PbI2 heterogeneous-cap-induced crystallization with a face-to-face configuration is proposed to obtain efficient CH3NH3PbI3 perovskite films during thermal annealing. The films with large-size and dominative (110)-oriented grains can effectively boost the photovoltaic performance of the perovskite solar cells.

Herein, we demonstrate that the facile face-down annealing route which effectively confines the evaporation of residual solvent molecules in one-step deposited precursor films can controllably enable the formation of (110) textured CH3NH3PbI3 films consisting of high-crystallinity well-ordered micrometer-sized grains that span vertically the entire film thickness. Such microstructural features dramatically decrease nonradiative recombination sites as well as greatly improve the transport property of charge carries in the films compared with that of the nontextured ones obtained by the conventional annealing route. As a consequence, the planar-heterojunction perovskite solar cells with these textured CH3NH3PbI3 films exhibit significantly enhanced power conversion efficiency (PCE) along with small hysteresis and excellent stability. The champion cell yields impressive PCE boosting to 18.64% and a stabilized value of around 17.22%. Particularly, it can maintain 86% of its initial value after storage for 20 days in ambient conditions with relative humidity of 10–20%. Our work suggests a facile and effective route for further boosting the efficiency and stability of low-cost perovskite solar cells.Keywords: CH3NH3PbI3; defects; face-down annealing; perovskite solar cells; texture development;

A homogeneous cap-mediated crystallization concept with face-to-face configuration is proposed here to modulate the crystallization kinetics of organolead triiodide perovskite (OTP) films in a controlled way. The introduced OTP caps, especially those with low surface roughness, play a dramatically positive role in retarding the nucleation rate, promoting the growth, and preventing the composition loss of OTP grains, fully facilitating the formation of pinhole-free OTP films with numerous desirable characteristics, such as greatly enlarged grains, vertically aligned grain boundaries, preferred (110) orientation, significantly improved crystallinity, and proper stoichiometry. As a consequence, planar-heterojunction solar cells incorporating these high-quality films deliver a promising average efficiency of 17.87%, showing a remarkable enhancement of approximately 30% compared with control samples. In particular, large fill factors can be routinely achieved in these high efficiency cells. This excellent performance mainly originates from greatly suppressed non-radiative recombination as well as from the greatly enhanced diffusion and transfer properties of charge carriers in the cells as a result of the improved quality of the OTP films. Our work presents an effective and useful strategy to fabricate high-performance planar heterojunction solar cells based on OTP materials.

As the light absorption layer, a CH3NH3PbI3 perovskite film with a fully and homogeneously covered flat surface is crucially important for perovskite photovoltaic devices. Herein, the sequential method is improved by introducing a spray-assisted solution process (SSP) instead of the commonly used dipping process in CH3NH3PbI3 perovskite film deposition. The morphology analysis for PbI2 films found that the airflow of the spray pre-wetting process provides a driving force for accelerating the dissolution and deep infiltration, which is helpful to form a mesoporous PbI2 scaffold. So the 2-propanol (IPA) pre-wetting has advantages to complete the reaction of PbI2 and CH3NH3I through the diffusion channel in the grain boundary and on the surface of the PbI2 film. Furthermore, the grain size and surface coverage of the film can be controlled effectively by a facile IPA pre-wetting process in the SSP. Meanwhile, the influence of the annealing process on the crystallization, chemical composition, microstructures and carrier transport properties of the perovskite films were investigated. Our results showed that an optimum annealing temperature and time can reduce crystal defects in the film and weaken the hysteresis phenomenon of the perovskite solar cell. Efficient mesoporous structured perovskite solar cells were fabricated with an optimal PCE of 14.2%, because of the improved perovskite film preparation method.

A dense and homogenous flat wide-bandgap (1.75 eV) CH3NH3PbI2.1Br0.9 perovskite film was prepared via a facile halide exchange route. The planar-heterojunction solar cell shows an optimal power conversion efficiency of 12.67% with negligible current hysteresis due to the film's large grains and vertically oriented grain boundaries.

Laser irradiation as a rapid crystallization approach was successfully introduced to prepare homogeneous, dense-grained CH3NH3PbI3 films. Planar-heterojunction solar cells employing these high-quality films showed the optimal efficiency of 17.8% with a remarkably high open-circuit voltage of 1.146 V.

