Defect passivation in zinc tin oxide: improving the transparency-conductivity trade-off and comparing with indium-based materials

Transparent conductive oxides (TCOs) are essential in technologies coupling light and electricity. Due to their good optoelectronic properties and the production scalability, Sn-doped indium oxide (In2O3:Sn) is the preferred TCO in industrial applications. Nonetheless indium is scarce in the Earth's crust and its availability might be compromised over the next decades. To address this issue, this doctoral project aims to (i) improve the optoelectronic properties of In-free TCOs seeking to match those of In2O3:Sn, and (ii) refine the optoelectronic properties of In2O3-based films to decrease the In consumption in applications. To accomplish this, we investigated the links between the defects, the microstructure and the optoelectronic properties of two families of TCOs, indium- and tin-based oxides. First, we studied the evolution in the optoelectronic properties and microstructure of amorphous zinc tin oxide (a-ZTO) when annealed up to 500C in oxidizing, neutral, and reducing atmospheres. We show that annealing in atmospheric pressure at temperatures > 300C decreases the detrimental subgap absorptance while increasing the electron mobility (mu). Thermal treatments in reducing atmospheres increase the free-carrier density (Ne) and the subgap absorptance. None of the thermal treatments resulted in important changes in the amorphous microstructure. Combining these results and density functional theory (DFT) calculations, oxygen deficiencies (VO) were identified as the source of detrimental subgap absorption. VO can act as donors but also as electron scattering centres. Based on these results, a-ZTO with ÎŒ up to 35 cm2/Vs, is demonstrated by high-temperature defect passivation. Profiting from the microstructure stability of a-ZTO, this material was used as a recombination junction in a tandem solar cell which require a high-temperature step in its processing. The passivation scheme might be problematic for temperature-sensitive technologies. Therefore, we demonstrated an alternative low-temperature passivation method, which relies on co-sputtering a-ZTO with SiO2 . Using two Sn-based oxides with different composition and microstructure: a-ZTO and SnO2 , and results of DFT calculations, we demonstrate that SiO2 contribution is twofold. (i) The oxygen from SiO2 passivate the VO in SnO2 and a-ZTO. (ii) The formation energy of the ionized VO is lowered by the silicon-atoms, enabling defects that do not contribute to the subgap absorptance. This passivation scheme improves the optical properties without affecting the electrical conductivity, overcoming the optoelectronic trade-off in Sn-based TCOs. Finally, we study an In-based high-ÎŒ TCO: Zr-doped In2O3. Films of In2O3:Zr with thickness from 15 nm-100 nm were sputtered in the amorphous state and annealed in different atmospheres. Annealing in air yields fully crystalline films with high transparency and a high-mu limited by phonon and ionized impurity scattering. 15 nm-thick films exhibit an average absorptance of < 0.5 % (between 390 nm-2000 nm) and an mu of 50 cm2/Vs increasing to 105 cm2/Vs for 100 nm-thick films. Alternatively, annealing in a neutral or reducing atmospheres result in higher mu for films thinner than 50 nm as a high Ne is maintained. The demonstration of thickness reduction while keeping high lateral mu makes In2O3:Zr is an alternative to reduce In in applications such as flexible displays, solar cells and light emitting diodes.

