Solid-state thermal neutron detectors with improved detection efficiencies and sensitivities are highly sought after for the detection of special nuclear materials (SNM). Due to the inherent nature of low flux of fission neutron emission from a potential SNM as well as the fact that the neutron flux is inversely proportional to the square of the distance from the SNM source, it is challenging in many cases to detect neutron signals at relatively large distances. Consequently, large size high efficiency detectors are desired to improve the detector performance and to provide a reasonable count rate (or sensitivity). We report here the successful synthesis by MOCVD of freestanding 10B enriched hexagonal boron nitride (h-BN) epilayers with a thickness of about 50 microns. The progress towards the realization of vertical photoconductor-like thermal neutron detectors with the detector area increasing from 1 mm × 1 mm to 3 mm × 3 mm while upholding a record high detection efficiency of about 53% among solid-state neutron detectors is described. The effects of resistivity, dark current density, and the carrier mobility-lifetime products on the device performance were monitored and optimized via MOCVD growth parameters. With improved material quality, the noise related dark counts have been significantly reduced. The work laid the groundwork for realizing large area detectors. Our results indicate that h-BN epilayers are highly promising for realizing sensitive solid-state thermal neutron detectors with expected advantages resulting from semiconductor technologies, including compact size, light weight, low operating voltage, and low cost.