Mahui

Halide‐ and nonhalide‐based guanidinium salts are explored to study the impact of counterions supplied along with the guanidinium cation on the photophysical properties of perovskite films and photovoltaic performance of perovskite solar cells.

The impacts of halide and nonhalide sources of guanidinium cations, including guanidinium chloride (GCl) ((NH₂)₃CCl) and guanidinium thiocyanate (GTC) ((NH₂)₃CSCN), are comparatively analyzed on the structural, morphological, and photophysical properties of (CsMAFA)PbBr _x I_3 − x (x = 0.17) (MA = methylammonium, FA = formamidinium) perovskite films. X‐ray diffraction (XRD) reveals that the formation of photoinactive phases depends on the nature of counterions (halide vs nonhalide). Furthermore, morphological analysis shows that with the addition of guanidinium salts, the apparent grain size decreases due to the enhancement of nucleation density and/or slow growth of perovskite structures. More importantly, the introduction of GCl leads to the fabrication of perovskite solar cells (PSCs), yielding a photovoltage as high as 1.16 V (1.1 V for reference). In contrast, the introduction of GTC minimally affects the photovoltage, underlining the significance of counterions in improving the photovoltage of PSCs. The present preliminary results of the density functional theory based theoretical investigation related to the effect of G cation on the structure of the perovskite system is presented. In summary, the insights gained through structural and morphological characterization helps to understand the critical role of counterions of guanidinium salts in PSCs.

This work provides a new strategy (by alternating the core) for fine‐tuning the energy levels of nonconjugated zwitterionic molecules and by which a series of stable and one‐step synthesized electron transport layers are obtained for achieving higher performance of polymer solar cells and reducing the cost of industrial manufacture.

Developing stable and cheap organic cathode interlayers (OCIs; to replace metal cathode and expensive OCIs for enhancing the device stability and reducing the manufacturing cost) is an important topic for commercial applications of polymer solar cells (PSCs). Herein, four one‐step synthesized organic electron transport layers (G‐Series electron‐transport layers [ETLs]) with a novel star‐shaped molecular structure consisting of a series of different heteroatom atoms as cores and sulfobetaine ions as a terminal substituent are explored. The energy levels can be finely tuned by applying different heteroatom atoms as cores. With the conventional device structure with poly[[2,6′‐4,8‐di(5‐ethylhexylthienyl)benzo[1,2‐b;3,3‐b]dithiophene][3‐fluoro‐2[(2‐ethylhexyl) carbonyl]thieno[3,4‐b]thiophenediyl]] (PTB7‐Th) as a donor and [6,6]‐phenyl‐C₇₁‐butyric‐acid‐methyl‐ester (PC₇₀BM) as an acceptor, the G‐C2‐based devices exhibit a power conversion efficiency (PCE) of 8.90% with Al as the top electrode, much higher than that of the corresponding Ca/Al‐based device (7.43%). Furthermore, G‐Series‐based solar cells are also more stable than the reference device based on Ca. In addition, these easy‐to‐get ETLs can be widely suitable for other PSCs based on different active layer systems. This work not only shows a new strategy for fine‐tuning energy levels of nonconjugated zwitterionic molecules but also provides simple and stable ETLs for low‐cost and high‐performance PSCs.

A simple dynamic antisolvent quenching process is used for the efficient and reliable fabrications of uniform and high‐quality 10 × 10 cm² large‐area perovskite films. The perovskite module fabricated using this technique achieves an efficiency approaching 18% and a certified efficiency of 17.4% with the aperture area of 53.64 cm².

Perovskite solar cells represent a promising photovoltaic technology, which achieves record power conversion efficiencies over 24%. However, a problem on the commercial processing is the unavoidable efficiency loss during the scalable fabrication of perovskite solar module. The efficient and reliable fabrications of high‐quality large‐area perovskite films guarantee commercialized up‐scaling of perovskite solar cells with high efficiency. Herein, a simple dynamic antisolvent quenching (DAS) process is presented to understand large‐area uniform perovskite films to obtain an efficient perovskite solar module. This method provides a facile and universal approach to fabricate cracks‐free and uniform large‐area mixed‐cation perovskite films. A champion module device (10 × 10 cm²) with efficiency of 17.82% (another module with certified efficiency of 17.4%) is obtained using DAS process.

