Indium zinc oxide window layer for high-efficiency Cu(In,Ga)Se2 solar cells

Publication date: 31 July 2017
Source:Thin Solid Films, Volume 634
Author(s): Richard Menner, Stefan Paetel, Wiltraud Wischmann, Michael Powalla
Cu(In,Ga)Se2 (CIGS) based solar cells are the most efficient thin-film solar cells today. At ZSW we typically employ a bilayer window onto CdS-buffered CIGS substrates consisting of high-resistive undoped ZnO and an Al-doped ZnO (AZO) by RF and DC magnetron sputtering, respectively. The sputtered AZO window layer still exhibits significant absorption in the visible and especially the near infrared wavelength region due to the high free carrier concentrations necessary for high film conductivity. We could achieve high conductivity at reduced carrier concentration by other transparent conductive oxide materials which exhibit much higher carrier mobility such as amorphous indium zinc oxide (IZO) instead of the AZO layer. Apart from its reported high mobility, additional advantages are the possible deposition at room temperature and the expected higher chemical stability against moisture, a characteristic more favourable for insufficiently encapsulated solar modules. Highly transparent and conductive films (>3000S/cm) with mobilities above 50cm²/(Vs) were achieved. Current-voltage measurements under simulated sunlight of CIGS-based solar cells with IZO window proved comparable efficiencies to AZO coated reference cells. A champion cell with 21.3% efficiency was achieved for CIGS with IZO. Damp heat testing of not encapsulated mini-modules clearly demonstrated enhanced stability against humidity for IZO window layers in comparison to AZO.