Mahui

The technological potential of emerging perovskite solar cells is determined by the stability of the power generated. Alkali salts used as interface modifiers positively affect device performance and stability and mitigate current–voltage hysteresis. Devices modified with potassium nitrate deliver a power conversion efficiency of 19.2% attributed to an improved charge carrier extraction and suppression of transient capacitive effects in device operation.

After demonstration of a 23% power conversion efficiency, a high operational stability is the next most important scientific and technological challenge in perovskite solar cells (PSCs). A potential failure mechanism is tied to a bias‐induced ion migration, which causes current–voltage hysteresis and a decay in the device performance over time. Herein, alkali salts are shown to mitigate hysteresis and stabilize device performance in n‐i‐p hybrid planar PSCs. Different alkali salts of potassium chloride, iodide, and nitrate as well as sodium chloride and iodide are deposited from aqueous solution onto the n‐type contact, based on SnO₂, prior to deposition of the perovskite absorber Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃. Introduction of potassium‐based alkali salts suppresses the current–voltage hysteresis and stabilizes the operational device stability at the maximum power point. This is attributed to the suppression of hole trapping at the n‐type selective transport layer (SnO₂)/perovskite interface observed by surface photovoltage spectroscopy, which is interpreted to reduce interfacial recombination and improve charge carrier extraction. The best and most stable performance of 19% is achieved using potassium nitrate as the interface modifier. Devices with higher and more stable performance exhibit substantially lower current transients, analyzed during maximum power point tracking.

A platinum(II) complexation strategy is developed to regulate the crystallinity of a newly designed s‐tetrazine‐containing wide‐bandgap copolymer donor PSFTZ, and optimize the morphology of the PSFTZ:Y6 active blend film, which boosts successfully the power conversion efficiency of the resulting nonfullerene polymer solar cells (NF‐PSCs) from 13.03% to 16.35%. 16.35% is the new record for single‐junction NA‐PSCs at present.

Abstract

A new strategy of platinum(II) complexation is developed to regulate the crystallinity and molecular packing of polynitrogen heterocyclic polymers, optimize the morphology of the active blends, and improve the efficiency of the resulting nonfullerene polymer solar cells (NF‐PSCs). The newly designed s‐tetrazine (s‐TZ)‐containing copolymer of PSFTZ (4,8‐bis(5‐((2‐butyloctyl)thio)‐4‐fluorothiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐3,6‐bis(4‐octylthiophen‐2‐yl)‐1,2,4,5‐tetrazine) has a strong aggregation property, which results in serious phase separation and large domains when blending with Y6 ((2,2′‐((2Z,2′Z)‐((12,13‐bis(2‐ethylhexyl)‐3,9‐diundecyl‐12,13‐dihydro‐[1,2,5]thiadiazolo[3,4‐e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2‐g]thieno[2′,3′:4,5]thieno[3,2‐b]indole‐2,10‐diyl)bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile)), and produces a power‐conversion efficiency (PCE) of 13.03%. By adding small amount of Pt(Ph)₂(DMSO)₂ (Ph, phenyl and DMSO, dimethyl sulfoxide), platinum(II) complexation would occur between Pt(Ph)₂(DMSO)₂ and PSFTZ. The bulky benzene ring on the platinum(II) complex increases the steric hindrance along the polymer main chain, inhibits the polymer aggregation strength, regulates the phase separation, optimizes the morphology, and thus improves the efficiency to 16.35% in the resulting devices. 16.35% is the highest efficiency for single‐junction PSCs reported so far.

Large excitonic optical nonlinearity in single‐crystalline 2D Ruddlesden–Popper perovskite (RPP) nanosheets characterized by a microscopic Z‐scan setup is reported. A room‐temperature excitonic Mott transition occurs near the exciton resonance of the thinnest quantum‐well RPPs, boosting the nonlinear response. The magnitude and sign of the nonlinear coefficients vary strongly with the excitation wavelength offering various nonlinear functionalities in the visible waveband.

