Tandem Devices Research Articles

Novel broad spectral response perovskite solar cells: A review of the current status and advanced strategies for breaking the theoretical limit efficiency

• This review described in detail for the first time how to enhance the overall performance of the devices by broadening the absorption spectrum of perovskite solar cells. • We discussed the working principles, characteristics and latest developments of narrow-band gap PSCs and PSCs doped with rare earth up-conversion luminescent materials, and put forward the corresponding strategy. • We analyzed the working principles of tandem PSCs and integrated perovskite/organic solar cells, reviewed their latest developments in detail, and proposed forward-looking opinions. Perovskite solar cells (PSCs) have revolutionized photovoltaic research. The power conversion efficiency (PCE) of PSCs has now reached 25.7%, which is comparable to current state-of-the-art silicon-based cells. However, PSCs can only utilize light of 300–850 nm, resulting in wasted near-infrared (NIR) light, which occupies 45%–50% of entire solar spectrum, which is one of the main reasons limiting the development of efficiency. Related strategies to broaden NIR spectroscopy to break the theoretical limit efficiency of PSCs have recently attracted extensive attention. This review firstly outlines theoretical basis for improving the NIR spectroscopy, then systematically summarizes some key strategies and research progress to improve NIR spectroscopy of PSCs. We firstly provided a comprehensive overview of historical research experiments on narrow-gap perovskite absorber layers, rare earth up-conversion, tandem devices, and integrated perovskite/organic solar cells and given constructive suggestions for exceeding limit efficiency of PSCs. Finally, based on the development status of PSCs with NIR utilization, the current issues, solutions and future development directions of important aspects to improve NIR utilization of PSCs are systematically discussed. This review lays the foundation for the efficiency of PSCs beyond the Shockley-Queisser limit, and provides a certain development prospect.

Study of the intermittent plasma structure around the divertor simulation experimental module in GAMMA 10/PDX

We have investigated the generation region of intermittent plasma structures, which could be due to the blob-like cross field transport around the divertor simulation experimental module (D-module) in the tandem mirror device GAMMA 10/PDX. A positive skewness of the ion saturation current was clearly seen when the electrode of the movable probe was located at the radial edge and just in front of the entrance limiter of the D-module. Fourier analysis and conditional averaging clarified that positive spikes intermittently appeared in the same region. This is the first indication that the phenomenon producing the high-density isolated plasma structures occurred in the upstream of the D-module. A negative skewness was also found, and large amplitude fluctuation was detected between regions showing positive and negative skewness. Furthermore, this and light emission fluctuations become stronger during the transient state from attached to detached state on the V-shaped target.

Combinatorial study to the physical properties of Cd1-xZnxTe thin films as budding absorber for solar cell applications

Cadmium telluride (CdTe) has long been recognized as promising absorber for single-junction solar cells, yet it suffers from snag of apposite contact. CdZnTe provides band gap tunability and can be used as absorber layer in tandem devices concerned. The impact of Zn content on physical properties of Cd1-xZnxTe thin films is indubitable and hence, present work demonstrates concentration evolution to physical properties of Cd1-xZnxTe thin films. Structural characterization depicted zinc blende cubic phased (220) plane as dominant reflection. The optical energy band gap (Eg) of films is attained within range 1.70–1.92 eV and electrical properties portrayed Ohmic character of films. Photoluminescence spectra showed single peak and topographical analysis demonstrated hill like-topographies. Field emission scanning electron microscopy (FESEM) maps exposed compact morphologies and compositional studies validated presence of constituent elements. The present study unveiled that maximum grain size, apt optical energy band gap and higher conductivity of Cd0.9Zn0.1Te films make these as budding absorbers for solar cell applications.

Perovskite/Silicon Tandem Solar Cells: Choice of Bottom Devices and Recombination Layers

Perovskite/silicon tandem solar cells have been intensively studied in recent years, and their efficiencies have rapidly increased owing to the numerous efforts in this field. A question about which type of silicon solar cell is the most suitable subcell in tandem devices emerges. Herein, three attractive silicon solar cells are summarized, including passivated-emitter rear-cell (PERC), tunnel oxide passivated contact (TOPCon), and heterojunction (HJT) cells. Their structures and features are elucidated for a clear understanding of the mechanism and potential of optimum performance. Moreover, the characteristics and performance of perovskite/silicon tandem cells with PERC, TOPCon, and HJT subcells are contrasted and discussed. The significant contribution of the passivation layer and structure design on both sides to photovoltaic properties of tandem devices is emphasized. Especially, the recombination layer between two subcells is analyzed in depth in terms of material chemistry, light absorption, and charge transport with a review on preparing the optimized structure.

