Vertical plane depth-resolved surface potential and carrier separation characteristics in flexible CZTSSe solar cells with over 12% efficiency

Cu2ZnSn(S,Se)4(CZTSSe)solar cells have resource distribution and economic advantages.The main cause of their low efficiency is carrier loss resulting from recombination of photo-generated electron and hole.To overcome this,it is important to understand their electron-hole behavior characteristics.To determine the carrier separation characteristics,we measured the surface potential and the local current in terms of the absorber depth.The elemental variation in the intragrains(IGs)and at the grain boundaries(GBs)caused a band edge shift and bandgap(Eg)change.At the absorber surface and subsurface,an upward Ec and Ev band bending structure was observed at the GBs,and the carrier separation was improved.At the absorber center,both upward Ec and Ev and downward Ec-upward Ev band bending structures were observed at the GBs,and the carrier separation was degraded.To improve the carrier separation and suppress carrier recombination,an upward Ec and Ev band bending structure at the GBs is desirable.

Interface optimization and defects suppression via Na F introduction enable efficient flexible Sb_(2)Se_(3) thin-film solar cells

Sb_(2)Se_(3) with unique one-dimensional(1D) crystal structure exhibits exceptional deformation tolerance,demonstrating great application potential in flexible devices.However,the power conversion efficiency(PCE) of flexible Sb_(2)Se_(3) photovoltaic devices is temporarily limited by the complicated intrinsic defects and the undesirable contact interfaces.Herein,a high-quality Sb_(2)Se_(3) absorber layer with large crystal grains and benign [hkl] growth orientation can be first prepared on a Mo foil substrate.Then NaF intermediate layer is introduced between Mo and Sb_(2)Se_(3),which can further optimize the growth of Sb_(2)Se_(3)thin film.Moreover,positive Na ion diffusion enables it to dramatically lower barrier height at the back contact interface and passivate harmful defects at both bulk and heterojunction.As a result,the champion substrate structured Mo-foil/Mo/NaF/Sb_(2)Se_(3)/CdS/ITO/Ag flexible thin-film solar cell delivers an obviously higher efficiency of 8.03% and a record open-circuit voltage(V_(OC)) of 0.492 V.This flexible Sb_(2)Se_(3) device also exhibits excellent stability and flexibility to stand large bending radius and multiple bending times,as well as superior weak light photo-response with derived efficiency of 12.60%.This work presents an effective strategy to enhance the flexible Sb_(2)Se_(3) device performance and expand its potential photovoltaic applications.

In-doping collaboratively controlling back interface and bulk defects to achieve efficient flexible CZTSSe solar cells

Focusing on the low open circuit voltage(V_(OC))and fill factor(FF)in flexible Cu_(2)ZnSn(S,Se)_(4)(CZTSSe)solar cells,indium(In)ions are introduced into the CZTSSe absorbers near Mo foils to modify the back interface and passivate deep level defects in CZTSSe bulk concurrently for improving the performance of flexible device.The results show that In doping effectively inhibits the formation of secondary phase(Cu(S,Se)_(2))and VSndefects.Further studies demonstrate that the barrier height at the back interface is decreased and the deep level defects(Cu_(Sn)defects)in CZTSSe bulk are passivated.Moreover,the carrier concentration is increased and the V_(OC) deficit(V_(OC,def))is decreased significantly due to In doping.Finally,the flexible CZTSSe solar cell with 10.01%power conversion efficiency(PCE)has been obtained.The synergistic strategy of interface modification and bulk defects passivation through In incorporation provides a new thought for the fabrication of efficient flexible kesterite-based solar cells.

Cross-linked polyelectrolyte reinforced SnO_(2)electron transport layer for robust flexible perovskite solar cells

SnO_(2)electron transport layer(ETL)is a vital component in perovskite solar cells(PSCs),due to its excellent photoelectric properties and facile fabrication process.In this study,we synthesized a water-soluble and adhesive polyelectrolyte with ethanolamine(EA)and poly-acrylic acid(PAA).The linear PAA was crosslinked by EA,forming a 3D network that stabilized the SnO_(2)nanoparticle dispersion.An organic–inorganic hybrid ETL is developed by introducing the cross-linked PAA-EA into SnO_(2)ETL,which prevents nano particle agglomeration and facilitates uniform SnO_(2)film formation with fewer defects.Additionally,the PAA-EA-modified SnO_(2)facilitated a uniform and compact perovskite film,enhancing the interface contact and carrier transport.Consequently,the PAA-EA-modified PSCs exhibited excellent PCE of 24.34%and 22.88%with high reproducibility for areas of 0.045 and 1.00 cm~2,respectively.Notably,owing to structure reinforce effect of PAA-EA in SnO_(2)ETL,flexible device demonstrated an impressive PCE of 23.34%while maintaining 90.1%of the initial PCE after 10,000 bending cycles with a bending radius of 5 mm.This successful approach of polyelectrolyte reinforced hybrid organic–inorganic ETL displays great potential for flexible,large-area PSCs application.

