External Quantum Efficiency Performance Research Articles

Tailoring of colloidal quantum dot layer thickness for highly efficient short-wavelength infrared photodiode

Colloidal quantum dots (CQDs) have emerged as promising materials for thin film photodiodes (TFPDs) in the short-wavelength infrared detection range, offering an alternative to III–V and HgCdTe-based TFPDs. However, optimizing the structure of CQD-based TFPDs remains a challenge, as it involves a delicate balance between reducing dark currents and enhancing carrier extraction efficiency. In this study, we explore the influence of varying the thickness of CQD layers to achieve a highly efficient photodiode. Our investigations reveal a continuous reduction in the dark current as the CQD layers become thicker, but we observe fluctuation in the external quantum efficiency (EQE). To shed light on this relationship between dark current density (Jdark) and EQE, we conduct capacitance measurements and employ optical simulations. From the capacitance measurements, they demonstrate an increased depletion width with varying CQD thickness, apart from layers exceeding 500 nm in thickness. Leveraging optical simulations, we propose an optimal thickness for CQD-based TFPDs and compare its EQE performance. The optimized CQD-based TFPD exhibits a Jdark of 4.1 μA/cm2 and EQE of 56.5%, and the highest specific detectivity, based on the assumption of shot noise dominance, is 1.78 × 1012 Jones at a wavelength of 1420 nm.

Degradation analysis of highly UV-resistant down-shifting layers for silicon-based PV module applications

In this work, the external quantum efficiency (EQE) of [Eu(bta)3me-phen] downshifters (DS) encapsulated by industrial procedures on photovoltaic (PV) modules is presented. The samples have been laminated using the DS layers faced-up and faced-down. The results obtained for the DS layers present an EQE enhancement of 1.2 % for the faced-up samples and 0.8 % for the faced-down samples. A climate chamber was used to evaluate the EQE performance during the ageing process; a temperature of 22 °C and relative humidity of 30 % was fixed. Several time cycles have been used to achieve a total UV dose of 454.94 kWh. After a 136.51 kWh dose, the increment of the EQE for the faced-up samples vanishes. In contrast, after a dose of 454.94 kWh, the EQE enhancement for samples with the DS layer faced down remains unaltered.

Realization of high-efficiency AlGaN deep ultraviolet light-emitting diodes with polarization-induced doping of the p-AlGaN hole injection layer

We investigate the polarization-induced doping in the gradient variation of Al composition in the p-Al0.75Ga0.25N/Al x Ga1−x N hole injection layer (HIL) for deep ultraviolet light-emitting diodes (DUV-LEDs) with an ultra-thin p-GaN (4 nm) ohmic contact layer capable of emitting 277 nm. The experimental results show that the external quantum efficiency (EQE) and wall plug efficiency (WPE) of the structure graded from 0.75 to 0.55 in the HIL reach 5.49% and 5.04%, which are improved significantly by 182% and 209%, respectively, compared with the structure graded from 0.75 to 0.45, exhibiting a tremendous improvement. Both theoretical speculations and simulation results support that the larger the difference between 0.75 and x in the HIL, the higher the hole concentration that should be induced; thus, the DUV-LED has a higher internal quantum efficiency (IQE). Meanwhile, as the value of x decreases, the absorption of the DUV light emitted from the active region by the HIL is enhanced, reducing the light extraction efficiency (LEE). The IQE and LEE together affect the EQE performance of DUV-LEDs. To trade off the contradiction between the enhanced IQE and decreased LEE caused by the decrease in Al composition, the Al composition in the HIL was optimized through theoretical calculations and experiments.

Analysis of size dependence and the behavior under ultrahigh current density injection condition of GaN-based Micro-LEDs with pixel size down to 3 μm

In this paper, the GaN-based green micro light-emitting diodes (Micro-LEDs) with various sizes (from 3 to 100 μm) were fabricated and electro-optically characterized. Atom layer deposition (ALD) passivation and potassium hydroxide (KOH) treatment were applied to eliminate the sidewall damage. The size dependence of Micro-LED was systematically analyzed with current-versus-voltage and current density-versus-voltage relationship. According to the favorable ideality factor results (<1.5), the optimized sidewall treatment was achieved when the device size shrank down to <10 μm. In addition, the external quantum efficiency (EQE) droop phenomenon, luminance and output power density characteristics were depicted up to the highest current density injection condition to date (120 kA cm−2), and 6 μm device exhibited an improved EQE performance with the peak EQE value of 16.59% at 20 A cm−2 and over 600k and 6M cd cm−2 at 1 and 10 A cm−2, indicating a greater brightness quality for over 3000 PPI multiple display application. Lastly, the blue shift of 6 μm device with elevating current density was observed in electroluminescence spectra and converted to CIE 1931 color space. The whole shifting track and color variation from 1 A cm−2 to 120 kA cm−2 were demonstrated by color coordinates.

