Shufeng Wang

Direct Observation of Long Electron-Hole Diffusion Distance in CH3NH3PbI3 Perovskite Thin Film

In high performance perovskite based solar cells, CH3NH3PbI3 is the key material. We carried out a study on charge diffusion in spin-coated CH3NH3PbI3 perovskite thin film by transient fluorescent spectroscopy. A thickness-dependent fluorescent lifetime was found. By coating the film with an electron or hole transfer layer, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) respectively, we observed the charge transfer directly through the fluorescence quenching. One-dimensional diffusion model was applied to obtain long charge diffusion distances in thick films, which is ~1.7u2009μm for electrons and up to ~6.3u2009μm for holes. Short diffusion distance of few hundreds of nanosecond was also observed in thin films. This thickness dependent charge diffusion explained the formerly reported short charge diffusion distance (~100u2009nm) in films and resolved its confliction to thick working layer (300–500u2009nm) in real devices. This study presents direct support to the high performance perovskite solar cells and will benefit the devices’ design.

Stable high-performance hybrid perovskite solar cells with ultrathin polythiophene as hole-transporting layer

Ultrathin polythiophene films prepared via electrochemical polymerization is successfully used as the hole-transporting material, substituting conventional HTM-PEDOT:PSS, in planar p-i-n CH3NH3PbI3 perovskite-based solar cells, affording a series of ITO/polythiophene/CH3NH3PbI3/C60/BCP/Ag devices. The ultrathin polythiophene film possesses good transmittance, high conductivity, a smooth surface, high wettability, compatibility with PbI2 DMF solution, and an energy level matching that of the CH3NH3PbI3 perovskite material. A promising power conversion efficiency of about 15.4%, featuring a high fill factor of 0.774, open voltage of 0.99 V, and short-circuit current density of 20.3 mA·cm−2 is obtained. The overall performance of the devices is superior to that of cells using PEDOT:PSS. The differences of solar cells with different hole-transfer materials in charge recombination, charge transport and transfer, and device stability are further investigated and demonstrate that polythiophene is a more effective and promising hole-transporting material. This work provides a simple, prompt, controllable, and economic approach for the preparation of an effective hole-transporting material, which undoubtedly offers an alternative method in the future industrial production of perovskite solar cells.

Hole-conductor-free planar perovskite solar cells with 16.0% efficiency

In the present work, a hole-conductor-free inverted-structure planar perovskite solar cell was fabricated by a solution process. Remarkably, the device showed a power conversion efficiency (PCE) up to 16.0%, which is higher than those of reported hole-conductor-free perovskite solar cells, and even higher than that of devices using NiOx as the hole conductor. Furthermore, the hole-conductor-free device was also found to reveal a very good stability.

Increasing open circuit voltage by adjusting work function of hole-transporting materials in perovskite solar cells

A series of conductive polymers, i.e., poly(3-methylthiophene) (PMT), poly(thiophene) (PT), poly(3-bromothiophene) (PBT) and poly(3-chlorothiophene) (PCT), were prepared via the electrochemical polymerization process. Subsequently, their application as hole-transporting materials (HTMs) in CH3NH3PbI3 perovskite solar cells was explored. It was found that rationally increasing the work function of HTMs proves beneficial in improving the open circuit voltage (Voc) of the devices with an ITO/conductive-polymer/CH3NH3PbI3/C60/BCP/Ag structure. In addition, the higher-Voc devices with a higher-work-function HTM exhibited higher recombination resistances. The highest open circuit voltage of 1.04 V was obtained from devices with PCT, with a work function of–5.4 eV, as the hole-transporting layer. Its power conversion efficiency attained a value of approximately 16.5%, with a high fill factor of 0.764, an appreciable open voltage of 1.01 V and a short circuit current density of 21.4 mA·cm–2. This simple, controllable and low-cost manner of preparing HTMs will be beneficial to the production of large-area perovskite solar cells with a hole-transporting layer.

The Dawn of Lead‐Free Perovskite Solar Cell: Highly Stable Double Perovskite Cs2AgBiBr6 Film

Abstract Recently, lead‐free double perovskites have emerged as a promising environmentally friendly photovoltaic material for their intrinsic thermodynamic stability, appropriate bandgaps, small carrier effective masses, and low exciton binding energies. However, currently no solar cell based on these double perovskites has been reported, due to the challenge in film processing. Herein, a first lead‐free double perovskite planar heterojunction solar cell with a high quality Cs2AgBiBr6 film, fabricated by low‐pressure assisted solution processing under ambient conditions, is reported. The device presents a best power conversion efficiency of 1.44%. The preliminary efficiency and the high stability under ambient condition without encapsulation, together with the high film quality with simple processing, demonstrate promise for lead‐free perovskite solar cells.