Improved interfacial adhesion for stable flexible inverted perovskite solar cells

Flexible perovskite solar cells have attracted widespread attention due to their unique advantages in lightweight,high flexibility,and easy deformation,which are suitable for portable electronics.However,the inverted(p-i-n)structured devices suffer from poor stability largely due to the low adhesion at the brittle interface(the hole transport layer/perovskite).Herein,zeolitic imidazolate framework-67(ZIF-67)is applied to inverted structured cells to optimize the interface and prolong the device lifetime.As a result,the flexible devices based on ZIF-67 obtain the champion power conversion efficiency of 20.16%.Over 1000 h under continuous light irradiation,the device retains 96%and 80%of its original efficiency without and with bias,respectively.Notably,devices show mechanical endurance with over 78%efficiency retention after 10,000 cycles of consecutive bending cycles(R=6 mm).The introduction of ZIF-67 suppresses the cracking in device bending,which results in improved environmental stability and bending durability.

Petri net based modeling and analysis for weldingflexible manufacturing cell

Due to the development of advanced manufacturing technology and the introduction of Smart Manufacturing notion in the field of modern industrial production, welding flexible manufacturing using robot technology has become the inevitable developing direction on welding automation. Based on a new intelligent arc welding flexible manufacturing cell (WFMC), its system structure and control policies are studied in this paper. Aiming at the different information flows among every subsystem and central monitoring computer in this WFMC, Petri net theory is introduced into the process of welding manufacturing. A discrete control model of WFMC has been constructed, in which the system's status is regarded as place and the control process is regarded as transition. Moreover, grounded on automation Petri net principle, the judging and utilizing of information obtained from welding sensors are imported into Petri net structure, which extends the traditional Petri net concepts. The control model and policies researched in this paper have established foundation for further intelligent real time control on welding flexible manufacturing cell and system.

Recent Progress of Electrode Materials for Flexible Perovskite Solar Cells

Flexible perovskite solar cells(FPSCs) have attracted enormous interest in wearable and portable electronics due to their high power-per-weight and low cost. Flexible and efficient perovskite solar cells require the development of flexible electrodes compatible with the optoelectronic properties of perovskite. In this review, the recent progress of flexible electrodes used in FPSCs is comprehensively reviewed. The major features of flexible transparent electrodes, including transparent conductive oxides, conductive polymer, carbon nanomaterials and nanostructured metallic materials are systematically compared. And the corresponding modification strategies and device performance are summarized. Moreover, flexible opaque electrodes including metal films, opaque carbon materials and metal foils are critically assessed. Finally, the development directions and difficulties of flexible electrodes are given.

Enhancing carrier transport in flexible CZTSSe solar cells via doping Li strategy

The passivation of non-radiative states and inhibition of band tailings are desirable for improving the open-circuit voltage(V_(oc))of CZTSSe thin-film solar cells.Recently,alkali metal doping has been investigated to passivate defects in CZTSSe films.Herein,we investigate Li doping effects by applying Li OH into CZTSSe precursor solutions,and verify that carrier transport is enhanced in the CZTSSe solar cells.Systematic characterizations demonstrate that Li doping can effectively passivate non-radiative recombination centers and reduce band tailings of the CZTSSe films,leading to the decrease in total defect density and the increase in separation distance between donor and acceptor.Fewer free carriers are trapped in the band tail states,which speeds up carrier transport and reduces the probability of deep-level defects capturing carriers.The charge recombination lifetime is about twice as long as that of the undoped CZTSSe device,implying the heterojunction interface recombination is also inhibited.Besides,Li doping can increase carrier concentration and enhance build-in voltage,leading to a better carrier collection.By adjusting the Li/(Li+Cu)ratio to 18%,the solar cell efficiency is increased significantly to 9.68%with the fill factor(FF)of 65.94%,which is the highest FF reported so far for the flexible CZTSSe solar cells.The increased efficiency is mainly attributed to the reduction of V_(oc)deficit and the improved CZTSSe/Cd S junction quality.These results open up a simple route to passivate non-radiative states and reduce the band tailings of the CZTSSe films and improve the efficiency of the flexible CZTSSe solar cells.