Organolead trihalide perovskites (OTPs) such as CH3NH3PbI3 (MAPbI3) have attracted much attention as the absorbing layer in solar cells and photodetectors (PDs). Flexible OTP devices have also been developed. Transparent electrodes (TEs) with higher conductivity, stability, and flexibility are necessary to improve the performance and flexibility of flexible OTP devices. In this work, patterned Au nanowire (AuNW) networks with high conductivity and stability are prepared and used as TEs in self-powered flexible MAPbI3 PDs. These flexible PDs show peak external quantum efficiency and responsivity of 60% and 321 mA/W, which are comparable to those of MAPbI3 PDs based on ITO TEs. The linear dynamic range and response time of the AuNW-based flexible PDs reach ∼84 dB and ∼4 μs, respectively. Moreover, they show higher flexibility than ITO-based devices, around 90%, and 60% of the initial photocurrent can be retained for the AuNW-based flexible PDs when bent to radii of 2.5 and 1.5 mm. This work suggests a high-performance, highly flexible, and stable TE for OTP flexible devices.Keywords: flexibility; metal nanowire network; perovskite; photodetector; transparent electrode

Large organolead triiodide perovskite (OTP) grains with little intragranular defects are beneficial to minimize carrier recombination, hence boosting cell performance. However, OTP films deposited by the widely used one-step spin-coating route are usually composed of small grains, because the poor thermal stability of OTP inherently restricts the processing window (temperature, time) during the film preparation, thus limiting grain coarsening in the film. Herein, the remarkable grain coarsening via Ostwald ripening in one-step deposited OTP films has been successfully realized by a facile and effective post-synthesis high-temperature heating treatment assisted with spin-coated CH3NH3I. By systematically investigating the heating treatment parameters, a high-quality OTP film with an enlarged average grain size from ∼280 nm to 1.2 μm, greatly enhanced crystallinity, and excellent stoichiometry is achieved. Benefiting from such improved features, this modified film shows significantly reduced defect states corresponding to the decrease of recombination centers, as well as enhanced carrier transport and injection properties, which lead to the dramatically boosted efficiency from 14.54% to 16.88% for planar-heterojunction solar cells. More importantly, the improved OTP film quality provides the possibility of thickening the absorber layer of cells to realize more sufficient absorption without serious aggravation of charge recombination. By further optimizing the thickness of the coarsened OTP films, highly efficient cells with relatively excellent reproducibility and an optimal efficiency of 19.24% are achieved.

A Ni3S2 coated indium tin oxide (ITO) core–shell structure on flexible carbon fabrics (CF@ITO@Ni3S2) was prepared by electrodepositing Ni3S2 nanosheets on ITO nanowire arrays grown on flexible carbon fabrics by chemical vapor deposition. The ITO nanowires on carbon fabrics formed a conductive support which offered a large contact surface with the electrolyte and hence was accessible for ion diffusion. Ni3S2 nanosheets were uniformly deposited on the ITO nanowire. The maximum mass loading of Ni3S2 on ITO nanowire array coated carbon fabrics could reach about a quadruple higher amount than that on the bare carbon fabrics. The prepared CF@ITO@Ni3S2 electrodes exhibited excellent capacitive performance compared with Ni3S2 coated bare carbon fabric electrodes (CF@Ni3S2). High areal capacitance of 3.85 F cm−2 and gravimetric capacitance of 1865 F g−1 were achieved when the mass loadings of Ni3S2 on the ITO nanowire arrays were around 4.12 mg cm−2 and 0.96 mg cm−2, respectively. The sample with 0.96 mg cm−2 of Ni3S2 could also deliver 1372 F g−1 when charge–discharge current density reached 50 mA cm−2, indicating the excellent rate capability of the structure. The assembled all-solid-state full cell based on symmetric electrodes obtained a relatively high areal capacitance of 736 mF cm−2 at 8 mA cm−2, which delivered a maximum energy density of 1.02 mW h cm−3 at a power density of 39.9 W cm−3. The outstanding capacitive performance suggests that the CF@ITO@Ni3S2 device was promising for application in an inexpensive energy storage system.

•The halide exchange route was used to transform CH3NH3PbI3 films into CH3NH3PbI3−xBrx films.•The morphologies and compositions of the CH3NH3PbI3−xBrx films could be controllably modified.•The optimal cell with CH3NH3PbI3−xBrx film showed substantially improved efficiency of 14.25% and stability in air.A facile halide exchange route based on CH3NH3Br solution post-treatment has been successfully employed to transform the two-step spin-coated CH3NH3PbI3 films into the CH3NH3PbI3−xBrx films, whose morphologies and compositions could be modified simultaneously. With the increase of CH3NH3Br solution concentration, the CH3NH3PbI3−xBrx films exhibited increased grain size, prolonged charge carrier lifetime, and enlarged bandgap with slightly reduced light absorption compared with the parent CH3NH3PbI3 film. The mesostructured perovskite solar cells based on the CH3NH3PbI3−xBrx films yielded the optimal power conversion efficiency (PCE) of 14.25%, which is much higher than that of device with the parent CH3NH3PbI3 film. In particular, the device based on the CH3NH3PbI3−xBrx film retained up to 93% of its original PCE after exposed to air for 14 days without any encapsulation, presenting a favorable stability. Our work suggests a novel and attractive way to fabricate high-performance perovskite solar cells with excellent stability.The perovskite solar cells based on CH3NH3PbI3−xBrx film via a facile halide exchange route showed significantly enhanced efficiency and stability in atmosphere.

