Using hydrogen‐doped In₂O₃films as a transparent back contact in (Ag,Cu)(In,Ga)Se₂solar cells

This study evaluates the potential of hydrogen‐doped In 2 O 3 (IOH) as a transparent back contact material in (Ag y ,Cu 1‐y )(In 1‐x ,Ga x )Se 2 solar cells. It is found that the presence of Na promotes the creation of Ga 2 O 3 at the back contact during (Ag y ,Cu 1‐y )(In 1‐x ,Ga x )Se 2 growth. An excessive Ga 2 O 3 formation results in a Ga depletion, which extends deep into the absorber layer. Consequently, the beneficial back surface field is removed and a detrimental reversed electrical field establishes. However, for more moderate Ga 2 O 3 amounts (obtained with reduced Na supply), the back surface field can be preserved. Characterization of corresponding solar cells suggests the presence of an ohmic back contact, even at absorber deposition temperatures of 550°C. The best solar cell with an IOH back contact shows a fill factor of 74% and an efficiency ( η ) of 16.1% (without antireflection coating). The results indicate that Ga 2 O 3 does not necessarily act as a transport barrier in the investigated system. Observed losses in open circuit voltage ( V OC ) as compared to reference samples with a Mo back contact are ascribed to a lower Na concentration in the absorber layer.