Ligang Yuan

The introduction of an intermediate layer to bridge different solar cell technologies, which have become mature for the optimization of single‐junction cells, is a critical technology for fabricating efficient hybrid tandem solar cells. The design and classification of an efficient interconnection layer that leverages the full potential of the hybrid tandems are discussed.

Abstract

Hybrid tandem solar cells offer the benefits of low cost and full solar spectrum utilization. Among the hybrid tandem structures explored to date, the most popular ones have four (simple stacking design) or two (terminal/tunneling layer addition design) terminal electrodes. Although the latter design is more cost‐effective than the former, its widespread application is hindered by the difficulty of preparing an interface between two solar cell materials. The oldest approach to the in‐series bonding of two or more bandgap solar cells relies on the introduction of a tunneling layer in multijunction III–V solar cells, but it has some limitations, e.g., the related materials/technologies are applicable only to III–V and certain other solar cells. Thus, alternative methods of realizing junction contacts based on the use of novel materials are highly sought after. Here, the strategies used to realize high‐performance tandem cells are described, focusing on interface control in terms of bonding two or more solar cells for tandem approaches. The presented information is expected to aid the establishment of ideal methods of connecting two or more solar cells to obtain the highest performance for different solar cell choices with minimized energy loss through the interface.

Herein, novel Ruddlesden–Popper Cs₂PbI₂Cl₂ nanosheets are synthesized and creatively employed as a multifunctional interface optimization material to improve the performance of CsPbI₂Br solar cells. Based on the heterostructured NSs/CsPbI₂Br/NSs inorganic film, an efficiency of 16.65% is obtained, which is one of the best reported for CsPbI₂Br solar cells, along with much‐enhanced UV, air, and thermal stabilities.

Abstract

Inorganic CsPbI₂Br perovskite solar cells (PSCs) have gained enormous research interest due to their excellent thermal and light stabilities. However, their unsatisfactory power‐conversion efficiency and poor intrinsic phase stability remain roadblocks to their further development. Herein, Cs₂PbI₂Cl₂ nanosheets (NSs) with the Ruddlesden–Popper (RP) structure are synthesized, and an NSs/CsPbI₂Br/NSs heterostructure is employed to enhance both the stability and efficiency of CsPbI₂Br solar cells. The novel Cs₂PbI₂Cl₂ NSs can not only passivate the top and bottom surfaces of the perovskite film and top surface of the TiO₂ film but also enhance the stability of the perovskite film. Based on the heterostructured NSs/CsPbI₂Br/NSs inorganic perovskite film, the efficiency of the CsPbI₂Br PSCs is improved from 15.02% to 16.65%. Moreover, the unencapsulated CsPbI₂Br devices with the NSs/CsPbI₂Br/NSs heterostructure sustain over 90% of their original efficiencies after being exposed to ambient conditions (≈25 °C and ≈35% RH) for 648 h. Both the UV‐light‐soaking stability (100 mW cm⁻¹ 365 nm UV light) and thermal stability (T = 85 °C) of the optimized devices are dramatically improved in comparison with their counterparts with only a 3D active layer. Therefore, this work promotes the application of RP inorganic perovskite nanocrystals in a range of perovskite optoelectronic devices.

Perovskite light‐emitting diodes attract much attention because of their advantages such as high color purity, easy and wide color tunability, and high photoluminescence quantum yields. The recent developments of ideal multifunctional charge transport layers are discussed by considering the fundamental limitations including defects, morphological and phase instability, high refractive index, and poor outcoupling of perovskites.

Abstract

Despite their low exciton‐binding energies, metal halide perovskites are extensively studied as light‐emitting materials owing to narrow emission with high color purity, easy/wide color tunability, and high photoluminescence quantum yields. To improve the efficiency of perovskite light‐emitting diodes (PeLEDs), much effort has been devoted to controlling the emitting layer morphologies to induce charge confinement and decrease the nonradiative recombination. The interfaces between the emitting layer and charge transporting layer (CTL) are vulnerable to various defects that deteriorate the efficiency and stability of the PeLEDs. Therefore, the establishment of multifunctional CTLs that can improve not only charge transport but also critical factors that influence device performance, such as defect passivation, morphology/phase control, ion migration suppression, and light outcoupling efficiency, are highly required. Herein, the fundamental limitations of perovskites as emitters (i.e., defects, morphological and phase instability, high refractive index with poor outcoupling) and the recent developments with regard to multifunctional CTLs to compensate such limitations are summarized, and their device applications are also reviewed. Finally, based on the importance of multifunctional CTLs, the outlook and research prospects of multifunctional CTLs for the further improvement of PeLEDs are discussed.

