Mahui

Spectrally tunable and narrow blue perovskite nanocrystals are realized via Rb‐doping in various nanoscale geometries of CsPbBr₃. This multi‐cation strategy enables highly emissive nanocrystals that exhibit stable electroluminescence throughout the blue range. The proposed approach opens the avenue for the development of deep‐blue perovskite quantum dots, necessary for the commercialization of high‐quality displays and lighting.

Abstract

Perovskite nanocrystals exhibit high photoluminescence quantum yields (PLQYs) and tunable bandgaps from ultraviolet to infrared. However, blue perovskite light‐emitting diodes (LEDs) suffer from color instability under applied bias. Developing narrow‐bandwidth deep‐blue emitters will maximize the color gamut of display technologies. Mixed anion approaches suffer from halide segregation that leads to their spectral instability. Here instead, a mixed cation strategy is employed whereby Rb⁺ is directly incorporated during synthesis into CsPbBr₃ nanocrystals. Blue‐emitting perovskite quantum dots (QDs) with stable photoluminescence, PLQYs greater than 60%, tunable emission from 460 to 500 nm, and narrow emission linewidths (<25 nm) are reported. The strategy retains a pure bromine crystal structure resulting in color‐pure stable electroluminescence at operating voltages of up to 10 V, peak external quantum efficiencies (EQEs) of 0.87% and 0.11% for sky‐blue (490 nm), and deep‐blue (464 nm) devices. The sky‐blue devices exhibit the highest combined luminance of 93 cd m⁻² at an EQE of 0.75%, the best reported to date of perovskite QD LEDs.

Methylammonium lead iodide (CH₃NH₃PbI₃) thin film displaying ferroelastic twin domains is a ferroic platform with strong photovoltaic action. A study about the light‐ferroic interaction in methylammonium lead iodide is presented, which extends fundamental understanding of light‐ferroic interaction and offers knowledge for the development on new generation photovoltaic and optical devices.

Abstract

Given the remarkable performance of hybrid organic–inorganic perovskites (HOIPs) in solar cells, light emitters, and photodetectors, the quest to advance the fundamental understanding of the photophysical properties in this class of materials remains highly relevant. Recently, the discovery of ferroic twin domains in HOIPs has renewed the debate of the ferroic effects on optoelectric processes. This work explores the interaction between light and ferroic twin domains in CH₃NH₃PbI₃. Due to strain and chemical inhomogeneities, photogenerated electrons and holes show a preferential motion in the ferroelastic twin domains. Density functional theory (DFT) shows that electrons and holes result in lattice expansion in CH₃NH₃PbI₃ differently. Hence, light generates strain in the ferroelastic domains due to preferential photocarrier motion, leading to a screening of strain variation. X‐ray diffraction studies verify the DFT simulations and reveal that the photoinduced strain is light intensity dependent, and the photoexcitation is a prerequisite of inducing strain by light. This work extends the fundamental understanding of light‐ferroic interaction and offers guidance for developing functional devices.

The power conversion efficiency of N2200‐based all‐polymer solar cells (all‐PSCs) can be drastically enhanced from ≈1% to ≈11% by simply changing the solvent from chlorobenzene and 2‐methyltetrahydrofuran (Me‐THF). In‐depth investigations reveal that the preaggregation of donor (PTzBI) and acceptor (N2200) polymers in 2‐Me‐THF is the key to enable such high performance for N2200‐based all‐PSC device.

Herein, all‐polymer solar cells (all‐PSCs) are studied based on PTzBI:N2200 system processed from two different solvents, chlorobenzene (CB) and 2‐methyltetrahydrofuran (Me‐THF). It is found that the preaggregation of the donor and acceptor polymers in Me‐THF is the key factor that enables a drastic enhancement in cell efficiency from ≈1% (processed by CB) to ≈11% (processed by Me‐THF). When using CB as the solvent, both donor and acceptor polymers are well dissolved and mostly disaggregated. In contrast, the donor and acceptor polymers both exhibit strong aggregation in Me‐THF. As a result, the donor and acceptor blend films processed from Me‐THF exhibit pure domains with appropriate molecular packing structure, which leads to high charge mobilities (10⁻³–10⁻⁴ cm² V⁻¹ s⁻¹) and fill factors (FFs; 75%), whereas the blend films processed by CB suffer from highly miscible and impure domains, hence decreasing the charge mobilities by 1–2 orders of magnitude compared with those of the corresponding pure films. The current work reveals that the polymer preaggregation is the key reason enabling optimal morphology and high performance in N2200‐based all‐PSCs, and this strategy may be potentially applied in other systems to optimize the morphology and performance of all‐PSCs.