Abstract

Materials with large optical nonlinearity, especially in the visible spectral region, are in great demand for applications in all‐optical information processing and quantum optics. 2D hybrid Ruddlesden−Popper‐type halide perovskites (RPPs) with tunable ultraviolet‐to‐visible direct bandgaps exhibit large nonlinear optical responses due to the strong excitonic effects present in their multiple quantum wells. Using a microscopic Z‐scan setup with femtosecond laser pulses tunable across the visible spectrum, it is demonstrated that single‐crystalline lead halide RPP nanosheets possess unprecedentedly large nonlinear refraction and absorption coefficients near excitonic resonances. A room‐temperature insulator (exciton)–metal (plasma) Mott transition is found to occur near the exciton resonance of the thinnest qunatum‐well RPPs, boosting the nonlinear response. Owing to the rapidly changing refractive index near resonance, a single RPP crystal can exhibit different nonlinear functionalities across the excitation spectrum. The results suggest that RPPs are efficient nonlinear materials in the visible waveband, indicating their potential use in integrated nonlinear photonic applications such as optical modulation and switching.

A uniform, large‐area deposition of colloidal quantum dot (CQD) ink is demonstrated by the tuning solute‐redistribution dynamics with the aid of an ultrasonic spray coating system. Electronically coupled CQD solids up to 100 cm² are fabricated, and are further applied to fabricate CQD solar cells that exhibit uniform and up to 8.10% power conversion efficiency over their whole area.

Abstract

Solution‐processed colloidal quantum dots (CQDs) are attractive materials for the realization of low‐cost and efficient optoelectronic devices. Although impressive CQD‐solar‐cell performance has been achieved, the fabrication of CQD films is still limited to laboratory‐scale small areas because of the complicated deposition of CQD inks. Large‐area, uniform deposition of lead sulfide (PbS) CQD inks is successfully realized for photovoltaic device applications by engineering the solute redistribution of CQD droplets. It is shown experimentally and theoretically that the solute‐redistribution dynamics of CQD droplets are highly dependent on the movement of the contact line and on the evaporation kinetics of the solvent. By lowering the friction constant of the contact line and increasing the evaporation rate of the droplets, a uniform deposition of CQD ink in length and width over large areas is realized. By utilizing a spray‐coating process, large‐area (up to 100 cm²) CQD films are fabricated with 3–7% thickness variation on various substrates including glass, indium tin oxide glass, and polyethylene terephthalate. Furthermore, scalable fabrication of CQD solar cells is demonstrated with 100 cm² CQD films which exhibits a notably high efficiency of 8.10%.

By performing H/F substitution on the pyrrolidinium cation, homochirality is introduced to the cation while maintaining the 1D perovskite framework, following which two enantiomeric ferroelectrics are obtained: (R)‐ and (S)‐3‐F‐(pyrrolidinium)CdCl₃. The T _c is successfully increased from 240 K in the parent (pyrrolidinium)CdCl₃ to 303 K in these two enantiomers, making the ferroelectricity applicable at room temperature.

Abstract

A ferroelectric with a high phase‐transition temperature (T _c) is an indispensable condition for practical applications. Over the past decades, both strain engineering and the isotope effect have been found to effectively improve the T _c within ferroelectric material systems. However, the former strategy seems to prefer working in inorganic ferroelectric thin films, while the latter is also limited to some certain systems, such as hydrogen‐bonded ferroelectrics. It is noted that a mono‐fluorinated molecule is geometrically very similar to its parent molecule and the substitution of H by an F atom can introduce a chiral center on the molecule to template or stabilize polar structures. Significantly, the barrier of rotation of the fluorinated organic molecules is raised, resulting in a remarkable increase in T _c. Herein, by applying the molecular design strategy of H/F substitution to the organic–inorganic perovskite ferroelectric (pyrrolidinium)CdCl₃ with a low T _c of 240 K, two high‐T _c chiral perovskite ferroelectrics, (R)‐ and (S)‐3‐F‐(pyrrolidinium)CdCl₃ are successfully synthesized, for which the T _c reaches 303 K. The significant enhancement of 63 K in T _c extends the ferroelectric working temperature range to room temperature. This finding provides a new effective way to regulate the T _c in ferroelectrics and to design high‐T _c molecular ferroelectrics.

Single‐emissive‐layer white organic light‐emitting diodes (WOLEDs) are demonstrated by using a small molecule, tris(4‐(phenylethynyl)phenyl)amine, without additional doping. By adjusting the annealing temperature, multiphotoluminescence is observed and various energy states are formed due to the change in the molecular configuration and packing with smaller spacing from heat‐induced rotation of the benzene rings.

