Monolithic Two-Terminal Tandem Solar Cells Using Sb2S3 and Solution-Processed PbS Quantum Dots Achieving an Open-Circuit Potential beyond 1.1 V.

Multijunction solar cells have the prospect of a greater theoretical efficiency limit than single-junction solar cells by minimizing the transmissive and thermalization losses a single absorber material has. In solar cell applications, Sb2S3 is considered an attractive absorber due to its elemental abundance, stability, and high absorption coefficient in the visible range of the solar spectrum, yet with a band gap of 1.7 eV, it is transmissive for near-IR and IR photons. Using it as the top cell (the cell where light is first incident) in a two-terminal tandem architecture in combination with a bottom cell (the cell where light arrives second) of PbS quantum dots (QDs), which have an adjustable band gap suitable for absorbing longer wavelengths, is a promising approach to harvest the solar spectrum more effectively. In this work, these two subcells are monolithically fabricated and connected in series by a poly(3,4-ethylene-dioxythiophene) polystyrene sulfonate (PEDOT:PSS)-ZnO tunnel junction as the recombination layer. We explore the surface morphology of ZnO QD films with different spin-coating conditions, which serve as the PbS QD cell's electron transport material. Furthermore, we examine the differences in photogenerated current upon varying the PbS QD absorber layer thickness and the electrical and optical characteristics of the tandem with respect to the stand-alone reference cells. This tandem architecture demonstrates an extended spectral response into the IR with an open-circuit potential exceeding 1.1 V and a power conversion efficiency of 5.6%, which is greater than that of each single-junction cell.

Chemically Engineered Titanium Oxide Interconnecting Layer for Multijunction Polymer Solar Cells.

We report chemically tunable n-type titanium oxides using ethanolamine as a nitrogen dopant source. As the amount of ethanolamine added to the titanium oxide precursor during synthesis increases, the Fermi level of the resulting titanium oxides (ethanolamine-incorporated titanium oxides) significantly changes from -4.9 eV to -4.3 eV, and their free charge carrier densities are enhanced by two orders of magnitudes, reaching up to 5 × 1018 cm-3. Unexpectedly, a basic ethanolamine reinforces not only the n-type properties of titanium oxides, but also their basicity, which facilitates acid-base ionic junctions in contact with acidic materials. The enhanced charge carrier density and basicity of the chemically tuned titanium oxides enable multi-junction solar cells to have interconnecting junctions consisting of basic n-type titanium oxides and acidic p-type PEDOT:PSS to gain high open-circuit voltages of 1.44 V and 2.25 V from tandem and triple architectures, respectively.

Ultra-High Concentration Vertical Homo-Multijunction Solar Cells for CubeSats and Terrestrial Applications.

This paper examines advances in ultra-high concentration photovoltaics (UHCPV), focusing specifically on vertical multijunction (VMJ) solar cells. The use of gallium arsenide (GaAs) in these cells increases their efficiency in a range of applications, including terrestrial and space settings. Several multijunction structures are designed to maximize conversion efficiency, including a vertical tunnel junction, which minimizes resistive losses at high concentration levels compared with standard designs. Therefore, careful optimization of interconnect layers in terms of thickness and doping concentration is needed. Homo-multijunction GaAs solar cells have been simulated and analyzed by using ATLAS Silvaco 5.36 R, a sophisticated technology computer-aided design (TCAD) tool aimed to ensure the reliability of simulation by targeting a high conversion efficiency and a good fill factor for our proposed structure model. Several design parameters, such as the dimensional cell structure, doping density, and sun concentrations, have been analyzed to improve device performance under direct air mass conditions AM1.5D. The optimized conversion efficiency of 30.2% has been achieved with investigated GaAs solar cell configuration at maximum concentration levels.

Exploring the impact of defect energy levels in CdTe/Si dual-junction solar cells using wxAMPS.

