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High-efficiency perovskite solar cells (PSCs) normally rely on costly, high purity (>99.99%), air-sensitive raw materials that vary batch-to-batch. The perovskite films and devices derived from conventional raw materials mixture method suffer from inferior reproducibility of optoelectronic properties and performance, as well as discounted promise towards low-cost scalable manufacturing. Distinguished from the direct mixing of raw materials, the preparation of perovskite films with precursors made by the redissolution of perovskite crystals holds the promise to make PSCs more affordable, reproducible, efficient and stable. The resultant perovskite films inherit the exceptional characteristics of the parent perovskite crystals, such as high crystallinity, high purity, accurate stoichiometric ratio, and low trap-state density, as well as good ambient and phase stability. Herein, we summarize recent progress on the employment of the perovskite crystals redissolution strategy for achieving low-cost, efficient perovskite-based solar-to-electricity conversion, which will help both popularize the redissolution strategy and reveal unprecedented advantages gained by its adoption.

ispartof: ADVANCED FUNCTIONAL MATERIALS vol:32 issue:52 status: published

Simplified structures of perovskite solar cells can reduce the fabrication and materials cost while simultaneously achieving high efficiencies, which are highly attractive for practical industrial production.

Over the past decade, organic‐inorganic metal halide perovskites have attracted tremendous research interest owing to their prominent optical and electric properties. These fantastic perovskite materials have been widely employed in a variety of optoelectronic devices, such as perovskite solar cells (PSCs), light‐emitting diodes (LEDs), photocatalysts, photodetectors and X‐ray imaging.[1]

Mapping perovskite trap states The high efficiency of hybrid inorganic-organic perovskite solar cells is mainly limited by defects that trap the charge carriers and lead to unproductive recombination. Ni et al. used drive-level capacitance profiling to map the spatial and energetic distribution of trap states in both polycrystalline and single-crystal perovskite solar cells. The interface trap densities were up to five orders of magnitude higher than the bulk trap densities. Deep traps were mainly located at the interface of perovskites and hole-transport layers, where processing created a high density of nanocrystals. These results should aid efforts aimed at avoiding trap-state formation or passivating such defects. Science , this issue p. 1352

Perovskite solar cells (PSCs) have witnessed an overwhelming success in their skyrocketed power conversion efficiencies (PCEs) within a short period, but a fully solution-processed fabrication of PSC multilayer stack has not been achieved. Herein, we report to fabricate the simplified PSCs with perovskite layer sandwiched and encapsulated between carbon-based electron transport layer (ETL) and counter electrode (CE) via a fully blade-coated process. A self-assembled monolayer of amphiphilic silane (AS) molecules on transparent conducting oxide (TCO) substrate appeals to the fullerene ETL deposition and preserves its integrity against the solvent damage induced by perovskite precursor. The AS serves as a“molecular glue”to strengthen the adhesion toughness at the TCO/ETL interface via robust chemical interaction and bonding, which remarkably facilitates the interfacial charge extraction, increases the PCEs by 77% and reduces the hysteresis. A champion PCE of 18.64% has been achieved for the fully-printed devices, which is one of the highest reported values for the carbon-based PSCs. AS-assisted interfacial linkage and carbon materials-assisted self-encapsulation remakably enhance the stability of the resultant PSCs, which did not experience any performance degradation when stored at ambient conditions for more than 3000 hours. This work provides a novel strategy to tackle the incompatibility issue of depositing each functional layer by using a full solution process and offers an additional promise of constructing efficient perovskite-based optoelectronics with prolonged lifespan in a low-cost manner.

To fabricate an effective antibacterial coating on the surfaces of diverse sanitary ceramic utensils, efficient titania photocatalysts with integrated features including high temperature anatase phase stability (>800 °C), excellent particle mobility for the formation of uniform thin coatings, high crystallinity and narrow particle size distribution are desirable. In this study, monodisperse dopant-free titania microspheres were synthesized with large pore size (∼80 nm) that remain anatase even after calcination at 900 °C. These titania microspheres were fabricated via a facile solvothermal treatment of amorphous spheres in the presence of 4.5 wt% ammonia solution and consisted of well-crystallized and faceted anatase nanocrystals with a uniform size of 24 nm. The anatase nanocrystals with high crystallinity and narrow crystal size distribution are responsible for their high temperature stability. The resulting anatase titania microspheres exhibited enhanced photocatalytic performance even after calcination at high temperature due to the retention of the anatase phase and the enhanced crystallinity. Such monodisperse anatase microspheres have potential to be applied as smart coating materials for antibacterial and self-cleaning applications.

