  We report on the progress in characterization of the Hamamatsu model
R11410-21 Photomultiplier tubes (PMTs) for XENON1T dark matter experiment. The
absolute quantum efficiency (QE) of the PMT was measured at low temperatures
down to -110 $^0$C (a typical the PMT operation temperature in liquid xenon
detectors) in a spectral range from 154.5 nm to 400 nm. At -110 $^0$C the
absolute QE increased by 10-15\% at 175 nm compared to that measured at room
temperature. A new low power consumption, low radioactivity voltage divider for
the PMTs is being developed. The measurement results showed that the PMT with
the current version of the divider demonstrated a linear response (within 5\%)
down to 5$\cdot$10$^4$ photoelectrons at a rate of 200 Hz. The radioactive
contamination induced by the PMT and the PMT voltage divider materials
satisfies the requirements for XENON1T detector not to exceed a total
radioactive contamination in the detector of 0.5 evts/year/1tonn. Most of the
PMTs received from the manufacturer showed a high quantum efficiency exceeding
30\%. In the mass production tests the measurements at room temperature showed
clear single photoelectron peaks for all PMTs been under study. The optimal
operation conditions were found at a gain of 2$\cdot$10$^6$. The operation
stability for most of the PMTs was also demonstrated at a temperature of -100
$^0$C. A dedicated setup was built for testing the PMTs in liquid xenon using
the XENON1T signal readout components including voltage dividers, cables and
feedthroughs. The PMTs tested in liquid xenon demonstrated a stable operation
for a time period of more than 5 months.
