Chen Weijie

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.

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.

Nitrogen‐doped carbon nano‐onions (N‐CNOs) are embedded in a modified graphene matrix (mGr). The mGr has superior electrical conductivity for charge transfer, and the N‐CNOs provide more catalytic active sites to improve electrocatalytic activity. The dye‐sensitized solar cell with the N‐CNOs/mGr as counter electrode exhibits low charge‐transfer resistance, long‐term stability, and a power conversion efficiency of 10.28%.

A desirable counter electrode material for dye‐sensitized solar cells (DSSCs) needs to have superior electrocatalytic activity, low charge‐transfer resistance, and long‐term stability. Herein, the development of a composite of nitrogen‐doped carbon nano‐onions with modified reduced graphene (N‐CNOs/mGr) to achieve these merits is reported. The mGr network has high electrical conductivity to improve charge transfer; the N‐CNOs with pyridinic and graphitic N provide more electrocatalytic active sites for the reduction of I ₃ ⁻ to I ⁻, and the carbon composite demonstrates excellent electrochemical stability. The constructed DSSC with the N‐CNOs/mGr electrode presents better long‐term stability and a higher power conversion efficiency of 10.28% than those devices with conventional Pt (6.54%) and mGr (5.11%) electrodes. Therefore, the all carbon‐based composite will open up new opportunities for a variety of electrochemical device applications.

Sb₂O₃/Ag/Sb₂O₃ multilayer electrodes are used in organic solar cells (OSCs) to demonstrate colorful semitransparent devices. Notably, the effect of thickness of each layer in the electrode on the optical properties and performance of organic solar cells is thoroughly investigated. By matching the electrodes with the absorption characteristics of the active layers, esthetically colored, semitransparent OSCs can be fabricated.

The pace of advancement and increasing power conversion efficiencies (PCEs) have provided the possibility for organic solar cells (OSCs) to be commercialized in a variety of applications, including semitransparent photovoltaics. The application of dielectric–metal–dielectric (DMD) transparent electrodes to OSCs is an effective way to achieve semitransparent OSCs with different colors. Herein, a DMD multilayer structure based on two Sb₂O₃ layers and silver (Ag) thin films as the top electrode is introduced. An ultrathin Sb₂O₃ layer is deposited between the electron transport layer and the Ag film as a bottom layer for the Sb₂O₃/Ag/Sb₂O₃ electrode; this layer inhibits Ag atom diffusion and aggregation, which leads to uniform formation of ultrathin Ag films. The thickness of the middle metal layer influences fill factor and short‐circuit current density values, which correlate well with device resistance and light reflection. For the top Sb₂O₃ layer, the thickness allows the selective transmittance of specific wavelengths of visible light while reflecting wavelengths that are not transmitted, creating a dichroic effect. OSCs are successfully fabricated using three kinds of colorful active layers in conjunction with Sb₂O₃/Ag/Sb₂O₃ electrodes, resulting in vividly colored devices with PCE of 6.33–7.88% and average visible transmittances of 23–30%.

A well aligned CsPbBr₃ micro–nanowire (MW) array is synthesized by controlling the growth of the intermediate CsBr MW array originated from the deposition of the 2D CsPb₂Br₅ phase. Furthermore, a high‐performance photodetector is demonstrated based on the CsPbBr₃ MW array.

Abstract

A large number of derivative phases in inorganic perovskites are reported with special structures and extraordinary performances in photoelectronic device applications. The reverse phase transition between derivative phases and perovskites usually induces recrystallization or forms mixed components. In this work, derivative phase‐induced growth of the CsPbBr₃ micro–nanowire (MW) array by utilizing phase transition of the 2D CsPb₂Br₅ phase is reported. Owing to its layered structure and phase transition, annealing of CsPb₂Br₅ at a temperature of 550 °C combined with solvent quenching leads to a templating effect to form a high‐quality CsBr MW array. Subsequent PbBr₂ deposition and the second annealing are employed to form aligned CsPbBr₃ MW arrays. Based on this method, a CsPbBr₃ MW array‐based photodetector is fabricated. The large grain size, less grain boundaries, and lower surface potential of the CsPbBr₃ MW array lead to high device performance with a responsivity of 7.66 A W⁻¹, a detectivity of ≈10¹² Jones, and long‐term operational stability over 1900 min.

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%.

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.