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There have been huge achievements of all‐perovskite tandem solar cells, which recently realized the highest power conversion efficiency of 24.8%. However, the complex device structure and complicated manufacture processes severely restrict the further development of all‐perovskite tandem solar cells. In this work, we successfully fabricated high‐efficiency hole transport material‐free (HTM‐free) Sn−Pb alloyed narrow bandgap perovskite solar cells (PSCs) by introducing guanidinium thiocyanate (GASCN) and hydroiodic acid (HI) into the perovskite precursor solution. GASCN and HI play a positive synergy effect during perovskite crystallization process resulting in larger grain size, fewer surface defects, and lower trap density to suppress the Sn2+ oxidation degradation. Furthermore, they could effectively adjust the energy level of perovskite materials, reduce the energy level difference between perovskite and ITO resulting in more efficiently transport of free hole charge carriers. As a result, with adding GASCN and HI, the achieved highest power conversion efficiency of HTM‐free devices increased from 12.58% to 17.85%, which is one of the highest PCEs among all values reported to date for the HTM‐free narrow‐bandgap (1.2–1.4 eV) Sn−Pb binary PSCs. Moreover, the optimized device shows improved environmental stability. Our additive strategy manifests a remarkable step towards the facile, cost‐efficient fabrication of HTM‐free perovskite‐based tandem solar cells with both high efficiency and simple fabrication process. [ABSTRACT FROM AUTHOR]

There have been a number of remarkable signs of progress achieved in tin–lead mixed narrow‐bandgap perovskite solar cells (PSCs) due to the high theoretical power conversion efficiency (PCE) and their promising application in tandem devices. Indeed, Sn–Pb mixed PSCs without a hole transport layer (HTL) also have been more attractive owing to lower cost and simplification of the device structure. However, the defects in perovskite film introduced by Sn2+ oxidation severely restrict device efficiency and stability.Herein, a small organic molecule, 4,4'‐sulfonyldiphenol, is employed to passivate perovskite (Eg = 1.26 eV) surface to decrease the interfacial defects and suppress the nonradiative carrier recombination. Furthermore, by regulating energy‐level alignment, charge carrier extraction is greatly facilitated. The device performance is significantly enhanced in that the champion PCE is enlarged to 21.43% with an open‐circuit voltage (Voc) of 0.876 V from only 18.02% with a Voc of 0.770 V. The stability of unencapsulated devices is improved substantially as well while retaining 80% PCE of its initial value after being stored in the glovebox for around 600 h. This facile but highly effective strategy successfully proposes the promising development of HTL‐free Sn–Pb mixed PSCs. [ABSTRACT FROM AUTHOR]

With the rapid development of the Internet of Things, convenient and portable self-powered devices are in great need. Among all substitutes that could provide clean and sustainable power, the flexible perovskite solar cells (FPSCs) are the most attractive with the characteristics of flexibility, lightweight, high power conversion efficiency, and low cost. In this review, the recent advances of FPSCs are summarized, focusing on the materials' assessment of flexible and durable substrate, transparent electrode, low-temperature processed charge transporting layer, and mechanically robust perovskite film, with device design interspersed in each part. Finally, the challenges of FPSCs in terms of higher efficiency, higher flexibility, higher stability, and scalable fabrication are summarized. [ABSTRACT FROM AUTHOR]

Highlights: Convincing candidates of flexible transparent electrodes are discussed in detail from the views of fabrication, properties and device performance. The progresses of flexible opaque electrodes used in flexible perovskite solar cells are provided. The future directions and challenges in developing flexible electrodes are highlighted. Flexible perovskite solar cells (FPSCs) have attracted enormous interest in wearable and portable electronics due to their high power-per-weight and low cost. Flexible and efficient perovskite solar cells require the development of flexible electrodes compatible with the optoelectronic properties of perovskite. In this review, the recent progress of flexible electrodes used in FPSCs is comprehensively reviewed. The major features of flexible transparent electrodes, including transparent conductive oxides, conductive polymer, carbon nanomaterials and nanostructured metallic materials are systematically compared. And the corresponding modification strategies and device performance are summarized. Moreover, flexible opaque electrodes including metal films, opaque carbon materials and metal foils are critically assessed. Finally, the development directions and difficulties of flexible electrodes are given. [ABSTRACT FROM AUTHOR]

Use of inorganic charge transport layer has demonstrated relatively stable perovskite solar cells (PSCs). NiOx is the most widely used inorganic hole transport layer in inverted PSCs and different techniques and doping in this layer have been reported to improve the performance of these devices. This manuscript describes the synthesis of NiOx thin film which act as hole transport layer on glass substrate at the initial stage and PSC devices were fabricated at the secondary stage. Effect of post deposition annealing temperature on the composition, tuning of the work function and aligning it with perovskite work function increase the hole transport efficiency and improve the open circuit voltage of devices from 0.96 to 1.08 V is reported. Ultraviolet photoelectron spectroscopy results were used to check the change in work function and X-ray photoelectron spectroscopy to probe the underlying reason for improvement of devices are also included. The charge transfer efficiency is checked by the results of time resolved photoluminescence spectra is also given. Devices with NiOx as hole transport layer and ZnSe as electron transport layer are also described and performance of devices is also included in this manuscript. [ABSTRACT FROM AUTHOR]

