Mahui

A bifunctional dye molecule, 5,15‐bis(2,6‐dioctoxyphenyl)‐10‐(bis(4‐hexylphenyl)‐amino‐20‐4‐carboxyphenylethynyl)porphyrinato]zinc(II) (YD₂‐o‐C8), is introduced into CsPbI₂Br PSCs. It not only broadens the light absorption range of the perovskite but also reduces the energy loss (E _loss) by interface passivation. As a result, the efficiency markedly enhances from 7.02% to 10.13%, featuring a short‐circuit current (J _SC) of 12.05 mA cm⁻² and a record‐high open‐circuit voltage (V _OC) of 1.37 V.

Inorganic lead halide perovskites are attracting increasing attention due to their much better thermal stability than the organic–inorganic hybrid perovskite materials. Thus, the low power conversion efficiency (PCE) is a key issue for the inorganic lead halide perovskite solar cells (PSCs). This is mainly due to their wider bandgap and larger energy loss (E _loss) in the devices. Herein, for solving this issue, a dye molecule‐assisted engineering using the dye of 5,15‐bis(2,6‐dioctoxyphenyl)‐10‐(bis(4‐hexylphenyl)‐amino‐20‐4‐carboxyphenylethynyl)porphyrinato]zinc(II) (YD₂‐o‐C8) is demonstrated. Results indicate that this molecule has a bifunctional effect, not only as a co‐sensitization layer for CsPbIBr₂ with broader absorption spectrum but also reduces the E _loss by interface passivation. Specifically, the light absorption range of the photoactive layer is broadened from 600 to nearly 680 nm. At the same time, the interfacial charge recombination is highly reduced. After optimizing, the champion PCE is enhanced from 7.02% to 10.13%, and record‐high open‐circuit voltage (V _OC) of 1.37 V and short‐circuit currents (J _SC) of 12.05 mA cm⁻² are achieved. This study opens a simple and efficient way to improve the efficiency of inorganic PSCs.

For developing low‐cost and high‐efficiency planar perovskite solar cells (PSCs), a straightforward low‐temperature chemical bath deposition process is developed to prepare a Co‐doped TiO₂ (Co‐TiO₂) electron transport layer (ETL); the optoelectrical properties of the TiO₂ ETL are significantly improved by Co‐doping. Finally, the efficiency of the PSCs is increased from 17.40% for TiO₂ to 19.10% for the Co‐TiO₂ ETL.

Planar hybrid perovskite solar cells (PSCs) attract great attention due to their obvious advantages of low‐temperature processing with a high power conversion efficiency (PCE) up to 23.32%. Here, Co‐doped TiO₂ (Co‐TiO₂) deposited by a straightforward low‐temperature chemical bath deposition (CBD) method is explored. Using Co‐TiO₂ as an electron transport layer (ETL) for the planar PSCs, the effects of doping on TiO₂ morphology, electronic properties, and solar cell performance are investigated. The PCE increases to 19.10% when the Co doping concentration is optimized at 5 mol%, an increase of 17.40% compared with that using the pristine TiO₂. Meanwhile, the notorious J–V hysteresis is suppressed to a greater extent. Considering that the low‐temperature CBD is comparable with continuous roll‐to‐roll processing, it makes the process and the Co‐TiO₂ ETL potential candidates for low‐cost commercialization.

A new small molecule (SM)‐acceptor, POIT‐IC4F, is developed. Due to the synergistic effects of side‐chain engineering and fluorination on the SM‐acceptor to simultaneously broaden spectral response and minimize voltage loss, the annealing‐free organic solar cells achieve a high device efficiency of 13.8%.

