Study of prolonged light-soaking (∼400 Hrs) effect on Pm-Si:H solar cell structures

Abstract

Different pm-Si:H structures were grown using PECVD technique in order to study the effect of light exposition on solar cell structures based on these materials. PIN and NIP structures 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 and exodiffusion experiments. 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, hydrogen stability, configuration and therefore the optical properties varied with the exposition time. The crystalline fraction of these structures is between 12 to 18% and increase with the exposition time. The optical gap decreases from 1.76 to 1.6 eV for the PIN structure while for the NIP decreases from 1.62 to 1.54 eV. The hydrogen stability and its amount bonded in these films are dependent of the device architecture. Hydrogen diffusion induces structural crystallization and 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 along with a shift on the exodiffusion peaks. Hydrogen diffusion is very different during light-soaking for both structures. We determined that the total hydrogen that effuses from PIN structure is lower than for the NIP, which is expected to cause less degradation of its optoelectronic properties under illumination, and a more stable device during operation.