Additives in metal halide perovskite films and their applications in solar cells

The booming growth of organic-inorganic hybrid lead halide perovskite solar cells have made this promising photovoltaic technology to leap towards commercialization.One of the most important issues for the evolution from research to practical application of this technology is to achieve high-throughput manufacturing of large-scale perovskite solar modules.In particular,realization of scalable fabrication of large-area perovskite films is one of the essential steps.During the past ten years,a great number of approaches have been developed to deposit high quality perovskite films,to which additives are introduced during the fabrication process of perovskite layers in terms of the perovskite grain growth control,defect reduction,stability enhancement,etc.Herein,we first review the recent progress on additives during the fabrication of large area perovskite films for large scale perovskite solar cells and modules.We then focus on a comprehensive and in-depth understanding of the roles of additives for perovskite grain growth control,defects reduction,and stability enhancement.Further advancement of the scalable fabrication of high-quality perovskite films and solar cells using additives to further develop large area,stable perovskite solar cells are discussed.

Synergistic stabilization of CsPbI_(3) inorganic perovskite via 1D capping and secondary growth

Cesium lead iodide(CsPbI_(3)) perovskite has gained great attention in the photovoltaic(PV) community because of its unique optoelectronic properties, good chemical stability and appropriate bandgap for sunlight harvesting applications. However, compared to solar cells fabricated from organic-inorganic hybrid perovskites, the commercialization of devices based on all-inorganic CsPbI_(3) perovskites still faces many challenges regarding PV performance and long-term stability. In this work, we discovered that tetrabutylammonium bromide(TBABr) post-treatment to CsPbI_(3) perovskite films could achieve synergistic stabilization with both TBA+cation intercalation and Br-doping. Such TBA^(+) cation intercalation leads to onedimensional capping with TBAPb I3 perovskite formed in situ, while the Br-induced crystal secondary growth helps effectively passivate the defects of CsPbI_(3) perovskite, thus enhancing the stability. In addition, the incorporation of TBABr can improve energy-level alignment and reduce interfacial charge recombination loss for better device performance. Finally, the highly stable TBABr-treated CsPbI_(3)-based perovskite solar cells show reproducible photovoltaic performance with a champion efficiency up to 19.04%, while retaining 90% of the initial efficiency after 500 h storage without encapsulation.

Strategies and methods for fabricating high quality metal halide perovskite thin films for solar cells

With the development of human society,the problems of environmental deterioration and energy shortage have become increasingly prominent.In order to solve these problems,metal halide perovskite solar cells(PSCs)stand out because of their excellent properties(i.e.,high optical absorption coefficient,long carrier lifetime and carrier diffusion length,adjustable band gap)and have been widely studied.PSCs with low cost,high power conversion efficiency and high stability are the future development trend.The quality of perovskite film is essential for fabricating PSCs with high performance.To provide a full picture of realizing high performance PSCs,this review focuses on the strategies for preparing high quality perovskite films(including antisolvent,Lewis acid-base,additive engineering,scaleable fabrication,strain engineering and band gap adjustment),and therefore to fabricate high performance PSCs and to accelerate the commercialization.

Up-Scalable Fabrication of SnO_(2)with Multifunctional Interface for High Performance Perovskite Solar Modules

Tin dioxide(SnO_(2))has been demonstrated as one of the promising electron transport layers for high-efficiency perovskite solar cells(PSCs).However,scalable fabrication of SnO_(2) films with uniform coverage,desirable thickness and a low defect density in perovskite solar mod-ules(PSMs)is still challenging.Here,we report preparation of high-quality large-area SnO_(2) films by chemical bath depo-sition(CBD)with the addition of KMnO_(4).The strong oxidiz-ing nature of KMnO_(4) promotes the conversion from Sn(II)to Sn(VI),leading to reduced trap defects and a higher carrier mobility of SnO_(2).In addition,K ions diffuse into the per-ovskite film resulting in larger grain sizes,passivated grain boundaries,and reduced hysteresis of PSCs.Furthermore,Mn ion doping improves both the crystallinity and the phase stability of the perovskite film.Such a multifunctional interface engineering strategy enabled us to achieve a power conversion efficiency(PCE)of 21.70% with less hysteresis for lab-scale PSCs.Using this method,we also fabricated 5×5 and 10×10 cm^(2) PSMs,which showed PCEs of 15.62% and 11.80%(active area PCEs are 17.26%and 13.72%),respectively.For the encapsulated 5×5 cm^(2) PSM,we obtained a T80 operation lifetime(the lifespan during which the solar module PCE drops to 80%of its initial value)exceeding 1000 h in ambient condition.

