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The market's increasing demand for portable electronic products has made the research in flexible devices very attractive. Flexible solar cells are one of the most eye-catching devices in the field of flexible photovoltaic devices. And the perovskite material has become a powerful candidate material for manufacturing flexible solar cells due to its excellent optical properties, low price and the ability to tolerate more stress accumulation. However, as an active layer, the perovskite film still faces the issue that cracks are easily generated when bending and encountering temperature changes for the limited flexibility. There are many factors that affect the mechanical stability of the perovskite layer: the characteristics of the material itself, the influence of the other layers in the device and the presence or absence of a protective layer. This work reviews some reported strategies of improving the perovskite bending durability, classifies them according to the influencing factors mentioned above, and describes them in detail respectively.

A dopant-free hole transport layer with high mobility and a low-temperature process is desired for optoelectronic devices. Here, we study a metal–organic framework material with high hole mobility and strong hole extraction capability as an ideal hole transport layer for perovskite solar cells. By utilizing lifting-up method, the thickness controllable floating film of Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2 at the gas–liquid interface is transferred onto ITO-coated glass substrate. The Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2 film demonstrates high compactness and uniformity. The root-mean-square roughness of the film is 5.5 nm. The ultraviolet photoelectron spectroscopy and the steady-state photoluminescence spectra exhibit the Ni3(HITP)2 film can effectively transfer holes from perovskite film to anode. The perovskite solar cells based on Ni3(HITP)2 as a dopant-free hole transport layer achieve a champion power conversion efficiency of 10.3%. This work broadens the application of metal–organic frameworks in the field of perovskite solar cells. Graphical Abstract

Interface strains and lattice distortion are inevitable issues during perovskite crystallization. Silane as a coupling agent is a popular connector to enhance the compatibility between inorganic and organic materials in semiconductor devices. Herein, a protonated amine silane coupling agent (PASCA-Br) interlayer between TiO2 and perovskite layers is adopted to directionally grasp both of them by forming the structural component of a lattice unit. The pillowy alkyl ammonium bromide terminals at the upper side of the interlayer provide well-matched growth sites for the perovskite, leading to mitigated interface strain and ensuing lattice distortion; meanwhile, its superior chemical compatibility presents an ideal effect on healing the under-coordinated Pb atoms and halogen vacancies of bare perovskite crystals. The PASCA-Br interlayer also serves as a mechanical buffer layer, inducing less cracked perovskite film when bending. The developed molecular-level flexible interlayer provides a promising interfacial engineering for perovskite solar cells and their flexible application.

Wearable devices are mainly based on plastic substrates, such as polyethylene terephthalate and polyethylene naphthalate, which causes environmental pollution after use due to the long decomposition periods. This work reports on the fabrication of a biodegradable and biocompatible transparent conductive electrode derived from bamboo for flexible perovskite solar cells. The conductive bioelectrode exhibits extremely flexible and light-weight properties. After bending 3000 times at a 4 mm curvature radius or even undergoing a crumpling test, it still shows excellent electrical performance and negligible decay. The performance of the bamboo-based bioelectrode perovskite solar cell exhibits a record power conversion efficiency (PCE) of 11.68%, showing the highest efficiency among all reported biomass-based perovskite solar cells. It is remarkable that this flexible device has a highly bendable mechanical stability, maintaining over 70% of its original PCE during 1000 bending cycles at a 4 mm curvature radius. This work paves the way for perovskite solar cells toward comfortable and environmentally friendly wearable devices.

Semiconductor nanomaterials with controlled morphologies and architectures are of critical importance for high-performance optoelectronic devices. However, the fabrication of such nanomaterials on polymer-based flexible electrodes is particularly challenging due to degradation of the flexible electrodes at a high temperature. Here we report the fabrication of nickel oxide nanopillar arrays (NiO x NaPAs) on a flexible electrode by vapor deposition, which enables highly efficient perovskite solar cells (PSCs). The NiO x NaPAs exhibit an enhanced light transmittance for light harvesting, prohibit exciton recombination, promote irradiation-generated hole transport and collection, and facilitate the formation of large perovskite grains. These advantageous features result in a high efficiency of 20% and 17% for the rigid and flexible PSCs, respectively. Additionally, the NaPAs show no cracking after 500 times of bending, consistent with the mechanic simulation results. This robust fabrication opens a new opportunity for the fabrication of a large area of high-performance flexible optoelectronic devices.

