Haiyang Li

Quantum dot white light emitting diodes with high scotopic/photopic ratios

Alloy core/shell CdxZn1-xS/ZnS quantum dots (QDs) are emerging as robust candidates for light-emitting diodes (LEDs), however the emission range of the current CdxZn1-xS/ZnS is quite limited, ranging from 390 to 470 nm. It still remains a challenging task to construct white LEDs based on current CdxZn1-xS/ZnS system. Here, a versatile ZnSe with a moderate band gap is introduced onto the Cd0.1Zn0.9S core. The ZnSe shell, on one hand, can passivate the core surface which leads to bright emissions. On the other hand, it is essential in extending the emission to red region so that the emission wavelengths of Cd0.1Zn0.9S/ZnS and Cd0.1Zn0.9S/ZnSe QDs can cover the whole visible region, which is very important for white LED applications. Two- and four-hump QD-based LEDs are computationally and experimentally investigated. Results show that four-hump quantum dot light-emitting diodes (QLED) have better performances than the two-hump one, in the luminous and the vision properties. The fabricated white LEDs (WLEDs) based on Cd0.1Zn0.9S/ZnS and Cd0.1Zn0.9S/ZnSe QDs exhibits a scotopic/photopic ratio (S/P) ratio as high as 2.52, which exceeds the current limit of 2.50 by common lighting technologies, a color rendering index of 90.3, a luminous efficacy of optical radiation of 460.78 lumen per unit optical power, and a correlated color temperature of 5454 K. These results suggest that CdxZn1-xS/ZnS and CdxZn1-xS/ZnSe quantum dots serving as emitters hold great promise for the next-generation white light source with better S/P ratio.

Role of ytterbium-erbium co-doped gadolinium molybdate (Gd 2 (MoO 4 ) 3 :Yb/Er) nanophosphors in solar cells.

Insufficient harvest of solar light energy is one of the obstacles for current photovoltaic devices to achieve high performance. Especially, conventional organic/inorganic hybrid solar cells (HSCs) based on PTB7 as p-type semiconductor can only utilize 400-800 nm solar spectrum. One effective strategy to overcome this obstacle is the introduction of up-conversion nanophosphors (NPs), in the virtue of utilizing the near infrared region (NIR) of solar radiation. Up-conversion can convert low-energy photons to high-energy ones through multi-photon processes, by which the solar spectrum is tailored to well match the absorptive domain of the absorber. Herein we incorporate erbium-ytterbium co-doped gadolinium molybdate (Gd2(MoO4)3, GMO), denoted as GMO:Yb/Er, into TiO2 acceptor film in HSCs to enhance the light harvest. Here Er3+ acts as activator while Yb-MoO4 2- is the joint sensitizer. Facts proved that the GMO:Yb/Er single crystal NPs are capable of turning NIR photons to visible photons that can be easily captured by PTB7. Studies on time-resolved photoluminescence demonstrate that electron transfer rate at the interface increases sharply from 0.65 to 1.42 × 109 s-1. As a result, the photoelectric conversion efficiency of the GMO:Yb/Er doped TiO2/PTB7 HSCs reach 3.67%, which is increased by around 25% compared to their neat PTB7/TiO2 counterparts (2.94%). This work may open a hopeful way to take the advantage of those conversional rare-earth ion doped oxides that function in tailoring solar light spectrum for optoelectronic applications.

Bright alloy type-II quantum dots and their application to light-emitting diodes

Type-II quantum dots (QDs) are emerging as a promising candidate for full color light sources owing to their advantages in achieving full color light by tuning the heterostructures. Despite the recent developments in type-II QDs, the choices of proper materials are limited for the composition of a high-quality QD and it still remains a big challenge to enhance the photoluminescence (PL) quantum yields (QYs) of type-II QDs for light-emitting diode (LED) applications. Here, we develop CdxZn1-xS/ZnSe/ZnS type-II QDs with a maximum quantum yield as high as 88.5%. Time-resolved PL results show that the ZnS shell suppresses non-radiative pathways by passivating the surface of CdxZn1-xS/ZnSe, thus leading to a high QY. Moreover, our results demonstrate that the outer ZnS also benefits the charge injection and radiative recombinations of the CdxZn1-xS/ZnSe. The LED based on green Cd0.2Zn0.8S/ZnSe/ZnS QDs achieves a current efficiency (CE) of 9.17cdA-1, an external quantum efficiency (EQE) of 8.78% and a low turn-on voltage of ∼2.3V.

Sodium Gadolinium Fluoride Nanophosphor-Based Solar Cells: Toward Subbandgap Light Harvesting and Efficient Charge Transfer

In this paper, we have synthesized erbium and ytterbium codoped sodium gadolinium fluoride (NaGdF4:Yb/Er) nanophosphors (NPs), aiming to extend the solar light harvest of PTB7 from visible into near-infrared. Evidence shows that Yb concentration plays an important role in upconversion, because it can inhibit the back energy transfer process and absorb considerable low-energy photons. Subsequently, NaGdF4:Yb/Er NPs have been incorporated into the photocatalytic titania (TiO2) nanoparticle layer to probe into electron transfer dynamics, and the photovoltaic performance of the assembled solar cells has been explored. The results show that NaGdF4:Yb/Er NPs excited at 976 nm present green and red emissions. After interfacing with bare or NP-doped electron donor TiO2, the lifetime of the emerged electron transfer has been shortened from 840 to 466 ps, and correspondingly, the electron transfer rate outstrips that of the bare TiO2 by a factor of 2.6. Consequently, an efficiency enhancement has been obtained with power conversion efficiency increasing to 3.61% from 2.81% of pure TiO2/PTB7. This work provides an efficient and facile approach to enhance the device performance by the codoping of robust rare-earth ions to widen the harvesting range of solar spectrum, boost electron transfer rate, and eventually strengthen the device performance.