A new photodetector design combines metal-organic frameworks and MXene materials to overcome silicon’s performance limits, enabling broad-spectrum light detection without external power.
(Nanowerk Spotlight) The efficient conversion of light into electrical signals is essential for many modern optoelectronic technologies, such as smartphone cameras, optical fiber communication, and environmental sensors. Photodetectors perform this crucial transformation, enabling applications ranging from optical communications to biological sensing. Yet silicon-based photodetectors have faced a persistent barrier to optimal performance: high-density surface states that generate excessive dark leakage currents. These unwanted currents significantly reduce both responsivity and detectivity, two critical metrics that determine photodetector quality.
Scientists have pursued various strategies to mitigate this limitation. Surface passivation techniques, interface buffer layers, and alternative semiconductor materials have all shown promise, yet each approach introduces new complications. Silicon remains the most attractive substrate material due to its abundance, well-established processing methods, and compatibility with existing electronics. However, conventional photodetector fabrication typically relies on high-temperature diffusion and ion implantation processes that reduce minority carrier lifetimes and introduce additional defects.
Two-dimensional materials have emerged as potential solutions due to their exceptional optical and electronic properties. Their dangling bond-free surfaces can form high-quality interfaces with other materials without strict lattice-matching requirements. Among these materials, metal-organic frameworks (MOFs) and MXenes show particular promise due to their tunable electronic properties. MOFs combine organic linkers with metal nodes to create structurally diverse materials with adjustable electronic characteristics. MXenes—two-dimensional transition metal carbides and nitrides—offer highly adaptable surface chemistry through various functional groups.
Until now, effectively integrating these advanced materials with silicon to create high-performance photodetectors has presented significant challenges. Researchers have struggled to form high-quality interfaces that maintain the intrinsic properties of each component while enabling efficient charge separation and transport.
A research team led by Yuansheng Ge, Quan Zhang, Jinlong Mu, and Jing Li has developed an innovative solution to this problem. Their approach, published in Advanced Science (“Solution-Processable Van Der Waals Heterojunctions on Silicon for Self-Powered Photodetectors with High Responsivity and Detectivity”), uses an all-solution-processable method to construct photodetectors through consecutive spray-coating of a conductive metal-organic framework (Cu₃(HHTP)₂) and metallic Ti₃C₂ MXene to form van der Waals dual junctions on silicon.
a) Synthesis schematic of Cu3(HHTP)2 on n-Si. b) Schematic illustration of the preparation of Ti3C2/Cu3(HHTP)2/n-Si photodetector. c) Comparison of performance parameters for various electrodes and attempts at using MXene or MOF materials in self-powered photodetectors. (Image: Reprinted from DOI:10.1002/advs.202500027, CC BY) (click on image to enlarge)
The researchers first fabricate a type I heterojunction by growing Cu₃(HHTP)₂ (where HHTP stands for 2,3,6,7,10,11-Hexahydroxybenzobenzene) thin films on n-type silicon using liquid-phase epitaxy. This creates an interface that facilitates unidirectional electron-hole separation. They then spray Ti₃C₂ MXene nanosheets onto this MOF layer, forming a secondary Schottky junction that further enhances charge separation.
This dual-junction configuration creates an electronic cascade that addresses the fundamental limitations caused by silicon surface states. The type I band alignment between Cu₃(HHTP)₂ and silicon directs photogenerated electrons toward the silicon layer, while the Schottky barrier between Ti₃C₂ and Cu₃(HHTP)₂ provides an additional driving force for charge separation.
X-ray diffraction analysis confirmed the successful synthesis of Cu₃(HHTP)₂ thin films with a hexagonal structure and 2D slip parallel superposition. High-resolution transmission electron microscopy revealed well-defined crystal lattices, indicating good crystallinity. The researchers controlled the thickness of the MOF layer by adjusting the number of liquid-phase epitaxy cycles, with 40 cycles yielding optimal performance.
The Ti₃C₂ MXene component was prepared through exfoliation of Ti₃AlC₂ using hydrochloric acid and lithium fluoride, followed by ultrasound-assisted processing. This created few-layer or single-layer nanosheets with lateral dimensions of 1-2 micrometers, which were then spray-coated onto the MOF surface.
Electronic characterization showed that Cu₃(HHTP)₂ functions as a p-type semiconductor with a direct band gap of 2.7 eV. When combined with n-type silicon, this forms a type I heterojunction with band edges positioned to facilitate efficient charge transfer. The work function difference between Cu₃(HHTP)₂ (4.86 eV) and Ti₃C₂ (4.56 eV) creates a Schottky barrier that further enhances charge separation.
First-principles calculations provided insights into the charge transfer dynamics at these interfaces. The researchers observed that in the Ti₃C₂/Cu₃(HHTP)₂/Si trilayer structure, electrons transfer stepwise from Ti₃C₂ through Cu₃(HHTP)₂ to silicon. This electronic cascade ensures that electrons flow efficiently toward the silicon electrode, minimizing recombination and maximizing photocurrent generation.
The photodetector demonstrates exceptional performance without requiring external power. Under 365 nm illumination, it achieves a high responsivity of 1.8 A W⁻¹ and a specific detectivity of 1.63 × 10¹² Jones. Its on/off ratio exceeds 3.9 × 10⁴ at an incident light power density of 330 μW cm⁻², indicating excellent sensitivity to light signals. Additionally, the device exhibits rise and fall times of 340 ms and 688 ms, respectively, demonstrating solid response speed.
Perhaps most impressively, the device maintains strong responsivity across wavelengths from 365 to 700 nm, spanning ultraviolet to visible light. This broad spectral response makes it suitable for diverse applications requiring light detection across different wavelength ranges.
The photodetector’s performance metrics set a benchmark for MOF-based optoelectronic devices. Compared to previous approaches using either MOFs or MXenes individually, the dual-junction strategy delivers superior results across all key parameters.
Beyond the impressive performance characteristics, this work introduces a straightforward approach for constructing high-quality van der Waals junctions on semiconductor surfaces. The solution-processable nature of the fabrication method offers significant advantages over conventional semiconductor processing, potentially reducing production costs while improving device performance.
This research demonstrates that carefully engineered interfaces through van der Waals heterojunctions can effectively address longstanding challenges in photodetector technology. By creating well-defined junctions that facilitate efficient charge separation and transport, the approach enables high-performance photodetection without external power sources.
As demand increases for efficient, self-powered sensing technologies across telecommunications, healthcare monitoring, and environmental sensing, this innovation provides a pathway toward devices that combine high performance with simpler fabrication requirements. The principles established here could inform the development of other optoelectronic devices, accelerating progress toward advanced technologies that leverage the unique properties of MOFs, MXenes, and other two-dimensional materials.
Get our Nanotechnology Spotlight updates to your inbox!
Thank you!
You have successfully joined our subscriber list.
Become a Spotlight guest author! Join our large and growing group of guest contributors. Have you just published a scientific paper or have other exciting developments to share with the nanotechnology community? Here is how to publish on nanowerk.com.
