Bottom Cell Research Articles

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.

Proposal and design of organic/CIGS tandem solar cell: Unveiling optoelectronic approaches for enhanced photovoltaic performance

Solar cells are promising devices for converting sunlight into electrical energy. Tandem solar devices, which combine multiple sub cells with complementary absorption spectra, offer a potential strategy to enhance the overall power conversion efficiency (PCE). In this work, the design and performance of organic/CIGS tandem solar devices are investigated. The initial tandem cell comprises two sub cells, namely an organic-based PM6:m-DTC-2 F top cell and CIGS bottom cell. The organic and CIGS cells show a PCE of 12.20% and 20.10%, respectively, where the initial results are based on calibrated cells reproduced from experimental studies. Accordingly, the initial tandem PM6:m-DTC-2 F/CIGS cell shows a PCE of 22.75%. The study focuses on optimizing the PCE via different approaches. Firstly, through the exploration of different hole transport layers (HTLs) of the front cell. Secondly, by investigating the matched current condition between the front and rear cells. Moreover, the doping effect of the CIGS film is investigated. Finally, optimization of the defect concentrations in the absorbers is introduced. These optimizations result in open circuit voltage (Voc) of 1.89 V with short circuit current (Jsc) of 17.55 mA/cm2, and a fill factor (FF) of 82.79%. Consequently, the PCE of the optimized tandem cell is further enhanced to 27.46%. Further, the stability of the proposed tandem cell is evaluated under varying temperatures, and its composition of flexible materials makes it a promising candidate for wearable applications.

Sodium-based di-chalcogenide: a promising material for tandem solar cells

Compounds based on chalcogen elements are widely studied currently due to their many interesting applications for electronic devices. The sodium-based dichalcogenide (NaNbS2) is a fascinating material with storage and conversion energy applications. In this paper, we conduct a first-principles investigation of the structural and thermodynamic stability and electronic properties of this material. We analyze a total of four structures to find the ground state using a fourth-order Birch–Murnaghan equation of state: the α and η related to the A-phase and the ζ 1 and ζ 2 related to the B-phase. We carefully address the exchange-correlation effects using the semi- local GGA-PBEsol targeted for solids. To analyze the electronic structure with higher accuracy, we implement the quasi-particle G o W o approximation. Our results for the fourth-order Birch–Murnaghan equation show that the most thermodynamically stable phase at zero temperature is α. To provide experimentalists insights about the possible routes to grow these materials, we calculated the convex hull of the α-model and ζ 1-model, finding that both are energetically stable. Finally, the calculated band gap with quasiparticle corrections for the α-model is 1.03 eV, which suggests possible applications of this material as a bottom cell in modern solar cells.

Design and parametric optimization of lead free perovskite based tandem solar cell for higher efficiency

A critical concern is the creation of effective and eco-friendly photovoltaic materials. The non-toxic perovskite substance methyl ammonium germanium tri-iodide (MAGeI3) has several applications in tandem solar cells. In this research, the efficiency of a multi-junction tandem solar cell with MAGeI3 as the absorbent layer in the top cell and FeSi2 as the absorbent layer in the bottom cell was examined by a thorough simulation utilising SCAPS-1D software. The simulation took into account several variables, including the thickness, energy level, bandgap, and carrier concentration of the various layers. According to the simulation’s findings, the efficiency is 27.25% and the fill factor (FF) is around 80%. MAGeI3 is a safe and effective substitute material compared to other lead-containing perovskite materials. This work emphasises the potential of (MAGeI3) as a productive and eco-friendly component for tandem solar cells.

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.

Reducing Hole Trap Density in Sn–Pb Perovskite Solar Cells via Molecular Phenylhydrazine

Narrow‐bandgap Sn–Pb‐alloyed perovskite is potential for a bottom cell in perovskite tandem solar cells. However, Sn‐contained perovskites tend to be readily oxidized in air atmosphere, which has ill influence on stability and photovoltaic performance. Herein, a method to suppress oxidation of divalent tin via post‐treatment of perovskite film with phenylhydrazine is reported. Phenylhydrazine‐treated FA0.5MA0.5Pb0.5Sn0.5I3 perovskite films effectively reduce the hole traps caused by the oxidation of Sn2+ on the perovskite surfaces. In addition, surface energetics are well aligned by post‐treatment, which is beneficial for voltage gain and charge transport. As a result, power conversion efficiency (PCE) is increased from 18.16% to 20.67% after post‐treatment due mainly to the significant improvement of open‐circuit voltage from 0.75 to 0.83 V. Furthermore, the device stability is improved, where 91.67% of initial PCE is maintained after 1000 h.

