Improvement of the MOCVD-grown InGaP-on-Si towards high-efficiency solar cell application

The thermal cycle annealing (TCA) for GaAs layer grown on Si substrate (GaAs/Si) increased the photoluminescence (PL) intensity of InGaP epilayer which was regrown on the GaAs/Si substrate by about 100 times. The full-width at half-maximum (FWHM) of double-crystal X-ray diffraction (DXRD) was decreased from 313 to 251 arcsec. From the electron-beam-induced current (EBIC) image measurements, the defect-related dark spots density (DSD) of the regrown InGaP layer was reduced by about 30% by using TCA GaAs/Si substrate. This means that TCA treatment for GaAs layer effectively increased the crystal quality of InGaP epilayer regrown on GaAs/Si substrate (InGaP/GaAs/Si). The PL intensity of InGaP epilayer was also enhanced due to the passivation of the residual defect-related nonradiative recombination centres by post-growth phosphine (PH₃/H₂=10%) plasma exposure.     

Quantum dot integration in heterostructure solar cells

InAs self-assembled quantum dots (SA-QDs) were incorporated into GaAlAs/GaAs heterostructure for solar cell applications. The structure was fabricated by molecular beam epitaxy on p-GaAs substrate. After the growth of GaAs buffer layer, multi-stacked InAs QDs were grown by self-assembly with a slow growth rate of 0.01 ML/s, which provides high dot quality and large dot size. Then, the structure was capped with n-GaAs and wide band gap n-GaAlAs was introduced. One, two or three stacks of QDs were sandwiched in the p–n heterojunction. The contribution of QDs in solar cell hetero-structure is the quantized nature and a high density of quantized states. I–V characterization was conducted in the dark and under AM1 illumination with 100 mW/cm² light power density to confirm the solar cell performance. Photocurrent from the QDs was confirmed by spectral response measurement using a filtered light source (1.1-μm wavelength) and a tungsten halogen lamp with monochromator with standard lock-in technique. These experimental results indicate that QDs could be an effective part of solar cell heterostructure. A typical I–V characteristic of this yet-to-be-optimized solar cell, with an active area of 7.25 mm², shows an open circuit voltage V_oc of 0.7 V, a short circuit current I_sc of 3.7 mA, and a fill factor FF of 0.69, leading to an efficiency η of 24.6% (active area).     

Use of Sb spray for improved performance of InAs/GaAs quantum dots for novel photovoltaic structures

Photoluminescence output from InAs/GaAs quantum dots has been improved by a Sb treatment immediately prior to capping with GaAs. Spectra taken at 300 and 80 K show a significant increase in output intensity when the quantum dots are exposed for 15 s under a Sb flux of approximately 0.1 monolayers per second, but this improvement is lost when the Sb exposure is extended to 30 s. There is no significant shift in the emission energies between samples indicating strain relief due to the cap layer is not responsible for the improvement. Analysis of temperature dependent photoluminescence taken between 80 and 300 K show increased activation energies at lower temperatures when an Sb spray is used, suggesting passivation of deep defect levels. For the higher temperature activation energy, corresponding to carrier escape from the QD to the barrier, whilst a 15 s Sb spray gives a substantial increase, the longer 30 s Sb spray sees the activation energy decrease, a result deduced to be due to Sb segregation providing shallow defect levels. A band structure including a very thin GaAsSb layer adjacent to the quantum dots is used to explain these results, with the 30 s Sb spray leading to shallow Sb segregation related defects and a lower activation energy. Depth dependent X-ray photoelectron spectroscopy data support the band structure proposed to explain the photoluminescence results and also reveals the highest concentration of Sb at the sample surface suggesting a ‘floating layer’ of Sb during growth of the GaAs cap. Some of the implications of these results, for growth of quantum dot samples and for two novel solar cell proposals, the intermediate band and hot carrier solar cells, are discussed.     

