Interface Reactive Sputtering of Transparent Electrode for High-Performance Monolithic and Stacked Perovskite Tandem Solar Cells

Sputtered indium tin oxide (ITO) fulfills the requirements of top transparent electrodes (TTEs) in semitransparent perovskite solar cells (PSCs) and stacked tandem solar cells (TSCs), as well as of the recombination layers in monolithic TSCs. However, the high-energy ITO particles will cause damage to the devices. Herein, the interface reactive sputtering strategy is proposed to construct cost-effective TTEs with high transmittance and excellent carrier transporting ability. Polyethylenimine (PEI) is chosen as the interface reactant that can react with sputtered ITO nanoparticles, so that, coordination compounds can be formed during the deposition process, facilitating the carrier transport at the interface of C60/PEI/ITO. Besides, the impact force of energetic ITO particles is greatly alleviated, and the intactness of the underlying C60 layer and perovskite layer is guaranteed. Thus, the prepared semitransparent subcells achieve a significantly enhanced power conversion efficiency (PCE) of 19.17%, surpassing those based on C60/ITO (11.64%). Moreover, the PEI-based devices demonstrate excellent storage stability, which maintains 98% of their original PCEs after 2000 h. On the strength of the interface reactive sputtering ITO electrode, a stacked all-perovskite TSC with a PCE of 26.89% and a monolithic perovskite-organic TSC with a PCE of 24.33% are successfully fabricated.

Highly Efficient Monolithic Perovskite/Perovskite/Silicon Triple-Junction Solar Cells

Wide-bandgap metal halide perovskites have demonstrated promise in multijunction photovoltaic (PV) cells. However, photoinduced phase segregation and the resultant low open-circuit voltage (Voc) have greatly limited the PV performance of perovskite-based multijunction devices. Here, a alloying strategy is reported to achieve uniform distribution of triple cations and halides in wide-bandgap perovskites by doping Rb+ and Cl- with small ionic radii, which effectively suppresses halide phase segregation while promoting the homogenization of surface potential. Based on this strategy, a Voc of 1.33 V is obtained from single-junction perovskite solar cells, and a VOC approaching 3.0 V and a power conversion efficiency of 25.0% (obtained from reverse scan direction, certified efficiency: 24.19%) on an 1.04 cm2 photoactive area can be achieved in a perovskite/perovskite/c-Si triple-junction tandem cell, where the certification efficiency is by far the greatest performance of perovskite-based triple-junction tandem solar cells. This work overcomes the performance deadlock of perovskite-based triple-junction tandem cells by setting a materials-by-design paradigm.

A Deep Dive into Cu2 ZnSnS4 (CZTS) Solar Cells: A Review of Exploring Roadblocks, Breakthroughs, and Shaping the Future

Renewable energy is crucial for sustainable future, and Cu2 ZnSnS4 (CZTS) based solar cells shine as a beacon of hope. CZTS, composed of abundant, low-cost, and non-toxic elements, shares similarities with Cu(In,Ga)Se2 (CIGS). However, despite its promise and appealing properties for solar cells, CZTS-based solar cells faces performance challenges owing to inherent issues with CZTS material, and conventional substrate structure complexities. This review critically examines these roadblocks, explores ongoing efforts and breakthroughs, providing insight into the evolving landscape of CZTS-based solar cells research. Furthermore, as an optimistic turn in the field, the review first highlights the crucial need to transition to a superstrate structure for CZTS-based single junction devices, and summarizes the substantial progress made in this direction. Subsequently, dive into the discussion about the fascinating realm of CZTS-based tandem devices, providing an overview of the existing literature as well as outlining the possible potential strategies for enhancing the efficiency of such devices. Finally, the review provides a useful outlook that outlines the priorities for future research and suggesting where efforts should concentrate to shape the future of CZTS-based solar cells.

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.

A Universal Perovskite/C60 Interface Modification via Atomic Layer Deposited Aluminum Oxide for Perovskite Solar Cells and Perovskite-Silicon Tandems

The primary performance limitation in inverted perovskite-based solar cells is the interface between the fullerene-based electron transport layers and the perovskite. Atomic layer deposited thin aluminum oxide (AlOX ) interlayers that reduce nonradiative recombination at the perovskite/C60 interface are developed, resulting in >60 millivolts improvement in open-circuit voltage and 1% absolute improvement in power conversion efficiency. Surface-sensitive characterizations indicate the presence of a thin, conformally deposited AlOx layer, functioning as a passivating contact. These interlayers work universally using different lead-halide-based absorbers with different compositions where the 1.55 electron volts bandgap single junction devices reach >23% power conversion efficiency. A reduction of metallic Pb0 is found and the compact layer prevents in- and egress of volatile species, synergistically improving the stability. AlOX -modified wide-bandgap perovskite absorbers as a top cell in a monolithic perovskite-silicon tandem enable a certified power conversion efficiency of 29.9% and open-circuit voltages above 1.92 volts for 1.17 square centimeters device area.

