Growth and characterisation of earth-abundant semiconductor nanostructures for solar energy harvesting

Zinc phosphide (Zn3P2) is a compound semiconductor based on earth-abundant elements with functional properties ideal for solar cell applications. Cheap, abundant, and renewable energy sources are increasingly imperative due to the imminent threat posed by climate change. So far, zinc phosphide has not been exploited due to limitations in the fabrication of a high-quality material as a consequence of challenges in growth, controllable doping, and heterostructure formation. The maximum conversion efficiency (~6%) was obtained 40 years ago. One route with potential to circumvent or minimise the impact of the limiting factors is the growth of zinc phosphide in the form of nanostructures. By growing the material into nanoscale objects one opens up new elastic strain relaxation mechanisms, minimise the interface area, and loosen the lattice-matching constraints for high-quality epitaxial growth. This thesis focuses on the growth and characterisation with electron microscopy of (i) zinc phosphide nanowires grown through a vapour-liquid-solid approach, and (ii) nanopyramids and thin films grown through selective area epitaxy and lateral epitaxial overgrowth. The first part of this thesis will introduce the reader to the motivation behind the research with regards to renewable energy and a review of zinc phosphide (Chapter 1), the growth of nanostructures (Chapter 2) with focus on molecular beam epitaxy and nan-owires, and then the use of electron microscopy for characterisation of these structures (Chapter 3). The second part of this thesis will focus on the scientific results from the investigations into the growth of zinc phosphide nanostructures. First, the epitaxial growth of zinc phosphide nanowires using an indium-catalysed vapour-liquid-solid approach was investigated (Chapter 4). The growth parameters and mechanism were explored, as well as their impact on the functional properties of the material. Four different nanowire morphologies were achieved, and a followup study focused on the structure and formation of the zigzag ones (Chapter 5). The structure of the zigzag nanowires was found to be analogous to the twin super-lattices found in e.g. III-V nanowires, with one exception. Instead of the standard twin, the zinc phosphide nanowires were found to contain a heterotwin based on a ~monolayer thick inclusion of indium at the mirror plane. It was shown that they did not influ-ence the optoelectronic properties. A more general model for the superlattice formation was also developed, taking into account the non-polar nature of zinc phosphide. The last study explored zinc phosphide growth by selective area epitaxy (Chapter 6). The growth was limited to nanoscale holes using a nano-patterned oxide mask, where it proceeds through a vapour-solid mechanism. Zinc phosphide then forms nanopyramids enclosed by (101) facets, its most energetically stable configuration. If allowed to grow for long enough, the pyramids would coalesce into a thin film, via so-called lateral epitaxial overgrowth. This approach constitutes a step forward in the quest of high-quality, reproducible, and tunable growth of zinc phosphide, and offers a new pathway for its use as an earth-abundant photovoltaic material. Finally, the work is summarised at the end alongside an outlook of future research building on the findings presented in this thesis (Chapter 7). The supplementary information to the different chapters can be found in appendices at the end of the thesis.

Transparent Passivating Contacts for Front Side Application in Crystalline Silicon Solar Cells

