Developments of c-Si1-xGex:H thin films as near-infrared absorber for thin film silicon solar cells

Abstract

Hydrogenated microcrystalline silicon germanium (c-Si1-xGex:H) thin films have been developed as alternative bottom sub-cell absorbers for multi-junction thin film silicon solar cells due to their narrower band-gaps and higher absorption coefficients than conventional hydrogenated microcrystalline silicon (c-Si:H) thin films. However, since the structure complexity was increased a lot by Ge incorporation, the influences of c-Si1-xGex:H film properties on Ge composition have not been understood yet. In this work, c-Si1-xGex:H thin films with various Ge content and similar crystalline volume fraction are fabricated by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD). The evolutions of c-Si1-xGex:H material properties by Ge incorporation are characterized by X-ray fluorescence spectrometry, Raman spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, absorption coefficient spectrum, and conductivity measurement. The results show that the properties of c-Si1-xGex:H thin films are strongly determined by Ge content. With the increase of Ge content, the absorption coefficient, (111) grain size, microstructure factor, and dark conductivity of c-Si1-xGex:H thin films increase, while the H content, (220) grain size, and photosensitivity of c-Si1-xGex:H thin film decrease. Then, c-Si1-xGex:H is used as the intrinsic layer in the single junction solar cells. The performances of c-Si1-xGex:H solar cells with different Ge content and two types of transparent conductive oxide (SnO2 and ZnO) substrates are systematically studied. The results indicate that although c-Si1-xGex:H thin films become more defective and less compact with Ge incorporation, c-Si1-xGex:H solar cells exhibit a significant improvement in near-infrared response, especially under the circumstances of thin cell thickness and inefficient light trapping structure. Meanwhile, by using ZnO substrates, initial efficiencies of 7.15% (Jsc=22.6 mA/cm2, Voc=0.494 V, FF=64.0%) and 7.01% (Jsc=23.3 mA/cm2, Voc=0.482 V, FF=62.4%) are achieved by c-Si0.9Ge0.1:H solar cell and c-Si0.73Ge0.27:H solar cell, respectively. Furthermore, the c-Si0.73Ge0.27:H solar cell is used as the bottom sub-cell of the double-junction solar cell, and a Jsc.bottom of 12.30 mA/cm2 can be obtained with the bottom sub-cell thickness as thin as 800 nm, which is even higher than that of c-Si:H bottom sub-cell with 1700 nm thickness. Finally, an initial efficiency of 10.28% is achieved in an a-Si:H/c-Si0.73Ge0.27:H double junction cell structure. It is demonstrated that by using the c-Si1-xGex:H solar cell as the bottom sub-cell in multi-junction thin film silicon solar cells, a higher tandem cell performance can be achieved with a thin thickness, which has a great potential for cost-effective photovoltaics.