We compare three representative high performance PV materials: halide perovskite MAPbI3, CdTe, and GaAs, in terms of photoluminescence (PL) efficiency, PL lineshape, carrier diffusion, and surface recombination and passivation, over multiple orders of photo-excitation density or carrier density appropriate for different applications. An analytic model is used to describe the excitation density dependence of PL intensity and extract the internal PL efficiency and multiple pertinent recombination parameters. A PL imaging technique is used to obtain carrier diffusion length without using a PL quencher, thus, free of unintended influence beyond pure diffusion. Our results show that perovskite samples tend to exhibit lower Shockley–Read–Hall (SRH) recombination rate in both bulk and surface, thus higher PL efficiency than the inorganic counterparts, particularly under low excitation density, even with no or preliminary surface passivation. PL lineshape and diffusion analysis indicate that there is considerable structural disordering in the perovskite materials, and thus photo-generated carriers are not in global thermal equilibrium, which in turn suppresses the nonradiative recombination. This study suggests that relatively low point-defect density, less detrimental surface recombination, and moderate structural disordering contribute to the high PV efficiency in the perovskite. This comparative photovoltaics study provides more insights into the fundamental material science and the search for optimal device designs by learning from different technologies.

Ordered 1D metal oxide structure is desirable in thin film solar cells owing to its excellent charge collection capability. However, the electron transfer in 1D electron transporting layer (ETL)-based devices is still limited to a submicrometer-long pathway that is vertical to the substrate. Here, an innovative closely packed rutile TiO nanowire (CRTNW) network parallel to the facet of fluorine-doped tin oxide (FTO) substrate is reported, which can serve as a 1D nanoscale electron transport pathway for efficient perovskite solar cells (PSCs). The PSC constructed using newly prepared CRTNW ETL achieves an impressive power conversion efficiency of 21.10%, which can be attributed to the facilitated electron extraction induced by the favorable junctions formed at FTO/ETL and ETL/perovskite interfaces and also the suppressed charge recombination originating from improved perovskite morphology with large grains, flat surface, and good surface coverage. The bifacial contact junctions engineering also enables large-area device fabrication. The PSC with 1 cm aperture yields an efficiency of 19.50% under one sun illumination. This work highlights the significance of controlling the orientation and packing density of the ordered 1D oxide nanostructured thin films for highly efficient optoelectronic devices in a large-scale manner.

Anchoring monolayer bilateral amines on perovskite passivates surface defects, reinforces grain boundaries, and enhances stability., The power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) are already higher than that of other thin film technologies, but laboratory cell-fabrication methods are not scalable. Here, we report an additive strategy to enhance the efficiency and stability of PSCs made by scalable blading. Blade-coated PSCs incorporating bilateral alkylamine (BAA) additives achieve PCEs of 21.5 (aperture, 0.08 cm2) and 20.0% (aperture, 1.1 cm2), with a record-small open-circuit voltage deficit of 0.35 V under AM1.5G illumination. The stabilized PCE reaches 22.6% under 0.3 sun. Anchoring monolayer bilateral amino groups passivates the defects at the perovskite surface and enhances perovskite stability by exposing the linking hydrophobic alkyl chain. Grain boundaries are reinforced by BAA and are more resistant to mechanical bending and electron beam damage. BAA improves the device shelf lifetime to >1000 hours and operation stability to >500 hours under light, with 90% of the initial efficiency retained.

The present work demonstrates a facile one-step process to fabricate thin films of amorphous titania nanowires on transparent conducting oxide substrates via hydrolysis of potassium titanium oxide oxalate in an aqueous solution at 90 °C. The resultant titania nanowire thin films (that have not undergone further annealing) are efficient electron transport layers in CH3NH3PbI3 perovskite solar cells, yielding full sun solar-to-electricity conversion efficiencies of up to 14.67% and a stabilized efficiency of 14.00% under AM 1.5G one sun illumination, comparable to high temperature sintered TiO2 counterparts. The high photovoltaic performance is attributed to the porous nanowire network that facilitates perovskite infiltration, its unique 1D geometry and excellent surface coverage for efficient electron transport, as well as suppressed charge recombination between FTO and perovskite.

