We analyze the results of an extensive characterization study involving electrical and optical measurements carried out on hydrogenated amorphous silicon (alpha-Si:H) thin film materials fabricated under a wide range of deposition conditions. By adjusting the synthesis parameters, we evidenced how conductivity, activation energy, electrical transport and optical absorption of an alpha-Si:H layer can be modified and optimized. We analyzed the activation energy and the pre-exponential factor of the dark conductivity by varying the dopant-to-silane gas flow ratio. Optical measurements allowed to extract the absorption spectra and the optical bandgap. Additionally, we report on the temperature dependence of the activation energy to satisfy the Meyer-Neldel rule. Finally, the influence of the individual films parameters upon the final performances of a single junction pin alpha-Si:H have been studied. The measurements show how a more than doubled enhancement in energy conversion efficiency can be obtained in an alpha-Si:H solar cell with a proper selection of synthesis conditions. (C) 2012 Elsevier B.V. All rights reserved.