Si Multijunction Research Articles

A modeling framework to quantify the intermediate layer impact in III–V//Si multijunction solar cells

Multijunction solar cells (MJSCs) are capable of converting sunlight to electricity more efficiently than single-junction solar cells. The intermediate scattering layers between the individual junctions contribute to high efficiency by impacting the generated currents, photon recycling (PR), as well as luminescent coupling (LC) in the device. The MJSC efficiency can be simulated using expressions that involve a simplified and idealized intermediate layer structure but cannot accurately reflect its actual performance. This work, however, aims to establish a systematic optical model for MJSCs with complicated intermediate layers. It begins with incorporating the LC and PR effects into the developed model, emphasizing requirements for the cut-off wavelength and long-wavelength transmission of the intermediate layer. Furthermore, a three-dimensional metallic nanocylinder array is designed as the intermediate layer to improve device performance. With the model, high-performance MJSCs can be designed and optimised by quantifying the impact of PR and LC on device parameters.

Two‐terminal III–V//Si triple‐junction solar cell with power conversion efficiency of 35.9 % at AM1.5g

AbstractIII–V//Si multijunction solar cells offer a pathway to increase the power conversion efficiency beyond the fundamental Auger limit of silicon single‐junctions. In this work, we demonstrate how the efficiency of a two‐terminal wafer‐bonded III–V//Si triple‐junction solar cell is increased from 34.1 % to 35.9 % under an AM1.5g spectrum, by optimising the III–V top structure. This is the highest reported efficiency to date for silicon‐based multijunction solar cell technologies. This improvement was accomplished by two main factors. First, the integration of a GaInAsP absorber in the middle cell increased the open‐circuit voltage by 51 mV. Second, a better current matching of all subcells enhanced the short‐circuit current by 0.7 mA/cm2. Two different growth directions, upright and inverted, were investigated. The highest cell efficiency of 35.9 % (Voc = 3.248 V, jsc = 13.1 mA/cm2, FF = 84.3 %) was achieved with an upright grown structure. Processing of upright structures requires additional bonding steps, which results in a reduced homogeneity of cell performance across the wafer. A detailed comparison with the currently best triple‐junction solar cell reveals future improvement opportunities and limits, considering voltage and current, respectively.