A numerical analysis of a CdTe/Si dual-junction solar cell in terms of defect density introduced at various defect energy levels in the absorber layer is provided. The impact of defect concentration is analyzed against the thickness of the CdTe layer, and variation of the top and bottom cell bandgaps is studied. The results show that CdTe thin film with defects density between 1014 and 1015 cm-3 is acceptable for the top cell of the designed dual-junction solar cell. The variations of the defect concentrations against the thickness of the CdTe layer indicate that the open circuit voltage, short circuit current density, and efficiency (ƞ) are more affected by the defect density at higher CdTe thickness. In contrast, the Fill factor is mainly affected by the defect density, regardless of the thin film's thickness. An acceptable defect density of up to 1015 cm-3 at a CdTe thickness of 300 nm was obtained from this work. The bandgap variation shows optimal results for a CdTe with bandgaps ranging from 1.45 to 1.7 eV in tandem with a Si bandgap of about 1.1 eV. This study highlights the significance of tailoring defect density at different energy levels to realize viable CdTe/Si dual junction tandem solar cells. It also demonstrates how the impact of defect concentration changes with the thickness of the solar cell absorber layer.

Performance assessment of a triple-junction solar cell with 1.0 eV GaAsBi absorber.

Group III-V semiconductor multi-junction solar cells are widely used in concentrated-sun and space photovoltaic applications due to their unsurpassed power conversion efficiency and radiation hardness. To further increase the efficiency, new device architectures rely on better bandgap combinations over the mature GaInP/InGaAs/Ge technology, with Ge preferably replaced by a 1.0 eV subcell. Herein, we present a thin-film triple-junction solar cell AlGaAs/GaAs/GaAsBi with 1.0 eV dilute bismide. A compositionally step-graded InGaAs buffer layer is used to integrate high crystalline quality GaAsBi absorber. The solar cells, grown by molecular-beam epitaxy, achieve 19.1% efficiency at AM1.5G spectrum, 2.51 V open-circuit voltage, and 9.86 mA/cm2 short-circuit current density. Device analysis identifies several routes to significantly improve the performance of the GaAsBi subcell and of the overall solar cell. This study is the first to report on multi-junctions incorporating GaAsBi and is an addition to the research on the use of bismuth-containing III-V alloys in photonic device applications.

One-step cladding metal oxide nanowires with a carbon-dots-embedded ZnO amorphous layer toward boosted photoelectrochemical water oxidation.

An effective method was proposed for constructing carbon dots (CDs)-sensitized multijunction composite photoelectrodes via one-step cladding a CDs-embedded ZnO amorphous overlayer on vertically aligned metal oxide nanowires. This strategy involved the double role of hexamethylenetetramine (HMTA) in the ethylene glycol (EG) solvent mixed with a controllable trace amount of water. In the water-deficient synthetic system, a limited portion of HMTA served as the pH buffer and hydroxyl source to force the hydrolytic process of zinc ions for the production of ZnO. The precipitated ZnO clusters were instantly capped by EG molecules through the activated alkoxidation reaction, and further crosslinked into an amorphous network surrounding the individual nanowires. Meanwhile, the excess HMTA was simultaneously depleted as the precursor for producing CDs in the EG solution through thermal condensation, which were packed in the gradually formed aggregates. We revealed that a CDs-embedded amorphous ZnO overlayer with an appropriate proportion of ingredient could be tailored through an optimal tradeoff between hydrolysis and condensation of HMTA. Benefiting from the synergy of the amorphous ZnO layer and the embedded CDs, the multijunction composite photoanodes exhibited significantly improved PEC performance and stability for water oxidation.

RAINBOW Organic Solar Cells: Implementing Spectral Splitting in Lateral Multi-Junction Architectures.

