Tandem Solar Cell Devices Research Articles

Thickness Optimization of Front and Recombination ITO in Monolithic Perovskite/Silicon Tandem Solar Cells

Optical losses of perovskite/silicon tandem solar cells can be effectively reduced by optimizing the thin‐film layer thicknesses. Herein, the thicknesses of DC sputtered indium tin oxide (ITO) films, which serve as the front electrode and the recombination layer connecting the subcells, are optimized to reach high transparency and good lateral charge transport simultaneously. Optical simulations of the full perovskite/silicon tandem solar cell stacks are performed to find the optimum recombination and front electrode ITO thicknesses for solar cells as well as modules. Implementation of the optimized 25 nm front electrode ITO thickness in semitransparent single‐junction perovskite solar cells increases the short‐circuit density by 1.5 mA cm−2 compared to the former reference thickness of 75 nm. Combined with an optimized 20 nm recombination ITO layer, high short‐circuit density of 20.3 mA cm−2 is reached in perovskite/silicon tandem solar cell devices, which is the highest reported value for planar front perovskite/silicon tandem solar cells to the best of knowledge. Further interface passivation enables 28.8% power conversion efficiency.

Current matching and filtered spectrum analysis of wide-bandgap/narrow-bandgap perovskite/CIGS tandem solar cells: A numerical study of 34.52 % efficiency potential

Perovskite (PVK) materials with wide-bandgap (WBG) play a crucial role in achieving high-performance tandem devices but phase segregation and open-circuit voltage (VOC) loss hinders in surpassing the efficiency of single-junction solar cells. Present work discloses a numerical simulation which has been carried out using a WBG material CH3NH3PbI3-xClx of bandgap (Eg) 1.65eV as an absorber layer of the top cell (TCELL) and a Cu(In,Ga)Se2-based cell having narrow-bandgap (NBG) of Eg (1.27eV) has been used as bottom cell (BCELL). The TCELL utilizes polyethyleneimine ethoxylated (PEIE) interfacial layer to promote charge-collection and charge-tunneling. The TCELL and BCELL have been optimized by various optimization processes such as simultaneous thickness variation of active-region with defect density (DD), variation of interface defect density (IDD) with solar-cell parameters a commendable power conversion efficiency (PCE) of 27.06 %, and 26.77 % has been achieved for respective solar-devices. Variations of numerous electron transport layer (ETL) and hole transport layer (HTL) have also been performed to acquire highly optimized TCELL. Thus, a perovskite/CIGS tandem solar cell (PVK/CIGS-TSC) device has been obtained using filtered-spectra analysis and current-matching (method which display a promising photovoltaic (PV) parameter with a high VOC of 2.29 V, short-circuit current density (JSC) of 17.41 mA/cm2, fill factor (FF) of 86.45 % and PCE of 34.47 %. These discoveries hold significant promise for the future development of TSCs.

Narrow Bandgap Metal Halide Perovskites for All-Perovskite Tandem Photovoltaics.

All-perovskite tandem solar cells are attracting considerable interest in photovoltaics research, owing to their potential to surpass the theoretical efficiency limit of single-junction cells, in a cost-effective sustainable manner. Thanks to the bandgap-bowing effect, mixed tin-lead (Sn-Pb) perovskites possess a close to ideal narrow bandgap for constructing tandem cells, matched with wide-bandgap neat lead-based counterparts. The performance of all-perovskite tandems, however, has yet to reach its efficiency potential. One of the main obstacles that need to be overcome is the─oftentimes─low quality of the mixed Sn-Pb perovskite films, largely caused by the facile oxidation of Sn(II) to Sn(IV), as well as the difficult-to-control film crystallization dynamics. Additional detrimental imperfections are introduced in the perovskite thin film, particularly at its vulnerable surfaces, including the top and bottom interfaces as well as the grain boundaries. Due to these issues, the resultant device performance is distinctly far lower than their theoretically achievable maximum efficiency. Robust modifications and improvements to the surfaces of mixed Sn-Pb perovskite films are therefore critical for the advancement of the field. This Review describes the origins of imperfections in thin films and covers efforts made so far toward reaching a better understanding of mixed Sn-Pb perovskites, in particular with respect to surface modifications that improved the efficiency and stability of the narrow bandgap solar cells. In addition, we also outline the important issues of integrating the narrow bandgap subcells for achieving reliable and efficient all-perovskite double- and multi-junction tandems. Future work should focus on the characterization and visualization of the specific surface defects, as well as tracking their evolution under different external stimuli, guiding in turn the processing for efficient and stable single-junction and tandem solar cell devices.

