Perovskites have appeared recently as a promising material in the field of solar-cell sensors due to its large light-absorption capability, its potentially inexpensive production cost, its semiconductor-like band-gap and its wide coating possibilities through techniques as solution processing. In the field of solar cells the efficiency has increased enormously in only two years showing the outstanding possibilities of this material. Even though, the efficiency measurements have been probed to be non-trivial due to hysteresis processes, revealing the complex behavior of this new material. As a consequence of its impressive performance as visible-light harvesters, people have started to investigate the possibility of using perovskite materials for X-ray and gamma ray detection. Improving the properties of the current X-ray detectors may allow to reduce the radiation risks during routine medical inspections as well as to decrease production costs for a large variety of industrial technologies in the fields of security, defense, radio astronomy, spectroscopy and research among many others.
The devices were fabricated at ICIQ using FTO (Fluorine doped Tin Oxide) coated glasses (8Ω/square). Dense titanium dioxide layer was deposited by spin coating over the previously cleaned FTO. Perovskite precursor solution was filtered and deposited by spin coating obtaining a 350 nm layer over the dense TiO2 layer. The HTM layers (spiro-OMeTAD or TAE-1) were filtered and deposited again in inert atmosphere by spin-coating methods onto the perovskite layer and similar HTM thickness was obtained (~100 nm). Finally, 80 nm of gold was deposited by thermal evaporation in an ultra-high vacuum chamber. Figure 4 shows the cross-section image of a diode using the perovskite material. The quantum efficiency was measured after the showing constant performance between 350nm and 770nm (Fig.5).
The diodes in Figure 4 where tested under the X-ray illumination measuring the voltage drop in a RLOAD=100MΩ resistance using a NI-USB 6289 device for the data acquisition (see Fig 6). Figure 6a illustrates the photo-response of the perovskite under an on and off illumination while Figure 6b depicts the correlation between the photo-current generated in the material and the intensity of the X-ray source. Figure 3 proves the detection of X-Rays with an extra-thin layer -500nm- of perovskite. This is the first time X-Rays response was stablished in Spain opening the road for new research.
The IGINITE program from the Barcelona Institute of Science and Technology (BIST) funded this research.
The Medipix collaboration, born at CERN in the 1990s, created an ASIC that reads out in a virtually noise-free manner the signals of X-rays, gammas and other particles, detected in a pixelated semiconductor sensor. Medipix accumulates hit counts in each pixel; in parallel, the collaboration created another family of devices, called TimePix, to record particle arrival time and/or the energy deposited per pixel. Over the years, both Medipix and Timepix chips evolved into more advanced versions that incorporated new features in order to fulfil demands from laboratories and commercial partners. IFAE played a significant role in developing readout circuitry for these chips. Medipix and TimePix devices are being used in many of applications such as space dosimetry, synchrotron light imaging, material analysis, and spectroscopic X-ray imaging, to mention just a few. In order to satisfy the requirements of newly foreseen applications, the Medipix collaboration started in 2016 on the next family of chips, Medipix 4 and Timepix 4. These devices, in 65 nm technology and using TSVs, will include the novelties of the third chip generation while offering higher count rates and finer time resolution, with more than one possible pixel format. IFAE joined the Medipix4 collaboration in 2017 and is participating in the design of the Timepix4 chip.