A novel strategy to neutralize charged point defects in organic‐inorganic hybrid perovskite materials is proposed for highly efficient and stable perovskite solar cells by using a zwitterionic L‐alanine additive, which can be passivated simultaneously with both positively and negatively charged defects because it contains both anion and cation functional groups in one molecule.

Abstract

Ionic defects (e.g., organic cations and halide anions), preferably residing along grain boundaries (GBs) and on perovskite film surfaces, are known to be a major source of the notorious environmental instability of perovskite solar cells (PeSCs). Although passivating ionic defects is desirable, previous approaches using Lewis base or acid molecules as additives suppress only the negatively or positively charged defects, thus leaving oppositely charged defects. In this work, both the cationic and anionic defects inside methyl ammonium lead tri‐iodide (MAPbI₃) are simultaneously passivated by introducing a zwitterionic form of the amino acid, L‐alanine, into the precursor solution as an additive. L‐alanine has both positive (NH₃ ⁺) and negative (COO⁻) functional groups at a specific solvent pH, thereby passivating both the cation and anion defects in MAPbI₃. The addition of L‐alanine increases the grain size of the perovskite crystals and lengthens the charge carrier lifetime (τ > 1 µs), leading to improved power conversion efficiencies (PCEs) of 20.3% (from 18.3% without an additive) for small‐area (4.64 mm²) devices and 15.6% (from 13.5%) for large‐area submodules (9.06 cm²). More importantly, the authors’ approach also significantly enhances the shelf storage and photoirradiation stabilities of PeSCs.

Bridge‐jointed 2D nanosheets are inserted between the methylammonium‐free perovskite and the dopant‐free hole transport layer (HTL) to form a scalable heterostructure, which preserves p‐type semiconduction of HTL and suppresses nonradiative‐recombination. Further, a perovskite solar module with an area of 35.80 cm² shows a certified efficiency of 15.3% and encapsulated modules retain over 91% of initial efficiency after damp heat test for 1000 h.

Abstract

Perovskite solar cell (PSC) modules employing a hole transport layer (HTL) without unstable dopants possess high potential for improving operational stability. However, the low efficiencies of the devices greatly limit their commercial applications owing to the lower efficacy of the dopant‐free HTL, introduced by the unintentional n‐doping effect of volatile ions from the halide‐rich perovskite surface. Here, a scalable heterostructure integrated by a methylammonium‐free perovskite film with an iodide‐rich surface, an ultrathin interlayer of bridge‐jointed graphene oxide nanosheets (BJ‐GO), and an HTL without additional ionic dopants is developed. In this heterostructure, the iodide ions are physically immobilized by the compact 2D network, and lead defects are chemically passivated by multiple coordination bonds. Moreover, the BJ‐GO with tunable surface energy enables a highly ordered HTL a considerably improved carrier mobility by an order of magnitude. Finally, the PSC module with an area of 35.80 cm² employing this heterostructure shows a certified efficiency of 15.3%. The encapsulated PSC modules retain over 91% of initial efficiency after the damp heat test at 85 °C and ≈85% relative humidity for 1000 h, while maintaining 90% of the initial value for 1000 h at the maximum power point under continuous 1‐Sun illumination at 60 °C.

The moisture instability and unscalable fabrication protocols are still unsolved and blocking FACs‐based perovskite solar cells’ further applications. Here, high‐quality FACsPbI₃ films are fabricated by crown ether tailoring (which chelated with Cs⁺/Pb²⁺ ions) to inhibit the moisture invasion and stabilize the α‐phase FACsPbI₃, producing large‐area perovskite films and improving solar module performance.

