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A strategy of combination with indacenodithieno[3,2‐b]selenophene and replacement of tetrachlorination by tetrabromination on the end group is developed for high‐performance nonfluorinated acceptors. The polymer solar cells based on TSeIC4Br achieve a higher power conversion efficiency of 11.92% than that of TSeIC4Cl due to the lower E _loss, enhanced J _sc, and higher V _oc simultaneously.

Herein, a pair of tetrahalogenated nonfullerene small molecular acceptors (NF‐SMAs) (TSeIC4Cl and TSeIC4Br) are designed and synthesized, with the same indacenodithieno[3,2‐b]selenophene central unit and two different dihalogenation terminal groups, respectively. The systematic investigation is achieved to reveal the impact of two different nondifluorinated terminal groups on the device performance of the resultant ITIC series NF‐SMAs. TSeIC4Br shows red‐shifted absorption range and higher frontier energy levels compared with that of TSeIC4Cl. Moreover, PM6:TSeIC4Br blend film exhibits more suitable phase‐separated morphology with more ordered molecular packing and improved miscibility compared with that of the tetrachlorinated counterpart. Importantly, PM6:TSeIC4Br‐based device exhibits a better power conversion efficiency (PCE) of 11.92%, with a higher open‐circuit voltage (V _oc) and an enhanced short‐circuit current density (J _sc) when compared with that of PM6:TSeIC4Cl‐based device (11.13%). Furthermore, the energy loss can be reduced by replacing the disubstituents of end group from chlorine to bromine atoms. The results demonstrate that incorporation of indacenodithieno[3,2‐b]selenophene and replacement of tetrachlorination by tetrabromination on the end group contributes to elevating the J _sc, reduce energy loss, and enhancing the PCE for its relevant ITIC series device simultaneously, which may give a new avenue for achieving high‐performance multihalogenated ITIC series NF‐SMAs.

A new series of dopant‐free hole‐transporting materials (HTMs) YZT1–YZT4 featuring porphyrin backbone is achieved in which the best power conversion efficiency (PCE) of YZT4 is 14.95% for doped and that of YZT1 is 13.10% for dopant‐free perovskite solar cells (PSCs) based on TiO₂ semiconductors.

A new series of structurally simple and easily accessible hole‐transporting materials (HTMs) YZT1–YZT4 using porphyrin backbone is devised for high‐performance perovskite solar cells (PSCs) with and without the aid of doping. The YZT‐series HTMs have either push–push or push–pull type planar linear molecular geometry with substitution of linear or branched alkylamine. UV–vis absorption, photoluminance (PL) quenching experiments, and theoretical studies all suggest a different pattern of molecular packing induced by molecular geometry and/or substituted chains. Nonetheless, both types of porphyrin HTMs perform well in TiO₂‐based PSCs when doped YZT4 with a power conversion efficiency (PCE) of 14.95% and undoped YZT1 with a PCE of 13.10%. The results clearly reveal the potential for porphyrin‐based HTMs for use in dopant‐free PSCs.

A dual‐functionalized bidentate molecule 2‐(2′‐thienyl)pyridine (2‐ThPy) is introduced to modulate perovskite crystallization and passivate halogen vacancy defects. Compared with monodentate counterparts, 2‐ThPy can anchor Pb²⁺ sites via S and N atomic bonding simultaneously. Consequently, 2‐ThPy‐treated CsPbI₂Br perovskite solar cells achieve a champion power conversion efficiency of 12.69% with negligible hysteresis and exhibit prominent moisture stability.

All inorganic mixed‐halide CsPbI₂Br perovskites with suitable bandgap and superior thermal durability have ignited rising interests in the field of perovskite solar cells (PSCs). However, the serious energy losses derived from deleterious trap‐assisted defects–induced notorious nonradiative recombination and inferior moisture durability are still the primary hindrance on the way to develop high‐performance CsPbI₂Br PSCs. Herein, a novel passivation strategy is presented by introducing dual‐functionalized bidentate molecule 2‐(2′‐thienyl)pyridine (2‐ThPy) to modulate perovskite crystallization and passivate halogen vacancy defects. Compared with monodentate counterparts, 2‐ThPy can anchor Pb²⁺ sites via S and N atomic bonding simultaneously, and the synthesized CsPbI₂Br films exhibit enlarged grain size, show advantages to passivate defect states, and dramatically reduce trap density, thereby lessening the detrimental carrier recombination. Consequently, a champion power conversion efficiency (PCE) of 12.69% with negligible hysteresis is delivered for the fabricated CsPbI₂Br PSCs treated with 2‐ThPy. Moreover, the moisture stability of CsPbI₂Br PSCs with 2‐ThPy is also greatly enhanced, and the device without encapsulation retains 92% of initial PCE value after 30 days aging under 25 °C and 40% relative humidity in ambient environment. The bidentate molecules passivation strategy paves a promising avenue to implement efficient and stable inorganic PSCs.

