Researchers have pushed mercury telluride nanocrystal photodiodes past a key performance limit by combining an ultrathin cadmium sulfide shell with revised cadmium-based interface chemistry, according to a study published in Advanced Materials.
Study: Surface Passivation of HgTe Nanocrystals Enabling E G /2Open-Circuit Voltage and Their Coupling to Dielectric Cavity for Narrow Detection. Image Credit: KPixMining/Shutterstock.com
The approach raises open-circuit voltage above half the optical bandgap and supports narrowband infrared detection in a dielectric cavity.
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Colloidal HgTe nanocrystals have been widely studied for infrared optoelectronics because their bandgap can be tuned across a broad range, and the required materials can be processed from solution. They also resist oxidation better than several other colloidal infrared systems.
Even so, HgTe photodiodes have remained limited by high dark current, modest thermal stability, and low open-circuit voltage. In the short-wave infrared range, reported VOC values have typically stayed around one-quarter to one-third of the bandgap, well below what has been achieved in better-passivated nanocrystal systems such as PbS and some perovskites.
Shell Passivation and Interface Control
The new study focuses on surface chemistry rather than solely device structure. The researchers grew an ultrathin CdS shell around HgTe nanocrystals to passivate surface defects while keeping the shell thin enough to avoid severely blocking charge transport.
They then built photodiodes using ITO and SnO2 on the electron-transport side and a hole-selective contact derived from Ag2Te nanocrystals on the other side. From there, they revised the chemical treatments used during processing.
In the optimized device, HgCl2 was replaced with CdCl2 in the HgTe/CdS ink, and HgBr2 was replaced with CdBr2 in the Ag2Te layer.
The change improved passivation, reduced interdiffusion across the stack, suppressed the formation of metallic silver, and helped create an Ag-doped CdTe-like hole-selective layer that acts as a unipolar barrier against electron leakage.
Voltage and Dark Current Improve Sharply
The electrical gains were substantial. With the optimized chemistry, dark current fell to about 10-7 A cm-2 at -0.5 V, while open-circuit voltage reached 420 mV. The authors say this is the first HgTe nanocrystal photodiode to exceed EG/2 in VOC.
The devices also delivered detectivity up to 1.5 × 1011 Jones at room temperature and response times below 200 ns. Responsivity remained below that of some of the best-reported HgTe nanocrystal diodes, but the overall device balance improved.
The paper also makes clear that the CdS shell was not an unqualified gain on its own. In an earlier core-shell version, the photocurrent dropped, likely because the shell introduced a transport barrier. The best results came only after the interface chemistry was further refined.
Clearer Interfaces Inside the Device
To understand the change, the researchers examined the internal structure of the diode stack.
XPS depth profiling showed that the optimized core-shell devices exhibited much less interdiffusion than conventional stacks, resulting in more clearly defined interfaces and better control over the doping profile.
Photoemission microscopy showed a similar shift in the electronic landscape. In the core-only material, the energy modulation across the junction was about 110 to 120 meV. In the core-shell system, it rose to about 230 meV.
The authors interpret that as evidence of a stronger built-in potential, though they note that the measurement probes only the top surface of a planarized structure and does not capture the full internal potential.
Narrow Detection At 1.55 µm
The second part of the study couples the improved photodiode to a dielectric Bragg cavity for narrowband detection near 1.55 µm, a wavelength relevant to telecom applications, spectroscopy, and low-background LiDAR.
In that configuration, the devices reached linewidths as narrow as 90 cm-1, far below the much broader response of a standard diode. The cavity also enhanced the optical field within the structure, helping concentrate absorption while taking advantage of the optimized diode’s low dark current.
The authors are careful not to overstate the result. They argue that the linewidth is limited in part by absorption in the diode itself, meaning a still narrower response could come at the cost of reduced responsivity.
Broader Significance
The results show that HgTe nanocrystal photodiodes can be improved through tighter control of surface chemistry and interfaces, not only by changing the optical design. The work addresses several problems at once: low VOC, dark current, thermal robustness, and spectral selectivity.
The study points to HgTe/CdS core-shell nanocrystals as a strong platform for compact infrared photodetectors, with potential applications in spectroscopy, LiDAR, and low-background imaging. It also prompts clear future work: improving absorption and responsivity without giving up the gains in voltage and noise performance.
Journal Reference
Colle, A., et al. (2026). Surface Passivation of HgTe Nanocrystals Enabling EG/2 Open-Circuit Voltage and Their Coupling to Dielectric Cavity for Narrow Detection. Advanced Materials, e73019. DOI: 10.1002/ADMA.73019