Complex lead halides with the perovskite structure has been extensively studied in recent years in terms of radiation hardness. Many researchers found surprisingly high resistance of various perovskite materials towards high energy electrons, protons, and hard ionization such as X-rays and gamma-rays. Superior radiation hardness makes a family of perovskite semiconductors an attractive candidate for single- and multijunction solar cells for the space application and as X-ray and gamma-ray detectors. As the space environment does not contain oxygen and moisture, perovskite solar cells are expected to have a longer operating lifetime, compared to terrestrial applications. What is more dangerous in space is the high level of ionization, which is characterized by the total ionizing dose (TID) parameter. It is estimated, that the accumulated dose can reach 1000 kRad (10 kGy) in 20 years of exploitation. One of the ways to study the radiation hardness of the materials to be used in space is by exposing them to ionization sources such as 60Co or 137Cs with photon energies 1.1 MeV and 662 keV respectively. A great number of publications are dedicated to the gamma ray stability of perovskite materials with various composition.
In a new paper published in Light: Science & Application, a team of scientists, led by Dr. Aleksandra Boldyreva the research scientist from Center of Energy Science and Technology, Skolkovo Institute of Science and Technology, Moscow, Russia and co-workers have focused on a gamma ray stability of a wide bandgap perovskite (1.75 eV) which can be applied as a top cell in tandem solar cells. It turned out, that small doses up to 10 kGy might passivate some of the negatively charged defects initially present in the perovskite films simultaneously activating other intrinsic defects. It was revealed by Admittance spectroscopy that dominating defects in perovskite film have the energy of 0.5 eV and with gamma ray dose the concentration of these defects rapidly decreases. At the same time the diffusion coefficient of these defects after 6 kGy dose increases by 2 orders of magnitude (Figure 1).
"We observed an unusual defect behavior, completely opposite to a Schottky-type defect (when the diffusion coefficient increases with its own concentration)."
Interestingly, further dose accumulation results in saturation of both parameters pointing on complex mechanism behind the interaction of gamma rays and perovskites.
"When gamma-rays with photon energies of 662 keV interact with the matter, the probability of the photoelectric effect is higher than the formation of Frankel pairs. The photoelectric effect occurs when electrons, excited by gamma photons, escape from the matter, leaving behind unrecombined holes. Perovskite polycrystalline films typically have many interfacial defects, some of which are negatively charged. These negatively charged defect states, after interaction with gamma-rays, annihilate, reducing non-radiative recombination. While the fraction of negatively charged defects is high, we observe improvement in recombination dynamics and reduction of defect states. At some point, there is more newly formed defects after interaction with gamma-rays than a number of passivated defects. At such point, further accumulation of dose results in degradation of film quality and a shift towards non-radiative recombination."
Fresh and exposed perovskite solar cells were further analyzed with annular dark-field scanning transmission electron microscopy (HAADF-STEM). Figure 2 shows that the 10 kGy exposed sample has more uniform distribution of iodine along the plane, while unexposed sample has a large accumulation of iodine next to the interface with hole transport layer.
"Combination of Admittance spectroscopy and HAADF-STEM analysis allowed to define that the dominating defects in the fabricated solar cells are iodine vacancies VI. Large amount of negatively charged iodine vacancies in pristine perovskite solar cells decreases with gamma ray exposure. At the same time, a drastic increase in diffusion coefficient points to migration through the freshly formed defects created by the gamma irradiation".