This review presents the present status and the future perspectives of Sn‐based perovskite solar cells. The strategies to find the breakthrough of highly efficient and robust Sn‐perovskite solar cells are discussed by focusing on current fabrication processes and defect physics scenario including compositional and dimensional engineering.

Sn‐based halide perovskites have attracted much interest due to their highly valuable electrical and optical properties. The promising optical and electrical properties of Sn‐based perovskites have enticed a lot of research to focus on developing the strategies and explore the in‐depth material characteristics. Sn‐halide perovskites exhibit apparent merits and demerits. The ideal electrical and optical properties are even better than that of Pb‐analogs, namely close‐to‐optimal bandgap, strong optical absorption, and good carrier mobilities. However, the present achievement of Sn‐halide perovskite solar cells is not satisfactory, which is commonly attributed to relatively low defect tolerance, fast crystallization, and oxidative instability. The efficiency of Sn‐based perovskites is far ahead, with a 9% power conversion efficiency (PCE), than the other (Ge, Bi, Sb, Cu, etc.) Pb‐free options but simultaneously lagging far behind Pb‐based analogs that have a 25.2% PCE. This review is aimed at presenting milestone works and revealing the pros and cons of Sn‐halide perovskites. In addition, the defect physics of Sn‐based perovskites is described. The improvement of open‐circuit voltage is a critical issue for Sn‐halide perovskites to compete with Pb‐based perovskites. The understanding of defect physics plays an instrumental role in designing strategies for efficient and robust Sn‐halide perovskite solar cells.

Publication date: December 2019

Source: Nano Energy, Volume 66

Author(s): Jiajia Wang, Hao Lin, Zilei Wang, Wenzhong Shen, Jichun Ye, Pingqi Gao

Graphical abstract

Publication date: December 2019

Source: Nano Energy, Volume 66

Author(s): Jigeon Kim, Bonkee Koo, Wook Hyun Kim, Jongmin Choi, Changsoon Choi, Sung Jun Lim, Jong-Soo Lee, Dae-Hwan Kim, Min Jae Ko, Younghoon Kim

Graphical abstract

We demonstrate that sodium acetate (NaOAc) directly generates short-chain OAc ions to exchange the long-chain oleate ligands of CsPbI₃ perovskite quantum dots (CsPbI₃-PQDs). NaOAc-based ligand exchange enables preservation of CsPbI₃-PQD size, minimization of surface trap states, and enhancement of electronic coupling in the resultant CsPbI₃-PQD solids. Consequently, NaOAc-treated CsPbI₃-PQD solar cells show improved device performance with 12.4% power conversion efficiency.

An electron‐deficient unit containing B←N bonds, namely BNIDT, is developed to construct polymer acceptors for photovoltaic applications. Desirable optoelectronic properties such as broad absorption profiles, low‐lying energy levels, ambipolar charge transport properties, and strong electron‐affinity are found for these polymers. All‐polymer solar cells using these B←N embedded polymers as acceptor materials exhibit an enhanced efficiency of 8.78%.

Abstract

In the field of all‐polymer solar cells (all‐PSCs), all efficient polymer acceptors that exhibit efficiencies beyond 8% are based on either imide or dicyanoethylene. To boost the development of this promising solar cell type, creating novel electron‐deficient units to build high‐performance polymer acceptors is critical. A novel electron‐deficient unit containing B←N bonds, namely, BNIDT, is synthesized. Systematic investigation of BNIDT reveals desirable properties including good coplanarity, favorable single‐crystal structure, narrowed bandgap and downshifted energy levels, and extended absorption profiles. By copolymerizing BNIDT with thiophene and 3,4‐difluorothiophene, two novel conjugated polymers named BN‐T and BN‐2fT are developed, respectively. It is shown that these polymers possess wide absorption spectra covering 350–800 nm, low‐lying energy levels, and ambipolar film‐transistor characteristics. Using PBDB‐T as the donor and BN‐2fT as the acceptor, all‐PSCs afford an encouraging efficiency of 8.78%, which is the highest for all‐PSCs excluding the devices based on imide and dicyanoethylene‐type acceptors. Considering that the structure of BNIDT is totally different from these classical units, this work opens up a new class of electron‐deficient unit for constructing efficient polymer acceptors that can realize efficiencies beyond 8% for the first time.