A 9% efficiency of flexible Mo-foil-based Cu2ZnSn(S,Se)4 solar cells by improving CdS buffer layer and heterojunction interface

Flexible Cu2ZnSn(S,Se)4(CZTSSe)solar cells show great potential applications due to low-cost,nontoxicity,and stability.The device performances under an especial open circuit voltage(VOC)are limited by the defect recombination of CZTSSe/CdS heterojunction interface.We improve the deposition technique to obtain compact CdS layers without any pinholes for flexible CZTSSe solar cells on Mo foils.The efficiency of the device is improved from 5.7%to 6.86%by highquality junction interface.Furthermore,aiming at the S loss of CdS film,the S source concentration in deposition process is investigated to passivate the defects and improve the CdS film quality.The flexible Mo-foil-based CZTSSe solar cells are obtained to possess a 9.05%efficiency with a VOC of 0.44 V at an optimized S source concentration of 0.68 mol/L.Systematic physical measurements indicate that the S source control can effectively suppress the interface recombination and reduce the VOCdeficit.For the CZTSSe device bending characteristics,the device efficiency is almost constant after1000 bends,manifesting that the CZTSSe device has an excellent mechanical flexibility.The effective improvement strategy of CdS deposition is expected to provide a new perspective for promoting the conversion efficiency of CZTSSe solar cells.

Recent advances of flexible perovskite solar cells

In few years only, the efficiency record of perovskite solar cells（PSCs） has raised quickly from 3.8% to over 22%. This emerging photovoltaic technology has primarily shown its great potential of industrialization. Flexible PSCs are thought to be one of the most priority options for mass production, related to the intrinsic advantage of perovskite thin films which could be deposited by facile solution processes at low temperature. Flexible PSCs have at least four advantages in comparison to the rigid counterpart：（1） it can generate higher power output at lighter weight,（2） it is easily portable,（3） it can be easily attached to architectures or textiles with diverse shapes, and（4） it is compatible with roll-to-roll fabrication in a large scale. In this review, we have summarized recent development of the key materials and technologies applied in flexible PSCs. The key materials including flexible substrates, transparent and conductive electrodes, and interfacial materials; some key technologies about roll-to-roll manufacture, encapsulation technology have been overviewed. Finally, a prospect on possible application directions of flexible PSCs has been discussed.

Solution‑Processed Transparent Conducting Electrodes for Flexible Organic Solar Cells with 16.61% Efficiency

Nonfullerene organic solar cells(OSCs)have achieved breakthrough with pushing the efficiency exceeding 17%.While this shed light on OSC commercialization,high-performance flexible OSCs should be pursued through solution manufacturing.Herein,we report a solution-processed flexible OSC based on a transparent conducting PEDOT:PSS anode doped with trifluoromethanesulfonic acid(CF3SO3H).Through a low-concentration and low-temperature CF3SO3H doping,the conducting polymer anodes exhibited a main sheet resistance of 35Ωsq−1(minimum value:32Ωsq−1),a raised work function(≈5.0 eV),a superior wettability,and a high electrical stability.The high work function minimized the energy level mismatch among the anodes,hole-transporting layers and electron-donors of the active layers,thereby leading to an enhanced carrier extraction.The solution-processed flexible OSCs yielded a record-high efficiency of 16.41%(maximum value:16.61%).Besides,the flexible OSCs afforded the 1000 cyclic bending tests at the radius of 1.5 mm and the long-time thermal treatments at 85°C,demonstrating a high flexibility and a good thermal stability.

Flexible perovskite solar cells:Materials and devices

Flexible perovskite solar cells(FPSCs)are supposed to play an important role in the commercialization of perovskite solar cells due to their unique properties,such as high efficiency,thin thickness and being compatible with roll to roll(R2R)process for mass production.At present,deformable and lightweight FPSCs have been successfully prepared and applied as power supply by integrating with different wearable and portable electronics,which opens a niche market for photovoltaics.In this mini review,we will introduce the recent progress of FPSCs from the aspect of small-area flexible devices,R2R processed devices with large scale and emerging flexible cells with deformability and stretchability.Finally,conclusion and outlook are provided.

