Tandem Structure Research Articles

Perovskite/Si tandem solar cells: Fundamentals, advances, challenges, and novel applications

AbstractThe world record device efficiency of single‐junction solar cells based on organic–inorganic hybrid perovskites has reached 25.5%. Further improvement in device power conversion efficiency (PCE) can be achieved by either optimizing perovskite films or designing novel device structures such as perovskite/Si tandem solar cells. With the marriage of perovskite and Si solar cells, a tandem device configuration is able to achieve a PCE exceeding the Shockley–Queisser limit of single‐junction solar cells by enhancing the usage of solar spectrum. After several years of development, the highest PCE of the perovskite/Si tandem cell has reached 29.5%, which is higher than that of perovskite‐ and Si‐based single‐junction cells. Here, in this review, we will (1) first discuss the device structure and fundamental working principle of both two‐terminal (2T) and four‐terminal (4T) perovskite/Si tandem solar cells; (2) second, provide a brief overview of the advances of perovskite/Si tandem solar cells regarding the development of interconnection layer, perovskite active layer, tandem device structure, and light management strategies; (3) third, discuss the challenges and opportunities for further developing perovskite/Si tandem solar cells. This review article, on the one hand, provides a comprehensive understanding to readers on the development of perovskite/Si tandems. On the other hand, it proposes various novel applications that may bring such tandems into the market in a near future.

Fusing dense and ReZero residual networks for super-resolution of retinal images

• A deep neural network (DRRN) for super-resolution of retinal image is built. • Use dense connections in the feature mapping to reuse feature information. • Exploit local and global residuals learning to recover retinal image details. • Use ReZero to dynamically learn the weight of the residual. Retinal images have important reference value for the analysis of cardiovascular diseases such as diabetes and hypertension. However, because of the large number of artifacts at the end of the blood vessel and the unclearness of the intersection of the blood vessel, it will cause the inaccuracy of the retinal blood vessel segmentation, thereby affecting the diagnosis of retinal diseases. To solve this problem, we proposed dense and ReZero residual networks for super-resolution of retinal images (DRRN). Firstly, we used a dense connection module to obtain more feature information. Then, we added local residual learning and global residual learning respectively to improve the resolution of retinal image blood vessels from coarse to fine. We also introduced ReZero (residual with zero initialization) to the local residual learning, which can dynamically facilitate well-behaved gradients in a deep network with dense connection modules to make the network converge faster. Finally, we used a tandem structure to reconstruct the local details of the retinal image. The results showed that our method can better reconstruct the details of the retinal image, and can accelerate the processing speed on STARE, DRIVE, and CHASE_DB1 datasets.

Efficient Organic Tandem Solar Cells Enabled by Solution‐Processed Interconnection Layer and Fine‐Tuned Active Layer

AbstractOrganic tandem solar cells (OTSCs) with the merits of overcoming the S‐Q limit of single‐junction, enhancing and expanding the sunlight harvesting, and minimizing the thermalization energy loss, provide a practical strategy to break through the power conversion efficiency (PCE). However, it is still challenge to realize the multilayered structure by all‐solution processing, especially shortage of interconnection layer (ICL), which physically and electronically connects front and rear subcells to accomplish the complicated tandem structure. Herein, all‐solution‐processed ICL of modified poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (m‐PEDOT) and titanium (diisopropoxide) bis(2,4‐pentanedionate) (TIPD) is invented for constructing inverted OTSCs, and by systematically managing the light and carriers within the stacked multilayers, highly efficient OTSCs are demonstrated. Compared with the traditional ICLs, it avoids the complicated vacuum evaporation and synthesis processes, guaranteeing low cost and high reproducibility. Furthermore, m‐PEDOT/TIPD ICL demonstrates excellent solvent resistance, high transmittance, and good conductivity. To enhance the light harvesting, absorption complementary donors and acceptors are selected as photoactive materials in front and rear subcells, and the transfer matrix formalism optical modeling is introduced to achieve the balanced short‐circuit current density of each subcell in OTSCs. By systematic device optimizations, the best PCE of 15.26% is achieved, among the best results reported for OTSCs.

