Nanolayer surface passivation schemes for silicon solar cells

Abstract

This thesis is concerned with nanolayer surface passivation schemes and corresponding deposition processes, for envisaged applications in crystalline silicon solar cells. Surface passivation, i.e. the reduction of electronic recombination processes at semiconductor surfaces, is essential for realizing high Si solar cell efficiencies. In turn, the increase in the energy conversion efficiency is a major driver for reducing the costs of photovoltaic electricity. However, at present, effective surface passivation schemes have been implemented in only a fraction of industrial Si solar cells. Therefore, the development of suitable surface passivation schemes and related technology is currently a key topic in photovoltaic research. This is underscored by the immense attention that aluminum oxide (Al2O3) has captured in recent years after being introduced as an effective surface passivation material in 2006. Al2O3 thin films appeared to have some advantages over contending technologies for the passivation of the rear side of p-type Si solar cells and for the passivation of the p+ emitter in n-type Si cells. Along with the use of Al2O3, atomic layer deposition (ALD)—with its benefits of submonolayer growth control and excellent uniformity and step-coverage—was also introduced as a novel deposition method in the field of Si photovoltaics. This thesis addresses topics ranging from the fundamental mechanisms that govern the properties of nanolayer surface passivation schemes to the industrial feasibility of the technology. These aspects are closely interwoven, as improved fundamental understanding forms the basis for developing, optimizing, implementing and commercializing the relevant technologies. The focus throughout the thesis is on Al2O3 and on ALD, both of which enabled new opportunities for developing and studying nanolayer surface passivation schemes. A share of the research was carried out in collaboration with strategic partners, including the solar cell manufacturer Q-Cells. The properties of the Al2O3 films were evaluated on multiple levels: Firstly, the surface passivation quality of the films was studied in relation to various technologically relevant parameters such as ALD conditions, annealing recipes, material properties and film thicknesses. It was found, for instance, that Al2O3 films with thicknesses down to 5-10 nm synthesized by plasma-assisted and thermal ALD induced ultralow surface recombination velocities, Seff <5 cm/s, on low resistivity n- and p-type Si in a relatively wide processing parameter window. Secondly, the chemical passivation (i.e., the reduction of the interface defect density) and field-effect passivation by negative fixed charges (i.e., the shielding of electrons from the surface), responsible for the passivation quality of Al2O3, are addressed in detail. While its negative fixed charge density is a distinguishing property relative to other relevant surface passivation materials, it is established that the effective chemical passivation, as demonstrated by very low defect densities of Dit <1011 cm-2 eV-1 at mid gap, also plays a key role in the Al2O3 passivation properties. Finally, the fundamental mechanisms controlling the chemical- and field-effect passivation were addressed experimentally using innovative approaches. For example, the diffusion of hydrogen present in the Al2O3 films (typically ~3 at.%) during annealing was studied with thermal effusion experiments and was correlated with the hydrogenation of interface defects. The latter, in combination with the presence of an interfacial SiO2 layer, are key to the low Dit values achieved by Al2O3. Regarding the field-effect passivation, the thickness of the interfacial SiO2 was identified as a key parameter controlling the negative fixed charge density associated with Al2O3. This experimental study relied on using an ALD SiO2 process for interface engineering with Angstrom resolution. It was combined with diagnostics such as electric-field induced second-harmonic generation for the contactless probing of the changes in charge distribution for thicker SiO2 interlayers. This thesis also addresses surface passivation stacks, such as Al2O3/a-SiNx:H, SiOx/a-SiNx:H and SiO2/Al2O3 stacks. The newly introduced SiO2/Al2O3 stacks are compatible with very low Seff values, regardless of the SiO2 synthesis method. In fact, the use of Al2O3 capping layers enabled an unprecedented high interface quality for low-temperature synthesized SiO2. This appears to be mainly related to a very effective hydrogenation of the remote Si/SiO2 interface during annealing. For both SiO2/Al2O3 and SiO2/a-SiNx:H stacks, field-effect passivation was reduced significantly compared to the corresponding single layers, which can avoid—sometimes undesirable—inversion conditions. Therefore, by using surface passivation stacks, not only the optical and chemical properties, but also the underlying passivation mechanisms can be controlled and optimized for solar cell and other electronic applications. Regarding solar cell processing, various aspects pertaining to the feasibility of Al2O3 and related technologies are addressed. For instance, the thermal stability of Al2O3-based passivation schemes proved to be adequate. Moreover, films deposited using batch- or spatial ALD methods—specifically designed by a number of companies to meet the throughput requirements for PV manufacturing—were shown to exhibit similar properties as obtained by single-wafer laboratory reactors. In addition, a pulsed-precursor PECVD process is reported as an alternative method for the fast deposition of Al2O3 and other materials. The development and understanding of Al2O3-based surface passivation schemes in conjunction with the new ways of investigating, controlling and manipulating their properties, as outlined in this thesis, are important for the ongoing developments in the field of photovoltaics aiming at higher efficiencies and lower costs per kilowatt-hour. Based on the recent announcements about enhanced efficiencies for industrial-type rear-passivated solar cells and the installation of high-throughput deposition systems for Al2O3 in solar cell pilot lines, it is expected that Al2O3-based surface passivation schemes will provide a leap in performance of a large share of commercially available solar modules in the coming years. In a broader context, the relevance of this thesis may extend to the field of (nano-)electronics in which the continuous reduction of device dimensions demands even more stringent requirements of thin film technology. Extrapolating the rapid developments in recent years, it is expected that ALD will play an increasingly important role for Si-based, but probably also other, photovoltaic applications in the near future.