Scalable solution coating of the absorber for perovskite solar cells

Perovskite-based solar cell technology has advanced significantly and the power conversion efficiencies are nowadays on par with commercialized photovoltaic technologies. To realize the potential of perovskite solar cells, the focus is now shifting to scalable fabrication technologies that will enable low-cost solution processing of perovskite solar cells over large areas and with high yields. This review article discusses the fundamental concerns that arise when transitioning from laboratory to large area solution coating, available scalable coating technologies, and their applicability to the fabrication of high-performance perovskite solar cells. We find that a significant amount of work has been done to test scalable coating technologies, but also that often the methods that led to highest-performing cells in the laboratory （e.g. antisolvent processing） show limited compatibility with scalable coating methods. To achieve a high-yield and low-cost process, development must emphasize a high degree of control provided by sequential conversion of perovskite films and engineering of additives that fine-tune coating properties of perovskite precursor inks.

The Main Progress of Perovskite Solar Cells in 2020-2021

Perovskite solar cells(PSCs)emerging as a promising photovoltaic technology with high efficiency and low manufacturing cost have attracted the attention from all over the world.Both the efficiency and stability of PSCs have increased steadily in recent years,and the research on reducing lead leakage and developing eco-friendly lead-free perovskites pushes forward the commercialization of PSCs step by step.This review summarizes the main progress of PSCs in 2020 and 2021 from the aspects of efficiency,stability,perovskite-based tandem devices,and lead-free PSCs.Moreover,a brief discussion on the development of PSC modules and its challenges toward practical application is provided.

Heterogeneous FASnI3 Absorber with Enhanced Electric Field for High-Performance Lead-Free Perovskite Solar Cells

Lead-free tin perovskite solar cells(PSCs)have undergone rapid development in recent years and are regarded as a promising ecofriendly photovoltaic technology.However,a strategy to suppress charge recombination via a built-in electric field inside a tin perovskite crystal is still lacking.In the present study,a formamidinium tin iodide(FASnI;)perovskite absorber with a vertical Sn;gradient was fabricated using a Lewis base-assisted recrystallization method to enhance the built-in electric field and minimize the bulk recombination loss inside the tin perovskites.Depth-dependent X-ray photoelectron spectroscopy revealed that the Fermi level upshifts with an increase in Sn;content from the bottom to the top in this heterogeneous FASnI;film,which generates an additional electric field to prevent the trapping of photo-induced electrons and holes.Consequently,the Sn;-gradient FASnI;absorber exhibits a promising efficiency of 13.82%for inverted tin PSCs with an open-circuit voltage increase of 130 mV,and the optimized cell maintains over 13%efficiency after continuous operation under 1-sun illumination for 1,000 h.

CsPbBr_(x)I_(3-x)thin films with multiple ammonium ligands for low turn-on pure-red perovskite light-emitting diodes

All-inorganic α-CsPbBr_(x)I_(3-x)perovskites featuring nano-sized crystallites show great potential for pure-red light-emitting diode(LED)applications.Currently,the CsPbBr_(x)I_(3-x)LEDs based on nano-sized α-CsPbBr_(x)I_(3-x)crystallites have been fabricated mainly via the classical colloidal route including a tedious procedure of nanocrystal synthesis,purification,ligand or anion exchange,film casting,etc.With the usually adopted conventional LED device structure,only high turn-on voltages(>2.7)have been achieved for CsPbBrxl3-x LEDs.Moreover,this mix-halide system may suffer from severe spectra-shift under bias.In this report,CsPbBr_(x)I_(3-x)thin films featuring nano-sized crystallites are prepared by incorporating multiple ammonium ligands in a one-step spin-coating route.The multiple ammonium ligands constrain the growth of CsPbBr_(x)I_(3-x)nanograins.Such CsPbBr_(x)I_(3-x)thin films benefit from quantum confinement.The corresponding CsPbBr_(x)I_(3-x)LEDs,adopting a conventional LED structure of indium-doped tin oxide(ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS)/CsPbBr_(x)I_(3-x)/[6,6]-phenyl C61 butyric acid methyl ester(PCBM)/bathocuproine(BCP)/AI,emit pure-red color at Commission Internationale de I'eclairage(CIE)coordinates of(0.709,0.290),(0.711,0.289),etc.,which represent the highest color-purity for reported pure-red perovskite LEDs and meet the Rec.2020 requirement at CIE(0.708,0.292)very well.The CsPbBr_(x)I_(3-x)LED shows a low turn-on voltage of 1.6 V,maximum external quantum efficiency of 8.94%,high luminance of 2,859 cd·m^(-2),and good color stability under bias.