A high-quality CH3NH3PbI3 film is crucial in the manufacture of a high-performance perovskite solar cell. Here, a recrystallization process via facile fumigation with DMF vapor has been successfully introduced to self-repair of CH3NH3PbI3 films with poor coverage and low crystallinity prepared by the commonly used one-step spin-coating method. We found that the CH3NH3PbI3 films with dendritic structures can spontaneously transform to the uniform ones with full coverage and high crystallinity by adjusting the cycles of the recrystallization process. The mesostructured perovskite solar cells based on these repaired CH3NH3PbI3 films showed reproducible optimal power conversion efficiency (PCE) of 11.15% and average PCE of 10.25 ± 0.90%, which are much better than that of devices based on the non-repaired CH3NH3PbI3 films. In addition, the hysteresis phenomenon in the current–voltage test can also be effectively alleviated due to the quality of the films being improved in the optimized devices. Our work proved that the fumigation of solvent vapor can modify metal organic perovskite films such as CH3NH3PbI3. It offers a novel and attractive way to fabricate high-performance perovskite solar cells.

Organic–inorganic lead halide perovskite compounds are very promising materials for high-efficiency perovskite solar cells. But how to fabricate high-quality perovksite films under controlled humidity conditions is still an important issue due to their sensitivity to moisture. In this study, we investigated the influence of ambient humidity on crystallization and surface morphology of one-step spin-coated perovskite films, as well as the performance of solar cells based on these perovskite films. On the basis of experimental analyses and thin film growth theory, we conclude that the influence of ambient humidity on nucleation at spin-coating stage is quite different from that on crystal growth at annealing stage. At the spin-coating stage, high nucleation density induced by high supersaturation prefers to appear under anhydrous circumstances, resulting in layer growth and high coverage of perovskite films. But at the annealing stage, the modest supersaturation benefits formation of perovskite films with good crystallinity. The films spin-coated under low relative humidity (RH) followed by annealing under high RH show an increase of crystallinity and improved performance of devices. Therefore, a mechanism of fast nucleation followed by modest crystal growth (high supersaturation at spin-coating stage and modest supersaturation at annealing stage) is suggested in the formation of high-quality perovskite films.Keywords: ambient humidity; crystal growth; film coverage; methylammonium lead trihalide; nucleation density;

Hematite (α-Fe2O3) is one of the most promising candidates for photoelectrodes in photoelectrochemical water splitting system. However, the low visible light absorption coefficient and short hole diffusion length of pure α-Fe2O3 limits the performance of α-Fe2O3 photoelectrodes in water splitting. Herein, to overcome these drawbacks, single-crystalline tin-doped indium oxide (ITO) nanowire core and α-Fe2O3 nanocrystal shell (ITO@α-Fe2O3) electrodes were fabricated by covering the chemical vapor deposited ITO nanowire array with compact thin α-Fe2O3 nanocrystal film using chemical bath deposition (CBD) method. The J–V curves and IPCE of ITO@α-Fe2O3 core–shell nanowire array electrode showed nearly twice as high performance as those of the α-Fe2O3 on planar Pt-coated silicon wafers (Pt/Si) and on planar ITO substrates, which was considered to be attributed to more efficient hole collection and more loading of α-Fe2O3 nanocrystals in the core–shell structure than planar structure. Electrochemical impedance spectra (EIS) characterization demonstrated a low interface resistance between α-Fe2O3 and ITO nanowire arrays, which benefits from the well contact between the core and shell. The stability test indicated that the prepared ITO@α-Fe2O3 core–shell nanowire array electrode was stable under AM1.5 illumination during the test period of 40 000 s.Keywords: core−shell; ITO; nanowire array; photoanode; water splitting; α-Fe2O3

The resistance of the perovskite CH3NH3PbI3 film was found to decrease significantly in seconds when the film was exposed to an NH3 atmosphere at room-temperature, and recover to its original value in seconds when out of the NH3 environment.