Both-Sided TOPCon Solar Cells – Towards a Lean Process Flow for Silicon Bottom Solar Cells

Silicon solar cells with both-side full-area passivating hole and electron contacts are viable candidates for application as bottom cells in tandem architectures. In this contribution, cells with poly-Si based contacts at both sides are investigated as a potential upgrade to the emerging i-TOPCon single junction devices featuring a passivating contact (poly-Si(n)/SiOx) only at the rear side and an alternative to the SHJ technology featuring a-Si based full-area passivating hole and electron contacts. We show that a higher thermal budget is needed to mediate between poly-Si(p)/SiOx hole and poly-Si(n)/SiOx electron contact when the same thermal interfacial SiOx is applied. This is addressed by using a PECVD oxide that is adapted to the needs of the hole and/or the electron contact. We present a proof-of-concept poly-Si(n)/SiOx/c-Si/SiOx/poly-Si(n) device with emphasis on a lean process flow. An all-PECVD process sequence for the hole contact, i.e., plasma oxides+i/p-a-Si deposited at 200°C in the same chamber was applied. With co-annealing of the contacts, no additional hydrogenation, and no edge insulation step, we observe no shunting and obtain efficiencies of up to 20.8% for M2 size cells, so far.

Key Aspects of p-Type TOPCon on Textured Surface for Silicon Bottom Cells in Tandem Devices

Both-sides TOPCon solar cells are an interesting candidate for a highly efficient and thermally robust Silicon (Si) bottom cell for tandem devices, such as Perovskite-Si solar cells. However, preparation of p-type TOPCon on a textured surface is necessary which is particularly challenging. This work aims to gain a deeper understanding of the limiting factors and thereby optimize the SiOx/poly-Si(p) contact to close the gap to its n-type counterpart. Using symmetrical lifetime samples, we first show that a high level of surface passivation can be achieved using thermally grown interfacial oxides of different thicknesses. The samples strongly benefit from an effective thermal activation of hydrogenation by means of fast-firing. Thus, a recombination current density J0s as low as 23.4 fA/cm² was achieved for p-type TOPCon on textured surface featuring an in situ Boron-doped poly-Si layer prepared by plasma enhanced chemical vapor deposition (PECVD). Moreover, we show that the passivation quality strongly depends on surface morphology. Smoothening the random pyramids’ valleys and – what is more – the tips has a positive impact on the surface passivation.

Optical Analysis of Perovskite/Silicon Tandem Solar Cells: Effect of Rear Side Grating and TOPCon Tunnel Junction

In the focus of the presented work is the analysis of a rear side reflection grating in context of a perovskite/silicon tandem solar cell. The typical configuration of a perovskite/Si tandem device requires a hole transport layer at the rear side of the silicon solar cell (p-i-n bottom cell structure). As such a poly-Si passivating contact like TOPCon is an attractive candidate. Until now, research faces the challenge to deposit p-TOPCon layers with good surface passivation properties on textured surfaces. A planar surface would avoid this issue and due to its smaller surface area intrinsically allowing for a better passivation. The optical disadvantage of the planar rear side can be eliminated by an appropriate optical grating at the rear side which enables ideal light trapping. In this work a new approach is developed to describe the optical properties of a diffraction grating as structured rear side reflector in a silicon bottom cell. The light distribution and the parasitic absorption per grating interaction are fitted to reflectance and absorptance measurements of a III-V on silicon triple-junction device with a rear grating structure. Compared to previous models, this new approach makes it possible to reliably quantify the different loss mechanisms in the spectral region above 1000 nm. An application of the simulation model to a perovskite/silicon tandem device shows the potential of the system with rear side grating. In addition, an integration of a TOPCon tunnel junction is evaluated and a process chain for the integration of a structured rear side reflector into the tandem system is discussed.

