• Energy Research

Abstract We succeeded in growing CZ monocrystalline silicon crystals with a longer lifetime than previously achieved. The MCZ technique was not used; instead, we employed melt-phobic quartz crucibles in a conventional CZ furnace. The improved lifetime is the result of reduced carbon incorporation into the growing crystals due to the suppression of SiO evaporation from the melt in the melt-phobic crucible. The melt-phobic effect of our crucibles has the potential to control the convection of molten silicon.

The impact of the post-implantation annealing conditions on the electrical characteristics of P-implanted homogeneous emitter silicon solar cells with aluminum-back surface field, 156 mm × 156 mm in size, is investigated. Based on a measurement of the internal quantum efficiency (IQE) and scanning capacitance microscopy observation, we discuss the behavior of the phosphorous prepared by annealing in a nitrogen atmosphere (PIA) and oxidation (PIO) after implantation of P and its impact on the cell characteristics. Implantation of P with step rotation was implemented while wafer made one rotation. A uniform thickness amorphous layer was formed in each side wall of the texture. The $\boldsymbol{p}$ - $\boldsymbol{n}$ junction depth of a P-implanted emitter with PIO is deeper than that of an emitter with PIA. Sheet resistance of P-implanted emitter with PIO shows higher value compared to that of PIA. The solar cell with PIO indicated a higher IQE at shorter wavelength. Solar cells with PIO show an improvement in the short-circuit current, open-circuit voltage, and conversion efficiency compared to cells subjected to PIA. A maximum conversion efficiency of 19.41% was obtained.

We attempted to improve the performance of passivated emitter and rear cells by increasing the quality of the aluminum back surface field (Al-BSF) beneath the local contact. We demonstrate that a thicker Al-BSF can be obtained by adding Si to the screen-printing Al paste, offering a greater penetration depth of several micrometers compared to the silicon-free Al paste, thereby increasing the open-circuit voltage. The presence of Si in the Al paste is believed to suppress the driving force for the strong lateral Si diffusion toward the Al layer during alloying, as evidenced by 50% decrease in the penetration depth of the Al–Si melt. The electron probe microanalyzer element analysis indicates that the Si-free Al paste produced the combination of hypo- and hypereutectic structures, while the use of the Al paste containing Si resulted in a hypoeutectic alloy, which has a strong presence of the primary α -Al dendrites. Different BSF thicknesses can be explained by that the liquid Al containing Si within the openings is saturated faster than the Si-free Al liquid under the same conditions. These findings allow a better understanding of the mass transport phenomena stimulated by the diffusion of Si and Al during the alloying process.

AbstractMultijunction (MJ) solar cells achieve very high efficiencies by effectively utilizing the entire solar spectrum. Previously, we constructed a III‐V//Si MJ solar cell using the smart stack technology, a unique mechanical stacking technology with Pd nanoparticle array. In this study, we fabricated an InGaP/AlGaAs//Si three‐junction solar cell with an efficiency of 30.8% under AM 1.5G solar spectrum illumination. This efficiency is considerably higher than our previous result (25.1%). The superior performance was achieved by optimizing the structure of the upper GaAs‐based cell and employing a tunnel oxide passivated contact Si cell. Furthermore, we examined the low solar concentration performance of the device and obtained a maximum efficiency of 32.6% at 5.5 suns. This performance is sufficient for realistic low concentration photovoltaic applications (below 10 suns). In addition, we characterize the reliability of the InGaP/AlGaAs//Si three‐junction solar cell with a damp heat test (85 °C and 85% humidity for 1000 h). It was confirmed that our solar cells have high long‐term stability under severe conditions. The results demonstrate the potential of GaAs//Si MJ solar cells as next‐generation photovoltaic cells and the effectiveness of smart stack technology in fabricating multijunction cells.

The “SMAC module” is a low-cost, high-efficiency photovoltaic module that integrates three techniques: a “SMart stack,” “Areal current matching,” and “solar Concentration.” This paper presents the result of a proof-of-concept study of the SMAC module conducted using device simulations and indoor experiments. The simulation results show that an SMAC module with a two-terminal GaAs/Si tandem solar cell can achieve an efficiency of approximately 30% and superior electricity generation per unit top cell area. The performance of the GaAs/Si solar cell developed in this study is similar to that of a GaAs/InGaAsP solar cell under concentrated artificial sunlight and is consistent with the simulation results. © 2016 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd.

