De relatie tussen de elektrische en optische eigenschappen van plasma-gedeponeerd zinkoxide

In dunne-film zonnecellen wordt zinkoxide gebruikt als toplaag, omdat dit materiaal tegelijk transparant en geleidend kan zijn. In dit werk zijn de elektrische eigenschappen van zinkoxide bepaald met een nieuwe optische methode. Op deze manier kregen we verbeterd inzicht in de elektrische geleiding van het zinkoxide en de relatie met de optische eigenschappen van het materiaal. Bovendien is het met dit model mogelijk de invloed van de twee belangrijkste verstrooiingsprocessen van vrije ladingen binnen het zinkoxide van elkaar te scheiden : de verstrooiing aan kristalgrenzen en de verstrooiing aan verontreinigingen

Optical modeling of plasma-deposited ZnO films : electron scattering at different length scales

In this work, an optical modeling study on electron scattering mechanisms in plasma-deposited ZnO layers is presented. Because various applications of ZnO films pose a limit on the electron carrier density due to its effect on the film transmittance, higher electron mobility values are generally preferred instead. Hence, insights into the electron scattering contributions affecting the carrier mobility are required. In optical models, the Drude oscillator is adopted to represent the free-electron contribution and the obtained optical mobility can be then correlated with the macroscopic material properties. However, the influence of scattering phenomena on the optical mobility depends on the considered range of photon energy. For example, the grain-boundary scattering is generally not probed by means of optical measurements and the ionized-impurity scattering contribution decreases toward higher photon energies. To understand this frequency dependence and quantify contributions from different scattering phenomena to the mobility, several case studies were analyzed in this work by means of spectroscopic ellipsometry and Fourier transform infrared (IR) spectroscopy. The obtained electrical parameters were compared to the results inferred by Hall measurements. For intrinsic ZnO (i-ZnO), the in-grain mobility was obtained by fitting reflection data with a normal Drude model in the IR range. For Al-doped ZnO (Al:ZnO), besides a normal Drude fit in the IR range, an Extended Drude fit in the UV-vis range could be used to obtain the in-grain mobility. Scattering mechanisms for a thickness series of Al:ZnO films were discerned using the more intuitive parameter "scattering frequency" instead of the parameter "mobility". The interaction distance concept was introduced to give a physical interpretation to the frequency dependence of the scattering frequency. This physical interpretation furthermore allows the prediction of which Drude models can be used in a specific frequency range

Atomic layer deposition for high-efficiency crystalline silicon solar cells

This chapter illustrates that Atomic Layer Deposition (ALD) is in fact an enabler of novel high-efficiency Si solar cells, owing to its merits such as a high material quality, precise thickness control, and the ability to prepare film stacks in a well-controlled way. It gives an overview of the field of Si solar cells, where for each concept. The chapter discusses the role of ALD in preparing passivation layers for homojunction Si solar cells. Special attention is given to the physics of surface passivation, the surface passivation by ALD Al2O3, ALD as a high-throughput deposition technique in the photovoltaic (PV) industry, and recent developments in the field of passivation layers prepared by ALD. The chapter focuses on transparent conductive oxides (TCOs) prepared by ALD for use in heterojunction Si solar cells, such as doped ZnO and In2O3 films

Quantification of pn-junction recombination in interdigitated back-contact crystalline silicon solar cells

Interdigitated back-contact (IBC) solar cells based on diffused crystalline silicon comprise a series of pn-junctions which border at the rear surface of the wafer. In this work, it is established that the presence of these pn-junctions can induce significant additional charge-carrier recombination, which affect the conversion efficiency of IBC cells through a reduction in fill factor and open-circuit voltage. Using specialized test structures with varying length of pn-junctions per area of solar cell (i.e., with varying junction density), the magnitude of the recombination at the pn-junction was determined. For nonpassivated rear surfaces, a second-diode recombination current density per unit of junction density J02 of ∼61 nA·junction–1·cm–1 was measured, whereas for surfaces which were passivated by either SiNx or Al2O3 /SiNx, J02 was reduced to ∼0.4 nA·junction–1·cm–1. Therefore, passivation of defects at the rear surface was proven to be vital in reducing this characteristic recombination current. Finally, by optimizing the p- and n-type dopant diffusion process recipes, J02 recombination could be suppressed. The improved doping recipes led to an increase in conversion efficiency of industrial “mercury” IBC solar cells by ∼1% absolute