Tandem domain structure determination based on a systematic enumeration of conformations

Protein structure determination is undergoing a change of perspective due to the larger importance taken in biology by the disordered regions of biomolecules. In such cases, the convergence criterion is more difficult to set up and the size of the conformational space is a obstacle to exhaustive exploration. A pipeline is proposed here to exhaustively sample protein conformations using backbone angle limits obtained by nuclear magnetic resonance (NMR), and then to determine the populations of conformations. The pipeline is applied to a tandem domain of the protein whirlin. An original approach, derived from a reformulation of the Distance Geometry Problem is used to enumerate the conformations of the linker connecting the two domains. Specifically designed procedure then permit to assemble the domains to the linker conformations and to optimize the tandem domain conformations with respect to two sets of NMR measurements: residual dipolar couplings and paramagnetic resonance enhancements. The relative populations of optimized conformations are finally determined by fitting small angle X-ray scattering (SAXS) data. The most populated conformation of the tandem domain is a semi-closed one, fully closed and more extended conformations being in minority, in agreement with previous observations. The SAXS and NMR data show different influences on the determination of populations.

Light extraction in tandem organic light emitting diodes

Since the invention of organic light emitting diodes (OLEDs), great research efforts have been dedicated to improving their efficiency and lifetime. For high-brightness applications, tandem OLED structures have advantages because of the lower current densities required to achieve high brightness. With the successful development of highly efficient charge generation layers, high brightness tandem OLEDs are used in displays and lighting. However, the major challenge for tandem OLEDs is the low light extraction efficiency, because about 50% of the light is trapped inside the device as waveguide modes. In this Perspective, we first review the recent works done on light extraction, analyze different waveguide mode extraction structures, and then identify the key factors determining the extraction efficiencies in tandem OLEDs.

Zinc-hyperdoped silicon nanocrystalline layers prepared via nanosecond laser melting for broad light absorption

Current techniques for hyperdoping of Si wafer surface by femtosecond laser or ion implantation have encountered several problems for device fabrication, such as severe lattice defects, shallow doping depth, poor electrical activity of impurities, difficult collection of charge carriers, or weak compatibility with present complementary metal–oxidesemiconductor processes. To provide a new way to solve these problems, in this study, Zn-hyperdoped Si layers were prepared on polycrystalline Si substrates by vacuum evaporation and nanosecond (ns) laser melting under different conditions of deposition and laser treatment. As a result, the hyperdoped Si layers of up to micron-level thickness with a broad light absorption (above 90% from the ultraviolet to the visible range and up to 54% for the near-infrared spectrum) and good electrical transport properties (electron mobility of up to 88.5 cm2⋅V−1s−1 and sheet resistance of ~ 100 Ω⋅square−1) were obtained. These hyperdoped Si layers can be flexibly superimposed on the existing Si optoelectronic devices: Such tandem structure avoids the damage of substrate and provides both the layers and substrate properties. In addition, the variation in the microstructural characteristics of the Zn-hyperdoped Si layers consisting of surface and cross-section structures, Zn impurity distribution and concentration, and crystallinity were carefully studied to clarify the relationships between their preparation conditions, microstructures, and physical properties. The results show that a matching between the thickness of Si-Zn deposited layer and the ns-laser irradiation fluence is crucial to achieve a full melting process and to obtain excellent Zn-hyperdoped Si layers. Their near-infrared absorption and electrical transport properties are determined by the combined effects of surface antireflection, Zn-hyperdoping, and nanocrystallization.

A SCAPS simulation investigation of non-toxic MAGeI3-on-Si tandem solar device utilizing monolithically integrated (2-T) and mechanically stacked (4-T) configurations