Highly efficient flexible perovskite solar cells with vacuum-assisted low-temperature annealed SnO2 electron transport layer

The demand for lightweight, flexible, and high-performance portable power sources urgently requires high-efficiency and stable flexible solar cells. In the case of perovskite solar cells(PSCs), most of the common electron transport layer(ETL) needs to be annealed for improving the optoelectronic properties,while conventional flexible substrates could barely stand the high temperature. Herein, a vacuumassisted annealing SnO_(2) ETL at low temperature(100℃) is utilized in flexible PSCs and achieved high efficiency of 20.14%. Meanwhile, the open-circuit voltage(V_(oc)) increases from 1.07 V to 1.14 V. The flexible PSCs also show robust bending stability with 86.8% of the initial efficiency is retained after 1000 bending cycles at a bending radius of 5 mm. X-ray photoelectron spectroscopy(XPS), atomic force microscopy(AFM), and contact angle measurements show that the density of oxygen vacancies, the surface roughness of the SnO_(2) layer, and film hydrophobicity are significantly increased, respectively. These improvements could be due to the oxygen-deficient environment in a vacuum chamber, and the rapid evaporation of solvents. The proposed vacuum-assisted low-temperature annealing method not only improves the efficiency of flexible PSCs but is also compatible and promising in the large-scale commercialization of flexible PSCs.

Review and perspective of materials for flexible solar cells

Thin-film flexible solar cells are lightweight and mechanically robust.Along with rapidly advancing battery technology,flexible solar panels are expected to create niche products that require lightweight,mechanical flexibility,and moldability into complex shapes,such as roof-panel for electric automobiles,foldable umbrellas,camping tents,etc.In this paper,we provide a comprehensive assessment of relevant materials suitable for making flexible solar cells.Substrate materials reviewed include metals,ceramics,glasses,and plastics.For active materials,we focus primarily on emerging new semiconductors including small organic donor/acceptor molecules,conjugated donor/acceptor polymers,and organometal halide perovskites.For electrode materials,transparent conducting oxides,thin metal films/nanowires,nanocarbons,and conducting polymers are reviewed.We also discuss the merits,weaknesses,and future perspectives of these materials for developing next-generation flexible photovoltaics.

Efficient flexible perovskite solar cells and modules using a stable SnO_(2)-nanocrystal isopropanol dispersion

The outstanding advantages of lightweight and flexibility enable flexible perovskite solar cells(PSCs)to have great application potential in mobile energy devices.Due to the low cost,low-temperature processibility,and high electron mobility,SnO_(2) nanocrystals have been widely employed as the electron transport layer in flexible PSCs.To prepare high-quality SnO_(2) layers,a monodispersed nanocrystal solution is normally used.However,the SnO_(2) nanocrystals can easily aggregate,especially after long periods of storage.Herein,we develop a green and cost-effective strategy for the synthesis of high-quality SnO_(2) nanocrystals at low temperatures by introducing small molecules of glycerol,obtaining a stable and well-dispersed SnO_(2)-nanocrystal isopropanol dispersion successfully.Due to the enhanced dispersity and super wettability of this alcohol-based SnO_(2)-nanocrystal solution,large-area smooth and dense SnO_(2) films are easily deposited on the plastic conductive substrate.Furthermore,this contributes to effective charge transfer and suppressed non-radiative recombination at the interface between the SnO_(2) and perovskite layers.As a result,a greatly enhanced power conversion efficiency(PCE)of 21.8%from 19.2%is achieved for small-area flexible PSCs.A large-area 5 cm×5 cm flexible perovskite solar mini-module with a champion PCE of 16.5%and good stability is also demonstrated via this glycerol-modified SnO_(2)-nanocrystal isopropanol dispersion approach.

Large-Area Gravure-Printed AgNWs Electrode on Water/Oxygen Barrier Substrate for Long-Term Stable Large-Area Flexible Organic Solar Cells

Large-area AgNWs electrodes(25 cm×10 cm)were fabricated through roll-to-roll printing on the polyvinyl alcohol(PVA)modified water and oxygen barrier substrate.The modification of the barrier film with PVA improved the wettability of silver nanowires on the barrier films and led to the formation of homogenous large-area AgNWs networks.The mechanical flexibility,especially the adhesion force between the silver electrode and the barrier film substrate was dramatically improved through PVA modification.The efficiency of 13.51%for the flexible OSCs with an area of 0.64 cm2 was achieved based on the PET/barrier film/PVA/AgNWs electrode.The long-term stability showed the flexible OSCs based on the PET/barrier film/PVA/AgNWs electrode have a significantly improved stability relative to the device on PET/AgNWs electrode,and comparable air stability as the rigid device with glass/ITO device.The unencapsulated devices maintained nearly 50%of the original efficiency after storage for 600 h in air.After a simple top encapsulation,the flexible devices remained at 60%of the initial efficiency after 2000 h in the air.Therefore,the flexible AgNWs electrode based on the barrier film would have the potential to improve the air storage stability of organic flexible solar cells.

