Non-toxic quantum dots pave way for SWIR sensors for consumer electronics

This breakthrough, detailed in Nature Photonics, hinges on a new technique for producing high-quality, non-toxic colloidal quantum dots compatible with complementary metal-oxide-semiconductor (CMOS) technology.

SWIR light, invisible to the human eye, promises enhanced reliability, functionality, and performance in various high-volume applications such as service robotics, automotive, and consumer electronics. SWIR-sensitive image sensors excel under challenging conditions like bright sunlight, fog, haze, and smoke. Additionally, they offer eye-safe illumination and enable material property detection through molecular imaging.

Traditional SWIR quantum dots, however, often contain hazardous heavy metals like lead or mercury, posing regulatory challenges under the European Union's Restriction of Hazardous Substances (RoHS) directive. To overcome this, ICFO's team, led by ICREA Prof. Gerasimos Konstantatos and including Yongjie Wang, Lucheng Peng, and Aditya Malla, collaborated with Qurv researchers Julien Schreier, Yu Bi, Andres Black, and Stijn Goossens. Their study introduces a novel synthesis method for silver telluride (Ag2Te) quantum dots, which are phosphine-free and maintain the advantageous properties of their heavy-metal counterparts.

The research initially focused on silver bismuth telluride (AgBiTe2) nanocrystals for photovoltaic devices. However, the accidental discovery of Ag2Te's strong, tuneable quantum-confined absorption redirected their efforts. The team developed a new synthesis process for phosphine-free Ag2Te quantum dots, circumventing the negative impact of phosphine on the optoelectronic properties critical for photodetection.

These newly synthesised quantum dots displayed remarkable performance, with distinctive excitonic peaks beyond 1,500nm. This achievement surpasses previous phosphine-based methods. The research then progressed to creating a basic laboratory-scale photodetector on an ITO-coated glass substrate. Yongjie Wang, a postdoctoral researcher at ICFO and the study's first author, highlighted the challenges in adapting the device setup for CMOS integration.

Their redesigned photodiode, now including a buffer layer, significantly enhanced performance. The resulting SWIR photodiode covered a spectral range from 350 to 1,600nm, a linear dynamic range over 118 dB, a -3dB bandwidth above 110kHz, and a room temperature detectivity around 10¹² Jones.

Gerasimos Konstantatos remarked: "To the best of our knowledge, the photodiodes reported here have for the first time realised solution-processed, non-toxic shortwave infrared photodiodes with figures of merit comparable to heavy-metal counterparts." He further noted the material's RoHS compliance and its potential for low-cost, high-performance SWIR photodetector applications.

In collaboration with Qurv, the researchers constructed a SWIR image sensor using this new photodiode, integrated with a CMOS-based read-out integrated circuit (ROIC) focal plane array (FPA). This marked the first demonstration of a non-toxic, room temperature-operating SWIR quantum dot-based image sensor. The sensor's effectiveness was confirmed by imaging silicon wafer transmission and the contents of opaque plastic bottles under SWIR light.

Gerasimos Konstantatos explained: “Accessing the SWIR with a low-cost technology for consumer electronics will unleash the potential of this spectral range with a huge range of applications including improved vision systems for the automotive industry (cars) enabling vision and driving under adverse weather conditions. SWIR band around 1.35-1.40µm, can provide an eye-safe window, free of background light under day/night condition, thus, further enabling long-range light detection and ranging (LiDAR), three-dimensional imaging for automotive, augmented reality and virtual reality applications.”

The team is now focused on enhancing photodiode performance through layer stack engineering and exploring new surface chemistries for Ag2Te quantum dots. These efforts aim to improve the material's performance, thermal, and environmental stability, facilitating its journey to market adoption.