In this study, solar cell capacitance simulator (SCAPS) was utilized to investigate a tandem solar device with Methyl-ammonium germanium iodide (MAGeI3), an organic perovskite, as top cell active layer, and crystalline silicon (c-Si) as the bottom cell. Validation studies were done against established single-junction device structures including MAPbI3 and c-Si. Our simulation-based results showcase a robust congruence with experimental data, with obtained power conversion efficiency (PCE) values of 20.19%, and 23.01% for MAPbI3 and c-Si based single-junction devices, respectively. For the thesis of this work, both four-terminal (4-T) and two terminal (2-T) perovskite-on-silicon tandem architectures were investigated. Our numerical simulation for the monolithic stacked 2-T tandem structure of MAGeI3-on-c-Si produced a PCE of 28.71 %, while a PCE of 32.2 % was obtained for the standard MAPbI3-on-c-Si. For the current matching condition in the 2-T tandem devices, the optimum thickness of MAGeI3 and MAPbI3 were found to be 626 nm and 223 nm respectively. For the mechanically stacked 4-T configurations, MAGeI3-on-c-Si and MAPbI3-on-c-Si produced PCE values of 29.85 % and 33.67%, respectively, with optimum top cell absorber thickness values of 1.7 µm and 1.4 µm, respectively. Each device architecture was optimised by carrying out extensive studies including modulations in absorber thickness, bulk defect density, interfacial defect density, and back contact metal work function. Our in-depth analyses highlight a remarkable potential for MAGeI3 to be utilized as the top cell of a perovskite-on-silicon solar device, boasting high efficiency and intrinsic non-toxicity.

A novel design of CTZS/Si tandem solar cell: a numerical approach

Multijunction or tandem solar cells can split the solar spectrum over several subcells with different bandgaps to convert sunlight into electricity more effectively than single-junction solar cells. The monolithic tandem design of third generation silicon solar energy materials is auspicious for photovoltaics. In this paper, the simulation-based studies of copper zinc tin sulfide/silicon (CZTS/Si) tandem cells based on CZTS as an upper subcell and silicon as a lower subcell absorber layer have been performed using SCAPS-1D. This study aims to evaluate the CZTS tandem cells’ performance based on the fact that both subcells are simulated to produce the best efficiency recorded at its bandgap. The simulation and optimization of the single-junction CZTS and Si solar cells were initially performed to fit the state-of-the-art records efficiency of 11.65% and 18.7%, respectively. Further, both the upper and lower cells have been evaluated at different thicknesses for tandem configuration after validation. Also, to obtain the same current for tandem structure, the upper subcell's performance is investigated at different thicknesses ranged from 0.1~1 µm while keeping the lower subcell thickness at 80 µm. Thus, at optimized upper absorber thickness of 0.191 µm and lower subcell 80 µm at transmitted spectrum the same current was obtained and gave an efficiency of 10.6% and 11.9%, respectively. The maximum efficiency of ~ 23 is obtained for tandem design with enhancing open circuit voltage 1.4 V.

Recent Progress of Organic Photovoltaics with Efficiency over 17%

The power conversion efficiency (PCE) of organic photovoltaics (OPVs) has exceeded 18% with narrow bandgap, non-fullerene materials Y6 or its derivatives when used as an electron acceptor. The PCE improvement of OPVs is due to strong photon harvesting in near-infrared light range and low energy loss. Meanwhile, ternary strategy is commonly recognized as a convenient and efficient means to improve the PCE of OPVs. In this review article, typical donor and acceptor materials in prepared efficient OPVs are summarized. From the device engineering perspective, the typical research work on ternary strategy and tandem structure is introduced for understanding the device design and materials selection for preparing efficient OPVs.

Optical design of TCO-free interconnecting layer for all-perovskite tandem solar cells

Organic–inorganic hybrid perovskite materials are excellent candidates as light absorbers in tandem solar cells with advantages of tunable bandgaps, high absorption coefficients, and facile and low-cost fabrication processes. As the key component of a tandem structure, the interconnecting layer (ICL) requires optical transparency, efficient carrier recombination, and facile up-scalability. To demonstrate the feasibility of an efficient TCO-free ICL in all-perovskite tandem solar cell devices, we have performed a comprehensive computational analysis using the transfer matrix method. The results suggest that varying the silver recombination layers up to 3 nm does not present a significant effect on the maximum achievable photocurrent (MAPC) of the cell. Moreover, the highest MAPC from the tandem cell with the lowest sensitivity to electron and hole transport layer thicknesses can be obtained by the transport layers with refractive indices within the range of 1.9–2.6. The findings are of direct practical relevance, i.e., with meeting our optical framework, a fabrication procedure can be merely directed toward the electrical optimization of the ICL. The presented methodology can serve as an introduction to benchmark ICL design for multi-junction excitonic solar cells.