By using a solvent‐mediated phase transformation process, a record certified 21.8% power conversion efficiency in pure‐iodide, alkaline‐metal‐free MA_0.5FA_0.5PbI₃ perovskite‐based solar cells is achieved.

Abstract

Composition and film quality of perovskite are crucial for the further improvement of perovskite solar cells (PSCs), including efficiency, reproducibility, and stability. Here, it is demonstrated that by simply mixing 50% of formamidinium (FA⁺) into methylammonium lead iodide (MAPbI₃), a highly crystalline, stable phase, and compact, polycrystalline grain morphology perovskite is formed by using a solvent‐mediated phase transformation process via the synergism of dimethyl sulfoxide and diethyl ether, which shows long carrier lifetime, low trap state density, and a record certified 21.8% power conversion efficiency (PCE) in pure‐iodide, alkaline‐metal‐free MA_0.5FA_0.5PbI₃ perovskite‐based PSCs. These PSCs show very high operational stability, with 85% PCE retention upon 1000 h 1 Sun intensity illumination. A 17.33% PCE module (6.5 × 7 cm²) is also demonstrated, attesting to the scalability of such devices.

Colloidal synthesis of all inorganic single‐crystalline β‐CsPbI₃ nanorods with an excellent photostability under 45–55% humidity displays the superior characteristics of fabricated inverted perovskite solar cells without any device passivation. Atomic resolution transmission electron micrography reveals the probable distribution of Cs, Pb, and I atoms in a single β‐phase CsPbI₃ nanorod.

Abstract

The synthesis of single‐crystalline β‐CsPbI₃ perovskite nanorods (NRs) using a colloidal process is reported, exhibiting their improved photostability under 45–55% humidity. The crystal structure of CsPbI₃ NRs films is investigated using Rietveld refined X‐ray diffraction (XRD) patterns to determine crystallographic parameters and the phase transformation from orthorhombic (γ‐CsPbI₃) to tetragonal (β‐CsPbI₃) on annealing at 150 °C. Atomic resolution transmission electron microscopy images are utilized to determine the probable atomic distribution of Cs, Pb, and I atoms in a single β‐phase CsPbI₃ NR, in agreement with the XRD structure and selected area electron diffraction pattern, indicating the growth of single crystalline β‐CsPbI₃ NR. The calculation of the electronic band structure of tetragonal β‐CsPbI₃ using density functional theory (DFT) reveals a direct transition with a lower band gap and a higher absorption coefficient in the solar spectrum, as compared to its γ‐phase. An air‐stable (45–55% humidity) inverted perovskite solar cell, employing β‐CsPbI₃ NRs without any encapsulation, yields an efficiency of 7.3% with 78% enhancement over the γ‐phase, showing its potential for future low cost photovoltaic devices.

Highly efficient flexible perovskite solar cells prepared by blade coating are reported. A dual hole transport layer comprised of “PEDOT:PSS/PTAA” is delicately designed, which forms a cascade energy level alignment, enabling markedly enhanced charge extraction. In conjugation with a morphology control by additive engineering, the scalable coated flexible solar cell shows an impressive efficiency of 19.41% with a record fill factor of 81%.

Abstract

Halide perovskites are one of the ideal photovoltaic materials for constructing flexible solar devices due to relatively high efficiencies for low‐temperature solution‐processed devices. However, the overwhelming majority of flexible perovskite solar cells are produced using spin coating, which represents a major hurdle for upscaling. Here, a scalable approach is reported to fabricate efficient and robust flexible perovskite solar cells on a polymer substrate. Thiourea is introduced into perovskite precursor solution to modulate the crystal growth, resulting in dense and uniform perovskite thin films on rough surfaces. As a decisive step, a cascade energy alignment is realized for the hole extraction layer by rationally designing a bilayer interface comprised of PEDOT:PSS/PTAA with a distinct offset in the highest occupied molecular orbital levels, enabling markedly enhanced charge extraction and spectral response. An efficiency as high as 19.41% and a record fill factor up to 81% are achieved for flexible perovskite devices processed by a scalable printing method. Equally important, the bilayer interface reinforces the bendability of the indium tin oxide substrate, leading to enhanced mechanical robustness of the flexible devices. These results underpin the importance of morphology control and interface design in constructing high‐performance flexible perovskite solar cells.