Abstract

FACs‐based (FA⁺, formamidinium and Cs⁺, cesium) perovskite solar cells have gained great attention due to their remarkable light and thermal stabilities toward practical application of perovskite modules. However, the moisture instability and difficulty in scalable fabrication are still the main obstacles blocking their photovoltaic applications in current status. Here, the employment of novel interaction between crown ether with metal cations is introduced to tailor the uniform growth and inhibit moisture invasion during the crystallization of α‐phase FACsPbI₃, yielding the successful synthesis of high‐quality perovskite films in a large scale. Consequently, perovskite solar cells (PSC) modules in the total area of 4 × 4 and 10 × 10 cm² are readily fabricated with respective champion efficiencies of 16.69% and 13.84% and excellent stability over 1000 h. This facile scaling‐up strategy assisted by crown ether has shown great promise for pursuing efficient and highly stable large‐area PSC modules.

Nature Photonics, Published online: 21 December 2020; doi:10.1038/s41566-020-00743-1

High‐quality and all‐inorganic CsPbI₂Br perovskite film with lower defects and improved hydrophobicity is prepared via a facile additive engineering with trace S₈, resulting in inverted solar cells with improved efficiency and stability.

Though prized for excellent thermal stability, inorganic perovskites are still behind organic/inorganic hybrid perovskites due to their high‐density defects and poor hydrophobicity. Herein, trace hydrophobic S₈ is used as additive to optimize the solution‐processed CsPbI₂Br perovskite film. A series of characterizations reveal that S₈ additive not only leads to retarded crystallization of α‐CsPbI₂Br perovskite at low temperature (<150 °C) via self‐formed Pb(S₈) _x ²⁺ intermediate but also induces efficient grain‐boundary passivation via distinctive PbS coordination interaction and reduced wettability on perovskite surface, which all point to the formation of the perovskite film with reduced defects and improved hydrophobicity. As a result, the inverted perovskite solar cells (PSCs) based on the optimized all‐inorganic perovskite of CsPbI₂Br:S₈ deliver an increased power conversion efficiency (PCE) from 12.76% to 14.46% as well as remarkably enhanced device stability under thermal or ambient condition. This work thus provides a simple way as well as new insights for boosting the performance of solution‐processed all‐inorganic perovskite.

In the structure of perovskite solar cells, N‐type semiconductor AgBiS₂ and dimethyl sulfoxide solvent mixed polyethylene glycol are used for perovskite film treatment. Finally, the perovskite solar cells with dual‐interfacial modification exhibite a remarkable improvement of power conversion efficiency from 18.58% to 21.19%, as well as show the excellent long‐term and moisture stability.

Although the research on perovskite solar cells (PSCs) has achieved rapid progress, its efficiency and stability still need to be further improved to meet the industrial requirements. The defects located inside the cells, on the surfaces, interfaces, or grain boundaries, will primarily affect carrier transportation through the formation of nonradiative recombination centers and hinder the further enhancement of the power conversion efficiency (PCE). Herein, a straightforward and simple defect passivation method is developed to increase the PCE and stability of PSCs. In the device, the N‐type semiconductor AgBiS₂ is introduced by thermal evaporation as a modified layer between the perovskite films and electron transport layer, which can improve the charge transport characteristic and bandgap optimization of PSCs. Simultaneously, dimethyl sulfoxide (DMSO) solvent mixed polyethylene glycol (PEG) is used for solvent annealing treatment, which can further improve the quality of perovskite film and reduce the trap density by increasing grain size and enhancing the crystallinity. As a result, the PSCs with dual‐interfacial modification exhibit a remarkable improvement in PCE from 18.58% to 21.19% with exceptional long‐term and moisture stability. This work provides an innovative insight for fabricating the stable and efficient PSCs toward the industrialization.

Perovskite solar cells (PSCs) have debuted as the photovoltaic devices with the most potential and progress is being made at an unprecedented pace. Meanwhile, additive engineering is continuously pushing the power conversion efficiency (PCE) and device stability to higher levels by passivating defects and regulating crystallization behaviors. Considering the scalable fabrication of PSCs in the following stage, seeking green additives for optimizing perovskites is extremely valuable and paramount. Herein, we pioneer a green additive engineering method using fumaric acid (FMAC) to optimize the three‐cation perovskites in order to obtain highly efficient PSCs. FMAC not only optimizes crystallization behaviors to endow the perovskite films with large grain size and few grain boundaries, but also forms a strong interaction with Pb²⁺/I⁻ of the perovskites, thereby stabilizing the [PbI₆]⁴⁻ octahedral framework of the perovskite crystal lattices and effectively passivating the surface defects. On this basis, FMAC improves the photoelectric properties of perovskites and in particular, suppresses the non‐radiative recombination. Consequently, the PCE of PSCs incorporating FMAC rises to 20.48%, exceeding that (19.18%) of the pristine device. In addition, FMAC also enhances the stability of PSCs. Therefore, we provide a significant strategy using a green additive to enhance the photovoltaic performance of PSCs.