Recent Advances in Wide Bandgap Perovskite Solar Cells: Focus on Lead-Free Materials for Tandem Structures

A tandem solar cell, which is composed of a wide bandgap (WBG) top sub-cell and a narrow bandgap (NBG) bottom subcell, harnesses maximum photons in the wide spectral range, resulting in higher efficiency than single-junction solar cells. WBG (>1.6 eV) perovskites are currently being studied a lot based on lead mixed-halide perovskites, and the power conversion efficiency of lead mixed-halide WBG perovskite solar cells (PSCs) reaches 21.1%. Despite the excellent device performance of lead WBG PSCs, their commercialization is hampered by their Pb toxicity and low stability. Hence, lead-free, less toxic WBG perovskite absorbers are needed for constructing lead-free perovskite tandem solar cells. In this review, various strategies for achieving high-efficiency WBG lead-free PSCs are discussed, drawing inspiration from prior research on WBG lead-based PSCs. The existing issues of WBG perovskites such as VOC loss are discussed, and toxicity issues associated with lead-based perovskites are also addressed. Subsequently, the natures of lead-free WBG perovskites are reviewed, and recently emerged strategies to enhance device performance are proposed. Finally, their applications in lead-free all perovskite tandem solar cells are introduced. This review presents helpful guidelines for eco-friendly and high-efficiency lead-free all perovskite tandem solar 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.

Regulating Crystal Orientation via Ligand Anchoring Enables Efficient Wide-Bandgap Perovskite Solar Cells and Tandems

Wide-bandgap (WBG) perovskite solar cells have attracted considerable interest for their potential applications in tandem solar cells. However, the predominant obstacles impeding their widespread adoption are substantial open-circuit voltage (VOC ) deficit and severe photo-induced halide segregation. To tackle these challenges, a crystal orientation regulation strategy by introducing dodecyl-benzene-sulfonic-acid as an additive in perovskite precursors is proposed. This method significantly promotes the desired crystal orientation, passivates defects, and mitigates photo-induced halide phase segregation in perovskite films, leading to substantially reduced nonradiative recombination, minimized VOC deficits, and enhanced operational stability of the devices. The resulting 1.66 eV bandgap methylamine-free perovskite solar cells achieve a remarkable power conversion efficiency (PCE) of 22.40% (certified at 21.97%), with the smallest VOC deficit recorded at 0.39 V. Furthermore, the fabricated semitransparent WBG devices exhibit a competitive PCE of 20.13%. Consequently, four-terminal tandem cells comprising WBG perovskite top cells and 1.25 eV bandgap perovskite bottom cells showcase an impressive PCE of 28.06% (stabilized 27.92%), demonstrating great potential for efficient multijunction tandem solar cell applications.

Bifacial and Semitransparent Sb2 (S,Se)3 Solar Cells for Single-Junction and Tandem Photovoltaic Applications

Thin-film solar cells are expected to play a significant role in the space industry, building integrated photovoltaic (BIPV), indoor applications, and tandem solar cells, where bifaciality and semitransparency are highly desired. Sb2 (S,Se)3 has emerged as a promising new photovoltaic (PV) material for its high absorption coefficient, tunable bandgap, and nontoxic and earth-abundant constituents. However, high-efficiency Sb2 (S,Se)3 solar cells exclusively employ monofacial architectures, leaving a considerable gap toward large-scale application in aforementioned fields. Here, a bifacial and semitransparent Sb2 (S,Se)3 solar cell and its extended application in tandem solar cells are reported. The transparent conductive oxides (TCOs) and the ultrathin inner n-i-p structure provide high long-wavelength transmittance. Despite the MnS/ITO Schottky junction, power conversion efficiencies (PCEs) of 7.41% and 6.36% are achieved with front and rear illumination, respectively, contributing to a great bifaciality of 0.86. Consequently, the reported device gains great enhancement in PV performance by exploiting albedo of surroundings and shows exceptional capability in absorbing tilt incident light. Moreover, an Sb2 (S,Se)3 /Si tandem solar cell with a PCE of 11.66% is achieved in preliminary trials. These exciting findings imply that bifacial and semitransparent Sb2 (S,Se)3 solar cells possess tremendous potential in practical applications based on their unique characteristics.

Pure-Phase, Large-Grained Wide-Band-Gap Perovskite Films for High-Efficiency, Four-Terminal Perovskite/Silicon Tandem Solar Cells

High-quality, stable perovskite films with a wide band gap between 1.65 and 1.80 eV are highly suitable for efficient and cost-competitive silicon-based tandem solar cells. Herein, we demonstrate that the combined strategies of the Pb(SCN)2 additive and air annealing can enable the Cs0.22FA0.78Pb(I0.85Br0.15)3 films with a wide band gap of 1.65 eV and favored properties including pure composition, high crystallinity, micro-sized grains, and reduced defects. With these desired films, the average efficiencies of semitransparent perovskite solar cells (PSCs) are boosted from (18.13 ± 0.31) to (20.35 ± 0.28)%. Further, the semitransparent PSC is used to assemble the four-terminal perovskite/TOPCon tandem solar cell. Benefiting from its excellent performance and preferred optical properties, the obtained tandem solar cell yields a milestone efficiency of 30.32%.

Inorganic Perovskite Surface Reconfiguration for Stable Inverted Solar Cells with 20.38% Efficiency and Its Application in Tandem Devices

Inorganic perovskite solar cells (IPSCs) have garnered attention in tandem solar cells (TSCs) due to their suitable bandgap and impressive thermal stability. However, the efficiency of inverted IPSCs has been limited by the high trap density on the top surface of inorganic perovskite film. Herein, a method for fabricating efficient IPSCs by reconfiguring the surface properties of CsPbI_2.85 Br_0.15 film with 2-amino-5-bromobenzamide (ABA) is developed. This modification not only exhibits the synergistic coordination of carbonyl (C=O) and amino (NH₂ ) groups with uncoordinated Pb²⁺ , but also the Br fills halide vacancies and suppresses the formation of Pb⁰ , effectively passivating the defective top surface. As a result, a champion efficiency of 20.38%, the highest efficiency reported for inverted IPSCs to date is achieved. Furthermore, the successful fabrication of a p-i-n type monolithic inorganic perovskite/silicon TSCs with an efficiency of 25.31% for the first time is demonstrated. Crucially, the unencapsulated ABA-treated IPSCs shows enhanced photostability, retaining 80.33% of its initial efficiency after 270 h, and thermal stability (maintain 85.98% of its initial efficiency after 300 h at 65 °C). The unencapsulated ABA-treated TSCs also retains 92.59% of its initial efficiency after 200 h under continuous illumination in ambient air.

