Molybdenum Thin Films Deposited by High Power Impulse Magnetron Sputtering for Back Contact Applications
(Room Room 104-A)
30 Apr 19
12:00 PM
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12:20 PM
Tracks:
High Power Impulse Magnetron Sputtering – HIPIMS
Molybdenum thin films used in chalcopyrite solar cells affect conversion efficiency through regulating Na diffusion rates and the texture of the Cu(InGa)Se2 absorber according to the microstructure and morphology of the back contact itself. Conventionally sputtered films achieve the lowest resistivity at low working pressure and exhibit high residual stress and poor adhesion due to the resulting high energy of the deposited flux. High Power Impulse Magnetron Sputtering was employed to ionise the sputtered flux, achieve high adatom mobility at low energy and influence the growth of Mo back contacts. Pulse durations in the range 60 to 1000 µs, sputtering voltages between 800 and 1500 V and deposition pressures of 2×10-3 mbar and 4×10-3 mbar resulted in ten-fold variations in the flux ratios of Mo1+/Mo0, Mo2+/Mo1+, Ar2+/Ar1+ and Mo1+/Ar1+ as determined by optical emission spectroscopy and time-resolved plasma-sampling energy- mass spectroscopy. The energy of metal and gas double- and single-charged ions reduced with pulse duration and increased with voltage. The microstructure of the films varied from open columnar with faceted tops to fully dense as observed by secondary electron microscopy. The presence of double-charged metal ions promoted larger grain size and random texture as determined from pole figures of the (211) and (110) diffraction peaks. The reflectivity of the films improved by 20% compared to industry-standard materials. The prevalence of double-charged metal ions lowered the resistivity of the films, with the lowest values reaching 12 µΩ-cm as observed by four-point probe measurements of 570 nm thick films. The correlation between resistivity, microstructure, crystallographic texture and deposition flux characteristics is discussed.