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Metal (titanium or Ti) nanopillar arrays (NaPAs), vertically protruding on a TiO₂ compact layer, function as an electron transporting layer in perovskite solar cells. Ti NaPA has highly hydrophilic surfaces passivated with TiO₂, high electron mobility, and low work function; hence it compensates the loss of light harvesting in perovskite and leads to highly efficient, antiaging photovoltaic performance.

Abstract

Electron transporting layers (ETLs), required to be optically transparent in perovskite solar cells (PSCs) having regular structures, possess a determinant effect on electron extraction and collection. Metal oxides (e.g., TiO₂) have overwhelmingly served as ETLs, but usually have low electron mobility (μ_e < 10⁻² cm² V⁻¹ s⁻¹) not favorable for photovoltaic conversion. Here, metal oxides are replaced with metals (e.g., Ti with μ_e ≈ 294 cm² V⁻¹ s⁻¹) that are sculptured via glancing angle deposition to be a close‐packed nanopillar array (NaPA), which vertically protrudes on a transparent electrode to obtain sufficient optical transmission for light harvesting in perovskite. Ti NaPAs, whose rough surfaces are passivated with 5 nm thick TiO₂ (i.e., Ti NaPAs@TiO₂) to suppress exciton recombination, lead to the champion power conversion efficiency (PCE) of 18.89% that is superior to that of MAPbI₃ PSCs without Ti NaPAs@TiO₂ or containing TiO₂ NaPAs@TiO₂, owing to high surface wettability, high μ_e, and relatively low work function of Ti. Furthermore, Ti NaPAs@TiO₂ effectively prevents the decomposition of MAPbI₃ to achieve long‐term shelf stability whereby 50‐day aging only causes 15% PCE degradation. This work paves the way toward widening the material spectrum, from semiconductors to metals, to generate a diverse range of ETLs for producing efficient optoelectronic devices with long‐term shelf stability.

Strain widely exists in metal–halide perovskites (MHPs), and the presence of the residual strain greatly influences the optoelectronic properties of MHP film. Recently, many studies have reported the key role of strain engineering in improving the photovoltaic performance of perovskite solar cells (PSCs). Herein, current understanding and advanced strategies of strain engineering in PSCs are systematically summarized.

Due to the impressive optoelectronic properties, metal–halide perovskites (MHPs) have drawn much attention in the field of next‐generation photovoltaics, and perovskite solar cells (PSCs) based on MHPs as light absorbers have reached a certified power conversion efficiency (PCE) of 25.5% in 2020. Despite the great progress, it is still challenging to fabricate high‐quality MHP films. Due to the “soft” ionic nature of MHPs, their polycrystalline films suffer from inevitable residual strain, which is found to not only be fatal to photovoltaic performance of PSCs, but also seriously accelerate the degradation of MHP film. As a result, understanding of strain in MHPs and the key role of strain engineering in improving the photovoltaic performance of PSCs have recently been extensively investigated. Herein, the recent progress of strain engineering in MHPs and their PSCs is systematically summarized. First, the origin of strain in MHPs and the impact of strain on the optoelectronic characteristics of MHPs are carefully discussed. Thereafter, the up‐to‐date studies focusing on strain engineering in PSCs are comprehensively reviewed. At last, the current challenges and future prospects in this field are highlighted.

The linear electro‐optic (EO) effect in organic perovskites is demonstrated. The crystal structures and polarizability can be modified by the rational design of the organic components to realize an enhancement of the EO performance. This work provides a molecular perspective on the design of organic perovskites for EO modulation and new applications in nonlinear optics.

