This document presents a study on an improved bi-alkali photocathode used on large hemispherical photomultiplier tubes (PMTs). The process has been applied on a sample of the Photonis XP1805, the 9 in. PMT used in the Pierre Auger Observatory experiment and compared to a sample of the standard production ones. Measurements of the photocathode sensitivity, the spectral response and the relative detection efficiency were performed on the samples of standard and improved PMTs to characterise this new photocathode process. Current measurements of the photocathode sensitivity show a mean improvement of around 19% at 400 nm. Comparative pulse measurements of the relative detection efficiency confirmed this result. Additional measurements were performed in order to determine if the new process could have secondary effects. The noise rate decay time after exposure lasts to a few tenths of hours. The dark count rate is more sensitive to temperature but remains lower than 10 kHz at 15 °C. The afterpulse probability is increased but remains very low with the type of tube used in this study.