Le Doyen, Prof. P. Kropf Imprimatur pour thèse de doctorat www.unine.ch/sciences Abstract Ethanol is used as a precursor during the growth of zinc oxide (ZnO) by low-pressure chemical vapor deposition (LPCVD). By adding ethanol, the surface of the deposited ZnO layer is flattened and its roughness is decreased about sevenfold. The layers be- come increasingly stressed and their resistivity grows significantly. The present work proposes an explanation for the observed behavior based on the catalytic decomposi- tion of ethanol at the ZnO surface and on the growth of selected crystal planes. By using ethanol for the last 10 % of the total ZnO layer growth only, sheet resistance is maintained and roughness is slightly decreased. The results indicate that such LPCVD ZnO bilayers could be a promising method to modify the ZnO surface morphology be- fore cell deposition, thus providing an alternative to argon plasma treatment, which is typically reported to improve solar cell parameters such as open-circuit voltage and fill factor. The introduction of an advanced LPCVD boron-doped ZnO (ZnO:B) into the pro- duction line at Bosch Solar Thin Film in Erfurt, Germany, is presented. Here the ZnO:B is deposited with a LPCVD production tool on 1:1 × 1:3 m2 glass substrates. Early results on a laboratory scale of this advanced transparent conductive oxide (ATCO) type were published by Ding et al. [1,2]. For an ATCO, they combined a highly doped ZnO:B seed layer with a low-doped ZnO:B bulk layer. Compared to a standard ZnO:B (Std-TCO), which is homogeneously doped across the total layer thickness, the ATCO possesses enhanced optical and electrical properties. Comparing the ATCO with the Std-TCO, equal in layer thickness (1800 nm) and sheet resistance (16 to 17 Ω), the ATCO possesses slightly better transmittance. Furthermore, the ATCO showed 37 % haze, which is 3 to 5 % absolute more haze compared to the Std-TCO. After ATCO optimization, a complete micromorph solar module was fabricated, incorporating this new advanced LPCVD ZnO:B layer as the front electrode. An initial module power of 153.7 W p was achieved. Here the unit W p indicates that the module was tested under standardized test conditions. The power output was determined at 25 ℃ with a light illumination power of 1000 W=m2. The light spectrum was adjust to be equivalent to an terrestrial solar spectrum with an air mass 1.5 characteristic. For the first time the present work introduces a novel degradation effect of the ZnO, which is observed for ZnO layers deposited on commercially available soda-lime glass. When an electric field is applied, the potential induced anodic degradation causes a 5 massive sheet resistance increase of the ZnO. The present work shows that the increase in resistance strongly affects the power output of small non-encapsulated and large en- capsulated micromorph thin-film photovoltaic modules, in which ZnO was used as the front electrode. The present work proposes that the evolution of gaseous oxygen at the anode (ZnO) may be responsible for the increased resistance of the ZnO. A model that describes this observation is presented and solutions to suppress the effect are proposed. The optical properties of non-degraded and anodically degraded ZnO:B are presented. The optical model used to simulate the infrared reflectance in the wavelength range between 1.2 and 25 µm is based on the Maxwell-Garnett effective-medium theory. The model is sensitive to the conditions at the grain boundaries of the investigated poly- crystalline ZnO:B films. The yielded results confirm the presence of defect-rich grain boundaries, especially after degradation. Furthermore, indications of a degraded ZnO layer next to the ZnO:B/glass interface with a different refractive index are found. Furthermore, based on Raman investigations, evidence for the creation of oxygen va- cancies, which correlate with a shift of the optical absorption edge of the ZnO:B, is presented. Investigations with scanning and transmission electron microscopy show microvoids at the grain boundaries after anodic degradation. This indicates that the grain/grain interfaces are the principle location of defects after degradation. Contents Contentsi List of Figures iii List of Tables vii 1 Introduction1 1.1 Contribution to the Field of Research...................3 2 Principles of Thin-Film Silicon PV5 2.1 Zinc Oxide - Deposition and Material Properties....................5 2.1.1 Deposition of Zinc Oxide by LPCVD...............6 2.1.2 Structural Properties........................ 