Pure organic TPB single crystal for direct X-ray detection

Abstract

For a long time, single crystals of the organic compound TPB (1,3,5-triphenylbenzene) have been viewed as a scintillator material for indirect X-ray detection. We here attempt to explore their potential for direct detection. A TPB single crystal with a size of up to 20 × 6 × 4 mm³ and without obvious growth steps was obtained using a solvent volatilization method. It has an orthorhombic structure with a high-quality FWHM of as low as 0.059 and preferred grown orientations along (131) and (220). AFM images clarify the two-dimensional nucleation growth mechanism. The fitted bandgap of the as-grown crystal is about 3.65 eV. TPB detectors with vertical and coplanar electrodes with a resistivity of 2.56 × 10¹³ Ω cm and 6.68 × 10¹³ Ω cm, respectively, were prepared. The mobilities for electrons (μ_e) and holes (μ_h) for the coplanar device were calculated to be 1.12 cm² V⁻¹ s⁻¹ and 1.02 cm² V⁻¹ s⁻¹, respectively. Under a 50 kV X-ray beam, the detection limit is as low as 0.35 μGy s⁻¹, and the sensitivity reaches 2.72 μC Gy⁻¹ cm⁻². The TPB devices have the ability to detect ²⁴¹Am@5.49 MeV α particles, showing the full-energy peak collected from the holes with an energy resolution of 39.13% at 700 V. The mobility lifetime product of the TPB devices for electrons (μτ)_e and holes (μτ)_h were calculated to be up to 2.24 × 10⁻⁵ cm² V⁻¹ and 4.04 × 10⁻⁵ cm² V⁻¹, respectively. The TPB detector could present a spectral response to ²⁴¹Am@59.5 keV γ-rays from the holes with an energy resolution of 44.25% at 1000 V, which shows the potential for both direct and indirect detection of high-energy radiation.