The initial stages of MAPbI₃ photodegradation prior to any significant change in light absorption are studied, with independent control of sample temperature and sunlight intensity (1–500 suns). Under the combined action of light and heat, a strong reduction of photoluminescence (PL) is observed. In contrast, illumination of perovskite films (with an intensity up to 500 suns) without heating induces considerable PL enhancement.

The initial stages of photo‐degradation of CH₃NH₃PbI₃ (MAPbI₃) thin films prior to any significant change in light absorption are studied in experiments with independent control of sample temperature and intensity of concentrated sunlight from 50 to 500 suns. Photo‐stability of the MAPbI₃ film is revealed to be extremely sensitive to the sample temperature. Under the combined action of light and heat (either by concentrated sunlight or by external heating), a strong reduction of the film photoluminescence (PL) without changes in the perovskite light absorption can be observed during the initial stages of degradation. In contrast, illumination of perovskite films (with intensity up to 500 suns) without heating (using chopped concentrated sunlight) induces considerable PL enhancement while the optical absorption spectrum remains unchanged. With accurate temperature control, aging under concentrated sunlight results in similar instability trends as that under 1 sun.

In this review, multifunctional molecules for perovskite solar cells (PSCs) are introduced. All the molecules can help to improve the performance of PSCs, such as forming low‐dimensional or dimensionally mixed perovskites and passivating defects, thus inducing good crystal growth behavior, improving the morphology of perovskite films, and facilitating charge transport. Eventually, PSCs with superior photoelectric properties and better stability can be obtained.

Organic–inorganic halide perovskite solar cells (PSCs) have recently attracted much attention with the recent certified power conversion efficiency (PCE) record exceeding 24%. To date, many approaches have been developed for producing high‐performance PSCs, in which the application of multifunctional molecules plays an important role. The multifunctional molecules can modify the morphology of perovskite films and/or passivate the surface defects through interactions with the perovskites' boundaries and/or the charge carrier extraction interfaces. As a result, both the PCEs and the stability of PSCs are improved. The recent progress in the development of multifunctional molecules‐incorporated PSCs is reviewed. The importance of further understanding of the role of the multifunctional molecules in the perovskite film formation process and defect passivation mechanism is discussed. Further research in terms of multifunctional molecules can help to develop high‐performance devices with long‐term stability for future practical applications of PSCs.

An aggregation‐induced emission (AIE) molecule is successfully employed as an effective hole transport material in an inverted planar perovskite solar cell. The improvement of perovskite crystallinity and the suppression of nonradiative recombination at the AIE/perovskite interface result in enhanced device performance and stability as compared with the poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS)‐based control one.

Organic hole‐transport materials (HTMs) are very promising for perovskite solar cells (PSCs) because the molecule structure is engineered via facile chemical routes. Herein, an aggregation‐induced emission (AIE) molecule, 2‐(2,7‐bis(4‐(bis(4‐methoxyphenyl)amino)phenyl)‐9H‐fluoren‐9‐ylidene)malononitrile (TFM), is successfully employed for the first time as a HTM in an inverted planar PSC, obtaining a promising device performance superior to that of the control device with poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS) HTM. An optimal power conversion efficiency (PCE) of 16.03% is obtained for the TFM‐based PSCs with a J _sc of 22.68 mA cm⁻², V _oc of 0.97 V and FF of 72.9%, while that of the control PEDOT:PSS‐based device is 14.95%. Steady‐state and time‐resolved photoluminescence results reveal suppressed nonradiative recombination at the TFM/perovskite interface that is attributed to the effective passivation of the uncoordinated Pb at the perovskite surface by the CN⁻ groups of TFM molecules, as confirmed by X‐ray photoelectronic spectroscopy measurements. In addition to the passivation, the hydrophobic character of TFM films also contributes to the improved device stability. The findings demonstrate the potential of AIE molecules in PSCs and also paves a novel way to improve device performance and stability by molecular structure engineering of AIE molecules in the future.

