Room temperature semiconductor detectors provide high energy resolution in gamma-ray spectroscopic applications, such as radioisotope identification. Materials such as TlBr and CsPbBr3, which provide ~2% FWHM, or better, energy resolution for 662-keV photopeak from Cs-137 irradiation at room temperature and high stopping power, nevertheless exhibit mixed electronic and ionic conduction that can lead to undesirable performance degradation. While the charge pulses generated by the gamma-ray interaction and a component of the leakage current are composed of a dominant electronic signal, the ionic component contributes to the current. It affects device stability via the migration of ionic species under applied bias and polarization at the semiconductor-electrode interface, interfering with charge collection. In this work, we characterize electronic and ionic conduction in single-crystal devices of TlBr and CsPbBr3 (CLB) using ionically blocking electrodes. The ionic contribution to the leakage current is evaluated from the devices' impedance spectra. In contrast, the electronic contribution is extracted from the asymptotic limit of the devices' current-voltage-time (iVt) plots for different bias values. The response to illumination with a 375-nm LED is also measured in the iVt plots. This work presents expressions for the temperature dependence of the conductivity, referencing previous work for TlBr. It also presents the gamma-ray spectra's bias dependence and evaluates ionic conduction's role in device burn-in, conditioning, and aging processes. Comparing the impedance spectra with and without LED illumination confirms that ions represent the majority charge carrier at room temperature for TlBr and CLB, which is consistent with previous work.
We demonstrate efficient solar cells based on an active layer of methylammonium lead iodide (MAPbI3) co-deposited via vapor transport deposition (VTD). In VTD organic and metal-halide precursor vapors are transported by carrier gas to a cooled substrate where film formation occurs. Our system design enables control over a range of process parameters allowing co-deposition and composition tuning of the final perovskite film. The impact of VTD processing conditions on film composition and solar cell performance will be discussed, with emphasis on tuning the lead iodide content in MAPbI3 films to realize power conversion efficiencies >10%.
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