Chalcogenide Perovskite, An Emerging Photovoltaic Material: Current Status and Future Perspectives

AbstractChalcogenide perovskite materials came into the picture of photovoltaic technology since 2015. Their admirable structural, electronic and optical properties make them highly promising for solar energy conversion. They have immense potential to solve the stability and toxicity issues of conventional perovskite solar cells. The maximum theoretical power conversion efficiency reported for them is about 30 % which is similar to CH3NH3PbI3 perovskite material. However their application in photovoltaics is limited by several challenges. In this review, we tried to provide an overview of the structural features of the chalcogenide perovskite materials and critically highlighted computational and synthetic accomplishments in this area. Various techniques developed for synthesizing these materials so far, have also been summarized. Along with that, the challenges associated in designing these materials for improved optoelectronic properties are also addressed. Serious research efforts are urgently needed for the realization of the dream of chalcogenide perovskite material based solar cells in terms of optimization of optoelectronic properties, synthetic cost, processing of thin films and application in tandem solar cell devices. This review may facilitate further progress in chalcogenide perovskite material based thin film solar cell in the future.

Simulation study of single solar cell structures based on the compositionally variable perovskite material CsSn(I1−xBrx)3 for tandem configured solar cells

This work involves simulating the performance of a single solar cell configured TiO2/CsSn(I1−xBrx)3/Cu2O, based on the lead-free, metallic CsSn(I1−xBrx)3 perovskite absorber, whose bandgap energy is tunable as a function of its bromine content. The simulation model used takes into account the variation of the physical parameters of the absorber as a function of its composition, represented by the ratio x, and also makes it possible to obtain the photovoltaic performance of the device studied as a function of x. The state of the front interface between the ETL electron-transport layer and the perovskite absorber was evaluated as a function of (x), by calculating the conduction bandgap offset and the misfit strain. Different electron transport layers were considered. The simulation results showed that the absorber composition influences all photovoltaic parameters, and that the choice of electron transport layer is important to achieve a compromise between device performance and the correct state of the front interface. The results indicate that CdZnS and CdS ETL materials correspond to bromine absorption ratios of x = 0 and x = 0.35 for which the conduction band offset values are − 0.03 eV and 0 eV respectively and the misfit strain 0.09 and 0.057, while ZnSe and ZnS correspond to ratios of x = 0.75 and x = 1 for which the conduction band offsets are 0 eV and 0.38 eV respectively and the strain 0.056 and 0.08. Based on these results, we determined the following optimal structures: CdZnS/CsSnI3/Cu2O and CdS/CsSn(I0.65Br0.35)3/Cu2O, which achieved yields of 18.1% and 18.5% respectively, as suitable for lower subcells with narrow bandgap energies, while the structures: ZnSe/CsSn(I0.25Br0.75)3/Cu2O and ZnS/CsSnBr3/Cu2O, which achieved efficiencies of 17.5% and 18% respectively, as top sub-cells with wide bandgap energies, in a tandem solar cell device. The aim is to develop highly efficient, non-toxic and stable single and tandem perovskite cells.

The Intermediate Connection of Subcells in Si-based Tandem Solar Cells.