A simple coalloying strategy is applied to partly substitute HC(NH₂)₂/CH₃NH₃ (FA/MA) and I⁻ in FA_0.85MA_0.15PbI₃ perovskite by Cs⁺ and Ac⁻ respectively, which is an effective way to improve the tolerance factor, crystallinity, electronic properties, and band structure of FA_0.85MA_0.15PbI₃ materials. Consequently, the coalloyed perovskite solar cells yield a champion power conversion efficiency of 21.95% with negligible hysteresis and high stability.

A simple coalloying strategy is applied to improve the efficiency and stability of FA_0.85MA_0.15PbI₃ perovskite solar cells (PSCs) by using cesium acetate (CsAc) as an additive. It is found that the simultaneous incorporation of cation (Cs⁺) and anion (Ac⁻) into the FA_0.85MA_0.15PbI₃ film is an effective approach to realize lattice contraction, grain size enlargement, photoelectric properties improvement, band structure modulation, and therefore the optimization of the efficiency and stability of PSCs. At optimal CsAc alloying, the FA_0.85MA_0.15PbI₃ PSCs achieve a maximum power conversion efficiency (PCE) of 21.95% and an average of over 21%. In addition, the alloyed PSCs retain 97% of their initial PCE values after aging for 55 days in air without encapsulation.

The power conversion efficiency of the carbon‐based perovskite solar cells is enhanced by 21.4% simply by interfacial post‐treatment with cesium acetate. The nonencapsulated device can remain stable for 4 months without observable degradation. The improved performance is attributed to the better matched energy levels and the reduced defect density.

The interface between the perovskite layer and carbon electrode is important for printable carbon‐based perovskite solar cells (PSCs) to improve the power conversion efficiency (PCE) and device stability. A series of acetate salts are employed to in situ post‐modify the interface between the perovskite layer and carbon electrode for printable carbon‐based PSCs by the post‐treatment method. Cesium acetate (CsAc) is identified to enhance the average PCE from 12.6% to 15.3%. The stabilized output PCE reaches 15.6%, and the highest open‐circuit voltage (V _OC) is 1.1 V, representing a new milestone in increasing the ratio of V _OC/E _g (E _g: bandgap of perovskite) to be 0.67 for the printable carbon‐based PSCs without hole transporting materials. Moreover, the device stability in air is also improved by CsAc post‐modification. The improved performance is attributed to the better matching of energy levels of the perovskite layer with a carbon electrode and reduced defect density in the perovskite layer via in situ produced methylammonium acetate and ion replacement. This simple and effective CsAc post‐treatment method opens a new promising direction for developing scalable carbon‐based PSCs.

Trihydrazine dihydriodide is successfully used as an additive for solution deposition of a formamidinium tin iodide (FASnI₃) perovskite layer, resulting in improved surface morphology and reduced carrier concentration. Using the derived FASnI₃ layer as a light absorber, a maximum power conversion efficiency of 8.48% is achieved in a planar‐heterojunction solar cell using common precursor SnI₂ with 99% purity.

The deposition of a uniform and dense tin‐based perovskite layer with low defect‐caused background carrier density is crucial for achieving efficient tin perovskite solar cells (PSCs). These defects are mainly caused by the rapid oxidation of Sn²⁺ to Sn⁴⁺ in tin perovskite during device fabrication. Herein, trihydrazine dihydriodide ((N₂H₄)₃(HI)₂) is used as an additive for solution deposition of a formamidinium tin iodide (FASnI₃) perovskite layer. The resultant FASnI₃ layer is homogeneous with full surface coverage; moreover, the content of Sn⁴⁺ is significantly reduced in the film from the SnI₂ precursor owing to the reductive property of (N₂H₄)₃(HI)₂. With the high‐quality FASnI₃ layer as a light absorber, planar‐heterojunction perovskite solar cells are fabricated, exhibiting a maximum power conversion efficiency of 8.48% and good reproducibility. This work opens new possibilities for achieving efficient lead‐free tin‐based perovskite solar cells.

Three novel low‐bandgap fused‐ring electron acceptors, BPIC, BPIC‐2Cl, and BPIC‐4Cl are designed based on a heptacyclic core, using phenyl‐substituted benzo[1,2‐b:4,5‐b′]dithiophene as the central unit, end‐capped with 1,1‐dicyano methylene‐3‐indanone (INCN), mono‐chlorinated INCN, and di‐chlorinated INCN moieties, respectively. The effects of chlorination on optical and electronic properties of molecules, film morphology, and photovoltaic device performance are investigated.