Improvement of life time of minority carriers in GaAs epi-layer grown on Ge substrate

A buffer layer structure on Ge substrate was studied for MOCVD growth of a high-quality GaAs layer. The buffer layer structure was designed taking into consideration both lattice constants and thermal expansion coefficients of GaAs and Ge. It consisted of a preliminarily grown thin layer of Al_xGa_1−xAs and a GaAs layer. Photoluminescence (PL) decay of a GaAs layer in an Alo_0.2Ga_0.8As-GaAs-Al_0.2Ga_0.8As double-hetero (DH) structure, which was grown on the buffer layer structure, was observed by time-resolved PL method to estimate the quality of epilayers in the DH structure. The PL decay time strongly depended on Al content (x) of the Al_xGa_1−x As preliminary layer, and the highest value was obtained when the x was 0.25. A PL decay time above 20 ns was successfully obtained for the DH structure grown on the buffer layer structure, which consisted of a 0.05 μm thick Al_0.25Ga_0.75As layer and a 1 μm thick GaAs layer. Although this value was half of that for the DH structure grown on GaAs substrate, it was much longer than the value of 3 ns for the DH structure grown on Ge substrate with a conventional GaAs buffer layer 1 μm thick.     

Thermal transport within quantum-dot nanostructured semiconductors

In this work, we aim at exploring the effects of the germanium quantum dot (QD) layer embedded in silicon thin films on the thermal transport property in use of the non-equilibrium molecular dynamics simulation tool. An attempt is made to distinguish and understand the effect of the QDs themselves and the effect of the wetting layer on which QDs are grown. In this study, we notice as often observed a significant increase in the thermal resistance due to heterogeneous interfaces. Moreover, it is found that a simple QD interface has a thermal resistance monotonically decreasing with increasing quantum dot density. It is probably because the QDs make the transition from one material to another smoother, alleviate the acoustic mismatch, and thus assist the energy transport. When the germanium QDs together with a germanium wetting layer is inserted into a silicon material, the involved interface thermal resistance decreases first but increases later with increasing quantum dot density. The competition between the roughness effect and the wave interference effect is employed to explain this variation trend. As far as the quantum-dot superlattice thin film is concerned, we find its effective thermal conductivity decreases monotonically with increasing quantum dot density and with decreasing film thickness. In all cases, the size of quantum dots affects little on the thermal resistance/conductivity.     

Optical properties of high-quality CuGaSe2 epitaxial layers examined by piezoelectric photoacoustic spectroscopy

The piezoelectric photoacoustic (PPA) signals for Cu-rich CuGaSe₂ (CGS) /GaAs (0 0 1) epitaxial layer (Cu/Ga=1.09–2.16) grown by molecular beam epitaxy (MBE) were successfully obtained at liquid-nitrogen temperature. The bandgap energies of CGS (A-band) decreased and GaAs was not almost changed with increasing the Cu/Ga ratios. This phenomenon was very similar to that of free exciton (FE) by photoluminescence (PL) and the lattice parameter c by X-ray diffraction (XRD) measurements.     

Analysis of photocurrent in the i-layer of GaAs-based n–i–p solar cell

A numerical investigation of the intrinsic layer effect on the improvement of GaAs n–i–p solar cell performances is presented. Solution of Poisson's equation together with continuity equations for electrons and holes allows the determination of carrier's density, electric field and recombination profiles within the i-layer. The analysis examines the effect of i-layer thickness on the electric field, recombination rate and collection efficiency. It is found that increasing the i-layer thickness increases the absorption while it reduces the electric field and increases the recombination rate. The three competing parameters have to be monitored simultaneously so as to obtain an optimal thickness. To achieve this, the variation of the total photocurrent is used as indicator. The photocurrent shows a sharp increase in the domain of very thin i-layers (<0.5 μm) then a saturation is reached for thicker layers (>1 μm), the simulation is performed for thicknesses up to 2 μm.     

Radioluminescent nuclear battery containing CsPbBr3 quantum dots: Application of a novel wave‐shifting agent

Radioluminescent nuclear battery has been widely studied for its miniaturization and long life. In this study, all‐inorganic perovskite quantum dots (CsPbBr₃ QDs) were selected as a novel wave‐shifting agent combined with liquid scintillator PPO (2,5‐diphenyloxazole). The QDs were used to regulate the emission spectrum to match different GaAs devices. The maximum output power of the RL nuclear battery was greatly enhanced by 1.91 to 2.53 times. Perovskite QDs with adjustable emission spectra were used as wave‐shifting agents and exhibited more excellent properties and application prospects than traditional wave‐shifting agents. The Monte Carlo method was used to simulate the energy deposition of fluorescent materials under various radioactive source models. Results verified the advantages of using liquid radioactive sources and liquid fluorescent materials. The application and reference value of perovskite QDs in nuclear detection and nuclear medical imaging were also discussed.     

