Plasmonic structure enhanced exciton generation at the interface between the perovskite absorber and copper nanoparticles.

The refractive index and extinction coefficient of a triiodide perovskite absorber (TPA) were obtained by fitting the transmittance spectra of TPA/PEDOT:PSS/ITO/glass using the transfer matrix method. Cu nanoplasmonic structures were designed to enhance the exciton generation in the TPA and to simultaneously reduce the film thickness of the TPA. Excitons were effectively generated at the interface between TPA and Cu nanoparticles, as observed through the 3D finite-difference time-domain method. The exciton distribution is advantageous for the exciton dissociation and carrier transport.

Bicyclopentadithiophene-Based Organic Semiconductor for Stable and High-Performance Perovskite Solar Cells Exceeding 22.

Well-performing organic-inorganic halide perovskites are susceptible to poor efficiency and instability due to their various defects at the interphases, grain boundaries (GBs), and surfaces. In this study, an in situ method is utilized for effectively passivating the under-coordinated Pb2+ defects of perovskite with new non-fullerene acceptors (NFAs) (INXBCDT; X = H, Cl, and Br) through their carbonyl and cyano functional groups during the antisolvent dripping process. It reveals that the bicyclopentadithiophene (BCDT) core with highly electron-withdrawing end-capping groups passivates GBs and boosts perovskite grain growth. This effective defect passivation decreases the trap density to increase the carrier recombination lifetime of the perovskite film. As a result, bromo-substituted dicyanomethylene indanone (INBr)-end-capped BCDT (INBrBCDT-b8; 3a)-passivated devices exhibit the highest power conversion efficiency (PCE) of 22.20% (vs those of 18.09% obtained for perovskite films without passivation) upon an optimized film preparation process. Note that devices treated with more soluble 2-ethylhexyl-substituted compounds (1a, 2a, and 3a) exhibit higher PCE than those treated with less soluble octyl-substituted compounds (1b, 2b, and 3b). It is also worth noting that BCDT is a cost-effective six-ring core that is easier to synthesize with a higher yield and therefore much cheaper than those with highly fused-ring cores. In addition, a long-term stability test in a glovebox for 1500 h reveals that the perovskite solar cells (PSCs) based on a perovskite absorber treated with compound 3a maintain ∼90% of their initial PCE. This is the first example of the simplest high-conjugation additive for perovskite film to achieve a PCE greater than 22% of the corresponding lead-based PSCs.

Broadband charge transfer dynamics in P3HT:PCBM blended film.

Broadband exciton dynamics in P3HT:PCBM blended film was observed by the femtosecond time-resolved photoluminescence sum-frequency technique. Onsager-Braun theory is applied to analyze the distribution of charge transfer radius at different energy levels. In our evaluation, the optimal diameter of P3HT fiber is about 14.3 nm for achieving the best exciton dissociation in P3HT:PCBM blended films. This technique can be readily used in the optimization of high-efficiency organic photovoltaics.

Large-Area Perovskite Film Prepared by New FFASE Method for Stable Solar Modules Having High Efficiency under Both Outdoor and Indoor Light Harvesting.

High-quality perovskite film is deposited on a 30 cm × 40 cm LiCoO2 -coated ITO/glass via newly developed freely falling anti-solvent extraction (FFASE) method followed by post watervapor annealing in an ambient atmosphere. Perovskite solar modules (PSMs, active area of 25.2 cm2 with mask) based on this high-quality film achieve the highest efficiency of 16.04 and 30.76% under 1 sun (100 mW cm-2 ) and 945 lux fluorescent light illumination, respectively. The encapsulated PSMs are stable at -20 to 80 °C thermal cycling and keep high efficiency at temperature as low as -20 °C and as high as 80 °C. When the encapsulated PSM is heated at 85 °C and 85% relative humidity under room lighting or heated at 60 °C under AM1.5 (100 mW cm-2 ) illumination for 1000 h, loses only ≈8% of its original efficiency. The high stability of PSMs is due to very high quality perovskite absorber being used. The underlying concept of the FFASE method for extracting the solvent from the large-area perovskite precursor film is that the whole precursor film contacts with the fresh anti-solvent during the crystallization stage.

Sol-Gel Prepared Spinel HTLs for Assembling 20% Efficiency Perovskite Solar Cell in Air Without Using Anti-Solvent and Toxic Solvent.