Silver‐Alloyed Low‐Bandgap CuInSe2 Solar Cells for Tandem Applications

Photovoltaic conversion efficiency of CuInSe2 solar cells is limited by low fill factor (FF) and open‐circuit‐voltage (V OC) values compared to Si or perovskite solar cells. Herein, small quantities of Ag to alloy CuInSe2 to improve its properties are used, such as enhanced grain growth, higher crystal quality, and less detrimental defects, to overcome the device limitations of low‐bandgap CuInSe2 absorbers. The impact of Ag on the electronic properties of the bulk material and the buffer–absorber interface is examined at different stoichiometric compositions. Ag alloying improves the morphology with larger grain sizes, extends carrier lifetimes, and elevates net doping densities. Ag alloying reduces Cu‐depleted ordered‐vacancy compounds at the buffer–absorber interface and causes a chalcopyrite phase of high‐crystal‐quality independent of the I/III ratio. It is suggested Ag affects the formation of the alkali‐rich surface layer (Rb–In–Se). The best solar cell with an Ag‐alloyed CuInSe2 absorber achieves a V OC over 600 mV and FF values of about 74%, which result in a power conversion efficiency of 18.7% for low‐bandgap energy of 1.0 eV with short‐circuit current above 42 mA cm−2. The advantage of Ag alloying on the device performance of CuInSe2 solar cells will impact future applications in tandem devices.

Influence of Component Properties on the Photovoltaic Performance of Monolithic Perovskite/Organic Tandem Solar Cells: Sub-Cell, Interconnecting Layer, and Photovoltaic Parameters.

Wide-bandgap perovskite sub-cells (WPSCs)-based tandem solar cells attract considerable interest because of their capability to surpass the Shockley-Queisser limit. Monolithic perovskite/organic tandem solar cells (POTSCs) integrating WPSCs and small-bandgap organic sub-cells (SOSCs) are famous compositions owing to their simple fabrication method and compatibility with flexible devices. Most studies on POTSCs focus on enhancing device efficiency by modifying one or two of the device components (WPSCs, SOSCs, and interconnecting layers). The characteristics of POTSCs are not extensively investigated so far, especially in terms of the influence of the device structure and component properties on the tandem device photovoltaic performance. In this study, the existing p-i-n type WPSC-based p-i-n POTSCs and n-i-p type WPSC-based n-i-p POTSCs are reviewed and their advantages and limitations are highlighted. Furthermore, the influence of the tandem device component properties (optical, electrical, and photovoltaic properties) on the photovoltaic parameters (open-circuit voltage, short-circuit current density, fill factor, and power conversion efficiency) and the existing device modification methods are discussed to provide comprehensive guidance for the development of POTSCs.

Investigation of High-Efficiency and Stable Carbon-Perovskite/Silicon and Carbon-Perovskite/CIGS-GeTe Tandem Solar Cells

The primary purpose of recent research on solar cells is to achieve a higher power conversion efficiency with stable characteristics. To push the developments of photovoltaic (PV) technology, tandem solar cells are being intensively researched, as they have higher power conversion efficiency (PCE) than single-junction cells. Perovskite solar cells (PSCs) are recently used as a top cell of tandem solar cells thanks to their tunable energy gap, high short circuit current, and low cost of fabrication. One of the main challenges in PSCs cells is the stability issue. Carbon perovskite solar cells (CPSCs) without a hole transport material (HTM) presented a promising solution for PSCs’ stability. The two-terminal monolithic tandem solar cells demonstrate the commercial tandem cells market. Consequently, all the proposed tandem solar cells in this paper are equivalent to two-terminal monolithic tandem devices. In this work, two two-terminal tandem solar cells are proposed and investigated using the SCAPS-1D device simulator. Carbon perovskite solar cell (CPSC) without hole transport material (HTM) is used as the top cell with a new proposed gradient doping in the perovskite layer. This proposal has led to a substantial enhancement of the stability issue known to be present in carbon perovskite cells. Moreover, a higher PCE, exceeding 22%, has been attained for the proposed CPSC. Two bottom cells are examined, Si and CIGS-GeTe solar cells. The suggested CPSC/Si and CPSC/CIGS-GeTe tandem solar cells have the advantage of having just two junctions, which reduces the complexity and cost of solar cells. The performance parameters are found to be improved. In specific, the PCEs of the two proposed cells are 19.89% and 24.69%, respectively.

