Chen Weijie

Despite the excellent photoelectronic properties of the all-inorganic cesium lead iodide (CsPbI₃) perovskite, which does not contain volatile and hygroscopic organic components, only a few CsPbI₃ devices are developed mainly owing to the frequent formation of an undesirable yellow δ-phase at room temperature. Herein, it is demonstrated that a small quantity of poly(ethylene oxide) (PEO) added to the precursor solution effectively inhibits the formation of the yellow δ-phase during film preparation, and promotes the development of a black α-phase at a low crystallization temperature. A systematic study reveals that a thin, dense, pinhole-free CsPbI₃ film is produced in the α-phase and is stabilized with PEO that effectively reduces the grain size during crystallization. A thin α-phase CsPbI₃ film with excellent photoluminescence is successfully employed in a light-emitting diode with an inverted configuration of glass substrate/indium tin oxide/zinc oxide/poly(ethyleneimine)/α-CsPbI₃/poly(4-butylphenyl-diphenyl-amine)/WO₃/Al, yielding the characteristic red emission of the perovskite film at 695 nm with brightness, external quantum efficiency, and emission band width of ≈101 cd m⁻², 1.12%, and 32 nm, respectively.

A small quantity of a poly(ethylene oxide) added in the precursor solution is beneficial for the development of all-inorganic CsPbI₃ perovskite in black α-phase with significantly improved ambient stability. Dense, uniform, and pinhole-free CsPbI₃ thin films consisting of tens of nanometers black α-phase crystals are successfully fabricated with excellent photophysical properties, leading to high performance light-emitting diodes.

High-quality pinhole-free perovskite film with optimal crystalline morphology is critical for achieving high-efficiency and high-stability perovskite solar cells (PSCs). In this study, a p-type π-conjugated polymer 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) is introduced into chlorobenzene to form a facile and effective template-agent during the anti-solvent process of perovskite film formation. The π-conjugated polymer PBDB-T is found to trigger a heterogeneous nucleation over the perovskite precursor film and passivate the trap states of the mixed perovskite film through the formation of Lewis adducts between lead and oxygen atom in PBDB-T. The p-type semiconducting and hydrophobic PBDB-T polymer fills in the perovskite grain boundaries to improve charge transfer for better conductivity and prevent moisture invasion into the perovskite active layers. Consequently, the PSCs with PBDB-T modified anti-solvent processing leads to a high-efficiency close to 20%, and the devices show excellent stability, retaining about 90% of the initial power conversion efficiency after 150 d storage in dry air.

p-Type π-conjugated polymer is introduced during the anti-solvent process to form high-quality pinhole-free perovskite films. Traps are passivated through Lewis adducts between the lead and oxygen atoms in the polymer. The hydrophobic polymer protects the perovskite grain boundaries against moisture invasion. The perovskite solar cells show efficiency reaching 20%, and high stability under storage, thermal stress (85 °C), and white-light illumination.

Lead-halide perovskites are well known to decompose rapidly when exposed to polar solvents, such as water. Contrary to this common-place observation, we have found that through introducing a suitable minor amount of water into the reaction mixture, we can synthesize stable CsPbBr₃ nanocrystals. The size and the crystallinity, and as a result the band gap tunability of the strongly emitting CsPbBr₃ nanocrystals correlate with the water content. Suitable amounts of water change the crystallization environment, inducing the formation of differently shaped perovskites, namely spherical NCs, rectangular nanoplatelets, or nanowires. Bright CsPbBr₃ nanocrystals with the photoluminescence quantum yield reaching 90 % were employed for fabrication of inverted hybrid inorganic/organic light-emitting devices, with the peak luminance of 4428 cd m⁻² and external quantum yield of 1.7 %.

Lead-halide perovskites are well known to decompose rapidly when exposed to polar solvents. Contrary to this, stable CsPbBr₃ nanocrystals could be synthesized through introducing a suitable minor amount of water into the reaction mixture. The size and the crystallinity, and as a result the band gap tunability of the strongly emitting CsPbBr₃ nanocrystals correlate with the water content.

In article number 1701935 by Taiho Park and co-workers, tetraethylene glycol (TEG) group is incorporated into a well-known high mobility polymer backbone to improve its solubility. The polymer, PTEG, exhibits high mobility and solubility in common organic solvents, showing the highest efficiency (19.8%) in the planar perovskite solar cell.

