Tandem quantum-dot photodiodes bring fast, glare-resistant 1550 nm LiDAR closer

LiDAR is quietly becoming one of the defining sensing technologies of the decade, powering autonomous driving, robotics, mixed reality, and industrial monitoring by timing the round trip of short laser pulses to reconstruct three-dimensional scenes. But as the industry pushes its operating wavelength from 905 nm toward 1550 nm, a band prized for eye safety and coherent sensing, it has run into a stubborn detector bottleneck. InGaAs photodiodes can see this infrared light, yet their high cost and complicated fabrication make them poorly suited to the large-area, low-cost detector arrays that consumer electronics and autonomous vehicles demand. Now a team led by Professor Liang Gao at the Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, reports in Light: Science & Applications a tandem PbSe colloidal quantum-dot photodiode that tackles the two most persistent problems of quantum-dot LiDAR detectors at once: background-light interference and slow response.

Colloidal quantum dots, semiconductor nanocrystals grown in solution, have long been attractive for infrared detection because their bandgaps can be tuned, they absorb strongly in the infrared, and they integrate readily with silicon readout circuits. They have already shown promise for two-dimensional infrared imaging. Moving to three-dimensional LiDAR, however, exposes two critical weaknesses. Conventional single-junction quantum-dot photodiodes cannot cleanly separate the 1550 nm laser echo from ambient visible light flooding the detector in a realistic outdoor scene, and their response speed has been limited by the device’s RC time constant, a major bottleneck for capturing the nanosecond pulses that time-of-flight ranging depends on.

The team’s answer is a device architecture that behaves as an integrated photonic-electronic link rather than relying on material optimization alone. Two matched quantum-dot photodiodes are stacked optically and connected electrically in series, so light passes through the front junction before reaching the rear junction. This geometry produces a built-in spectral filtering effect. Visible light is absorbed strongly in the front junction, generating carriers there while leaving the rear junction largely dark; because the two sub-junctions are wired in series, this photocurrent mismatch suppresses the visible-light response. Light at 1550 nm, by contrast, penetrates more deeply and generates balanced photocurrents in both junctions, so the target LiDAR signal passes through essentially intact.

The same series connection solves the speed problem from a different direction. Cascading two junctions halves the effective junction capacitance, lowering the RC time constant that normally throttles high-mobility quantum-dot photodiodes. Compared with simply thickening the absorber or shrinking the device area, the tandem approach cuts capacitance while keeping a similar carrier transit path, offering a structural route to faster response without giving up light-collection efficiency.

The measured results back the concept. The optimized tandem PbSe quantum-dot photodiodes achieved a response time of 1.01 ns, a linear dynamic range above 99.6 dB, and a specific detectivity of 2.29 × 10^11 Jones at 1550 nm. The tandem architecture reduced the visible-range external quantum efficiency by roughly 13-fold relative to single-junction devices while maintaining comparable response at the target wavelength, exactly the asymmetry a LiDAR detector needs when a faint laser echo must survive a flood of sunlight.

The researchers went beyond benchtop characterization and built a working 1550 nm LiDAR system around the tandem photodiodes. In the ranging experiment, the device enabled centimeter-level spatial resolution. Under strong broadband background illumination, the tandem photodiodes retained 97.9% of their pulse signal intensity, outperforming single-junction quantum-dot photodiodes tested under the same interference condition, a result that speaks directly to the outdoor environments where LiDAR must actually operate.

“Our tandem quantum-dot photodiode provides a structural solution to two long-standing challenges in colloidal quantum-dot LiDAR detectors: background-light interference and response-speed limitation,” the researchers summarize. “By integrating optical pre-processing and electronic post-processing in a single device, the tandem architecture enables fast and spectrally selective detection at 1550 nm.”

The significance of the work lies in what it replaces. A 1550 nm LiDAR module today typically leans on InGaAs photodiodes, whose fabrication costs and process complexity limit their use in the consumer-scale arrays that next-generation high-performance LiDAR envisions. A solution-processed quantum-dot detector that is simultaneously fast, spectrally selective, and interference-resistant changes that calculus, pointing toward low-cost, integrated 1550 nm detectors compatible with silicon-based readout circuits.

The path from laboratory demonstration to deployed hardware still runs through further advances in quantum-dot surface passivation, thin-film uniformity, array fabrication, and silicon readout integration. But if those engineering steps mature, the tandem colloidal quantum-dot photodiode could find applications well beyond autonomous driving, extending to robotic vision, consumer depth sensing, mixed reality, and infrared imaging, where a detector that ignores visible glare while resolving nanosecond infrared pulses would be a quietly transformative component.

Subject of Research: Tandem colloidal quantum-dot photodiodes for spectrally selective, fast 1550 nm LiDAR detection

Article Title: Spectral-selective quantum-dot photodiodes for 1550 nm LiDAR

Article References: Spectral-selective quantum-dot photodiodes for 1550 nm LiDAR. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: LiDAR, colloidal quantum dots, PbSe photodiodes, 1550 nm detection, tandem device architecture, spectral selectivity, response time, time-of-flight ranging, autonomous driving, infrared detectors, background-light interference, Light: Science & Applications