An electron‐deficient unit containing B←N bonds, namely BNIDT, is developed to construct polymer acceptors for photovoltaic applications. Desirable optoelectronic properties such as broad absorption profiles, low‐lying energy levels, ambipolar charge transport properties, and strong electron‐affinity are found for these polymers. All‐polymer solar cells using these B←N embedded polymers as acceptor materials exhibit an enhanced efficiency of 8.78%.

Abstract

In the field of all‐polymer solar cells (all‐PSCs), all efficient polymer acceptors that exhibit efficiencies beyond 8% are based on either imide or dicyanoethylene. To boost the development of this promising solar cell type, creating novel electron‐deficient units to build high‐performance polymer acceptors is critical. A novel electron‐deficient unit containing B←N bonds, namely, BNIDT, is synthesized. Systematic investigation of BNIDT reveals desirable properties including good coplanarity, favorable single‐crystal structure, narrowed bandgap and downshifted energy levels, and extended absorption profiles. By copolymerizing BNIDT with thiophene and 3,4‐difluorothiophene, two novel conjugated polymers named BN‐T and BN‐2fT are developed, respectively. It is shown that these polymers possess wide absorption spectra covering 350–800 nm, low‐lying energy levels, and ambipolar film‐transistor characteristics. Using PBDB‐T as the donor and BN‐2fT as the acceptor, all‐PSCs afford an encouraging efficiency of 8.78%, which is the highest for all‐PSCs excluding the devices based on imide and dicyanoethylene‐type acceptors. Considering that the structure of BNIDT is totally different from these classical units, this work opens up a new class of electron‐deficient unit for constructing efficient polymer acceptors that can realize efficiencies beyond 8% for the first time.

A new lead‐free halide Rb₂CuBr₃ scintillator with 1D crystal structure is presented. It exhibits self‐trapped exciton emission with a large Stokes shift (0.91 eV). Thus, it has near‐unity photoluminescence quantum yield (98.6%) and a high radioluminescence light yield of ≈91 056 photons per MeV.

Abstract

Scintillators are widely utilized for radiation detections in many fields, such as nondestructive inspection, medical imaging, and space exploration. Lead halide perovskite scintillators have recently received extensive research attention owing to their tunable emission wavelength, low detection limit, and ease of fabrication. However, the low light yields toward X‐ray irradiation and the lead toxicity of these perovskites severely restricts their practical application. A novel lead‐free halide is presented, namely Rb₂CuBr₃, as a scintillator with exceptionally high light yield. Rb₂CuBr₃ exhibits a 1D crystal structure and enjoys strong carrier confinement and near‐unity photoluminescence quantum yield (98.6%) in violet emission. The high photoluminescence quantum yield combined with negligible self‐absorption from self‐trapped exciton emission and strong X‐ray absorption capability enables a record high light yield of ≈91056 photons per MeV among perovskite and relative scintillators. Overall, Rb₂CuBr₃ provides nontoxicity, high radioluminescence intensity, and good stability, thus laying good foundations for potential application in low‐dose radiography.

3D nanoprinting of organic–inorganic metal halide perovskites is realized with guiding crystallization using a femtoliter ink meniscus in mid‐air.