Abstract

Electrical‐to‐optical signal conversion is widely employed in information technology and is implemented using on‐chip optical modulators. State‐of‐the‐art modulator technologies are incompatible with silicon manufacturing techniques: inorganic nonlinear crystals such as LiNbO₃ are integrated with silicon photonic chips only using complex approaches, and hybrid silicon–LiNbO₃ optical modulators show either low bandwidth or high operating voltage. Organic perovskites are solution‐processed materials readily integrated with silicon photonics; and organic molecules embedded within the perovskite scaffold allow in principle for high polarizability. However, it is found that the large molecules required for high polarizability also require an increase of the size of the perovskite cavity: specifically, using the highly polarizable DR²⁺ (R = H, F, Cl) in the A site necessitates the exploration of new X‐site options. Only by introducing BF₄ ^– as the X‐site molecule is it possible to synthesize (DCl)(NH₄)(BF₄)₃, a material exhibiting a linear EO coefficient of 20 pm V^–1, which is 10 times higher than that of metal halide perovskites and is a 1.5 fold enhancement compared to reported organic perovskites. The EO response of the organic perovskite approaches that of LiNbO₃ (r _eff ≈ 30 pm V^–1) and highlights the promise of rationally designed organic perovskites for use in efficient EO modulators.

We disclosed a key finding to modulate the crystallization kinetics of FASnI₃ through a non‐classical nucleation mechanism based on pre‐nucleation clusters. A direct link between the colloids in the perovskite precursor solution and final optoelectronic quality of the perovskite films was established. Finally, power conversion efficiency of 11.39 % was obtained for FASnI₃‐based perovskite solar cells.

Abstract

Tin halide perovskites are rising as promising materials for lead‐free perovskite solar cells (PSCs). However, the crystallization rate of tin halide perovskites is much faster than the lead‐based analogs, leading to more rampant trap states and lower efficiency. Here, we disclose a key finding to modulate the crystallization kinetics of FASnI₃ through a non‐classical nucleation mechanism based on pre‐nucleation clusters (PNCs). By introducing piperazine dihydriodide to tune the colloidal chemistry of the FASnI₃ perovskite precursor solution, stable clusters could be readily formed in the solution before nucleation. These pre‐nucleation clusters act as intermediate phase and thus can reduce the energy barrier for the perovskite nucleation, resulting in a high‐quality perovskite film with lower defect density. This PNCs‐based method has led to a conspicuous photovoltaic performance improvement for FASnI₃‐based PSCs, delivering an impressive efficiency of 11.39 % plus improved stability.

Despite the conspicuous achievements in perovskite solar cells (PSCs), the further improvement of power conversion efficiencie (PCE) is hindered by substantially detrimental carrier recombination resulyted from the high interfacial charge defect density and inferior charge transport kinetics. Herein, we develop an interface engineering strategy to introduce Lewis base thiophene or thiazole modified C₃N₄ layer at electron transfer layer (ETL)/perovskite interface to constitute a stepwise energy band alignment and passivate defects at interfaces of perovskite film. Attributed to its well‐matched energy level with TiO₂ and perovskite, the charge extraction efficiency and charge transfer dynamics can be remakbly promoted, greatly inhibittng charge recombination at interface. Besides, thiophene and thiazole can donate the lone pair electrons in S or N atoms to under‐coordinated Pb²⁺, which effectively passivates the electronic trap states caused by halogen vacancies, thereby greatly minimizing trap‐assisted non‐radiative recombination in the PSC devices. Eventually, thiazole‐C₃N₄/perovskite based devices acquire an outstanding efficiency of 19.23%, supported by an enhanced open‐circuit voltage (V_OC) of 1.11 V with improved moisture stability. This work provides an avenue for the interfacial energy level modulation and defect passivation strategies on the rational interface microstructure design for meliorating the performance of PSCs.

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Exceptional water‐stability has been confirmed in DMASnBr3 and exploited in photocatalysis to enhance the hydrogen photogeneration of graphitic carbon nitride.

