ICMCTF2010 Session H4/C4: Thin Films for Photovoltaics: Synthesis and Characterization

Like amorphous silicon, the microcrystalline silicon (μc-Si) can be demonstrated by plasma enhanced chemical vapor deposition. Hence, fabrication of μc-Si solar cells could be based on equipment for amorphous silicon (a-Si) solar cells. In addition, μc-Si has other advantages over a-Si. For example, the absorption coefficient of μc-Si is higher than a-Si in red light and infrared wavelengths, and the carriers mobility in μc-Si is also larger than a-Si. But for μc-Si, there are still a lot of defects inside it. These defects will lead to recombination of photo-generated carriers and the efficiency of μc-Si solar cells is limited. We have designed a new structure – the three-terminal μc-Si solar cell, a back-to-back pin-nip solar cell by the simulation tool, ISE. As compared with the typical μc-Si pin solar cells, the three-terminal μc-Si solar cell can increase the average electric field in the device. The electric field can help separation of photo-generated electron-hole-pairs and decrease the recombination velocity efficaciously. The comparison between the typical cell and the three-terminal μc-Si cell will be shown in this conference. The efficiency of the three-terminal μc-Si solar cell larger than 10 % can be expected.

The performance of CuInS₂ (CIS) thin film solar cells can be enhanced through the addition of Na into the CIS layer. In almost all of the cases, Na originates from substrate glass or a separate NaF layer that is deposited before, during, or after the growth of CIS, followed by thermal annealing. In the study, we have used an alternative approach by which NaF is incorporated into the CIS layer through co-evaporation. Cu, In, and NaF were first co-evaporated at desired ratios to allow the formation of precursor films having various nominal concentrations of Na. The composition of the precursor films was determined using inductively coupled plasma-mass spectrometer. The precursor films were then sulfurized to form CIS absorber layers in the same evaporation chamber. During the sulfurization, samples were removed at different temperatures for analysis in order to determine the desired sulfurization schedules. The morphology of Na-doped CIS layers was characterized using scanning electron microscopy and the crystalline structure was investigated using grazing incident X-ray diffraction. Secondary ion mass spectrometry was used to determine the concentration profiles along the film thickness. The optical and electrical properties of the absorber layers were investigated using UV-Visible spectroscopy and Hall measurement system, respectively. Effects of Na concentration on the absorber layer characteristics are addressed and discussed.

We prepared polycrystalline AgInSe₂ thin films by vacuum evaporation on Si(100) substrate at a high temperature using the stochiometric powder. The thin films were characterized by X-ray diffraction and Uv-vis-NIR spectroscopy .For the fabrication of densely distributed one dimensional nanostructures of Silver Indium selenide on Si substrates, the thermally evaporated films of AIS on Si (1 0 0) substrate were irradiated by incident 200MeV Ag⁺ ions at a fluence of 5 X 10¹¹ Ion/cm². At elevated substrate temperatures AIS were featured by the nanorods -like structure. The optical and structural properties of the irradiated films were studied using UV–visible absorption spectroscopy , atomic force microscopy (AFM) , Field emission scanning electron microscopy (FESEM ) and XRD .The controlled fabrication of such densely distributed one dimensional nanorods on Si substrate using ion beam technique, we believe, would open up a variety of applications such as nanoelectronics and optoelectronics devices.

Porous TiO₂ layers are known to be produced on titanium by the plasma electrolytic oxidation (PEO) process. Such coatings can be considered as potential candidates to use as surface electrode materials in photovoltaic devices, e.g. dye-sensitized solar cells (DSSCs), where the specific surface area of the TiO₂ electrode is required to be as large as possible for efficient dye absorption. However as shown in the literature and our previous work [1], the as-formed PEO layer does not provide a large enough specific area and thus the photovoltaic efficiency of the assembled device is low. The aim of this study is therefore to develop a nano-structured TiO₂ surface layer based on PEO treated titanium by using alkaline and thermal annealing post-treatment procedures. The specific surface area is expected to be increased due to re-formation of the original PEO layer in NaOH solution and the anatase content to be enhanced due to crystallisation during subsequent annealing. Microstructure of the post-treated TiO₂ and the photovoltaic efficiency of the assembled device are examined as a function of alkali concentration, bath temperature, and soaking time.

Experimental results show that, after the alkaline post-treatment, a ’nano-flaky’ morphology was developed over the oxide surface. Higher alkali concentration, bath temperature and soaking time enhanced the roughening of the PEO treated surface and at the same time a thicker reformed layer can be obtained. However, a possibility of crack development in the oxide layer increases, compromising the photovoltaic efficiency. Following the alkaline post-treatment in 1.25 M NaOH solution at 40^oC for 12 h, the assembled device had an ultimate photovoltaic efficiency of 0.329% in contrast to that of 0.061% presented by the as-formed PEO TiO₂. Further annealing at temperatures above 300^oC significantly enhanced the formation of the anatase phase in the oxide layer and a maximum photovoltaic efficiency of 2.194% can be achieved when annealed at 400^oC.

[1] P-J Chu, A Yerokhin, A Leyland, A Matthews, J-L He, Through-thickness Microstructural Characterization of the Plasma Electrolytic Oxidized Titanium Oxide Fabricated on Metal Titanium, Abstracts of ICMCTF 2009, paper GP-6, p 99.

The application of the technique to the characterization of thin layers on conductive substrates or non-conductive substrates such as glasses and ceramics is investigated. Issues related to heat and thermal damage as well as coupling efficiency are shown and solutions to overcome these issues are presented resulting from theoretical and experimental characterizations of the RF GD plasma and its interaction with the material’s surface.

The new RF coupler allows sputtering of up to 5mm thick glasses or ceramics. It provides more than adequate performance for useful analysis as examples from solar cells and coated ceramics will illustrate. Other Semiconductor, Photovoltaic and PVD/CVD applications will be presented.