III-V Thin-Film Solar Cells Bonded to Si Substrates via Metal Grids

Ⅲ-Ⅴ on Si multijunction (MJ) solar cells are promising as next-generation solar cells since they can provide high efficiency with low cost in comparison with conventional Si and Ⅲ-Ⅴ MJ cells. In fabricating such III-V/Si MJ cells, Ⅲ-Ⅴ subcells are placed on Si bottom cell by hybrid approaches such as surface-activated bonding (SAB). The bonding interfaces with lower interface resistances are strongly required so as to achieve better performance of hybrid MJ cells.It was found that the resistance across the directly-bonded III-V/Si interfaces in MJ cells was higher than the resistance in junctions made of heavily-doped substrates because of the thin heavily-doped bonding layers in actual subcell structures. It was reported that indium tin oxide (ITO) films as intermediate layers between III-V and Si subcells played a role of lowering the series resistance of MJs [1]. It was found, however, that the resistance of GaAs//ITO/Si junctions increased by annealing them [2]. Furthermore, the quantum efficiency of Si bottom cells was lowered by inserting the ITO layers because of the free carrier absorption of ITO layers.These problems are assumed to be avoided by using metal grids in combination with dielectric materials such as SiO2 and SiN. Figure 1(a) shows a schematic process for fabricating Ⅲ-Ⅴ/metal grid/Si MJ cells. Metal grid structures work as ohmic contacts for both of Ⅲ-Ⅴ and Si layers. Dielectric materials, such as SiO2 and SiN, are likely to work as passivation for Si subcells. The metal grids must be slightly thicker than the dielectric materials so as to ensure successful bonding of semiconductor to metal grid. It is also notable that impacts of shadow loss due to the metal grid can be minimized by aligning the emitter contacts on the top cells to the metal grids. We recently fabricated Si//metal grid/Si and GaAs//metal grid/Si junctions by SAB and reported on their electrical characteristics [3]. In this work, we fabricate III-V cells bonded on conductive Si substrates via metal grids and examine their photovoltaic properties.We prepared n-on-p InGaP/GaAs double-junction (2J) epi layers grown on GaAs substrates. The epi substrates were bonded to p-Si substrates via metal grids using SAB. 2J cells were fabricated by removing the GaAs substrates and forming emitter and base contacts on the top and bottom surfaces, respectively. The top view of the fabricated cells is shown in Fig. 1(b). Note that the emitter contacts were aligned to metal grids. Their current-voltage (I-V) characteristics under the solar irradiance of AM1.5G/one sun, which were obtained by using an in-house solar simulator, are shown in Fig. 1(c). The observed open-circuit voltage and short-circuit current (2.47 V and 10.9 mA/cm2) indicate that the fabricated cells normally work as 2J cells. Although the structure and fabrication process are not yet optimized, the results that we obtained suggests that the bonding via metal grids are promising for fabricating hybrid MJ cells with low series resistance.[1] Naoteru Shigekawa, Tomoya Hara, Tomoki Ogawa, Jianbo Liang, Takefumi Kamioka, Kenji Araki, and Masafumi Yamaguchi, "GaAs/Indium Tin Oxide/Si Bonding Junctions for III-V-on-Si Hybrid Multijunction Cells With Low Series Resistance", IEEE J. Photovolt. 8 (3), pp. 879-886 (2018).[2] Tomoya Hara, Tomoki Ogawa, Jianbo Liang, Kenji Araki, Takefumi Kamioka, and Naoteru Shigekawa, "Electrical properties of GaAs//indium tin oxide/Si junctions for III-V-on-Si hybrid multijunction cells", Jpn. J. Appl. Phys. 57, 08RD05 (2018).[3]Takashi Hishida, Jianbo Liang, and Naoteru Shigekawa, "Low-resistance semiconductor/semiconductor junctions with intermediate metal grids for III-V-on-Si multijunction solar cells", Jpn. J. Appl. Phys. 59, SB, SBBB04 (2020). Figure 1

III‐V//Si multijunction solar cells with 30% efficiency using smart stack technology with Pd nanoparticle array

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.

Dual-junction GaAs solar cells and their application to smart stacked III–V//Si multijunction solar cells

We report high-quality dual-junction GaAs solar cells grown using solid-source molecular beam epitaxy and their application to smart stacked III–V//Si quadruple-junction solar cells with a two-terminal configuration for the first time. A high open-circuit voltage of 2.94 eV was obtained in an InGaP/GaAs/GaAs triple-junction top cell that was stacked to a Si bottom cell. The short-circuit current density of a smart stacked InGaP/GaAs/GaAs//Si solar cell was in good agreement with that estimated from external quantum efficiency measurements. An efficiency of 18.5% with a high open-circuit voltage of 3.3 V was obtained in InGaP/GaAs/GaAs//Si two-terminal solar cells.

Four‐Terminal Perovskite/Silicon Multijunction Solar Modules

Multijunction solar cells employing perovskite and crystalline‐silicon (c‐Si) light absorbers bear the exciting potential to surpass the efficiency limit of market‐leading single‐junction c‐Si solar cells. However, scaling up this technology and maintaining high efficiency over large areas are challenging as evidenced by the small‐area perovskite/c‐Si multijunction solar cells reported so far. In this work, a scalable four‐terminal multijunction solar module design employing a 4 cm2 semitransparent methylammonium lead triiodide perovskite solar module stacked on top of an interdigitated back contact c‐Si solar cell of identical area is demonstrated. With a combination of optimized transparent electrodes and efficient module design, the perovskite/c‐Si multijunction solar modules exhibit power conversion efficiencies of 22.6% on 0.13 cm2 and 20.2% on 4 cm2 aperture area. Furthermore, a detailed optoelectronic loss analysis along with strategies to enhance the performance is discussed.

