Wearable Health Monitoring with Ultrathin Self-Powered Heavy-Metal-Free Cu-In-Se Quantum Dot-Photodetectors

Photodetectors (PDs), that transform light signals into electronic signals, are essential components in wearable electronics, finding wide-ranging applications in biological imaging, optical communication, and health monitoring. In particular, wearable health monitoring devices utilizing photoplethysmography (PPG) technology, which employs a light source and PDs to measure blood circulation variations, offer significant potential for non-invasive, cost-effective, and continuous real-time monitoring of vital signs and cardiac function. To enable effective real-time vital sign monitoring, PDs must exhibit high specific detectivity (<i>D*</i>), rapid response time, and high mechanical deformability. Consequently, the development of mechanically deformable PDs with superior device performance and response times has been a paramount objective. While prior research advanced flexible and stretchable PDs, challenges endure, including limited deformability with thick inorganic films and reduced performance in organic PDs. Furthermore, most photodetectors rely on external power sources to generate photocurrent, restricting the evolution of point-of-care type wearable systems capable of independent, wireless, and sustainable operation.<br/>Colloidal quantum dots (QDs) are highlighted for PDs, boasting size-tunable electrical and optical properties, high photo-absorption coefficients, narrow emission and absorption bandwidth, and robust photo- and air-stability. Nevertheless, the practical application of QD-PDs in wearable electronics faces challenges. Existing QD-PDs exhibit limited mechanical deformability due to their relatively thick light absorption layers (typically several hundreds of nanometers). Furthermore, the conventional use of toxic heavy-metal-containing QDs, including lead, mercury, and cadmium chalcogenide QDs, raises concerns about impact on human health.<br/>Recently, heavy-metal-free Cu-In-Se (CISe) QDs been highlighted as a promising material for eco-friendly optoelectronic devices. CISe QDs offer advantageous characteristics, including a direct-bandgap structure, high absorption coefficient, broad absorption range spanning from ultra-violet to near-infrared, stable phase structure, cost-effectiveness, and nontoxicity. However, a dearth of research exists regarding the selection of optimal charge transport materials, specifically for the electron transport layer (ETL) and hole transport layer (HTL) materials, tailored to CISe QD-PDs. The electron mobility of typical ETLs is generally three orders of magnitude higher than that of general HTLs, causing the charge imbalance and recombination of photogenerated electrons and holes, leading to poor device performance. Therefore, the development of high-quality HTLs is essential to achieve high-performance CISe QD-PDs.<br/>In this study, we present ultrathin self-powered CISe QD-PDs for the wearable PPG-based health monitoring system. We constructed PDs with CISe QDs capped with iodide ions as ultrathin light absorption layers (~40 nm), <i>p</i>-type colloidal MoS₂ nanosheets blended with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) as HTLs, and <i>n</i>-type ZnO nanoparticles as ETLs. Fabricated ultrathin PDs (~120 nm except electrodes) feature high <i>D*</i> of 2.10×10¹² Jones, linear dynamic range of 102 dB, and spectral sensitive region from 250 to 1,050 nm at 0 V bias. These PDs utilize built-in potential created through the photovoltaic effect, facilitating the efficient separation and transfer of photogenerated electron−hole pairs without requiring an external power source. Their performance rivals that of conventional QD-PDs employing thick Pb-chalcogenide QD layers and typical self-powered PDs from previous researches. Furthermore, PDs fabricated on flexible substrates exhibit outstanding mechanical flexibility and device performance, surpassing that of flexible PDs employing alternative semiconductor materials. This realize skin-attachable PPG sensors capable of real-time vital sign monitoring.