11 2.1.3 Electrical Properties........................ 12 2.1.4 Optical Properties......................... 14 2.1.5 Dielectric Function of Polycrystalline LPCVD ZnO:B...... 16 2.2 Silicon Thin Films - Deposition and Material Properties.................... 18 2.2.1 Deposition of Silicon Thin Films by Plasma-Enhanced Chemical Vapor Deposition.......................... 18 2.2.2 Amorphous Silicon......................... 20 2.2.3 Microcrystalline Silicon....................... 21 2.3 Micromorph Solar Cells and Modules................... 22 2.3.1 Concept of a micromorph solar cell................ 23 2.3.2 Working principles of a solar cell and characteristic I(V) parameters 24 2.3.3 Module Fabrication......................... 28 3 Techniques of Characterization 31 3.1 Optical Characterization.......................... 31 3.1.1 Ultraviolet-Visible Spectroscopy.................. 31 3.1.2 Fourier Transform Infrared Spectroscopy............. 32 3.1.3 Raman Spectroscopy........................ 34 3.2 Electrical Characterization......................... 34 3.2.1 Four-Point Probe Resistance Measurement............ 34 3.2.2 Hall Effect Measurement...................... 34 3.3 Structural Characterization........................ 36 3.3.1 Atomic Force Microscopy...................... 36 3.3.2 X-Ray Diffraction Analysis (XRD)................ 37 3.3.3 Secondary Ion Mass Spectroscopy (SIMS)............ 37 i ii Contents 3.4 Characterization of Solar Cells and Modules............... 38 3.4.1 Current-Voltage Characterization of Solar Cells and Modules............................... 38 3.4.2 External Quantum Efficiency (EQE)............... 39 4 Ethanol-enriched LPCVD ZnO bilayers: Properties and Growth { A Potential Electrode for Thin Film Solar Cells 41 4.1 Background................................. 41 4.2 Experimental................................ 43 4.3 Results.................................... 43 4.3.1 ZnO single layers deposited using a mixture of ethanol, water and DEZ............................... 43 4.3.2 Ethanol-enriched bilayers...................... 51 4.4 Conclusion.................................. 54 5 Integration of Advanced-TCO into the Production Line at Bosch 57 5.1 Experimental................................ 58 5.2 Results.................................... 59 5.2.1 Variation of process temperature................. 60 5.2.2 Variation of water-diethylzinc ratio................ 62 5.2.3 Variation of process pressure.................... 63 5.2.4 Variation of seed layers deposition time / thickness....... 64 5.2.5 Variation of bulk layer doping................... 65 5.2.6 Comparison of ATCO vs. Std-TCO................ 67 5.2.7 Result on module power using ATCO............... 69 5.3 Conclusion.................................. 70 6 Anodic degradation of ZnO:B on soda-lime glass 71 6.1 Background................................. 71 6.2 Experimental................................ 73 6.3 Anodic degradation of a large size module in the climate chamber... 75 6.4 Anodic degradation of a unencapsulated mini module on the hot plate. 77 6.5 Degradation of samples with ZnO:B on glass only............ 79 6.5.1 Electrical changes of glass/ZnO:B samples............ 79 6.5.2 Optical changes of glass/ZnO:B samples............. 81 6.6 The glass substrate as a source of oxygen during degradation treatment 92 6.7 Model of anodic degradation of ZnO:B at the glass/ZnO:B interface.. 93 6.8 Improvements................................ 97 6.9 Conclusion.................................. 100 7 Summary 103 References 105 A IR spectral reflectivity response of low-iron soda-lime glass ix Publications xiii Acknowledgment xv Curriculum Vitae xvii List of Figures 1.1 Net price reduction of solar modules based on micromorph technology.2 2.1 Demonstration