A cathodoluminescence study of InP/InGaP axially heterostructured NWs for tandem solar cells

Axially heterostructured nanowires (NWs) constitute a promising platform for advanced electronic and optoelectronic nanodevices. The presence of different materials in these NWs introduces a mismatch resulting in complex strain distributions susceptible of changing the band gap and carrier mobility. The growth of these NWs presents challenges related to the reservoir effect in the catalysts droplet that affect to the junction abruptness, and the occurrence of undesired lateral growth creating core–shell heterostructures that introduce additional strain. We present herein a cathodoluminescence (CL) analysis on axially heterostructured InP/InGaP NWs with tandem solar cell structure. The CL is complemented with micro Raman, micro photoluminescence (PL), and high resolution transmission electron microscopy measurements. The results reveal the zinc blende structure of the NWs, the presence of a thin InGaP shell around the InP bottom cell, along with its associated strain, and the doping distribution.

Design and performance investigation of CIGS/SWCNT tandem solar cell for efficiency improvement

This research work demonstrates the design and simulation of highly efficient tandem solar cell by using SCAPS-1D software. This tandem solar cell has been designed with a higher bandgap Copper Indium Gallium Diselenide (CIGS) absorber as a top cell and a lower bandgap Single Wall Carbon Nano Tube (SWCNT) absorber as a bottom cell, which are arranged in the series connection. Firstly, both top and bottom solar cells were analysed under standalone afterward, tandem configuration was developed with the top cell illumined by AM 1.5 spectrum and the bottom cell illumined by AM 1.5 filtered by top cell spectrum. The effects of absorber thickness, doping density, and bandgap on the output characteristics of the solar cell are also investigated. The current matching of both cells is attained by tuning the bandgap and thickness of Cu(In1-xGax)Se2 absorber of the upper cell. The remarkable efficiency of 38.91 % (Voc = 1.3441 V, Jsc = 37.87 mA/cm2 and FF = 76.41 %) is attained of the proposed CIGS/SWCNT tandem device at a thickness of 860 nm and 2000 nm for absorber of top and bottom cell respectively. The results of this research suggest a route for the fabrication of CIGS/SWCNT tandem solar cell with high efficiency.

Scalable all‐perovskite double‐ and triple‐junction solar modules: Modeling for configuration optimization

AbstractWe modeled the photovoltaic conversion of all‐perovskite (PVK) double‐ and triple‐junction solar modules to clarify the configurations suitable for the monolithically series‐interconnected structure, which offers high scalability by fully exploiting the advantages of the thin‐film modules over wafer‐based crystalline‐silicon modules. We first formulated the photovoltaic processes of single cells and modules by reference to previously reported data, next optimized the module structure parameters including the bandgaps of PVKs, cell widths, and transparent‐electrode thicknesses, and then evaluated the annually averaged conversion efficiencies (ηannual) defined by the ratio of the annual energy yield to the annual insolation in outdoor environments using a meteorological database. The double‐junction four‐terminal (2J‐4T) module overcomes the shortcomings involved in the two‐terminal module consisting of series‐connected top and bottom cells, providing higher ηannual and more options of the top‐cell bandgap; the latter allows us to select a more durable PVK composition. However, the dual output (four terminals) is practically a serious drawback. The double‐junction voltage‐matched (2J‐VM) configuration eliminates this drawback, that is, realizes the single output (two terminals) with taking over the advantages of 2J‐4T, and hence, 2J‐VM would be the most promising candidate. However, when the VM configuration is applied to the triple‐junction modules, the ohmic loss and optical loss in the transparent electrodes used for the three submodules are more detrimental. To mitigate this shortcoming, we proposed a new configuration of the triple‐junction series/parallel‐connecting voltage‐matched (3J‐SPVM) module. This uses only two substrates with securing high ηannual and other advantages of the VM configuration, which contributes to cost reduction. Consequently, 3J‐SPVM is potentially the most promising configuration for widespread use.

