A comprehensive model for the electronic transport in polycrystalline ZnO:B thin films grown by low pressure chemical vapor deposition is presented. The optical mobilities and carrier concentration calculated from reflectance spectra using the Drude model were compared with the data obtained by Hall measurements. By analyzing the results for samples with large variation of grain size and doping level, the respective influences on the transport of potential barriers at grain boundaries and intragrain scattering could be separated unambiguously. A continuous transition from grain boundary scattering to intragrain scattering is observed for doping level increasing from 3 ϫ 10 19 to 2 ϫ 10 20 cm −3Transparent conductive oxides ͑TCOs͒ are an essential part of thin-film silicon solar cells. To contact the cell and act as a transparent window, TCOs have to exhibit a high conductivity and a high optical transmittance. In addition, they have to scatter the light at the TCO-cell interface in order to increase the effective absorption of light within the active layers.1,2 Boron-doped zinc oxide ͑ZnO:B͒ layers deposited by low pressure chemical vapour deposition ͑LPCVD͒ have been intensively developed in our institute.3 This material is especially attractive for thin-film solar cell technology, because of its low cost, and of the wide availability of its constituent raw materials. Furthermore, the LPCVD technique is well suited for large-scale device fabrication. 4 These films are constituted of large grains with a pronounced 9-18 preferential crystallographic direction. The extremities of the grains appear at the growing surface as large pyramids, which yield an as-grown rough surface texture that efficiently diffuses the light that enters into the solar cell. 1,3The optical and electrical properties of TCO films have been extensively investigated and recently reviewed. Ellmer 5 and Minami 6 discussed the limit of the resistivity of such films by analyzing reported data for ZnO films deposited by different techniques. They found that the electron mobility in undoped films is mainly limited by grain boundary scattering, whereas for doped layers intragrain scattering mechanism is predominant. But the transition between these two mechanisms within a given ZnO film series could not be evidenced yet. In the present work, the scattering mechanism limiting the electron mobility is determined by the comparison, of the value of the optical mobility ͑as evaluated by using the classical Drude model͒ with the value of the Hall mobility. When both values are markedly different, the mechanism limiting the electron mobility is attributed to grain boundary scattering. When both values are similar, the limiting mechanism is attributed to intragrain scattering. This approach is commonly applied to other TCOs. [7][8][9][10][11] The wide range of carrier density easily achievable in boron-doped LPCVD ZnO by varying the gas flow ratio allows us to observe within one single film system the transition from one transport mechanism to the other. T...

Zinc oxide (ZnO) is now often used as a transparent conductive oxide for contacts in thinfilm silicon solar cells. This paper presents a study of ZnO material deposited by the lowpressure 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 1C. 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.

The authors report on the fabrication of microcrystalline silicon p-i-n solar cells with efficiencies close to 10%, using glass coated with zinc oxide (ZnO) deposited by low pressure chemical vapor deposition (LPCVD).LPCVD front contacts were optimized for p-i-n microcrystalline silicon solar cells by decreasing the free carrier absorption of the layers and increasing the surface roughness. These modifications resulted in an increased current density of the solar cell but also in significantly reduced fillfactor (FF) and open-circuit voltage (Voc). In order to avoid these reductions, a new surface treatment of the ZnO is introduced. It transforms profoundly the surface morphology by turning the typical V-shaped valleys of the LPCVD ZnO into U-shaped valleys and by erasing from the surface small-sized pyramids and asperities. As a result, for fixed deposition parameters, the p-i-n microcrystalline silicon solar cell efficiency increased from 3.3% to 9.2%Further optimization of the microcrystalline silicon solar cell on this 'new' type of LPCVD ZnO front contact has led to an efficiency of 9.9%.

Doped ZnO layers deposited by low-pressure chemical vapour deposition technique have been studied for their use as transparent contact layers for thin-film silicon solar cells.Surface roughness of these ZnO layers is related to their light-scattering capability; this is shown to be of prime importance to enhance the current generation in thin-film silicon solar cells. Surface roughness has been tuned over a large range of values, by varying thickness and/or doping concentration of the ZnO layers.A method is proposed to optimize the light-scattering capacity of ZnO layers, and the incorporation of these layers as front transparent conductive oxides for p-i-n thin-film microcrystalline silicon solar cells is studied.

Low pressure chemical vapour deposition (LP-CVD) ZnO as front transparent conductive oxide (TCO), developed at IMT, has excellent light-trapping properties for a-Si:H p-i-n single-junction and 'micromorph' (amorphousymicrocrystalline silicon) tandem solar cells. A stabilized record efficiency of 9.47% has independently been confirmed by NREL for an amorphous silicon singlejunction p-i-n cell (;1 cm ) deposited on LP-CVD ZnO coated glass. Micromorph tandem cells with an initial efficiency of