Abstract

Water‐stable metal halide perovskites could foster tremendous progresses in several research fields where their superior optical properties can make differences. In this work we report clear evidence of water stability in a lead‐free metal halide perovskite, namely DMASnBr₃, obtained by means of diffraction, optical and X‐ray photoelectron spectroscopy. Such unprecedented water‐stability has been applied to promote photocatalysis in aqueous medium, in particular by devising a novel composite material by coupling DMASnBr₃ to g‐C₃N₄, taking advantage from the combination of their optimal photophysical properties. The prepared composites provide an impressive hydrogen evolution rate >1700 μmol g⁻¹ h⁻¹ generated by the synergistic activity of the two composite costituents. DFT calculations provide insight into this enhancement deriving it from the favorable alignment of interfacial energy levels of DMASnBr₃ and g‐C₃N₄. The demonstration of an efficient photocatalytic activity for a composite based on lead‐free metal halide perovskite in water paves the way to a new class of light‐driven catalysts working in aqueous environments.

Herein, the chemistry of tin perovskite compounds for the fabrication of high‐efficiency nontoxic solar cells is described. The reaction mechanisms among the compounds and additives present in the Sn perovskite films are discussed to correlate with the device performance.

Lead (Pb)‐based perovskite solar cells (Pb‐PSCs) have been recorded with a fascinating power conversion efficiency (PCE) of 25.5%. However, the presence of toxic Pb in the perovskite absorber material hinders the commercial aspects of Pb‐PSCs as a promising and efficient new generation of solar cells. Fortunately, theoretical calculations have predicted that tin (Sn)‐based perovskite solar cells (Sn‐PSCs) could have superior performance comparable to the Pb‐PSCs. Recently, many approaches have been reported for developing efficient Sn‐PSCs but yet they have shown the best PCE of 13.24%. This low PCE compared to Pb‐PSCs might be because Sn‐PSCs have been approached in the same way as Pb‐PSCs. However, from a chemistry viewpoint, the understanding of Sn‐PSCs might be very different from that of Pb‐based ones. Herein, the fundamental knowledge of the chemistry and coordination chemistry of Sn^II compounds and their structural properties is described. Then, an insight is provided into understanding the recent trends of Sn perovskite formation using various Lewis base additives in the precursor solution and incorporation as a cation in the perovskite lattice. Finally, the influence of utilizing Lewis base additives on the device dynamics is discussed.

A low‐temperature solution‐processed, annealing‐free, amorphous metal oxyhydroxide cathode interlayer is used to facilitate charge extraction and suppress interfacial charge recombination in inverted perovskite photovoltaics, delivering a power conversion efficiency of 21.3%.

The state‐of‐the‐art high‐performance perovskite solar cells (PSCs) with inverted p‐i‐n device structure normally use crystalline metal oxide materials or organic small molecules as the cathode interlayer between the fullerene layer and metal electrode. However, these interlayers are made by either high‐temperature or complicated vacuum‐assisted fabrication process, and in many cases, they are not efficient and effective enough to simultaneously extract the electrons and suppress the interfacial charge recombination. Herein, for the first time, a facile low‐temperature solution‐processed strategy is demonstrated to fabricate an amorphous metal oxyhydroxide (a‐MOH) thin film, which is used as a robust cathode interlayer in inverted PSCs. The a‐MOH interlayer not only facilitates electron extraction and collection via “energy‐favorable” electron tunneling, but also suppresses the interfacial charge recombination via effective hole blocking and electron backflow inhibition. As a result, the PSCs based on a‐MOH interlayer achieve a stabilized power conversion efficiency (PCE) of 21.1% and retain 93% of initial PCE after continuous one‐sun illumination for 500 hours.

State‐of‐the‐art perovskite and organic solar cells use inorganic hole transport materials (HTMs) due to their superior electronic properties. These HTMs are, however, expensive and prone to degradation. A range of robust inorganic HTMs are emerging, that provide a trade‐off between efficiency, stability, and cost, and are critically reviewed herein.

Interfaces in perovskite and organic solar cells play a central role in advancing efficiency and prolong device durability. They improve charge transport/transfer from the absorber layer to the collecting electrodes, while also blocking the opposite charge carriers, minimize voltage losses by suppressing charge recombination. and may act as buffer/protective layers and nanomorphology regulators for the absorber layer. One such interface is formed by the hole transport layer (HTL) and the organic/perovskite absorber. These HTLs typically consist of organic semiconductors, which, although are solution processable at low temperatures and allow perfect energy‐level alignment with the absorber layer and therefore efficient charge collection, are prone to degradation in ambient conditions and under continuous light exposure. In a quest for robust alternatives, inorganic materials such as metal oxides, graphene oxide, bronzes, copper thiocyanate, and transition metal dichalcogenides are actively investigated. However, their hole extraction capability is inferior compared with organic semiconductors as they possess specific energetics leading to significant charge extraction barriers and moderate charge collection. To achieve further advancements in their hole transporting capabilities, strongly interconnecting knowledge of their synthesis, electronic properties, and device performance metrics is required.

