Significant progress has been made in perovskite solar cells by various effective film-forming methods. Solution-process methods for the perovskite film appropriate under ambient conditions are desired to be explored for the practical industrialization. Here, a secondary crystal growth strategy is developed for the fabrication of perovskite film in ambient atmosphere. By this method, the conversion from PbI2 to CH3NH3PbI3 on the planar substrate can be completed, overcoming the limitation of standard sequential deposition. After secondary growth, high-quality crystals are obtained and compact densely to form a pinhole-free film. Exceeding 17% of power conversion efficiency is achieved for planar CH3NH3PbI3 devices by controlling the reaction time of two stages carefully. This method can be easily controlled, reproduced and performed in the ambient, which meets the industrial requirements for highly efficient, low cost planar perovskite solar cells.

Organic-inorganic hybrid perovskite solar cells (SCs) have emerged as one of the most promising contenders to traditional silicon solar cells, due to their active layers outstanding photoelectric properties, such as appropriate direct bandgap, balanced high carrier mobility and long carrier diffusion length, the identified power conversion efficiency (PCE) has reached to 22.7%. But the toxic lead, a key component in the archetypical light harvesting material, is a large obstacle to commercialization. Herein, we reviewed the recent progress in lead-free perovskite (-like) SCs according to the valent difference of metal ions in absorber material, e.g., bivalent (Sn2+, Ge2+, Cu2+, Sr2+), trivalent (Bi3+, Sb3+), tetravalent (Sn4+) and hybrid valent (e.g. Ag+ and Bi3+). Finally, we gave an outlook on the tactic to achieve high performance lead-free perovskite (-like) SCs. (c) 2018 Elsevier Ltd. All rights reserved.

The electron transport layer (ETL), as an important component of planar perovskite solar cells (P-PSCs), can effectively extract photon-generated electrons from perovskites and convey them to the cathode; by this token, its properties directly determine the photovoltaic performances of P-PSCs. Herein, we introduce a ZnO/SnO2 double electron transport layer for CH3NH3PbI3-based P-PSCs, achieving a high open circuit voltage (V-OC) of 1.15 V with the power conversion efficiency (PCE) of 19.1% when the SnO2-based devices have a V-OC of 1.07 V and a PCE of 18.0%; to the best of our knowledge, this is the highest V-OC obtained by using an inorganic electron transport layer for pure CH3NH3PbI3-based P-PSCs so far. This result demonstrates that a higher Fermi energy (E-F) and conduction band minimum (E-CBM) of ETL could drive a higher V-OC and a better PCE.

The perovskite is a class of material with crystalline structure similar to CaTiO3. In recent years, the organic-inorganic hybrid metallic halide perovskite has been widely investigated as a promising material for a new generation photovoltaic device, whose power conversion efficiency (PCE) record reaches 22.7%. One of its underlying morphological characteristics is the twin domain within those sub-micron sized crystal grains in perovskite thin films. This is important for discussion since it could be the key for understanding the fundamental mechanism of the device's high performance, such as long diffusion distance and low recombination rate. This review aims to summarize studies on twin domains in perovskite thin films, in order to figure out its importance, guide the current studies on mechanism, and design new devices. Firstly, we introduce the research history and characteristics of widely known twin domains in inorganic perovskite BaTiO3. We then focus on the impact of the domain structure emerging in hybrid metallic halide perovskite thin films, including the observation and discussion on ferroelectricity/ferroelasity. The theoretical analysis is also presented in this review. Finally, we present a spectroscopic method, which can reveal the generality of twin domains within perovskite thin films. We anticipate that this summary on the structural and physical properties of organometallic halide perovskite will help to understand and improve the high-performance of photovoltaic devices.

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.