Low-Pressure Chemical Vapor Deposited Zinc Oxide Films

Laura Ding

Polycrystalline zinc oxide (ZnO) films, prepared by low-pressure chemical vapor deposition are investigated in this thesis. ZnO belongs to the class of transparent conductive oxide materials, as it is transparent to light from the visible to the near-infrared range and is simultaneously electrically conductive. These two properties allow ZnO films to be integrated in thin-film silicon solar cells, as well as other opto-electronic devices. In particular, thin-film solar cells require a contact layer covering the whole active layer in order to efficiently collect the photogenerated electrical current, and therefore, the film electrode at the front of the cell has to be a window to the sunlight, in addition to being able to transport current. ZnO’s electrical conductivity is usually improved by doping with impurities like boron, but the film transparency is reduced, and consequently, the current that the solar cell can produce decreases. Thus, the first point investigated in this thesis is to find ways to relax the trade-off between transparency and conductivity. Instead of doping, which increases the electron concentration, the electron mobility has to be improved. To do so, the electron transport is first studied and modeled for standard films, in order to better define and understand its limitations. Then, a new way of preparing ZnO bilayer films is shown to lead to enhanced transparency and light scattering, for a maintained conductivity compared to uniformly doped films. Alternatively, various treatments (hydrogen plasma, vacuum annealing and ultraviolet exposure) are tested, all of which reduce the density of defects that cause electron scattering in the ZnO; treated films present an improved conductivity and very high mobilities close to 60 cm2V−1s−1, which corresponds to the mobility value in single crystals of equivalent doping. In addition, these films absorb less than 2% of the visible-to-infrared light, which makes them very promising ultra-transparent electrodes, for solar cells or other applications. For use in thin-film silicon solar cells, two additional criteria have to be met besides trans- parency and conductivity. First, because the front electrode is also the substrate for the deposition of the rest of the cell layers, its surface morphology has to be favorable to the growth of silicon. Moreover, this surface has to present a rough texture capable of diffusing and trapping the light into the cell, and increasing its chance of being absorbed. These charac- teristics can be incompatible, but are nevertheless necessary to reach thin-film solar cells with high efficiencies. So, the second point of interest of this thesis is to adapt the surface of ZnO layers by investigating their preparation method. Combinations of different growth regimes in multilayers are proposed to enable a smoother controllable of the surface without resort to additional etching treatments, leading to improved solar cell electrical performance. Finally, the stability of ZnO films exposed to various conditions that modify their electrical and optical properties is investigated. First, ZnO is exposed to different oxidizing conditions (storage and annealing in air, exposure to moisture, plasma of carbon dioxide). The loss in conductivity from the interaction of the oxygen species with ZnO is characterized by a new infrared spectroscopic method and a hydrogen plasma treatment is investigated as a means to reverse or partially prevent the degradation in conductivity. Then, the effect of simulated sunlight exposure on ZnO film properties in solar cells is investigated. The resistivity of the front ZnO electrode decreases due to ultraviolet light absorption, and the resistivity of the back electrode increases due to the chemisorption of oxygen species, in absence of ultraviolet light. The front electrode transparency is also degraded; this effect is more pronounced for doped ZnO and its impact on the current photogeneration of solar cells is demonstrated. Therefore, a specific optimization of the front and back electrodes appears necessary. This thesis contributes on the one side to an in-depth understanding of the opto-electrical properties of ZnO films prepared by low-pressure chemical vapor deposition. On the other side, the different proposed approaches enable more flexibility in the development of more efficient and stable electrodes, not only for solar cells, but for many other applications such as electroluminescent diodes, flat-panel displays, touch-screens or transparent transistors.

EPFL2013

A multi-technique approach to study the microstructural properties of tin-based transparent conductive oxides