Characterization of various tandem solar cells: Protocols, issues, and precautions

In the search for a more efficient solar cell, various types of tandem solar cells (TSCs) have been actively developed worldwide as the performances of the single junction solar cells approach their theoretical limits. Meanwhile, various materials and structures are adopted in TSCs, which makes their characterizations and comparison difficult. Along with the classical monolithic TSC, which exhibits two electrical contacts, devices with three or four electrical contacts have been widely studied as a more performing alternative of commercialized solar cells. For a fair and accurate evaluation of the device performance of TSCs, understanding the effectiveness and limitations of the characterization of the different types of TSCs is crucial. In this paper, we summarize various types of TSCs and discuss their characterization methods.

Innovative Approaches to Semi-Transparent Perovskite Solar Cells

Perovskite solar cells (PSCs) are advancing rapidly and have reached a performance comparable to that of silicon solar cells. Recently, they have been expanding into a variety of applications based on the excellent photoelectric properties of perovskite. Semi-transparent PSCs (ST-PSCs) are one promising application that utilizes the tunable transmittance of perovskite photoactive layers, which can be used in tandem solar cells (TSC) and building-integrated photovoltaics (BIPV). However, the inverse relationship between light transmittance and efficiency is a challenge in the development of ST-PSCs. To overcome these challenges, numerous studies are underway, including those on band-gap tuning, high-performance charge transport layers and electrodes, and creating island-shaped microstructures. This review provides a general and concise summary of the innovative approaches in ST-PSCs, including advances in the perovskite photoactive layer, transparent electrodes, device structures and their applications in TSC and BIPV. Furthermore, the essential requirements and challenges to be addressed to realize ST-PSCs are discussed, and the prospects of ST-PSCs are presented.

Pivotal Routes for Maximizing Semitransparent Perovskite Solar Cell Performance: Photon Propagation Management and Carrier Kinetics Regulation

Semitransparent perovskite solar cells (ST-PSCs) are ideal candidates for building-integrated photovoltaics (BIPV) and tandem solar cells (TSCs) owing to their tunable bandgap and high visible transparency. The best power conversion efficiency (PCE) of ST-PSCs is close to 15% with an average visible transmittance of over 20%, which still lags far behind the PCE of normal opaque PSCs. This can be attributed to the poor light utilization efficiency (LUE) of ST-PSCs. Herein, the pivotal routes for maximizing LUE of ST-PSCs in terms of photon propagation management and carrier kinetics regulation are systematically rationalized. First, the fundamental theoretical analyses on optical processes and electronic properties are provided. Then, insights on photon propagation management measures and carrier kinetics regulation strategies are provided. Furthermore, a summary of the promising commercial application of ST-PSCs in BIPV and TSCs is provided. Finally, the main progress of ST-PSCs is briefly summarized, and the directions for the commercialization of ST-PSCs are proposed.

Twenty-Two Percent Efficient Pb-Free All-Perovskite Tandem Solar Cells Using SCAPS-1D

Herein, we reported the simulation study of lead (Pb)-free all-perovskite tandem solar cells using SCAPS-1D. Tandem solar cells are comprised of two different cells which are known as the top cell and the bottom cell. We simulated tandem solar cells using methyl ammonium germanium iodide (MAGeI₃) as the top subcell absorber layer due to its wide band gap of 1.9 eV. Further, FA_0.75MA_0.25Sn_0.25Ge_0.5I₃ = FAMASnGeI₃ was used as the bottom subcell absorber layer due to its narrow band gap of 1.4 eV. The tandem solar cells were simulated with MAGeI₃ as the top cell and FAMASnGeI₃ as the bottom subcell using SCAPS-1D. Various electro-transport layers (ETLs) i.e., titanium dioxide, tin oxide, zinc oxide, tungsten trioxide, and zinc selenide, were used to examine the impact of ETL on the efficiency of tandem solar cells. The observations revealed that TiO₂ and ZnSe have more suitable band alignment and better charge-extraction/transfer properties. A reasonably improved efficiency of 23.18% and 22.4% have been achieved for TiO₂ and ZnSe layer-based tandem solar cells, respectively.

Mixed Solvents Assisted Post-Treatment Enables High-Efficiency Single-Junction Perovskite and 4T Perovskite/CIGS Tandem Solar Cells

The interface between the perovskite layer and the hole transport layer (HTL) plays a vital role in hole extraction and electron blocking in perovskite solar cells (PSCs), and it is particularly susceptible to harmful defects. Surface passivation is an effective strategy for addressing the above concerns. However, because of its strong polarity, isopropyl alcohol (IPA) is used as a solvent in all of the surface treatment materials reported thus far, and it frequently damages the surface of perovskite. In this paper, a method is proposed for dissolving the passivation materials, for example, guanidine bromide (GABr), in mixed solvents (1:1) of IPA and toluene (TL), which can efficiently passivate interface and grain boundary defects by minimizing the IPA solubility of the perovskite surface. As a result, all the performance parameters Voc, Jsc, and FF are improved, and the power conversion efficiency (PCE) increased from 20.1 to 22.7%. Moreover, combining the PSCs with GABr post-treatment in mixed solvents with copper indium gallium selenide (CIGS) solar cells, a 4-terminal (4T) perovskite/CIGS tandem device is realized and a PCE of 25.5% is achieved. The mixed solvent passivation strategy demonstrated here, hopefully, will open new avenues for improving PSCs' efficiency and stability.