Abstract

White organic light‐emitting diode (WOLED) technology has attracted considerable attention because of its potential use as a next‐generation solid‐state lighting source. However, most of the reported WOLEDs that employ the combination of multi‐emissive materials to generate white emission may suffer from color instability, high material cost, and a complex fabrication procedure which can be diminished by the single‐emitter‐based WOLED. Herein, a color‐tunable material, tris(4‐(phenylethynyl)phenyl)amine (TPEPA), is reported, whose photoluminescence (PL) spectrum is altered by adjusting the thermal annealing temperature nearly encompassing the entire visible spectra. Density functional theory calculations and transmission electron microscopy results offer mechanistic understanding of the PL redshift resulting from thermally activated rotation of benzene rings and rotation of 4‐(phenylethynyl) phenyl)amine connected to the central nitrogen atom that lead to formation of ordered molecular packing which improves the π–π stacking degree and increases electronic coupling. Further, by precisely controlling the annealing time and temperature, a white‐light OLED is fabricated with the maximum external quantum efficiency of 3.4% with TPEPA as the only emissive molecule. As far as it is known, thus far, this is the best performance achieved for single small organic molecule based WOLED devices.

The factors affecting the V _OC in 2D perovskite cells with different [PbI₆]⁴⁻ layer sheets (n = 2–4) are elucidated. Nonradiative recombination at the perovskite/C₆₀ interface is found to dominate except for the n = 2 system where the bulk recombination determines the properties of the cell. Substantial V _OC gains through suppression of interfacial recombination at the top interface are expected.

Abstract

2D Ruddlesden–Popper perovskite (RPP) solar cells have excellent environmental stability. However, the power conversion efficiency (PCE) of RPP cells remains inferior to 3D perovskite‐based cells. Herein, 2D (CH₃(CH₂)₃NH₃)₂(CH₃NH₃) _{n −1}Pb _n I_{3 n +1} perovskite cells with different numbers of [PbI₆]⁴⁻ sheets (n = 2–4) are analyzed. Photoluminescence quantum yield (PLQY) measurements show that nonradiative open‐circuit voltage (V _OC) losses outweigh radiative losses in materials with n > 2. The n = 3 and n = 4 films exhibit a higher PLQY than the standard 3D methylammonium lead iodide perovskite although this is accompanied by increased interfacial recombination at the top perovskite/C₆₀ interface. This tradeoff results in a similar PLQY in all devices, including the n = 2 system where the perovskite bulk dominates the recombination properties of the cell. In most cases the quasi‐Fermi level splitting matches the device V _OC within 20 meV, which indicates minimal recombination losses at the metal contacts. The results show that poor charge transport rather than exciton dissociation is the primary reason for the reduction in fill factor of the RPP devices. Optimized n = 4 RPP solar cells had PCEs of 13% with significant potential for further improvements.

Alkali metals are reported to influence the device performance of Cu(In,Ga)Se₂ (CIGSe) solar cells substantially. The present work shows that although Na and K are known to segregate to grain boundaries (GBs) in polycrystalline CIGSe thin films, no indications can be found that these alkali metals exhibit any passivating effect on GBs.

Thin‐film solar cells based on Cu(In,Ga)Se₂ (CIGSe) absorber layers reach conversion efficiencies of above 20%. One key to this success is the incorporation of alkali metals, such as Na and K, into the surface and the volume of the CIGSe thin film. The present work discusses the impact of Na and K on the grain‐boundary (GB) properties in CIGSe thin films, i.e., on the barriers for charge carriers, Φ_b, and on the recombination velocities at the GBs, s _GB. First, the physics connected with these two quantities as well as their impact on the device performance are revised, and then the values for the barrier heights and recombination velocities are provided from the literature. The s _GB values are measured by means of a cathodoluminescence analysis of Na‐/K‐free CIGSe layers as well as on CIGSe layers on Mo/sapphire substrates, which are submitted to only NaF or only KF postdeposition treatments. Overall, passivating effects on GBs by neither Na nor K can be confirmed. The GB recombination velocities seem to remain on the same order of magnitude, in average about 10³–10⁴ cm s⁻¹, irrespective of whether CIGSe thin films are Na‐/K‐free or Na‐/K‐containing.