Highly efficient tandem PHOLEDs with lithium-doped BPhen/NDP-9-doped TAPC as a charge generation layer

The development of large-area organic light-emitting diode (OLED) displays requires a highly efficient tandem device architecture and an easily processable charge generation layer (CGL) with a low voltage drop and high optical transparency. In this study, we investigated and applied a doped organic n-CGL/p-CGL using thermal vacuum deposition in tandem OLED devices. A doping concentration of 1.0 wt.% for Li in 4, 7-Diphenyl-1, 10-phenanthroline (BPhen) was optimal for the n-CGL with 8 wt.% for 2-(7-dicyanomethylene-1,3,4,5,6,8,9,10-octafluoro-7H-pyrene-2-ylidene)-malononitrile (NDP-9)-doped N,N-bis(4-methylphenyl)benzenamine (TAPC) as a p-CGL. Maximum luminous efficiencies of 42.5 and 63.4 cd/A and a 4,000 cd/m2 current density for the target luminance values of 11.2 and 6.5 mA/cm2 were demonstrated for double-stack and triple-stack tandem blue phosphorescent OLED devices, respectively. Implementing these highly efficient tandem device structures will improve the overall lifetime of OLED displays by lowering their operating current density at the target luminance.

Toward practical solar-driven photocatalytic water splitting on two-dimensional MoS2 based solid-state Z-scheme and S-scheme heterostructure

Solar-driven splitting of water to hydrogen production is one of the utmost auspicious and cost-effective technologies providing sustainable fuel as it necessitates only sunlight and water. This substantial technology can be achieved in a two step-photoexcitation system using an artificial Z-scheme process. The key to comprehend this reaction is to discover an effective and robust photocatalyst which preferably makes sufficient use of solar energy. Benefiting from the ultrahigh surface area, appropriate band potentials and unique physiochemical properties of 2D layered structure, MoS2 has attained intensive research attention for hydrogen evolution reaction. The comprehensive review summarizes the recent progression in solid mediator Z-scheme based MoS2 particularly in the field of water splitting. The review initiates with the important bases of Z-scheme water splitting mechanism in addition to structural and optoelectronic properties of MoS2. In addition, detailed discussion on the solid-mediator Z-schemes to further improve the photocatalytic efficacy and limit the back (water-forming) reactions. The review mainly focuses on the solid-mediator based Z-schematic systems involving new tandem structures with an efficient synergistic effect to achieve both wide-range light absorptivity and charge pairs transference. In addition, a new S-scheme heterostructing is briefly introduced to understand the charge-transport route clearly and vividly. The aim of wide-spreading water splitting devices from lab-scale to wide-range potential applications has been considered as ‘artificial photosynthetic leaf-to-tree challenge’. Finally, a summary and outlook on the challenges and future research topics on MoS2 based Z-scheme heterostructure photocatalysts are presented.

Pd-mediated mechanical stack of III–V solar cells fabricated via hydride vapor phase epitaxy

In this study, we present the first demonstration of mechanical stacking of solar cells grown via hydride vapor phase epitaxy (HVPE) to fabricate tandem cells. Smart stack technology, in which Pd nanoparticles were used as the bonding mediator, was used for bonding the cells. To prepare the smart stack, the top cells of the tandem structure were peeled from the substrate using the epitaxial lift-off technique. For this purpose, the cell structure was grown on an AlAs sacrificial layer. GaAs solar cells with good performance were fabricated using HVPE, in which the surface roughened by AlAs was restored to flatness by growing a thick GaAs film. The GaAs//InGaAs tandem cells achieved by bonding the GaAs solar cells grown via HVPE with the InGaAs solar cells grown by metal–organic vapor phase epitaxy exhibited a conversion efficiency of 22.6%.

Fast Response Organic Tandem Photodetector for Visible and Near-Infrared Digital Optical Communications.

