shuhui

Conjugated polymers act as a hydrophobic interlayer with superb hole mobility between perovskite and doped spiro‐OMeTAD, enhancing the device stability and performance.

Hybrid organic–inorganic perovskite (HOIP) solar cells have achieved a certified power conversion efficiency (PCE) of 22.7%, which commonly use doped spiro‐OMeTAD as hole transport materials (HTMs). However, the additives in spiro‐OMeTAD can absorb moisture and cause the degradation of HOIP layers, leading to severe air‐instability of devices. Herein, conjugated polymers of PD‐10‐DTTE‐7 as a new effective interlayer between perovskite and doped spiro‐OMeTAD to achieve air‐stable efficient perovskite solar cells are reported. Its hydrophobic nature can effectively prevent the penetration of moisture and additives. Its superb hole mobility (9.54 cm² V⁻¹ s⁻¹, ≈10⁵ times higher than spiro‐OMeTAD) and suitable highest occupied molecular orbital level (−5.33 eV) are preferable to the hole injection and transport at the interface thus enhancing the device PCE. As a result, the MAPbI₃ solar cells with the PD‐10‐DTTE‐7 interlayer achieve remarkable device air‐stability and enhanced PCE, compared with the devices without the interlayer. These results provide a feasible approach to enhance solar cell stability and performance simultaneously.

The sulfonic zwitterion combines the functions of morphology tailoring and defect passivation together into one kind of functional molecule, and this “all‐in‐one” system provides a facile but effective pathway for the fabrication of high‐performance perovskite solar cells.

Abstract

Uniform and high‐electronic‐quality perovskite thin films are essential for high‐performance perovskite devices. Here, it is shown that the 3‐(decyldimethylammonio)‐propane‐sulfonate inner salt (DPSI), which is a sulfonic zwitterion, plays dual roles in tuning the crystallization behavior and passivating the defects of perovskites. The synergistic effect of crystallization control and defect passivation remarkably suppresses pinhole formation, reduces the charge trap density, and lengthens the carrier recombination lifetime, and thereafter boosts the small‐area (0.08 cm²) planar perovskite device efficiency to 21.1% and enables a high efficiency of 18.3% for blade‐coating large‐area (1 cm²) devices. The device also shows good light stability, which remains at 88% of the initial efficiency under continuous unfiltered AM 1.5G light illumination for 480 h. These findings provide an avenue for simultaneous crystallization control and defect passivation to further improve the performance of perovskite devices.

The impact of Rashba effects in halide perovskites is still under debate. Using femtosecond transient absorption and photoluminescence, it is shown that luminescence from hot carriers is weaker than that of cold carriers, as expected from strongly radiative transitions in direct gap semiconductors. Several possible resolutions to this, including lattice dynamics that overcome Rashba splittings at room temperature are considered.

Abstract

The generation and recombination of charge carriers in semiconductors through photons controls photovoltaic and light‐emitting diode operation. Understanding of these processes in hybrid perovskites has advanced, but remains incomplete. Using femtosecond transient absorption and photoluminescence, it is observed that the luminescence signal shows a rise over 2 ps, while initially hot photogenerated carriers cool to the band edge. This indicates that the luminescence from hot carriers is weaker than that of cold carriers, as expected from strongly radiative transitions in direct gap semiconductors. It is concluded that the electrons and holes show a strong overlap in momentum space, despite recent proposals that Rashba splitting leads to a band offset suppressing such an overlap. A number of possible resolutions to this, including lattice dynamics that remove the Rashba splitting at room temperature, and localization of luminescence events to length scales below 10 nm are considered.

Ultrastable wide‐bandgap conjugated polymers are obtained via effective isolation and encapsulation of a chain based on a supramolecular self‐encapsulation mechanism, which preserves the fundamental optoelectronic property of the conjugated backbone core and also improves the spectral and morphological stability. Thickness‐insensitive emission behavior enables its excellent film‐processing ability with a higher reproducibility. Larger‐scale flexible deep‐blue electroluminescent devices with a single‐chain excitonic behavior are also fabricated.