Abstract

As competing with the established silicon technology, organic–inorganic metal halide perovskites are continually gaining ground in optoelectronics due to their excellent material properties and low‐cost production. The ability to have control over their shape, as well as composition and crystallinity, is indispensable for practical materialization. Many sophisticated nanofabrication methods have been devised to shape perovskites; however, they are still limited to in‐plane, low‐aspect‐ratio, and simple forms. This is in stark contrast with the demands of modern optoelectronics with freeform circuitry and high integration density. Here, a nanoprecision 3D printing is developed for organic–inorganic metal halide perovskites. The method is based on guiding evaporation‐induced perovskite crystallization in mid‐air using a femtoliter ink meniscus formed on a nanopipette, resulting in freestanding 3D perovskite nanostructures with a preferred crystal orientation. Stretching the ink meniscus with a pulling process enables on‐demand control of the nanostructure's diameter and hollowness, leading to an unprecedented tubular‐solid transition. With varying the pulling direction, a layer‐by‐layer stacking of perovskite nanostructures is successfully demonstrated with programmed shapes and positions, a primary step for additive manufacturing. It is expected that the method has the potential to create freeform perovskite nanostructures for customized optoelectronics.

Incorporating dicyanobenzothiadiazole into polymer yields an n‐type semiconductor DCNBT‐IDT, which exhibits a narrow bandgap of 1.43 eV and a high absorption coefficient of 6.15 × 10⁴ cm⁻¹. The DCNBT‐IDT‐based all‐polymer solar cells achieve a remarkable power conversion efficiency of 8.32% with a small energy loss of 0.53 eV and a photoresponse of up to 870 nm.

Abstract

Currently, n‐type acceptors in high‐performance all‐polymer solar cells (all‐PSCs) are dominated by imide‐functionalized polymers, which typically show medium bandgap. Herein, a novel narrow‐bandgap polymer, poly(5,6‐dicyano‐2,1,3‐benzothiadiazole‐alt‐indacenodithiophene) (DCNBT‐IDT), based on dicyanobenzothiadiazole without an imide group is reported. The strong electron‐withdrawing cyano functionality enables DCNBT‐IDT with n‐type character and, more importantly, alleviates the steric hindrance associated with typical imide groups. Compared to the benchmark poly(naphthalene diimide‐alt‐bithiophene) (N2200), DCNBT‐IDT shows a narrower bandgap (1.43 eV) with a much higher absorption coefficient (6.15 × 10⁴ cm⁻¹). Such properties are elusive for polymer acceptors to date, eradicating the drawbacks inherited in N2200 and other high‐performance polymer acceptors. When blended with a wide‐bandgap polymer donor, the DCNBT‐IDT‐based all‐PSCs achieve a remarkable power conversion efficiency of 8.32% with a small energy loss of 0.53 eV and a photoresponse of up to 870 nm. Such efficiency greatly outperforms those of N2200 (6.13%) and the naphthalene diimide (NDI)‐based analog NDI‐IDT (2.19%). This work breaks the long‐standing bottlenecks limiting materials innovation of n‐type polymers, which paves a new avenue for developing polymer acceptors with improved optoelectronic properties and heralds a brighter future of all‐PSCs.

The use of liquid exfoliated 2D WS₂ and MoS₂ as hole‐transporting layers (HTLs) in ultrahigh efficiency organic solar cells is reported. WS₂ yields cells with higher power conversion efficiency (PCE), fill‐factor, and short‐circuit current than MoS₂ and poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate). When WS₂ is introduced as HTL in PBDB‐T‐2F:Y6:PC₇₁BM organic solar cells, a maximum PCE value of 17% is achieved.

Abstract

The application of liquid‐exfoliated 2D transition metal disulfides (TMDs) as the hole transport layers (HTLs) in nonfullerene‐based organic solar cells is reported. It is shown that solution processing of few‐layer WS₂ or MoS₂ suspensions directly onto transparent indium tin oxide (ITO) electrodes changes their work function without the need for any further treatment. HTLs comprising WS₂ are found to exhibit higher uniformity on ITO than those of MoS₂ and consistently yield solar cells with superior power conversion efficiency (PCE), improved fill factor (FF), enhanced short‐circuit current (J _SC), and lower series resistance than devices based on poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) and MoS₂. Cells based on the ternary bulk‐heterojunction PBDB‐T‐2F:Y6:PC₇₁BM with WS₂ as the HTL exhibit the highest PCE of 17%, with an FF of 78%, open‐circuit voltage of 0.84 V, and a J _SC of 26 mA cm⁻². Analysis of the cells' optical and carrier recombination characteristics indicates that the enhanced performance is most likely attributed to a combination of favorable photonic structure and reduced bimolecular recombination losses in WS₂‐based cells. The achieved PCE is the highest reported to date for organic solar cells comprised of 2D charge transport interlayers and highlights the potential of TMDs as inexpensive HTLs for high‐efficiency organic photovoltaics.

