Multicolor silicon-quantum-dot light-emitting diodes

Quantum dots (QDs) based on different elements and compounds feature great promise for novel applications such as biolabeling or optoelectronic devices. A team of researchers recently reported efficient light-emitting diodes based on semiconductor QDs,¹ while another group realized tunable QD lasers.² However, the toxicity of the elements used for these devices—such as cadmium sulfide, cadmium selenide, and their lead-containing counterparts—is a severe drawback for many applications and may even impede the commercialization of these novel QD-based applications.³

Among the more promising candidates, silicon quantum dots (SiQDs) seem to be ideally suited to this task due to silicon's non-toxicity, earth abundance, and its domination of the microelectronics and photovoltaics industry. As bulk silicon is an indirect semiconductor, it is only possible to achieve significant light emission under strong confinement conditions occurring for SiQDs with a size of about 5nm or less.⁴ Recent reports of very high photoluminescence quantum yields,^{5, 6} novel synthesis routes, colloidal stability, and lack of cytotoxicity of SiQDs^{7, 8} have done much to encourage study of LEDs based on these nanoparticles.^9–11

Using separation by size-selective precipitation, as recently reported,⁵ we were able to synthesize allylbenzene-capped SiQDs of different sizes and achieve surprisingly narrow size distributions. In our study, the resulting quantum dots showed bright and intense photoluminescence (PL) with peaks at 680nm, 650nm and 625nm, corresponding to approximately 1.8nm-, 1.6nm-, and 1.3nm-sized SiQDs, respectively.⁵ We used these novel, efficiently luminescent, and colloidally stable SiQDs as part of a light-emitting material in hybrid organic-QD-based light-emitting diodes: see Figure 1(a) for a schematic representation of the device architecture.

Figure 1. Device architecture and electroluminescence (EL) measurements. (a) Schematic representation of the stack of silicon-quantum-dot light-emitting diodes (SiLEDs).¹² (b) EL intensity over time at constant current of 1.6mA/cm² for size-separated and not-size-separated silicon quantum dots for light emission, respectively. (c) EL spectra as a function of the applied voltage (3.5, 4.5, 8, and 10V). We achieved a reduced shift of the emission wavelength of about 15nm using size-separated samples. The inset shows a shift of the EL maximum averaged over three different SiLEDs. LiF/Al: Lithium fluoride/aluminium. TPBi: 1,3,5-tris(N-phenylbenzimidazol-2,yl)benzene. SiQDs: Silicon quantum dots. Poly-TPD: Poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine]. PEDOT:PSS: Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate). ITO: Indium tin oxide. a.u.: Arbitrary units.

Compared to II-VI semiconductor QDs, and despite very promising first reports on SiQD-based light-emitting diodes (SiLEDs),^9–11 the tunability of the emission wavelength and the relatively short lifetimes of the devices under operation remained a challenge. We developed a new way to achieve both significantly longer lifetimes and advanced tunability by using size-separated SiQDs.¹²

The use of size-separated, nearly monodisperse SiQDs enabled us to produce bright and highly efficient SiLEDs featuring homogenous electroluminescence (EL) in the red spectral region that is stable over extended periods of time. We compared devices built with and without size-separated samples and showed that size separation significantly improves the long-term stability of the devices: see Figure 1(b). We achieved operation times of over 40 hours and of 15 hours for devices built with and without size-separated samples, respectively. Further, the corresponding 1/e-lifetime increased by a factor of 30. We also observed a drastically reduced shift of the maximum of the emission wavelength as a function of the applied voltage: see Figure 1(c). While others reported large shifts of the EL of over 50nm,¹¹ our devices feature only a small shift of 15nm, which might be further reduced by using higher-precision size separation.¹³

In addition to the discussed red-emitting SiLEDs, we fabricated devices luminescing at different colors depending on the particles' sizes. Figure 2(a) shows the PL and EL of the three corresponding QDs solutions and SiLEDs, respectively. There is an excellent agreement between the PL and EL of different emitters, confirming that the emission indeed originates from the SiQDs. We can easily tune the emission wavelength of the devices from the deep red (680nm) down to the orange/yellow (625nm) spectral region by changing the size of the used size-separated SiQDs. Figure 2(b) shows a photograph of the described devices encapsulated and driven under ambient conditions. The luminescent areas are 5 × 5mm each and show the EL of 1.8nm- and 1.6nm-sized SiQDs, respectively.

Figure 2. Emission properties of SiLEDs featuring different emission wavelengths. (a) Comparison of electroluminescence (EL) and photoluminescence (PL) of the different-sized emitters. (b) SiLEDs connected to a 9V battery in series to an ohmic resistor. We took the individual photographs at ambient lighting conditions and did not modify them with any image-processing software.

In summary, we have achieved bright, efficient, and long-term stable EL from silicon quantum dots for the first time. By using size-separated nanoparticles, we were able to significantly improve the performance of our SiLEDs. Besides increasing device lifetimes under operation, we showed that the voltage-dependent shift of the EL is much less pronounced when using nanoparticle samples containing only particles of nearly the same size. We also fabricated defect-free luminescent areas by simple solution processing and, using size-separated particles of different diameters, realized SiLEDs emitting at different wavelengths.

There is much potential here. New classes of functional surface molecules could allow access to an even broader spectral-tuning range. Additionally, surface molecules featuring advantageous electric properties for a better charge transport inside the LED might significantly boost the electrical conductivity of the SiQD layer and hence lead to even more enhanced device performance. Further work will concentrate on better understanding the degradation processes occurring in the LED as well as on novel chemical approaches to make the nanoparticles more stable against chemical degradation.

G.A.O. thanks the Natural Sciences and Engineering Council of Canada and the Ministry of Science, Research and the Arts of Baden-Württemberg for strong and sustained support of this work at University of Toronto and Karlsruhe Institute of Technology, respectively. F.M.-F. acknowledges generous support by the Karlsruhe School of Optics & Photonics.