Optimal growth procedure of GaInP/GaAs heterostructure for high-efficiency solar cells

We have developed an optimal growth procedure for gas-source MBE production of a GaInP/GaAs heterointerface. The interface quality is crucial to obtaining high-performance GaAs solar cells with a GaInP barrier layer because minority carrier lifetime depends strongly on the interface structure. In situ Reflective High-Energy Electron Diffraction (RHEED) observation during the growth across the GaInP/GaAs heterointerface revealed that the phosphorus atoms are replaced by arsenic atoms in the near-interface region of the GaInP layer, and a transient layer acting as a carrier trap is formed. Introduction of a GaP layer into the interface was found to be effective in suppressing carrier loss. From Composition Analysis by Thickness Fringe-Transmission Electron Microscopy (CAT-TEM) images, it was also found that the optimum thickness of inserted GaP to avoid the generation of misfit dislocations is 1 nm.     

Delta doping and positioning effects of type II GaSb quantum dots in GaAs solar cell

GaSb quantum dot (QD) solar cell structures were grown by molecular beam epitaxy on GaAs substrates. We investigate the reduction in open-circuit voltage and study the influence of the location of QD layers and their delta doping within the solar cell. Devices with 5 layers of delta-doped QDs placed in the intrinsic, n- and p-regions of a GaAs solar cell are experimentally investigated, and the deduced values of J_sc, V_oc, fill factor, efficiency (η) are compared. A trade-off is needed to minimize the V_oc degradation while maximizing the short circuit current density (J_sc) enhancement due to sub-bandgap absorption. The voltage recovery is attributed to the removal of the QDs from the high-field region which reduces SRH recombination. The devices with p- or n-doped QDs placed in the flat band potential (p- or n-region) show a recovery in J_sc and V_oc compared to devices with delta-doped QDs placed in the depletion region. However, there is less photocurrent arising from the absorption of sub-band gap photons. Furthermore, the long wavelength photoresponse of the n-doped QDs placed in the n-region shows a slight improvement compared to the control cell. The approach of placing QDs in the n-region of the solar cell instead of the depletion region is a possible route towards increasing the conversion efficiency of QD solar cells.     

Self-sacrificing template synthesis of CdS quantum dots/Cd-Hap composite photocatalysts for excellent H2 production under visible light

Well dispersive CdS quantum dots (QDs) were successfully in-situ grown on cadmium hydroxyapatite (Cd₅(PO₄)₃OH, Cd-Hap) assembled rods through a self-sacrificing hydrothermal method. No any nocuous organic ligands were used in such self-sacrificing route, allowing for a green approach to prepare CdS QDs with clean surfaces and enough active sites. The deposition of CdS QDs onto Cd-Hap surfaces led to a dramatically enhanced performance in H₂ production under visible light irradiation as compared to bulk CdS nanoparticles. The optimal CdS QDs/Cd-Hap composite displayed a H₂ evolution rate of 14.1 μmol h^?1 without using any noble metal cocatalyst, which was about 4.2 times higher than that of pristine CdS. The apparent quantum efficiency for CdS QDs/Cd-Hap composite was up to 18%. It was also found that CdS QDs/Cd-Hap composite can continuously generate H₂ from water in the presence of electron donors for more than 125 h. The enhanced photocatalytic performance of CdS QDs/Cd-Hap composites could be attributed to the high charge separation efficiency resulting from the efficient capture of photoinduced electrons by oxygen vacancies in Cd-Hap rods and the quantum confinement effect of CdS QDs with strong redox capacity as well as the increased active sites.     

Interfacial oxide layer mechanisms in the generation of electricity and hydrogen by solar photoelectrochemical cells

The effect of an oxide layer of tunneling dimensions, together with localized interface states at the optically-illuminated semiconductor-electrolyte interface, on the transport of electrons and holes from the semiconductor bands into the redox levels in solution is calculated analytically. The interface states are considered to affect the charge transport primarily in an electrostatic manner, by adjusting their charge with changes in the semiconductor surface potential.

A photovoltage, V, is developed in the oxidized semiconductor electrode, which divides between the space-charge region of the semiconductor V_s and the oxide layer V_ox. The majority-carrier current for a given total photovoltage depends primarily on V_s, and is therefore reduced from its value without an oxide layer. This has the effect of substantially increasing the open-circuit voltage and the photovoltaic conversion efficiency of a photo-electrochemical solar cell. The oxide layer is also expected to reduce, or in some cases to eliminate, the requirement for a supplementary external bias voltage in the production of hydrogen or other fuels from solar energy, using photosynthetic devices of this type.     