While multi-junction geometries have the potential to boost the efficiency of organic solar cells, the experimental gains yet obtained are still very modest. This work proposes an alternative spectral splitting device concept in which various individual semiconducting junctions with cascading bandgaps are laid side by side, thus the name RAINBOW. Each lateral sub-cell receives a fraction of the spectrum that closely matches the main absorption band of the given semiconductor. Here, simulations are used to identify the important material and device properties of each RAINBOW sub-cell. Using the resulting design rules, three systems are selected, with narrow, medium, and wide effective bandgaps, and their potential as sub-cells in this geometry is experimentally investigated. With the aid of a custom-built setup that generates spectrally spread sunlight on demand, the simulations are experimentally validated, showing that this geometry can lead to a reduction in thermalization losses and an improvement in light harvesting, which results in a relative improvement in efficiency of 46.6% with respect to the best sub-cell. Finally, a working proof-of-concept monolithic device consisting of two sub-cells deposited from solution on the same substrate is fabricated, thus demonstrating the feasibility and the potential of the RAINBOW solar cell concept.

Maximizing Electric Power through Spectral-Splitting Photovoltaic-Thermoelectric Hybrid System Integrated with Radiative Cooling.

As zero-emission technologies, a daytime radiative cooling (RC) strategy developed recently, and photovoltaic (PV) and thermoelectric (TE) technologies have aroused great interest to reduce fossil fuel consumption and carbon emissions. How to integrate these state-of-the-art technologies to maximise clean electricity from the sun and space remains a huge challenge, and the limit efficiency is still unclear. In this study, a spectral-splitting PV-TE hybrid system integrated with RC is proposed to maximise clean electricity from the sun and space without any emissions. For the sun acting as a typical constant heat-flux heat source, the current thermoelectric theory overestimates the thermoelectric efficiency highly since the theory is based on constant temperature-difference conditions. A new theory based on heat-flux conditions is employed to achieve maximum thermoelectric efficiency. The PV-TE hybrid system with RC is superior to the conventional hybrid system, not only in terms of higher efficiency but also in its 24-h operation capacity. In a system with a single-junction cell, the total efficiency with 30 suns (39.4%) is higher than the theoretical PV efficiency at 500 suns (38.2%). In a hybrid system with four-junction cells, total efficiency is over 65% which is superior to most current photoelectric and thermal power systems.

Semi-monolithic Integration of All-Chalcopyrite Multijunction Solar Conversion Devices via Thin-Film Bonding and Exfoliation.

We report on a semi-monolithic integration method to circumvent processing incompatibility between materials of dissimilar classes and combine them into multijunction devices for photovoltaic and photoelectrochemical applications. Proof-of-concept all-chalcopyrite tandems were obtained by consecutive transfer of fully integrated unpatterned 1.85 eV CuGa3Se5 and 1.13 eV CuInGaSe2 PV stacks from their Mo/soda lime glass substrates onto a new single host substrate. This transfer approach consists of two key steps: (1) bonding of the solar stack (face down) onto a handle (e.g., SnO2:F, FTO) using a transparent conductive composite and (2) delamination of the solar stack at the chalcopyrite/Mo interface by employing a wedge-based exfoliation technique. Upon transfer onto FTO, a CuGa3Se5 champion device demonstrated near-coincident photocurrent density-voltage characteristic with a baseline measurement. Then, the exfoliated CuGa3Se5 single-junction stack transferred onto FTO served as the new host onto which a second fully processed CuInGaSe2 stack was bonded (face down) onto and liberated from its Mo/SLG substrate, leading to a complete transfer of both sub-cells onto one FTO substrate. A champion semi-monolithic tandem device exhibited a power conversion efficiency of 5.04% with an open-circuit voltage, a short-circuit current density, and a fill factor of 1.24 V, 7.19 mA/cm2, and 56.7%, respectively. This first-time demonstration of a fully operational semi-monolithic device provides a new avenue to combine thermally, mechanically, and/or chemically incompatible thin-film material classes into tandem photovoltaic and photoelectrochemical devices while maintaining state-of-the-art sub-cell processing.

Multi-Junction Solar Cells and Nanoantennas.