Self-doping by Ti3+ is a useful method to expand the light response of TiO2 into the visible light region. However, to obtain a stable Ti3+-doped TiO2 seems to be a challenge due to the easy oxidation of Ti3+ during the heterogeneous reaction. Here, we propose a simple carbon coating route to stabilize the Ti3+-doped TiO2, in which both the Ti3+ and precursor of the carbon coating layer were in situ formed from the hydrothermal hydrolysis of titanium isopropoxide. The carbon coated Ti3+-doped TiO2 exhibited excellent stability for photocatalytic hydrogen production. Based on electron paramagnetic resonance (EPR) analysis, the proposed stabilizing mechanism is that the conductive carbon coating layer as a barrier layer prevents the H2O and O2 from diffusing into the surface of the photocatalyst, which can oxidize the surface O vacancies and Ti3+ in TiO2. Our findings offer a simple route to prepare a highly stable TiO2-based photocatalyst with visible light response.

Easily fabricated, directly transferred thin nanoporous gold was first used as a back electrode for hole-transport-material-free perovskite solar cells. In order to infiltrate CH3NH3PbI3 into the pores of mesoporous layers and the nanoporous gold back electrode, three ways, namely one-step spin-coating deposition, sequential deposition, and two-step spin-coating deposition were introduced to fabricate CH3NH3PbI3. The devices containing well infiltrated CH3NH3PbI3 show the highest power conversion efficiency of 7.99%.

•The CH3NH3PbI3 films show a smooth surface of RMS only16 nm.•PSC's average Voc has been raised up from 0.823±0.105 V to 0.940±0.008 V.•The average PCE of mesostructure PSC could be promoted by 25% approximately.A homogenous coverage of the CH3NH3PbI3 light absorption layer is crucially important for a high-performance metal–organic halide perovskite solar cell (PSC). Herein, we found that a facile spray-assisted process instead of the commonly used dipping process in a two-step spin-coating method can effectively reduce the roughness (of RMS only 16 nm), enhance the coverage of the light absorption layer and eliminate the pinholes in the layer. The experimental results demonstrated that the PSC's average open-circuit voltage (Voc) has been raised up from 0.823±0.105 V to 0.940±0.008 V by the spray-assisted process. It benefits from the low leakage possibility between the hole-transport-material (HTM) and the mesoporous TiO2 layer when a smooth and pinhole-free light absorption layer has successfully formed. Finally the average power conversion efficiencies of the mesostructure PSCs could be promoted by 25% approximately.A facile spray-assisted fabrication of homogenous flat CH3NH3PbI3 films (of RMS only 16 nm and pinhole free) for high performance mesostructure perovskite solar cells.

We have developed a facile and compatible method to in situ fabricate uniform metal nanowire networks on substrates. The as-fabricated metal nanowire networks show low sheet resistance and high transmittance (2.2 Ω sq–1 at T = 91.1%), which is equivalent to that of the state-of-the-art metal nanowire networks. We demonstrated that the transmittance of the metal networks becomes homogeneous from deep-ultraviolet (200 nm) to near-infrared (2000 nm) when the size of the wire spacing increases to micrometer size. Theoretical and experimental analyses indicated that we can improve the conductivity of the metal networks as well as keep their transmittance by increasing the thickness of the metal films. We also carried out durability tests to demonstrate our as-fabricated metal networks having good flexibility and strong adhesion.Keywords: electrospinning; flexibility; metal nanowire network; transparent conductive electrode;

In situ grown nickel sulfide and cobalt sulfide hierarchical nanospheres on F-doped SnO2 (FTO) substrates exhibited comparable catalytic activities to sputtering Pt on the counter electrodes for dye-sensitized solar cells (DSSCs). The fresh cells with the nickel sulfide and cobalt sulfide on the counter electrodes could reach power conversion efficiencies of 6.81% and 6.59% respectively, approaching an efficiency of 6.85% based on the sputtering Pt counter electrode. Both nickel sulfide and cobalt sulfide counter electrodes could maintain the cell's relatively high performance in the long-term stability test in 504 hours.

One-dimensional (1D) assembly of TiO2 nanoparticles (NPs) was successfully achieved by an improved electrospinning technique. Electrospinning precursor solution was prepared with conventional TiO2 NPs and polyvinylpyrrolidone dispersed in ethanol. The newly developed 1D assembly of nanoparticles (AS-NP) has been introduced into the photoanode in dye-sensitized solar cells (DSSCs). Compared to the traditional disordered stacking of TiO2 NPs, the AS-NPs bring in faster charge transport and longer electron lifetime, as well as a higher light scattering ability (especially in the wavelength range from 500 nm to 650 nm). It is exhibited that the AS-NPs can enhance electron transport and light scattering, while retaining the merits of NP morphology. Consequently, the efficiency of a cell based on an AS-NP/NP bilayer photoanode could be improved by about 15% in comparison to a reference cell made of absolute TiO2 NPs.