Simulation of β-Ga2O3 vertical Schottky diode based photodetectors revealing average hole mobility of 20 cm2 V−1 s−1

With a wide bandgap of ∼4.85 eV, high chemical and thermal stability, and melt growth availability, β-Ga2O3 has been found in a large number of solar blind photodetector (SBP) applications including missile guidance, flame detection, water purification, and intersatellite communication. The modelling of a Schottky diode (SD) based SBPs is crucial in order to reach high external quantum efficiency (EQE), especially for self-powered applications and also to extract hole mobility in these devices. The EQE performance of β-Ga2O3 vertical SD SBPs with various Schottky contact finger spacings is obtained using highly controversial hole mobility values reported in the literature. By modelling experimentally demonstrated EQE values of the existing β-Ga2O3 vertical SD SBPs, average nonequilibrium hole mobility value of ∼20 cm2 V−1 s−1 is extracted, which is quite higher than the claimed theoretical value of 1 × 10−6 cm2 V−1 s−1 and motivates for the efforts of technologically important p-type β-Ga2O3. By modelling the efficiency of full Schottky metal covered vertical SD SBPs by using hole mobility value of 20 cm2 V−1 s−1, internal quantum efficiency of 92% is obtained at an optimum n-type doping concentration of 1 × 1016 cm−3.

Influence of Zn–Si donor–acceptor pair traps in invert InGaP/GaAs dual-junction solar cell by MOCVD

In In(Ga)P materials, Zn atoms diffuse into the adjacent Si doping layer at high temperatures, forming Zn–Si donor–acceptor pairs which function as non-radiation carrier recombination centers. The traps lead to decreased carrier diffusion length and separation efficiency in the space charge region of InGaP subcell, resulting in degradation of photoelectric conversion efficiency. In this paper, by adopting an InGaP p–i–n junction structure, the invert growth thin film cell was able to reach the same external quantum efficiency performance as the upright InGaP/GaAs dual-junction solar cell. The Zn doping peak was not observed at the Si doping interface layer in the electrochemical capacitance–voltage measurement. A 675 nm peak was observed at 5 and 30 K and absent in 77 K, as was resolved by low-temperature PL measurement. The peak showed a strong correlation with the Zn diffusion into adjacent Si doping InGaP layer. We speculate that the 675 nm peak represented the energy of Zn–Si donor–acceptor pair traps.

Silicon quantum dot solar cell using top-down approach

The current trend of research in the area of third-generation photovoltaics is to increase the efficiency of solar cell device adopting alternate and novel ways such as use of quantum dots to absorb maximum solar spectrum. A new and low-cost top-down approach to fabricate silicon quantum dot solar cell (QDSC) using spin coating of Si QDs embedded in flowable oxide is proposed. Si QDs with diameter smaller than 8 nm were synthesized using top-down approach by ultrasonication of freestanding porous silicon films obtained by anodization of Si. Systematic measurements of current density–voltage (J–V), capacitance–voltage (C–V) and external quantum efficiency (EQE) were carried out. The QDSC exhibits photovoltaic effect, hysteresis in C–V characteristics and improved EQE performance in short wavelength as compared with a reference c-Si cell with same structure, but without QD layer. The novelty and simplicity of the QDSC fabrication process make these results important.

Design of integrated III-nitride/non-III-nitride tandem photovoltaic devices

The integration of III-nitride and non-III-nitride materials for tandem solar cell applications can improve the efficiency of the photovoltaic device due to the added power contributed by the III-nitride top cell to that of high-efficiency multi-junction non-III-nitride solar cells if the device components are properly designed and optimized. The proposed tandem solar cell is comprised of a III-nitride top cell bonded to a non-III-nitride, series-constrained, multi-junction subcell. The top cell is electrically isolated, but optically coupled to the underlying subcell. The use of a III-nitride top cell is potentially beneficial when the top junction of a stand-alone non-III-nitride subcell generates more photocurrent than the limiting current of the non-III-nitride subcell. Light producing this excess current can either be redirected to the III-nitride top cell through high energy photon absorption, redirected to the lower junctions through layer thickness optimization, or a combination of both, resulting in improved total efficiency. When the non-III-nitride cell’s top junction is the limiting junction, the minimum power conversion efficiency that the III-nitride top cell must contribute should compensate for the spectrum filtered from the multi-junction subcell for this design to be useful. As the III-nitride absorption edge wavelength, λN, increases, the performance of the multi-junction subcell decreases due to spectral filtering. In the most common spectra of interest (AM1.5 G, AM1.5 D, and AM0), the technology to grow InGaN cells with λN &amp;lt; 520 nm is found to be sufficient for III-nitride top cell applications. The external quantum efficiency performance, however, of state-of-the-art InGaN solar cells still needs to be improved. The effects of surface/interface reflections are also presented. The management of these reflection issues determines the feasibility of the integrated III-nitride/non-III-nitride design to improve overall cell efficiency.

Modeling of multi-junction solar cells for estimation of EQE under influence of charged particles using artificial neural networks

Abstract External quantum efficiency (EQE) of a solar cell provides information on the internal operations of the solar cells which can be used in optimization of solar cell design. The EQE of solar cells for space applications is adversely affected by the influence of charged particles in space. Usually numerical model based software, e.g., PC1D, are used to estimate the EQE and fitted with the measured EQE to obtain degradation performance of space solar cells. However, the accuracy of these models may be limited due to complex phenomena and interactions occurring between the junctions of the solar cells and the nonlinear influence of charged particles. In this paper we propose an artificial neural network (ANN)-based model to estimate the EQE performance of triple-junction InGaP/GaAs/Ge solar cells under the influence of a wide range of charged particles. Using the experimental data from Sato et al. [1] , it is shown that the ANN-based models provide a better estimate of the EQE than the PC1D model [1] in terms of mean square error and correlation coefficient.