The Effect of the Illumination Intensity on the Performance of Si Multijunction based Integrated Photoelectrochemical water Splitting Devices

Abstract We present a study of the effect of illumination intensity on the performance of a photovoltaic-biased electrochemical (PV-EC) device for solar hydrogen production based on a triple junction thin film silicon solar cell. The influence of the illumination intensity was studied for the solar cell as well as for an integrated PV-EC device. We show that while the open circuit voltage decreases with a reduction in intensity, the triple junction solar cell still provides a sufficient voltage to drive spontaneous water splitting. Moreover, a slight improvement in the fill factor at lower intensities can relax the requirements to the utilized co-catalysts. As a consequence, the difference in the performance of PV-EC devices featuring very active (Pt/RuO 2 ) and less active (Ni/Co 3 O 4 ) catalyst materials decreases when the illumination intensity is reduced.

Post passivation light trapping back contacts for silicon heterojunction solar cells.

Light trapping in crystalline silicon (c-Si) solar cells is an essential building block for high efficiency solar cells targeting low material consumption and low costs. In this study, we present the successful implementation of highly efficient light-trapping back contacts, subsequent to the passivation of Si heterojunction solar cells. The back contacts are realized by texturing an amorphous silicon layer with a refractive index close to the one of crystalline silicon at the back side of the silicon wafer. As a result, decoupling of optically active and electrically active layers is introduced. In the long run, the presented concept has the potential to improve light trapping in monolithic Si multijunction solar cells as well as solar cell configurations where texturing of the Si absorber surfaces usually results in a deterioration of the electrical properties. As part of this study, different light-trapping textures were applied to prototype silicon heterojunction solar cells. The best path length enhancement factors, at high passivation quality, were obtained with light-trapping textures based on randomly distributed craters. Comparing a planar reference solar cell with an absorber thickness of 280 μm and additional anti-reflection coating, the short-circuit current density (JSC) improves for a similar solar cell with light-trapping back contact. Due to the light trapping back contact, the JSC is enhanced around 1.8 mA cm-2 to 38.5 mA cm-2 due to light trapping in the wavelength range between 1000 nm and 1150 nm.

Assessment of Rear-Surface Processing Strategies for III–V on Si Multijunction Solar Cells Based on Numerical Simulations

The manufacturing of high-efficiency III–V on Si multijunction solar cells needs the development of hybrid, i.e., adapted to both families of materials, solar cell processing techniques, able to extract the full photovoltaic potential of both the subcells. This fact especially impacts the processing of the silicon rear surface of the tandem, which cannot receive treatments commonly used in the single-junction Si solar cell industry [Al-back surface field (BSF), thermal SiO2, and so on], since these would result in an excessive thermal load that would deteriorate the III–V upper layers (top cell, tunnel junction, and buffer layer). However, the Si bottom cell requires an advanced design with good rear passivation, a good ohmic contact, and good carrier selectivity, so that its contribution to the efficiency of the tandem is maximized. Accordingly, in this paper, several low-temperature compatible rear-surface passivation techniques for the Si bottom subcell in a monolithic III–V/Si tandem solar cell are explored. In particular, aluminum BSFs, passivated emitter and rear cell (PERC)-like architecture, passivated emitter and rear locally diffused (PERL)-like architecture formed with low thermal loads, and heterojunction with intrinsic thin layer (HIT)-like processes are assessed using numerical simulations, and a comparison of the Si bottom cell performance for the mentioned alternatives in a GaAsP/Si dual-junction solar cell is presented.