Low-temperature sol-gel prepared ZnCo2 O4 spinel-based thin films are developed as high-performance hole transporting layer (HTL) for coating perovskite film (NA-Psk) from the basic MAPbI3 /ACN/CH3 NH2 solution in air without using anti-solvent. Inverted PSC based on 2 mole% (vs Zn) Cu2+ doped ZnCo2 O4 (2%Cu@ZnCo2 O4 ) HTL and NA-Psk absorber exhibit the maximum power conversion efficiency (PCE) of 20.0% with no current hysteresis while the cell based on ZnCo2 O4 and PEDOT:PSS HTL (using NA-Psk absorber) achieves the PCE of 15.79% and 12.3% with a current hysteresis index of 9.8% and 32.4%, respectively. Without encapsulation, PSCs based on 2%Cu@ZnCo2 O4 , ZnCo2 O4 , and PEDOT:PSS HTLs maintain 90%, 77%, and 12%, respectively of the original efficiency by standing in ambient atmosphere (temperature: 20-25 °C, RH:30%-40%) for 1800 h. Large area (10 cm × 10 cm substrate) perovskite mini-module (PSM) with PCE over 15% is also demonstrated by using sol-gel prepared 2%Cu@ZnCo2 O4 HTL. The poor photovoltaic performance of PEDOT:PSS HTL is due to the basic MAPbI3 /ACN/CH3 NH2 solution will deprotonate the acidic PEDOT:PSS to reduce its conductivity whereas ZnCo2 O4 HTL are not affected by basic perovskite precursor solution.

Enantiotropic nematics from cross-like 1,2,4,5-tetrakis(4'-alkyl-4-ethynylbiphenyl)benzenes and their biaxiality studies.

The theoretically predicted optimum length/breadth/width ratio for maximizing shape biaxiality was investigated experimentally by the facile and successful synthesis of cross-shaped compound 3, which showed enantiomeric nematic phase behavior. This cross-like core structure could alternatively be viewed as two fused V-shaped mesogens, which have recently immerged as a new direction in biaxial nematic research, at the bending tips that can act as a new structure for biaxial investigations. Whilst the thermal analysis data of compound 3 did not meet the expected theoretical values for biaxial nematics, surface-induced biaxiality was evidenced by optical studies. Cluster-size analysis within the nematic phase of compound 3 revealed the formation of meta-cybotactic nematics, which approached the cluster sizes of cybotactic nematics. The split small-angle 2D X-ray diffraction patterns of magnetic-field-aligned samples indicated that the nematic phase was composed of small smectic C-like clusters with the tilting of molecules within the clusters. The wide-temperature-range enantiomeric nematic phase of cross-like compound 3 enabled the molecular skeleton to serve as an alternative skeleton to bent-rod mesogens, which exhibited nematic phases with the potential competition of transitions to higher-order liquid-crystalline phases and crystallization, for future biaxial investigations.

Multifunctional Ionic Fullerene Additive for Synergistic Boundary and Defect Healing of Tin Perovskite to Achieve High-Efficiency Solar Cells.

A series of new ionic fullerene derivatives (C60-RNH3-X; X = Cl, Br, or I) were designed especially for using as additives for tin perosvkite (TPsk, with chemical formula of FA0.98EDA0.01SnI3) to form TPsk-C60-RNH3-X bulk heterojunction (BHJ) films. Inverted tin-perovskite solar cells (TPSCs) based on BHJ TPsk-C60-RNH3-Br absorber achieved the highest power conversion efficiency up to 11.74% with very high FF of 73%, without current hysteresis and stable in a glovebox. The designed spherical ionic fullerene halide additive, sitting in the grain boundaries of the TPsk film, can not only improve the quality of the TPsk film and change the valence band energy to match better with the PEDOT:PSS hole transporter but also be a carrier transporting connector between tin-perovskite grains, the defects/traps passivation/healing agent by interacting with Sn2+ ions and filling the halogen vacancies. The functions of the ionic fullerene halide additive were revealed with XRD patterns, SEM images, element mapping, UPS spectra, infrared spectra, AFM, and SCLC data. Being able to passivate newly generated defects during device operation or sitting on the shelf is an important step to improve the long-term stability of TPSCs. If a passivation agent can move dynamically during cell operation or storage to heal the defects of perovskite, the instability problem of TPSCs can be alleviated. The spherical ionic fullerene halide could be one of the ideal passivation agents satisfying this purpose.

Synergistic Engineering of Conduction Band, Conductivity, and Interface of Bilayered Electron Transport Layers with Scalable TiO2 and SnO2 Nanoparticles for High-Efficiency Stable Perovskite Solar Cells.