Tandem solar cells are rationally designed and fabricated by stacking multiple subcells to achieve power conversion efficiency well above the Shockley-Queisser (SQ) limit. There is a large selection pool for the subcell candidates, such as Si, III-V, Kesterite, Perovskite, and organic solar cells. A series of different combinations of these subcells have been successfully demonstrated in practical tandem solar cell devices. However, there has not been a systematic summary of how to connect subcells in a tandem solar cell using a practical, cost-effective, and efficiency-beneficial fashion. In this work, the connection manners of subcells within a tandem cell are classified into three main categories, performing sequential growth, using the physical connection, and applying an intermediate layer, focusing on systematical description of intermediate layers using different materials. The advantages and disadvantages of these connection methods and their applicability to tandem cell types are further elaborated using two typical example models, III-V/Si and Perovskite inclusive tandem cell devices. Eventually, this work can provide useful guidance on how to carry out a suitable intermediate connection in the design of tandem solar cells depending on the selected subcells and device structure.

Optical simulations and optimization of perovskite/CI(G)S tandem solar cells using the transfer matrix method

In this work we employ the transfer matrix method for the analysis of optical materials properties to simulate and optimize monolithic tandem solar cell devices based on CuIn1−x Ga x Se2, CI(G)S, and perovskite (PVK) absorbers. By finding models that fit well the experimental data of the CI(G)S solar cell, the semitransparent perovskite solar cell (PSC) and the PVK/CI(G)S monolithic tandem solar cell, we were able to perform a detailed optical loss analysis that allowed us to determine sources of parasitic absorption. We found better substitute materials for the transport layers to increase the power conversion efficiency and, in case of semitransparent PSCs, sub-bandgap transmittance. Our results set guidelines for the monolithic PVK/CI(G)S tandem solar cells development, predicting an achievable efficiency of 30%.

Laser-based monolithic series interconnection of two-terminal perovskite-CIGSe tandem solar cells: determination of the optimal scribe line properties

To achieve a monolithic series interconnection of tandem solar cell devices consisting of a perovskite top cell and a CIGSe bottom cell, a two-terminal interconnection scheme is introduced that includes an additional, fourth patterning step, the so-called iso-cut, which separates the window layer stack between the two solar cells. The implementation of this interconnection scheme requires a process development for a total of four structuring steps, which was achieved by systematically varying the laser parameters. Based on a detailed characterization of the individual scribe line properties with respect to their scribe line depth, morphology, electrical functionality, chemical composition and their influence on adjacent and underlying layers, the optimal patterning parameters and suitable process windows were derived for each step, which is a prerequisite for a loss-free monolithic series interconnection in a tandem module.

Numerical modeling and performance analysis of Sb-based tandem solar cell structure using SCAPS – 1D

Antimony sulfo-selenide, Sb2(S-Se)3including Sb2S3 and Sb2Se3 are known as the binary type metal chalcogenides semiconductors. They carry abundant elemental storage which is nontoxic in nature, having good stability at elevated temperature and have acceptable physical specifications regarding light absorbance substances in photovoltaic devices. Exploiting from the earlier study related to thin film devices and from the recent nanostructured photovoltaics, the Sb2S3 and Sb2Se3is being considered as an absorber layer for hetero-junction type planer solar cells or in sensitized-architecture solar cells. To show a perspective of this sphere of study, this paper exhibits some distinctive studies of the physical parameters of the Sb2(S,Se)3 device. In this optimization process, Sb2Se3 and Sb2S3 used as a tandem structure in the conventional thin film solar cell structure. Different parameters like; thickness, radiative recombination, carrier concentration and series/shunt resistance of the device has been optimized using SCAPS-1D software sequentially. After the optimization of all the above discussed parameters, a reasonable power conversion efficiencies of 13.2% was achieved. With the optimized value, how to achieve a device with good PCE is discussed. Finally, this paper analyses a detail study about the performance of antimony sulphide-selenide based tandem solar cell device for photovoltaic applications.

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.

Dry-wet hybrid deposition of wide-bandgap mixed-halide perovskites for tandem solar cell applications

The tandem architecture of solar cells is expected to be one of the promising strategies for further improving the performance of photovoltaic devices. While a dry-wet hybrid deposition method is highly promising for fabricating perovskite top cells on textured silicon bottom cells, the bandgap tunability of this deposition method has not been investigated in detail. Here, we show the systematic investigation on mixed-halide perovskite films fabricated by the dry-wet hybrid deposition method on a PbBr2 underlayer. The thermally deposited PbBr2 underlayer is demonstrated to be beneficial for promoting reaction with an organic halide precursor solution, resulting in a dense morphology with large perovskite grains. Furthermore, the hybrid deposition method is also shown to be a straightforward way for wide-range bandgap tuning as it can facilitate halogen mixing. These findings provide useful guidelines for further improving the power conversion efficiency of multijunction tandem solar cell devices.

