微晶硅锗薄膜作为近红外光吸收层在硅基薄膜太阳电池中的应用

采用射频等离子体增强化学气相沉积技术,制备了具有一定晶化率不同Ge含量的氢化微晶硅锗(μcSi1-xGex:H)薄膜.通过Ⅹ射线荧光谱、拉曼光谱、X射线衍射谱、傅里叶红外谱、吸收系数谱和电导率的测试,表征了μc-Si_(1-x)Ge_x:H的材料微结构随Ge含量的演变.研究表明:提高Ge含量可以增强μc-Si_(1-x)Ge_x:H薄膜的吸收系数.将其应用到硅基薄膜太阳电池的本征层中可以有效提高电池的短路电流密度(J_(sc)).特别是在电池厚度较薄或陷光不充分的情况下,长波响应的提高会更为显著.应用ZnO衬底后,在Ge含量分别为9%和27%时,μc-Si_(1-x)Ge_x:H太阳电池的转换效率均超过了7%.最后,将μc-Si_(1-x)Ge_x:H太阳电池应用在双结叠层太阳电池的底电池中,发现μc-Si_(0.73)Ge_(0.27):H底电池在厚度为800 nm时即可得到比1700 nm厚微晶硅(μc-Si:H)底电池更高的长波响应.以上结果体现μc-Si_(1-x)Ge_x:H太阳电池作为高效近红外光吸收层,在硅基薄膜太阳电池中应用的前景.

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

Hydrogenated microcrystalline silicon germanium (μc-Si_1-xGe_x: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 (μc-Si:H) thin films. However, since the structure complexity was increased a lot by Ge incorporation, the influences of μc-Si_1-xGe_x:H film properties on Ge composition have not been understood yet. In this work, μc-Si_1-xGe_x: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 μc-Si_1-xGe_x: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 μc-Si_1-xGe_x: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 μc-Si_1-xGe_x:H thin films increase, while the H content, (220) grain size, and photosensitivity of μc-Si_1-xGe_x:H thin film decrease. Then, μc-Si_1-xGe_x:H is used as the intrinsic layer in the single junction solar cells. The performances of μc-Si_1-xGe_x: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 μc-Si_1-xGe_x:H thin films become more defective and less compact with Ge incorporation, μc-Si_1-xGe_x: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% (J_sc=22.6 mA/cm², V_oc=0.494 V, FF=64.0%) and 7.01% (J_sc=23.3 mA/cm², V_oc=0.482 V, FF=62.4%) are achieved by μc-Si_0.9Ge_0.1:H solar cell and μc-Si_0.73Ge_0.27:H solar cell, respectively. Furthermore, the μc-Si_0.73Ge_0.27:H solar cell is used as the bottom sub-cell of the double-junction solar cell, and a J_sc.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 μc-Si:H bottom sub-cell with 1700 nm thickness. Finally, an initial efficiency of 10.28% is achieved in an a-Si:H/μc-Si_0.73Ge_0.27:H double junction cell structure. It is demonstrated that by using the μc-Si_1-xGe_x: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.