Functional Layers of Inverted Flexible Perovskite Solar Cells and Effective Technologies for Device Commercialization

Perovskite solar cells (PSCs) are now one of the most promising solar cells due to advantages such as high‐power conversion efficiency (PCE), low cost, and ease of fabrication. Among PSCs, flexible perovskite solar cells (FPSCs) are lightweight and bendable, making them easier to transport and install than rigid PSCs, and hence can be used in wearable and portable electronics, space energy systems, multifunctional integrated buildings, unmanned systems, and other applications. Compared with regular n–i–p FPSCs, inverted p–i–n FPSCs possess advantages of reliable operational stability, reduced hysteresis effect, low‐temperature fabrication process, and strong potential for tandem devices. At present, the PCEs of single‐junction and tandem‐inverted FPSCs have reached 21.76% and 24.7%, respectively, indicating promising commercial applications. In this review, first, the developments of device functional layers including flexible substrates, flexible conductive electrodes, charge transport layers, and perovskite active layers in inverted FPSCs are elucidated and discussed thoroughly. Then, the technologies for accelerating commercialization of inverted FPSCs are summarized in detail. Finally, perspectives on the development and commercialization of inverted FPSCs are provided.

Monolithic Perovskite/Si Tandem Solar Cells—Silicon Bottom Cell Types and Characterization Methods

AbstractAs a more efficient alternative to the crystalline Si (c‐Si) cell whose performance is approaching its theoretical limit, perovskite (PVSK)/Si tandem solar cells (TSCs) are attracting attention from academia and industry. The tandem device, which can benefit from the maturity of the c‐Si cells and the high performance of the PVSK solar cell, has undergone tremendous development. The expectation for its commercialization is increasing as the device shows conversion efficiencies surpassing those of single‐junction solar cells. In this perspective article, the recent progress of the PVSK/Si TSC is summarized from a viewpoint balanced between academia and industry, by regrouping reported devices depending on the types of Si bottom cells: passivated emitter rear contact, heterojunction with intrinsic thin layer, and polysilicon‐passivated contact. In addition, opto‐electronic characterization methods of the PVSK/Si TSCs are reviewed, which can provide an in‐depth analysis of the subcells that can facilitate the development.

Energy Yield Modeling for Optimization and Analysis of Perovskite‐Silicon Tandem Solar Cells Under Realistic Outdoor Conditions

AbstractA comprehensive algorithm for calculating the energy yield (EY) is used as the most important figure‐of‐merit in determining the capabilities of PV devices in realistic operation. The model is based on advanced optical modeling and extensive opto‐thermo‐electrical characterization. It takes the realistic environmental and device installation data fully into account, as well as the complete set of device specifications including all relevant temperature‐induced variations. A detailed analysis and optimization of two‐terminal perovskite‐silicon tandem devices are done in terms of long‐term electrical energy production at different geographical locations from distinct Köppen–Geiger–Photovoltaic climate zones (KGPV). It is shown that the optimal perovskite bandgap (EG,PK) in a tandem device operating under realistic conditions is in all cases higher than the one optimized under STC, yet does not differ significantly from location to location, which is beneficial from the manufacturing point of view. The optimal EG,PK is influenced primarily by the spectral distribution of incident irradiance, and not so much by the operating temperature conditions. Moreover, a clear linear dependency between the optimal EG,PK and the weighted average photon energy of the incident irradiance is demonstrated, which presents an important rule for rapid tandem design.