Abstract

Controlling chain behavior through smart molecular design provides the potential to develop ultrastable and efficient deep‐blue light‐emitting conjugated polymers (LCPs). Herein, a novel supramolecular self‐encapsulation strategy is proposed to construct a robust ultrastable conjugated polydiarylfluorene (PHDPF‐Cz) via precisely preventing excitons from interchain cross‐transfer/coupling and contamination from external trace H₂O/O₂. PHDPF‐Cz consists of a mainchain backbone where the diphenyl groups localize at the 9‐position as steric bulk moieties, and carbazole (Cz) units localize at the 4‐position as supramolecular π‐stacked synthon with the dual functionalities of self‐assembly capability and hole‐transport facility. The synergistic effect of the steric bulk groups and π‐stacked carbazoles affords PHDPF‐Cz as an ultrastable property, including spectral, morphological stability, and storage stability. In addition, PHDPF‐Cz spin‐coated gelation films also show thickness‐insensitive deep‐blue emission with respect to the reference polymers, which are suitable to construct solution‐processed large‐scale optoelectronic devices with higher reproducibility. High‐quality and uniform deep‐blue emission is observed in large‐area solution‐processed films. The electroluminescence shows high‐quality deep‐blue intrachain emission with a CIE (0.16, 0.12) and a very narrow full width at half‐maximum of 32 nm. Finally, large‐area and flexible polymer light‐emitting devices with a single‐molecular excitonic behavior are also fabricated. The supramolecular self‐encapsulation design provides an effective strategy to construct ultrastable LCPs for optoelectronic applications.

Publication date: 16 January 2019

Source: Joule, Volume 3, Issue 1

Author(s): Wanchun Xiang, Zaiwei Wang, Dominik J. Kubicki, Wolfgang Tress, Jingshan Luo, Daniel Prochowicz, Seckin Akin, Lyndon Emsley, Jiangtao Zhou, Giovanni Dietler, Michael Grätzel, Anders Hagfeldt

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Electron backscatter diffraction (EBSD) combined with carrier lifetime, mobility, and diffusion length measurements, shows that optical and scanning electron microscopy images cannot accurately predict grain boundary positions or their resulting properties. Grain boundaries passivated by amorphous perovskite display increased photoluminescence lifetime and intensity. This suggests that crystallographic (not only chemical) effects play an important role in halide perovskite grain boundary properties.

Abstract

Grain boundaries play a key role in the performance of thin‐film optoelectronic devices and yet their effect in halide perovskite materials is still not understood. The biggest factor limiting progress is the inability to identify grain boundaries. Noncrystallographic techniques can misidentify grain boundaries, leading to conflicting literature reports about their influence; however, the gold standard – electron backscatter diffraction (EBSD) – destroys halide perovskite thin films. Here, this problem is solved by using a solid‐state EBSD detector with 6000 times higher sensitivity than the traditional phosphor screen and camera. Correlating true grain size with photoluminescence lifetime, carrier diffusion length, and mobility shows that grain boundaries are not benign but have a recombination velocity of 1670 cm s⁻¹, comparable to that of crystalline silicon. Amorphous grain boundaries are also observed that give rise to locally brighter photoluminescence intensity and longer lifetimes. This anomalous grain boundary character offers a possible explanation for the mysteriously long lifetime and record efficiency achieved in small grain halide perovskite thin films. It also suggests a new approach for passivating grain boundaries, independent of surface passivation, to lead to even better performance in optoelectronic devices.

Publication date: 19 December 2018

Source: Joule, Volume 2, Issue 12

Author(s): Tiankai Zhang, Mingzhu Long, Minchao Qin, Xinhui Lu, Si Chen, Fangyan Xie, Li Gong, Jian Chen, Ming Chu, Qian Miao, Zefeng Chen, Wangying Xu, Pengyi Liu, Weiguang Xie, Jian-bin Xu

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Publication date: 16 January 2019

Source: Joule, Volume 3, Issue 1

Author(s): Bo Chen, Zhengshan Yu, Kong Liu, Xiaopeng Zheng, Ye Liu, Jianwei Shi, Derrek Spronk, Peter N. Rudd, Zachary Holman, Jinsong Huang

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This paper reports an improvement in light conversion efficiency of a single crystalline Si solar cell by over‐coating it with copper modified mesoporous TiO₂ particles. J‐V curves, diffused reflectance, short circuit current ratio versus wavelength, and recombination lifetime studies revealed that copper modified mesoporous TiO₂ performs better than mesoporous TiO₂ particles. The improvement in light conversion efficiency is attributed to surface passivation.