•Cr,B-codoped-SrTiO3 was successfully synthesized by one-step hydrothermal method.•Cr,B-codoped-SrTiO3 had stronger visible light absorption and smaller band gap (2.07 eV) than Cr-doped-SrTiO3.•The strong interaction of Cr 3d with substitutional B 2p levels lead to the stronger visible light absorption and smaller band gap.•Cr,B-codoped SrTiO3 photocatalyst showed higher photocatalytic activity for hydrogen evolution (15.4 μmol/h) than that over Cr-doped SrTiO3 (9.3 μmol/h).Cr,B-codoped-SrTiO3 was successfully synthesized by one-step hydrothermal method. XRD, FTIR, Raman and XPS results showed that B existed in the forms of both substitutional B for O and interstitial B in the bulk of SrTiO3. UV-Vis absorption spectra showed that Cr,B-codoped-SrTiO3 had strong visible light absorption and small band gap (2.07 eV). Theoretical calculation pointed out that the strong p–d repulsion of substitutional B 2p with Cr 3d was responsible for the narrowing of band gap and enhanced visible light absorption. Our results indicate that the codoping of Cr and B offers a novel route to modify the light response of SrTiO3. The photocatalytic activity for hydrogen production over Cr,B-codoped SrTiO3 photocatalyst is 15.4 μmol/h, which is higher than that over Cr-doped SrTiO3 (9.3 μmol/h) synthesized by the similar method. The higher activity for Cr,B-codoped SrTiO3 is attributed to its smaller band gap than Cr-doped SrTiO3.

A novel porous fluorine doped tin oxide (PFTO) conductive framework was introduced to counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). When modified by platinum (Pt) or carbon (C), the PFTO conductive framework displays high catalytic activity to I−/I3− redox couples. Power conversion efficiencies of 6.09% and 5.81% were obtained in the DSSCs based on Pt and C modified PFTO CEs respectively, which were close to that of DSSCs with Pt coated FTO glass (6.05%) and Pt sheet (6.26%) CEs. Maximum limiting performances of the CEs were obtained from the polarization curves. The CE based on PFTO showed higher maximum limiting power conversion efficiency (∼20%) compared with the planar FTO substrate Pt CE (∼18%), with the increase of its surface area and electrocatalytic activity.

Vertically oriented CuInS2 nanosheet thin films were prepared via a facile one-step solvothermal process and used in dye-sensitized solar cells (DSSCs) as counter electrodes. The catalytic activity of the CuInS2 films based on different precursor concentrations was investigated using electrochemical methods. DSSCs based on optimized CuInS2 thin film as counter electrodes reached a power conversion efficiency of 6.33%, comparable to that of sputtering Pt (6.07%).

A dye-free photoelectrochemical (PEC) solar cell based on BiVO4 oxide film was developed. Without a bias, a high incident photo-to-current conversion efficiency (IPCE) of 43% was obtained and the solar energy conversion efficiency of the cell was 14 times higher than the record efficiency of Fe2O3 PEC solar cells. Ultra-fast transient absorption spectroscopy was utilized to investigate the mechanism of the different photoelectric performances. The results revealed that there was a longer lifetime of photogenerated carriers in BiVO4 than that in Fe2O3. It was helpful to develop other suitable photoelectrode materials for dye-free PEC solar cells.Highlights► A dye-free PEC solar cell based on BiVO4 oxide film. ► An IPCE of 43%. ► The efficiency is 14 times higher than the record efficiency of Fe2O3 solar cells. ► Ultra-fast transient absorption spectroscopy was used to investigate the mechanism. ► There is a longer lifetime of photogenerated carriers in BiVO4 than that in Fe2O3.