A simple, synergistic engineering of the conduction band (CB), conductivity, and interface of TiO2-based bilayered electron transport layers (ETLs) via scalable TiO2 and SnO2 nanoparticles processed at low temperature (≤ 100 °C) for regular planar perovskite solar cells (PSCs) was developed. The bottom layer (Lt-TiO2:SnO2 nanocomposite film) was prepared by spin coating from the ethanol suspension of small ground TiO2 nanoparticles with big ground SnO2 nanoparticles as the additive. The top C-SnO2 layer (spin-coated from the concentrated commercial SnO2 nanoparticles (C-SnO2 NPs, 20 wt %, 7 nm in size suspended in H2O)) can be regarded as an interlayer between Lt-TiO2:SnO2 and perovskite (Psk) absorbers. Bilayered Lt-TiO2:SnO2/C-SnO2 ETLs are dense films with a cascade CB, good conductivity, facile electron extraction/transport ability, and a highly hydrophilic surface for depositing high-quality Psk films. Regular planar PSCs based on Lt-TiO2:SnO2/C-SnO2 ETLs combined with a (FAI)0.90(PbI2)0.94(MABr)0.10(PbBr2)0.10 absorber and a spiro-OMeTAD hole transporter achieved the highest power conversion efficiency of 22.04% with a negligible current hysteresis. The champion cell lost less than 3% of the initial efficiency under continuous room lighting (1000 lux) for 1000 h (lost 10% after 2184 h) without encapsulation under an inert atmosphere. Four related low-temperature-processed ETLs (Lt-TiO2/C-SnO2, Lt-C-SnO2, Lt-TiO2:SnO2, and Lt-TiO2) were fabricated using the same metal oxide nanoparticle suspensions and studied simultaneously to reveal the function of each metal oxide in the bilayered Lt-TiO2:SnO2/C-SnO2 ETLs. In the bottom Lt-TiO2:SnO2 layer, small TiO2 nanoparticles were needed for making a dense film, and highly conducting big SnO2 nanoparticles are used to increase the conductivity of ETLs and a handy electron transport path for reducing the charge accumulation and series resistance of the cell. A top C-SnO2 layer (regarded as an interlayer between Psk and Lt-TiO2:SnO2) was used to extract/transport electrons facilely, to form a bilayered ETL with a cascade CB, and to create a hydrophilic surface to deposit high-quality Psk films to enhance the photovoltaic performance of the PSCs. This study provides a blueprint for designing good-performance ETLs for high-efficiency, stable regular planar PSCs using various sized nanoparticles prepared in a very simple and low-cost way.

Phenol assisted deaggregation of polyaniline chains: simple route to high quality polyaniline film.

High conducting polyaniline films were readily prepared by in situ chemical oxidative polymerization/deposition of aniline in the presence of a very small amount of organic additive such as phenol. The conductivity of a thin ( approximately 150 nm) polyaniline film synthesized in the presence of 0.01 wt % of phenol ( r-PANI) is an order of magnitude higher (as well as better conducting homogeneity) than that of a film (PANI) obtained from the conventional method without an additive. r-PANI also has better adhesion and electrochemical stability/reversibility, more transparency in the visible-light region, and faster/easier doping/dedoping response compared to PANI. The function of phenol molecule is to avoid the formation of the inter- and/or intrachain hydrogen bonding during the growth of the polyaniline chains. The deaggregation/reducing intrachain hydrogen bonding of polyaniline chains by phenol molecules was revealed with IR, SAXS, and SEM data. All these data supported that phenol does assist the deaggregation of polyaniline chains during the growth of polymer chains or nanorods.

A Method for the Preparation of Highly Oriented MAPbI3 Crystallites for High-Efficiency Perovskite Solar Cells to Achieve an 86% Fill Factor.

The quality of the perovskite absorber is known to be the most crucial parameter for the photovoltaic performance of perovskite solar cells. By combining the one-step anti-solvent engineering method followed by gas blowing, MAPbI3 film containing highly oriented multi-crystalline nanograins (150∼500 nm) was made first. A user-friendly, simple, large-throughput, and reproducible post-solvent annealing (made by treating the film with anti-solvent containing H2O under spinning) was used to enlarge the perovskite grains up to 1.5 µm. Inverted (p-i-n) perovskite solar cells based on this highly ordered, large-grain MAPbI3 film achieve the highest efficiency of 21% with an extremely high fill factor (FF) of 86%. The high-efficiency cell shows almost no current hysteresis and is stable under 1 sun illustration in a glovebox or standing in the ambient atmosphere (20∼25 °C, ca. 30% humidity) under room lighting (T5 lamp, 500 lux). A creative method combining the gas blowing with quick and simple post-treatment to prepare a highly oriented MAPbI3 film with large multi-crystalline grains to achieve excellent photovoltaic performance was demonstrated. This creative film-preparation method was also successfully applied to fabricate large area MAPbI3 film for high-efficiency perovskite mini-modules. Being able to control the crystallization and growth of perovskite crystallites definitely makes the fabrication of perovskite solar cells more reproducible.

