Investigation of Impurity Neutralization and Defect Passivation in Polycrystalline Silicon Solar Cells
- PDF / 2,686,643 Bytes
- 13 Pages / 420.48 x 639 pts Page_size
- 56 Downloads / 216 Views
INVESTIGATION OF IMPURITY NEUTRALIZATION AND DEFECT PASSIVATION IN POLYCRYSTALLINE SILICON SOLAR CELLS LAWRENCE L.KAZMERSKI Solar Energy Research Institute, 1617 Cole Boulevard, Golden, CO 80401 ABSTRACT The passivation of grain boundaries and the neutralization of impurities in polycrystalline silicon, important processes for the improvement of performance of devices fabricated from this material, are discussed. The incorporation of hydrogen into grain boundaries is investigated using surface analysis methods. Volume-mapping techniques are used to identify the bonding mechanisms of the hydrogen in oxygen-free and oxygen-rich intergrain regions. Interactions between shallow acceptors (B,AI,Ga and In) and hydrogen in polycrystalline Si are studied. The bonding mechanisms involved in the acceptor neutralization process at the grain boundaries are evaluated using microanalytical techniques. Differences in the incorporation of molecular and atomic hydrogen, and corresponding variation in electrical passivation of grain boundaries, are observed. Volume-indexed AES and Auger difference spectroscopy data are complemented by scanning tunneling microscope images to confirm the direct hydrogen-silicon bonding in boron-doped grain boundaries. INTRODUCTION A significant contribution to the recent improvements in Si solar cell performances (e.g., -22% terrestrial conversion efficiency for single crystal and -17% for polycrystalline Si) [1,2] has been the increased understanding and control of impurities in the semiconductor material and at device interfaces [3-6]. This applies both to purposely-placed impurities for doping and electrical passivation, and for those which inadvertently occur during the Si-production or device fabrication processes. Certainly, the ability to control such impurity species depends upon the ability to detect them, not only their concentrationlevels, but also their ex-
act spatiallocationswithin the cell structure--especially at critical interfaces--to the hundreds of Angstroms resolution. The observations that hydrogen saturates or satisfies dangling bonds have led to its use to compensate defects and amorphous material [3,7]. Hydrogen has been successfully utilized to improve the electrical characteristics of polycrystalline solar cells [8-12], with some typical current-voltage data shown in Fig. 1 for untreated and hydrogen-processed devices. Recently, it has been discovered that hydrogen can be used effectively to neutralize deep impurities and shallow acceptors in elemental and compound semiconductors [13- 22]. It is the purpose of this paper to examine the mechanisms involved in hydrogen treatments for passivation (used herein to refer to the electrical improvement at defects) and neutralization (referring to the mechanisms involved with impurities). Specifically, high spatial-resolution surface analysis techniques, scanning Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS), are applied to the investigation of the chemistry of the grain boundaries as a function of imp
Data Loading...