High‐Efficiency Solution‐Processed Two‐Terminal Hybrid Tandem Solar Cells Using Spectrally Matched Inorganic and Organic Photoactive Materials

AbstractAlthough the power conversion efficiency (PCE) of inorganic perovskite‐based solar cells (PSCs) is considerably less than that of organic‐inorganic hybrid PSCs due to their wider bandgap, inorganic perovskites are great candidates for the front cell in tandem devices. Herein, the low‐temperature solution‐processed two‐terminal hybrid tandem solar cell devices based on spectrally matched inorganic perovskite and organic bulk heterojunction (BHJ) are demonstrated. By matching optical properties of front and back cells using CsPbI2Br and PTB7‐Th:IEICO‐4F BHJ as the active materials, a remarkably enhanced stabilized PCE (18.04%) in the hybrid tandem device as compared to that of the single‐junction device (9.20% for CsPbI2Br and 10.45% for PTB7‐Th:IEICO‐4F) is achieved. Notably, the PCE of the hybrid tandem device is thus far the highest PCE among the reported tandem devices based on perovskite and organic material. Moreover, the long‐term stability of inorganic perovskite devices under humid conditions is improved in the hybrid tandem device due to the hydrophobicity of the PTB7‐Th:IEICO‐4F back cell. In addition, the potential promise of this type of hybrid tandem device is calculated, where a PCE of as much as ≈28% is possible by improving the external quantum efficiency and reducing energy loss in the sub‐cells.

Numerical investigation of CIGS thin-film solar cells

The high efficiency of thin Cu(In1−xGax)Se2 solar cells is investigated using the Atlas SILVACO-TCAD tools. The CIGS model is based on an experimental study and takes into account the physical properties, dimensions, and thicknesses of the different layers. The main features model used consists of the thermionic emission concept that was used to describe the carrier transport model across the buffer/absorber heterojunction. Thermionic emission was found to be an important mechanism for carrier transport across the hetero-interface since it produced a matching behaviour for the cell under study with experimental behaviour, thus validating the considered CIGS model for further investigation. To improve efficiency, magnesium fluoride (MgF2) is used as the preferred anti-reflective coating to maximize the photocurrent from the incident light intensity. The effect of varying the single-layer antireflective coating thickness variation, surface recombination velocity at the hetero-interface between CdS and CIGS layers, absorber thickness variation, and gallium content in the Cu(In1−xGax)Se2 on cell performance are studied under AM 1.5G, 1 sun illumination. The novelty of this research includes the configuration of the p-CIGS/n-CdS cells with thin CIS material used for the back surface field layer to improve efficiency. The result has shown that a cell performance of 25% efficiency can be achieved using a 1 µm thick absorber and an MgF2 anti-reflective layer. This result can further aid the development of ultrathin, flexible, and tandem solar cell devices.

Hybrid III-V/SiGe solar cells grown on Si substrates through reverse graded buffers

In the search for a hybrid III-V/Si photovoltaic technology, a tandem GaAsP/SiGe solar cell grown on silicon substrate have been developed using SiGe/Ge reverse graded buffers. Reverse buffer layers can be made thinner than conventional designs, favour the reduction of the threading dislocation density and allow the growth of thicker active layers in the same growth time, but also facilitate the appearance of cracks. In this work, experimental results of functional tandem solar cell devices manufactured with this approach and strategies for their optimization are presented.

