Bas W. H. van de Loo

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 ph...

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.

Boron-Doped Silicon Surfaces From B

A p+ - doping method for silicon solar cells is presented whereby boron atoms from a pure boron (PureB) layer deposited by chemical vapor deposition using B2H6 as precursor were thermally diffused into silicon. The applicability of this doping process for the doped surfaces of silicon solar cells was evaluated in terms of surface morphology after thermal diffusion, the boron dopant profiles, and sheet resistances, as well as the recombination parameter

_{\bf 2}

J_{0{\rm p+}}

H

, when the doped layers were passivated by Al2O3 films prepared by atomic layer deposition. Adequate surface passivation could be achieved with a surface recombination contribution to

_{\bf 6}

J_{0{\rm p+}}

Passivated by ALD Al

of <20 fA/cm2 for most samples. However, when a boron-rich layer (BRL) was present at the Si surface, a much higher recombination current density was observed, proving that a BRL was detrimental to the high-quality surface passivation of boron-diffused surfaces. It was found that sufficient O2 in the furnace during the thermal diffusion process could eliminate any potential residual BRL, while excessive O2 concentration results in boron depletion and a higher sheet resistance. Therefore, in addition to optimizing the initial PureB layer thickness and thermal budget to control the dopant profiles, the O2 concentration during the diffusion must also be well controlled.

_{\bf 2}

The interplay between hydrogenation and passivation of poly-Si/SiOx contacts to n-type Si wafers is studied using atomic layer deposited Al2O3 and anneals in forming gas and nitrogen. The poly-Si/SiOx stacks are prepared by thermal oxidation followed by thermal crystallization of a-Si:H films deposited by plasma-enhanced chemical vapor deposition. Implied open-circuit voltages as high as 710 mV are achieved for p-type poly-Si/SiOx contacts to n-type Si after hydrogenation. Correlating minority carrier lifetime data and secondary ion mass spectrometry profiles reveals that the main benefit of Al2O3 is derived from its role as a hydrogen source for chemically passivating defects at SiOx; Al2O3 layers are found to hydrogenate poly-Si/SiOx much better than a forming gas anneal. By labelling Al2O3 and the subsequent anneal with different hydrogen isotopes, it is found that Al2O3 exchanges most of its hydrogen with the ambient upon annealing at 400 °C for 1 h even though there is no significant net change in its total hydrogen content.The interplay between hydrogenation and passivation of poly-Si/SiOx contacts to n-type Si wafers is studied using atomic layer deposited Al2O3 and anneals in forming gas and nitrogen. The poly-Si/SiOx stacks are prepared by thermal oxidation followed by thermal crystallization of a-Si:H films deposited by plasma-enhanced chemical vapor deposition. Implied open-circuit voltages as high as 710 mV are achieved for p-type poly-Si/SiOx contacts to n-type Si after hydrogenation. Correlating minority carrier lifetime data and secondary ion mass spectrometry profiles reveals that the main benefit of Al2O3 is derived from its role as a hydrogen source for chemically passivating defects at SiOx; Al2O3 layers are found to hydrogenate poly-Si/SiOx much better than a forming gas anneal. By labelling Al2O3 and the subsequent anneal with different hydrogen isotopes, it is found that Al2O3 exchanges most of its hydrogen with the ambient upon annealing at 400 °C for 1 h even though there is no significant net change in it...

O

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.

_{\bf 3}

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.