The synthesis and characterization of copper (I) selenocyanate (CuSeCN) and its application as a solution-processable hole-transport layer (HTL) material in transistors, organic light-emitting diodes, and solar cells are reported. Density-functional theory calculations combined with X-ray photoelectron spectroscopy are used to elucidate the electronic band structure, density of states, and microstructure of CuSeCN. Solution-processed layers are found to be nanocrystalline and optically transparent (>94%), due to the large bandgap of ≥3.1 eV, with a valence band maximum located at −5.1 eV. Hole-transport analysis performed using field-effect measurements confirms the p-type character of CuSeCN yielding a hole mobility of 0.002 cm² V⁻¹ s⁻¹. When CuSeCN is incorporated as the HTL material in organic light-emitting diodes and organic solar cells, the resulting devices exhibit comparable or improved performance to control devices based on commercially available poly(3,4-ethylenedioxythiophene):polystyrene sulfonate as the HTL. This is the first report on the semiconducting character of CuSeCN and it highlights the tremendous potential for further developments in the area of metal pseudohalides.

Copper (I) selenocyanate is successfully synthesized, studied, and applied as a wide bandgap hole-transporting material in transistors, organic solar cells, and light-emitting diodes, for the first time. Resulting devices exhibit excellent operating characteristics highlighting the tremendous potential of metal pseudohalides as a new class of highly transparent p-type semiconductors.

3D inverse opal (3D-IO) oxides are very appealing nanostructures to be integrated into the photoelectrodes of dye-sensitized solar cells (DSSCs). Due to their periodic interconnected pore network with a high pore volume fraction, they facilitate electrolyte infiltration and enhance light scattering. Nonetheless, preparing 3D-IO structures directly on nonflat DSSC electrodes is challenging. Herein, 3D-IO TiO₂ structures are prepared by templating with self-assembled polymethyl methacrylate spheres on glass substrates, impregnation with a mixed TiO₂:SiO₂ precursor and calcination. The specific surface increases from 20.9 to 30.7 m² g⁻¹ after SiO₂ removal via etching, which leads to the formation of mesopores. The obtained nanostructures are scraped from the substrate, processed as a paste, and deposited on photoelectrodes containing a mesoporous TiO₂ layer. This procedure maintains locally the 3D-IO order. When sensitized with the novel benzothiadiazole dye YKP-88, DSSCs containing the modified photoelectrodes exhibit an efficiency of 10.35% versus 9.26% for the same devices with conventional photoelectrodes. Similarly, using the ruthenium dye N719 as sensitizer an efficiency increase from 5.31% to 6.23% is obtained. These improvements originate mainly from an increase in the photocurrent density, which is attributed to an enhanced dye loading obtained with the mesoporous 3D-IO structures due to SiO₂ removal.

A significant improvement of the photocurrent density and hence the power conversion efficiency of dye-sensitized solar cells (DSSCs) is achieved by integrating mesoporous 3D inverse opal TiO₂ nanostructures into the photoelectrode. Using an organic dye for sensitization leads to efficiencies up to 10.35%, which is the highest value reported using inverse opal structures as photoactive component in the photoelectrode.

Researchers have recently revealed that hybrid lead halide perovskites exhibit ferroelectricity, which is often associated with other physical characteristics, such as a large nonlinear optical response. In this work, the nonlinear optical properties of single crystal inorganic–organic hybrid perovskite CH₃NH₃PbBr₃ are studied. By exciting the material with a 1044 nm laser, strong two-photon absorption-induced photoluminescence in the green spectral region is observed. Using the transmission open-aperture Z-scan technique, the values of the two-photon absorption coefficient are observed to be 8.5 cm GW⁻¹, which is much higher than that of standard two-photon absorbing materials that are industrially used in nonlinear optical applications, such as lithium niobate (LiNbO₃), LiTaO₃, KTiOPO₄, and KH₂PO₄. Such a strong two-photon absorption effect in CH₃NH₃PbBr₃ can be used to modulate the spectral and spatial profiles of laser pulses, as well as to reduce noise, and can be used to strongly control the intensity of incident light. In this study, the superior optical limiting, pulse reshaping, and stabilization properties of CH₃NH₃PbBr₃ are demonstrated, opening new applications for perovskites in nonlinear optics.

The two-photon absorption properties of CH₃NH₃PbBr₃ are investigated by exciting the material with a 1044 nm laser. Such a strong two-photon absorption effect can be used to modulate the spectral and spatial profiles of laser pulses. In this study, the superior optical limiting, pulse reshaping, and stabilization properties of CH₃NH₃PbBr₃ are demonstrated.