Photovoltaic technology is currently at the heart of the energy transition in our pursuit to lean off fossil-fuel-based energy sources. Understanding the workings and trends of the technology is crucial, given the reality. With most conventional PV cells constrained by the Shockley-Queisser limit, new alternatives have been developed to surpass it. One of such variations are heterojunction cells, which, by combining different semiconductor materials, break free from the previous constraint, leveraging the advantages of both compounds. A subset of these cells are multi-junction cells, in their various configurations. These build upon the heterojunction concept, combining several junctions in a cell-a strategy that has placed them as the champions in terms of conversion efficiency. With the aim of modelling a multi-junction cell, several optic and optoelectronic models were developed using a Finite Element Tool. Following this, a study was conducted on the exciting and promising technology that are nanoantenna arrays, with the final goal of integrating both technologies. This research work aims to study the impact of the nanoantennas' inclusion in an absorbing layer. It is concluded that, using nanoantennas, it is possible to concentrate electromagnetic radiation near their interfaces. The field's profiles might be tuned using the nanoantennas' geometrical parameters, which may lead to an increase in the obtained current.

In Situ Grown Nanocrystalline Si Recombination Junction Layers for Efficient Perovskite-Si Monolithic Tandem Solar Cells: Toward a Simpler Multijunction Architecture.

The perovskite-Si tandem is an attractive avenue to attain greater power conversion efficiency (PCE) than their respective single-junction solar cells. However, such devices generally employ complex stacks with numerous deposition steps, which are rather unattractive from an industrial perspective. Here, we develop a simplified tandem architecture consisting of a perovskite n-i-p stack on a silicon heterojunction structure without applying the typically used indium-tin-oxide (ITO) recombination junction (RJ) layer between the top and bottom cells. It is demonstrated that an n-type hydrogenated nanocrystalline silicon (nc-Si:H) grown in situ on an amorphous silicon hole contact layer of the bottom cell acts as an efficient RJ layer, leading to a high open-circuit voltage (VOC) of >1.8 V and a PCE of 21.4% without optimizing the optical design. Compared to the tandem cell with an ITO RJ layer, the nc-Si:H RJ layer not only improves light management but also achieves a higher VOC due to superior contact properties with an overlying SnO2 electron transport layer of the perovskite top cell. Omitting the costly material and its deposition step offers the opportunity toward realizing industrially feasible high-efficiency tandem solar cells.

Integration of Si Heterojunction Solar Cells with III-V Solar Cells by the Pd Nanoparticle Array-Mediated "Smart Stack" Approach.

This paper describes the way to fabricate two-terminal tandem solar cells using Si heterojunction (SHJ) bottom cells and GaAs-relevant III-V top cells by "smart stack", an approach enabling the series connection of dissimilar solar cells through Pd nanoparticle (NP) arrays. It was suggested that placing the Pd NP arrays directly on typical SHJ cells results in poor tandem performance because of the insufficient electrical contacts and/or deteriorated passivation quality of the SHJ cells. Therefore, hydrogenated nanocrystalline Si (nc-Si:H) layers were introduced between Pd NPs and SHJ cells to improve the electrical contacts and preserve the passivation quality. Such nc-Si:H-capped SHJ cells were integrated with InGaP/AlGaAs double-junction cells, and a certified efficiency of 27.4% (under AM 1.5 G) was achieved. In addition, this paper addresses detailed analyses of the 27.4% cell. It was revealed that the cell had a relatively large gap at the smart stack interface, which limited the short-circuit current density (thereby the efficiency) of the cell. Therefore, higher efficiency would be expected by reducing the interfacial gap distance, which is governed by the height of the Pd NPs.

Reaching the Ultimate Efficiency of Solar Energy Harvesting with a Nonreciprocal Multijunction Solar Cell.

The Landsberg limit represents the ultimate efficiency limit of solar energy harvesting. Reaching this limit requires the use of nonreciprocal elements. The existing device configurations for attaining the Landsberg limit, however, are very complicated. Here, we introduce the concept of a nonreciprocal multijunction solar cell and show that such a cell can reach the Landsberg limit in the idealized situation where an infinite number of layers are used. We also show that such a nonreciprocal multijunction cell outperforms a standard reciprocal multijunction cell for a finite number of layers. Our work significantly simplifies the device configuration required to reach the ultimate limit of solar energy conversion and points to a pathway toward using nonreciprocity to improve solar energy harvesting.