In the present work, an improvement in light‐conversion efficiency by coating spherical particles of mesoporous TiO₂ (MT) and copper‐modified mesoporous TiO₂ (CMT) on single‐crystalline Si solar PV cell is observed. The studies are carried out by spin coating with different concentrations of MT (0.25 to 1.5%) and CMT (0.25 to 1.0%) particles on bare cells independently and compared with a bare solar cell throughout the work. It is observed that in the case of MT coatings, the conversion efficiency increases initially and reaches a maximum of 9.77% at a coating concentration of 1.0%; thereafter, it decreases with an increase in coating concentration. This decrease in efficiency may be attributed to coagulation of MT particles, whereas in the case of CMT particles the conversion efficiency reached a maximum of 8.85% at 0.5% concentration and decreases thereafter. This study indicates that a higher efficiency can be achieved at lower concentrations of CMT coating compared to MT coatings on a bare solar cell. The increase in efficiency may be attributed to better surface passivation and/or trapping of electrons at the metal sites by Cu in CMT. Recombination lifetime measurements results corroborated that CMT particles show better surface passivation as a result of which there is an increase in efficiency. Diffused reflectance and short‐circuit current ratio versus wavelength studies on MT and CMT also revealed that CMT particles perform better than MT particles.

High‐crystallinity quasi‐2D perovskite films with oriented structure are fabricated by using 3‐bromobenzylammonium iodide, leading to perovskite solar cells with a high efficiency of 18.20%. Moreover, the unencapsulated devices exhibit excellent moisture resistance, retaining 82% of the initial efficiency after 2400 h under ambient conditions. Even after immersion into water for 60 s, the unsealed device shows little decay.

Abstract

Quasi‐2D layered organometal halide perovskites have recently emerged as promising candidates for solar cells, because of their intrinsic stability compared to 3D analogs. However, relatively low power conversion efficiency (PCE) limits the application of 2D layered perovskites in photovoltaics, due to large energy band gap, high exciton binding energy, and poor interlayer charge transport. Here, efficient and water‐stable quasi‐2D perovskite solar cells with a peak PCE of 18.20% by using 3‐bromobenzylammonium iodide are demonstrated. The unencapsulated devices sustain over 82% of their initial efficiency after 2400 h under relative humidity of ≈40%, and show almost unchanged photovoltaic parameters after immersion into water for 60 s. The robust performance of perovskite solar cells results from the quasi‐2D perovskite films with hydrophobic nature and a high degree of electronic order and high crystallinity, which consists of both ordered large‐bandgap perovskites with the vertical growth in the bottom region and oriented small‐bandgap components in the top region. Moreover, due to the suppressed nonradiative recombination, the unencapsulated photovoltaic devices can work well as light‐emitting diodes (LEDs), exhibiting an external quantum efficiency of 3.85% and a long operational lifetime of ≈96 h at a high current density of 200 mA cm⁻² in air.

Surface‐terminating grain boundaries in perovskite oxides are identified to be exceptional in oxygen evolution electrocatalysis. Atomic‐scale direct observation and density functional theory calculations demonstrate that symmetry‐broken atom displacement in oxygen octahedra at grain boundaries strongly correlates with easier charge transfer between metals and oxygen for remarkable enhancement of the oxygen electrocatalytic efficiency.

