Structure of microcrystalline solar cell materials: What can we learn from electron microscopy?
- PDF / 2,425,474 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 103 Downloads / 151 Views
A24.1.1
Structure of microcrystalline solar cell materials: What can we learn from electron microscopy? M. Luysberg and L. Houben Institut of Solid State Research and Ernst-Ruska Centre for Microscopy and Spectroscopy with Electrons, Research Centre Jülich, 52425 Jülich, Germany, ABSTRACT Microcrystalline silicon and its group IV alloys are widely explored as absorber layers in thin film solar cells. Despite the extended research in recent years the fundamental understanding of the relation between macroscopical properties, i.e. electrical and optical properties, and the microstructure is poor. Clearly, the structure of microcrystalline materials, consisting of a phase mixture between “amorphous” material, crystalline grains, and voids, is complex. To demonstrate the strengths and limitations of transmission electron microscopy on microcrystalline materials, we will discuss different techniques employed to investigate grain sizes and morphologies, crystallographic orientations, amorphous volume fractions, and lateral arrangements of crystallites. In particular, we focus on the potential for analyzing the structure of grain boundaries and the amorphous phase in microcrystalline silicon and silicon carbide by the most advanced techniques in atomic resolution imaging in the transmission electron microscope. INTRODUCTION The implementation of silicon and its group IV alloys e.g. into a solar cell allows to make use of the individual optical or electrical properties of each material in one device. For instance, the use of microcrystalline silicon (µc-Si) p- and n- layers within an otherwise amorphous solar cell, improves the performance because of the high conductivity of microcrystalline Si compared to amorphous material. Also, the variation of the band gap upon composition in a-Si based group IV alloy enables tailoring the optical properties. Microcrystalline silicon and its alloys can be grown by various methods which are compatible with low-temperature deposition on low-cost substrates, e.g. plasma-enhanced or hotwire chemical vapor deposition (PECVD [1-3], HWCVD [4-6]). Common to the low-temperature growth is that the crystallinity of the films can be varied from highly crystalline to amorphous, depending on the deposition parameters. Especially the dilution of the groupIV based precursor gases in hydrogen is used to tailor the phase composition [7, 8]. The microstructure typically Figure 1. Schematics of the microstructure of Si obtained upon a decrease in hydrogen obtained in the range of highly crystalline to dilution is schematically shown in predominantely amorphous growth conditions (from Fig. 1, where on the left hand side the left to right).
A24.1.2
columnar structures are highly crystalline samples, but on the right hand side almost amorphous films contain only small crystallites embedded in an amorphous matrix. In between these two extreme cases, the transition region is characterized by columnar crystallites of a length significantly smaller than the film thickness. Solar cells made with intrinsic abs
Data Loading...