Charting the Irreversible Degradation Modes of Low Bandgap Pb‐Sn Perovskite Compositions for De‐Risking Practical Industrial Development

AbstractThe commercialization of a solar technology necessitates the fulfillment of specific requirements both regarding efficiency and stability to enter and gain space in the photovoltaic market. These aims are heavily dependent on the selection of suitable materials, which is critical for suppressing any reliability risks arising from inherent instabilities. Focusing on the absorber material, herein the most suitable low bandgap lead‐tin composition candidate for all‐perovskite tandem applications is investigated by studying their degradation mechanisms with both widely available and advanced characterization techniques. Three irreversible degradation processes are identified in narrow bandgap Pb‐Sn perovskite absorbers: 1) Tin (Sn) oxidation upon air exposure, 2) methylammonium (MA) loss upon heat exposure, and 3) formamidinium (FA) and cesium (Cs) segregation leading to impurity phase formation. From an industrial perspective, it is proposed to refocus attention on FASn0.5Pb0.5I3 which minimizes all three effects while maintaining a suitable bandgap for a bottom cell and good performance. Moreover, a practical and highly sensitive characterization method is proposed to monitor the oxidation, which can be deployed both in laboratory and industrial environments and provide useful information for the technological development process, including, the effectiveness of encapsulation methods, and the acceptable time windows for air exposure.

Lithography‐free fabrication of a local contact interlayer for Si‐based tandem solar cells

AbstractCurrently, the Si solar cell market share is dominated by PERC solar cells. Although the efficiency of PERC solar cells has been steadily increasing, it is expected to reach the practical efficiency limit in the near future. The thin film/PERC Si tandem cell technique can be one of the solutions to overcome the single‐cell efficiency limit. In this study, we developed a novel interlayer fabrication technology for the diffused junction Si solar cells of the PERC and Al BSF cell architectures. We combined laser contact opening (LCO) and laser‐induced forward transfer (LIFT) processes to fabricate local contact opening with low contact resistance while maintaining the high passivation performance of the Si bottom cell. The dielectric‐passivated emitter of the Si solar cell was ablated locally by the LCO process, and subsequently, the Ti nanoparticles were transferred selectively by the LIFT process to the opened emitter region followed by transparent conducting oxide deposition. Laser process parameters were carefully optimized to fabricate low‐loss interlayers. We applied the developed interlayer fabrication technology to the Si bottom cells of Al BSF and PERC cells. Finally, we demonstrated successfully the perovskite/PERC Si tandem cell with an interlayer developed in this study. The developed interlayer fabrication technology does not include a photolithography step and vacuum deposition processes for buffer metals; thus, we expect it may be more compatible with the mass production of thin film/diffused junction Si tandem solar cells.

Bandgap Pairing in Three‐Terminal Tandem Solar Cells: From Limiting Efficiency to Voltage‐Matched Device Performance

Three‐terminal tandem solar cells (3 T TSCs) have recently sparked increasing interest as they feature a lean monolithic device architecture similar to two‐terminal TSCs and, like four‐terminal TSCs, do not require current matching for optimal operation. In this contribution, detailed balance limit calculations for different combinations of top and bottom cell bandgaps are conducted to determine the optimum bandgap pairing and limiting efficiency of 3T TSCs. An experimental realization of a 3T TSC with perovskite and silicon sub‐cells and a combined efficiency of 28.9% is presented and used to derive a realistic parameterization for non‐radiative recombination. Herein, the optimum bandgap pairing and resulting maximum efficiency under voltage‐matched conditions for voltage‐matching ratios of 1:2 and 2:3, which is relevant for stringing and module integration of 3T TSCs, are further determined. To this end, non‐radiative recombination is incorporated in the model and quantified by matching theoretical open‐circuit voltages and those of real‐world high‐efficiency solar cells based on different absorber materials (and thus bandgaps), including the perovskite top cell of the best in‐house 3T TSC.

Mechanically Stacked Solar Cells for Concentrator Photovoltaics

The power output of dual-junction mechanically stacked solar cells comprising different sub-cell materials in a terrestrial concentrating photovoltaic module has been evaluated. The ideal bandgap combination of both cells in a stack was found using EtaOpt. A combination of 1.4 eV and 0.7 eV has been found to produce the highest photovoltaic conversion efficiency under the AM1.5 Direct Solar Spectrum with x500 concentration. As EtaOpt does not consider the absorption profile of solar cell materials; the practical power output per unit area of a dual junction mechanically stacked solar cell has been modelled considering the optical absorption co-efficients and thicknesses of the individual solar cells. The model considered a GaAs top cell and a Ge, GaSb, Ga0.47In0.53As or Si bottom cell. It was found that GaSb gives the highest power contribution as a bottom cell in a dual junction configuration followed by Ge and GaInAs. While the additional power provided by a Si bottom cell is less than these it remains a suitable candidate for a bottom cell owing to its lower cost.