High-Performance 1 cm2 Perovskite-Organic Tandem Solar Cells with a Solvent-Resistant and Thickness-Insensitive Interconnecting Layer

Organic solar cells (OSCs) and perovskite solar cells (PVSCs) are promising candidates for next-generation thin film photovoltaic technologies. The integration of OSCs with PVSCs in tandem devices is now attracting significant attention due to their similar fabrication procedures and the potential to afford a higher device performance. Here, a thickness-insensitive and solvent-resistant interconnecting layer is developed to efficiently connect perovskite and organic subcells with low contact resistance. The resultant perovskite-organic tandem devices maintain high efficiencies over a wide thickness range of the interconnecting layer, from ∼20 nm to ∼50 nm, providing an easily fabricated, solvent-resistant platform to integrate perovskite and organic active layers with low-temperature solution processing techniques. The tandem devices containing an ultrathin PVSC and a typical non-fullerene OSC give a maximum efficiency of 19.2%, which is much higher than those of the single-junction devices. Moreover, highly reproducible 1 cm² perovskite-organic tandem devices are achieved using the thickness-insensitive and solvent-resistant interconnecting layer, and an efficiency of 17.8% is realized. These 1 cm² tandem solar cells are used successfully in solar-to-hydrogen systems to afford a solar-to-fuel conversion efficiency of 11.2%. Overall, these advances represent significant progress in the design of ultrathin and facile solution processed perovskite-organic tandem solar cells.

CsPbCl3 -Cluster-Widened Bandgap and Inhibited Phase Segregation in a Wide-Bandgap Perovskite and its Application to NiOx -Based Perovskite/Silicon Tandem Solar Cells

Nickel oxide (NiO_x ) is an attractive hole-transport material for efficient and stable p-i-n metal-halide perovskite solar cells (PSCs). However, an undesirable redox reaction occurs at the NiO_x /perovskite interface, which results in a low open-circuit voltage (V_OC ), instability, and phase separation of the NiO_x -based wide-bandgap perovskite (Br > 20%). In order to simultaneously address the abovementioned phase separation problem and redox chemistry at the perovskite/NiO_x interface, the bandgap is widened from 1.64 to 1.67 eV by adding inorganic CsPbCl₃ -clusters (3 mol%) to the Cs₂₂ Br₁₅ perovskite precursor solution. Moreover, adding extra 2 mol% CsCl enriches the NiO_x /perovskite interface with Cl, thereby preventing the redox reaction at the interface, while controlling the Br content to within 15% improves the photostability of the wide-bandgap perovskite. Consequently, the power conversion efficiency (PCE) of a single-junction p-i-n PSC increases from 17.82% to 19.76%, which leads to the fabrication of highly efficient monolithic p-i-n-type NiO_x -based perovskite/silicon tandem solar cells with PCEs of up to 27.26% (certified PCE: 27.15%). The perovskite to an n-i-p-type perovskite/silicon tandem solar cell is also applied to deliver a V_OC of 1.93 V and a final efficiency of 25.5%. These findings provide critical insight into the fabrication of highly efficient and stable wide-bandgap perovskites.

Wide-Bandgap Organic-Inorganic Lead Halide Perovskite Solar Cells

Under the groundswell of calls for the industrialization of perovskite solar cells (PSCs), wide-bandgap (>1.7 eV) mixed halide perovskites are equally or more appealing in comparison with typical bandgap perovskites when the former's various potential applications are taken into account. In this review, the progress of wide-bandgap organic-inorganic hybrid PSCs-concentrating on the compositional space, optimization strategies, and device performance-are summarized and the issues of phase segregation and voltage loss are assessed. Then, the diverse applications of wide-bandgap PSCs in semitransparent devices, indoor photovoltaics, and various multijunction tandem devices are discussed and their challenges and perspectives are evaluated. Finally, the authors conclude with an outlook for the future development of wide-bandgap PSCs.

Monolithic All-Perovskite Tandem Solar Cells with Minimized Optical and Energetic Losses

Perovskite-based multijunction solar cells are a potentially cost-effective technology that can help surpass the efficiency limits of single-junction devices. However, both mixed-halide wide-bandgap perovskites and lead-tin narrow-bandgap perovskites suffer from non-radiative recombination due to the formation of bulk traps and interfacial recombination centers which limit the open-circuit voltage of sub-cells and consequently of the integrated tandem. Additionally, the complex optical stack in a multijunction solar cell can lead to losses stemming from parasitic absorption and reflection of incident light which aggravates the current mismatch between sub-cells, thereby limiting the short-circuit current density of the tandem. Here, an integrated all-perovskite tandem solar cell is presented that uses surface passivation strategies to reduce non-radiative recombination at the perovskite-fullerene interfaces, yielding a high open-circuit voltage. By using optically benign transparent electrode and charge-transport layers, absorption in the narrow-bandgap sub-cell is improved, leading to an improvement in current-matching between sub-cells. Collectively, these strategies allow the development of a monolithic tandem solar cell exhibiting a power-conversion efficiency of over 23%.