Two-dimensional (2D) ZnO nanosheet arrays were prepared via vanadium (V)-doping assisted hydrothermal method, and then the nanosheet was successfully converted to a nanorod-on-nanosheet ZnO hierarchical structure by treating with Na2S solution and subsequent hydrothermal reaction. Hierarchical films with different nanorod growth time (1–8 h) were prepared and their photovoltaic properties were also investigated after electrodeposition of CdSe quantum dots. For the hierarchical nanorod-on-nanosheet ZnO films, increasing the ZnO nanorod growth time can enormously enlarge the length of branched nanorods and light-scattering ability, resulting in better light-harvesting efficiency and higher photo-generated electron concentration, which leads to higher short-circuit current density ( J sc) and open-circuit voltage ( V oc). However, further increasing nanorod growth time to 8 h leads to the over-dense coverage of nanorods, which is harmful for light-harvesting efficiency and leads to severe electron recombination, eventually diminishes the power conversion efficiency (PCE). With the optimized nanorod modification and Cu2S counter electrode, the PCE reaches a maximum value of 4.26%, which to the best of our knowledge, is among the highest PCE record for CdSe sensitized solar cells based on ZnO photoanodes.

Currently, blade-coated perovskite solar cells (PSCs) with high power conversion efficiencies (PCEs), that is, greater than 20%, normally employ methylammonium lead tri-iodide with a sub-optimal bandgap. Alloyed perovskites with formamidinium (FA) cation have narrower bandgap and thus enhance device photocurrent. However, FA-alloyed perovskites show low phase stability and high moisture sensitivity. Here, it is reported that incorporating 0.83 molar percent organic halide salts (OHs) into perovskite inks enables phase-pure, highly crystalline FA-alloyed perovskites with extraordinary optoelectronic properties. The OH molecules modulate the crystal growth, enhance the phase stability, passivate ionic defects at the surface and/or grain boundaries, and enhance the moisture stability of the perovskite film. A high efficiency of 22.0% under 1 sun illumination for blade-coated PSCs is demonstrated with an open-circuit voltage of 1.18 V, corresponding to a very small voltage deficit of 0.33 V, and significantly improved operational stability with 96% of the initial efficiency retained under one sun illumination for 500 h.

Halide perovskite materials have achieved overwhelming success in various optoelectronic applications, especially perovskite solar cells and perovskite-based light-emitting diodes (P-LEDs), owing to their outstanding optical and electric properties. It is widely believed that flat and mirror-like perovskite films are imperative for achieving high device performance, while the potential of other perovskite morphologies, such as the emerging textured perovskite, is overlooked, which leaves plenty of room for further breakthroughs. Compared to flat and mirror-like perovskites, textured perovskites with unique structures, e.g., coral-like, maze-like, column-like or quasi-core@shell assemblies, are more efficient at light harvesting and charge extraction, thus revolutionizing the pathways toward ultrahigh performance in perovskite-based optoelectronic devices. Employing a textured perovskite morphology, the record of external quantum efficiency for P-LEDs is demonstrated as 21.6%. In this research news, recent progress in the utilization of textured perovskite is summarized, with the emphasis on the preparation strategies and prominent optoelectronic properties. The impact of the textured morphology on light harvesting, carrier dynamic management, and device performance is highlighted. Finally, the challenges and great potential of employing these innovative morphologies in fabricating more efficient optoelectronic devices, or creating a new energy harvesting and conversion regime are also provided.

Recent years have witnessed an incredibly high fever in metal halide perovskite materials due to their promising applications in a wide range of optoelectronic applications. The morphologies and optoelectronic properties of the perovskite layers play critical roles in affecting the optoelectronic performances. This review summarizes the recent advances in the fabrication of a variety of perovskite morphologies and their promising progress achieved in different optoelectronic applications, including solar cells, light-emitting diodes, photodetectors, lasers, photocatalysis, X-ray detectors/imagers, and luminescent solar concentrators. Several blossoming representatives, including 0D perovskite quantum dots, 1D perovskite nanowires, 2D perovskite nanosheets/nanoplatelets, and 3D textured perovskite assemblies (i.e., cuboid-like, inverse opal-like, coral-like, or maze-like morphologies, etc.), are highlighted to demonstrate their fascinating properties and outstanding capabilities for efficient optoelectronic applications. Finally, a perspective on the remaining challenges and future directions of fabricating unique perovskite morphologies for next-generation high-performance optoelectronic devices is provided.