Cesium lead iodide (CsPbI₃) perovskite, an all-inorganic halide perovskite, is synthesized on a platinum-coated silicon substrate for an ultra-low operating voltage resistive switching memory device by Soo Young Kim, Ho Won Jang, and co-workers in article number 1705783. An electrochemical metallization mechanism involving metal conducting filaments is proposed to explain the resistive switching behavior which can be applied to next-generation synaptic devices.

Optimizing the interfacial contacts between the photoactive layer and the electrodes is an important factor in determining the performance of organic solar cells (OSCs). A charge-selective layer with tailored electrical properties enhances the charge collection efficiency and interfacial stability. Here, the potential of hydrogenated TiO₂ nanoparticles (H-TiO₂ NPs) as an efficient electron-selective layer (ESL) material in OSCs is reported for the first time. The H-TiO₂ is synthesized by discharge plasma in liquid at atmospheric pressure, which has the benefits of a simple one-pot synthesis process, rapid and mild reaction conditions, and the capacity for mass production. The H-TiO₂ exhibits high conductivity and favorable energy level formation for efficient electron extraction, providing a basis for an efficient bilayer ESL system composed of conjugated polyelectrolyte/H-TiO₂. Thus, the enhanced charge transport and extraction efficiency with reduced recombination losses at the cathode interfacial contacts is achieved. Moreover, the OSCs composed of H-TiO₂ are almost free of light soaking, which has been reported to severely limit the performance and stability of OSCs based on conventional TiO₂ ESLs. Therefore, H-TiO₂ as a new efficient, stable, and cost-effective ESL material has the potential to open new opportunities for optoelectronic devices.

This study demonstrates the potential of hydrogenated TiO₂ (H-TiO₂) as an efficient electron-selective layer in optoelectronic devices. The H-TiO₂ is simply one-pot mass-produced using a discharge plasma system in liquid at atmospheric pressure. The H-TiO₂ exhibits high conductivity and favorable energy level formation, resulting in the high-efficiency and light-soaking-free organic solar cells.

In the past years, hybrid perovskite materials have attracted great attention due to their superior optoelectronic properties. In this study, the authors report the utilization of cobalt (Co²⁺) to partially substitute lead (Pb²⁺) for developing novel hybrid perovskite materials, CH₃NH₃Pb_1-xCo_xI₃ (where x is nominal ratio, x = 0, 0.1, 0.2 and 0.4). It is found that the novel perovskite thin films possess a cubic crystal structure with superior thin film morphology and larger grain size, which is significantly different from pristine thin film, which possesses the tetragonal crystal structure, with smaller grain size. Moreover, it is found that the 3d orbital of Co²⁺ ensures higher electron mobilities and electrical conductivities of the CH₃NH₃Pb_1-xCo_xI₃ thin films than those of pristine CH₃NH₃Pb₄ thin film. As a result, a power conversion efficiency of 21.43% is observed from perovskite solar cells fabricated by the CH₃NH₃Pb_0.9Co_0.1I₃ thin film. Thus, the utilization of Co, partially substituting for Pb to tune physical properties of hybrid perovskite materials provides a facile way to boost device performance of perovskite solar cells.

The utilization of cobalt (Co²⁺) to partially substitute lead (Pb²⁺) for developing novel hybrid perovskite materials and perovskite solar cells is reported. A power conversion efficiency of 21.43% is observed from perovskite solar cells fabricated by the CH₃NH₃Pb_0.9Co_0.1I₃ thin film, which is due to it possessing a cubic crystal structure with superior thin film morphology and larger grain size.

Increasing the power conversion efficiency (PCE) of the two-dimensional (2D) perovskite-based solar cells (PVSCs) is really a challenge. Vertical orientation of the 2D perovskite film is an efficient strategy to elevate the PCE. In this work, vertically orientated highly crystalline 2D (PEA)₂(MA)_n–1Pb_nI_3n+1 (PEA= phenylethylammonium, MA = methylammonium, n = 3, 4, 5) films are fabricated with the assistance of an ammonium thiocyanate (NH₄SCN) additive by a one-step spin-coating method. Planar-structured PVSCs with the device structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/(PEA)₂(MA)_n–1Pb_nI_3n+1/[6,6]-phenyl-C61-butyric acid methyl ester/bahocuproine/Ag are fabricated. The PCE of the PVSCs is boosted from the original 0.56% (without NH₄SCN) to 11.01% with the optimized NH₄SCN addition at n = 5, which is among the highest PCE values for the low-n (n < 10) 2D perovskite-based PVSCs. The improved performance is attributed to the vertically orientated highly crystalline 2D perovskite thin films as well as the balanced electron/hole transportation. The humidity stability of this oriented 2D perovskite thin film is also confirmed by the almost unchanged X-ray diffraction patterns after 28 d exposed to the moisture in a humidity-controlled cabinet (H_r = 55 ± 5%). The unsealed device retains 78.5% of its original PCE after 160 h storage in air atmosphere with humidity of 55 ± 5%. The results provide an effective approach toward a highly efficient and stable PVSC for future commercialization.