Abstract

A grain boundary forms as an internal interface when two crystalline grains with mutually different crystallographic orientations are in direct contact with each other. As a result, atomic arrangement at grain boundaries differs from that of the bulk, showing serious displacements deviating from the original symmetric positions. As these symmetry‐broken configurations are difficult to achieve in the bulk crystals, grain boundaries are considered distinctive platforms that can exhibit new physical properties. By using both sintered polycrystals with various grain sizes and thin films on bicrystal substrates, it is directly verified that surface‐terminating grain boundaries in LaCoO₃ and LaMnO₃ are exceptional in oxygen evolution electrocatalysis, showing more than an order of magnitude higher activity. A combination of atomic‐scale structure observation and density functional theory calculations demonstrates that the displacement of atoms in metal–oxygen octahedra correlates with significant splitting of the degenerate transition‐metal 3d orbitals, and subsequently much easier charge transfer between metals and oxygen is attained. In addition to identifying the grain boundaries as strikingly active sites, the findings suggest that symmetry breaking by atom displacements in metal–oxygen octahedra is an efficient approach to remarkably enhance the oxygen electrocatalytic efficiency in perovskite oxides.

The light intensity dependence of interfacial charge transfer efficiency in methylammonium lead iodide (MAPI₃) perovskite bilayers with electron/hole transfer layers is investigated using a range of steady‐state and time‐resolved optical techniques, allowing quantification of the kinetic competition between charge transfer versus recombination and trapping under irradiation conditions relevant to device operation.

Abstract

This study addresses the dependence of charge transfer efficiency between bilayers of methylammonium lead iodide (MAPI₃) with PC₆₁BM or poly(3,4‐ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) charge transfer layers on excitation intensity. It analyzes the kinetic competition between interfacial electron/hole transfer and charge trapping and recombination within MAPI₃ by employing a range of optical measurements including steady‐state (SS) photoluminescence quenching (PLQ), and transient photoluminescence and absorption over a broad range of excitation densities. The results indicate that PLQ measurements with a typical photoluminescence spectrometer can yield significantly different transfer efficiencies to those measured under 1 Sun irradiation. Steady‐state and pulsed measurements indicate low transfer efficiencies at low excitation conditions (<5E + 15 cm⁻³) due to rapid charge trapping and low transfer efficiencies at high excitation conditions (>5E + 17 cm⁻³) due to fast bimolecular recombination. Efficient transfer to PC₆₁BM or PEDOT:PSS is only observed under intermediate excitation conditions (≈1 Sun irradiation) where electron and hole transfer times are determined to be 36 and 11 ns, respectively. The results are discussed in terms of their relevance to the excitation density dependence of device photocurrent generation, impact of charge trapping on this dependence, and appropriate methodologies to determine charge transfer efficiencies relevant to device performance.

Zn_0.8Cd_0.2S nanoparticles (ZCS) are doped in [6,6]‐phenyl‐C61‐butyric acid methyl (PCBM) to form an inorganic/organic hybrid as an efficient electron transport layer (ETL) in p‐i‐n planar perovskite solar cells. The ZCS@PCBM interlayer improves electron extraction, enhances electron transportation, and suppresses charge recombination. The shielding effect of ZCS nanoparticles can keep the perovskite from erosion by ambient moisture, thus improving the device stability.

In this study, an inorganic/organic hybrid, Zn_0.8Cd_0.2S nanoparticles (ZCS) embedded in [6,6]‐phenyl‐C61‐butyric acid methyl (PCBM), as an efficient electron transport layer (ETL) for air‐processed p‐i‐n perovskite solar cells (PSCs) has been demonstrated, and the doping effect and doping mechanism are systematically studied. As compared to PCBM, ZCS@PCBM ETL exhibits improved electron extraction at the perovskite/ETL interface, increased electron transportation within the ETL, enhanced charge collection efficiency, and suppressed interfacial charge recombination, resulting in significantly improved power conversion efficiency (PCE) from 14.41 to 17.18% by 19.2%. Interestingly, the ZCS nanoparticles can protect the perovskite layer from erosion by ambient moisture, and 82% of the initial PCE for the non‐encapsulated devices with ZCS@PCBM ETL is retained after 500 h storage in the atmosphere (humidity 30–60%) versus only 13% of the initial PCE for the PCBM ETL without ZCS doping.