Organic photodetectors (OPDs), which usually work as photodiodes, photoconductors, or phototransistors, have emerged as candidates for next-generation light sensing. However, low response speed caused by low carrier mobility and resistance-capacitance (RC) time constant, severely hinders the commercialization of OPDs. Herein, the authors demonstrate a state-of-the-art OPD with a record response speed of 146.8ns by employing tandem structure to simultaneously reduce both the carrier transit time and RC time constant of the device, which is faster than that of previously reported OPDs as far as they know. Moreover, benefitting from the multi-level barrier enhancement and voltage division engendered by tandem structure, an ultralow noise current of 7.82 × 10-14 A Hz-1/2 is obtained, as well as a wide detection range in 300-1000nm. In addition, the tandem OPDs are successfully integrated into the optical communication system as signal receivers, demonstrating the precise digital signal communication from visible to near-infrared light. It is believed that tandem OPDs have promising application potential in the wireless transmission system.

Experimental study of combined transparent solar panel and large Fresnel lens concentrator based hybrid PV/thermal sunlight harvesting system

Solar energy is immensely available during daytime on the earth. It is a sustainable, renewable and green energy. Improvement in efficiency of harnessing solar energy is the utmost concern of the present research due to its renewable nature. This article presents a complete utilization of solar irradiation over the broad spectrum of solar energy efficiently. The design, development and implementation of the hybrid solar photo voltaic & thermal (SPVT) system are reported, in which a tandem structure of transparent solar panel and large Fresnel lens concentrator are mounted for sunlight harvesting. In this hybrid system a semi-transparent solar panel mounted on the top for PV application and large Fresnel lens is for concentrating the transmitted light onto a solar thermal absorber unit for heating water efficiently. The partial visible portion of solar energy is converted into voltage generation using semi transparent solar panel (TSP) while remaining transmitted radiations is further concentrated by a large Fresnel lens which was fixed below semi-transparent solar panel. Concentrated sunlight was focused on solar thermal unit which was utilized for water heating. Two different color filter semi-transparent solar panel modules were integrated with the system for testing and reported that the red color filter solar panel generates more power and overall efficiency as compared to other one. Experimental results using the prototype developed system are reported. Due to simultaneous electricity generation and domestic hot water application the hybrid system becomes more efficient, robust and cost effective for sustainable development of the country.

Aerodynamic wake oscillator for modeling flow-induced vibration of tandem cylinders with short spans

The tandem cylinders immersed in an oncoming flow undergo violent flow-induced vibration (FIV). The cylinders with short spans, in the specific conditions of airflow and mass damping, experience the galloping-like but self-limited vibration. It promotes a coupling instability, controlled by the shear layers and wake vortices. The peculiar behavior of FIV for tandem cylinders is the interference between transverse galloping and vortex-induced vibration (VIV), which lacks a modeling approach through classical theories. Based on the quasi-steady assumptions, a mathematical model including two semicoupled structural oscillators and a reduced wake oscillator was established to simulate the vibration responses. A nonlinear aerodynamic damping coefficient and an empirical interfered force were introduced to the model, revealing the unexplored interference mechanism between two cylinders with short spans. The model with calibrated parameters can reproduce the nonlinear and bifurcated vibration responses of cylinders, providing a quantitative agreement with the experimental data. Although the model only works for high Scruton numbers and loses high fidelity in predicting the vibration amplitudes of the downstream cylinder in several cases, the vital velocity thresholds, amplitude-velocity trends and maximum amplitudes can be precisely obtained. This model is conducive to preacquiring the FIV responses of tandem industrial structures, including steel stacks, bladeless wind turbines, low-power energy-harvesting devices, etc.

Monolithic Perovskite-Carrier Selective Contact Silicon Tandem Solar Cells Using Molybdenum Oxide as a Hole Selective Layer

Monolithic perovskite–silicon tandem solar cells with MoOx hole selective contact silicon bottom solar cells show a power conversion efficiency of 8%. A thin 15 nm-thick MoOx contact to n-type Si was used instead of a standard p+ emitter to collect holes and the SiOx/n+ poly-Si structure was deposited on the other side of the device for direct tunneling of electrons and this silicon bottom cell structure shows ~15% of power conversion efficiency. With this bottom carrier selective silicon cell, tin oxide, and subsequent perovskite structure were deposited to fabricate monolithic tandem solar cells. Monolithic tandem structure without ITO interlayer was also compared to confirm the role of MoOx in tandem cells and this tandem structure shows the power conversion efficiency of 3.3%. This research has confirmed that the MoOx layer simultaneously acts as a passivation layer and a hole collecting layer in this tandem structure.