Abstract This paper presents the development of industrial-sized p-type passivated emitter and rear cells (PERCs) with a selective emitter (SE) structure. We focus on different assisted passivation schemes and use nitric acid oxidation of silicon (NAOS) to form an ultrathin SiO2 layer. The results show that all I–V parameters of the PERCs are significantly modified when SiO2 is present on the front and/or rear sides of the cell. For front-emitter passivation by NAOS-SiO2, we observe an increase in the open-circuit voltage (Voc) and short-circuit current density (Jsc) and strong enhancement of the internal quantum efficiency (IQE) for short-wavelength photons. This can be attributed to the high level of chemical passivation, as indicated by the reduced interface trap density (Dit), due to the reduced Shockley–Read–Hall recombination. By contrast, all I–V parameters associated with SiO2 rear passivation decrease due to the increased Dit, which results in enhanced surface recombination. The different types of surface passivation for n-type (front) and p-type (rear) surfaces are presumably due to different doping surface concentrations. These results suggest that NAOS surface pretreatment is a very promising technique to improve the level of surface passivation and thereby enhance the performance of industrial-sized PERCs.

This paper introduces a selective phosphorus emitter formed by screen-printed resist masking combined with a wet chemical etch-back process for an industrial-sized passivated emitter and rear cell (PERC). Applying the selective emitter (SE) concept is expected to decrease the recombination losses at the front surface of the PERC cell. With the SE structure, we observed an increase in the open-circuit voltage $(V_{{\rm{oc}}})$ and short-circuit current density $(J_{{\rm{sc}}})$ , but a decrease in the fill factor (FF), compared with homogeneous emitter cells. The sheet resistance $(R_{{\rm{sheet}}})$ of the lightly doped emitter (n+-emitter) by the etch-back process had a considerable impact on the $V_{{\rm{oc}}}$ and $J_{{\rm{sc}}}$ , which we attributed to the reduced emitter saturation current density $(J_{0e})$ caused by a reduction in the Auger recombination in the n+ -emitter. The diminished FF was owing to the higher $R_{{\rm{sheet}}}$ , resulting in an increased series resistance of the cell. These results suggest that an SE structure made by the wet etch-back process is a very promising technology to improve the conversion efficiency of industrial-sized PERC solar cells.

BBr3 and POCl3 thermal diffusion is a widely used technique for p- and n-type emitter formation in bifacial solar cell fabrication. However, single-side doping of BBr3 or POCl3 is difficult to achieve through gas diffusion carried out at high temperatures. Thus, removal of the unexpected doped layer on one side is undertaken in the fabrication of bifacial solar cells. We have implemented a spin-etching technique to remove the doped layer; however, the removal leads to a planar surface, and the related optical losses affect the bifaciality of the cell through a lowering of the short-circuit current. In this paper, we demonstrate a new procedure to maintain the pyramid texture on the side from which the doped layer is removed. The effect of the texture-maintaining removal process was examined using scanning electron microscopy (SEM) and reflectance measurements. SEM images show that the edges of the texture pyramids become wider. The optical losses due to the wider edges were investigated through reflection measurements. To investigate the potential of this new procedure, an n-type bifacial cell was fabricated on a 156 mm × 156 mm 180-μm-thick n-type pseudo Czochralski-Si wafer, and the current–voltage parameters were compared with the single-side planar n-type bifacial solar cell.

Effects of surface passivation at a SiO2/phosphorus-doped layer (n+-layer) front interface were investigated. Two kinds of cells with different surface concentration were fabricated. Surface potential at the interface was changed by applying bias voltage (VF). Both open-circuit voltage and short-circuit current of the cell with n+-layer concentration of 3 × 1018 cm−3 depended on VF. Internal quantum efficiency of this cell in short- and medium-wavelength range was changed by applying VF. It was shown that cell performance was improved by the accumulation of electrons at the interface. To consider the work function difference between a material on the SiO2 film and the n+-layer is important, and cell performance can be further improved by applying VF to passivate the SiO2n+-layer interface.

AbstractWe studied the effects of thermal annealing on the interfacial properties of atomic-layer-deposited alumina (Al2O3) films and aluminum oxide/silicon nitride (AlOx/SiNx) stacks on silicon. Thermal treatment was found to have a significant effect on the lifetime improvement, owing to full activation of field-effect passivation, which can be realized with an increased flat-band voltage (Vfb). The presence of a capping SiNx layer itself and/or during deposition is suspected to cause distribution of unstable negatively charged traps in the AlOx/SiNx stacks, resulting in a slight reduction in lifetime with inversely improved Vfb. The interface trap density, an indicator of chemical passivation, remains at the same order of magnitude after thermal treatment and SiNx deposition, suggesting that atomic-layer deposition induces a high passivation quality. These results suggest that consideration of thermal-annealing effects and SiNx deposition conditions are crucial for improving the quality of passivation stacks at the rear of passivated emitter rear contact (PERC) cells and, consequently, the high performance of PERC cells.