Josua Andreas Stückelberger

In this thesis, we present the development and characterization of novel approaches to carrier-selective passivating contacts for crystalline silicon (c-Si) solar cells. In our first approach, we benefit from a wide-bandgap material, namely mixed-phase silicon oxide (SiOx). High hydrogen dilution during the plasma-enhanced chemical vapor deposition of a SiOx layer leads to the growth of vertically oriented silicon filaments embedded in a silicon oxide matrix. The resulting mixed-phase material combines transparency and vertical conduction through the layer. We propose a layered contact structure starting with an ultra-thin chemically grown SiOx (~1.2 nm) at the c-Si wafer surface, followed by a phosphorus-doped bilayer of the mixed-phase SiOx, finishing with a nanocrystalline silicon layer that ensures contact with the metallization. With such a stack, an electron-selective passivating contact with a saturation current density J0 of 5.3 fA/cm2 is achieved on n-type wafers with hydrogenation, whereas on p-type wafers the contact reached 5.8 fA/cm2 even without hydrogenation. The contact is analyzed in detail with respect to its structure and its change upon thermal treatment. Two different growth regions are detected: one with the vertically oriented silicon inclusions and a second one in which the silicon phases have more lateral growth. Crystallization and atomic redistribution are analyzed during in-situ annealing in an electron microscope, leading to a better understanding of the change in distribution of phosphorus and oxygen as well as the nucleation sites. The influence of the annealing temperatures and dwell times as well as the initial doping concentration on the resulting in-diffused region and the passivation quality is reported. We use simulations to relate the influence of these parameters on passivation to the different recombination mechanisms. From the analyzed phosphorus doping profiles, an interesting observation is reported. The chemical oxide varies in thickness and density depending on the wafer polarity as well as the base resistivity. Proof-of-concept cells including this novel structure demonstrate the suitability for electron-selective passivating contacts at the device level with conversion efficiencies of 19.0% on planar wafers and 20.1% on textured wafers. Promisingly high short-circuit current densities (Jsc) of 35 mA/cm2 (40 mA/cm2) on planar (textured) with low parasitic absorption indicate its potential as a front contact in next-generation solar cells. Additionally, the beneficial feature of the smooth refractive index change within the layers that leads to a low reflectance over a broad wavelength range, similar to a built-in anti-reflection coating, is presented. A second approach to reach highly transparent electron-selective passivating contacts is demonstrated by the use of a plasma process based on the precursor gases phosphine (PH3), silicon tetrafluoride (SiF4), hydrogen (H2), and argon (Ar). The beneficial effect of fluorine in terms of passivation without the need for additional hydrogenation processes is shown, reaching implied open-circuit voltages of 730 mV on planar surfaces and 707 mV on textured surfaces. These layers are implemented in first proof-of-concept working devices with efficiencies of 19.5% on single-side-textured wafers and Jsc values of up to 40 mA/cm2.

EPFL2018

Crystalline Order in Compound Semiconductors in Nanoscale Structures and Thin Films

Mahdi Zamani

Semiconductors materials and devices are essential building blocks for many of the technologies deeply embedded in modern life. Improving the performance of semiconductor devices requires a deeper understanding of the fundamental mechanisms controlling the crystalline structure of the semiconductor materials. This thesis focuses on two compound semiconductor systems in the form of ðð3ð2 thin films and ðºððŽð nanowires and tries to establish methods for minimizing the defects in these structures by the careful control of order at the atomic scale. In the first part of this work, we have focused on Zn3 P2 thin films. The combination of suitable material properties and the abundancy of its constituting elements makes this platform a promising candidate for large-scale and scalable photovoltaic applications. Despite this, Zn3 P2 is not adopted by the solar industry due to the many technical challenges in the growth of this material. In this work, by careful control of the state of the interface between the thin films and the substrates, we have provided a method for the growth of thick and monocrystalline Zn3 P2 thin films with superior quality. This is crucial for the realization of any successful photovoltaic cell based on this material. Although molecular beam epitaxy is used in this study, the process provided is generalizable and could also be exploited for the other growth methods. A host of different characterization techniques, including electron microscopy and spectroscopy, Raman spectroscopy, photoluminescence spectroscopy and x-ray diffraction are used to assess the different aspects of the thin films. It is observed that monocrystalline films have better optical properties compared to polycrystalline thin films, making them more suitable for photovoltaic applications. In the second part of the thesis, we have focused on improving the crystalline quality of ðºððŽð nanowires. Similar to other III-V semiconductors, ðºððŽð hosts an internal electrical dipole known as polarity. Arsenide and phosphide nanowires often grow along (111)ðµ direction, which implies the termination of (111) bilayer by group V elements. However, this configuration is often defective and results in polytypism. On the other hand, (111)ðŽ growth, which refers to group III termination, results in a crystalline structure that is largely free of defects. However, growth in this polarity is rarely reported. In this work, we try to understand the reason for this elusiveness and the superior crystalline quality associated with A-polar growth. To do that, we have established a new framework that explicitly focuses on the atomic structure and order of liquid-solid interface in nanowires grown by vapor-liquid-solid method. The atomic structure of this interface has been ignored up to now in the literature due to the technical challenges for investigating such fragile atomic order. Here, we employ a combination of experimental observations via electron microscopy and spectroscopy and machine-learning-based molecular dynamics simulations that were developed by external collaborators. The results of this study provide a unique insight into the fundamental aspects of nanowire growth. Key words: compound semiconductors, photovoltaic cells, molecular beam epitaxy, thin films, nanowires, growth polarity, liquid-solid interface, liquid ordering