The use of liquid exfoliated 2D WS₂ and MoS₂ as hole‐transporting layers (HTLs) in ultrahigh efficiency organic solar cells is reported. WS₂ yields cells with higher power conversion efficiency (PCE), fill‐factor, and short‐circuit current than MoS₂ and poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate). When WS₂ is introduced as HTL in PBDB‐T‐2F:Y6:PC₇₁BM organic solar cells, a maximum PCE value of 17% is achieved.

Abstract

The application of liquid‐exfoliated 2D transition metal disulfides (TMDs) as the hole transport layers (HTLs) in nonfullerene‐based organic solar cells is reported. It is shown that solution processing of few‐layer WS₂ or MoS₂ suspensions directly onto transparent indium tin oxide (ITO) electrodes changes their work function without the need for any further treatment. HTLs comprising WS₂ are found to exhibit higher uniformity on ITO than those of MoS₂ and consistently yield solar cells with superior power conversion efficiency (PCE), improved fill factor (FF), enhanced short‐circuit current (J _SC), and lower series resistance than devices based on poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) and MoS₂. Cells based on the ternary bulk‐heterojunction PBDB‐T‐2F:Y6:PC₇₁BM with WS₂ as the HTL exhibit the highest PCE of 17%, with an FF of 78%, open‐circuit voltage of 0.84 V, and a J _SC of 26 mA cm⁻². Analysis of the cells' optical and carrier recombination characteristics indicates that the enhanced performance is most likely attributed to a combination of favorable photonic structure and reduced bimolecular recombination losses in WS₂‐based cells. The achieved PCE is the highest reported to date for organic solar cells comprised of 2D charge transport interlayers and highlights the potential of TMDs as inexpensive HTLs for high‐efficiency organic photovoltaics.

Secondary amine, dimethylamine is intentionally included in MAPbI₃ perovskite to improve the rigidity and steric hindrance for water adsorption, giving rise to reduced defect density and enhanced hydrophobicity. Solar cells based on this perovskite structure demonstrate a record certified power conversion efficiency of 20.8% for NiO _x ‐based inverted perovskite solar cells and excellent operational stability under continuous light soaking.

Abstract

Large‐bandgap perovskites offer a route to improve the efficiency of energy capture in photovoltaics when employed in the front cell of perovskite–silicon tandems. Implementing perovskites as the front cell requires an inverted (p–i–n) architecture; this architecture is particularly effective at harnessing high‐energy photons and is compatible with ionic‐dopant‐free transport layers. Here, a power conversion efficiency of 21.6% is reported, the highest among inverted perovskite solar cells (PSCs). Only by introducing a secondary amine into the perovskite structure to form MA_{1− x} DMA _x PbI₃ (MA is methylamine and DMA is dimethylamine) are defect density and carrier recombination suppressed to enable record performance. It is also found that the controlled inclusion of DMA increases the hydrophobicity and stability of films in ambient operating conditions: encapsulated devices maintain over 80% of their efficiency following 800 h of operation at the maximum power point, 30 times longer than reported in the best prior inverted PSCs. The unencapsulated devices show record operational stability in ambient air among PSCs.

The cooperative relationship between spin, energy, and polarization parameters is revealed to maximize triplet‐to‐singlet conversion based on high‐efficiency exciplex organic light‐emitting diodes (OLEDs) with the EQE_max over 21%. This cooperative relationship provides a critical guideline to further advance the development of organic light‐emitting diodes.