Enhancing the short-circuit current and efficiency of organic solar cells using MoO3 and CuPc as buffer layers

Efficient bulk-heterojunction (BHJ) (regioregular poly (3-hexylthiophene) (P3HT): (6, 6)-phenyl C₆₁ butyric acid methyl ester (PCBM)) solar cells were fabricated with molybdenum trioxide (MoO₃) and copper phthalocyanine (CuPc) as buffer layers. The insertion of MoO₃ layer was found to be critical to the device performance, effectively extracting holes to prevent the exciton quenching and reducing the interfacial resistance because of alignment of energy levels. The introduction of CuPc buffer layer was observed to be ameliorative for device performance, further enlarging the visible absorption spectra range of the devices. The effect of the MoO₃ and CuPc layer thickness on device performance was studied. The optimized thickness was achieved when MoO₃ layer was 12 nm and CuPc layer was 6 nm, resulting in optimized power conversion efficiency (PCE) of 3.76% under AM1.5G 100 mW/cm² illumination.     

Transport phenomena within the liquid phase of a laboratory-scale circular methanol pool fire

The effects of altering the lower thermal boundary condition of a methanol pool from −5 °C to 50 °C was investigated within a 90 mm diameter and 12 mm deep quartz burner under steady state burning condition in a quiescent air environment. Both the burning rate and the flame height were observed to increase by 15% with increasing bottom temperature over this range of bottom boundary conditions. The temperature and velocity within the liquid were measured by a single thermocouple traversed through the pool and PIV, respectively, in order to better understand the transport of mass and energy in the liquid. Temperature measurements revealed a distinct two-layer vertical thermal structure with the upper layer of the pool being almost uniform and near the boiling temperature of the fuel, while the lower layer experienced an increasing temperature gradient as the bottom boundary temperature was lowered. The thickness of the thermally uniform layer increased as the bottom temperature was increased. The measured fluid velocity showed a complementary two-layer structure with the upper layer being dominated by a pair of counter-rotating vortices that kept this portion of the liquid well mixed and transferred heat from the hot pool wall to the pool center, while the flow in the lower layer was uniformly low in value and vertical. A model was presented to aid in understanding the energy transfer within the liquid phase. In the lower layer, the Peclet Number was in the order of unity and required that the energy transfer throughout the liquid phase to be modeled as a combination of conduction and convection. Using this physical model, the change in burning rate over the full 55 °C change in bottom temperature was predicted within 2%, thereby supporting the proposed mechanism for energy transfer into the pool’s depth.     

Effects of CdS buffer layers on photoluminescence properties of Cu(In,Ga)Se2 solar cells

Photoluminescence (PL) have been studied on Cu(In,Ga)Se₂ (CIGS) thin films, CdS/CIGS and CIGS solar cells, to clarify the carrier recombination process. The chemical-bath deposition (CBD) of the CdS buffer layer on the CIGS thin film leads to (i) the enhancement of near-band-edge PL intensity by a factor of 2–3, (ii) change in energy of the defect-related PL and (iii) the slight change in the decay time. They are related not only to the minimization of the surface recombination but also to the modification of native defects at the Cu-poor surface of CIGS by the occupation of Cd atom at the Cu site. A donor–acceptor pair PL at low-temperature and temperature-dependent PL have been studied. They are discussed in terms of the impurity and defect levels created in the CIGS film during the CBD-CdS process.     

N型单晶硅衬底上非晶硅/单晶硅异质结太阳电池计算机模拟

采用德国HMI研发的AFORS-HET软件模拟了N型衬底非晶硅,单晶硅异质结太阳电池的特性,结果表明随着发射层厚度的增加,短路电流下降,电池的短波响应变差.在非晶硅,单晶硅异质结界面处加入不同的界面态密度(Dit).发现当Dit1012cm-2·eV-1时,电池的开路电压和填充因子均大幅减小,导致电池效率降低.当在非晶硅,单晶硅异质结界面处加入本征非晶缓冲层后,电池性能明显改善,但是缓冲层厚度应控制在30nm以内.模拟的a-Si/i-a-Si:H/c-Si/i-a-Si:H/n a-Si双面异质结太阳电池的最高转换效率达到28.47%.     