Poly(3,4-ethylene dioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is a widely utilized hole-transporting material (HTM) in planar photovoltaic devices, such as organic solar cells (OSCs) and perovskite solar cells (PSCs). However, the hygroscopic nature of PEDOT:PSS aqueous dispersions may restrict their future application. Therefore, it is necessary to develop other effective and stable HTMs to achieve high-performance photovoltaic devices. Herein, we demonstrate a facile route to deposit solution-processed MoO3, GeO2, V2O5 and CrO3 thin films as hole-transporting layers by directly dissolving their commercial powders in deionized water. Among these, the solution-processed V2O5 (sV2O5) film exhibited the highest work function of 5.2 eV, and the best hydrophobicity, with a contact angle of 77.2°. The sV2O5-based OSCs and PSCs presented power conversion efficiencies (PCEs) of 8.36% and 14.13%, respectively. Notably, the PEDOT:PSS V2O5 composite HTM based device obtained a maximum PCE of 18.03% with a Voc exceeding 1.0 V. These aqueous-solution-processed HTMs have potential applications in green and low-cost photovoltaic devices by virtue of their simple and ecofriendly preparations.

Interface engineering is an efficient method for improving the performance of planar perovskite solar cells (PSCs). In this paper, the performance of PSCs was improved significantly by introducing 4,7-diphenyl-1,10-phenanthroline (Bphen) doped with bis(2-methyldibenzo-[f,h]quinoxaline) (Ir(MDQ)2(acac)) to modify the interface between perovskite (CH3NH3PbI3−xClx)/PCBM (phenyl-C61-butyric acid methyl ester) and an Ag electrode. The power conversion efficiency (PCE) was enhanced up to 15.87%, compared with 10.77% for the reference device without interlayer modification. It was found that the enhanced PCE was attributed to the better interface contact between the perovskite and Ag cathode. A suitable interface roughness is beneficial for reducing the leakage current and the probability of carrier recombination, resulting in an enhanced fill factor and thus improved device efficiency.

A new three-armed imidazolium phenoxy ionic liquid (TIPIL) has been successfully designed, synthesized and characterized. The thermal property, amorphous feature and ionic conductivity of TIPIL were also investigated. 1-Ethyl-3-methylimidazolium iodide (EMII), a typical ionic conductor, could be effectively inhibited from crystallization by TIPIL, which has been characterized by several testing. Thus, as a crystal growth inhibitor, TIPIL was used into EMII based solid-state dye-sensitized solar cells (ssDSSCs). Compared with the reference device, the DSSC containing TIPIL showed improved photovoltaic conversion efficiency up to 5.37%, and displayed excellent long-term stability. These results provide us a feasible method to explore new crystal growth inhibitors for ssDSSCs.

In this study, we report the growth of molybdenum oxide (MoOx) film by polymer-assisted deposition (PAD), an environmentally friendly strategy in an aqueous system. The MoOx film has good crystal quality and is dense and smooth. The transparency of the film is >95% in the wavelength range of 300–900 nm. The device based on P3HT:PCBM absorber material was fabricated. The solar cell with PAD-MoOx as an anode interfacial layer exhibits great performance, even better than that of a solar cell with PEDOT:PSS or evaporated MoOx as an anode interfacial layer. More importantly, the solar cells based on the growth of MoOx have a longer term stability than that of solar cells based on PEDOT:PSS. These results demonstrate the aqueous PAD technology provides an alternative strategy not only for the thin films’ growth of applied materials but also for the solution processing for the low-cost fabrication of future materials to be applied in the field of solar cells.

Monodisperse CdS quantum dots (QDs) are synthesized by thermal decomposition of organic complexes in the system of the cost-effective commercial 0# diesel at 200 °C. The prepared CdS QDs have a good dispersion and high crystallization. When the CdS QDs are doped into the blends of poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6, 6)C61 (PCBM) for hybrid solar cells (HSCs), the HSCs achieve about 25% increase of power conversion efficiency in comparison to the reference device without the CdS QDs. The improvement of the cell performance mainly attributes to the increased short-circuit current density arising from the absorption enhancement in the wavelength range of 350–550 nm by introducing the synthesized CdS QDs into the P3HT: PCBM active layer.