A new heptacyclic core based on phenyl‐substituted benzo[1,2‐b:4,5‐b']dithiophene (BDT) is designed and paired with 1,1‐dicyano methylene‐3‐indanone (INCN) end group to construct a nonfullerene acceptor, BPIC. The strong aggregation and large phase separation in the poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione))]) (PBDB‐T):BPIC blend cause inefficient exciton dissociation and ineffective charge transport, resulting in a low 11.12% power conversion efficiency (PCE) with low short‐circuit current density (J _SC) and fill factor (FF). To finely control the active‐layer nanomorphology, the chlorine atom is introduced into the INCN termini, and di‐chlorinated BPIC‐2Cl and tetra‐chlorinated BPIC‐4Cl are synthesized. It is an interesting phenomenon that, unlike other literature reports, while the di‐chlorination reduces crystallinity and phase‐separation scale, further chlorination increases crystallinity and phase separation. The PBDB‐T:BPIC‐2Cl device exhibits suitable molecular packing and nearly ideal nanoscale phase separation, which facilitates exciton dissociation and charge transport and thus yields the higher PCE of 12.63% with significantly improved J _SC and FF. PBDB‐T:BPIC‐4Cl device, however, exhibits strong stacking intensity and excessively large phase separation, leading to the clearly reduced J _SC, FF, and PCE of only 8.23%. This work demonstrates that novel phenyl‐substituted BDT core and delicated chlorination strategy provides powerful tools for high‐performance nonfullerene acceptors in organic solar cells.

A systematic study of structurally similar fullerene derivatives shows that even minor modifications in their structure have a strong impact on their performance as electron transport layer (ETL) materials for perovskite solar cells. The best ETL significantly improves ambient stability of the devices for >800 h presumably due to an optimal size/shape of the solubilizing addend enabling compact molecular packing.

It is known that the operation lifetime of perovskite solar cells can be extended by orders of magnitude if properly selected hole‐transport and electron transport layers provide good isolation for the perovskite absorber preventing evaporation of volatile species (e.g., photoinduced) from the active layer and blocking the diffusion of aggressive moisture and oxygen from the surrounding environment. Herein, a systematic study of a family of structurally similar fullerene derivatives as electron transport layer (ETL) materials for p‐i‐n perovskite solar cells is presented. It is shown that even minor modifications of the molecular structure of the fullerene derivatives have a strong impact on their electrical performance and, particularly, ambient stability of the devices. Indeed, an optimally functionalized fullerene derivative applied as an ETL enables stable operation of perovskite solar cells when exposed to air for >800 h, which is manifested in retention of 90% of the original photovoltaic performance. In contrast, the reference devices with phenyl‐C₆₁‐butyric acid methyl ester as the ETL degraded almost completely within less than 100 h of air exposure. Most probably, the side chains of the best‐performing fullerene ETL materials are filling the gaps between the carbon spheres, thus preventing the diffusion of oxygen and moisture inside the device.

Bi₂Te₃ nanoplates with a tunable energy structure are introduced in inorganic perovskite solar cells (PSCs), accelerating hole transport by the matched band alignment. Confirmed by systematic measurements, charge recombination is largely suppressed due to lower trap density and higher carrier mobility. The optimal PSC with Bi₂Te₃ exhibits highly decreased V _OC loss and enhanced long‐term stability over 50 days.

