Paper EN1+TF-WeA2
Improving the Damp-Heat Stability of Copper Indium Gallium Diselenide Solar Cells

Wednesday, November 2, 2011, 2:20 pm, Room 103

While copper indium gallium diselenide (CIGS) thin film solar cells with laboratory efficiencies exceeding 20 % have been reported, these high efficiencies may degrade with time as the devices are exposed to humid environments. It is well known that grain boundary diffusion of water through the ZnO to the CIGS-CdS interface is implicated in long-term degradation of the solar cell performance.¹ This penetration must be reduced or stopped to increase the solar cell lifetime. Herein, we show that amorphous tin dioxide (SnO₂) layers deposited by radio frequency (RF) magnetron sputtering on top of the completed CIGS solar cells can significantly increase the device lifetime by forming a barrier against water diffusion. Specifically, with approximately 0.2 micron and thicker SnO₂ layers deposited on top of the completed CIGS solar cells we have demonstrated that initially 11.1 % efficient CIGS solar cells lose less than 7 % of this peak efficiency and still exhibit efficiencies greater than 10 % (factor of 10%) after 150 hours at 85 ^oC and 85 % relative humidity. In comparison, under identical test conditions, the solar cells without the SnO₂ layer lost nearly 80 % of their initial efficiency within 24 hours after commencing the test. We studied the effects of deposition conditions and film thickness for different film structures on the solar cell stability in damp-heat tests. The deposited SnO₂ films tend to be amorphous when deposited at room temperature or when the films are thin, but show increased crystallinity for thicker films and films deposited at 150 ^oC. We found that solar cells coated with polymorphous SnO₂ films exhibit better damp-heat stability than those coated with polycrystalline films. By polymorphous we mean films that consist of nanocrystalline SnO₂ embedded in amorphous SnO₂. We attribute this difference to the lack of grain boundary diffusion in polymorphous SnO₂ films. Replacing the crystalline ZnO window layer with a SnO₂ film can provide further protection of the CIGS solar cells. We demonstrate a 8.2±0.2 % efficient CIGS solar cell with a SnO₂ window layer. Same solar cell fabrication process and CIGS film with ZnO window layer resulted in 8.2±0.6 % overall efficiency. The open circuit voltages of the two cells were the same indicating that the band alignment with the SnO₂ film is suitable for CIGS. These SnO₂ films were deposited using magnetron sputtering at 5 mTorr and 150-250 W RF power using Ar as the sputtering gas without substrate heating.