Evolution of Structural and Optical Properties on PIN and NIP pm-Si:H Devices During 400 Hrs of Light-Soaking
- PDF / 5,602,548 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 92 Downloads / 185 Views
Evolution of Structural and Optical Properties on PIN and NIP pm-Si:H Devices During 400 Hrs of Light-Soaking Leon Hamui1, Pere Roca i Cabarrocas2 and Guillermo Santana1 1 Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México. A.P. 70-360, Coyoacán, C.P. 04510, México, D.F. 2 Laboratoire de Physique des Interfaces et des Couches Minces, CNRS, Ecole Polytechnique, 91128 Palaiseau, France ABSTRACT Pm-Si:H PIN and NIP solar cells structures grown using plasma enhanced chemical vapor deposition (PECVD) technique were analyzed during 400 hrs of light-soaking exposition. The evolution of the structural and optical properties was observed and characterized by Raman spectroscopy, spectroscopic ellipsometry. The effect observed is related to defects creation due to induced hydrogen diffusion, break of Si-H bonds and the generation of dangling bonds that causes less passivated films. The film microstructure, and therefore the optical properties varied with the exposition time. The crystalline fraction of these structures presents a slight decrease and it is observed to be between 15 to 24% for the PIN and 5 to 10% for the NIP. The optical gap increases from 1.66 to 1.68 eV for the PIN structure while for the NIP no significant change is observed during light-soaking. Hydrogen diffusion during lights soaking generates a decrease on the absorption properties of the films which in turn is expected to reduce the device efficiency during operation. In this work we show that long range motion of hydrogen during light-soaking causes a hydrogen rearrangement on the film and microstructure changes. We determined that there is not an pronounced change on the film structure during prolonged light exposition related to the stability of the pm-Si:H films. The PIN structure properties are more affected during light soaking in comparison to the NIP structure which is expected to cause less degradation of its optoelectronic properties under illumination, and a more stable device during operation. INTRODUCTION Hydrogenated polymorphous silicon (pm-Si:H), which consists of silicon nanocrystals embedded in an amorphous silicon (a-Si:H) matrix, is a material suitable for solar cell devices. The latter is because it exhibits better electronic transport and stability properties compared to those of the conventional a-Si:H [1, 2]. Most of the improved properties related to these materials are a consequence of the hydrogen ability to passivate defects and impurities in the Si network, removing or reducing the intra-bandgap levels with this interaction [3]. Reversible Staebler-Wronski effect is one of the most significant phenomena that affects the stability and performance of amorphous silicon thin films solar cells [4]. This light induced degradation causes changes in structural and electronic properties of the material [4-6]. Bonded hydrogen atoms may break during light exposure and diffuse through the amorphous network generating an increase of the dangling bonds. On the other hand, diffusing hydrogen atoms may be tr
Data Loading...