Herein, three small molecule (SM)‐acceptors (POIT‐IC, POIT‐IC2F, and POIT‐IC4F) are developed by combining the side‐chain engineering located on the sp³‐hybridized carbon atoms of the fused‐ring core and the fluorination of end groups. From ITIC to POIT‐IC, POIT‐IC2F, and then to POIT‐IC4F, the SM‐acceptors show gradually broadened absorption spectra, increased maximum extinction coefficient, crystallinity, and electron mobilities due to the synergistic effects of side‐chain engineering and fluorination. Compared with nonfluorinated ITIC and POIT‐IC, as fluorination broadens the molecular spectra, POIT‐IC2F and POIT‐IC4F with alkoxyphenyl side chains show less decreased LUMO levels than IT‐IC2F and IT‐IC4F with alkylphenyl side chains, which are conducive to both higher V _oc and J _sc for organic solar cells (OSCs). Combined with polymer donor PM6, the POIT‐IC4F‐based OSCs achieve a device efficiency of up to 13.8% with a high V _oc of 0.91 V and J _sc of 20.9 mA cm⁻², which are significantly higher than that of the control OSCs based on ITIC (8.9%), POIT‐IC (10.1%), or IT‐IC4F (12.2%). An efficiency of 13.8% is one of the highest PCEs reported for the annealing‐free OSCs. Our results show that the synergistic effects of side‐chain engineering and fluorination on SM‐acceptor can simultaneously broaden spectral response and minimize voltage loss of OSCs and ultimately achieve high device efficiency.

Solution‐processable Ga₂O₃ nanocrystals are developed as a novel electron‐transporting layer for high‐performance perovskite solar cells. The smooth film of Ga₂O₃ nanocrystals offers a better interface with perovskite and improves charge transport efficiency. The Ga₂O₃‐based device shows negligible hysteresis unlike the TiO₂‐based analogue.

The electron‐transporting layer (ETL) plays a very important role in perovskite solar cells (PSCs). The traditional TiO₂ ETL exhibits drawbacks such as complex preparation process and low stability. Devices incorporating TiO₂ as the ETL also show large hysteresis that limits their performance. Herein, Ga₂O₃ nanocrystals (NCs), prepared by a solution process, are applied as an ETL in n‐i‐p planar structured PSCs. The Ga₂O₃‐based devices exhibit negligible hysteresis and achieve higher performance than the TiO₂‐based devices. Due to better energy level matching and smoother surface morphology, films of Ga₂O₃ NCs make good interfacial contact with the perovskite top layer, improving the charge transport efficiency. The perovskite layer also exhibits high crystallinity. Unlike TiO₂, which is commonly prepared by high‐temperature sintering or solution hydrolysis, films of Ga₂O₃ NCs can be prepared by solution spin‐coating at a low temperature. This greatly reduces the complexity of fabrication and improves device performance.

Perovskite solar cells are modified by dropping alkylammonium solutions over CH₃NH₃PbI₃ films and lead to an increase in the stability after exposure to humidity. In the presence of the alkylammonium chains, the bulk perovskite is converted to a 2D/3D structure that helps the device to retain its performance for longer.

Hybrid organic and inorganic perovskite solar cells lack long‐term stability, and this negatively impacts the widespread application of this emerging and promising photovoltaic technology. Herein, aiming to increase the stability of perovskite films based on CH₃NH₃PbI₃ and to deeply understand the formation of 2D structures, solutions of alkylammonium chlorides containing 8, 10, and 12 carbons are introduced during the spin‐coating on the surface of 3D perovskite films leading to the in situ formation of 2D structures. It is possible to identify the chemical formulae of some 2D structures formed by X‐ray diffraction and UV–vis analysis of the modified films. Interestingly, the increase in the stability of the CH₃NH₃PbI₃ films due to the formation of a 2D + 3D perovskite network is only possible in planar TiO₂ substrates. The increase in stability of the CH₃NH₃PbI₃ films follows the surfactant molecule order: octylammonium (8C) > decylammonium (1 °C) > dodecylammonium (12C) chlorides > standard. An increase of 17.6% in the lifetime of the devices assembled with octylammonium‐modified perovskite film is observed compared with that of the standard device, which is directly linked to the improvement of the charge carrier lifetimes obtained from time‐correlated single photon counting measurements.