Characterization of ZnSiP2 Films Grown on Si Substrate by Liquid Phase Epitaxy: Morphology, Composition, and Interface Microstructure

The exploration and growth of novel cost-effective top cell materials is one of the great hot topics for the development of Si-based tandem solar cells. ZnSiP2 was thought to be a promising candidate of the top cell material for the Si tandem solar cell. In this work, ZnSiP2 single crystal film with the stoichiometric ratio was successfully grown on a Si substrate by the liquid phase epitaxy method for the first time. The surface morphology, thickness, composition, and the orientation of the film were characterized by SEM, EDS, EPMA, and the high-resolution XRD. The results show that a continuous epitaxial ZnSiP2 film with the thickness of 3 μm was grown on the Si substrate. In addition to this continuous film, many island single crystals were also observed on the surface of the film. The high-resolution XRD φ scan results indicated that the film as well as these isolated islands crystallized with the same orientation, c-orientation with respect to the (100) Si substrate, without any misoriented domains. The microstructure of the ZnSiP2/Si interface characterized by the high-resolution transmission electron microscopy image, as well as the selected area electron diffraction patterns of the ZnSiP2 thin films and Si substrates, also confirmed the epitaxial growth of ZnSiP2 on the Si substrate. The electrical conductivity σ, Hall mobility μH, and carrier concentrations n of the ZnSiP2 film were 1.38 × 10–5 Ω–1 cm–1, 94.5 cm–2 V–1 s–1, and 9.11 × 109 cm–3, respectively. Our work paves the way for the development of ZnSiP2 silicon-based tandem solar cell devices.

Evaluating the Intrinsic Material Stability at the Semiconductor/Electrolyte Interface for Solar Fuel Production

Stability is one of the most critical issues that determines the material selection for components used in integrated solar fuel generators. Although a variety of III-V semiconducting compounds with suitable band gaps can be used in tandem solar-cell devices, their material robustness in acidic/alkaline electrolyte under solar-driven water-splitting/solar-free dark conditions remains ambiguous. Thus, it is urgent and useful to evaluate whether these material have serious corrosion issues and whether their corrosion can be mitigated via mechanistic understanding. Herein, in a rigorous manner, we present a comprehensive study to understand the fundamental corrosion chemistry of these technologically-important semiconductors. Various characterization techniques including inductively coupled plasma mass spectroscopy (ICP-MS), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), and atomic-force microscopy (AFM) are combined to directly reveal the physical/chemical transformations occurring at the semiconductor/electrolyte interface. Combined with considerations of thermodynamic pourbaix diagrams, their sophisticated corrosion behaviors can be understood under varying conditions. Eventually, rational protection strategies are proposed to realize prolonged lifetime for sustained and practical solar fuel generation.

Growth kinetics of GaxIn(1−x)P nanowires using triethylgallium as Ga precursor

GaxIn(1−x)P nanowire arrays are promising for various optoelectronic applications with a tunable band-gap over a wide range. In particular, they are well suited as the top cell in tandem junction solar cell devices. So far, most GaxIn(1−x)P nanowires have been synthesized by the use of trimethylgallium (TMGa). However, particle assisted nanowire growth in metal organic vapor phase epitaxy is typically carried out at relatively low temperatures, where TMGa is not fully pyrolysed. In this work, we developed the growth of GaxIn(1−x)P nanowires using triethylgallium (TEGa) as the Ga precursor, which reduced Ga precursor consumption by about five times compared to TMGa due to the lower homogeneous pyrolysis temperature of TEGa. The versatility of TEGa is shown by synthesis of high yield GaxIn(1−x)P nanowire arrays, with a material composition tunable by the group III input flows, as verified by x-ray diffraction measurements and photoluminescence characterization. The growth dynamics of GaxIn(1−x)P nanowires was assessed by varying the input growth precursor molar fractions and growth temperature, using hydrogen-chloride as in situ etchant. We observed a complex interplay between the precursors. First, trimethylindium (TMIn) inhibits Ga incorporation into the nanowires, resulting in higher In composition in the grown nanowires than in the vapor. Second, the growth rate increases with temperature, indicating a kinetically limited growth, which from nanowire effective binary volume growth rates of InP and GaP can be attributed to the synthesis of GaP in GaxIn(1−x)P. We observed that phosphine has a strong effect on the nanowire growth rate with behavior expected for a unimolecular Langmuir–Hinshelwood mechanism of pyrolysis on a catalytic surface. However, growth rates increase strongly with both TEGa and TMIn precursors as well, indicating the complexity of vapor–liquid–solid growth for ternary materials. One precursor can affect the decomposition of another, and each precursor can affect the wetting properties and catalytic activity of the metal particle.