The molecular picture of amplified spontaneous emission of star-shaped functionalized-truxene derivatives

Organic optical gain materials are the basis for organic solid-state lasers. However, the fundamental mechanism for Amplified Spontaneous Emission (ASE) is absent. Herein, three star-shaped molecules based on a truxene core with π-conjugated arms are studied to illuminate the influence of the molecular structure on ASE performance. We found that the three compounds demonstrated different ASE characteristics. The strong conjugated linkage in the molecular arms enhanced ASE, while the attenuated conjugated linkage deteriorates ASE for the given molecular structures. Based on the theoretical analysis, the conjugative coupling suppresses the low-frequency vibration, which is beneficial to the formation of an effective “four-level” energy system for ASE. On the contrary, the poor conjugative coupling brings out a mass of low-frequency modes, and hence, continuous vibronic energy sublevels will ruin the molecular “four-level” energy system. Our study presented a clear picture to clarify the ASE mechanism and offers valuable guidance for the design of new organic optical gain materials.

A combinational molecular design to achieve highly efficient deep-blue electrofluorescence

A deep-blue emitter 1-(10-(4-methoxyphenyl)anthracen-9-yl)-4-(10-(4-cyanophenyl)anthracen-9-yl)tetraphenylethene (TPEA) has been successfully prepared by a combinational molecular design, which contains triplet–triplet fusion (TTF) and hybridized local charge transfer (HLCT) characteristics to increase the ratio of triplet excitons used. The tetraphenylethene (TPE) moiety contributes the emitter with an aggregation-induced emission (AIE) property to enhance the solid-state luminescence efficiency. The crystallographic structure shows that the two anthracene groups are twisted from the central TPE moiety, which effectively prevents a bathochromic shift of the emission. In addition, we adopted a donor–acceptor (D–A) structure to improve the charge balance in organic light-emitting diodes (OLEDs). The material possesses high thermal stability with a glass transition temperature (Tg) of 155 °C. Based on all these advantages, a high performance of the non-doped device was achieved with a turn-on voltage (Von) of 2.6 V at a luminance of 1 cd m−2, a maximum power efficiency (ηPE,max) of 11.1 lm W−1, a maximum current efficiency (ηCE,max) of 9.9 cd A−1, and a low current efficiency roll-off even at 1000 cd m−2. Moreover, a deep-blue emission with Commission Internationale de lEclairage (CIE) coordinates of (0.15, 0.09), a maximum external quantum efficiency (ηext,max) of 8.0% and the highest ηPE,max of 7.3 lm W−1 among all the TTF and HLCT deep-blue emitters were obtained by doping TPEA into the host of bis-4-[(N-carbazolyl)phenyl]-phenylphosphine oxide (BCPO). These results indicate that the combinational molecular design is promising for highly efficient deep-blue emitters.

Tin Compensation for the SnS Based Optoelectronic Devices

In this paper we report the growth of high quality SnS thin films with good crystallinity deposited on two-dimensional (2D) mica substrates. It is believed that the 2D nature of SnS, with strong intra-layer covalent bonds and weak inter-layer van der Waals interactions, is responsible for its relative insensitivity to lattice mismatch. We also investigated the reduction of Sn vacancies in the material using Sn-compensation technique during the material growth process. The experimental results clearly demonstrated substantial enhancements in the electrical and structural properties for films deposited using Sn-compensation technique. A mobility of 51u2009cm2u2009u2009V−1u2009s−1 and an XRD rocking curve full width at half maximum of 0.07° were obtained. Sn-compensated SnS/GaN:Si heterojunctions were fabricated and significant improvement in both the I-V characteristics and the spectral responsivities of the devices were characterized.

Enhanced photoluminescence and the self-assembled fibrillar nanostructure of 5-(cholesteryloxy)methyl-8-hydroxyquinoline lithium in a gel state

Soluble 5-(cholesteryloxy)methyl-8-hydroxyquinoline lithium(I) (LiChQ) was synthesized through the modification of 8-hydroxyquinoline lithium (LiQ) with cholesterol, and showed about 3 times more enhanced luminescence than pristine LiQ. When increasing the concentration of LiChQ up to 1 wt% in non-protic solvents, nanoscale fibers of 30–100 nm diameter were formed through self-assembly in a super-gel state. There was a red shift in the absorption of the gel in comparison to the solution, which indicates that LiChQ tends to form a J-aggregate in the gel state. We also investigated the gelation process of LiChQ using the Lippert–Mataga equation. We suggest that LiChQ has potential applications in luminescent devices and/or as a template for nanostructured optoelectronic materials.

Applications of ferroelectrics in photovoltaic devices

Ferroelectric materials exhibiting anomalous photovoltaic properties are one of the foci of photovoltaic research. We review the foundations and recent progress in ferroelectric materials for photovoltaic applications, including the physics of ferroelectricity, nature of ferroelectric thin films, characteristics and underlying mechanism of the ferroelectric photovoltaic effect, solar cells based on ferroelectric materials, and other related topics. These findings have important implications for improving the efficiency of photovoltaic cells.摘要具有反常光伏效应的铁电材料是光伏研究的重点之一. 本文综述了铁电材料在光伏应用中的研究进展, 包括铁电性的物理基础、铁电薄膜的性质、铁电光伏效应的特点和内在机制、铁电材料太阳电池等. 这些发现对于进一步提高光伏电池的效率具有重要意义.