The linking topology effect and the doping effect on the optical and electronic properties of a series of carbazole‐based hole‐transport materials (HTMs) with 2,7‐substitution and 3,6‐substitution are systematically investigated. The results clearly demonstrate that the 2,7‐substituted carbazole‐based HTMs display higher hole mobility and conductivity, thereby exhibiting better device performance in both solid‐state dye‐sensitized solar cells and perovskite solar cells.

Carbazole is a promising core for the molecular design of hole‐transport materials (HTMs) for solid‐state mesoscopic solar cells (ssMSCs), such as solid‐state dye‐sensitized solar cells (ssDSSCs) and perovskite solar cells (PSCs) due to its low cost and excellent optoelectronic properties of its derivatives. Although carbazole‐based HTMs are intensely investigated in ssMSCs and promising device performance is demonstrated, the fundamental understanding of the impact of linking topology on the properties of carbazole‐based HTMs is lacking. Herein, the effect of the linking topology on the optical and electronic properties of a series of carbazole‐based HTMs with 2,7‐substitution and 3,6‐substitution is systematically investigated. The results demonstrate that the 2,7‐substituted carbazole‐based HTMs display higher hole mobility and conductivity among this series of analogous molecules, thereby exhibiting better device performance. In addition, the conductivity of the HTMs is improved after light treatment, which explains the commonly observed light‐soaking phenomenon of ssMSCs in general. All these carbazole‐based HTMs are successfully applied in ssMSCs and one of the HTMs X50‐based devices yield a promising efficiency of 6.8% and 19.2% in ssDSSCs and PSCs, respectively. This study provides guidance for the molecular design of effective carbazole‐based HTMs for high‐performance ssMSCs and related electronic devices.

Three novel polytriphenylamine‐based polymers (H‐Z1, H‐Z2, and H‐Z3) are designed and applied as hole‐transport layers in all‐inorganic perovskite solar cells. Due to the gradual deepening of the highest occupied molecular orbital energy levels from H‐Z1, H‐Z2 to H‐Z3, the energy loss (E _loss) can be decreased from 0.69, 0.64, to 0.62 eV for H‐Z1, H‐Z2, and H‐Z3, respectively.

The energy loss (E _loss) control via interfacial engineering is a significant indispensible methodology to realize high‐performance all‐inorganic perovskite solar cells (PVSCs). Herein, three novel polytriphenylamine‐based polymer derivatives (H‐Z1, H‐Z2, and H‐Z3) are synthesized, and the energy levels of these polymers are tuned feasibly through introducing the electron‐withdrawing group of trifluoromethyl in the triphenylamine (TPA) unit. These very deep HOMO energy levels are very beneficial for improving the open‐circuit voltages (V_ocs) in PVSCs with the potentially decreased E _losss. Due to the gradual deepening of HOMO energy levels from H‐Z1, H‐Z2 to H‐Z3, the V_ocs are elevated from 1.23, 1.28 to 1.30 V, respectively, where the E _loss s are decreased from 0.69, 0.64, to 0.62 eV for H‐Z1, H‐Z2, and H‐Z3, respectively. Interestingly, both of the H‐Z1‐ and H‐Z2‐based devices show the highest PCEs, over 14%, in all‐inorganic PVSCs, which are effectively comparable to the results of reference device using Spiro‐OMeTAD as HTL. Thus, through the efficient atomic engineering and chemical modification in corresponding p‐typed polymers, excellent hole transport polymers are achieved for high‐performance and stable PVSCs with very low E _loss.

In article no. 1900094, Doo Kyung Moon and co‐workers newly synthesize a chlorinated thiophenebased donor polymer, P(Cl), which has a relatively low synthetic complexity. P(Cl) shows an excellent power conversion efficiency and high atmospheric stability in non‐fullerene polymer solar cells.

