Plasma treatment (PT) of the buffer layer for highly H2-diluted hydrogenated amorphous silicon (a-Si:H) absorption layers is proposed as a technique to improve efficiency and mitigate light-induced degradation (LID) in a-Si:H thin film solar modules. The method was verified for a-Si:H single-junction and a-Si:H/microcrystalline silicon (µc-Si:H) tandem modules with a size of 200 × 200 mm2 (aperture area of 382.5 cm2) under long-term light exposure. H2 PT at the p/i interface was found to eliminate non-radiative recombination centers in the buffer layer, and plasma-enhanced chemical vapor deposition at low radio-frequency power was found to suppress the generation of defects during the growth of a-Si:H absorption layers on the treated buffer layers. With optimized H2 PT of the a-Si:H single-junction module, the stabilized short circuit current and fill factor increased, and the stabilized open circuit voltage moves beyond its initial value. The results demonstrate 7.7% stabilized efficiency and 10.5% LID for the a-Si:H single-junction module and 10.82% stabilized efficiency and 7.76% LID for the a-Si:H/µc-Si:H tandem module. Thus, the growth of an a-Si:H absorption layer on a H2 PT buffer layer can be considered as a practical method for producing high-performance Si thin film modules. Copyright © 2012 John Wiley & Sons, Ltd.