Phenylhydrazine hydrochloride is introduced into FASnI₃‐based perovskite solar cells (where FA = NH₂CHNH₂ ⁺) in order to reduce the existing Sn⁴⁺ and prevent the further degradation of the FASnI₃. Consequently, the champion device shows a high power conversion efficiency up to 11.4%, a long‐term storage stability over 2300 h, and an efficiency recovery capability after being exposed to air.

Abstract

The development of tin (Sn)‐based perovskite solar cells (PSCs) is hindered by their lower power conversion efficiency and poorer stability compared to the lead‐based ones, which arise from the easy oxidation of Sn²⁺ to Sn⁴⁺. Herein, phenylhydrazine hydrochloride (PHCl) is introduced into FASnI₃ (FA = NH₂CH  NH₂ ⁺) perovskite films to reduce the existing Sn⁴⁺ and prevent the further degradation of FASnI₃, since PHCl has a reductive hydrazino group and a hydrophobic phenyl group. Consequently, the device achieves a record power conversion efficiency of 11.4% for lead‐free PSCs. Besides, the unencapsulated device displays almost no efficiency reduction in a glove box over 110 days and shows efficiency recovery after being exposed to air, due to a proposed self‐repairing trap state passivation process.

A small molecule of 4,4′,4″,4′″‐(pyrazine‐2,3,5,6‐tetrayl) tetrakis (N,N‐bis(4‐methoxyphenyl) aniline) (PT‐TPA) is applied to effectively p‐dope the FA _x MA_{1− x} PbI₃ (FA:HC(NH₂)₂; MA:CH₃NH₃) perovskite surface, with obvious conductivity and carrier concentration increase. After applying PT‐TPA into perovskite solar cells, the doping‐induced band bending at the perovskite surface facilitates hole extraction to the hole‐transport layer and expels electrons toward the cathode, which reduces surface charge recombination. The optimized devices demonstrate a stabilized efficiency of 22.9%.

Abstract

Tailoring the doping of semiconductors in heterojunction solar cells shows tremendous success in enhancing the performance of many types of inorganic solar cells, while it is found challenging in perovskite solar cells because of the difficulty in doping perovskites in a controllable way. Here, a small molecule of 4,4′,4″,4″′‐(pyrazine‐2,3,5,6‐tetrayl) tetrakis (N,N‐bis(4‐methoxyphenyl) aniline) (PT‐TPA) which can effectively p‐dope the surface of FA _x MA_{1− x} PbI₃ (FA: HC(NH₂)₂; MA: CH₃NH₃) perovskite films is reported. The intermolecular charge transfer property of PT‐TPA forms a stabilized resonance structure to accept electrons from perovskites. The doping effect increases perovskite dark conductivity and carrier concentration by up to 4737 times. Computation shows that electrons in the first two layers of octahedral cages in perovskites are transferred to PT‐TPA. After applying PT‐TPA into perovskite solar cells, the doping‐induced band bending in perovskite effectively facilitates hole extraction to hole transport layer and expels electrons toward cathode side, which reduces the charge recombination there. The optimized devices demonstrate an increased photovoltage from 1.12 to 1.17 V and an efficiency of 23.4% from photocurrent scanning with a stabilized efficiency of 22.9%. The findings demonstrate that molecular doping is an effective route to control the interfacial charge recombination in perovskite solar cells which is in complimentary to broadly applied defect passivation techniques.

Incorporating silver atoms into the inorganic halide perovskite Cs₃Bi₂Br₉ to form Cs₂AgBiBr₆ eliminates the strong localization of electron–hole pairs, makes the electronic band distribution more dispersible, and further changes the photoelectric properties including band structure, exciton binding energy, charge carrier mobility, and carrier relaxation lifetime, leading to a remarkable enhancement in photocatalytic hydrogen evolution under visible light.