Abstract

Experimental studies to reveal the cooperative relationship between spin, energy, and polarization through intermolecular charge‐transfer dipoles to harvest nonradiative triplets into radiative singlets in exciplex light‐emitting diodes are reported. Magneto‐photoluminescence studies reveal that the triplet‐to‐singlet conversion in exciplexes involves an artificially generated spin‐orbital coupling (SOC). The photoinduced electron parametric resonance measurements indicate that the intermolecular charge‐transfer occurs with forming electric dipoles (D^+•→A^−•), providing the ionic polarization to generate SOC in exciplexes. By having different singlet‐triplet energy differences (ΔE _ST) in 9,9′‐diphenyl‐9H,9′H‐3,3′‐bicarbazole (BCzPh):3′,3′″,3′″″‐(1,3,5‐triazine‐2,4,6‐triyl)tris(([1,1′‐biphenyl]‐3‐carbonitrile)) (CN‐T2T) (ΔE _ST = 30 meV) and BCzPh:bis‐4,6‐(3,5‐di‐3‐pyridylphenyl)‐2‐methyl‐pyrimidine (B3PYMPM) (ΔE _ST = 130 meV) exciplexes, the SOC generated by the intermolecular charge‐transfer states shows large and small values (reflected by different internal magnetic parameters: 274 vs 17 mT) with high and low external quantum efficiency maximum, EQE_max (21.05% vs 4.89%), respectively. To further explore the cooperative relationship of spin, energy, and polarization parameters, different photoluminescence wavelengths are selected to concurrently change SOC, ΔE _ST, and polarization while monitoring delayed fluorescence. When the electron clouds become more deformed at a longer emitting wavelength due to reduced dipole (D^+•→A^−•) size, enhanced SOC, increased orbital polarization, and decreased ΔE _ST can simultaneously occur to cooperatively operate the triplet‐to‐singlet conversion.

Secondary amine, dimethylamine is intentionally included in MAPbI₃ perovskite to improve the rigidity and steric hindrance for water adsorption, giving rise to reduced defect density and enhanced hydrophobicity. Solar cells based on this perovskite structure demonstrate a record certified power conversion efficiency of 20.8% for NiO _x ‐based inverted perovskite solar cells and excellent operational stability under continuous light soaking.

Abstract

Large‐bandgap perovskites offer a route to improve the efficiency of energy capture in photovoltaics when employed in the front cell of perovskite–silicon tandems. Implementing perovskites as the front cell requires an inverted (p–i–n) architecture; this architecture is particularly effective at harnessing high‐energy photons and is compatible with ionic‐dopant‐free transport layers. Here, a power conversion efficiency of 21.6% is reported, the highest among inverted perovskite solar cells (PSCs). Only by introducing a secondary amine into the perovskite structure to form MA_{1− x} DMA _x PbI₃ (MA is methylamine and DMA is dimethylamine) are defect density and carrier recombination suppressed to enable record performance. It is also found that the controlled inclusion of DMA increases the hydrophobicity and stability of films in ambient operating conditions: encapsulated devices maintain over 80% of their efficiency following 800 h of operation at the maximum power point, 30 times longer than reported in the best prior inverted PSCs. The unencapsulated devices show record operational stability in ambient air among PSCs.

In article number https://doi.org/10.1002/adma.2019032391903239, Junsheng Yu, Wei Huang, Ziyi Ge, Tobin J. Marks, Antonio Facchetti, and co‐workers present a spontaneous passivation method to greatly improve the performance of perovskite solar cells (PSCs) by using a zwitterionic small‐molecule electrolyte. This bottom‐up passivation is a novel and promising strategy to overcome outstanding issues impeding PSC advances in the future.

A spirobixanthene‐based dendrimer, DH1, is designed and synthesized. DH1 with the hyperbranched structure shows a large molecular size of up to 1.9 nm. The amorphous DH1 is the first dendrimer‐type HTM applied for MAPbI₃ perovskite solar cells, obtaining a power conversion efficiency of 17.13%. This work demonstrates that a quasiglobular dendrimer with a large molecular size is a promising design approach for excellent HTMs.

A dendrimer based on a spirobixanthene core, termed DH1, is designed and synthesized as a hole‐transporting material (HTM) for perovskite solar cells (PSCs). DH1 showing a hyperbranched structure with methoxydiphenylamine carbazole dendrons stretching outward along the para‐phenylene spacer acquires a large molecular size of up to 1.9 nm, which favors good thermal stability and amorphous property. The thus obtained DH1‐based pinhole‐free film as a hole‐transport layer results in a power conversion efficiency of 17.13% and reduced hysteresis behavior of MAPbI₃‐based planar PSCs. This work provides the first example of the use of dendrimer‐type HTM for PSC application, demonstrating a promising approach to design HTMs in a quasiglobular dendrimer with a large molecular size.

