Electrically-driven single-photon sources based on colloidal quantum dots with near-optimal antibunching at room temperature

Abstract

Photonic quantum information requires high-purity, easily accessible, and scalable single-photon sources. Here, we report an electrically driven single-photon source based on colloidal quantum dots. Our solution-processed devices consist of isolated CdSe/CdS core/shell quantum dots sparsely buried in an insulating layer that is sandwiched between electron-transport and hole-transport layers. The devices generate single photons with near-optimal antibunching at room temperature, i.e., with a second-order temporal correlation function at zero delay (g ⁽²⁾(0)) being <0.05 for the best devices without any spectral filtering or background correction. The optimal g ⁽²⁾(0) from single-dot electroluminescence breaks the lower g ⁽²⁾(0) limit of the corresponding single-dot photoluminescence. Such highly suppressed multi-photon-emission probability is attributed to both novel device design and carrier injection/recombination dynamics. The device structure prevents background electroluminescence while offering efficient single-dot electroluminescence. A quantitative model is developed to illustrate the carrier injection/recombination dynamics of single-dot electroluminescence.

Fig. 1

Optical properties of a single-photon device. a Schematic diagram of the key components of the device. b, Electroluminescence (driven at 2.4 V) and c, the corresponding photoluminescence (excited by 450 nm continuous wave laser) microscopic images. Scale bar: 5 μm. Green circles in b and c are shown as visual guides to highlight the relevant dots. d Electroluminescence spectrum (blue curve, driven at 2.8 V). The noise counts from the detector are also plotted (red curve). e g ⁽²⁾(τ) curve of a quantum-dot driven at 2.6 V. f Averaged electroluminescence counts of a single dot under different driving voltages (y-axis in log-scale)

Fig. 2

Optimization of the isolating film thickness. a Electroluminescence spectra of a device inheriting the conventional LED structure, i.e., isolated quantum dots directly sandwiched between the Poly-TPD and ZnO (without the PMMA layer, see inset for schematic drawing), at 3.0 V, 2.5 V, and 2.0 V (from top to bottom). The intensity of electroluminescence spectrum at 2.0 V is magnified by a factor of 20. b Electroluminescence spectra of the devices with isolated quantum dots embedded in PMMA layers with thicknesses of 17 nm, 12 nm, 10.4 nm and 0 nm, respectively (from top to bottom). The intensity of electroluminescence spectrum of device with a PMMA layer thickness of 17 nm is magnified by a factor of 50. The spectra are taken at a bias of 2.8 V. c Voltage-dependent integrated electroluminescence intensity from the isolated quantum dots (red squares) and Poly-TPD (blue triangles) in a device with a 12-nm PMMA layer. For quantitative comparison, all spectra were acquired from a 1-mm² emitting area of the devices. Red shaded areas (600–660 nm) and blue shaded areas (400–500 nm) in a and b illustrate the integration ranges used for quantum dot and Poly-TPD electroluminescence intensity calculations shown in (c), respectively

Fig. 3

Single-dot electroluminescence and single-dot photoluminescence. a Lowest g ²(0) values from the two sets of 9 quantum dots under electro-excitation (blue) and photo-excitation (red), respectively, The g ⁽²⁾(0) value equals the average of four data points around zero time delay and error bars depict the standard error of these four measurements. b Carrier dynamics of photoluminescence for a non-blinking quantum dot. The violet curved arrows indicate the generation of a new electron–hole pair by absorbing a single excitation photon, while the straight red arrows indicate the recombination of one individual electron–hole pair. c Schematic drawings of carrier dynamics of electroluminescence. Here $∣\mathrm{g}⟩,∣{\mathrm{e}}^{-}⟩,∣{\mathrm{h}}^{+}⟩,∣{\mathrm{n}e}^{-}⟩,∣{\mathrm{n}h}^{+}⟩,∣\mathrm{X}⟩,∣{\mathrm{X}}^{-}⟩,$ $∣{\mathrm{X}}^{+}⟩,∣{\mathrm{X}}^{2+}⟩,∣{\mathrm{X}}^{2-}⟩,\mathrm{and}∣\mathrm{B}X⟩$ represent ground state, single-electron state, single-hole state, n-electron state, n-hole state, exciton state, negatively charged-exciton state, positively charged-exciton state, three-hole-one-electron state, three-electron-one-hole state, and bi-exciton state, respectively. Note that the rare cases of one dot accumulating more than four carriers are ignored. The curved blue (green) up (down) arrows indicate the injection of a hole (electron) into a quantum dot

Fig. 4

Single-dot g ⁽²⁾(τ) at different excitation conditions. a,b Antibunching curves of single-dot photoluminescence and electroluminescence at different excitation levels, respectively. N _av in a, which is proportional to the excitation power, stands for the number of photons absorbed per lifetime cycle of excitons. c The dependence of simulated g ⁽²⁾(0) on the relative pumping rates (k _X = 25 μs⁻¹ is the decay rate of an exciton) for photoluminescence (red curve) and electroluminescence at the situation of balanced charge injection (blue curve). Red squares and blue triangles are experimental data for photoluminescence and electroluminescence from respective single-quantum dots, respectively. Standard deviations were calculated based on the values of four data points around zero time delay and plotted as error bars