Two-Terminal Perovskite-Based Tandem Solar Cells for Energy Conversion and Storage

The organic-inorganic hybrid perovskite solar cells present a rapid improvement on power conversion efficiency from 3.8% to 25.5% in the past decades. Owing to the tuneable bandgaps, low-cost, and ease of fabrication, perovskites become ideal candidate materials for fabricating tandem solar cells, especially for efficient and high-voltage monolithic two-terminal devices. In this review, an overview of recent advances in various monolithic perovskite-based tandem solar cells with a focus on the key challenges is provided. Subsequently, the recombination layer materials, construction of wide-bandgap perovskite layer, stability of narrow-bandgap, and current matching principle in tandems are highlighted in order to optimize the output voltage and conversion efficiency of tandem solar cells. Finally, the recent progress is summarized with a focus on potential applications of tandem solar cells for energy conversion and storage, including hydrogen production by water splitting, CO₂ reduction, supercapacitors, and rechargeable batteries, benefiting from the adjustable output voltage of tandem solar cells. It is hoped that this work can offer a feasible strategy to explore more possibilities for fabricating new two-terminal tandem solar cells with high voltage and high conversion efficiency for energy conversion and storage.

Recent Progress in Developing Monolithic Perovskite/Si Tandem Solar Cells

Monolithic perovskite/Silicon tandem solar cells have reached a certified efficiency of 29. 1% in recent years. In this review, we discuss material design for monolithic perovskite/Si tandem solar cells, with the focus on the top-cell development to improve their performance. Firstly, we introduce different types of transparent electrodes with high transmittance and low sheet-resistance used in tandem solar cells. We then discuss the development of the wide-bandgap perovskite absorber for top-cells, especially the strategies to obtain the perovskite layers with good efficiency and stability. In addition, as a special functional layer in tandem solar cells, the recombination layers play an important role in device performance, wherein different configurations are summarized. Furthermore, tandem device cost analysis is discussed. This review summarizes the progress of monolithic perovskite/Silicon tandem solar cells in a pragmatic perspective, which may promote the commercialization of this technology.

Choose Your Own Adventure: Fabrication of Monolithic All-Perovskite Tandem Photovoltaics

Metal halide perovskites (MHPs) have transfixed the photovoltaic (PV) community due to their outstanding and tunable optoelectronic properties coupled to demonstrations of high-power conversion efficiencies (PCE) at a range of bandgaps. This has motivated the field to push perovskites to reach the highest possible performance. One way to increase the efficiency is by fabricating multijunction solar cells, which can split the solar spectrum, reducing thermalization loss. Low-cost all-perovskite tandems have a real chance to soon exceed 30% PCE, which could transform the PV industry. Achieving this goal requires the identification of perovskite sub-cells that are both highly efficient and can be effectively integrated. Herein, it is discussed how to navigate the multiple-choice adventure in choosing between the myriad of options and considerations present when deciding what perovskite materials, contact layers, and processing tools to use. Some of the potential fabrication pitfalls often encountered in MHP based tandem PVs are highlighted, so that they can hopefully be avoided in the future.

Recent Progress in Metal Halide Perovskite-Based Tandem Solar Cells

Metal halide perovskite (MHP)-based tandem solar cells are a promising candidate for use in cost-effective and high-performance solar cells that can compete with fossil fuels. To understand the research trends for MHP-based tandem solar cells, a general introduction to single-junction and multiple-junction MHP solar cells and the configuration of tandem devices is provided, along with an overview of the recent progress regarding various MHP-based tandem cells, including MHP/crystalline silicon, MHP/CuInGaS, MHP/organic photovoltaic, MHP/quantum dot, and all-perovskite tandem cell. Future research directions for MHP-based tandem solar cells are also discussed.

Compositional Engineering Study of Lead-Free Hybrid Perovskites for Solar Cell Applications

Hybrid organic-inorganic perovskite solar cells (HOIPs), especially CH₃NH₃PbI₃ (MAPbI₃), have received tremendous attention due to their excellent power conversion efficiency (25.2%). However, two fundamental hurdles, long-term stability and lead (Pb) toxicity, prevent HOIPs from practical applications in the solar industry. To overcome these issues, compositional engineering has been used to modify cations at A- and B-sites and anions at the X-site in the general form ABX₃. In this work, we used the density functional theory (DFT) to incorporate Rb, Cs, and FA at the A-site to minimize the volatile nature of MA, while the highly stable Ca²⁺ and Sr²⁺ were mixed with the less stable Ge²⁺ and Sn²⁺ at the B-site to obtain a Pb-free perovskite. To further enhance the stability, we mixed the X-site anions (I/Br). Through this approach, we introduced 20 new perovskite species to the lead-free perovskite family and 7 to the lead-containing perovskite family. The molecular dynamic (MD) simulations, enthalpy formation, and tolerance and octahedral factor study confirm that all of the perovskite alloys we introduced here are as stable as pristine MAPbI₃. All Pb-free perovskites have suitable and direct band gaps (1.42-1.77 eV) at the Γ-point, which are highly desirable for solar cell applications. Most of our Pb-free perovskites have smaller effective masses and exciton binding energies. Finally, we show that the introduced perovskites have high absorption coefficients (10⁵ cm^-1) and strong absorption efficiencies (above 90%) in a wide spectral range (300-1200 nm), reinforcing their significant potential applications. This study provides a new way of searching for stable lead-free perovskites for sustainable and green energy applications.