Atomic-layer deposited passivation schemes for c-Si solar cells

A review of recent developments in the field of passivation of c-Si surfaces is presented, with a particular focus on materials that can be prepared by atomic layer deposition (ALD). Besides Al2O3, various other novel passivation schemes have recently been developed, such as Ga2O3, Ta2O5, SiO2/Al2O3, HfO2/Al2O3 and TiO2, which altogether can passivate a wider variety of doped and textured Si surfaces. Moreover, they are interesting candidates in the emerging field of passivating contacts, for instance as novel tunnel oxides. In this field, ALD can offer some distinct advantages, such as a precise control in film thickness, composition and even area-selective deposition

Status and prospects for atomic layer deposited metal oxide thin films in passivating contacts for c-Si photovoltaics

In the field of photovoltaics, atomic layer deposition (ALD) is mostly known for its success in preparing Al2O3-based surface passivation layers for c-Si homojunction cells. In the last years, many novel types of c-Si heterojunctions have appeared, referred to as passivating contacts. In these concepts, metal oxide thin films are used for surface passivation, carrier selectivity and as transparent conductive oxide. This leads to the question whether the success of ALD for homojunctions can be translated into this new field as well. Therefore, this work provides an overview of these new concepts, and highlights both the current role and prospects of ALD in this field

Silicon surface passivation by transparent conductive zinc oxide

Surface passivation is essential for high-efficiency crystalline silicon (c-Si) solar cells. Despite the common use of transparent conductive oxides (TCOs) in the field of solar cells, obtaining surface passivation by TCOs has thus far proven to be particularly challenging. In this work, we demonstrate outstanding passivation of c-Si surfaces by highly transparent conductive ZnO films prepared by atomic layer deposition. Effective surface recombination velocities as low as 4.8 cm/s and 11 cm/s are obtained on 3 Ω cm n- and p-type (100) c-Si, respectively. The high levels of surface passivation are achieved by a novel approach by using (i) an ultrathin SiO2 interface layer between ZnO and c-Si, (ii) a sacrificial Al2O3 capping layer on top of the ZnO film during forming gas annealing, and (iii) the extrinsic doping of the ZnO film by Al, B, or H. A combination of isotope labeling, secondary-ion mass spectrometry, and thermal effusion measurements showed that the sacrificial Al2O3 capping layer prevents the effusion of hydrogen from the crystalline ZnO and the underlying Si/SiO2 interface during annealing, which is critical in achieving surface passivation. After annealing, the Al2O3 capping layer can be removed from the ZnO film without impairing the high levels of surface passivation. The surface passivation levels increase with increased doping levels in ZnO, which can be attributed to field-effect passivation by a reduction in the surface hole concentration. The ZnO films of this work are suitable as a transparent conductor, an anti-reflection coating, and a surface passivation layer, which makes them particularly promising for simplifications in future solar cell manufacturing

Passivating contacts for crystalline silicon solar cells: from concepts and materials to prospects

To further increase the conversion efficiency of crystalline silicon (c-Si) solar cells, it is vital to reduce the recombination losses associated with the contacts. Therefore, a contact structure that simultaneously passivates the c-Si surface while selectively extracting only one type of charge carrier (i.e., either electrons or holes) is desired. Realizing such passivating contacts in c-Si solar cells has become an important research objective, and an overview and classification of work to date on this topic is presented here. Using this overview, we discuss the design guidelines for passivating contacts and outline their prospects