Studies of various alkali metal oxides containing calcium silicate glasses as substrate for solar cell application

Abstract

Extensive research is going on in the field of thin film solar cells (TFSCs) to improve its efficiency and reduce the cost. Various groups observed that the film grown on soda lime glass (SLG) substrate had a <112> oriented texture with absence of aggregate structures and aggregate boundaries due to (i)compressive stress between the film and substrate because of a better matched coefficient of thermal expansion (CTE), and (ii) contamination of absorber layer with sodium (Na) diffused from SLG substrate. Various theoretical, chemical and simulation models are proposed to explain that Na acts as a catalyst in the absorber layer to passivate the Se vacancies and reduce the defect trap states, which directly influence and improve the efficiency and performance of the TFSC. The extent of this effect is further explored by depositing a NaF layer of various thicknesses on different substrates and concluded that performance is best at 0.14 at% Na diffusion through SLG. Influence of Li and K pre and post-deposition is found to enhance the performance of cells. Therefore, a glass substrate of a TFSC must have: (i) Glass transition temperature (Tg)> 550°C (ii) presence of Na, K, Li (iii) CTE ~ 5-8×10-6 K-1 (iv) Low cost The present thesis describes the preparation of the alkali oxide containing calcium silicate glasses, wherein sodium oxide is systematically replaced by lithium and potassium oxide, via melt quench technique. The structural, optical, mechanical, thermal and electrical properties of the as-prepared glasses are studied using to determine their suitability as substrate for thin-film solar cells. The synthesized glasses were prepared as substrate to deposit Molybdenum thin film to study the diffusion of alkali ions from glass to Mo-film. The thesis is divided into six chapters with a list of cited references at the end of each chapter. Chapter 1 presents an introduction to glasses, its characteristic features, components of glasses and their effect on glass structure and their properties. Various engineering applications of glasses are discussed in detail, concluded with the need and importance of glass in thin film photovoltaics. The evolution of thin film photovoltaics to fulfil the highly escalating energy needs of the whole world is elaborated. Various components of thin film solar cells are discussed highlighting the importance of substrate materials and Mo as back contact electrode. Finally, the suitability of glass as substrate material is described in terms of specific properties such as mechanical strength, thermal stability, coefficient of thermal expansion, electrical properties and cost effectiveness. Chapter 2 presents the detailed literature survey related to the type of substrate materials employed for the thin film solar cells so far. Role of alkali ion incorporation in CIGS/CZTS based thin film solar cells is reviewed in view of its effect on the efficiency of thin film solar cells. Conventionally used substrate materials for thin films are reviewed to conclude that alkali oxide containing glass is the most promising and cost-effective mode. A brief review of alkali oxide containing glass compositions is presented to study their effect on the desired properties for substrate materials. On the basis of the complete literature review, objectives of the thesis are determined and presented at the end of this chapter. Chapter 3 elaborates the information of glass compositions and the raw materials taken to synthesize them. The procedure followed for the synthesis of glasses using melt quench technique is given in detail. The preparation of substrate for thin film deposition, from synthesized glasses is also discussed in detail. The characterization techniques employed for studying the properties of glass samples are discussed in detail such as X-ray diffraction (XRD), Fourier transform infrared-spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), differential thermal analyser (DTA), Impedance Analyser, UV-vis spectroscopy, Vickers hardness testing technique. Details of Sputtering machine used to deposit the Mo-film, field-emission scanning electron microscopy (FE-SEM) used to determine the thickness and morphology of the Molybdenum thin film are discussed. Secondary ion mass spectroscopy (SIMS) used to study the alkali diffusion from glass to thin film is also elaborated in this chapter. Chapter 4 deals with the results and discussion part of the present work. The chapter is divided into two parts. Part I focuses on the effect of variation in type of alkali oxide contained in glass composition on the structural, mechanical, optical, thermal and electrical properties of glasses which is investigated in detail using various characterization techniques. Self-diffusivity of alkali ions within glass is estimated using impedance spectroscopy technique. The size of the alkali ion does not directly affect the structural, optical and electrical properties of the glasses. The diffusivity of sodium ions in sodium oxide containing glass is the highest. Part II consists of the detailed investigation of effect of presence of another alkali oxide with sodium oxide in glass composition on various properties of the glasses in light of mixed alkali effect. Systematically substituting the sodium oxide by smaller sized lithium oxide and bigger sized potassium oxide into the present composition of glasses exhibits typical non-linearity in properties with respect to variation in composition. Its prospective effect on transport properties of alkali ions in glasses is given. The mixed alkali effect within glass enhances the conductivity with the substitution of lithium oxide, however, deteriorates on the introduction of potassium oxide in the glass composition. Chapter 5 reports the growth and optimization of Molybdenum (Mo) thin film using DC magnetron sputtering under the influence of different sputtering parameters. The bilayer Mo-film with appropriate thickness, structure, morphology, adhesion and resistivity is optimized as the deposition power of 75W for 40 min high pressure deposition followed by 20 min low pressure deposition. Chapter 6 elaborates the diffusion of different alkali ions from glass substrate to Mo film using SIMS. It shows that, as the self diffusion of Na in sodium oxide containing glass is highest, the extent of out-diffusion of Na ions from glass to Mo film is the highest. The out-diffusion of Li and K ions is low. Further, it presents the comparative study of sodium ion diffusion from commercially available soda lime glass and synthesized sodium calcium silicate (NCS) glass, respectively to the Mo-film deposited onto it and found that the extent of diffusion in NCS glass is higher than commercially available SLG.. The lattice and grain boundary diffusion coefficients for alkali ions are estimated by fitting the diffusion profile using error function and Whipple’s Analysis. Profile in the interfacial region is fitted to estimate the interfacial access of alkali ions. The diffusion mechanism of alkali ions follow Harrison’s type B diffusion kinetics. The composition of substrate material is found to critically influence the out-diffusion of alkali ions to the overlayers. Chapter 7 elucidates the conclusion of the entire work done in the present thesis and future scope of the current study. The major conclusion of this work is that, based on all types of preliminary investigations, the synthesized glasses fulfil the requirement of properties to be suitably used as substrate for thin film solar cells, with suitable mechanical strength, glass transition temperature higher than 540°C, coefficient of thermal expansion close to SLG, and matches well with that of the Mo/CZTS thin films. The Mo-film optimized using sputtering conditions is appropriate for use as back contact electrode of the CZTS/CIGS based thin film solar cell with thickness of 1 micron, (110) oriented BCC structure, leafy morphology, good adhesion and resistivity of the order of 10-4 Ωcm. Single alkali ion in glass composition shows better diffusivity in case of Na and Li ions as compared to mixed alkali glasses. Comparative study of Na diffusion from SLG and NCS glass shows that the extent of diffusion in NCS glass is higher than SLG. Thus, our synthesized NCS glass is a suitable and promising candidate for the application as substrate for thin film solar cell. Future scope of the study includes the comparative study of grain boundary characteristics of the CZTS film deposited onto SLG and synthesized glasses, and to study the effect of composition on the performance of solar cell deposited onto the synthesized substrates.