In article no. 1900063, Kai Wang and co‐workers report the spin‐dependent electron‐hole recombination and dissociation in nonfullerene acceptor ITIC‐based binary and ternary organic bulk heterojunction systems. ITIC itself exhibits a negative magneto‐photocurrent due to the exciton‐charge reaction. The effect becomes critically important for electron‐hole dissociation in both systems at large fields and longer photo‐excitation wavelengths.

In article no. 1900091, Lai Feng and co‐workers demonstrate that solution‐processed two‐dimensional Nb₂O₅(001) nanosheets can be combined with PC₆₁BM and employed as a double‐layered electron transport layer for inverted inorganic CsPbI₂Br perovskite solar cells with high performance and excellent stability.

Solar RRL, Volume 3, Issue 7, July 2019.

Annealing a poly(ethylene oxide) film over its glass transition temperature leads to the formation of a cross‐linking complex with metal ions at the tin oxide quantum dot and perovskite interface, which passivates the interface defects for enhanced electron transfer from the perovskite layer to cathode.

Interface engineering is critical for achieving high‐efficiency and high‐stability perovskite solar cells (PSCs). Herein, a new interface engineering approach—poly(ethylene oxide) (PEO) modification of SnO₂ quantum dot (QD) film—to improve electron transport is introduced. It is found that when the PEO film is annealed over its glass‐transition temperature, the ether‐oxygen unshared electron pair in the PEO film activates to form a crosslinking complex with metal ions at the SnO₂ QD and perovskite interface, which triggers heterogeneous nucleation over the perovskite precursor film and is beneficial for achieving uniform and dense perovskite films. PEO is also shown to passivate the bulk defects of perovskite films and the interface defects between SnO₂ QD and perovskite, which promotes electron‐transferring from the perovskite layer to cathode. PSCs based on SnO₂ QD with PEO treatment exhibit an enhanced efficiency, leading to a champion PCE of 20.23%, with good reproducibility and stability.

A hole‐transporting material (HTM), termed V1160, based on four TPD‐type fragments connected by a Tröger's base structural core, is synthesized, characterized, and applied as an HTM in perovskite solar cells. Demonstrating an over 18% power conversion efficiency, the fully amorphous nature of V1160, suggesting further studies in TPD‐based materials, is warranted.

One of the obstacles to the commercialization of perovskite solar cells (PSCs) is the high price and morphological instability of the most common hole‐transporting material (HTM) Spiro‐OMeTAD. Herein, a novel HTM, termed V1160, based on four N,N′‐bis(3‐methylphenyl)‐N,N′‐diphenylbenzidine (TPD)‐type fragments, fused by a Tröger's base core, is synthesized and successfully applied in PSCs. Investigation of the optical, thermal, and photoelectrical properties shows that V1160 is a suitable candidate for application as an HTM in PSCs. A promising power conversion efficiency (PCE) of over 18% is demonstrated, which is only slightly lower than that of Spiro‐OMeTAD. Moreover, V1160‐based devices exhibit improved performances in dopant‐free configurations and superior stability. Favorable morphological properties in combination with a simple synthesis make V1160 and related materials promising for HTM applications.

An amorphous passivation layer using phenethylammonium iodide for amply covering the surface and grain boundaries of the CH₃NH₃PbI₃ film, results in the reduction of trap density and suppression of nonradiative recombination.

Organic–inorganic lead halide perovskite solar cells have realized a rapid increase of power conversion efficiency (PCE) in the past few years. However, their performance still suffers trap‐assisted decline due to defects at the surface and grain boundaries of the perovskite film. Herein, a phenethylammonium iodide‐lead iodide (PEAI‐PbI₂) passivation layer is formed on the CH₃NH₃PbI₃ perovskite film. The characterization results indicate that the PEAI covering layer leads to the reduction of surface defects and suppression of nonradiative recombination. By manipulating this surface passivation method, a remarkably improved V _OC of 1.16 V and an enhanced PCE of 20.8% are achieved.