EPFL2021

Etude par microscopie électronique de particules nanostructurées d'oxalate de cobalt précipitées en milieu aqueux

The understanding of material self-assembling and self-organization mechanisms, at mesoscale, is crucial for nanotechnologies development. Such structures, hierarchically organized in superlattices or colloïdal crystals, are observed in inorganic or organo-metallic precipitates. The classical characterization, using electron microscopy and X-ray diffraction, of a synthesized powder is inadequate to describe the hierarchically organized polycrystalline structure of the final particles. Here, we have highlighted the successive steps that lead to the spontaneous formation of a cobalt oxalate dihydrate (COD) colloïdal crystals that precipitate in aqueous media. The characterization of the final particles, made by X-ray and electronic diffraction, has allowed us to determine the controversial crystalline phase of the precipitated COD. The β and γ COD phases has been discriminated by comparing experimental results with simulations of X-ray diffractogram and electron diffraction patterns for these both structures. The precipitated COD corresponds to the γ phase indicating that it comes from solid dehydration of cobalt oxalate tetrahydrate. The final powder characterization, performed by X-ray diffraction (XRD), atomic force microscopy (AFM), low voltage high resolution scanning electron microscopy (LVHRSEM) and transmission electron microscopy (TEM), gives some diverging results about monocrystalline or polycrystalline nature of the precipitates. XRD and AFM analysis show a polycrystalline structure of particles whereas LVHRSEM, TEM and electron diffraction observations indicate a monocrystalline structure. This apparent contradiction has led us to elaborate a new characterization method allowing to follow the growth and the structural evolution of the precipitate as a function of time. The cryo- preparation techniques are well adapted as they allow freezing and direct observation of solid state suspension by electron microscopy. The freeze/drying method has been modified and used for both SEM and TEM, and has allowed us to study the morphological and structural evolution of the COD precipitate from nanoscale to mesoscale. In addition, TEM analysis of samples prepared by classical techniques has been performed to complement cryo-electron microscopy observations. The combination of these two methods show that COD precipitation is a complex multi-step process leading to the formation of a core/shell heterostructure. The anisotropic core is porous and partially crystalline. It is formed by agglomeration of isolated primary particles and agregated ones (15-20 nm sized secondary particles). The crystalline shell corresponds to the final particles faces and is made up by the layer by layer self-assembling and alignment of 5-7 nm sized crystalline primary particles. These nanoparticles are aligned and perfectly ordered in strings of nanograins in the layer structure. The self-assembling occurs in the last step of growth where we have a lower ionic strength and supersaturation inducing slower kinetics of growth and aggregation. Moreover, the crystalline order in the primary and secondary particles increase with the time of reaction consistent with a continuous process of primary particle nucleation and growth. Our results show that self-assembling occurs layer by layer with terraces, kinks and steps that are present on the faces. This reminds the layer by layer crystalline growth models but in this case, the buiding blocks are colloïdal instead of atomic or molecular. Based on these different results, we propose an original model describing the COD precipitate growth. In addition, we have studied the poly(methacrylic acid-sodium salt) effect on the COD precipitation. This additive (PMMA) appeared to be a very good growth inhibitor. The PMMA effect on the final particles morphology is dramatic. Its concentration variation in solution slows down the nanoparticles aggregation rates along a [101] preferential direction. The crystal habit can then be controlled and we have synthesized platelets, cubes and rods with tailored length. The use of cryogenic electron microscopy has been shown to be a powerful tool for the understanding of time dependent aspects of particle growth. The use of such techniques for the study of other systems, where the final precipitate appearance may hide particle substructures, will help to elucidate growth mechanisms and allow finer control of nanostructured materials for many applications. The complex self-assembling and self-organization processes could then be highlighted and studied on a nanometer to micrometer scale.

EPFL2004