Perovskite solar cells based on screen-printed thin films.

One potential advantage of perovskite solar cells (PSCs) is the ability to solution process the precursors and deposit films from solution1,2. At present, spin coating, blade coating, spray coating, inkjet printing and slot-die printing have been investigated to deposit hybrid perovskite thin films3-6. Here we expand the range of deposition methods to include screen-printing, enabled by a stable and viscosity-adjustable (40-44,000 cP) perovskite ink made from a methylammonium acetate ionic liquid solvent. We demonstrate control over perovskite thin-film thickness (from about 120 nm to about 1,200 nm), area (from 0.5 × 0.5 cm2 to 5 × 5 cm2) and patterning on different substrates. Printing rates in excess of 20 cm s-1 and close to 100% ink use were achieved. Using this deposition method in ambient air and regardless of humidity, we obtained the best efficiencies of 20.52% (0.05 cm2) and 18.12% (1 cm2) compared with 20.13% and 12.52%, respectively, for the spin-coated thin films in normal devices with thermally evaporated metal electrodes. Most notably, fully screen-printing devices with a single machine in ambient air have been successfully explored. The corresponding photovoltaic cells exhibit high efficiencies of 14.98%, 13.53% and 11.80% on 0.05-cm2, 1.00-cm2 and 16.37-cm2 (small-module) areas, respectively, along with 96.75% of the initial efficiency retained over 300 h of operation at maximum power point.

Chemical bath deposition of SnO2films on PEN/ITO substrates for efficient flexible perovskite solar cells.

Flexible perovskite solar cells (f-PSCs) have achieved significant success. However, high-quality tin dioxide (SnO2) electron transport layers (ETLs) fabricated via chemical bath deposition (CBD) have not been achieved on flexible PEN/ITO substrates. This limitation is primarily due to the corrosion of the poor-quality ITO layer by the strongly acidic CBD solution. Here, we analyzed the reasons for the poor corrosion resistance of ITO films on PEN substrate from multiple perspectives, such as element composition, microstructure, and crystallinity. Then, we proposed a modified CBD method for SnO2films suitable for flexible PEN/ITO substrates. We employed SnCl2·2H2O as the tin source and regulated the pH of the CBD solution by NH3·H2O, which effectively avoided the corrosion of the ITO layer by the CBD solution and achieved high-quality SnO2films on the ITO layers. Compared to the commercial SnO2dispersion, the SnO2films prepared by this method have smaller grains and higher transmittance. As a result, we achieved an unprecedented power conversion efficiency (PCE) of 20.71% for f-PSCs fabricated on PEN/ITO substrates with SnO2ETLs by CBD method. This breakthrough facilitates the development of high-performance f-PSCs by a low-cost and large-scale chemical bath deposition of high-quality ETLs on flexible substrates.

Precursor engineering for efficient and stable perovskite solar cells.

The perovskite film prepared by the two-step spin coating method is widely used in photovoltaic devices due to its good film morphology and great reproducibility. However, there usually exists excessive lead iodide (PbI2) in the perovskite film for this method, which is believed to passivate the grain boundaries (GBs) to increase the efficiency of the perovskite solar cells. Nevertheless, the excessive PbI2at the GBs of perovskite is believed to induce the decomposition of the perovskite film and undermine the long-term stability of devices. In this study, we utilize precursor engineering to realize the preparation of perovskite solar cells with high efficiency and stability. The concentration of organic salts (AX: A = MA+, FA+; X = I-, Cl-) in the precursor solution for the second step of the two-step spin coating method is adjusted to optimize the perovskite light-absorbing layer so that the excessive PbI2is converted into perovskite to obtain a smooth and pinhole-free perovskite film with high performance. Our results indicate that by adjusting the concentration of AX in the precursor solution, PbI2in the film could be completely converted into perovskite without excessive AX residue. Both the efficiency and stability of the perovskite solar cells without excessive PbI2have been significantly improved. A planar perovskite solar cell with the highest power conversion efficiency (PCE) of 21.26% was achieved, maintaining about 90% of the initial PCE after 300 h of storage in a dry air environment and in the dark, about 76% of the initial PCE after 300 h of continuous illumination of 1 Sun.