Tin and Mixed Lead-Tin Halide Perovskite Solar Cells: Progress and their Application in Tandem Solar Cells

Metal halide perovskites have recently attracted enormous attention for photovoltaic applications due to their superior optical and electrical properties. Lead (Pb) halide perovskites stand out among this material series, with a power conversion efficiency (PCE) over 25%. According to the Shockley-Queisser (SQ) limit, lead halide perovskites typically exhibit bandgaps that are not within the optimal range for single-junction solar cells. Partial or complete replacement of lead with tin (Sn) is gaining increasing research interest, due to the promise of further narrowing the bandgaps. This enables ideal solar utilization for single-junction solar cells as well as the construction of all-perovskite tandem solar cells. In addition, the usage of Sn provides a path to the fabrication of lead-free or Pb-reduced perovskite solar cells (PSCs). Recent progress in addressing the challenges of fabricating efficient Sn halide and mixed lead-tin (Pb-Sn) halide PSCs is summarized herein. Mixed Pb-Sn halide perovskites hold promise not only for higher efficiency and more stable single-junction solar cells but also for efficient all-perovskite monolithic tandem solar cells.

Two-Dimensional BAs/InTe: A Promising Tandem Solar Cell with High Power Conversion Efficiency

Tandem solar cells (SCs) connecting two subcells with different absorption bands have the potential to reach the commercialized photovoltaic standard. However, the performance improvement of tandem architectures is still a challenge, primarily owing to the mismatch of band gaps in two subcells. Here, we demonstrate a two-dimensional (2D) BAs/InTe-based tandem SC, which could achieve solar-to-electric conversion efficiency higher than 30%. First, the narrow band gap of hexagonal single-layer BX (X = P and As) and wide band gap of single-layer YZ (Y = Ga and In, Z = S, Se, and Te) are found to have high thermodynamic stability based on density functional theory calculations. Next, considering narrow and wide band gaps at the HSE06 functional, single-layer BX/YZ-based tandem SCs are built to effectively capture a broad-band solar spectrum by combining such two subcells. Since the band gap of single-layer BAs matches well with that of the InTe monolayer, the power conversion efficiency of BAs/InTe-based tandem SC can reach as high as 30.2%. Moreover, it is important to note that the used materials, including few-layer GaZ and InSe, have been experimentally prepared, which strongly supports the high feasibility of the designed 2D tandem SCs in this work. Our constructed 2D-material-based devices can be competitive in realizing commercialized high-performance tandem SCs.

Recent Progress in Molecular Design of Fused Ring Electron Acceptors for Organic Solar Cells

The quest for sustainable energy sources has led to accelerated growth in research of organic solar cells (OSCs). A solution-processed bulk-heterojunction (BHJ) OSC generally contains a donor and expensive fullerene acceptors (FAs). The last 20 years have been devoted by the OSC community to developing donor materials, specifically low bandgap polymers, to complement FAs in BHJs. The current improvement from ≈2.5% in 2013 to 17.3% in 2018 in OSC performance is primarily credited to novel nonfullerene acceptors (NFA), especially fused ring electron acceptors (FREAs). FREAs offer unique advantages over FAs, like broad absorption of solar radiation, and they can be extensively chemically manipulated to tune optoelectronic and morphological properties. Herein, the current status in FREA-based OSCs is summarized, such as design strategies for both wide and narrow bandgap FREAs for BHJ, all-small-molecule OSCs, semi-transparent OSC, ternary, and tandem solar cells. The photovoltaics parameters for FREAs are summarized and discussed. The focus is on the various FREA structures and their role in optical and morphological tuning. Besides, the advantages and drawbacks of both FAs and NFAs are discussed. Finally, an outlook in the field of FREA-OSCs for future material design and challenges ahead is provided.

Advances in Solution-Processed Multijunction Organic Solar Cells

The efficiency of organic solar cells can benefit from multijunction device architectures, in which energy losses are substantially reduced. Herein, recent developments in the field of solution-processed multijunction organic solar cells are described. Recently, various strategies have been investigated and implemented to improve the performance of these devices. Next to developing new materials and processing methods for the photoactive and interconnecting layers, specific layers or stacks are designed to increase light absorption and improve the photocurrent by utilizing optical interference effects. These activities have resulted in power conversion efficiencies that approach those of modern thin film photovoltaic technologies. Multijunction cells require more elaborate and intricate characterization procedures to establish their efficiency correctly and a critical view on the results and new insights in this matter are discussed. Application of multijunction cells in photoelectrochemical water splitting and upscaling toward a commercial technology is briefly addressed.

A Semitransparent Inorganic Perovskite Film for Overcoming Ultraviolet Light Instability of Organic Solar Cells and Achieving 14.03% Efficiency

Organic solar cells (OSCs) can be unstable under ultraviolet (UV) irradiation. To address this issue and enhance the power conversion efficiency (PCE), an inorganic-perovskite/organic four-terminal tandem solar cell (TSC) based on a semitransparent inorganic CsPbBr₃ perovskite solar cell (pero-SC) as the top cell and an OSC as bottom cell is constructed. The high-quality CsPbBr₃ photoactive layer of the planar pero-SC is prepared with a dual-source vacuum coevaporation method, using stoichiometric precursors of CsBr and PbBr₂ with a low evaporation rate. The resultant opaque planar pero-SC exhibits an ultrahigh open-circuit voltage of 1.44 V and the highest reported PCE of 7.78% for a CsPbBr₃ -based planar pero-SC. Importantly, the devices show no degradation after 120 h UV light illumination. The related semitransparent pero-SC can almost completely filter UV light and well maintain photovoltaic performance; it additionally shows an extremely high average visible transmittance. When it is used to construct a TSC, the top pero-SC acting as a UV filter can utilize UV light for photoelectric conversion, avoiding the instability problem of UV light on the bottom OSC that can meet the industrial standards of UV-light stability for solar cells, and leading to the highest reported PCE of 14.03% for the inorganic-perovskite/organic TSC.

