Identification and Suppression of Point Defects in Bromide Perovskite Single Crystals Enabling Gamma-Ray Spectroscopy.

Methylammonium lead tribromide (MAPbBr3) stands out as the most easily grown wide-band-gap metal halide perovskite. It is a promising semiconductor for room-temperature gamma-ray (γ-ray) spectroscopic detectors, but no operational devices are realized. This can be largely attributed to a lack of understanding of point defects and their influence on detector performance. Here, through a combination of crystal growth design and defect characterization, including positron annihilation and impedance spectroscopy, the presence of specific point defects are identified and correlated to detector performance. Methylammonium (MA) vacancies, MA interstitials, and Pb vacancies are identified as the dominant charge-trapping defects in MAPbBr3 crystals, while Br vacancies caused doping. The addition of excess MABr reduces the MA and Br defects and so enables the detection of energy-resolved γ-ray spectra using a MAPbBr3 single-crystal device. Interestingly, the addition of formamidinium (FA) cations, which converted to methylformamidinium (MFA) cations by reaction with MA+ during crystal growth further reduced MA defects. This enabled an energy resolution of 3.9% for the 662 keV 137Cs line using a low bias of 100 V. The work provides direction toward enabling further improvements in wide-bandgap perovskite-based device performance by reducing detrimental defects.

Identification of lead vacancy defects in lead halide perovskites.

Perovskite photovoltaics advance rapidly, but questions remain regarding point defects: while experiments have detected the presence of electrically active defects no experimentally confirmed microscopic identifications have been reported. Here we identify lead monovacancy (VPb) defects in MAPbI3 (MA = CH3NH3+) using positron annihilation lifetime spectroscopy with the aid of density functional theory. Experiments on thin film and single crystal samples all exhibited dominant positron trapping to lead vacancy defects, and a minimum defect density of ~3 × 1015 cm-3 was determined. There was also evidence of trapping at the vacancy complex [Formula: see text] in a minority of samples, but no trapping to MA-ion vacancies was observed. Our experimental results support the predictions of other first-principles studies that deep level, hole trapping, [Formula: see text], point defects are one of the most stable defects in MAPbI3. This direct detection and identification of a deep level native defect in a halide perovskite, at technologically relevant concentrations, will enable further investigation of defect driven mechanisms.

Distance determination in heterogeneous DNA model systems by pulsed EPR.

Many biological systems, especially those based on nucleic acids, are structurally heterogeneous in solution. We demonstrate here the ability to measure multiple distances, of between 2 and 7 nm, from a heterogeneous mixture of double-spin-labeled DNA duplexes. We have constructed a DNA distance ruler based on the attachment of nitroxide spin labels to 2'-amino-modified nucleosides. The distribution of distances between the spin labels was obtained by Tikhonov regularization analysis of the dipolar coupling evolution data measured by using the electron paramagnetic resonance method, pulsed-electron double resonance (PELDOR). Optimization of the conditions and techniques used in the preparation of the samples has allowed us to increase the sensitivity and reduce aggregation artifacts. As a result, we have been able to demonstrate deconvolution of distances from structurally heterogeneous samples and show the limits of the technique by examining data derived from up to five DNA duplexes, in a single mixture, in which the concentration of each species was as low as 5 microM.