APCVD of dual layer transparent conductive oxides for photovoltaic applications

We report the atmospheric pressure chemical vapour deposition (APCVD) of a dual layer transparent conductive oxide (TCO). This combines a fluorine doped tin oxide (FTO) base layer with a fluorine doped zinc oxide (FZO) top layer, where we seek to utilise the respective advantages of each material and the differences in their associated industrial deposition process technologies. Deposition of a 250 nm thick FZO layer on FTO was enough to develop features seen with FZO only layers. The crystallographic orientation determined by the FZO dopant concentration. Changes to the deposition parameters of the underlying FTO layer effected stack roughness and carrier concentration, and hence optical scattering and absorption. Photovoltaic cells have been fabricated using this TCO structure showing promising performance, with efficiencies as high as 10.21% compared to reference FTO only values of 9.02%. The bulk of the coating was FTO, providing the majority of conductivity and the large surface features associated with this material, whilst keeping the overall cost low by utilising the very fast growth rates achievable. The FTO was capped with a thinner FZO layer to provide a top surface suitable for wet chemical or plasma etching, allowing the surface morphology to be tuned for specific applications. (C) 2015 Elsevier B.V. All rights reserved.

L'oxyde de zinc par dépôt chimique en phase vapeur comme contact électrique transparent et diffuseur de lumière pour les cellules solaires

Sylvie Faÿ

Zinc oxide (ZnO) is a material that belongs to the family of Transparent Conductives Oxides (TCO). Its non-toxicity and the abundant availability in the Earth's crust of its components make it an ideal candidate as electrical transparent contact for thin-film amorphous and/or microcrystalline silicon solar cells. The Low-Pressure Chemical Vapour Deposition (LP-CVD) method allows one to obtain rough ZnO layers, which can effectively scatter the light that passes through them. This high scattering capacity increases the path of the light within the solar cell and therefore enhances its probability to be absorbed by the solar cell, and consequently the photogenerated current. This thesis work studies in detail the LP-CVD technology used for deposition of ZnO layers, which are employed as TCO layers for amorphous, microcrystalline and "micromorph" (microcrystalline/amorphous tandem) solar cells developped at the Institute of Microtechnology of the University of Neuchâtel. The chapter 3 is a detailed study of the growth of ZnO layers on a glass substrate. This study reveals a columnar microstructure of the ZnO layers, which are composed of large conical monocrystals. The tops of these monocrystals emerge at the surface of the ZnO layers as pyramids, which give to such layers their rough aspect and therefore their capacity of scattering the light. The two following chapters represent a study of the effect of variations in deposition parameters on the optical properties (transparency and scattering power), electrical properties (conductivity), and structural properties of ZnO layers. The deposition parameters varied thereby are the various gas flows, the substrate temperature, and the process pressure. In particular, a strong influence of the substrate temperature on the microstructure of the ZnO layers is observed. A variation of ten degrees of this deposition temperature leads to abrupt variations of the optical and electrical properties of the layers. From these various studies, a strong correlation is established, between the scattering power, the electrical properties, and the microstructure of the ZnO layers deposited by LP-CVD: The higher the pyramidal grains at the surface of the layers, the higher the scattering capacity of these layers. The larger the monocrystals within the bulk of the layers, the higher the mobility of the free electrons. This enhanced mobility leads to an increase of the conductivity of the layers. Finally, the chapter 6 of this thesis work describes the various situations where LP-CVD ZnO layers are used as transparent electrical contacts and light scattering layers in thin-film silicon solar cells. It is schown that, by incorporating ZnO layers developped in this work in the photovoltaic devices, an increase of 8% in the photogenerated current is obtained, compared to the performances of solar cells using standard commercially available TCO.

EPFL2003

Low pressure chemical vapour deposition of ZnO layers for thin-film solar cells: Temperature-induced morphological changes

Cédric Bucher, Sylvie Faÿ, Arvind Shah

Zinc oxide (ZnO) is now often used as a transparent conductive oxide for contacts in thin-film silicon solar cells. This paper presents a study of ZnO material deposited by the low-pressure chemical vapour deposition technique, in a pressure range below the pressures usually applied for the deposition of this kind of material. A temperature series has been deposited, showing a morphological transition around 150 °C. ZnO samples deposited with temperatures just higher than this transition are constituted of large grains highly oriented along a single crystallographic orientation. These "monocrystals" lead to low resistivity values, showing a clear correlation between the size of the surface grains and the electrical performance of corresponding films. Additionally, these large grains also yield ZnO layers with high transparency and high light-scattering power, specially suitable for solar cell technology based on thin-film silicon. © 2004 Elsevier B.V. All rights reserved.

2005

c-texture versus a-texture low pressure metalorganic chemical vapor deposition ZnO films: Lower resistivity despite smaller grain size

Arthur Brian Aebersold, Duncan Alexander, Christophe Ballif, Laura Ding, Lorenzo Fanni, Monica Morales Masis, Sylvain Nicolay

Recently, it has been shown that it is possible to tune the morphology of zinc oxide films deposited by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) while preserving good electrical conductivity. Here a closer look is taken at films deposited under two different deposition conditions; one leading to LP-MOCVD a-texture (i.e., with the a-axis perpendicular to the substrate), the other resulting in c-texture (i.e., with the c-axis perpendicular to the substrate), with the aim of correlating their structural and electrical characteristics. We introduce the concept of a "selection layer" to indicate the initial region of growth that precedes the establishment of a clear preferential crystallographic film orientation. With a strong preferential c-texture of initial nucleation the selection layer for c-texture films is minimal (

Elsevier2014

L'oxyde de zinc par dépôt chimique en phase vapeur comme contact électrique transparent et diffuseur de lumière pour les cellules solaires

Sylvie Faÿ

EPFL2003