To solve the thermal instability issue of organic–inorganic hybrid perovskites, all‐inorganic perovskite solar cells (PSCs) have been featured in the spotlight. However, their power conversion efficiencies (PCEs) are far from satisfactory due to the substantially radiative and nonradiative recombination of charge carriers in the common‐structured devices. Herein, bismuth telluride (Bi₂Te₃) nanoplates are designed as an interlayer between cesium lead halide (CsPbBrI₂) and 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,90‐spirobifluorene (Spiro‐OMeTAD) to reduce the notorious trap states and charge recombination. Confirmed by systematic electrochemical and photoelectrical techniques, the Bi₂Te₃ interlayer optimizes hole extraction and transport efficiency because of the matched band level structure and drastically reduces trap defect densities. Prolonged effective lifetime and shorter diffusion time induced by the Bi₂Te₃ interlayer reveal less electron–hole recombination and more efficient carrier transport, which lead to a larger photocurrent and less open circuit voltage loss of PSCs. The all‐inorganic PSCs with the optimal Bi₂Te₃ interlayer exhibit a highly enhanced PCE of 11.96%. Moreover, Bi₂Te₃ also acts as a blocking layer for the migration of iodide ions, silver, and moisture, resulting in a considerable device stability of more than 70% of initial PCE after 50 days without extra encapsulation. This low‐cost and facile method for efficient and stable all‐inorganic PSCs offers great promise as a next‐generation renewable energy source.

The large grains and high crystallinity of Pb(Ac)₂‐doped α‐CsPbI₂Br active layers with CsBr passivation is realized by a two‐step annealing process. The corresponding planar all‐inorganic CsPbI₂Br perovskite solar cells exhibit a long‐term ultrahigh power conversion efficiency of 16.37%, with a substantially improved V _OC of 1.271 V.

All‐inorganic CsPbI₂Br perovskite has attracted increasing attention, owing to its outstanding thermal stability and suitable bandgap for optoelectronic devices. However, the substandard power conversion efficiency (PCE) and large energy loss (E _loss) of CsPbI₂Br perovskite solar cells (PSCs) caused by the low quality and high trap density of perovskite films still limit the application of devices. Herein, the post‐treatment of evaporating cesium bromide (CsBr) is utilized on top of the perovskite surface to passivate the CsPbI₂Br–hole‐transporting layer interface and reduce E _loss. The results of microzone photoluminescence indicate that the evaporated CsBr gathered at the grain boundaries of CsPbI₂Br layers and Br‐enriched perovskites (CsPbI _x Br_3−x , x < 2) are formed, which can provide protection for CsPbI₂Br. Therefore, the gaps between crystal grains are filled up, and the recombination loss of the all‐inorganic CsPbI₂Br PSCs is reduced accordingly. The champion device exhibits high open‐circuit voltage and a PCE of 1.271 V and 16.37%, respectively. This is the highest reported PCE among all‐inorganic CsPbI₂Br PSCs reported so far. In addition, the stability of CsPbI₂Br PSCs is effectively improved by CsBr passivation, and the device without encapsulation can retain 86% of its initial PCE after 1368 h of storage, which is beneficial for practical applications.

Interface engineering is widely recognized as an effective strategy to improve the efficiency and stability of perovskite solar cells. This review is intended to provide a deep understanding of interface design principles for highly efficient and stable perovskite photovoltaic devices and a timely overview for state‐of‐the‐art interfacial materials in this rapidly developing field.

Exceptionally high efficiencies for organic–inorganic hybrid perovskite solar cells (PSCs) have been achieved. However, their operational stability still needs to be improved. The intrinsic instability of halide perovskites caused by the presence of volatile organic cations, as well as the degradation of hybrid perovskites induced by the adverse permeation of environmental water (H₂O)/oxygen (O₂) and the undesired ion diffusion or migration are the major reasons. Beyond strengthening perovskites themselves, interface engineering is now regarded as a valid strategy to prolong device lifetime by preventing the undesired degradation pathways. This comprehensive review highlights the utilization of practical interface engineering for enhancing the efficiency and stability of organic–inorganic lead halide PSCs. First, the impacts of interface design on the energy‐level alignment and carrier dynamics are overviewed. Second, recent progresses on the development of interfacial materials for simultaneously achieving high efficiency and stability of PSCs are summarized. At last, the interfacial layer design principles along with future outlook of this rapidly developing field are discussed.