The champion dye-sensitized solar cells (DSSCs) based on TiO2 nanoparticles nearly reach the limit of photo-current density using the black dye or zinc porphyrin dye as sensitizer. However, the way to make ordinary DSSCs more efficient as well as to understand the mechanism is still essential. Here we present an elegant UV irradiation treatment of TiO2 nanosheets to enhance the performance of DSSCs based on the TiO2 nanosheets via room temperature removal of inorganic surfactants and reconstruction of the (001) surface of TiO2 nanosheets, killing two birds with one stone. UV irradiation was utilized to remove the fluorine-surfactant on the surface of anatase TiO2 nanosheets with a high percentage of exposed {001} facets which were synthesized with the aid of hydrofluoric acid. The nanosheets treated with UV irradiation for 40 min had the advantage of improving the photoelectric conversion efficiency of DSSCs by 17.6%, compared to that without UV treatment when they were introduced into DSSCs as photoanode materials. The improved efficiency was ascribed to more dye adsorption. A theoretical calculation proposed that UV irradiation induced microfaceted steps on the TiO2 surface by two domain (1 × 4) reconstruction after UV irradiating the (1 × 1) (001) surface. The microfaceted steps increase the active surface area of the TiO2 nanosheets by increasing the exposure of titanium atoms and engendering active sites.

A ZnO compact layer prepared by a sol–gel method was introduced into a photoelectrode at the interface between fluorine-doped tin oxide (FTO) substrate and a mesoporous ZnO layer in ZnO-based dye-sensitized solar cells (DSSCs). The ZnO compact layer was characterized by field-emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and UV-Vis spectroscopy. The compact layer increased the photoelectric conversion efficiency of ZnO-based DSSCs by 20%. An electrochemical impedance spectroscopy (EIS) study demonstrated that the compact layer strikingly reduced the interfacial resistance in the device by enhancing the conductive contact between nanocrystalline ZnO and FTO substrate. The photocurrent density–voltage characteristics in the dark suggests that the compact ZnO layer also plays the role of a blocking layer suppressing the charge recombination, which is illustrated by the suppression of dark current density. The two effects effectively elevate the short circuit current density (JSC) and open circuit voltage (VOC), and finally improve the overall conversion efficiency of ZnO-based DSSCs.

A series of new-type of donor–π–acceptor dyes (TCT-1–6) utilizing 1,3,5-triazine as π spacers were synthesized. These dyes were characterized by 1H NMR, ESI-MS, EA, and X-ray crystallography. Their photovoltaic performances were also investigated. An overall photon-to-electron conversion efficiency of 1.8% was achieved with the DSSC based on the dye TCT-1(Jsc = 3.33 mA/cm2, Voc = 757 mV, FF = 71.8%) under AM 1.5G illumination (100 mW/cm2).

The assembly of NiO nanodiscs (namely nanoflowers) as well as the dispersed NiO nanodiscs have been successfully synthesized via the thermal decomposition of Ni(OH)2 obtained from different Ni sources in non-basic solution. The route is environment-friendly. The materials were characterized by X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM) and N2 adsorption–desorption. The porous structures with pore size around 6 nm can be observed on the single NiO disc. The nanoflowers exhibit better performance than nanodiscs in the electrochemical test and water treatment experiments, due to much more available surface areas and spaces formed in the NiO nanoflowers.Research highlights▶ NiO nanodiscs and nanoflowers have been controllably fabricated via the thermal decomposition of Ni(OH)2 by using different Ni sources in non-basic solution for anion-assisted effect. ▶ The route is environment-friendly. ▶ The nanoflowers exhibit better performance than the nanodiscs when they are applied in electrochemical test and water treatment.

Na1−xLaxTa1−xCrxO3 and NaTa1−xCrxO3 (x = 0.01, 0.03, 0.05 and 0.10) have been synthesized by a solid state reaction method. These photocatalysts can produce H2 in the presence of methanol under visible light irradiation (λ > 420 nm). The photocatalytic activities of Na1−xLaxTa1−xCrxO3 are much higher than those of NaTa1−xCrxO3, respectively. Especially, the H2 evolution rate of Na0.9La0.1Ta0.9Cr0.1O3 is 2.2 μmol h−1, which is nearly 4 times higher than that of NaTa0.9Cr0.1O3 (0.6 μmol h−1). The improved activities of Na1−xLaxTa1−xCrxO3 compared with NaTa1−xCrxO3 can be ascribed to two factors: one is smaller particle size and higher specific surface area which is caused by the doping of lanthanum; the other is that Na1−xLaxTa1−xCrxO3 has less Cr6+, which is induced by codoping of lanthanum and chromium.

Facilitated by TiO2 particles absorbing La3+ in hydrosol, La-doped TiO2 was prepared by a sol-hydrothermal method. Electron paramagnetic resonance and Brunauer−Emmett−Teller (BET) surface area analysis showed that the obtained La-doped anatase TiO2 surface provided a higher density of oxygen vacancies without a change in the BET surface area. A theoretical calculation was carried out to explain the generation mechanism of the increased oxygen vacancies. The results showed that the La-doped anatase TiO2 (101) surface tends to engender oxygen vacancies. The photoelectric conversion efficiency of dye-sensitized solar cells fabricated from 1 mol % La-doped TiO2 reached 6.72%, which gave an efficiency improved by 13.5% compared with that of the cells fabricated from pure TiO2. The improvement in the efficiency was ascribed to more dye absorbed on the surface of TiO2.