One-Pot Synthesis of One-Dimensional Multijunction Semiconductor Nanochains from Cu1.94S, CdS, and ZnS for Photocatalytic Hydrogen Generation.

Chains of alternating semiconductor nanocrystals are complex nanostructures that can offer control over photogenerated charge carriers dynamics and quantized electronic states. We develop a simple one-pot colloidal synthesis of complex Cu1.94S-CdS and Cu1.94S-ZnS nanochains exploiting an equilibrium driving ion exchange mechanism. The chain length of the heterostructures can be tuned using a concentration dependent cation exchange mechanism controlled by the precursor concentrations, which enables the synthesis of monodisperse and uniform Cu1.94S-CdS-Cu1.94S nanochains featuring three epitaxial junctions. These seamless junctions enable efficient separation of photogenerated charge carriers, which can be harvested for photocatalytic applications. We demonstrate the superior photocatalytic activity of these noble metal free materials through solar hydrogen generation at a hydrogen evolution rate of 22.01 mmol g-1 h-1, which is 1.5-fold that of Pt/CdS heterostructure photocatalyst particles.

Liquid-Based Multijunction Molecular Solar Thermal Energy Collection Device.

Photoswitchable molecules-based solar thermal energy storage system (MOST) can potentially be a route to store solar energy for future use. Herein, the use of a multijunction MOST device that combines various photoswitches with different onsets of absorption to push the efficiency limit on solar energy collection and storage is explored. With a parametric model calculation, it is shown that the efficiency limit of MOST concept can be improved from 13.0% to 18.2% with a double-junction system and to 20.5% with a triple-junction system containing ideal, red-shifted MOST candidates. As a proof-of-concept, the use of a three-layered MOST device is experimentally demonstrated. The device uses different photoswitches including a norbornadiene derivative, a dihydroazulene derivative, and an azobenzene derivative in liquid state with different MOSTproperties, to increase the energy capture and storage behavior. This conceptional device introduces a new way of thinking and designing optimal molecular candidates for MOST, as much improvement can be made by tailoring molecules to efficiently store solar energy at specific wavelengths.

Study of the Cross-Influence between III-V and IV Elements Deposited in the Same MOVPE Growth Chamber.

We have deposited Ge, SiGe, SiGeSn, AlAs, GaAs, InGaP and InGaAs based structures in the same metalorganic vapor phase epitaxy (MOVPE) growth chamber, in order to study the effect of the cross influence between groups IV and III-V elements on the growth rate, background doping and morphology. It is shown that by adopting an innovative design of the MOVPE growth chamber and proper growth condition, the IV elements growth rate penalization due to As "carry over" can be eliminated and the background doping level in both IV and III-V semiconductors can be drastically reduced. In the temperature range 748-888 K, Ge and SiGe morphologies do not degrade when the semiconductors are grown in a III-V-contaminated MOVPE growth chamber. Critical morphology aspects have been identified for SiGeSn and III-Vs, when the MOVPE deposition takes place, respectively, in a As or Sn-contaminated MOVPE growth chamber. III-Vs morphologies are influenced by substrate type and orientation. The results are promising in view of the monolithic integration of group-IV with III-V compounds in multi-junction solar cells.

Characterization and Design of Photovoltaic Solar Cells That Absorb Ultraviolet, Visible and Infrared Light.

The world is witnessing a tide of change in the photovoltaic industry like never before; we are far from the solar cells of ten years ago that only had 15-18% efficiency. More and more, multi-junction technologies seem to be the future for photovoltaics, with these technologies already hitting the mark of 30% under 1-sun. This work focuses especially on a state-of-the-art triple-junction solar cell, the GaInP/GaInAs/Ge lattice-matched, that is currently being used in most satellites and concentrator photovoltaic systems. The three subcells are first analyzed individually and then the whole cell is put together and simulated. The typical figures-of-merit are extracted; all the I-V curves obtained are presented, along with the external quantum efficiencies. A study on how temperature affects the cell was done, given its relevance when talking about space applications. An overall optimization of the cell is also elaborated; the cell's thickness and doping are changed so that maximum efficiency can be reached. For a better understanding of how varying both these properties affect efficiency, graphic 3D plots were computed based on the obtained results. Considering this optimization, an improvement of 0.2343% on the cell's efficiency is obtained.