Pathways toward commercial perovskite/silicon tandem photovoltaics.

Perovskite/silicon tandem solar cells offer a promising route to increase the power conversion efficiency of crystalline silicon (c-Si) solar cells beyond the theoretical single-junction limitations at an affordable cost. In the past decade, progress has been made toward the fabrication of highly efficient laboratory-scale tandems through a range of vacuum- and solution-based perovskite processing technologies onto various types of c-Si bottom cells. However, to become a commercial reality, the transition from laboratory to industrial fabrication will require appropriate, scalable input materials and manufacturing processes. In addition, perovskite/silicon tandem research needs to increasingly focus on stability, reliability, throughput of cell production and characterization, cell-to-module integration, and accurate field-performance prediction and evaluation. This Review discusses these aspects in view of contemporary solar cell manufacturing, offers insights into the possible pathways toward commercial perovskite/silicon tandem photovoltaics, and highlights research opportunities to realize this goal.

Enhanced efficiency in two-terminal all-perovskite tandem solar cells via binary functional high polymer doping strategy

An enhanced power conversion efficiency (PCE) of 26.1 % has been achieved in two-terminal (2T) all-perovskite tandem perovskite solar cells (TPSCs) via a binary functional high polymer doping strategy. By incorporating methylene diphenyl diisocyanate polyurethane (MDI-PU) and PJ71 into the wide bandgap (WBG) and narrow bandgap (NBG) perovskite layers, respectively, the perovskite crystalline quality of them was significantly improved. The optimized PCE of the WBG top cell and the NBG bottom cell achieved 17.6 % and 20.9 %, respectively. After continuous dark storage of 150 days in nitrogen-filled glovebox, the PCE of optimized TPSC remained 94.2 % of initial. TPSCs on flexible substrates of polyethylene naphthalate (PEN) and silk-derived substrate (SDS) were also studied. With the binary polymer doping strategy, the PEN-based TPSC achieved a champion PCE of 22.2 %, and exhibited excellent anti-bending ability compared with control. The champion SDS-based TPSC achieved the PCE of 11.6 %, and showed enhanced anti-stretching ability. We demonstrate that strong coordinate bonding formed between the O atom in the carbonyl group of MDI-PU and the Pb2+ ions at the grain boundaries (GBs) in WBG perovskite. For the NBG layer, the S atoms in the thiophene structure of PJ71 interact with the Pb2+ and Sn2+ ions in the NBG perovskite and effectively passivate the GB defects. The study suggests the potential for developing new generation multi-functional optoelectronic high polymers to further boost the performance of 2T all-perovskite TPSCs.

Possibility of highly efficient 2D–3D perovskite/CIGS tandem solar cells with over 30% efficiency

Tandem photovoltaics consisting of a wide-bandgap top cell and a narrow-bandgap bottom cell have shown great potential to exceed current single-junction photovoltaics.

Improvement of the Sb2Se3/Si tandem cell and simulation in SCAPS

In the work, tandem solar cells were designed to achieve improved efficiency using Sb2Se3/Si as the materials. The work involved simulating the Sb2Se3/Si structures, where Sb2Se3 served as the top cell and Si as the bottom cell, under AM1.5 G solar spectrum illumination using the one-dimensional device simulator Scaps. The obtained results demonstrated a significant increase in the efficiency of the Sb2Se3/Si tandem solar cell. Initially, the top cell had an efficiency of 14.29%, while the bottom cell had an efficiency of 11.22% when considered as single cells in a row. When used in tandem, the efficiency of the Sb2S3/Si tandem cell improved to 14.6%. Additionally, there were notable improvements in the electrical parameters of the solar cell. The overall efficiency (ƞ) reached 21.41%, the open-circuit voltage (Voc) was 1.37volts, the short-circuit current density (Jsc) was 19.78 mA/cm2, and the fill factor (FF) reached 78.62%. These results highlight the success of the Sb2S3/Si tandem solar cell in achieving higher efficiency and improved electrical performance compared to single cells.