Kaicheng Zhang

Robust superhydrophobic TiO2@fabrics for UV shielding, self-cleaning and oil–water separation

Inspired by the surface geometry and composition of the lotus leaf with its self-cleaning behavior, in this work, a TiO2@fabric composite was prepared via a facile strategy for preparing marigold flower-like hierarchical TiO2 particles through a one-pot hydrothermal reaction on a cotton fabric surface. In addition, a robust superhydrophobic TiO2@fabric was further constructed by fluoroalkylsilane modification as a versatile platform for UV shielding, self-cleaning and oil–water separation. The results showed TiO2 particles were uniformly distributed on the fibre surface with a high coating density. In comparison with hydrophobic cotton fabric, the TiO2@fabric exhibited a high superhydrophobic activity with a contact angle of ∼160° and a sliding angle lower than 10°. The robust superhydrophobic fabric had high stability against repeated abrasion without an apparent reduction in contact angle. The as-prepared composite TiO2@fabric demonstrated good anti-UV ability. Moreover, the composite fabric demonstrated highly efficient oil–water separation due to its extreme wettability contrast (superhydrophobicity/superoleophilicity). We expect that this facile process can be readily and widely adopted for the design of multifunctional fabrics for excellent anti-UV, effective self-cleaning, efficient oil–water separation, and microfluidic management applications.

Room-temperature mixed-solvent-vapor annealing for high performance perovskite solar cells

In this paper, we introduce a room-temperature mixed-solvent-vapor annealing (rtMSVA) method to fabricate high performance perovskite solar cells (pero-SCs) based on MAPbI3−xClx without the need for thermal annealing (TA). An ultra-smooth perovskite thin-film with high crystallinity was obtained by the DMF/CB mixed-solvent (1 : 20, v/v) vapor annealing at room-temperature without TA and the power conversion efficiency (PCE) of the pero-SCs reached 16.4%. More importantly, the reproducibility of the PCEs is quite good among 40 different devices. Furthermore, large active area pero-SCs were fabricated with the rtMSVA method. The PCEs of the pero-SCs based on ITO and flexible PET/Ag mesh electrodes with an active area of 1.21 cm2 reached 11.01% and 7.5%, respectively. We anticipate that rtMSVA would very possibly become a promising crystallization method for the fabrication of large area pero-SCs in the near future.

Water-Soluble 2D Transition Metal Dichalcogenides as the Hole-Transport Layer for Highly Efficient and Stable p–i–n Perovskite Solar Cells

As a hole-transport layer (HTL) material, poly(3,4-ethylenedioxythiophene):polystyrene-sulfonate (PEDOT:PSS) was often criticized for its intrinsic acidity and hygroscopic effect that would in the long run affect the stability of perovskite solar cells (Pero-SCs). As alternatives, herein water-soluble two-dimensional (2D) transition metal dichalcogenides (TMDs), such as MoS2 and WS2 were used as HTLs in p-i-n Pero-SCs. It was found that the content of 1T phase in 2D TMDs HTLs is centrally important to the power conversion efficiencies (PCEs) of Pero-SCs, and the 1T-rich TMDs (as achieved from exfoliation and without postheating) lead to much higher PCEs. More importantly, as PEDOT:PSS was replaced by 2D TMDs, both the PCEs and stability of Pero-SCs were significantly improved. The highest PCEs of 14.35 and 15.00% were obtained for the Pero-SCs with MoS2 and WS2, respectively, whereas the Pero-SCs with PEDOT:PSS showed a highest PCE of only 12.44%. These are up to date the highest PCEs using 2D TMDs as HTLs. After being stored in a glovebox for 56 days, PCEs of the Pero-SCs using MoS2 and WS2 HTLs remained 78 and 72%, respectively, whereas the PCEs of Pero-SCs with PEDOT:PSS almost dropped to 0 over 35 days. This study demonstrates that water-soluble 2D TMDs have great potential for application as new generation of HTLs aiming at high performance and long-term stable Pero-SCs.

Comprehensive Study of Sol–Gel versus Hydrolysis–Condensation Methods To Prepare ZnO Films: Electron Transport Layers in Perovskite Solar Cells

Owing to the high charge mobility and low processing temperature, ZnO is regarded as an ideal candidate for electron transport layer (ETL) material in thin-film solar cells. For the film preparation, the presently dominated sol-gel (SG) and hydrolysis-condensation (HC) methods show great potential; however, the effect of these two methods on the performance of the resulting devices has not been investigated in the same frame. In this study, the ZnO films made through SG and HC methods were applied in perovskite solar cells (Pero-SCs), and the performances of corresponding devices were compared under parallel conditions. We found that the surface morphologies and the conductivities of the films prepared by SG and HC methods showed great differences. The HC-ZnO films with higher conductivity led to relatively higher device performance, and the best power conversion efficiencie (PCE) of 12.9% was obtained; meanwhile, for Pero-SCs based on SG-ZnO, the best PCE achieved was 10.9%. The better device performance of Pero-SCs based on HC-ZnO should be attributed to the better charge extraction and transportation ability of HC-ZnO film. Moreover, to further enhance the performance of Pero-SCs, a thin layer of pristine C60 was introduced between HC-ZnO and perovskite layers. By doing so, the quality of perovskite films was improved, and the PCE was elevated to 14.1%. The preparation of HC-ZnO film involves relatively lower-temperature (maximum 100 °C) processing; the films showed better charge extraction and transportation properties and can be a more promising ETL material in Pero-SCs.