An ingenious surface chlorination treatment method is used to passivate the interface defects of perovskite/zinc oxide (ZnO), which effectively reduces the interface charge recombination loss and improves the poor interface chemical characteristics. Thus, the fabricated zinc oxide–chlorine (ZnO–Cl)‐based device achieves an enhanced efficiency and suppressed hysteresis, as well as strengthened stability in perovskite solar cells.

Defect states on the zinc oxide (ZnO) surface cause severe interfacial charge recombination and perovskite decomposition during device operation, which inevitably leads to efficiency loss and poor device stability, making the usage of ZnO in perovskite solar cells (PSCs) problematic. Herein, a simple and effective method of inorganic chlorination treatment is used to passivate the surface defects of the ZnO electron transport layer. It is shown that chlorine (Cl) effectively fills the oxygen vacancy defects of ZnO, suppressing charge recombination and facilitating charge transport at the perovskite/ZnO interface. Therefore, the resulting CH₃NH₃PbI₃‐based device achieves an enhanced power conversion efficiency with suppressed hysteresis. Meanwhile, the chlorination of the ZnO surface protects the perovskite layer from decomposition, thus improving device stability. Herein, an ingenious method is developed to further improve the device performance of ZnO‐based PSCs and useful guidance is provided for the development of other perovskite optoelectronics, especially those with ZnO as the charge transport layer.

Recent progress in the fabrication of CsPbIBr₂ perovskite films and their applications in halide perovskite solar cells are overviewed, with special attention paid to the fabrication technique modification of CsPbIBr₂ films and the resulting performance enhancement of the photovoltaic devices.

CsPbIBr₂ photovoltaic materials attract remarkable attention in the field of all‐inorganic halide perovskite solar cells (HPSCs) due to their superior humidity stability and heat endurance. Since the first report in 2016, the power conversion efficiency (PCE) of CsPbIBr₂‐based HPSCs (Cs‐HPSCs) has increased from 4.7% to 11.53% with an almost 2.5‐fold leap in a short time. Cs‐HPSCs have also become one of the most researched materials in the all‐inorganic perovskite family. Here, the crystal structure and spectrum properties of CsPbIBr₂ are first elucidated to provide a preliminary overview. Subsequently, significantly modified strategies, including various assisting procedures for spin coating, interface engineering, and element impurity doping for superior perovskites and better‐performing cells are meticulously introduced. Overall, the development process of the CsPbIBr₂ materials is focused on, and the feasible strategies to improve fabrication techniques for superior perovskite films and corresponding device PCEs are emphatically summarized, with the aim to provide some constructive guidelines for the rapid development of Cs‐HPSCs.

The current status and recent advances in perovskite‐based tandem solar cells, including perovskite–silicon, perovskite–perovskite, and perovskite–copper indium gallium selenide (CIGS) integrations, are comprehensively reviewed. Different configurations, key issues regarding the photoelectric properties, and material design are discussed. The critical role of perovskite bandgap optimization, interface engineering, and recombination layers are analyzed to outline the roadmaps for future investigations.

Metal halide perovskite‐based solar cells have achieved rapidly increasing efficiencies of up to 23.7%. However, it is still far away from the Shockley–Quiesser limit of 33.16%. Tandem solar cells, consisting of two subcells with complementary absorption, are suggested as an alternative to beat this limit due to the fact that a maximum efficiency of 42% can be reached using two subcells with bandgaps of 1.9 eV/1.0 eV, opening up a great potential to develop perovskite‐based tandem solar cells. In this review, the current status of and recent advances in perovskite‐based tandem solar cells are highlighted, including perovskite–silicon, perovskite–perovskite, and perovskite–copper indium gallium selenide (CIGS) integrations. Different configurations, key issues regarding the photoelectric properties, present efficiency limitations, and material design are discussed. The critical role of perovskite bandgap optimization, interface engineering, and recombination layers are also analyzed to outline the roadmaps for future investigation. The current challenging issues and future perspectives are also provided. It is hoped that the findings will provide new perspectives for perovskite‐based tandem solar cells with an unprecedented performance and the opportunity for commercialization.

