Review of progress on a-Si alloy solar cell research

Abstract

Abstract Materials and device studies have focused on the development of a-SiGe:H and a-SiC alloys for high performance multijunction solar cells and submodules. Outdoor measurements of a-SiC:H/a-Si:H/a-SiGe:H cells have yielded 10.7% conversion efficiency for a triple junction cell which contained a 1.4 eV bandgap bottom cell. a-SiC/a-SiGe submodules with a 2500 A thick bottom junction have shown efficiencies as high as 7.7% (active area 900 cm2) and single junction submodules have yielded a conversion efficiency of 9.8%. Stacked junction devices show a clear improvement in stability, compared to equivalent single junction devices. Tandem junction devices with a conversion efficiency of 9% have been prepared which exhibit only an 11% performance loss after 1000 hours continuous AM 1.5 illumination at 40 °C. Analysis indicates that the net degradation of a stacked device is the mean of the component single junction devices. p-Layers prepared from B(CH3) have been extensively studied. The optical bandgap of the p-layer increases by approximately 0.1 eV at a given value of conductivity compared to films prepared from diborane. Devices prepared from the feedstock have shown open circuit voltages as high as 0.943 and fill factors as high as 0.74 and unoptimized devices with an efficiency of 11.5% have been measured. In contrast microcrystalline alloys show little or no improvement in open circuit voltages. The steady state photocarrier grating (SSPG), photoconductivity and photothermal defection spectrometry have been used to understand and improve the properties of a-SiGe:H films prepared using a wide variety of feedstocks and deposition conditions. Studies have shown that trace levels of boron have a marked influence on the ambipolar diffusion length of both a-SiGe:H and a-Si:H films. In single chamber systems boron doping occurs due to carryover from the p-layer deposition.