In perovskite solar cells, the morphology of the porous TiO2 electron transport layer (ETL) largely determines the quality of the perovskites. Here, we chose micro-scale TiO2 (0.2 mu m) and compared it with the conventional nanoscale TiO2 (20 nm) in relation to the crystallinity of perovskites. The results show that the micro-scale TiO2 is favorable for increasing the grain size of the perovskites and enhancing the light scattering. However, the oversized TiO2 results in an uneven surface. The evenness of the perovskites can be improved by nanoscale TiO2, while the crystallinity and compactness are not as good as those of the films based on micro-scale TiO2. To combine the advantages of both micro-scale and nanoscale TiO2, by mixing 0.2 mu m/20nm TiO2 with a ratio of 1 : 2 as the composite ETL, the device average power conversion efficiency was increased to 11.2% from 9.9% in the case of only 20 nm TiO2. (C) 2018 The Japan Society of Applied Physics

A deep-blue emitter 1-(10-(4-methoxyphenyl) anthracen-9-yl)-4-(10-(4-cyanophenyl) anthracen-9yl) 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 1C. Based on all these advantages, a high performance of the non-doped device was achieved with a turnon voltage (Von) of 2.6 V at a luminance of 1 cd m(-2), a maximum power efficiency (ZPE, max) of 11.1 lm W-1, a maximum current efficiency (ZCE, 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 l'E ` clairage (CIE) coordinates of (0.15, 0.09), a maximum external quantum efficiency (Zext, max) of 8.0% and the highest ZPE, 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.

Ultraviolet (UV) photodetectors with high responsivity and fast response are crucial for practical applications. Double perovskite Cs2AgBiBr6 has emerged as a promising optoelectronic material due to its excellent physics and photoelectric properties. However, no work is reported based on its film for photodetector applications. Herein, an ITO/SnO2/Cs2AgBiBr6/Au hole-transport layer free planar heterojunction device is fabricated for photodetector application. The device is self-powered with two responsivity peaks at 350 and 435 nm, which is suitable for ultraviolet-A (320-400 nm) and deep-blue light detecting. A high responsivity of 0.11 A W-1 at 350 nm and a quick response time of less than 3 ms are obtained, which is significantly higher than other semiconductor oxide heterojunction-based UV detectors. More importantly, the stability is significantly better than most of the hybrid perovskite photodetectors reported so far. Its photocurrent shows no obvious degradation after more than 6 months storage in ambient conditions without any encapsulation. Consequently, the utilization of Cs2AgBiBr6 film is a practical approach for high performance, large-area lead-free perovskite photodetector applications. For the mechanism, it is found that photogenerated carriers in Cs2AgBiBr6 film are separated at the Cs2AgBiBr6/SnO2 heterojunction interface by its built-in field. The low toxicity and high stability of this double perovskite active layer make it very promising for practical applications.

Combining rigid twisted spirobifluorene with two strongly electron-withdrawing terpyridine moieties to form a ``(A)(n)-D-(A)(n)'' structure is an effective way to achieve electron transport materials (ETMs) with high triplet energy, suitable frontier orbital levels, excellent thermal stability and electrochemical stability for long-lived and highly efficient organic light-emitting diodes (OLEDs), 2,2'-di([2,2':6',2 `'-terpyridin]-4'-yl)-9,9'-spirobi[fluorene] (22-TPSF) and 2,7-di([2,2':6',2 `'-terpyridin]-4'-yl)-9,9'-spirobifluorene (27-TPSF), both of which are better than the conventional ETM 1,3,5-tris(N-phenylbenzimidazol-2-yl-benzene (TPBi). In addition, the crystal packing mode in their single crystals undergoes a significant transformation from the sandwich arrangement of 22-TPSF into the brick wall arrangement of 27-TPSF, causing a big difference in electron transport mobility, which changes from 0.012 to 0.104 cm(2) V-1 s(-1) as calculated through density functional theory. This variation leads to a phenomenon where the 22-TPSF based devices display a lower maximum external quantum efficiency of 22.9% and a shorter half-life (T-50) of 173925 hours at an initial luminance of 100 cd m(-2) than the 27-TPSF based devices. These findings highlight the great potential of the ETM structured as ``(A)(n)-D-(A)(n)'' using the terpyridine and spirobifluorene moieties; moreover, the positional isomerism effect allows remarkable tuning of the electron transport mobility and makes an obvious influence on OLED performance and lifetime.