Protonation of polyethylenimine ethoxylated (PEIE) can effectively passivate the chemical reaction between the PEIE and a nonfullerene (NF) active layer. As a result, the PEIE can work very efficiently as a low‐work‐function interface for NF solar cells. These flexible solar cells exhibit power conversion efficiency up to 12.5% with a room‐temperature‐processed PEIE interface.

Abstract

Nonfullerene (NF) organic solar cells (OSCs) have been attracting significant attention in the past several years. It is still challenging to achieve high‐performance flexible NF OSCs. NF acceptors are chemically reactive and tend to react with the low‐temperature‐processed low‐work‐function (low‐WF) interfacial layers, such as polyethylenimine ethoxylated (PEIE), which leads to the “S” shape in the current‐density characteristics of the cells. In this work, the chemical interaction between the NF active layer and the polymer interfacial layer of PEIE is deactivated by increasing its protonation. The PEIE processed from aqueous solution shows more protonated N⁺ than that processed from isopropyl alcohol solution, observed from X‐ray photoelectron spectroscopy. NF solar cells (active layer: PCE‐10:IEICO‐4F) with the protonated PEIE interfacial layer show an efficiency of 13.2%, which is higher than the reference cells with a ZnO interlayer (12.6%). More importantly, the protonated PEIE interfacial layer processed from aqueous solution does not require a further thermal annealing treatment (only processing at room temperature). The room‐temperature processing and effective WF reduction enable the demonstration of high‐performance (12.5%) flexible NF OSCs.

Low‐cost counter materials for dye‐sensitized and perovskite solar cells are summarized, with a focus on the regular patterns that appear in their intrinsic features and structural design.

Abstract

It is undoubtable that the use of solar energy will continue to increase. Solar cells that convert solar energy directly to electricity are one of the most convenient and important photoelectric conversion devices. Though silicon‐based solar cells and thin‐film solar cells have been commercialized, developing low‐cost and highly efficient solar cells to meet future needs is still a long‐term challenge. Some emerging solar‐cell types, such as dye‐sensitized and perovskite, are approaching acceptable performance levels, but their costs remain too high. To obtain a higher performance–price ratio, it is necessary to find new low‐cost counter materials to replace conventional precious metal electrodes (Pt, Au, and Ag) in these emerging solar cells. In recent years, the number of counter‐electrode materials available, and their scope for further improvement, has expanded for dye‐sensitized and perovskite solar cells. Generally regular patterns in the intrinsic features and structural design of counter materials for emerging solar cells, in particular from an electrochemical perspective and their effects on cost and efficiency, are explored. It is hoped that this recapitulative analysis will help to make clear what has been achieved and what still remains for the development of cost‐effective counter‐electrode materials in emerging solar cells.

The development of lead‐free perovskites has attracted increasing attention. The design rules for lead‐free perovskite materials with diverse structures are presented. The structure–property relationships and optical‐, electric‐, and magnetic‐related applications of these lead‐free perovskites are summarized. Based on these structure–property relationships, strategies for multifunctional perovskite design are proposed.

Abstract

Lead halide perovskites have emerged as promising semiconducting materials for different applications owing to their superior optoelectronic properties. Although the community holds different views toward the toxic lead in these high‐performance perovskites, it is certainly preferred to replace lead with nontoxic, or at least less‐toxic, elements while maintaining the superior properties. Here, the design rules for lead‐free perovskite materials with structural dimensions from 3D to 0D are presented. Recent progress in lead‐free halide perovskites is reviewed, and the relationships between the structures and fundamental properties are summarized, including optical, electric, and magnetic‐related properties. 3D perovskites, especially A₂B⁺B³⁺X₆‐type double perovskites, demonstrate very promising optoelectronic prospects, while low‐dimensional perovskites show rich structural diversity, resulting in abundant properties for optical, electric, magnetic, and multifunctional applications. Furthermore, based on these structure–property relationships, strategies for multifunctional perovskite design are proposed. The challenges and future directions of lead‐free perovskite applications are also highlighted, with emphasis on materials development and device fabrication. The research on lead‐free halide perovskites at Linköping University has benefited from inspirational discussions with Prof. Olle Inganäs.