Technology computer aided design of 29.5% efficient perovskite/interdigitated back contact silicon heterojunction mechanically stacked tandem solar cell for energy-efficient applications

A 29.5% efficient perovskite/SiC passivated interdigitated back contact silicon heterojunction (IBC-SiHJ) mechanically stacked tandem solar cell device has been designed and simulated. This is a substantial improvement of 40% and 15%, respectively, compared to the transparent perovskite solar cell (21.1%) and Si solar cell (25.6%) operated individually. The perovskite solar cell has been used as a top subcell, whereas 250- and 25-μm-thick IBC-SiHJ solar cells have been used as bottom subcells. The realistic technology computer aided design analysis has been performed to understand the physical processes in the device and to make reliable predictions of the behavior. The performance of the top subcell has been obtained for different acceptor densities and hole mobility in Spiro-MeOTAD along with the impact of counter electrode work function. To incorporate the effect of material quality, the influence of carrier lifetimes has also been studied for perovskite top and IBC-SiHJ bottom subcells. The optical and electrical behavior of the devices has been obtained for both standalone as well as tandem configuration. Results reported in this study reveal that the proposed four-terminal tandem device may open a new door for cost-effective and energy-efficient applications.

SnS Thin Films Prepared by Chemical Spray Pyrolysis at Different Substrate Temperatures for Photovoltaic Applications

The preparation and analysis of morphological, structural, optical, vibrational and compositional properties of tin monosulfide (SnS) thin films deposited on glass substrate by chemical spray pyrolysis is reported herein. The growth conditions were evaluated to reduce the presence of residual phases different to the SnS orthorhombic phase. X-ray diffraction spectra revealed the polycrystalline nature of the SnS films with orthorhombic structure and a preferential grain orientation along the (111) direction. At high substrate temperature (450°C), a crystalline phase corresponding to the Sn2S3 phase was observed. Raman spectroscopy confirmed the dominance of the SnS phase and the presence of an additional Sn2S3 phase. Scanning electron microscopy (SEM) images reveal that the SnS film morphology depends on the substrate temperature. Between 250°C and 350°C, SnS films were shaped as rounded grains with some cracks between them, while at substrate temperatures above 400°C, films were denser and more compact. Energy-dispersive x-ray spectroscopy (EDS) analysis showed that the stoichiometry of sprayed SnS films improved with the increase of substrate temperature and atomic force microscopy micrographs showed films well covered at 350°C resulting in a rougher and bigger grain size. Optical and electrical measurements showed that the optical bandgap and the resistivity decreased when the substrate temperature increased, and smaller values, 1.46 eV and 60 Ω cm, respectively, were attained at 450°C. These SnS thin films could be used as an absorber layer for the development of tandem solar cell devices due to their high absorbability in the visible region with optimum bandgap energy.

Oxygenated CdS Buffer Layers Enabling High Open‐Circuit Voltages in Earth‐Abundant Cu2BaSnS4 Thin‐Film Solar Cells

Earth‐abundant Cu2BaSnS4 (CBTS) thin films exhibit a wide bandgap of 2.04–2.07 eV, a high absorption coefficient > 104 cm−1, and a p‐type conductivity, suitable as a top‐cell absorber in tandem solar cell devices. In this work, sputtered oxygenated CdS (CdS:O) buffer layers are demonstrated to create a good p–n diode with CBTS and enable high open‐circuit voltages of 0.9–1.1 V by minimizing interface recombination. The best power conversion efficiency of 2.03% is reached under AM 1.5G illumination based on the configuration of fluorine‐doped SnO2 (back contact)/CBTS/CdS:O/CdS/ZnO/aluminum‐doped ZnO (front contact).