Surface‐modified NiO _x nanoparticles (NPs) as hole transport materials in n‐i‐p‐structured perovskite solar cells are studied. The modified NiO _x NPs disperse well in chlorobenzene, and their film forms smooth and pinhole‐free layers, which show good electrical conductivity and improved extraction properties. The power conversion efficiency is improved from 5.5% to 13.1%.

Modified NiO _x nanoparticles (NPs) developed via surface engineering are applied to a hole transport layer (HTL) in n‐i‐p‐structured perovskite solar cells (PSCs). Hexanoic acid (HA) as a surfactant improves the dispersibility of NiO _x NPs in chlorobenzene (CB). The conductivity of the NiO _x ‐HA film is 1.20×10−5S cm−1, which is superior to that of 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) with dopants. The NiO _x ‐HA film shows better hole extraction properties compared with the pristine NiO _x film. The NiO _x ‐HA NPs form closely packed and pinhole‐free films, leading to improved device performance with a power conversion efficiency from 5.5% to 13.1%.

Nature Materials, Published online: 01 July 2019; doi:10.1038/s41563-019-0416-2

Low-dimensional tin-halide perovskites exhibit strong temperature dependence of luminescence decay time that translates into high sensitivity over a wide range of temperatures and as such can be used in high-resolution remote thermography.

The crystallization pathways of mixed perovskites under spin‐coating are investigated via in situ grazing‐incidence wide‐angle X‐ray scattering (GIWAXS), which reveals the existence of an “annealing window”. The as‐cast film should be timely annealed within the annealing window to achieve a high‐quality perovskite film. The incorporation of Cs⁺ can remarkably extend the annealing window, thereby improving the device performance and reproducibility.

Abstract

Mixed perovskites have achieved substantial successes in boosting solar cell efficiency, but the complicated perovskite crystal formation pathway remains mysterious. Here, the detailed crystallization process of mixed perovskites (FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃) during spin‐coating is revealed by in situ grazing‐incidence wide‐angle X‐ray scattering measurements, and three phase‐formation stages are identified: I) precursor solution; II) hexagonal δ‐phase (2H); and III) complex phases including hexagonal polytypes (4H, 6H), MAI–PbI₂–DMSO intermediate phases, and perovskite α‐phase. The correlated device performance and ex situ characterizations suggest the existence of an “annealing window” covering the duration of stage II. The spin‐coated film should be annealed within the annealing window to avoid the formation of hexagonal polytypes during the perovskite crystallization process, thus achieving a good device performance. Remarkably, the crystallization pathway can be manipulated by incorporating Cs⁺ ions in mixed perovskites. Combined with density functional theory calculations, the perovskite system with sufficient Cs⁺ will bypass the formation of secondary phases in stage III by promoting the formation of α‐phase both kinetically and thermodynamically, thereby significantly extending the annealing window. This study provides underlying reasons of the time sensitivity of fabricating mixed‐perovskite devices and insightful guidelines for manipulating the perovskite crystallization pathways toward higher performance.

The rare‐earth element ytterbium (Yb) is substituted into the B site of a cubic ABX₃ perovskite lattice in place of lead. The resulting CsYbI₃ nanocrystals exhibit strong excitonic emission with high quantum yield and the potential for use in hybrid photodetectors as a photoactive layer. Such lead‐free CsYbI₃ nanocrystals offer tremendous opportunities in optical and optoelectronic applications.

