Improving the damp-heat stability of copper indium gallium diselenide solar cells with a semicrystalline tin dioxide overlayer

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. Specifically, it is well known that water can diffuse to the CIGS-CdS-ZnO heterojunction. This penetration must be reduced or stopped to increase the solar cell lifetime. Herein, we show that tin dioxide layers deposited on top of completed CIGS solar cells can significantly increase the device lifetime by forming a barrier against water diffusion. Specifically, in accelerated damp-heat tests, our best results showed that initially 8-12% efficient CIGS solar cells did not decay from this peak efficiency even after 240 h at 85 °C and 85% relative humidity. In comparison, under identical test conditions, the solar cells without the tin dioxide layer lost nearly 80% of their initial efficiency, within 24 h after commencing the test. We deposited the tin dioxide films by radio frequency magnetron sputtering from tin dioxide targets at 5 mTorr. Semicrystalline SnO ₂ films deposited at room temperature had SnO ₂ nanocrystals embedded in amorphous SnO ₂ without grain boundaries. The semicrystalline films exhibited better damp-heat stability than crystalline films deposited at higher temperature. We infer from the slow open circuit voltage decay that water permeation to the p-n junction is reduced when semicrystalline SnO ₂ overlayers are used to protect the solar cell. We attribute this difference in damp heat stability to the lack of grain boundary water diffusion in semicrystalline SnO ₂ films.