In this report, an improved method to estimate the equivalent circuit parameters in the dye-sensitized solar cells (DSSCs) is introduced. It is founded that, several different groups of values of equivalent circuit parameters can fit well to the same experiment-measured I–V curve. Furthermore, the gap between some parameter values in those different groups is so large that it reaches up to several orders of magnitude. To eliminate this uncertainty of parameter estimation, an improved method, which based on both the extention of measured range and the computer simulation of current–voltage (I–V) characteristic curves, is proposed to ensure uniquely the values of DSSCs equivalent circuit parameters. A series of I–V curves which derived from the estimated parameters by this improved method fit well to the corresponding experiment-measured I–V curves. The results indicate that, there exclusively exists one group of parameter values for a special DSSCs equivalent circuit, thus demonstrating the validity of the improved method proposed in this work.

As the light absorption layer, a CH3NH3PbI3 perovskite film with a fully and homogeneously covered flat surface is crucially important for perovskite photovoltaic devices. Herein, the sequential method is improved by introducing a spray-assisted solution process (SSP) instead of the commonly used dipping process in CH3NH3PbI3 perovskite film deposition. The morphology analysis for PbI2 films found that the airflow of the spray pre-wetting process provides a driving force for accelerating the dissolution and deep infiltration, which is helpful to form a mesoporous PbI2 scaffold. So the 2-propanol (IPA) pre-wetting has advantages to complete the reaction of PbI2 and CH3NH3I through the diffusion channel in the grain boundary and on the surface of the PbI2 film. Furthermore, the grain size and surface coverage of the film can be controlled effectively by a facile IPA pre-wetting process in the SSP. Meanwhile, the influence of the annealing process on the crystallization, chemical composition, microstructures and carrier transport properties of the perovskite films were investigated. Our results showed that an optimum annealing temperature and time can reduce crystal defects in the film and weaken the hysteresis phenomenon of the perovskite solar cell. Efficient mesoporous structured perovskite solar cells were fabricated with an optimal PCE of 14.2%, because of the improved perovskite film preparation method.

The resistance of the perovskite CH3NH3PbI3 film was found to decrease significantly in seconds when the film was exposed to an NH3 atmosphere at room-temperature, and recover to its original value in seconds when out of the NH3 environment.

In situ grown nickel sulfide and cobalt sulfide hierarchical nanospheres on F-doped SnO2 (FTO) substrates exhibited comparable catalytic activities to sputtering Pt on the counter electrodes for dye-sensitized solar cells (DSSCs). The fresh cells with the nickel sulfide and cobalt sulfide on the counter electrodes could reach power conversion efficiencies of 6.81% and 6.59% respectively, approaching an efficiency of 6.85% based on the sputtering Pt counter electrode. Both nickel sulfide and cobalt sulfide counter electrodes could maintain the cell's relatively high performance in the long-term stability test in 504 hours.

The champion dye-sensitized solar cells (DSSCs) based on TiO2 nanoparticles nearly reach the limit of photo-current density using the black dye or zinc porphyrin dye as sensitizer. However, the way to make ordinary DSSCs more efficient as well as to understand the mechanism is still essential. Here we present an elegant UV irradiation treatment of TiO2 nanosheets to enhance the performance of DSSCs based on the TiO2 nanosheets via room temperature removal of inorganic surfactants and reconstruction of the (001) surface of TiO2 nanosheets, killing two birds with one stone. UV irradiation was utilized to remove the fluorine-surfactant on the surface of anatase TiO2 nanosheets with a high percentage of exposed {001} facets which were synthesized with the aid of hydrofluoric acid. The nanosheets treated with UV irradiation for 40 min had the advantage of improving the photoelectric conversion efficiency of DSSCs by 17.6%, compared to that without UV treatment when they were introduced into DSSCs as photoanode materials. The improved efficiency was ascribed to more dye adsorption. A theoretical calculation proposed that UV irradiation induced microfaceted steps on the TiO2 surface by two domain (1 × 4) reconstruction after UV irradiating the (1 × 1) (001) surface. The microfaceted steps increase the active surface area of the TiO2 nanosheets by increasing the exposure of titanium atoms and engendering active sites.