Abstract

Lead‐free inorganic halide perovskites have triggered appealing interests in various energy‐related applications including solar cells and photocatalysis. However, why perovskite‐structured materials exhibit excellent photoelectric properties and how the unique crystalline structures affect the charge behaviors are still not well elucidated but essentially desired. Herein, taking inorganic halide perovskite Cs₃Bi₂Br₉ as a prototype, the significant derivation process of silver atoms incorporation to induce the structural transformation from Cs₃Bi₂Br₉ to Cs₂AgBiBr₆, which brings about dramatic differences in photoelectric properties is unraveled. It is demonstrated that the silver incorporation results in the co‐operated orbitals hybridization, which makes the electronic distributions in conduction and valence bands of Cs₂AgBiBr₆ more dispersible, eliminating the strong localization of electron–hole pairs. As consequences of the electronic structures derivation, exhilarating changes in photoelectric properties like band structure, exciton binding energy, and charge carrier dynamics are verified experimentally and theoretically. Using photocatalytic hydrogen evolution activity under visible light as a typical evaluation, such crystalline structure transformation contributes to a more than 100‐fold enhancement in photocatalytic performances compared with pristine Cs₃Bi₂Br₉, verifying the significant effect of structural derivations on the exhibited performances. The findings will provide evidences for understanding the origin of photoelectric properties for perovskites semiconductors in solar energy conversion.

A methodology is developed and applied to study interfacial recombination losses in lead‐halide perovskite‐based solar cells. The study uses a variation of hole‐transport layers in combination with photoelectron spectroscopy, steady‐state and transient photoluminescence, and device simulations to show how to quantify recombination losses due to nonideal band alignment and variations in recombination dynamics.

Abstract

The interfaces between absorber and transport layers are shown to be critical for perovskite device performance. However, quantitative characterization of interface recombination has so far proven to be highly challenging in working perovskite solar cells. Here, methylammonium lead halide (CH₃NH₃PbI₃) perovskite solar cells are studied based on a range of different hole‐transport layers, namely, an inorganic hole‐transport layer CuO_x, an organic hole‐transport layer poly(triarylamine) (PTAA), and a bilayer of CuO_x/PTAA. The cells are completed by a [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM)/bathocuproine/Ag electron contact. Energy levels are characterized using photoelectron spectroscopy and recombination dynamics by combining steady‐state photoluminescence and transient photoluminescence with numerical simulations. While the PTAA‐based devices hardly show any interface recombination losses and open‐circuit voltages >1.2 V, substantial losses are observed for the samples with a direct CuO_x/perovskite interface. These losses are assigned to a combination of energetic misalignment at the CuO_x/perovskite interface coupled with increased interface recombination velocities at the perovskite/PCBM interface.

This work provides a simple, effective, and low‐cost method to fabricate a ZnO nanoparticle electron transport layer with a thickness higher than 130 nm. The doping of insulating polymer, polystyrene can not only modify the firm quality of ZnO to improve device performance, but also optimize the reproducibility, mechanical endurance, and ambient stability of the polymer‐based solar cells.

Abstract

The optimization of interfacial layer plays a critical role in the ultimate use of polymer‐based solar cells (PSCs). By introducing an insulating polymer, polystyrene (PS), into the ZnO nanoparticles (NPs) with large particle size, an electron transport layer (ETL) with a thickness of more than 130 nm is produced. The doping of PS not only improves the film quality of ZnO NPs to generate a denser, smoother, and more uniform ETL, but also increases the contact properties between the hydrophilic ZnO and hydrophobic active layer. In comparison to control devices, the power conversion efficiencies (PCEs), short circuit current densities, and fill factors of PSCs with the PS‐modified ETL for a typical fullerene system PTB7‐Th:PC₇₁BM and, also, a nonfullerene system PBDB‐T:ITIC are increased, with PCEs from 8.49% to 9.54% and 10.03% to 11.05%, respectively. The reproducibility, mechanical endurance, and ambient stability of the PSCs with the PS‐modified ZnO NP ETL are significantly improved. The combination of the insulating polymer and ZnO NPs provides a simple, low‐cost way to realize the commercialization of high performance, flexible PSCs.

A Cs₄PbI₆‐mediated method is developed to fabricate cesium (Cs)‐rich perovskite films. It is also found that ≈15% alloying with the organic formamidine (FA) cation can sufficiently stabilize the perovskite phase with excellent phase and UV‐irradiation stability. FA_0.15Cs_0.85PbI₃‐based perovskite solar cells achieve a champion power conversion efficiency of 17.5%.