The present work establishes a facile process for one-step hydrothermal growth of vertically aligned anatase cactus-like branched TiO2 (CBT) arrays on a transparent conducting oxide (TCO) substrate. Various CBT morphologies are obtained by adjusting the potassium titanium oxide oxalate (PTO) reactant concentration (from 0.05 M to 0.15 M) and this yields a morphologically-controllable branched TiO2 arrays geometry. The CBT arrays consist of a vertically oriented nanowire (NW) or nanosheet (NS) stem and a host of short nanorod (NR) branches. The hierarchical CBT arrays demonstrate their excellent candidatures as photoanodes, which are capable of exhibiting high light-harvesting efficiency in dye-sensitized solar cells (DSSCs). Consequently, DSSCs based on 7 μm long optimized CBT arrays (0.05 M PTO), which are assembled with high density and high aspect-ratio NR branches, exhibit an impressive power conversion efficiency of 6.43% under AM 1.5G one sun illumination. The high performance can be attributed to the prominent light-harvesting efficiency, resulting from larger surface area and superior light-scattering capability.

Perovskite solar cells (PSCs) without an electron transport layer (ETL) exhibit fascinating advantages such as simplified configuration, low cost, and facile fabrication process. However, the performance of ETL-free PSCs has been hampered by severe charge carrier recombination induced either by current leakage (insufficient perovskite film coverage) or inferior charge extraction. Herein, an additive-assisted morphological engineering strategy is used to construct an intriguing bilayer perovskite film featuring a dense bottom layer and a maze-like top layer. Such maze-like perovskite films enable the construction of ETL-free PSCs with a PCE of 18.5% and negligible hysteresis, which can be attributed to the higher crystallinity and superior light-harvesting capability of the resultant perovskite film, as well as facilitated hole extraction at the hole transport layer (HTL)/perovskite interface. This work provides a simple approach to modify the perovskite film morphology and demonstrates the correlation between facilitated charge-carrier extraction and high-performance ETL-free perovskite photovoltaics.

The efficiencies of the hybrid organic-inorganic perovskite solar cells have been rapidly approaching the benchmarks held by the leading thin-film photovoltaic technologies. Arguably, one of the most important factors leading to this rapid advancement is the ability to manipulate the microstructure of the perovskite layer and the adjacent functional layers within the device. Here, an analysis of the nucleation and growth models relevant to the formation of perovskite films is provided, along with the effect of the perovskite microstructure (grain sizes and voids) on device performance. In addition, the effect of a compact or mesoporous electron-transport-layer (ETL) microstructure on the perovskite film formation and the optical/photoelectric properties at the ETL/perovskite interface are overviewed. Insight into the formation of the functional layers within a perovskite solar cell is provided, and potential avenues for further development of the perovskite microstructure are identified.

Microscopic design and morphological engineering of the semiconducting metal oxide as electron-transporting layers (ETLs) is of vital importance for optical enhancement, photonic structuring, and charge collection optimization within optoelectronic devices. Herein, nanowire-coated, branched macroporous titania (BMT) thin films are reported as a new type of ETL prepared by using silica spheres as a sacrificial template, followed by a sol–gel and subsequent alkaline-assisted etching process. The BMT films feature 3D hierarchical structures and interconnected networks with tunable pore sizes, branch densities, and film thicknesses. The titania films are employed as ETLs in perovskite solar cells (PSCs), resulting in remarkable power conversion efficiencies (PCEs) of 20.1%; a noticeable 16% increase compared with titania nanowire (TNW) ETL-based counterparts (PCE = 17.3%). The superior device performance of the BMT-based PSCs can be attributed to the maximized light harvesting and charge collection capabilities. These beneficial properties are derived from the effective infiltration of the perovskite precursor into the titania macropores, efficient light confinement within the macropore structure, and the textured perovskite capping layer, as well as enhanced charge transport and reduced charge recombination through the BMT architecture. This work demonstrates a simple and effective approach for constructing branched macroporous metal-oxide photoelectrodes toward high-performance photovoltaic devices.