Abstract

Lead‐(Pb‐) halide perovskite nanocrystals (NCs) are interesting nanomaterials due to their excellent optical properties, such as narrow‐band emission, high photoluminescence (PL) efficiency, and wide color gamut. However, these NCs have several critical problems, such as the high toxicity of Pb, its tendency to accumulate in the human body, and phase instability. Although Pb‐free metal (Bi, Sn, etc.) halide perovskite NCs have recently been reported as possible alternatives, they exhibit poor optical and electrical properties as well as abundant intrinsic defect sites. For the first time, the synthesis and optical characterization of cesium ytterbium triiodide (CsYbI₃) cubic perovskite NCs with highly uniform size distribution and high crystallinity using a simple hot‐injection method are reported. Strong excitation‐independent emission and high quantum yields for the prepared NCs are verified using photoluminescence measurements. Furthermore, these CsYbI₃ NCs exhibit potential for use in organic–inorganic hybrid photodetectors as a photoactive layer. The as‐prepared samples exhibit clear on–off switching behavior as well as high photoresponsivity (2.4 × 10³ A W⁻¹) and external quantum efficiency (EQE, 5.8 × 10⁵%) due to effective exciton dissociation and charge transport. These results suggest that CsYbI₃ NCs offer tremendous opportunities in electronic and optoelectronic applications, such as chemical sensors, light emitting diodes (LEDs), and energy conversion and storage devices.

A type of perovskite bifunctional device (PBD) with high photovoltaic (PV) and electroluminescence (EL) performance is developed. Interfacial energy‐band engineering between the perovskite and hole‐transport layer (HTL) is performed to turn the n‐type surface of the perovskite into p‐type and also correct the misalignment to form a well‐defined n–i–p heterojunction.

Abstract

Currently, photovoltaic/electroluminescent (PV/EL) perovskite bifunctional devices (PBDs) exhibit poor performance due to defects and interfacial misalignment of the energy band. Interfacial energy‐band engineering between the perovskite and hole‐transport layer (HTL) is introduced to reduce energy loss, through adding corrosion‐free 3,3′‐(2,7‐dibromo‐9H‐fluorene‐9,9‐diyl) bis(n,n‐dimethylpropan‐1‐amine) (FN‐Br) into a HTL free of lithium salt. This strategy can turn the n‐type surface of perovskite into p‐type and thus correct the misalignment to form a well‐defined N–I–P heterojunction. The tailored PBD achieves a high PV efficiency of up to 21.54% (certified 20.24%) and 4.3% EL external quantum efficiency. Free of destructive additives, the unencapsulated devices maintain >92% of their initial PV performance for 500 h at maximum power point under standard air mass 1.5G illumination. This strategy can serve as a general guideline to enhance PV and EL performance of perovskite devices while ensuring excellent stability.

Impressive progress has been made in the last few years on producing perovskite photovoltaics using scalable printing and coating technologies. The key developments, such as the control of nucleation and crystal growth of the perovskite thin film, which have enabled this rapid progress in coated and printed perovskite photovoltaics, are highlighted.

Abstract

Hybrid organic–inorganic metal halide perovskite semiconductors provide opportunities and challenges for the fabrication of low‐cost thin‐film photovoltaic devices. The opportunities are clear: the power conversion efficiency (PCE) of small‐area perovskite photovoltaics has surpassed many established thin‐film technologies. However, the large‐scale solution‐based deposition of perovskite layers introduces challenges. To form perovskite layers, precursor solutions are coated or printed and these must then be crystallized into the perovskite structure. The nucleation and crystal growth must be controlled during film formation and subsequent treatments in order to obtain high‐quality, pin‐hole‐free films over large areas. A great deal of understanding regarding material engineering during the perovskite film formation process has been gained through spin‐coating studies. Based on this, significant progress has been made on transferring material engineering strategies to processes capable of scale‐up, such as blade coating, spray coating, inkjet printing, screen printing, relief printing, and gravure printing. Here, an overview is provided of the strategies that led to devices deposited by these scalable techniques with PCEs as high as 21%. Finally, the opportunities to fully close the shrinking gap to record spin‐coated solar cells and to scale these efficiencies to large areas are highlighted.

The rare‐earth element ytterbium (Yb) is substituted into the B site of a cubic ABX₃ perovskite lattice in place of lead. The resulting CsYbI₃ nanocrystals exhibit strong excitonic emission with high quantum yield and the potential for use in hybrid photodetectors as a photoactive layer. Such lead‐free CsYbI₃ nanocrystals offer tremendous opportunities in optical and optoelectronic applications.