Maximizing Current Density in Monolithic Perovskite Silicon Tandem Solar Cells

Perovskite silicon tandem solar cells can overcome the efficiency limit of silicon single‐junction solar cells. In two‐terminal perovskite silicon tandem solar cells, current matching of subcells is an important requirement. Herein, a current‐matched tandem solar cell using a planar front/ rear side‐textured silicon heterojunction bottom solar cell with a p–i–n perovskite top solar cell that yields a high certified short‐circuit current density of 19.6 mA cm−2 is reported. Measures taken to improve the device are guided by optical simulation and a derived optical roadmap toward maximized tandem current density. To realize current matching of the two subcells, variation of the perovskite bandgap from ≈1.68 to 1.64 eV and thickness is investigated. Spectrometric characterization, in which current–voltage curves of tandem devices are recorded at systematically varied spectral irradiance conditions, is applied to determine the current matching point. In addition, remaining device limitations such as nonradiative recombination at the perovskite's interfaces are analyzed. Replacing the hole transport layer PTAA by 2PACz results in an overall certified power conversion efficiency of up to 26.8%. Precise simulation based on the device structure is essential as it provides efficient paths toward improving the device efficiency.

Design and optimization of four-terminal mechanically stacked and optically coupled silicon/perovskite tandem solar cells with over 28% efficiency

Silicon/perovskite tandem devices are believed to be a favorite contender for improving cell performance over the theoretical maximum value of single-junction photovoltaic (PV) cells. The present study evaluates the design and optimization of four-terminal (4-T) mechanically stacked and optically coupled configurations using SCAPS (solar cell capacitance simulator). Low-cost, stable, and easily processed semitransparent carbon electrode-based perovskite solar cells (c-PSCs) without hole transport material (HTM) and highly efficient crystalline silicon (c-Si) PV cells were utilized as top and bottom cells, respectively. The wide bandgap multi-cation perovskite Csx(FA0.4MA0.6)1−xPbI2.8Br0.2 and a low bandgap c-Si were employed as light-harvesting layers in the top and bottom cells, respectively. The impact of perovskite thickness and doping concentrations were examined and optimized for both tandem configurations. Under optimized conditions, thicknesses of 1000 nm and 1100 nm are the best values of the perovskite absorber layer for 4-T mechanically stacked and optically coupled arrangements, respectively. Likewise, 1 × 1017 cm−3 doping concentration of top cells revealed the highest performance in both structures. With these optimized parameters under tandem configurations, efficiency values of 28.38% and 29.34% were obtained in 4-T mechanically and optically coupled tandems, respectively. Results suggest that by optimizing perovskite thickness and doping concentration, the proposed designs using HTM-free c-PSCs could enhance device performance.

Wide-Bandgap Perovskite Solar Cell Using a Fluoride-Assisted Surface Gradient Passivation Strategy.

Wide-bandgap (1.68 eV) perovskite solar cells (PSCs) are important components of perovskite/Si tandem devices. However, the efficiency of wide bandgap PSCs has been limited by their huge open-circuit voltage (VOC) deficit due to non-radiative recombination. Deep-level acceptor defects are identified as the major killers of VOC, and they can be effectively improved by passivation with ammonium salts. Theoretical calculation predicts that increasing the distance between F and -NH3+ of fluorinated ammonium can dramatically enhance the electropositivity of -NH3+ terminals, thus providing strong adsorption onto the negatively charged IA and IPb anti-site defects. Characterizations further confirm that surface gradient passivation employing p-FPEAI demonstrates the most efficient passivation effect. Consequently, a record-efficiency of 21.63% with the smallest VOC deficit of 441 mV is achieved for 1.68 eV-bandgap inverted PSCs. Additionally, a flexible PSC and 1 cm2 opaque device also deliver the highest PCEs of 21.02% and 19.31%, respectively.

Wide‐Bandgap Perovskite Solar Cell Using a Fluoride‐Assisted Surface Gradient Passivation Strategy

AbstractWide‐band gap (1.68 eV) perovskite solar cells (PSCs) are important components of perovskite/Si tandem devices. However, the efficiency of wide band gap PSCs has been limited by their huge open‐circuit voltage (Voc) deficit due to non‐radiative recombination. Deep‐level acceptor defects are identified as the major killers of Voc, and they can be effectively improved by passivation with ammonium salts. Theoretical calculation predicts that increasing the distance between F and −NH3+ of fluorinated ammonium can dramatically enhance the electropositivity of −NH3+ terminals, thus providing strong adsorption onto the negatively charged IA and IPb anti‐site defects. Characterizations further confirm that surface gradient passivation employing p‐FPEAI demonstrates the most efficient passivation effect. Consequently, a record‐efficiency of 21.63 % with the smallest Voc deficit of 441 mV is achieved for 1.68 eV‐band gap inverted PSCs. Additionally, a flexible PSC and 1 cm2 opaque device also deliver the highest PCEs of 21.02 % and 19.31 %, respectively.