Publication date: September 2019

Source: Nano Energy, Volume 63

Author(s): Fengyou Wang, Yuhong Zhang, Meifang Yang, Jinyue Du, Leilei Xue, Lili Yang, Lin Fan, Yingrui Sui, Jinghai Yang, Xiaodan Zhang

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Publication date: September 2019

Source: Nano Energy, Volume 63

Author(s): Xianyong Zhou, Manman Hu, Chang Liu, Luozheng Zhang, Xiongwei Zhong, Xiangnan Li, Yanqing Tian, Chun Cheng, Baomin Xu

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Publication date: September 2019

Source: Nano Energy, Volume 63

Author(s): Lin Xu, Xinfu Chen, Junjie Jin, Wei Liu, Biao Dong, Xue Bai, Hongwei Song, Peter Reiss

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Research on planar perovskite solar cells (PSCs) in (inverted) p–i–n configuration, using transparent p-type front-electrodes, is strongly emerging. NiO_x has been demonstrated to be one of the most promising candidates to be employed as a hole transport layer (HTL) in these devices, however, its low intrinsic conductivity and unmatched Fermi level with respect to the perovskite layer limit the performance of the PSCs. Extrinsic doping of NiO_x HTLs is a versatile and powerful strategy to mitigate these shortcomings, which, within the past three years, led to significantly enhanced power conversion efficiencies (exceeding 20%). In this review, we present a comprehensive overview of the strategies applied to improve the performance of NiO_x HTLs used in inverted PSCs with special emphasis on the properties modulation induced by extrinsic doping. Current challenges and perspectives for exploiting these HTLs in high-efficiency inverted PSCs are also given.

Publication date: November 2019

Source: Organic Electronics, Volume 74

Author(s): Mohammad Adil Afroz, Ritesh Kant Gupta, Rabindranath Garai, Maimur Hossain, Suyash Pati Tripathi, Parameswar Krishnan Iyer

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Publication date: November 2019

Source: Organic Electronics, Volume 74

Author(s): Amruth C, Beata Luszczynska, Marek Zdzislaw Szymanski, Jacek Ulanski, Ken Albrecht, Kimihisa Yamamoto

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Molecular doping is demonstrated as a powerful strategy for enhancing electrical conductivity in arrays of electronically coupled CsPbI₃ nanocrystals. The high surface‐area‐to‐volume ratio enables physisorbed organic redox molecules to inject charge carriers into a nanocrystal array. p‐Type doping improves both ground‐state conductivity and photoconductivity, enabling pronounced performance enhancements to both transistors and phototransistors.

Abstract

Doping of semiconductors enables fine control over the excess charge carriers, and thus the overall electronic properties, crucial to many technologies. Controlled doping in lead‐halide perovskite semiconductors has thus far proven to be difficult. However, lower dimensional perovskites such as nanocrystals, with their high surface‐area‐to‐volume ratio, are particularly well‐suited for doping via ground‐state molecular charge transfer. Here, the tunability of the electronic properties of perovskite nanocrystal arrays is detailed using physically adsorbed molecular dopants. Incorporation of the dopant molecules into electronically coupled CsPbI₃ nanocrystal arrays is confirmed via infrared and photoelectron spectroscopies. Untreated CsPbI₃ nanocrystal films are found to be slightly p‐type with increasing conductivity achieved by incorporating the electron‐accepting dopant 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F₄TCNQ) and decreasing conductivity for the electron‐donating dopant benzyl viologen. Time‐resolved spectroscopic measurements reveal the time scales of Auger‐mediated recombination in the presence of excess electrons or holes. Microwave conductance and field‐effect transistor measurements demonstrate that both the local and long‐range hole mobility are improved by F₄TCNQ doping of the nanocrystal arrays. The improved hole mobility in photoexcited p‐type arrays leads to a pronounced enhancement in phototransistors.

Incorporation of the molecular Lewis acid bis(pentafluorophenyl)zinc [Zn(C₆F₅)₂] into a high‐hole‐mobility organic small‐molecule/polymer blend yields transistors with a maximum mobility of 21.5 cm² V⁻¹ s⁻¹. Analysis of the materials and devices reveals that Zn(C₆F₅)₂ acts as a simultaneous p‐dopant and microstructure modifier. Density functional theory calculations predict that the formation of molecular complexes mediates these synergistic effects.

