Properties of amorphous silicon passivation layers for all back contact c-Si heterojunction solar cells
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Properties of amorphous silicon passivation layers for all back contact c-Si heterojunction solar cells Lulu Zhang1,2, Ujjwal Das1, Jesse Appel1, Steve Hegedus1, Robert Birkmire1,2 1 Institute of Energy Conversion, University of Delaware, Newark DE 19716 2 Department of Physics and Astronomy, University of Delaware, Newark DE 19716 ABSTRACT Low temperature deposited Interdigitated All Back Contact a-Si:H/c-Si Heterojunction (IBC-SHJ) devices are a promising approach for high efficiency, low cost solar cells on thin wafers. Thin intrinsic a-Si:H films (i-a-Si:H) deposited below 300°C provide excellent surface passivation and high Voc. However, the optical properties of a-Si:H layers and electronic band alignment at the heterointerface are critical to reduce optical losses and transport barriers in IBCSHJ solar cells. At the front illumination surface, a wide band gap (Eg) i-a-Si:H layer with good passivation is desirable for high Voc and Jsc while at the rear surface a narrower Eg i-a-Si:H layer with good passivation is required for higher FF and Voc as seen in 2D numerical simulation. Various substrate temperature, H2/SiH4 dilution ratio and plasma power conditions were explored to obtain i-a-Si:H with good passivation and desired Eg. All the deposited films are characterized by Variable Angle Spectroscopic Ellipsometry (VASE) to determine Eg and thickness and by Fourier Transform Infrared spectroscopy (FTIR) to estimate hydrogen content and microstructure factor. Passivation qualities are examined by quasi-steady state photoconductance (QSS-PC) measurement. The i-layer Eg, was varied in the range from ~1.65eV to 1.91eV with lifetime >1 ms. Lowest Eg is obtained just prior to the structure transition from amorphous to epitaxial-like growth. The FF of IBC-SHJ devices improved from 20% to 70% as Eg of the a-Si:H rear passivation layer decreased from 1.78 to 1.65 eV. INTRODUCTION Interdigitated All Back Contact Silicon Hetero-Junction (IBC-SHJ) solar cells combine the advantages of back contact designs with c-Si/a-Si:H heterojunction technology, which have potentials to reach 26% efficiency at lower cost [1]. The all back contact design eliminates front grid shading loss resulting in high short circuit current (Jsc), reduces grid resistance leading to improve fill factor (FF) by increasing contact coverage and thickness, and simplifies interconnection between cells when integrated into a module. Additionally, silicon heterojunction technology utilizes a low-temperature,
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