2D materials for conducting holes from grain boundaries in perovskite solar cells

Grain boundaries in organic-inorganic halide perovskite solar cells(PSCs)have been found to be detrimental to the photovoltaic performance of devices.Here,we develop a unique approach to overcome this problem by modifying the edges of perovskite grain boundaries with flakes of high-mobility two-dimensional(2D)materials via a convenient solution process.A synergistic effect between the 2D flakes and perovskite grain boundaries is observed for the first time,which can significantly enhance the performance of PSCs.We find that the 2D flakes can conduct holes from the grain boundaries to the hole transport layers in PSCs,thereby making hole channels in the grain boundaries of the devices.Hence,2D flakes with high carrier mobilities and short distances to grain boundaries can induce a more pronounced performance enhancement of the devices.This work presents a cost-effective strategy for improving the performance of PSCs by using high-mobility 2D materials.

A novel dopant for spiro-OMeTAD towards efficient and stable perovskite solar cells

2,2’,7,7’-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9’-spirobifluorene(spiro-OMeTAD), as the most commonly used hole transport material(HTM), plays a significant role in the normal structured(n-i-p) high-efficiency perovskite solar cells(PSCs). In general, it is prepared by a halogen solvent(chlorobenzene, CBZ) and needs an ion dopant(lithium bis(trifluoromethanesulfonyl)imide, Li-TFSI) to improve its conductivity and hole mobility. However, such a halogen solvent is not environmentally friendly and the widely used LiTFSI dopant would affect the stability of PSCs. Herein, we develop a non-halogen solvent-tetrahydrofuran(THF)-prepared spiro-OMeTAD solution with a new p-type dopant,potassium bis(fluorosulfonyl)imide(K-FSI), to apply into PSCs. By this strategy, high-hole-mobility spiro-OMeTAD film is achieved. Meanwhile, the potassium ions introduced by diffusion into perovskite surface passivate the interfacial defects. Therefore, a hysteresis-free champion PSC with an efficiency of 21.02% is obtained, along with significantly improved stability against illumination and ambient conditions. This work provides a new strategy for HTMs toward hysteresis-free high-efficiency and stable PSCs by substituting dopants.

Increase the rigidity and hydrophobicity of perovskite by a molecular design

Organic-inorganic hybrid perovskite solar cells are raising as the most promising next generation solar cells due to their excellent efficiency and low-cost fabrication.Organic components in A-site(ABX3)play an important role in the structure of perovskite materials.For example,it has been reported that the introduction of dimethylamine(DMA)or formamidine(FA)into perovskite structure results in the phase change from tetragonal to cubic[1–3].However,up to now,the relation between the introduced molecules and performance of perovskite solar cells is unclear.

Organic additive engineering toward efficient perovskite light-emitting diodes

Perovskite materials with excellent optical and electrical properties are promising for light-emitting diodes.In the field of perovskite light-emitting diodes(PeLEDs),organic materials additive engineering has been proved to be an effective scheme for enhancing efficiency and stability in PeLEDs.Most impressively,the reported external quantum efficiency of PeLEDs based on perovskite-organic composite has reached over 20%.Herein,we will review the important progress of the organic materials'additive-modified PeLEDs and discuss the remaining problems and challenges and the key research direction in the near future.

A redox shuttle imparts operational durability to perovskite solar cells

The family of perovskite materials are described by the formula ABX3, where A are monovalent cations, B are divalent cations, and X are anions.