It is very important to study the crystallization of hybrid organic-inorganic perovskites because their thin films are usually prepared from solution. The investigation on the growth of perovskite films is however limited by their polycrystallinity. In this work, methylammonium lead triiodide single crystals grown from solutions with different methylammonium iodide (MAI):lead iodide (PbI2) ratios were investigated. We observed a V-shaped dependence of the crystallization onset temperature on the MAI:PbI2 ratio. This is attributed to the MAI effects on the supersaturation of precursors and the interfacial energy of the crystal growth. At low MAI:PbI2 ratio (<1.7), more MAI leads to the supersaturation of the precursors at lower temperature. At high MAI:PbI2 ratio, the crystal growing plans change from (100)-plane dominated to (001)-plane dominated. The latter have higher interfacial energy than the former, leading to a higher crystallization onset temperature. [ABSTRACT FROM AUTHOR]

The mechanical properties of formamidinium lead halide perovskites (FAPbX3, X=Br or I) grown by inverse-temperature crystallization have been studied by nanoindentation. The measured Young's moduli (9.7-12.3 GPa) and hardnesses (0.36-0.45 GPa) indicate good mechanical flexibility and ductility. The effects of hydrogen bonding were evaluated by performing ab initio molecular dynamics on both formamidinium and methylammonium perovskites and calculating radial distribution functions. The structural and chemical factors influencing these properties are discussed by comparison with corresponding values in the literature for other hybrid perovskites, including double perovskites. Our results reveal that bonding in the inorganic framework and hydrogen bonding play important roles in determining elastic stiffness. The influence of the organic cation becomes more important for structures at the limit of their perovskite stability, indicated by high tolerance factors. [ABSTRACT FROM AUTHOR]

The Cover Picture shows a graphical representation of the use of the nanoindentation technique to probe the intrinsic mechanical properties of hybrid perovskites. The dynamic nature of the lead‐halide perovskite framework gives rise to mechanical flexibility, leading to relatively low Young's moduli and hardnesses. The structural–mechanical relationships were further explored by X‐ray crystallography and density functional theory calculations, where the effect of structural anisotropy, the size of the A cations and hydrogen bonding on the material's response under uniaxial stress were addressed. More details can be found in the Communication by Sun et al. on page 3740 in Issue 19, 2017 (DOI: 10.1002/cssc.201700991). [ABSTRACT FROM AUTHOR]

Perovskite solar cells (PSCs) have attracted much attention due to their high power conversion efficiency (PCE) and low cost fabrication. Since the PCE is critically related to the perovskite crystal morphology and quality, controlling the perovskite crystal formation is becoming more important. In this work, we chose PDDA polyelectrolyte as the crystal capping agent to modulate the perovskite thin film formation. The PDDA capping agent significantly enhanced the short-circuit current ( J sc ) and fill factor (FF) of the corresponding PSCs, which produced an optimal PCE of 14.1% compared to 12.5% of control device without the modifying agent. The effect of PDDA agent is examined by various techniques. It was found that improved surface morphology, enhanced crystallinity, and decreased grain boundary related defects are contributed to the performance and stability enhancement. [ABSTRACT FROM AUTHOR]

Planar perovskite solar cells (PSCs) with poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) have attracted much interest because they can be fabricated by low temperature process and exhibit high power conversion efficiency (PCE). The PCE is strongly dependent on the interface properties between the PCBM layer and cathode. In this work, solution-processable material, rhodamine 101, was used as the interfacial layer between the PCBM layer and Ag cathode. These interfacial materials significantly increase the fill factor so as to the PCE of the PSCs. The optimal power conversion efficiencies are 13.2% (CH 3 NH 3 PbI 3 ) and 14.8% (CH 3 NH 3 PbI 3−x Cl x ) for the devices with rhodamine 101 as the electron collection interlayer, saliently higher than that (8.5% and 12.0%) of control devices without an electron-collection interlayer. The effect of the interfacial material is carefully examined by time-resolved photoluminance spectroscopy (TR-PL), ultraviolet photoelectron spectroscopy (UPS), and kelvin probe force microscopy (KPFM) techniques. Decreased work-function and energy barrier, enhanced electron extraction and prolonged free carrier lifetime all contributed to the photovoltaic performance improvement. [ABSTRACT FROM AUTHOR]

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has been widely used as the hole transport material in optoelectronic devices. To avoid the cross talk among different crossbars, PEDOT:PSS with low conductivity is required. It thus has a high loading of the non-conductive PSSH. The PSSH-to-PEDOT weight ratio is 6 for Clevios P VP Al 4083 that is the most popular polymer as the hole transport layer. However, the acidic PSSH brings severe problems to the device stability and performance. Here, PEDOT:PSS solutions with low acidity can be prepared through a facile treatment of PEDOT:PSS solution by probe ultrasonication. Two grades of PEDOT:PSS, Clevios PH1000 and Clevios P, with a PSSH-to-PEDOT weight ratio of 2.5 were treated by probe ultrasonication. The ultrasonication can lower the viscosity and the colloidal sizes of PEDOT:PSS solutions and conductivity of PEDOT:PSS films. The pH value of probe-ultrasonicated Clevios P was 2.12, higher than that (1.77) of pristine Clevios P VP Al 4083. The ultrasonication-treated PEDOT:PSS solutions were used as hole transport layer in polymer solar cells and perovskite solar cells. The photovoltaic performances of these solar cells are comparable to that of control devices employing Clevios P VP Al 4083 PEDOT:PSS as the hole transport layer. [ABSTRACT FROM AUTHOR]