Numerical simulation of the effect of the Al molar fraction and thickness of an AlxGa1−xAs window on the sensitivity of a p+–n–n+ GaAs solar cell to 1 MeV electron irradiation

In this paper numerical simulation has been used to predict the effect of the thickness and aluminium (Al) mole fraction of an AlGaAs layer, used as a window for a p⁺–n–n⁺ GaAs solar cell under AM0 illumination and exposed to 1 MeV electron irradiation. Such solar cells are used in satellites and undergo severe degradation in their performance due to induced structural defects. The irradiation-induced defects are modelled as energy levels in the energy gap of GaAs. To predict this effect, the spectral response is evaluated for different electron irradiation fluences for two types of cells. In the first a narrow Al_0.31Ga_0.69As window is a small part of the p⁺ layer while in the second type the whole window is an Al_xGa_1?xAs layer with a gradual Al mole fraction. The obtained results show that the Al_xGa_1?xAs window with a gradual Al mole fraction improves the resistance of the solar cell to electron irradiation especially in the short wavelengths range.     

Simulation of laminar impinging jet on a porous medium with a thermal non-equilibrium model

This work shows numerical simulations of an impinging jet on a flat plate covered with a layer of a porous material. Macroscopic equations for mass and momentum are obtained based on the volume-average concept. Two macroscopic models are employed for analyzing energy transport, namely the one-energy equation model, based on the Local Thermal Equilibrium assumption (LTE), and the two-energy equation closure, where distinct transport equations for the fluid and the porous matrix follow the Local Non-Thermal Equilibrium hypothesis (LNTE). The numerical technique employed for discretizing the governing equations was the finite volume method with a boundary-fitted non-orthogonal coordinate system. The SIMPLE algorithm was used to handle the pressure–velocity coupling. Parameters such as porosity, porous layer thickness, material permeability and thermal conductivity ratio were varied in order to analyze their effects on flow and heat transport. Results indicate that for low porosities, low permeabilities, thin porous layers and for high thermal conductivity ratios, a different distribution of local Nusselt number at the wall is calculated depending on the energy model applied. The use of the LNTE model indicates that it is advantageous to use a layer of highly conducting and highly porous material attached to the hot wall.     

Designing performance enhanced nuclear battery based on the Cd-109 radioactive source

A dual-effect nuclear battery based on the radio-voltaic and radioluminescence effect was developed, which has the ability to convert nuclear energy into electrical energy with two different modes. Performance-enhanced nuclear batteries are mainly based on the addition of ZnS:Cu radio-luminescent layer to Cd-109 X-ray radioactive source and GaAs radio-voltaic layer. In order to explore the response relationship between the mode of energy conversion and the electrical performance of nuclear battery, the physical model was established to research the deposition energy distribution by using Monte Carlo method. The addition of the radio-luminescent material increases the effective energy deposition of the X-rays and the optimized thickness of ZnS:Cu in such a dual-effect nuclear battery should be set to 560 μm. The current–voltage characteristic curves of the batteries before and after performance optimization were utilized to investigate the electrical properties. Through a comprehensive comparison of Cd-109 nuclear batteries with or without radio-luminescent layer, the simulated results are consistent with experimental results. The results indicate that the electrical performance of dual-effect nuclear battery is significantly higher than that of single radio-voltaic nuclear battery. Moreover, the energy conversion efficiency increases from 0.079% (single radio-voltaic nuclear battery) to 0.119% (dual-effect nuclear battery). The improved performance of the dual-effect nuclear battery provides potential applications for space-based autonomous remote sensors and continuous low-power generation technologies.     

多级鼓泡式加湿除湿型太阳能海水淡化装置研究

提出一种多级鼓泡式加湿除湿型海水淡化装置。该装置主要由多曲面太阳能聚光系统、加湿层和除湿层以及相应的泵和管路组成。经聚光器加热后的高温空气在风机驱动下分别进入加湿层和除湿层,热风穿过各级筛板及筛孔,产生气泡,增大了空气与水的接触面积,强化了传热传质过程。在不同天气下对装置进行实验研究,结果表明：在晴朗天气下,装置的太阳能利用率最高可达0.41,淡化装置效率最大可达1.23,最大产水速率为3.66 kg/h,全天产水量为17.08 kg;在非晴朗天气下,装置产水量为12.43 kg。