Ralph Mohr

Crucial parameters and device physics of amorphous silicon alloy tandem solar cells

Abstract We have previously shown that tandem solar cells exhibit higher efficiency and better long-term stability than single junction cells. We shall discuss crucial parameters for achieving high performance solar cells and certain aspects of device physics associated with the tandem structure. We will present data on what effect changes in intrinsic layer thicknesses, p + and n + layer thicknesses, back reflector quality, and solar spectral variation have on these multijunction devices.

Status of Fluorinated Amorphous Silicon Alloy Multi-Junction Solar Cells

We present in this paper updated solar cell performance data on fluorinated amorphous silicon and fluorinated amorphous silicon-germanium alloys. We have achieved a 9.7% conversion efficiency for a singlejunction device using a-Si:Ge:F:H alloy having an optical band gap of 1.5 eV. We have also achieved 12.5% and 13.0% efficiencies, respectively, for dual-band-gap tandem and triple devices using a-Si:F:H and a-Si:Ge:F:H alloys. These represent the highest values reported to date for their respective configurations. Multi-junction solar cells exhibit excellent stability not only in terms of light-induced effect but also in terms of prolonged heating at elevated temperatures.

Physics of High Efficiency Multijunction Solar Cells

We have shown that high efficiency solar cells can be obtained by incorporating materials of different band gaps in a multijunction configuration. This configuration gives rise to high efficiency due to spectrum splitting as well as superior stability due to thin top cells. We have fabricated multijunction solar cells and acheived a 13% conversion efficiency using 1.7eV a-Si:F:H and 1.5 eV a-Si:Ge:F:H materials in a three-cell triple configuration. This device was deposited onto a stainless steel substrate coated with a back reflector; it also incorporates a microcrystalline p + layer. The physical properties of each of the solar cell layers and their role in the device physics of a high efficiency solar cell will be described.