Transparent Conductive Adhesives for Tandem Solar Cells Using Polymer-Particle Composites

Transparent conductive adhesives (TCAs) can enable conductivity between two substrates, which is useful for a wide range of electronic devices. Here, we have developed a TCA composed of a polymer-particle blend with ethylene-vinyl acetate as the transparent adhesive and metal-coated flexible poly(methyl methacrylate) microspheres as the conductive particles that can provide conductivity and adhesion regardless of the surface texture. This TCA layer was designed to be nearly transparent, conductive in only the out-of-plane direction, and of practical adhesive strength to hold the substrates together. The series resistance was measured at 0.3 and 0.8 Ω cm² for 8 and 0.2% particle coverage, respectively, while remaining over 92% was transparent in both cases. For applications in photovoltaic devices, such as mechanically stacked multijunction III-V/Si cells, a TCA with 1% particle coverage will have less than 0.5% power loss due to the resistance and less than 1% shading loss to the bottom cell.

Compositionally Graded Absorber for Efficient and Stable Near-Infrared-Transparent Perovskite Solar Cells

Compositional grading has been widely exploited in highly efficient Cu(In,Ga)Se2, CdTe, GaAs, quantum dot solar cells, and this strategy has the potential to improve the performance of emerging perovskite solar cells. However, realizing and maintaining compositionally graded perovskite absorber from solution processing is challenging. Moreover, the operational stability of graded perovskite solar cells under long-term heat/light soaking has not been demonstrated. In this study, a facile partial ion-exchange approach is reported to achieve compositionally graded perovskite absorber layers. Incorporating compositional grading improves charge collection and suppresses interface recombination, enabling to fabricate near-infrared-transparent perovskite solar cells with power conversion efficiency of 16.8% in substrate configuration, and demonstrate 22.7% tandem efficiency with 3.3% absolute gain when mechanically stacked on a Cu(In,Ga)Se2 bottom cell. Non-encapsulated graded perovskite device retains over 93% of its initial efficiency after 1000 h operation at maximum power point at 60 °C under equivalent 1 sun illumination. The results open an avenue in exploring partial ion-exchange to design graded perovskite solar cells with improved efficiency and stability.

Single-graded CIGS with narrow bandgap for tandem solar cells

Multi-junction solar cells show the highest photovoltaic energy conversion efficiencies, but the current technologies based on wafers and epitaxial growth of multiple layers are very costly. Therefore, there is a high interest in realizing multi-junction tandem devices based on cost-effective thin film technologies. While the efficiency of such devices has been limited so far because of the rather low efficiency of semitransparent wide bandgap top cells, the recent rise of wide bandgap perovskite solar cells has inspired the development of new thin film tandem solar devices. In order to realize monolithic, and therefore current-matched thin film tandem solar cells, a bottom cell with narrow bandgap (~1 eV) and high efficiency is necessary. In this work, we present Cu(In,Ga)Se₂ with a bandgap of 1.00 eV and a maximum power conversion efficiency of 16.1%. This is achieved by implementing a gallium grading towards the back contact into a CuInSe₂ base material. We show that this modification significantly improves the open circuit voltage but does not reduce the spectral response range of these devices. Therefore, efficient cells with narrow bandgap absorbers are obtained, yielding the high current density necessary for thin film multi-junction solar cells.

Nonfullerene Tandem Organic Solar Cells with High Open-Circuit Voltage of 1.97 V

Small-molecule nonfullerene-based tandem organic solar cells (OSCs) are fabricated for the first time by utilizing P3HT:SF(DPPB)₄ and PTB7-Th:IEIC bulk heterojunctions as the front and back subcells, respectively. A power conversion efficiency of 8.48% is achieved with an ultrahigh open-circuit voltage of 1.97 V, which is the highest voltage value reported to date among efficient tandem OSCs.

Stable Low-Bandgap Pb-Sn Binary Perovskites for Tandem Solar Cells

A low-bandgap (1.33 eV) Sn-based MA_0.5 FA_0.5 Pb_0.75 Sn_0.25 I₃ perovskite is developed via combined compositional, process, and interfacial engineering. It can deliver a high power conversion efficiency (PCE) of 14.19%. Finally, a four-terminal all-perovskite tandem solar cell is demonstrated by combining this low-bandgap cell with a semitransparent MAPbI₃ cell to achieve a high efficiency of 19.08%.

Evaluation of Small Molecules as Front Cell Donor Materials for High-Efficiency Tandem Solar Cells

Three small molecules as front cell donors for tandem cells are thoroughly evaluated and a high power conversion efficiency of 11.47% is achieved, which demonstrates that the oligomer-like small molecules offer a good choice for high-performance tandem solar cells.

CH3 NH3 PbBr3 -CH3 NH3 PbI3 Perovskite-Perovskite Tandem Solar Cells with Exceeding 2.2 V Open Circuit Voltage

Perovskite-perovskite tandem solar cells with open-circuit voltages of over 2.2 V are reported. These cost-effective, solution-processible perovskite hybrid tandem solar cells with high open-circuit voltages are fabricated by the simple lamination of a front planar MAPbBr3 perovskite cell and a back MAPbI3 planar perovskite solar cell.

High Efficiency Tandem Thin-Perovskite/Polymer Solar Cells with a Graded Recombination Layer

Perovskite-containing tandem solar cells are attracting attention for their potential to achieve high efficiencies. We demonstrate a series connection of a ∼ 90 nm thick perovskite front subcell and a ∼ 100 nm thick polymer:fullerene blend back subcell that benefits from an efficient graded recombination layer containing a zwitterionic fullerene, silver (Ag), and molybdenum trioxide (MoO3). This methodology eliminates the adverse effects of thermal annealing or chemical treatment that occurs during perovskite fabrication on polymer-based front subcells. The record tandem perovskite/polymer solar cell efficiency of 16.0%, with low hysteresis, is 75% greater than that of the corresponding ∼ 90 nm thick perovskite single-junction device and 65% greater than that of the polymer single-junction device. The high efficiency of this hybrid tandem device, achieved using only a ∼ 90 nm thick perovskite layer, provides an opportunity to substantially reduce the lead content in the device, while maintaining the high performance derived from perovskites.