Effect of Br content on phase stability and performance of H2N=CHNH2Pb(I1-x Br x )3 perovskite thin films.

Pristine and Br-doped H2N = CHNH2Pb(I1-x Br x )3 (FAPb(I1-x Br x )3, Br content x = 0, 0.05, 0.15, 0.2, 0.3, and 0.4) films were prepared. The effect of Br-doping on phase stability, defect density, and performance of FAPb(I1-x Br x )3 was investigated by x-ray diffraction (XRD), scanning electron microscopy, ultraviolet-visible-near infrared absorbance spectroscopy, x-ray photoemission spectroscopy (XPS), Kelvin probe force microscopy (KPFM), positron annihilation spectroscopy, and current density-voltage (J-V) characteristics. The XRD measurements exhibit the enhancement of perovskite phase stability at x = 0.05. However, the phase stability decreases gradually with Br content (x) over 0.05. The increase of Br-doping content leads to the downshifting of both valence band (VB) position (indicated by XPS) and Fermi level (illustrated by KPFM). The energy level shifts are most probably due to the increase of Br 4p orbital content in VB and the change of self-doping levels. Doppler broadening spectra of positron annihilation radiation of the samples reveal that, the defect densities of Br-doped samples are much lower than that of pristine FAPbI3. For FAPb(I0.95Br0.05)3 sample, a high photoelectric conversion efficiency of 17.12% (25.7% higher than that of undoped sample) is successfully achieved. The significant enhancement of photoelectric conversion efficiency realized by Br-doping is attributed to the improvement of morphology, high phase stability, and low defect densities.

Influence of hole transport material/metal contact interface on perovskite solar cells.

Interfaces have a significant impact on the performance of perovskite solar cells. This work investigated the influence of hole transport material/metal contact interface on photovoltaic behaviours of perovskite solar devices. Different hole material/metal contact interfaces were obtained by depositing the metal under different conditions. High incident kinetic energy metal particles were proved to penetrate and embed into the hole transport material. These isolated metal particles in hole transport materials capture holes and increase the apparent carrier transport resistance of the hole transport layer. Sample temperature was found to be of great significance in metal deposition. Since metal vapour has a high temperature, the deposition process accumulated a large amount of heat. The heat evaporated the additives in the hole transport layer and decreased the hole conductivity. On the other hand, high temperature may cause iodization of the metal contact.

Fast Fabrication of a Stable Perovskite Solar Cell with an Ultrathin Effective Novel Inorganic Hole Transport Layer.

With the aim of fabricating simple, reproducible, and scalable perovskite solar cells (PSCs) with least time consumption, a novel CoOx hole transport layer (HTL) was first proposed and introduced in this work. The CoOx HTL thickness was minimized to about 10 nm with complete coverage on the FTO substrate (F-doped SnO2) by direct current magnetron sputtering. The ultrathin HTL could minimize the incident light loss caused by cobalt ion absorption and reduce the carrier transport loss by shortening the transport path. Copper was incorporated into the CoOx lattice to address the low conductivity of the CoOx film and the energy-level mismatch between CoOx and the perovskite material. On the basis of cobalt-copper binary oxide (Co1-yCuyOx), the highest power conversion efficiency (PCE) of about 10% was achieved, which was acceptable for mass production. Moreover, the deposition of such Co1-yCuyOx films takes only 2 min without size limitation of substrates. A well-functioned device based on the Co1-yCuyOx HTL could hence be fabricated within 100 min. Excellent stability was demonstrated as well, with over 90% of the initial PCE remaining after being stored in a dark and humid environment (relative humidity 60%) for 12 days.

One step spray-coated TiO2 electron-transport layers for decent perovskite solar cells on large and flexible substrates.