The advantage of phenylethylammonium (PEA⁺) in forming pinhole-free 2D (PEA)₂(methylammonium (MA))_n−1Pb_nI_3n+1 n = 3, 4, 5) perovskite film with vertical orientation and high crystallinity under assistance of an ammonium thiocyanate additive by one-step spin-coating method is demonstrated. The optimized planar-structured perovskite solar cell based on vertically oriented (PEA)₂(MA)₄Pb₅I₁₆ (n = 5) film presents the best power conversion efficiency of 11.01% with excellent stability.

Three acceptor–donor–acceptor type nonfullerene acceptors (NFAs), namely, F–F, F–Cl, and F–Br, are designed and synthesized through a halogenation strategy on one successful nonfullerene acceptor FDICTF (F–H). The three molecules show red-shifted absorptions, increased crystallinities, and higher charge mobilities compared with the F–H. After blending with donor polymer PBDB-T, the F–F-, F–Cl-, and F–Br-based devices exhibit power conversion efficiencies (PCEs) of 10.85%, 11.47%, and 12.05%, respectively, which are higher than that of F–H with PCE of 9.59%. These results indicate that manipulating the absorption range, crystallinity and mobilities of NFAs by introducing different halogen atoms is an effective way to achieve high photovoltaic performance, which will offer valuable insight for the designing of high-efficiency organic solar cells.

Through a halogenation strategy onto the end-capping group in the FDICTF-based small-molecule acceptor, red-shifted absorptions, increased crystallinities, and higher charge mobilities are achieved. The device based on F–Br with power conversion efficiency of 12.05% and remarkable FF of 76% is one of only a few organic solar cells with efficiencies over 12% reported to date.

A bifunctional conjugated organic molecule 4-(aminomethyl) benzoic acid hydroiodide (AB) is designed and employed as an organic cation in organic–inorganic halide perovskite materials. Compared with the monofunctional cation benzylamine hydroiodide (BA) and the nonconjugated bifunctional organic molecule 5-ammonium valeric acid, devices based on AB-MAPbI₃ show a good stability and a superior power conversion efficiency of 15.6% with a short-circuit current of 23.4 mA cm⁻², an open-circuit voltage of 0.94 V, and a fill factor of 0.71. The bifunctional conjugated cation not only benefits the growth of perovskite crystals in the mesoporous network, but also facilitates the charge transport. This investigation helps explore new approaches to rational design of novel organic cations for perovskite materials.

A bifunctional conjugated organic molecule AB is designed and employed as an organic cation in perovskite materials. Compared with the monofunctional cation BA and the nonconjugated bifunctional cation AVA, the devices based on AB-MAPbI₃ show superior efficiency with good stability. The bifunctional-conjugated cation not only benefits the growth of perovskite crystals in the mesoporous network, but also facilitates the charge transport.

Adding cesium (Cs) and rubidium (Rb) cations to FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃ hybrid lead halide perovskites results in a remarkable improvement in solar cell performance, but the origin of the enhancement has not been fully understood yet. In this work, time-of-flight, time-resolved microwave conductivity, and thermally stimulated current measurements are performed to elucidate the impact of the inorganic cation additives on the trap landscape and charge transport properties within perovskite solar cells. These complementary techniques allow for the assessment of both local features within the perovskite crystals and macroscopic properties of films and full devices. Strikingly, Cs-incorporation is shown to reduce the trap density and charge recombination rates in the perovskite layer. This is consistent with the significant improvements in the open-circuit voltage and fill factor of Cs-containing devices. By comparison, Rb-addition results in an increased charge carrier mobility, which is accompanied by a minor increase in device efficiency and reduced current–voltage hysteresis. By mixing Cs and Rb in quadruple cation (Cs-Rb-FA-MA) perovskites, the advantages of both inorganic cations can be combined. This study provides valuable insights into the role of these additives in multiple-cation perovskite solar cells, which are essential for the design of high-performance devices.

Time-resolved microwave conductivity, time-of-flight, and thermally stimulated current measurements reveal that Cs reduces the trap density in hybrid lead halide perovskites. Rb additives enhance the charge carrier mobility, but show minor effects on the trap landscape. The increase in open-circuit voltage in multiple-cation perovskite solar cells can be related to a reduced trap density through Cs-incorporation.