Jionghua Wu

Submicron GaMn quasicrystals in ferromagnetic GaAs

GaMn icosahedral particles with quasicrystalline order have been found in Mn implanted and rapidly annealed GaAs by means of selected-area electron diffraction, high-resolution, and dark-field electron microscopy. The orientation relationship between the submicron quasicrystals with the icosahedral (2/mu200935) symmetry and the face-centered cubic (fcc) GaAs matrix is determined to be: i5 (the fivefold inversion axis in (2/mu200935))//[110]GaAs, i3 (the threefold inversion axis)//[111]GaAs, and i2 (the twofold axis)//[121]GaAs. The statistics of these structural studies, combined with magnetic force microscopy, indicate that the submicron quasicrystals are ferromagnetic.

DMF as an Additive in a Two-Step Spin-Coating Method for 20% Conversion Efficiency in Perovskite Solar Cells

DMF as an additive has been employed in FAI/MAI/IPA (FA= CH2(NH2)2, MA = CH3NH3, IPA = isopropanol) solution for a two-step multicycle spin-coating method in order to prepare high-quality FAxMA1-xPbI2.55Br0.45 perovskite films. Further investigation reveals that the existence of DMF in the FAI/MAI/IPA solution can facilitate perovskite conversion, improve the film morphology, and reduce crystal defects, thus enhancing charge-transfer efficiency. By optimization of the DMF amount and spin-coating cycles, compact, pinhole-free perovskite films are obtained. The nucleation mechanisms of perovskite films in our multicycle spin-coating process are suggested; that is, the introduction of DMF in the spin-coating FAI/MAI/IPA solution can lead to the formation of an amorphous phase PbX2-AI-DMSO-DMF (X = I, Br; A = FA, MA) instead of intermediate phase (MA)2Pb3I8·2DMSO. This amorphous phase, similar to that in the one-step method, can help FAI/MAI penetrate into the PbI2 framework to completely convert into the perovskite. As high as 20.1% power conversion efficiency (PCE) has been achieved with a steady-state PCE of 19.1%. Our work offers a simple repeatable method to prepare high-quality perovskite films for high-performance PSCs and also help further understand the perovskite-crystallization process.

Fluorinated fused nonacyclic interfacial materials for efficient and stable perovskite solar cells

Three fused-ring n-type semiconductors based on 6,6,12,12-tetrakis(4-hexylphenyl)-indacenobis(dithieno[3,2-b;2,3-d]thiophene) end-capped with 1,1-dicyanomethylene-3-indanone substituted by different numbers of fluorine atoms (INIC series) are employed as interfacial materials to modify the surface of the perovskite film in inverted planar perovskite solar cells (PSCs). Due to fast interfacial charge extraction and efficient trap passivation, PSCs based on INIC series exhibit a maximum power conversion efficiency of 19.3% without any hysteresis, which is superior to control devices without INIC series (16.6%). Moreover, the strong water-resistance ability of fluorinated INIC significantly enhances the ambient stability of the PSCs. The effects of fluorine atom number on the device performance are discussed.

Investigation on the role of Lewis bases in the ripening process of perovskite films for highly efficient perovskite solar cells

Introduction of Lewis acid–base adducts is an efficient way for achieving high quality perovskite films. In this work, we investigate the influence of Lewis bases on the annealing process of perovskite films, and three Lewis bases with different melting points and boiling points including DMSO, urea and thiourea are chosen. The interaction between the Lewis base and PbI2 as well as the residual time of Lewis bases in perovskite films while annealing has been investigated. In the meantime, directly spin-coating urea or thiourea on perovskite films is also performed to study the effect of Lewis bases on the perovskite crystallinity and morphology during annealing. It is found that the existence of Lewis bases in the annealing process could significantly promote the perovskite grain growth. DMSO can be removed within a few minutes whereas the residual time of thiourea is longer than that of DMSO and urea, consistent with their boiling points. As a result, the grain sizes of the thiourea-based perovskite film increase continuously whereas the DMSO-based sample shows an obvious ripening effect only in the first few minutes. To take advantage of the ripening effect, synergistic systems of DMSO and a small amount of urea or thiourea are further adopted for fabricating perovskite solar cells (PSCs). The device based on DMSO/urea presents the best PCE of 20.06%, higher than 18.8% for the device based on DMSO and 8.35% for the device based on urea.

Photocharge accumulation and recombination in perovskite solar cells regarding device performance and stability

Photocharge accumulation and recombination in perovskite solar cells have been systematically investigated in this paper by electrochemical spectroscopy and transient photocurrent/photovoltage methods. It is found that the non-equilibrium photocharges stored in the selective charge transport layers follow a backward recombination mechanism. That is, the photocharges are first captured by the interface defects corresponding to the fast photovoltage decay, while the bulk charge recombination instead of the diffusion process dominates the slow photovoltage decay process. Further investigation reveals that the device degradation preferentially takes place at the interface under working conditions, which thus can confirm the importance of interface engineering to enhance the device stability.