Proton Radiation Hardness of Perovskite Tandem Photovoltaics.

Monolithic [Cs0.05(MA0. 17FA0. 83)0.95]Pb(I0.83Br0.17)3/Cu(In,Ga)Se2 (perovskite/CIGS) tandem solar cells promise high performance and can be processed on flexible substrates, enabling cost-efficient and ultra-lightweight space photovoltaics with power-to-weight and power-to-cost ratios surpassing those of state-of-the-art III-V semiconductor-based multijunctions. However, to become a viable space technology, the full tandem stack must withstand the harsh radiation environments in space. Here, we design tailored operando and ex situ measurements to show that perovskite/CIGS cells retain over 85% of their initial efficiency even after 68 MeV proton irradiation at a dose of 2 × 1012 p+/cm2. We use photoluminescence microscopy to show that the local quasi-Fermi-level splitting of the perovskite top cell is unaffected. We identify that the efficiency losses arise primarily from increased recombination in the CIGS bottom cell and the nickel-oxide-based recombination contact. These results are corroborated by measurements of monolithic perovskite/silicon-heterojunction cells, which severely degrade to 1% of their initial efficiency due to radiation-induced recombination centers in silicon.

Enhancing the photoresponse by CdSe-Dye-TiO2 -based multijunction systems for efficient dye-sensitized solar cells: A theoretical outlook.

This work presents theoretical modeling of some systems, using density functional theory (DFT), for enhancing the photoresponse of a dye-sensitized solar cell. The optimization of the dye (NKX 2587) as well as the dye derivatives was carried out using B3LYP and 6-311g (d,p) level of theory, using DFT as incorporated in Gaussian 03 level of programming. The HOMO-LUMO energy gaps are lower for (CdSe)13 -Dye-(TiO2 )6 multijunction systems in comparison with both the isolated dyes as well as dye-TiO2 systems. The absorption peaks were found to be mostly red-shifted for (CdSe)13 -Dye-(TiO2 )6 multijunction systems with respect to the Dye-TiO2 systems, indicating the enhancement of the absorption behavior of the dye sensitizer by its interaction with the CdSe framework. The results thus indicate some sort of co-sensitization of the TiO2 by the dye sensitizer as well as the CdSe quantum dot and are hence expected to increase the efficiency of the solar device. © 2019 Wiley Periodicals, Inc.

Polar-Induced Selective Epitaxial Growth of Multijunction Nanoribbons for High-Performance Optoelectronics.

Semiconductor heterostructures are basic building blocks for modern electronics and optoelectronics. However, it still remains a great challenge to combine different semiconductor materials in single nanostructures with tailored geometry and chemical composition. Here, a polar-induced selective epitaxial growth method is reported to alternately grow CdS and CdS xSe1- x heterostructure nanoribbons (NRs) side by side in the lateral direction, with the heterointerface (junction) number to be well controlled. Transmission electron microscopy (TEM) and spatial-resolved µ-PL spectra are employed to characterize the heterostructure NRs, which indicate that the achieved NRs are high-quality heterostructures with sharp interfaces. Kelvin probe force microscopy (KPFM) and femtosecond pump-probe characterizations further confirm the efficient charge-transfer process across the interfaces in the multijunction NRs. Photodetectors based on the achieved NRs are realized and systematically investigated, demonstrating junction number-dependent optoelectronic response behaviors. NRs with more junctions exhibit more superior device performances, reflecting the important roles of the high-quality interface regions. Based on this multijunction NRs device, high on-off ratio (107) and remarkable responsivity (1.5 × 105 A/W) are demonstrated, both of which represent the best results compared to the reported CdS, CdSe, and their heterostructures. These novel multijunction NRs may find broad applications in future integrated photonics and optoelectronics devices and systems.