Device Optimization of a Pb-Free All Perovskite Tandem Solar Cell with 29.59% Power Conversion Efficiency

This study has focused on simulating a highly efficient lead-free all-perovskite tandem solar cell using the SCAPS 1D device simulation tool. In the tandem structure, the top cell used Cs2AgBi0.75Sb0.25Br6 (Eg = 1.80 eV), and the bottom cell used FAMASnGeI3 (Eg = 1.40 eV) as the absorber material. Also, ZnO and NiOx were used as the electron transport layer (ETL) and the hole transport layer (HTL), respectively. Primarily the individual top cell and bottom cell have been optimized. The highest efficiency of the top cell was found to be 17.13% with performance parameters VOC = 1.23V, JSC = 15.57 mA/cm2 and FF = 89.39%, whereas the optimized efficiency of the bottom cell was found to be 17.58% with performance parameters VOC = 0.85V, JSC= 27.38 mA/cm2 and FF = 75.34%. The thickness of the absorber of the top cell and bottom cell shows a significant impact on the device performance and the optimum thickness for the Cs2AgBi0.75Sb0.25Br6 absorber layer was found to be 400 nm, whereas for the FAMASnGeI3 layer was found to be 200 nm. Careful optimization of the tandem device has resulted in an improvement of the performance and obtained an efficiency of 29.59 %, with JSC of 17.50 mA/cm2, VOC of 2.09 V, and FF of 80.87%. The final simulated device sums up tremendous potential for the fabrication of a highly efficient PSC device using lead-free perovskite materials towards excelling commercialization.

Bonding III–V/Textured‐Silicon Monolithic Flexible Tandem Devices

AbstractIII–V/silicon tandem solar cells simultaneously have the potential advantages of high efficiency and low cost. Bonding is an effective way to realize tandem at both ends. However, it is difficult to realize vacuum bonding in industrialized micrometer‐sized pyramid silicon cells. Up to know, all bonding in III–V/silicon tandem solar cells is based on planar silicon cells. Here, a transparent conductive adhesive (TCA) based on micron particle size is designed and implemented, which realizes the bonding of III–V cell and textured silicon cell for the first time. The TCA consists of epoxy adhesive 301 (Epoxy‐301) as transparent adhesive and silver‐coated flexible polymethylmethacrylate microspheres as conductive particles. The average contact resistance of TCA is 0.17 Ω cm2, and the transmittance exceeds 91% in the range of 800—1200 nm. In addition, TCA shows good stability in the etching process of GaAs substrates. Monolithic III–V/silicon device exhibits 25.1% power conversion efficiency (PCE) on thin and textured silicon heterojunction solar cells, which enables flexible properties of tandem cells. Compared with silicon subcell and III–V subcell, the performance of tandem device is improved by 28.3% and 8.6%, respectively. This strategy provides a broad research window for improving the performance of tandem solar cells based on industrialized textured‐silicon bottom cells.

Recent Developments of Polymer Solar Cells with Photovoltaic Performance over 17%

AbstractWith the emergence of ADA'DA‐type (Y‐series) non‐fullerene acceptors (NFAs), the power conversion efficiencies (PCEs) of organic photovoltaic devices have been constantly refreshed and gradually reached 20% in recent years (19% for single junction and 20% for tandem device). The acceptors possess specific design concept, which greatly enrich the NFA types and have excellent compatibility with many donor materials. It is gratifying to note that the previously underperforming donor materials combine with these regulated acceptors to shine again. Nowadays, the concept of modular design is widely used in the research of acceptors and donors, injecting new vitality into the field of organic photovoltaics. Furthermore, these acceptors also promote the research of multicomponent devices, tandem devices, bilayer devices, processing solvent engineering, and additive engineering. Herein, the latest progresses of polymer solar cells with efficiency over 17% are briefly reviewed from the aspects of active material design, interface material development, and device technology. At last, the opportunities and challenges of organic photovoltaic commercialization in the future are discussed.