Abstract

The stability issue is still one of the main limitations of the commercialization of perovskite photovoltaics. The mixed cation FA _x Cs_{1 −x} PbI₃ has shown great promise owing to its improved thermal and moisture stability. However, the study of FA _x Cs_{1 −x} PbI₃ is concentrated on formamidine (FA)‐rich perovskite, whereas cesium (Cs)‐rich FA _x Cs_{1 −x} PbI₃ perovskites are barely studied due to the inevitable phase separation when Cs > 30 mol%. Here, a Cs₄PbI₆‐mediated method is developed to synthesize Cs‐rich FA _x Cs_{1 −x} PbI₃ perovskites. It is demonstrated that Cs₄PbI₆ intermediate phase has a low Cs cation diffusion barrier and therefore offers a fast ion exchange with the preformed FA‐rich perovskite phase to finally form the Cs‐rich FA _x Cs_{1 −x} PbI₃ perovskite. The results indicate that ≈15% alloying with organic FA cations can sufficiently stabilize the perovskite phase with excellent phase and UV‐irradiation stability. The FA_0.15Cs_0.85PbI₃ perovskite solar cells achieve a champion power conversion efficiency of 17.5%, showing the great potential of Cs‐based perovskites for efficient and stable solar cells.

Conjugated polyelectrolytes (CPEs) are studied as interlayers in perovskite‐based solar cells. By modulating the ionic density in CPEs, wetting, perovskite crystal growth, and interfacial defect passivation are optimized, achieving 18.38% efficiency for a large‐area (1 cm²) device with negligible hysteresis and stable power output.

Abstract

A series of anionic conjugated polyelectrolytes (CPEs) is synthesized based on poly(fluorene‐co‐phenylene) by varying the side‐chain ionic density from two to six per repeat units (MPS2‐TMA, MPS4‐TMA, and MPS6‐TMA). The effect of MPS2, 4, 6‐TMA as interlayers on top of a hole‐extraction layer of poly(bis(4‐phenyl)‐2,4,6‐trimethylphenylamine (PTAA) is investigated in inverted perovskite solar cells (PeSCs). Owing to the improved wettability of perovskites on hydrophobic PTAA with the CPEs, the PeSCs with CPE interlayers demonstrate a significantly enhanced device performance, with negligible device‐to‐device dependence relative to the reference PeSC without CPEs. By increasing the ionic density in the MPS‐TMA interlayers, the wetting, interfacial defect passivation, and crystal growth of the perovskites are significantly improved without increasing the series resistance of the PeSCs. In particular, the open‐circuit voltage increases from 1.06 V for the PeSC with MPS2‐TMA to 1.11 V for the PeSC with MPS6‐TMA. The trap densities of the PeSCs with MPS2,4,6‐TMA are further analyzed using frequency‐dependent capacitance measurements. Finally, a large‐area (1 cm²) PeSC is successfully fabricated with MPS6‐TMA, showing a power conversion efficiency of 18.38% with negligible hysteresis and a stable power output under light soaking for 60 s.

CdIn₂S₄/In₂S₃ bulk heterojunction nanosheet arrays are designed as photoanodes of photoelectrochemical cells, which have high transparency and high separation efficiency up to 90%. This photoanode is integrated with a perovskite solar cell to form an unbiased solar water‐splitting system, delivering a solar to hydrogen conversion efficiency of 3.3%.

Abstract

The integration of photoelectrochemical photoanodes and solar cells to build an unbiased solar‐to‐hydrogen (STH) conversion system provides a promising way to solve the energy crisis. The key point is to develop highly transparent photoanodes, while its bulk separation efficiency (η_sep.) and surface injection efficiency are as high as possible. To resolve this contradiction, first a novel CdIn₂S₄/In₂S₃ bulk heterojunctions in the interior of nanosheets is designed as a photoanode with high transparency and an ultrahigh η_sep. up to 90%. Furthermore, decorating the ultrathin amorphous SnO₂ layer by atomic layer deposition, the surface oxygen‐evolution kinetics of the photoanode are increased significantly. As a result, the onset potential of the photoanode shifts negatively to 0.02 V vs RHE, and the photocurrent density boosts to 2.98 mA cm⁻² at 1.23 V vs RHE, which is ten times higher than that of pristine CdIn₂S₄. Such a high‐performance photoanode enables the integrated metal sulfide photoanode–perovskite solar cell system to deliver a STH conversion efficiency of 3.3%.

Publication date: October 2020

Source: Nano Energy, Volume 76

Author(s): Jianming Yang, Qinye Bao, Liang Shen, Liming Ding

Publication date: October 2020

Source: Nano Energy, Volume 76

Author(s): Gaoda Chai, Yuan Chang, Zhengxing Peng, Yanyan Jia, Xinhui Zou, Dian Yu, Han Yu, Yuzhong Chen, Philip C.Y. Chow, Kam Sing Wong, Jianquan Zhang, Harald Ade, Liwei Yang, Chuanlang Zhan