Herein, the correlation between chemical structures and physicochemical properties of organic hole transporting materials, which plays fundamental roles in supervising future molecular design and synthesis for perovskite solar cells, is outlined. To move in the practical line of perovskite solar marketing, interfacial contact materials are key parts in case of stability and efficiency.

Perovskite solar cells (PSCs) with advantages of exceptional photovoltaic performance and facile solution‐processed fabrication have shown great potential in future scalable application. After about a decade of rapid development, this new PSCs technology demonstrates over 25% efficiency, a comparable performance with traditional silicon solar cells. Further, the development of PSCs in the direction of scalable production still highly relies on designing innovative materials with low cost and high efficiency. Recently, a great number of functional organic molecules as hole transport materials (HTMs) have been designed, synthesized, and studied in PSCs, including molecules with planar structure, 3D geometry, or different core units. Discovering the correlation between their chemical structures and physicochemical properties plays a fundamental role in supervising future molecular design and synthesis. Herein, recent advances in organic molecular HTMs with various structures in typical and reverse PSCs device configuration are summarized, including doped and doping‐free materials. By evaluating the structural modification and analyzing their effects on photovoltaic performance, the goal is to generate universal strategies for preparing low‐cost and efficient HTMs, paving the way for future scalable application of PSCs.

Inorganic CsPbI3 perovskite is the most competitive candidate to hybrid perovskites. However, its poor phase stability, hydrophobicity and high‐density defects have limited the development of CsPbI3 perovskite solar cells (PSCs). To overcome these obstacles for achieving high‐performance CsPbI3 PSCs, additive engineering has been widely employed. Herein, the progress of additive engineering in CsPbI3 PSCs is systematically reviewed.

All‐inorganic perovskite solar cells (PSCs) have attracted a lot of attention in the past few years because of their preeminent thermal stability compared with organic–inorganic hybrid PSCs. Among all kinds of all‐inorganic perovskites, CsPbI₃ perovskite with a proper bandgap of ≈1.7 eV becomes the most competitive candidate. However, its poor phase stability, hydrophobicity, and high‐density defects have limited the development of CsPbI₃ PSCs. To overcome these obstacles for achieving high‐performance CsPbI₃ PSCs, additive engineering has been widely used, which has rapidly promoted the power conversion efficiency (PCE) to over 19%. Herein, the progress of additive engineering in CsPbI₃ PSCs is systematically reviewed. First, the roles of additives in CsPbI₃ PSCs are introduced, including improving phase stability, increasing moisture resistance, and passivating defects. Then, the additive engineering is categorized (additive engineering in perovskites and at perovskite/hole transport layer interfaces) and reviewed in detail. Finally, future research directions on additive engineering are suggested for further enhancing stability and improving PCE.

The power conversion efficiency of the market‐dominating silicon photovoltaics approaches its theoretical limit. Bifacial solar operation with harvesting additional light impinging on the module back and the perovskite/silicon tandem device architecture are among the most promising approaches for further increasing the energy yield from a limited area. Here, the energy output of perovskite/silicon tandem solar cells in monofacial and bifacial operation is calculated, for the first time considering luminescent coupling between two sub‐cells. For energy yield calculations idealized solar cells are studied at both, standard testing as well as realistic weather conditions in combination with a detailed illumination model for periodic solar panel arrays. Typical experimental photoluminescent quantum yield values reveal that more than 50% of excess electron‐hole pairs in the perovskite top cell can be utilized by the silicon bottom cell by means of luminescent coupling. As a result, luminescent coupling strongly relaxes the constraints on the top‐cell bandgap in monolithic tandem devices. In combination with bifacial operation, the optimum perovskite bandgap shifts from 1.71 eV to the range 1.60‐1.65 eV where already high‐quality perovskite materials exist. The results can hence change a paradigm in developing the optimum perovskite material for tandem solar cells.

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