Spray-coating as a facile and quantitative method was introduced to prepare thin and continuous TiO2 compact layers on different substrates for perovskite solar cells. The as-prepared film is highly transparent and smooth, which is of significance in perovskite solar cells to decrease incident light loss and facilitate the film cast and electric contact. The compact TiO2 layer shows excellent performance when coated with perovskite and assembled into a device. Since it provides unlimited substrate size, patterning function and the TiO2 used for spray-coating is well crystallized, this method has huge potential for mass production and great adaptability for a variety of applications.

Enhanced electrical property of Ni-doped CoOx hole transport layer for inverted perovskite solar cells.

Ultrathin Ni doped CoOx films were prepared by direct current co-sputtering at room temperature as inorganic hole transport materials for inverted perovskite solar cells. P-type doping was designed to adjust the valence band position of CoOx to match the that of CH3NH3PbI3, which would effectively eliminate the interface barrier. Moreover, the hole extraction ability would be enhanced and the power conversion efficiency of the devices hence increased from 3.68% to 9.60%. The optimized performance was also accompanied by decent stability as a result of its intrinsic stability.

Mesostructured perovskite solar cells based on highly ordered TiO2 network scaffold via anodization of Ti thin film.

An anodized TiO2 interconnected network was fabricated and utilized as a mesoporous scaffold and electron transporter in perovskite solar cells. By modifying the synthesis parameters, the morphological features of the interconnected TiO2 nanostructures can be widely tuned and precisely controlled. The functional properties of the anodized TiO2 network are found to be severely influenced by morphology as well as the extent of oxidation. The device with the optimized TiO2 network exhibits superior electron extraction and transferability, resulting in conspicuous enhancement of the photocurrent and power conversion efficiency (PCE). This work proposes a promising and facile method for improving the performance of perovskite solar cells.

Vacuum-Assisted Drying Process for Screen-Printable Carbon Electrodes of Perovskite Solar Cells with Enhanced Performance Based on Cuprous Thiocyanate as a Hole Transporting Layer.

Carbon-based perovskite solar cells without a hole transport layer (HTL) are considered to be highly stable and of low cost. However, the deficient interface contact and inferior hole extraction capability restrict the further improvement of the device efficiency. Introducing a hole transporting layer, such as cuprous thiocyanate (CuSCN), can enhance the hole extraction ability and improve the interface contact. However, our further studies indicated that-at a certain temperature-for carbon-based solar cells, in the CuSCN layer, the diffusion of SCN- into the perovskite film would produce more interfacial defects and aggravate nonradiative recombination, thus hindering the carrier transport. We further disclosed the reasons for performance attenuation during the thermal treatment of carbon electrodes, proposed a vacuum-assisted drying process for carbon electrodes to suppress the destructive effect, and finally, achieved an enhanced efficiency for perovskite solar cells with a CuSCN inorganic HTL and screen-printable carbon electrode. Also, the unencapsulated perovskite solar cell demonstrated over 80% efficiency retention after being stored in an ambient atmosphere (45-70% relative humidity (RH)) for over 1000 h and maintained over 85% efficiency retention for 309 h of 1-sun irradiation under a continuous nitrogen flow under open-circuit conditions.

CsPbI2Br Perovskite Solar Cells Based on Carbon Black-Containing Counter Electrodes.

CsPbI2Br perovskite solar cells (PSCs) based on carbon electrodes (CEs) are considered to be low-cost and thermally stable devices. Nevertheless, the insufficient contact and energy level mismatch between the CsPbI2Br layer and CE hinder the further enhancement of the cell efficiency. Herein, a carbon black (CB) interlayer was introduced between the perovskite layer and CE. The hole extraction was facilitated due to the larger contact area and suitable energy band alignment in the CsPbI2Br/CB interface. Further investigation indicated the diffusion of CB nanoparticles from the CE or CB layer to the CsPbI2Br film after a certain period of time. We disclosed the formation of a CB-CsPbI2Br bulk heterojunction structure due to the carbon diffusion, which resulted in an efficiency enhancement. As a result, a record efficiency of 13.13% is achieved for carbon-based inorganic PSCs. This work also reveals that the diffusion of CB nanoparticles in CB-containing PSCs is universal and inevitable, although this kind of diffusion results in the enhancement of cell efficiency.