The scrupulous design of nanoarchitectures and smart hybridization of specific active materials are closely related to the overall photovoltaic performance of an anode electrode. Here we present a solution-based strategy for the fabrication of well-aligned metal oxide-based nanowire-nanosheet-nanorod hyperbranched arrays on transparent conducting oxide substrates. For these hyperbranched arrays, we observe a twofold increment in dye adsorption and enhanced light trapping and scattering capability compared with the pristine titanium dioxide nanowires, and thus a power conversion efficiency of 9.09% is achieved. Our growth approach presents a strategy to broaden the photoresponse and maximize the light-harvesting efficiency of arrays architectures, and may lead to applications for energy conversion and storage, catalysis, water splitting and gas sensing.

Hierarchical anatase TiO(2) nano-architecture arrays consisting of long TiO(2) nanowire trunk and numerous short TiO(2) nanorod branches on transparent conductive fluorine-doped tin oxide glass are successfully synthesized for the first time through a facile one-step hydrothermal route without any surfactant and template. Dye-sensitized solar cells based on the hierarchical anatase TiO(2) nano-architecture array photoelectrode of 18 μm in length shows a power conversion efficiency of 7.34% because of its higher specific surface area for adsorbing more dye molecules and superior light scattering capacity for boosting the light-harvesting efficiency. The present photovoltaic performance is the highest value for the reported TiO(2) nanowires array photoelectrode.

The vertically oriented anatase single crystalline TiO2 nanostructure arrays (TNAs) consisting of TiO2 truncated octahedrons with exposed {001} facets or hierarchical TiO2 nanotubes (HNTs) consisting of numerous nanocrystals on Ti-foil substrate were synthesized via a two-step hydrothermal growth process. The first step hydrothermal reaction of Ti foil and NaOH leads to the formation of H-titanate nanowire arrays, which is further performed the second step hydrothermal reaction to obtain the oriented anatase single crystalline TiO2 nanostructures such as TiO2 nanoarrays assembly with truncated octahedral TiO2 nanocrystals in the presence of NH4F aqueous or hierarchical TiO2 nanotubes with walls made of nanocrystals in the presence of pure water. Subsequently, these TiO2 nanostructures were utilized to produce dye-sensitized solar cells in a backside illumination pattern, yielding a significant high power conversion efficiency (PCE) of 4.66% (TNAs, J(SC) = 7.46 mA cm(-2), V(OC) = 839 mV, FF = 0.75) and 5.84% (HNTs, J(SC) = 10.02 mA cm(-2), V(OC) = 817 mV, FF = 0.72), respectively.

Hierarchical TiO2, ZnO, SnO2 and Zn2SnO4 photoelectrodes have been prepared and shown significant photovoltaic performance in dye (quantum dot)-sensitized soalr cells. The electron transport and recombination of the solar cells have been investigated in detail.

Hierarchical one-dimensional SnO2 nanofibers (NFs) consisting of nanoparticles (NPs) have been synthesized and employed as an efficient light scattering layer to fabricate double layered dye-sensitized solar cells (DSSCs). The photovoltaic performance (5.44%) of the DSSC based on the SnO2 NPs (14.5 μm)/NFs (4.0 μm) double layered photoanode showed a 26.3% enhancement compared to that of a SnO2 NPs photoanode (18.5μm, 4.30%). Intensity modulation photocurrent spectroscopy (IMPS), intensity-modulated voltage spectroscopy (IMVS) and open-circuit voltage decay (OCVD) spectroscopy were used to investigate the kinetic process of electron transport and recombination within SnO2 NPs or NPs/NFs based DSSCs, revealing a faster electron transport rate and a lower recombination rate in the SnO2 NPs/NFs photoanode. The UV-vis diffuse reflectance spectroscopy results showed that the SnO2 NPs/NFs possess better light scattering ability which enhanced their photovoltaic performance compared to the SnO2 NPs. Furthermore, an overall power conversion efficiency of 6.31% (with a Jsc of 16.78 mA cm−2, Voc of 711 mV and FF of 0.53) was achieved for the SnO2 NPs/NFs double layered photoanode with an additional SnO2 blocking layer, which suppressed electron recombination between the FTO glass and the electrolyte.