  This work evaluates the viability of polyimide and parylene-C for passivation
of lithium-drifted silicon (Si(Li)) detectors. The passivated Si(Li) detectors
will form the particle tracker and X-ray detector of the General Antiparticle
Spectrometer (GAPS) experiment, a balloon-borne experiment optimized to detect
cosmic antideuterons produced in dark matter annihilations or decays.
Successful passivation coatings were achieved by thermally curing polyimides,
and the optimized coatings form an excellent barrier against humidity and
organic contamination. The passivated Si(Li) detectors deliver $\lesssim\,4$
keV energy resolution (FWHM) for 20$-$100 keV X-rays while operating at
temperatures of $-$35 to $-45\,^{\circ}$C. This is the first reported
successful passivation of Si(Li)-based X-ray detectors operated above cryogenic
temperatures.