Application of Cesium on the Restriction of Precursor Crystallization for Highly Reproducible Perovskite Solar Cells Exceeding 20% Efficiency

In this study, we systematically explored the mixed-cation perovskite Cs x(MA0.4FA0.6)1- xPbI3 fabricated via sequential introduction of cations. The details of the effects of Cs+ on the fabrication and performance of inorganic-organic mixed-cation perovskite solar cells examined in detail in this study are beyond the normal understanding of the adjusting band gap. It is found that a combined intercalation of Cs+ and dimethyl sulfoxide (DMSO) in PbI2-DMSO precursor film formed a strong and steady coordinated intermediate phase to retard PbI2 crystallization, suppress yellow nonperovskite δ-phase, and obtain a highly reproducible perovskite film with less defects and larger grains. The Cs-contained triple-cation-mixed perovskite Cs0.1(MA0.4FA0.6)0.9PbI3 devices yield over 20% reproducible efficiencies, superior stabilities, and fill factors of around 0.8 with a very narrow distribution.

Electropolymerization Porous Aromatic Framework Film As a Hole-Transport Layer for Inverted Perovskite Solar Cells with Superior Stability

PAF-86 film is electropolymerized (EP) by targeted monomer M1 tethered bifunctional carbozolyl moieties which not only serve in electron donation but also provide effective electrochemical (EC) active sites. The resulting PAF-86 film possesses a fairly compact surface, remarkable stability, efficient hole extraction capacity, and hole-transporting materials (HTMs) for inverted heterojunction perovskite solar cells (PSCs). Likewise, our investigation shows that PAF-86 film based perovskite solar cells (PSCs) retained about 80% power conversion efficiency (PCE) without encapsulation in air, and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) based PSCs devices reduce to 4% under the same conditions. More impressively, the electropolymerization approach is convenient, controlled, and operated at ambient conditions which elude post heat-treatments and are appropriate for industrial application.

Solvent-engineering toward CsPb(IxBr1−x)3 films for high-performance inorganic perovskite solar cells

All-inorganic perovskites have aroused great interest recently because of their prominent photoelectric properties and thermal stability for stable, highly efficient perovskite solar cells. Herein, a two-step multiple spin coating method has been developed for the first time to prepare cesium halide perovskite films. In particular, dimethyl sulfoxide (DMSO) is introduced into CsBr/methanol precursor solution to increase the solubility of CsBr; in the meantime, it can also remarkably influence the morphologies, optical properties and photovoltaic performance of CsPb(IxBr1−x)3 (0 < x < 1) films. By regulating the content of DMSO, as high as 13.27% power conversion efficiency (PCE) has been achieved with a steady-state PCE of 12.5% under optimal conditions, which can maintain 98% of the initial PCE for 480 h under ambient conditions without encapsulation. The synergistic effect of the solvent-engineering method can provide a new access for good quality all-inorganic perovskite thin films for high performance optoelectronic devices.

Fullerene derivative anchored SnO2 for high-performance perovskite solar cells

A fullerene derivative, 9-(1-(6-(3,5-bis(hydroxymethyl)phenoxy)-1-hexyl)-1H-1,2,3-triazol-4-yl)-1-nonyl [60]fullerenoacetate (C9), is employed to anchor the surface of a SnO2 electron transport layer (ETL) in planar heterojunction perovskite solar cells (PSCs). C9 suppresses charge recombination due to efficient passivation of oxygen-vacancy-related defects on the surface of the SnO2 ETL through the formation of a Lewis adduct between the under-coordinated Sn in SnO2 and hydroxyl terminal groups in C9. The C9 modifying layer enhances the extraction of photogenerated charge carriers due to the negligible injection barrier and strong electron affinity. C9 can also improve the quality of the perovskite film with enlarged grain size, reduced grain boundary and improved crystallinity. The PSCs based on C9-modified SnO2 exhibit improved photovoltaic performance with efficiency up to 21.3% with negligible hysteresis, which is superior to the control devices based on bare SnO2 (20.0%). Moreover, the ambient, photo and electric-field stability of the modified devices is also enhanced.

New two-dimensional porous graphitic carbon nitride nanosheets for highly efficient photocatalytic hydrogen evolution under visible-light irradiation

Aiming at tedious preparation procedures for 2D porous graphitic carbon nitride (2DPCN) nanosheets, a one-step thermal polymerization method based on a supramolecular precursor has been developed. As-prepared 2DPCNs exhibit enlarged specific surface areas with rich reaction sites, better crystallinity, stronger visible-light harvesting capability, aligned energy levels and faster charge transfer. A superior photocatalytic hydrogen evolution rate of 220 μmol h−1 under visible-light (λ > 420 nm) irradiation, has been achieved. In the meantime, an apparent quantum yield of 1.3% at 490 nm is also obtained, indicating that 2DPCN possesses green light activity. Our one-step thermal polymerization method based on a supramolecular precursor provides a new way to develop C3N4 photocatalysts with different morphologies and better photoelectric properties.