Origami-inspired perovskite X-ray detector by printing and folding

X-ray detectors are crucial for various applications in medicine, industry, and science. Efficient X-ray detection is challenging due to the low interaction probability of hard X-ray photons, requiring thick absorber layers in traditional planar detectors. These thick layers, however, can hinder efficient charge carrier collection due to trapping and recombination. Perovskite semiconductors offer a promising alternative due to their efficient X-ray absorption, high conversion efficiency, and good charge transport. They are also printable onto flexible substrates using low-cost solution processing. This research explores an origami-inspired folded device architecture to overcome the limitations of conventional planar designs. The key concept is to utilize relatively thin perovskite absorbers on a flexible substrate, then fold the device to reorient the absorber parallel to the X-ray beam. This maximizes X-ray absorption efficiency without requiring excessively thick layers. The folded design also promises high spatial resolution due to the physical separation of pixels. This study builds upon previous theoretical and experimental work on flexible perovskite X-ray detectors, aiming to demonstrate a high-performance origami-inspired folded detector. The fabrication of a foldable perovskite sensor array is presented, and the detector is characterized in both planar and folded configurations under 50 kVp–150 kVp X-ray radiation. The potential spatial resolution is also evaluated, along with an investigation into the device's operational stability under continuous X-ray exposure.

The introduction extensively reviews the existing literature on X-ray detectors, highlighting the challenges of using thick absorber layers in conventional planar designs and the advantages of perovskite semiconductors as direct conversion X-ray absorbers. It cites various research papers showcasing different approaches to building high-performance perovskite-based X-ray detectors, including those using solution processing and printing methods. The review also mentions the use of perovskites in other photodetectors. Furthermore, the introduction mentions previous work on edge-on configurations and the authors’ own prior research on theoretical modeling and inkjet printing of flexible perovskite X-ray detectors, establishing the context and background of the current study. It also explains why the origami-inspired folded design is superior to a stacked design for scalability and practical implementation.

The fabrication of the origami-inspired perovskite X-ray detector involved several steps. First, folding lines were engraved into a 25-µm-thick polyethylene naphthalate (PEN) substrate using a laser cutter. After cleaning, a gold bottom electrode was thermally evaporated, followed by a sputtered NiO hole transport layer. A 6-µm-thick triple cation perovskite (TCP) absorber was then inkjet-printed. An electron transport layer of C60 fullerene and bathocuproine (BCP) was then thermally evaporated, and finally, a gold top electrode was deposited. Aerosol jet printing was used to create silver lines connecting the pixel electrodes to the external readout electronics. The folding of the detector foil was carefully performed. A second PEN foil was placed on top to prevent shorts and provide protection during the folding process. The electrical characterization was performed using a Keithley 2450 SourceMeter, measuring current-voltage (I-V) curves in the dark and under X-ray radiation (50 kVp and 150 kVp). X-ray sensitivity analysis involved measuring the time-resolved current under pulsed X-ray radiation at various dose rates. To evaluate the spatial resolution, a moving edge method was employed with a tungsten edge and 150 kVp X-ray radiation, measuring the presampled edge spread function (ESF) to calculate the modulation transfer function (MTF). The operational stability was assessed by continuously exposing the folded detector to 150 kVp X-ray radiation for extended periods and monitoring its response. The theoretical X-ray sensitivity was predicted using a model incorporating detector efficiency, photon energy, and mass energy absorption coefficient of air, using the SPEKTR 3.0 toolkit for X-ray spectrum simulation and a custom simulation framework for other parameters. The modulation transfer function was calculated using both numerical (derivative and fast Fourier transform) and analytical methods.

The planar detector exhibited X-ray sensitivities of 25–35 µC/(Gy_aircm²) under 50 kVp–150 kVp X-ray radiation. Remarkably, the folded detector demonstrated significantly increased sensitivities, reaching values of several hundred µC/(Gy_aircm²) and a record high of 1409 µC/(Gy_aircm²) at 150 kVp. This high performance was achieved without photoconductive gain and with zero external bias. The folded detector exhibited a potential spatial resolution exceeding 20 lp/mm under 150 kVp X-ray radiation, significantly better than many other perovskite and other detector materials. This high spatial resolution was assessed using the presampled MTF. The operational stability tests revealed a highly stable detector response for over 19 hours of continuous 150 kVp X-ray irradiation, accumulating a dose exceeding 126.8 Gyair. This stable operation was achieved under ambient conditions without additional encapsulation, suggesting high environmental stability. The study successfully demonstrated the concept of a folded perovskite X-ray detector with exceptional performance, opening avenues for future development.

The significant improvement in X-ray sensitivity achieved by the folded detector architecture validates the design's effectiveness in enhancing X-ray absorption. The high spatial resolution demonstrated further underscores the advantages of this approach. The absence of photoconductive gain suggests the high sensitivity is an intrinsic property of the folded design rather than a consequence of amplification mechanisms. The exceptional operational stability points to the robust nature of the device and its potential for practical applications. The achieved performance surpasses that of various previously reported perovskite and other X-ray detectors, making this origami-inspired approach highly competitive. Future work should focus on reducing noise and improving the fill factor by using thinner substrates and optimizing pixel design. The integration of printed thin-film transistors could enable the creation of large-area, low-cost X-ray detector systems.

This research successfully demonstrated a proof-of-concept for a high-performance origami-inspired folded perovskite X-ray detector. The folded architecture dramatically improves X-ray sensitivity and achieves exceptional spatial resolution without the need for photolithography. The device's remarkable operational stability under continuous high-dose X-ray irradiation further strengthens its commercial viability. Future research should focus on addressing limitations such as noise reduction and fill factor improvement, paving the way for large-area, low-cost, high-performance X-ray detector systems.

The study primarily focused on a single-pixel demonstration. While the potential for high spatial resolution was shown through the presampled MTF, the actual spatial resolution of a larger array would be limited by pixel pitch and aliasing effects. Further investigation is needed to optimize pixel design for improved fill factor and to thoroughly characterize the noise characteristics of the detector in a larger array configuration. The long-term stability under real-world conditions with encapsulation should also be investigated more thoroughly.