Impact of the Inflow Population From Outbreak Areas on the COVID-19 Epidemic in Yunnan Province and the Recommended Control Measures: A Preliminary Study.

Background: COVID-19 developed into a global pandemic in 2020 and poses challenges regarding the prevention and control capabilities of countries. A large number of inbound travelers from other regions could lead to a renewed outbreak of COVID-19 in the local regions. Globally, as a result of the imbalance in the control of the epidemic, all countries are facing the risk of a renewed COVID-19 outbreak brought about by travelers from epidemic areas. Therefore, studies on a proper management of the inbound travelers are urgent. Methods: We collected a total of 4,733,414 inbound travelers and 174 COVID-19 diagnosed patients in Yunnan province from 21 January 2020 to 20 February 2020. Data on place of origin, travel history, age, and gender, as well as whether they had suspected clinical manifestations for inbound travelers in Yunnan were collected. The impact of inbound travelers on the local epidemic was analyzed with a collinear statistical analysis and the effect of the control measures on the epidemic was evaluated with a sophisticated modeling approach. Results: Of the 174 COVID-19 patients, 60.9% were not from Yunnan, and 76.4% had a history of travel in Hubei. The amount of new daily cases in Yunnan was significant correlated with the number of inbound travelers from Hubei and suspected cases among them. Using Susceptible-Exposed-Infectious-Recovered (SEIR) model analysis, we found that the prevention and control measures dropped the local R0 down to 1.07 in Yunnan province. Conclusions: Our preliminary analysis showed that the proper management of inbound travelers from outbreak areas has a significantly positive effect on the prevention and control of the virus. In the process of resettlement, some effective measures taken by Yunnan province may provide an important reference for preventing the renewed COVID-19 outbreak in other regions.

A general precipitation strategy for large-scale synthesis of molybdate nanostructures.

A general precipitation strategy has been developed for the large-scale synthesis of molybdate nanostructures, and a series of molybdate nanostructures such as Fe(2)(MoO(4))(3) nanoparticles, ZnMoO(4) nanoplates, MnMoO(4) nanorods and CoMoO(4) nanowires have been successfully prepared.

Synthesis and magnetic properties of Co-Sn-O nanorings.

A novel Co-Sn-O nanoring structure was synthesized via a simple hydrothermal route; the difference in diffusion rates of ions was considered to be the main reason for the formation of such a ring structure.

Fabrication of well-defined water-soluble core/shell heteronanostructures through the SiO2 spacer.

A generalized approach for the fabrication of core/shell heteronanostructures through a simple chemical bath deposition method assisted by the bridging of SiO(2) spacer has been developed and a slow heterogeneous nucleation mechanism identified as the key to the success of this method.

Efficient Bulk Heterojunction CH3NH3PbI3-TiO2 Solar Cells with TiO2 Nanoparticles at Grain Boundaries of Perovskite by Multi-Cycle-Coating Strategy.

A novel bulk heterojunction (BHJ) perovskite solar cell (PSC), where the perovskite grains act as donor and the TiO2 nanoparticles act as acceptor, is reported. This efficient BHJ PSC was simply solution processed from a mixed precursor of CH3NH3PbI3 (MAPbI3) and TiO2 nanoparticles. With dissolution and recrystallization by multi-cycle-coating, a unique composite structure ranging from a MAPbI3-TiO2-dominated layer on the substrate side to a pure perovskite layer on the top side is formed, which is beneficial for the blocking of possible contact between TiO2 and the hole transport material at the interface. Scanning electron microscopy clearly shows that TiO2 nanoparticles accumulate along the grain boundaries (GBs) of perovskite. The TiO2 nanoparticles at the GBs quickly extract and reserve photogenerated electrons before they transport into the perovskite phase, as described in the multitrapping model, retarding the electron-hole recombination and reducing the energy loss, resulting in increased VOC and fill factor. Moreover, the pinning effect of the TiO2 nanoparticles at the GBs from the strong bindings between TiO2 and MAPbI3 suppresses massive ion migration along the GBs, leading to improved operational stability and diminished hysteresis. Photoluminescence (PL) quenching and PL decay confirm the efficient exciton dissociation on the heterointerface. Electrochemical impedance spectroscopy and open-circuit photovoltage decay measurements show the reduced recombination loss and improved carrier lifetime of the BHJ PSCs. This novel strategy of device design effectively combines the benefits of both planar and mesostructured architectures whilst avoiding their shortcomings, eventually leading to a high PCE of 17.42% under 1 Sun illumination. The newly proposed approach also provides a new way to fabricate a TiO2-containing perovskite active layer at a low temperature.

