Infrared Optoelectronics Based on Colloidal Nanocrystals

Traditional infrared (IR) technology has relied on the epitaxial growth of CdHgTe (MCT) and InGaAs since 1960s. While these epitaxial materials are well-established, they have not become cost-effective products for the consumer market, limiting their applications. Solution-processed nanocrystals (NCs) exhibit tunable optical properties spanning from the UV to THz spectra, making them suitable for applications such as single photon emission, biolabeling, and down conversion for displays. Although the visible spectrum is commonly utilized, the potential of NCs in the IR range has been largely overlooked. For the infrared spectral range, organic semiconductors are intrinsically inefficient, leaving inorganic NCs as the most cost-effective and efficient option. Here, we present our latest achievements in infrared light-emitting diodes (LEDs) and photodiodes (PDs), operating across various wavelengths using tunable emissive colloidal materials. Our focus has been on CdHgSe nanoplatelets (NPLs) and RoHS-compliant InAs quantum dots (QDs).<br/>Mercury chalcogenides (eg. HgSe and HgTe) exhibit the most efficient emission in the near-IR to mid-IR range. However, these materials are inherently fragile, making it extremely challenging to grow a shell over them. To address this, we developed an innovative synthesis method starting with CdSe NPLs. We performed cation exchange, replacing Cd with Hg, and subsequently grew a thick layer of CdZnS over the resulting Cd_xHg_1-xSe NPLs. This procedure allows fine tuning by adjusting the concentration of Hg cations and the stoichiometric ratio of Cd/Hg, mirroring the epitaxial growth of MCT. Utilizing this tunable short-wave infrared (SWIR) emitting material, which achieves a photoluminescence quantum yield (PLQY) of 55%, we designed and fabricated LEDs that emit at wavelengths ranging from 1200 nm to 1700 nm. This builds on our previous work, where we achieved an external quantum efficiency (EQE) of 7.5% at 1300 nm.¹ Here, we also showed the potential of using the same material as the active layer of PD, with an EQE of 25% at 1200 nm wavelength.<br/>In parallel, with an innovative synthesis of InAs/ZnSe core/shell QDs we increased the thickness of ZnSe thick shell. Synthesizing InAs QDs has traditionally been a complex process, typically requiring the use of highly reactive, flammable, toxic, and expensive chemicals such as tris-trimethylsilyl arsine (TMS-As). In recent years, researchers have sought to replace TMS-As with cheaper, safer, and less reactive arsenic precursors. Among the alternatives explored, tris(dimethylamino)-arsine (amino-As) has shown the most promise. Using amino-As along with Alane N,N-dimethylethylamine as a reducing agent and ZnCl₂ as an additive, we developed a method to synthesize InAs QDs and InAs/ZnSe core/shell QDs with a shell thickness 1.5 monolayers which exhibit photoluminescence (PL) at 860 nm and a PLQY of 42% ± 4%.² Utilizing these QDs, we produced an LED with a turn-on voltage of 2.7V, an EQE of 5.5%, and a maximum radiance of 0.2 Wsr⁻¹cm⁻².³ Building on these findings, we refined our synthesis process to create InAs/ZnSe QDs with a tunable ZnSe shell thickness of up to 7 monolayers, achieving a remarkable PLQY of approximately up to 70% ± 7% and a PL peak at 900 nm in solution.⁴ The electronic structure of the thick-shell QDs resembles that of type-I heterostructures, enhancing exciton confinement in the core region due to the ZnSe layer. We utilized these efficient QDs for making LEDs. The champion LED reaches an EQE of 13.3% and radiance of 12 Wsr⁻¹cm⁻², figures-of-merit that are comparable to devices based on complex core/multi-shell InAs QDs obtained via a tris-trimethylsilyl (TMS) arsine route.⁵<br/><br/>1. Prudnikau, A. <i>et al.</i> <b><i>Adv. Funct. Mater.</i></b> 34, (2024)<br/>2. Zhu, D. <i>et al.</i> <b><i>J. Am. Chem. Soc.</i></b> 144, 10515–10523 (2022)<br/>3. De Franco, M. <i>et al.</i> <b><i>ACS Energy Lett.</i> </b>7, 3788–3790 (2022)<br/>4. Zhu, D. <i>et al.</i> <b><i>Adv. Mater.</i></b> 35, (2023)<br/>5. Roshan, H. <i>et al.</i> <b><i>Adv. Sci.</i></b> (2024) doi:10.1002/advs.202400734