An all‐layer‐inorganic perovskite solar cell (PSC) based on inorganic CsPbI₂Br perovskite absorber layer and tailored NiO hole‐transporting layer (HTL) is fabricated. The tailored NiO nanocrystalline films exhibit uniform, pinhole‐free morphologies, efficient charge‐extraction capabilities, and intrinsic chemical stability, which gives the whole photovoltaic device a high efficiency and much improved stability compared with PSCs based on the organic HTLs.

Cesium‐based inorganic perovskite solar cells (PSCs) have attracted great attention due to the superior thermal stability of the light absorbers. However, the reported devices normally contain organic charge‐transporting layers (CTLs), such as spiro‐OMeTAD, which is expensive and highly sensitive to ambient atmosphere and temperature. It is of great significance to develop inorganic CTLs with low cost and robust stability. To date, it is still a big challenge to achieve high‐quality inorganic CTL films via the solution process, especially for the hole‐transporting layer (HTL) in conventional n‐i‐p structures. Herein, tailored NiO nanocrystalline films as HTLs in an all‐layer‐inorganic CsPbI₂Br‐based PSCs are developed, which exhibit uniform, pinhole‐free morphologies and efficient charge‐extraction capabilities. Consequently, the as‐constructed all‐layer‐inorganic PSCs, with an optimal power conversion efficiency (PCE) of 15.14% and a stabilized power output of 14.82%, present robust long‐term thermal stability: retained 85% of their initial PCEs after a thermal treatment at 85 °C in the dark in a nitrogen atmosphere with encapsulation for 1000 h, greatly surpassing the performance of the PSCs based on the organic HTLs.

New natriumion‐functionalized carbon nano‐dots (CNDs@Na) are rationally designed for planar inverted perovskite solar cells as an interface modification layer to reduce interfacial defects. CNDs@Na interfacial modification passivates surface trap states and reduces trap density at the interface, which facilitates photogenerated holes extraction and suppresses charge recombination.

Realizing the full potential of perovskite photovoltaic requires stringent control over nonradiative losses in the devices. Herein, the interfacial carrier recombination of inverted planar perovskite solar cells (PSCs) is suppressed using rationally designed natriumion‐functionalized carbon nano‐dots (CNDs@Na). The binding effect of carbon dots on Na⁺ inhibits the interstitial occupancy of alkali cations and reduces the microstrain of the polycrystalline film. Furthermore, modified surface wettability improves the ordering and crystal size of perovskite, which restrains ion diffusion and improves interfacial contact, leading to reduced interfacial charge recombination. Consequently, the effective interfacial passivation and crystallization control enhance the photovoltaic performance and long‐term stability of PSCs, resulting in an efficiency of over 20% with negligible hysteresis.

A small‐molecule acceptor (IDTS‐4F) is designed for a ternary approach, which enables the simultaneous increase in open‐circuit voltage and short‐circuit current density without sacrificing fill factor. The two acceptors form homogeneous acceptor phases, which synergize them with the increase in phase purity and crystallinity and the reduction in domain size, whereas the charge mobilities and recombinations are maintained.

Herein, an A–D–A‐type nonfullerene acceptor (named IDTS‐4F) with an alkyl thiophenyl side chain and dimethoxy thiophene bridging unit is reported. The use of an alkyl thiophenyl group is important, as the insertion of sulfur atoms can slightly downshift the highest occupied molecular orbital (HOMO) level of the molecule and allows IDTS‐4F to match with state‐of‐the‐art donor polymer PM6 (or PM7). Compared with conventional nonfullerene acceptors, IT‐4F, the IDTS‐4F molecule, has a smaller optical bandgap and higher lowest unoccupied molecular orbital (LUMO) level, which are beneficial to increase the V _oc and J _sc of the devices. Nonfullerene organic solar cell devices are fabricated using IDTS‐4F. Although the binary device based on IDTS‐4F exhibits a lower fill factor (FF, 70%), the ternary device by incorporating 0.2 of IDTS‐4F and 0.8 of IT‐4F (with PM6 as the donor polymer) can simultaneously achieve a higher V _oc and J _sc, while maintaining the high FF (77%) of IT‐4F based system. Morphology characterizations indicate the formation of homogeneous film morphology, the large increase in phase purity and crystallinity, and the reduction in domain size upon addition of crystalline IDTS‐4F, while the electron/hole mobilities and recombination losses of the IT‐4F system are both maintained.