Tandem Solar Cells from Accessible Low Band-Gap Polymers Using an Efficient Interconnecting Layer

Tandem solar cell architectures are designed to improve device photoresponse by enabling the capture of wider range of solar spectrum as compared to single-junction device. However, the practical realization of this concept in bulk-heterojunction polymer systems requires the judicious design of a transparent interconnecting layer compatible with both polymers. Moreover, the polymers selected should be readily synthesized at large scale (>1 kg) and high performance. In this work, we demonstrate a novel tandem polymer solar cell that combines low band gap poly isoindigo [P(T3-iI)-2], which is easily synthesized in kilogram quantities, with a novel Cr/MoO3 interconnecting layer. Cr/MoO3 is shown to be greater than 80% transparent above 375 nm and an efficient interconnecting layer for P(T3-iI)-2 and PCDTBT, leading to 6% power conversion efficiencies under AM 1.5G illumination. These results serve to extend the range of interconnecting layer materials for tandem cell fabrication by establishing, for the first time, that a thin, evaporated layer of Cr/MoO3 can work as an effective interconnecting layer in a tandem polymer solar cells made with scalable photoactive materials.

Polymer homo-tandem solar cells with best efficiency of 11.3%

Rational materials design and interface engineering are both essential to realize a high performance for tandem cells. Two identical bulk heterojunctions are connected in series using novel interconnection layers combining pH-neutral conjugated polyelectrolytes and a thin film of ZnO nanoparticles by a solution process. The best performing tandem cells achieve a power conversion efficiency of 11.3%, with 25% enhancement in efficiency compared with single cells, which arises primarily from the increased light absorption.

Tandem solar cells made from amorphous silicon and polymer bulk heterojunction sub-cells

A tandem solar cell based on a combination of an amorphous silicon (a-Si) and polymer solar cell (PSC) is demonstrated. As these tandem devices can be readily fabricated by low-cost methods, they require only a minor increase in the total manufacturing cost. Therefore, a combination of a-Si and PSC provides a compelling solution to reduce the cost of electricity produced by photovoltaics.

Enhanced fill factor of tandem organic solar cells incorporating a diketopyrrolopyrrole-based low-bandgap polymer and optimized interlayer

We demonstrate that reproducible results can be obtained from tandem solar cells based on the wide-bandgap poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2',1',3'-benzothiadiazole] (PCDTBT) and the diketopyrrolopyrrole (DPP)-based narrow bandgap polymer (DT-PDPP2T-TT) with a decyltetradecyl (DT) and an electron-rich 2,5-di-2-thienylthieno[3,2-b]thiophene (2T-TT) group fabricated using an optimized interlayer (ZnO NPs/ph-n-PEDOT:PSS) [NPs: nanoparticles; ph-n: pH-neutral PEDOT: poly(3,4-ethylenedioxythiophene); PSS: polystyrene sulfonate]. The tandem cells are fabricated by applying a simple process without thermal annealing. The ZnO NP interlayer operates well when the ZnO NPs are dispersed in 2-methoxyethanol, as no precipitation and chemical reactions occur. In addition to the ZnO NP film, we used neutral PEDOT:PSS as a second interlayer which is not affect to the sequential deposited bulk heterojunction (BHJ) active layer of acidification. The power conversion efficiency (PCE) of a tandem device reaches 7.4 % (open-circuit voltage VOC =1.53 V, short-circuit current density JSC =7.3 mA cm(-2) , and fill factor FF=67 %). Furthermore, FF is increased to up to 71 % when another promising large bandgap (bandgap ∼1.94 eV) polymer (PBnDT-FTAZ) is used. The surface of each layer with nanoscale morphology (BHJ1/ZnO NPs film/ph-n-PEDOT:PSS/BHJ2) was examined by means of AFM analysis during sequential processing. The combination of these factors, efficient DPP-based narrow bandgap material and optimized interlayer, leads to the high FF (average approaches 70 %) and reproducibly operating tandem BHJ solar cells.

A vertically integrated solar-powered electrochromic window for energy efficient buildings

A solution-processed self-powered polymer electrochromic/photovoltaic (EC/PV) device is realized by vertically integrating two transparent PV cells with an ECD. The EC/PV cell is a net energy positive dual functional device, which can be reversibly switched between transparent and colored states by PV cells for regulating incoming sunlight through windows. The two PV cells can individually, or in pairs, generate electricity.

Performance-improved thin-film a-Si:H/μc-Si:H tandem solar cells by two-dimensionally nanopatterning photoactive layer

Tandem solar cells consisting of amorphous and microcrystalline silicon junctions with the top junction nanopatterned as a two-dimensional photonic crystal are studied. Broadband light trapping, detailed electron/hole transport, and photocurrent matching modulation are considered. It is found that the absorptances of both junctions can be significantly increased by properly engineering the duty cycles and pitches of the photonic crystal; however, the photocurrent enhancement is always unevenly distributed in the junctions, leading to a relatively high photocurrent mismatch. Further considering an optimized intermediate layer and device resistances, the optimally matched photocurrent approximately 12.74 mA/cm2 is achieved with a light-conversion efficiency predicted to be 12.67%, exhibiting an enhancement of over 27.72% compared to conventional planar configuration.