Dual Sub‐Cells Modification Enables High‐Efficiency n–i–p Type Monolithic Perovskite/Organic Tandem Solar Cells

AbstractMonolithic perovskite/organic tandem solar cells (POTSCs) have attracted increasing attention owing to ability to overcome the Shockley–Queisser limit. However, compromised sub‐cells performance limits the tandem device performance, and the power conversion efficiency (PCE) of POTSCs is still lower than their single‐junction counterparts. Therefore, optimized sub‐cells with minimal energy loss are desired for producing high‐efficiency POTSCs. In this study, an ionic liquid, methylammonium acetate (MAAc), is used to modify wide‐bandgap perovskite sub‐cells (WPSCs), and bathocuproine (BCP) is used to modify small‐bandgap organic solar cells. The Ac− group of MAAc can effectively heal the Pb defects in the all‐inorganic perovskite film, which enables a high PCE of 17.16% and an open‐circuit voltage (Voc) of 1.31 V for CsPbI2.2Br0.8‐based WPSCs. Meanwhile, the BCP film, inserted at the ZnO/organic bulk‐heterojunction (BHJ) interface, acts as a space layer to prevent direct contact between ZnO and the BHJ while passivating the surface defects of ZnO, thereby mitigating ZnO defect‐induced efficiency loss. As a result, PM6:CH1007‐based SOSCs exhibit a PCE of 15.46%. Integrating these modified sub‐cells enable the fabrication of monolithic n–i–p structured POTSCs with a maximum PCE of 22.43% (21.42% certified), which is one of the highest efficiencies in such type of POTSCs.

Electrochemically Deposited CZTSSe Thin Films for Monolithic Perovskite Tandem Solar Cells with Efficiencies Over 17%

In spite of the high potential economic feasibility of the tandem solar cells consisting of the halide perovskite and the kesterite Cu2ZnSn(S,Se)4 (CZTSSe), they have rarely been demonstrated due to the difficulty in implementing solution‐processed perovskite top cell on the rough surface of the bottom cells. Here, we firstly demonstrate an efficient monolithic two‐terminal perovskite/CZTSSe tandem solar cell by significantly reducing the surface roughness of the electrochemically deposited CZTSSe bottom cell. The surface roughness (Rrms) of the CZTSSe thin film could be reduced from 424 to 86 nm by using the potentiostatic mode rather than using the conventional galvanostatic mode, which can be further reduced to 22 nm after the subsequent ion‐milling process. The perovskite top cell with a bandgap of 1.65 eV could be prepared using a solution process on the flattened CZTSSe bottom cell, resulting in the efficient perovskite/CZTSSe tandem solar cells. After the current matching between two subcells involving the thickness control of the perovskite layer, the best performing tandem device exhibited a high conversion efficiency of 17.5% without the hysteresis effect.

Bulk Incorporation with 4‐Methylphenethylammonium Chloride for Efficient and Stable Methylammonium‐Free Perovskite and Perovskite‐Silicon Tandem Solar Cells

AbstractMethylammonium (MA)‐free perovskite solar cells have the potential for better thermal stability than their MA‐containing counterparts. However, the efficiency of MA‐free perovskite solar cells lags behind due to inferior bulk quality. In this work, 4‐methylphenethylammonium chloride (4M‐PEACl) is added into a MA‐free perovskite precursor, which results in greatly enhanced bulk quality. The perovskite crystal grains are significantly enlarged, and defects are suppressed by a factor of four upon the incorporation of an optimal concentration of 4M‐PEACl. Quasi‐2D perovskites are formed and passivate defects at the grain boundaries of the perovskite crystals. Furthermore, the perovskite surface chemistry is modified, resulting in surface energies more favorable for hole extraction. This facile approach leads to a steady state efficiency of 23.7% (24.2% in reverse scan, 23.0% in forward scan) for MA‐free perovskite solar cells. The devices also show excellent light stability, retaining more than 93% of the initial efficiency after 1000 h of constant illumination in a nitrogen environment. In addition, a four‐terminal mechanically stacked perovskite‐silicon tandem solar cell with champion efficiency of 30.3% is obtained using this MA‐free composition. The encapsulated tandem devices show excellent operational stability, retaining more than 98% of the initial performance after 42 day/night cycles in an ambient atmosphere.