Abstract

Incorporating the molecular organic Lewis acid tris(pentafluorophenyl)borane [B(C₆F₅)₃] into organic semiconductors has shown remarkable promise in recent years for controlling the operating characteristics and performance of various opto/electronic devices, including, light‐emitting diodes, solar cells, and organic thin‐film transistors (OTFTs). Despite the demonstrated potential, however, to date most of the work has been limited to B(C₆F₅)₃ with the latter serving as the prototypical air‐stable molecular Lewis acid system. Herein, the use of bis(pentafluorophenyl)zinc [Zn(C₆F₅)₂] is reported as an alternative Lewis acid additive in high‐hole‐mobility OTFTs based on small‐molecule:polymer blends comprising 2,7‐dioctyl[1]benzothieno [3,2‐b][1]benzothiophene and indacenodithiophene–benzothiadiazole. Systematic analysis of the materials and device characteristics supports the hypothesis that Zn(C₆F₅)₂ acts simultaneously as a p‐dopant and a microstructure modifier. It is proposed that it is the combination of these synergistic effects that leads to OTFTs with a maximum hole mobility value of 21.5 cm² V⁻¹ s⁻¹. The work not only highlights Zn(C₆F₅)₂ as a promising new additive for next‐generation optoelectronic devices, but also opens up new avenues in the search for high‐mobility organic semiconductors.

The interplay between the wide‐energy‐gap host and the thermally activated delayed fluorescence (TADF) sensitizer in fluorescent organic light‐emitting diodes is comprehensively studied. This reveals that an efficient TADF host and recombination on it are both required for multiple sensitizing processes, which leads to a maximum external quantum efficiency/power efficiency of 24.2%/89.5 lm W⁻¹.

Abstract

Comprising an emitting layer (EML) constituting a wide‐energy‐gap host, a thermally activated delayed fluorescence (TADF) sensitizer and a conventional fluorescent dopant, TADF‐sensitizing‐fluorescence organic light‐emitting diodes (TSF‐OLEDs) highly depend on component interplay to maximize their performance, which, however, is still under‐researched. Taking the host type (TADF or non‐TADF) and the recombination position (on the host or on the TADF sensitizer) into consideration, the interplay of host and TADF sensitizer is comprehensively studied and manipulated. A wide‐energy‐gap host with TADF and recombination of charges on it are both required to maximize device performances by triggering multiple sensitizing processes to eliminate exciton losses. Based on those findings, a maximum external quantum efficiency (EQE)/power efficiency (PE) of 23.2%/76.9 lm W⁻¹ is realized with a newly developed TADF host, significantly outperforming the reference devices. Further device optimization leads to unprecedently high EQE/PE of 24.2%/89.5 lm W⁻¹ and a half‐lifetime of over 400 h at an initial luminance of 2000 cd m⁻², with the peak PE being the highest value among the reported TSF‐OLEDs. This work reveals the importance of manipulating the component interplay in EMLs, opening a new avenue toward highly efficient TSF‐OLEDs.

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.

The impact of light on the stability of perovskite solar cells (PSCs) is comprehensively investigated. Elevated device temperature and excess charge carriers are the driving forces for defect formation and PSC device degradation under illumination, not the photovoltage or strain. Cooling the device and operating at maximum power point can improve PSC stability.

Abstract

With power conversion efficiencies now reaching 24.2%, the major factor limiting efficient electricity generation using perovskite solar cells (PSCs) is their long‐term stability. In particular, PSCs have demonstrated rapid degradation under illumination, the driving mechanism of which is yet to be understood. It is shown that elevated device temperature coupled with excess charge carriers due to constant illumination is the dominant force in the rapid degradation of encapsulated perovskite solar cells under illumination. Cooling the device to 20 °C and operating at the maximum power point improves the stability of CH₃NH₃PbI₃ solar cells over 100× compared to operation under open circuit conditions at 60 °C. Light‐induced strain originating from photothermal‐induced expansion is also observed in CH₃NH₃PbI₃, which excludes other light‐induced‐strain mechanisms. However, strain and electric field do not appear to play any role in the initial rapid degradation of CH₃NH₃PbI₃ solar cells under illumination. It is revealed that the formation of additional recombination centers in PSCs facilitated by elevated temperature and excess charge carriers ultimately results in rapid light‐induced degradation. Guidance on the best methods for measuring the stability of PSCs is also given.