Abstract

Lead‐(Pb‐) halide perovskite nanocrystals (NCs) are interesting nanomaterials due to their excellent optical properties, such as narrow‐band emission, high photoluminescence (PL) efficiency, and wide color gamut. However, these NCs have several critical problems, such as the high toxicity of Pb, its tendency to accumulate in the human body, and phase instability. Although Pb‐free metal (Bi, Sn, etc.) halide perovskite NCs have recently been reported as possible alternatives, they exhibit poor optical and electrical properties as well as abundant intrinsic defect sites. For the first time, the synthesis and optical characterization of cesium ytterbium triiodide (CsYbI₃) cubic perovskite NCs with highly uniform size distribution and high crystallinity using a simple hot‐injection method are reported. Strong excitation‐independent emission and high quantum yields for the prepared NCs are verified using photoluminescence measurements. Furthermore, these CsYbI₃ NCs exhibit potential for use in organic–inorganic hybrid photodetectors as a photoactive layer. The as‐prepared samples exhibit clear on–off switching behavior as well as high photoresponsivity (2.4 × 10³ A W⁻¹) and external quantum efficiency (EQE, 5.8 × 10⁵%) due to effective exciton dissociation and charge transport. These results suggest that CsYbI₃ NCs offer tremendous opportunities in electronic and optoelectronic applications, such as chemical sensors, light emitting diodes (LEDs), and energy conversion and storage devices.

A type of perovskite bifunctional device (PBD) with high photovoltaic (PV) and electroluminescence (EL) performance is developed. Interfacial energy‐band engineering between the perovskite and hole‐transport layer (HTL) is performed to turn the n‐type surface of the perovskite into p‐type and also correct the misalignment to form a well‐defined n–i–p heterojunction.

Abstract

Currently, photovoltaic/electroluminescent (PV/EL) perovskite bifunctional devices (PBDs) exhibit poor performance due to defects and interfacial misalignment of the energy band. Interfacial energy‐band engineering between the perovskite and hole‐transport layer (HTL) is introduced to reduce energy loss, through adding corrosion‐free 3,3′‐(2,7‐dibromo‐9H‐fluorene‐9,9‐diyl) bis(n,n‐dimethylpropan‐1‐amine) (FN‐Br) into a HTL free of lithium salt. This strategy can turn the n‐type surface of perovskite into p‐type and thus correct the misalignment to form a well‐defined N–I–P heterojunction. The tailored PBD achieves a high PV efficiency of up to 21.54% (certified 20.24%) and 4.3% EL external quantum efficiency. Free of destructive additives, the unencapsulated devices maintain >92% of their initial PV performance for 500 h at maximum power point under standard air mass 1.5G illumination. This strategy can serve as a general guideline to enhance PV and EL performance of perovskite devices while ensuring excellent stability.

A new methodology for the calibration of oxygen nonstoichiometry in perovskite thin films to their oxygen vibrational modes is introduced. This is achieved by actively pumping oxygen in and out of Sr(Ti,Fe)O_{3− y} thin films integrated into an electrochemical cell and measuring in situ Raman spectra.

Abstract

Effective integration of perovskite films into devices requires knowledge of their electro‐chemomechanical properties. Raman spectroscopy is an excellent tool for probing such properties as the films' vibrational characteristics couple to the lattice volumetric changes during chemical expansion. While lattice volumetric changes are typically accessed by analyzing Raman shifts as a function of pressure, stress, or temperature, such methods can be impractical for thin films and do not capture information on chemical expansion. An in situ Raman spectroscopy technique using an electrochemical titration cell to change the oxygen nonstoichiometry of a model perovskite film, Sr(Ti,Fe)O_{3− y}  , is reported and the lattice vibrational properties are correlated to the material's chemical expansion. How to select an appropriate Raman vibrational mode to track the evolution in oxygen nonstoichiometry is discussed. Subsequently, the frequency of the oxygen stretching mode around Fe⁴⁺ is tracked, as it decreases during reduction as the material expands and increases during reoxidation as the material shrinks. This methodology of oxygen pumping and in situ Raman spectroscopy of oxide films enables future in operando measurements even for small material volumes, as is typical for applications of films as electrodes or electrolytes utilized in electrochemical energy conversion or memory devices.