Deep insights into the advancements and applications of perovskite based photovoltaic cells

The organometal halide perovskite materials have a blend of surprising optoelectronic properties, for example high value of absorption coefficient and abrupt optical retention edge, lifetime, long charge carrier diffusion length and many more. Brought in conjunction with the capacity for manufacturing at low temperature, likewise from the solution, devices based on perovskite, particularly solar cells have been contemplated seriously with striking advancements in performance, in the course of recent years. The amalgamation of minimal effort, high efficiency and extra applications gives incredible potential to commercialization of these cells. The applications and performance of perovskite cells frequently relate with the structures of the device. Numerous creative structures of the devices were produced, targeting for vast scale manufacture, diminishing creation cost, upgrading the PCE and subsequently expanding the prospective for future applications. This paper outlines the various advanced structures of PSC, challenges confronted by these PSCs and their future perspectives. The commercial applications of PSC are additionally talked about in this paper.

Structural engineering of cathodes for improved Zn-ion batteries

Aqueous zinc-ion batteries(ZIBs) are attracting considerable attention because of their low cost,high safety and abundant anode material resources.However,the major challenge faced by aqueous ZIBs is the lack of stable and high capacity cathode materials due to their complicated reaction mechanism and slow Zn-ion transport kinetics.This study reports a unique 3 D ’flower-like’ zinc cobaltite(ZnCo_(2)O_(4-x)) with enriched oxygen vacancies as a new cathode material for aqueous ZIBs.Computational calculations reveal that the presence of oxygen vacancies significantly enhances the electronic conductivity and accelerates Zn^(2+) diffusion by providing enlarged channels.The as-fabricated batteries present an impressive specific capacity of 148.3 mAh g^(-1) at the current density of 0.05 A g^(-1),high energy(2.8 Wh kg^(-1)) and power densities(27.2 W kg^(-1)) based on the whole device,which outperform most of the reported aqueous ZIBs.Moreover,a flexible solid-state pouch cell was demonstrated,which delivers an extremely stable capacity under bending states.This work demonstrates that the performance of Zn-ion storage can be effectively enhanced by tailoring the atomic structure of cathode materials,guiding the development of low-cost and eco-friendly energy storage materials.

Efficient fully laser-patterned flexible perovskite modules and solar cells based on low-temperature solution-processed SnO2/mesoporous-TiO2 electron transport layers

Efficient flexible perovskite solar cells and modules were developed using a combination of SnO2 and mesoporous-TiO2 as a fully solution-processed electron transport layer （ETL）. Cells using such ETLs delivered a maximum power conversion efficiency （PCE） of 14.8%, which was 30% higher than the PCE of cells with only SnO2 as the ETL. The presence of a mesoporous TiO2 scaffold layer over SnO2 led to higher rectification ratios, lower series resistances, and higher shunt resistances. The cells were also evaluated under 200 and 400 lx artificial indoor illumination and found to deliver maximum power densities of 9.77 μW/cm^2 （estimated PCE of 12.8%） and 19.2 μW/cm^2 （estimated PCE of 13.3%）, respectively, representing the highest values among flexible photovoltaic technologies reported so far. Furthermore, for the first time, a fully laser-patterned flexible perovskite module was fabricated using a complete three-step laser scribing procedure （P1, P2, P3） with a PCE of 8.8% over an active area of 12 cm^2 under an illumination of 1 sun.

“Reinforced concrete”-like flexible transparent electrode for organic solar cells with high efficiency and mechanical robustness

Flexible transparent electrodes(FTEs) with robust mechanical stability are crucial for the industrial application of flexible organic solar cells(OSCs). However, their production remains challenging owing to the difficulty in balancing the conductivity,transmittance, and adhesion of FTEs to substrates. Herein, we present the so-called “reinforced concrete” strategy which finetunes the structure of silver nanowires(Ag NWs)-based FTEs with polydopamine(PDA) possessing good adhesion properties and moderate reducibility. The PDA reduces Ag+to form silver nanoparticles(Ag NPs) which grow like “rivets” at the Ag NW junction sites;PDA stabilizes the Ag NW skeleton and improves the adhesion between the Ag NWs and polyethylene terephthalate(PET) substrate and interface layer. The obtained Ag NW:PDA:Ag NP FTE exhibits excellent optoelectronic properties and high mechanical stability. The resulting flexible OSCs exhibit 17.07% efficiency, high flexibility during 10,000 bending test cycles, and robust peeling stability. In addition, this “reinforced concrete”-like FTE provides great advantages for the production of large-area flexible OSCs, thereby paving a new way toward their commercial application.