We have improved the ac-dc transfer differences and enhanced the sensitivity in our thin-film multijunction thermal converters (TFMJTCs) in vacuum. The ac-dc transfer differences of the TFMJTCs are less than 1 μV/V down to 1 Hz and 10 μV/V down to 0.2 Hz owing to the increased thermal time constant. The measured output voltage exceeds 120 mV at 10^-4 Pa owing to the reduction of heat conduction to the air. The sensitivity of the device, defined as the ratio of the output voltage to the input power, is 2.4 V/W, which is approximately three times larger than that measured at ambient pressure. Numerical simulations of the heat transfer of our TFMJTC explain the experimental results well.