Evolution of silicon bulk lifetime during III–V‐on‐Si multijunction solar cell epitaxial growth

AbstractThe evolution of Si bulk minority carrier lifetime during the heteroepitaxial growth of III–V on Si multijunction solar cell structures via metal‐organic chemical vapor deposition (MOCVD) has been analyzed. In particular, the impact on Si lifetime resulting from the four distinct phases within the overall MOCVD‐based III–V/Si growth process were studied: (1) the Si homoepitaxial emitter/cap layer; (2) GaP heteroepitaxial nucleation; (3) bulk GaP film growth; and (4) thick GaAsyP1‐y compositionally graded metamorphic buffer growth. During Phase 1 (Si homoepitaxy), an approximately two order of magnitude reduction in the Si minority carrier lifetime was observed, from about 450 to ≤1 µs. However, following the GaP nucleation (Phase 2) and thicker film (Phase 3) growths, the lifetime was found to increase by about an order of magnitude. The thick GaAsyP1‐y graded buffer was then found to provide further recovery back to around the initial starting value. The most likely general mechanism behind the observed lifetime evolution is as follows: lifetime degradation during Si homoepitaxy because of the formation of thermally induced defects within the Si bulk, with subsequent lifetime recovery due to passivation by fast‐diffusing atomic hydrogen coming from precursor pyrolysis, especially the group‐V hydrides (PH3, AsH3), during the III–V growth. These results indicate that the MOCVD growth methodology used to create these target III–V/Si solar cell structures has a substantial and dynamic impact on the minority carrier lifetime within the Si substrate. Copyright © 2015 John Wiley & Sons, Ltd.

Recent trends of patents on front TCOs for highly efficient thin-film Si photovoltaic devices.

The recent trends of US patents on the front transparent electrode of thin-film silicon (Si) photovoltaic (PV) devices are reviewed. The various transparent conductive oxide (TCO) materials have been invented to satisfy a multifunctional prerequisite for the front electrode: high electrical conductivity, high optical transparency, effective light trapping, anti-reflection effect, and diffusion barrier. The recent surge of filed patents reflects the great importance of the front TCO technology for high efficiency thin-film Si PV devices. Among the TCO materials, properties of commercially available F-doped tin oxide (SnO2:F)-coated glass substrates are compared. SnO2:F-coated glass substrates share 20-30% of the cost for production of thin-film Si PV modules - evaluated values from mass production at KISCO. Therefore, the cost and technological innovation must be established for cost-effective mass production of large-area thin-film Si multijunction PV modules.

Theoretical energy yield of GaAs-on-Si tandem solar cells

ABSTRACTIII-V on Si multijunction solar cells represent an alternative to traditional compound III-V multijunction cells as a promising way to achieve high efficiencies. A theoretical study on the energy yield of GaAs/Si tandem solar cells is performed to assess the performance potential and sensitivity to spectral variations. Recorded time-dependent spectral irradiance data in two locations (Singapore and Denver) were used. We found that a 4-terminal contact scheme with thick top cell confers distinctive advantages over a 2-terminal scheme, giving a yield potential 21% higher than the 2-terminal scheme in Singapore and 17% higher in Denver. The theoretical energy yield benefit of a 4-terminal device emphasizes the need for further technology development in this design space.

Advances in amorphous silicon photovoltaic technology

With the advent of new multijunction thin film solar cells, amorphous silicon photovoltaic technology is undergoing a commercial revival with about 30 megawatts of annual capacity coming on-line in the next year. These new a−Si multijunction modules should exhibit stabilized conversion efficiencies on the order of 8%, and efficiencies over 10% may be obtained in the next several years. The improved performance results from the development of amorphous and microcrystalline silicon alloy films with improved optoelectronic properties and from the development of more efficient device structures. Moreover, the manufacturing costs for these multijunction modules using the new large-scale plants should be on the order of $1 per peak watt. These modules may find widespread use in solar farms, photovoltaic roofing, as well as in traditional remote applications.