Nucleation mediated interfacial precipitation for architectural perovskite films with enhanced photovoltaic performance.

Perovskite films are a promising candidate for future highly efficient and low-cost solar cells. The long diffusion length of charge carriers in the perovskite film makes its architecture fabrication seem unnecessary, while the rapid crystallization process increases the difficulty in its architecture fabrication. Here we show the fabrication of perovskite architectures through a nucleation mediated interfacial precipitation method with the proper immiscible anti-solvent. Consecutively evolved architectures from tri-layer porous films to bilayer dense films are obtained. The interfacial precipitation provides the possibility of controlling the crystallization process of perovskite films, while the secondary nucleation is the origin of the porous architecture. The nucleation mediation can be a novel bottom-up approach to fabricate architectural perovskite films. The tri-layer architectural perovskite film exhibits excellent light absorption in the range of 500-800 nm and good photovoltaic performance with 8.2% enhancement in efficiency compared with the bilayer film for the corresponding solar cells.

Achieving High Current Density of Perovskite Solar Cells by Modulating the Dominated Facets of Room-Temperature DC Magnetron Sputtered TiO2 Electron Extraction Layer.

The short circuit current density of perovskite solar cell (PSC) was boosted by modulating the dominated plane facets of TiO2 electron transport layer (ETL). Under optimized condition, TiO2 with dominant {001} facets showed (i) low incident light loss, (ii) highly smooth surface and excellent wettability for precursor solution, (iii) efficient electron extraction, and (iv) high conductivity in perovskite photovoltaic application. A current density of 24.19 mA cm-2 was achieved as a value near the maximum limit. The power conversion efficiency was improved to 17.25%, which was the record value of PSCs with DC magnetron sputtered carrier transport layer. What is more, the room-temperature process had a great significance for the cost reduction and flexible application of PSCs.

Ultrasmooth Perovskite Film via Mixed Anti-Solvent Strategy with Improved Efficiency.

Most antisolvents employed in previous research were miscible with perovskite precursor solution. They always led to fast formation of perovskite even if the intermediate stage existed, which was not beneficial to obtain high quality perovskite films and made the formation process less controllable. In this work, a novel ethyl ether/n-hexane mixed antisolvent (MAS) was used to achieve high nucleation density and slow down the formation process of perovskite, producing films with improved orientation of grains and ultrasmooth surfaces. These high quality films exhibited efficient charge transport at the interface of perovskite/hole transport material and perovskite solar cells based on these films showed greatly improved performance with the best power conversion efficiency of 17.08%. This work also proposed a selection principle of MAS and showed that solvent engineering by designing the mixed antisolvent system can lead to the fabrication of high-performance perovskite solar cells.

Novel Perovskite Solar Cell Architecture Featuring Efficient Light Capture and Ultrafast Carrier Extraction.

A new perovskite solar cell (PSC) structure with a functionalized interface between perovskite and a hole transport material has been proposed in this report. The short circuit current density of PSC was notably enhanced with the novel architecture (with an increase of 8.7%), and a power conversion efficiency (PCE) of 16.93% was achieved. With the increased perovskite/hole conductor interface, hysteresis suppression was observed. The advantages of this structure in light-harvesting efficiency, trap density, and carrier separation rate were proved by various characterization and analysis studies. It is noteworthy that a PCE of 14.67% was achieved with poly(3-hexyl-thiophene), which to our knowledge is the highest performing PSC based on this material.