The linear dichroism conversion phenomenon is reported in quasi‐1D hexagonal perovskite chalcogenide BaTiS₃, which also shows a record level of optical anisotropy in the visible range. Wavelength‐dependent polarization‐resolved Raman spectroscopy and first‐principles calculations further confirm the orthogonal cross‐over of the linear dichroism polarity in this material. This discovery could lead to novel photonic devices for multispectral imaging, sensing, and communication.

Abstract

Anisotropic photonic materials with linear dichroism are crucial components in many sensing, imaging, and communication applications. Such materials play an important role as polarizers, filters, and waveplates in photonic devices and circuits. Conventional crystalline materials with optical anisotropy typically show unidirectional linear dichroism over a broad wavelength range. The linear dichroism conversion phenomenon has not been observed in crystalline materials. The investigation of the unique linear dichroism conversion phenomenon in quasi‐1D hexagonal perovskite chalcogenide BaTiS₃ is reported. This material shows a record level of optical anisotropy within the visible wavelength range. In contrast to conventional anisotropic optical materials, the linear dichroism polarity in BaTiS₃ makes an orthogonal change at an optical wavelength corresponding to the photon energy of 1.78 eV. First‐principles calculations reveal that this anomalous linear dichroism conversion behavior originates from the different selection rules of the parallel energy bands in the BaTiS₃ material. Wavelength‐dependent polarized Raman spectroscopy further confirms this phenomenon. Such a material, with linear dichroism conversion properties, could facilitate the sensing and control of the energy and polarization of light, and lead to novel photonic devices such as polarization‐wavelength selective detectors and lasers for multispectral imaging, sensing, and optical communication applications.

A new type of ACI perovskite is prepared through the alternating ordering of BEA²⁺ and MA⁺ cations in the interlayer space (B‐ACI). The high exciton extraction efficiency and a narrow distribution of quantum well widths of B‐ACI perovskite enable a device with a record efficiency of 17.39%. Furthermore, the devices show stronger resistance to humidity, heating, and light soaks than previous equivalents.

Abstract

Low‐dimensional Ruddlesden–Popper (LDRP) perovskites are a current theme in solar energy research as researchers attempt to fabricate stable photovoltaic devices from them. However, poor exciton dissociation and insufficiently fast charge transfer slows the charge extraction in these devices, resulting in inferior performance. 1,4‐Butanediamine (BEA)‐based low‐dimensional perovskites are designed to improve the carrier extraction efficiency in such devices. Structural characterization using single‐crystal X‐ray diffraction reveals that these layered perovskites are formed by the alternating ordering of diammonium (BEA²⁺) and monoammonium (MA⁺) cations in the interlayer space (B‐ACI) with the formula (BEA)_0.5MA _n PbnI_3n+1. Compared to the typical LDRP counterparts, these B‐ACI perovskites deliver a wider light absorption window and lower exciton binding energies with a more stable layered perovskite structure. Additionally, ultrafast transient absorption indicates that B‐ACI perovskites exhibit a narrow distribution of quantum well widths, leading to a barrier‐free and balanced carrier transport pathway with enhanced carrier diffusion (electron and hole) length over 350 nm. A perovskite solar cell incorporating BEA ligands achieves record efficiencies of 14.86% for (BEA)_0.5MA₃Pb₃I₁₀ and 17.39% for (BEA)_0.5Cs_0.15(FA_0.83MA_0.17)_2.85Pb₃(I_0.83Br_0.17)₁₀ without hysteresis. Furthermore, the triple cations B‐ACI devices can retain over 90% of their initial power conversion efficiency when stored under ambient atmospheric conditions for 2400 h and show no significant degradation under constant illumination for over 500 h.