We report oxygen vacancies (OVs) rich hexagonal ReO3 nanostructured electrocatalysts for efficient hydrogen generation. Through a simple argon plasma exposure, OVs are introduced into the ReO3 nanoparticles (NP) and nanosheets to enhance electrocatalytic activities with decreasing overpotentails from 157 mV and 178 mV to 138 mV and 145 mV at the current density of 10 mA cm-2, respectively. As-processed OVs rich ReO3 NP exhibit a good stability during electrochemical measurements for 20 h in acidic electrolyte. The huge active surface area, abundant OVs and excellent conductivity contribute to the performance according to the experimental data. Further theoretical calculations show that the abundant OVs adsorb H with lower Gibbs free energy facilitating hydrogen evolution.

The underlayer plays an important role for organic–inorganic hybrid perovskite formation and charge transport in perovskite solar cells (PSCs). Here, we employ a classical organic small molecule, 5,6,11,12-tetraphenyltetracene (rubrene), as the underlayer of perovskite films to achieve 15.83% of power conversion efficiency with remarkable moisture tolerance exposed to the atmosphere. Experiments demonstrate rubrene hydrophobic underlayer not only drives the crystalline grain growth of high quality perovskite, but also contributes to the moisture tolerance of PSCs. Moreover, the matching energy level of the desirable underlayer is conductive to extracting holes and blocking electrons at anode in PSCs. This introduction of organic small molecule into PSCs provides alternative materials for interface optimization, as well as platform for flexible and wearable solar cells.

It will be interesting and valuable information can be achieved if a direct comparison between organic light emitting devices (OLEDs) fabricated by vacuum evaporated method (vac) and solution-based manufacturing processes (sol) was realized. Small molecular OLEDs with a mixed organic layer structure (MOLOLEDs) make it possible for direct comparison between devices with the same materials but fabricated by the two processing methods. This article shows a direct comparison of the luminescence characteristics, charge conduction, and device physics between MOLOLEDs fabricated by vac- and sol-processing techniques. It gives an elementary explain how the organic/metal interfaces influence the charge conduction and device performance.

The relative densities of SiCln (n = 0–2) in SiCl4 radio frequency (rf) glow discharge plasma are measured by mass spectrometry. The effects of discharge parameters, i.e., rf power, discharge pressure, substrate temperature, and SiCl4 flow rate on the relative densities of SiCln (n = 0–2) are investigated in detail. An optimum configuration of discharge parameters (low rf power, high discharge pressure, low substrate temperature, and low flow rate), which enhanced the formation of SiCln (n = 0–2) radicals, is searched by a great deal of measurements and discussions. In the optimum configuration of discharge parameters, we measure the spatial distribution of SiCln (n = 0–2) radicals in the most optimized plasma parameters. The experimental results reveal that Si and SiCl may be the dominant precursors in forming the thin film.

We report the synthesis of carbon nanotube/TiC hybrid fibers using a polymer-assisted chemical solution approach. Ti metal ions are bound to aqueous polyethyleneimine (PEI) to form precursor solution. Amphiphilic PEI with Ti easily permeates the CNT fibers. Upon annealing in a controlled atmosphere, a homogeneous TiC network is formed in the CNT fibers. The obtained CNT/TiC hybrid fibers show prominent enhancement in mechanical strength and electrical conductivity. The tensile strength and conductivity of CNT/TiC fibers can be improved to 0.67 GPa and 1650 S cm(-1) at room temperature, respectively. More importantly, a tensile modulus as high as 420 GPa has been achieved for the CNT/TiC fibers. Analysis shows that the cross-linking matrix of hard TiC plays a significant role in the improvement of mechanical strength. Furthermore, the electrons are transported in the CNT/TiC fiber by a three dimensional hopping mechanism.

Due to the good photocatalytic activity, the TiO2@CNTs thin film is highly desirable to apply to the self-cleaning glass for green intelligent building. Here, the TiO2@CNTs thin film has been successfully achieved by polymer-assisted approach of an aqueous chemical solution method. The polymer, polyethylenimine, aims to combine the Ti4+ with CNTs for film formation of TiO2@CNTs. The resultant thin film was uniform, highly transparent, and super-hydrophilic. Owing to fast electron transport and effectively hindering electron-hole recombination, the TiO2@CNTs thin film has nearly twofold photocatalytic performance than pure TiO2. The TiO2@CNTs thin films show a good application for self-cleaning glasses. Electronic supplementary material The online version of this article (doi:10.1186/s11671-016-1674-4) contains supplementary material, which is available to authorized users.