Alkali incorporation into Cu(In,Ga)Se^sub 2^ determined by crystal orientation of Mo back contact: Implications for highly efficient photovoltaic devices

The structural, optical, and electrical properties of molybdenum (Mo) layers play a major role in Cu(In,Ga)Se2 (CIGS) solar cell performance. The Mo layer works as a transport gate for diffusion of alkali ion from the soda-lime glass substrate to the back contact in CIGS solar cells. In the present work, Mo back contacts are controlled to exhibit two different orientations: (110)-oriented and randomly oriented. The influence of these orientations on CIGS absorbers and resulting solar cells is investigated. In situ thermo-Raman spectroscopy and secondary ion mass spectrometry results indicate that the greater amount of alkali ions are found in the CIGS absorber with randomly oriented Mo back contact than in the (110)-orientated Mo back contact. The resulting different Na incorporations significantly affect the performance of the resulting devices. Devices with the randomly oriented Mo back contact exhibit superior device performances to the devices with (110)-oriented Mo back contact. With comprehensive device characterizations, an alkali ion release determined by the orientation of the Mo back contact affects the recombination mechanism in the CIGS bulk and the back contact properties at the CIGS/Mo interface.