Federica Landucci

Transparent conductive oxides (TCOs) are semiconductor-like materials that exhibit high electrical conductivity and high optical transparency combined. They are adopted in various applications ranging from gas sensors, to electrochromic windows, to photovoltaic cells. Indium-based TCOs represent the industry standard. Nevertheless, indium is among the less abundant elements in the earth crust and forecasts based on its current consumption envisage an urgent need to replace it. Tin-based TCOs are a promising alternative, since their opto- electronic characteristics mimic the ones of indium-based materials. This thesis aims to investigate the link between optoelectronic and microstructural properties of tin dioxide and zinc tin oxide (ZTO) with a composition Zn0.05Sn0.30O0.65 and their stability when submitted to thermal treatments. Indeed, lots of practi- cal applications require the TCO to operate in high temperature conditions. To conduct this study, a combination of analytical techniques, such as transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X- ray diffraction (XRD), electron paramagnetic resonance (EPR) and differential scanning calorimetry (DSC) was employed. Amorphous SnO2 and ZTO were deposited by RF sputtering and annealed up to 1050°C in different atmospheres. The influence of annealing temperature and atmosphere were decoupled and led us to an in-depth comprehension of the mechanisms governing the optoelectronic properties of both materials. When annealed in air, between room temperature and 300°C, ZTO exhibits increased mobility and carrier concentration with respect to the as-deposited state. This increase, investigated with DSC, was ascribed to a structural relaxation that allows point defects to release electrons in conduction band. Between 300°C and 500°C atmospheric oxygen passivates oxygen vacancies, drastically decreasing the carrier concentration and therefore causing a large drop of the conductivity. EPR experiments allowed to ascribe the drop in conductivity to the decrease of carrier concentration, which occurs slightly before the phase change. At 570°C (and 930°C for the case of vacuum annealing) the phase change occurs and the ZTO crystallizes in the rutile form of SnO2. The material becomes completely insulating. When the temperature is increased to 1050°C, evaporation of zinc is observed. In order to improve the electrical conductivity of ZTO at high temperature, a doping strategy was implemented starting from DFT calculations conducted by a partner group, who screened among the entire periodic table, which elements are the best candidates to act as n-dopants for ZTO. Bromine and iodine were retained, since they were found to be the most energetically favorable to become substitutional defects for a tin site. An exploratory doping route is therefore presented and the treated samples analyzed with TEM, EDX and UV-VIS-IR spectroscopy. Finally, the structural properties of an indium-based TCO (zirconium-doped indium oxide) were investigated and used as a benchmark to propose a crystallization model for the tin-based, as well as the indium-based materials. The influence of pa- rameters such as the material thickness, annealing atmosphere and temperature and deposition pressure are discussed for both materials.

EPFL2019

Zr-doped indium oxide electrodes: Annealing and thickness effects on microstructure and carrier transport

Christophe Ballif, Mathieu Gérard Boccard, Aïcha Hessler-Wyser, Quentin Thomas Jeangros, Federica Landucci, Monica Morales Masis, Oscar Esteban Rucavado Leandro

Zr-doped indium oxide (In2O3:Zr) has been shown to satisfy the requirements of low resistance, wide band gap, and high infrared transmittance for application as a front contact in broadband solar cells. However, the reduction of indium usage in front of transparent electrodes is still an unsatisfied requirement. With the goal of reducing the amount of indium while leveraging its properties, in this work, In2O3 :Zr films with reduced thickness compared to those standardly used in solar cells are studied. 100 to 15-nm-thick films were sputtered at room temperature and annealed in distinct atmospheres to study the links between thickness, microstructure, and optoelectronic properties. As-deposited films exhibit an amorphous microstructure embedding bixbyite In2O3 nanocrystals. Annealing in neutral (N-2) or reducing atmosphere (H-2) allows a slight growth of these crystallites but the layers remain mostly amorphous. Whereas annealing in air results in polycrystalline films with an average grain lateral size ranging from 350 to 500 nm. The large crystalline grains formed during air annealing lead to increased electron mobility for all thickness: up to 100 cm(2)V(-1)s(-1) for 100-nm-thick films and up to 50 cm(2)V(-1)s(-1) for 15-nm-thick films, which is remarkable for such thin polycrystalline films. Conversely, H-2 annealing ensures high free-carrier densities (>1 x 10(20) cm(-3)) but not high mobilities, still achieving conductivities between 1000 and 2000 S cm(-1), with the films less than 50-nm-thick keeping high broadband transmittance. The possibility of thinning down In2O3:Zr to a few tens of nanometers while keeping both high lateral conductivity and good transparency makes this material a promising candidate to reduce the amount of indium in optoelectronic applications, such as flexible touch screens and solar cells.

AMER PHYSICAL SOC2019