Frequency Band Characteristics of a Triboelectric Nanogenerator and Ultra-Wide-Band Vibrational Energy Harvesting.

Micromechanical vibration, as one of the most prevalent forms of energy in an ambient environment, has surpassing application potentials as the power source for self-powered electronics. A triboelectric nanogenerator (TENG) can effectively convert vibrational energy to electricity, which has the unique benefit of a wide-band over a traditional vibration energy harvester due to the contact electrification mechanism. Herein, the frequency band characteristics of vibrational TENG (V-TENG) were systematically elaborated. The mechanical model of V-TENG was established to explore its working mechanism for wide-band vibrational energy harvesting. By simulation analysis and experimental validation, the bandwidth dependence of V-TENG on acceleration magnitude, proof mass, stiffness, and gap distance was investigated in detail. With optimized structural parameters, an ultra-wide-band vibration energy harvester (UVEH) was developed by a tandem spring-mass structure. Within the ultra-wide-band range from 3 to 45 Hz, the UVEH can invariably illuminate 36 serial light-emitting diodes (LEDs) and charge a 33 μF capacitor to 1.5 V within 35 s. This work has quantitatively studied frequency band characteristics of V-TENG and provided a promising strategy for wide-band vibrational energy harvesting from a machine, bridge, water wave, and human motion.

Photon redistribution of two-terminal perovskite/Si tandem solar cells induced by the optical coupling layer for higher power conversion efficiency

The tandem structure is promising to break the Shockley–Queisser limit of single-junction solar cells. Optical management is essential in two-terminal (2T) tandem devices to equalize the current of each sub-cell and enhance the performance. In this work, the photon redistribution effect caused by the optical coupling layer (OCL) and its impact on 2T perovskite/Si tandem solar cells are systematically investigated. We modulate the overall optical field distribution to balance the current of the two sub-cells by structural modulation. The introduction of an OCL affects the photon distribution in the 2T tandem device, which disturbs the original current match and complicates the optical management. In addition to the antireflection function, the photon redistribution effect due to the coupling effects exists for each type of OCL. Various materials (LiF, MgF2, Al2O3 and ZnO) are employed as OCLs and compared. It is shown that they influence the device performance to different degrees. Compared to LiF and MgF2, the influence of the 2T tandem device on short-circuit current density is more significant when Al2O3 and ZnO act as OCLs. By investigating the optical mechanism and then improving the optical management, an optimized device performance can be achieved. We believe that it is necessary to consider the coupling effect in the structural design of 2T tandem solar cells.

Suitable Top Cell Partners for Copper Indium Gallium Selenide‐Based Tandem Solar Cells to Achieve >30% Efficiency

A four‐terminal tandem structure based on the copper indium gallium selenide (CIGS) solar cell is a viable approach to achieve high efficiency. For this purpose, a dual‐junction configuration, consisting of the experimental CIGS cell with a record efficiency of 23.35% as the bottom cell and wide bandgap thin‐film solar absorbers as the top cell, is evaluated by modeling. Herein, the effects of luminescence efficiency, optical absorption, bandgap, thickness, and light trapping of the top cell on the designed tandem performance are investigated. The results show that the optimum bandgap and the thickness of the absorber layer for the top cell are between 1.7 and 2 eV and 0.5 μm, respectively. Light trapping for less than 1 μm‐thick top cell has a strong impact on cell performance. The analytical model for the tandem solar cell is provided to recognize the suitable materials as the partner for the CIGS cell. The analysis shows that the gallium arsenide (GaAs), copper zinc tin sulfide (CZTS), cadmium telluride (CdTe), and copper gallium selenide (CGS) thin‐film solar cells can be considered suitable top cells to achieve a tandem cell with more than 30% efficiency. The Al0.31Ga0.69As/CIGS tandem solar cells with 35.9% efficiency are also proposed.