PbI2 heterogeneous-cap-induced crystallization with a face-to-face configuration is proposed to obtain efficient CH3NH3PbI3 perovskite films during thermal annealing. The films with large-size and dominative (110)-oriented grains can effectively boost the photovoltaic performance of the perovskite solar cells.

A dense and homogenous flat wide-bandgap (1.75 eV) CH3NH3PbI2.1Br0.9 perovskite film was prepared via a facile halide exchange route. The planar-heterojunction solar cell shows an optimal power conversion efficiency of 12.67% with negligible current hysteresis due to the film's large grains and vertically oriented grain boundaries.

Vertically oriented CuInS2 nanosheet thin films were prepared via a facile one-step solvothermal process and used in dye-sensitized solar cells (DSSCs) as counter electrodes. The catalytic activity of the CuInS2 films based on different precursor concentrations was investigated using electrochemical methods. DSSCs based on optimized CuInS2 thin film as counter electrodes reached a power conversion efficiency of 6.33%, comparable to that of sputtering Pt (6.07%).

A homogeneous cap-mediated crystallization concept with face-to-face configuration is proposed here to modulate the crystallization kinetics of organolead triiodide perovskite (OTP) films in a controlled way. The introduced OTP caps, especially those with low surface roughness, play a dramatically positive role in retarding the nucleation rate, promoting the growth, and preventing the composition loss of OTP grains, fully facilitating the formation of pinhole-free OTP films with numerous desirable characteristics, such as greatly enlarged grains, vertically aligned grain boundaries, preferred (110) orientation, significantly improved crystallinity, and proper stoichiometry. As a consequence, planar-heterojunction solar cells incorporating these high-quality films deliver a promising average efficiency of 17.87%, showing a remarkable enhancement of approximately 30% compared with control samples. In particular, large fill factors can be routinely achieved in these high efficiency cells. This excellent performance mainly originates from greatly suppressed non-radiative recombination as well as from the greatly enhanced diffusion and transfer properties of charge carriers in the cells as a result of the improved quality of the OTP films. Our work presents an effective and useful strategy to fabricate high-performance planar heterojunction solar cells based on OTP materials.

Laser irradiation as a rapid crystallization approach was successfully introduced to prepare homogeneous, dense-grained CH3NH3PbI3 films. Planar-heterojunction solar cells employing these high-quality films showed the optimal efficiency of 17.8% with a remarkably high open-circuit voltage of 1.146 V.

Large organolead triiodide perovskite (OTP) grains with little intragranular defects are beneficial to minimize carrier recombination, hence boosting cell performance. However, OTP films deposited by the widely used one-step spin-coating route are usually composed of small grains, because the poor thermal stability of OTP inherently restricts the processing window (temperature, time) during the film preparation, thus limiting grain coarsening in the film. Herein, the remarkable grain coarsening via Ostwald ripening in one-step deposited OTP films has been successfully realized by a facile and effective post-synthesis high-temperature heating treatment assisted with spin-coated CH3NH3I. By systematically investigating the heating treatment parameters, a high-quality OTP film with an enlarged average grain size from ∼280 nm to 1.2 μm, greatly enhanced crystallinity, and excellent stoichiometry is achieved. Benefiting from such improved features, this modified film shows significantly reduced defect states corresponding to the decrease of recombination centers, as well as enhanced carrier transport and injection properties, which lead to the dramatically boosted efficiency from 14.54% to 16.88% for planar-heterojunction solar cells. More importantly, the improved OTP film quality provides the possibility of thickening the absorber layer of cells to realize more sufficient absorption without serious aggravation of charge recombination. By further optimizing the thickness of the coarsened OTP films, highly efficient cells with relatively excellent reproducibility and an optimal efficiency of 19.24% are achieved.

Self-doping by Ti3+ is a useful method to expand the light response of TiO2 into the visible light region. However, to obtain a stable Ti3+-doped TiO2 seems to be a challenge due to the easy oxidation of Ti3+ during the heterogeneous reaction. Here, we propose a simple carbon coating route to stabilize the Ti3+-doped TiO2, in which both the Ti3+ and precursor of the carbon coating layer were in situ formed from the hydrothermal hydrolysis of titanium isopropoxide. The carbon coated Ti3+-doped TiO2 exhibited excellent stability for photocatalytic hydrogen production. Based on electron paramagnetic resonance (EPR) analysis, the proposed stabilizing mechanism is that the conductive carbon coating layer as a barrier layer prevents the H2O and O2 from diffusing into the surface of the photocatalyst, which can oxidize the surface O vacancies and Ti3+ in TiO2. Our findings offer a simple route to prepare a highly stable TiO2-based photocatalyst with visible light response.