Film Grain-Size Related Long-Term Stability of Inverted Perovskite Solar Cells.

The power conversion efficiency (PCE) of the perovskite solar cell is high enough to be commercially viable. The next important issue is the stability of the device. This article discusses the effect of the perovskite grain-size on the long-term stability of inverted perovskite solar cells. Perovskite films composed of various sizes of grains were prepared by controlling the solvent annealing time. The grain-size related stability of the inverted cells was investigated both in ambient atmosphere at relative humidity of approximately 30-40 % and in a nitrogen filled glove box (H2 O<0.1 ppm, O2 <10 ppm). The PCE of the solar cell based on a perovskite film having the grain size larger than 1 µm (D-10) decreases less than 10 % with storage in a glove box and less than 15 % when it was stored under an ambient atmosphere for 30 days. However, the cell using the perovskite film composed of small (∼100 nm) perovskite grains (D-0) exhibits complete loss of PCE after storage under the ambient atmosphere for only 15 days and a PCE loss of up to 70 % with storage in the glove box for 30 days. These results suggest that, even under H2 O-free conditions, the chemical- and thermal-induced production of pin holes at the grain boundaries of the perovskite film could be the reason for long-term instability of inverted perovskite solar cells.

A perovskite cell with a record-high-V(oc) of 1.61 V based on solvent annealed CH3NH3PbBr3/ICBA active layer.

A high open-circuit voltage inverted perovskite solar cell based on a CH3NH3PbBr3 absorber and ICBA acceptor is reported. The CH3NH3PbBr3 film fabricated under ambient atmosphere at a moderate temperature (∼100 °C) using a two-step spin-coating method is composed of aggregated nano-grains. Upon solvent annealing of the CH3NH3PbBr3/ICBA film, the efficiency of the resulting cell increases from 1.71% to 7.50% with a remarkably high open circuit voltage (Voc) of ca. 1.60 V. ICBA acts not only as a high LUMO acceptor to realize high Voc but also as a mending agent to increase the efficiency of the cell by penetrating into the defects/voids of the CH3NH3PbBr3 film via solvent annealing as evidenced by TRPL, XPS and SEM data. Solvent annealing of the active layer was proved to be simple and effective device engineering to improve the efficiency of the perovskite cell based on a low quality film and the Voc of the inverted perovskite cell can be tuned by the LUMO level of the acceptor were revealed. The CH3NH3PbBr3/ICBA film is semi-transparent with an average 50% transmittance under visible light. The moderatetemperature processed CH3NH3PbBr3 solar cell with high Voc and a semi-transparent absorber has great potential for application as the top cell in a tandem solar cell.

Surface Engineering of ZnO Thin Film for High Efficiency Planar Perovskite Solar Cells.

Sputtering made ZnO thin film was used as an electron-transport layer in a regular planar perovskite solar cell based on high quality CH3NH3PbI3 absorber prepared with a two-step spin-coating. An efficiency up to 15.9% under AM 1.5G irradiation is achieved for the cell based on ZnO film fabricated under Ar working gas. The atmosphere of the sputtering chamber can tune the surface electronic properties (band structure) of the resulting ZnO thin film and therefore the photovoltaic performance of the corresponding perovskite solar cell. Precise surface engineering of ZnO thin film was found to be one of the key steps to fabricate ZnO based regular planar perovskite solar cell with high power conversion efficiency. Sputtering method is proved to be one of the excellent techniques to prepare ZnO thin film with controllable properties.

Fabrication of sub-100 nm conducting polyaniline wire on a polymer substrate based on friction nanolithography.

Friction Nanolithography (FRNL) is used to fabricate polyaniline nano-wire on a flexible, insulating, and highly transparent substrate. FRNL uses SPM tip to scratch the substrate to form a trench with higher friction force. Polyaniline nano-wire is selectively deposited on the trench. Sub-100 nm polyaniline wire is fabricated based on FRNL.