Changing the alkyl chain position of a small‐molecule donor provides optimized conformation, improved phase aggregation, and enhanced photovoltaic properties. The strategy affords 12.3% efficiency single‐junction all‐small‐molecule organic solar cells (ASM OSCs) with reduced recombination and enhanced carrier lifetimes. The power conversion efficiency of 12.3% is higher than all reported single‐junction ASM OSCs.

Molecular stacking plays an important role in defining the active layer morphology in all‐small‐molecule organic solar cells (ASM OSCs). However, the precise control of donor/acceptor stacking to afford optimal phase separation remains challenging. Herein, the molecular stacking of a small‐molecule donor is tuned by changing the alky chain position to match a high‐performance small‐molecule nonfullerene acceptor (NFA), Y6. The alky chain engineering not only affects the planarity of the small‐molecule donor, but also the molecular aggregation and the active layer morphology, and thus the photovoltaic performance. Notably, single‐junction ASM OSCs with 12.3% power conversion efficiency (PCE) are achieved. The PCE of 12.3% is among the top efficiencies of single‐junction ASM OSCs reported in the literature to date. The results highlight the importance of fine‐tuning the molecular structure to achieve high‐performance ASM OSCs.

A quasi‐two‐dimensional perovskite film with stable domain distribution is prepared based on a new spacer. The film containing pure bromide perovskite exhibits enhanced deep‐blue fluorescence with quantum yield of 77% by low‐dimensional component engineering. As a result, the corresponding light‐emitting diodes deliver stable deep‐blue emission with a peak external quantum efficiency of 2.6%.

Abstract

Compared to efficient green and near‐infrared light‐emitting diodes (LEDs), less progress has been made on deep‐blue perovskite LEDs. They suffer from inefficient domain [various number of PbX₆ ⁻ layers (n)] control, resulting in a series of unfavorable issues such as unstable color, multipeak profile, and poor fluorescence yield. Here, a strategy involving a delicate spacer modulation for quasi‐2D perovskite films via an introduction of aromatic polyamine molecules into the perovskite precursor is reported. With low‐dimensional component engineering, the n₁ domain, which shows nonradiative recombination and retarded exciton transfer, is significantly suppressed. Also, the n₃ domain, which represents the population of emission species, is remarkably increased. The optimized quasi‐2D perovskite film presents blue emission from the n₃ domain (peak at 465 nm) with a photoluminescence quantum yield (PLQY) as high as 77%. It enables the corresponding perovskite LEDs to deliver stable deep‐blue emission (CIE (0.145, 0.05)) with an external quantum efficiency (EQE) of 2.6%. The findings in this work provide further understanding on the structural and emission properties of quasi‐2D perovskites, which pave a new route to design deep‐blue‐emissive perovskite materials.

In bulk heterojunctions with small energetic offsets between donor and acceptor materials, the donor polymer can assist the electron transport by providing “bridges” or a “shortcut” for electron transport across the small‐molecular domains and facilitates the overall electron transport. This finding can be also applied to other fields to tune the charge transport property of organic materials or slush blends.

Abstract

Conventional organic solar cell (OSC) systems have significant energy offsets between the donor and acceptor both at the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. Because of this, in a bulk heterojunction (BHJ) system, electrons typically transport in acceptors, whereas holes typically transport in donors. It is not favorable for electrons to hop back and forth between the donor and acceptor because the hopping is energetically disfavored. In such conventional OSC systems, the addition of donor polymer to acceptor films should typically reduce the electron mobility. In this study, a surprisingly large increase (up to 30×) in electron mobility is observed in an OSC blend when introducing a polymer donor into small molecular acceptor. By ruling out morphology reasons, it is shown that the donor polymer can assist the electron transport by providing “bridges” or a “shortcut” for electron transport across the domains of small molecular acceptors. This can happen because, for these systems, the LUMO offset is small. The study shows the benefits of donor‐assisted electron transport in BHJ systems with small energetic offsets. This finding could be also applied to other fields to tune the optimized charge transport property of organic materials or slush blends.