Tuning Surface Energy of Conjugated Polymers via Fluorine Substitution of Side Alkyl Chains: Influence on Phase Separation of Thin Films and Performance of Polymer Solar Cells

Different contents of fluorine in side alkyl chains were incorporated into three conjugated polymers (namely, PBDTTT-f13, PBDTTT-f9, and PBDTTT-f5) whose backbones consist of benzodithiophene donors and thienothiophene acceptors. These three fluorinated polymers, in comparison with the well-known analogue PTB7-Th, show comparable energy levels and optical band gaps. However, the fluorination of side alkyl chains significantly changed the surface energy of bulk materials, which leads to distinctly different self-assembly behaviors and phase separations as being mixed with PC71BM. The increased mismatch in surface energies between the polymer and PC71BM causes larger scale phase domains, which makes a sound explanation for the difference in their photovoltaic properties.

Interfacial engineering via inserting functionalized water-soluble fullerene derivative interlayers for enhancing the performance of perovskite solar cells

For n–i–p type perovskite solar cells (Pero-SCs), one of the limitations which hinder the improvement of device performance is the extremely low electrical conductivity of the TiO2 electron transport layer (ETL). Herein, two novel functionalized water-soluble fullerene derivatives C60O∼18(OH)∼10(NH2)∼8 (f-C60) and C70O∼18(OH)∼10(NH2)∼10 (f-C70) were prepared and utilized as buffer layers between ITO and a vacuum-deposited fullerene C60 ETL in Pero-SCs. After inserting f-C60 and f-C70 layers, the Pero-SCs showed an enhanced crystallinity of the perovskite film, more efficient charge transport, reduced recombination and a decreased charge-transport resistance. Therefore, a summit power conversion efficiency (PCE) of 16.97% was achieved for the device with the f-C60/C60 ETL, which was ∼24% higher than that of the control device (13.71%). The PCE of the Pero-SC based on the f-C70/C60 ETL also exhibited a 16% enhancement. This work demonstrated the great potential of f-C60 and f-C70 for application as ETLs in Pero-SCs.

Catechol derivatives as dopants in PEDOT:PSS to improve the performance of p–i–n perovskite solar cells

For planar p–i–n perovskite solar cells (Pero-SCs), the bottom hole transporting layer (HTL) material is crucially important, since it can greatly affect the device performance in two aspects: (1) hole collection and transportation and (2) the crystallinity of the perovskite layer formed on it. Herein, a series of catechol derivatives, L-3,4-dihydroxyphenylalanine (DOPA), norepinephrine (NE) and 3,4-dihydroxybenzhydrazide (DOBD), were employed as dopants in PEDOT:PSS and applied as HTLs, and the influence on performance of p–i–n Pero-SCs was systematically studied. It is found that all these three catechols can improve the power conversion efficiency (PCE) of the Pero-SCs, among which DOBD shows far better performance than the other two. Under optimized conditions, a PCE of 17.46% was achieved for the p–i–n Pero-SCs using DOBD-doped PEDOT:PSS as the HTL. The investigations on morphology, fluorescence and electrochemical impedance spectra indicate that the PCE improvement should be mainly attributed to the facilitated charge collection and transportation due to the doped HTL and the enhanced crystallinity of the perovskite films. This line of research demonstrates that the easily accessible catechols can be employed as an excellent dopant in PEDOT:PSS for application as HTLs in Pero-SCs and opens a novel avenue for further improving the performance of the devices.

Chemical Modification of n-Type-Material Naphthalene Diimide on ITO for Efficient and Stable Inverted Polymer Solar Cells

To provide orthogonal solvent processable surface modification and improve the device stability of bulk-heterojunction polymer solar cells (PSCs), n-type semiconducting material naphthalene diimide (NDI) was chemically introduced onto the ITO surface as a cathode interlayer (CIL) using 3-bromopropyltrimethoxysilane (BrTMS) as a coupling agent. After modification, the work function of ITO can be decreased from 4.70 to 4.23 eV. The modified ITO cathode was applied in inverted PSCs based on PTB7-Th:PC71BM. With the CIL modification, a champion power conversion efficiency (PCE) of 5.87% was achieved, showing a dramatic improvement compared to that of devices (PCE = 3.58%) without CIL. More importantly, with these chemical bonded interlayers, the stability of inverted PSCs was greatly enhanced. The improved PCE and stability can be attributed to the increased open-circuit voltage and the formation of robust chemical bonds in NDI-TMS films, respectively. This study demonstrated that chemical modification of ITO with n-type semiconducting materials provides an avenue for not only solving the solvent orthogonal problem but also improving the device performance in terms of the PCE and the stability.

Semi-transparent perovskite solar cells: unveiling the trade-off between transparency and efficiency

Semi-transparent perovskite solar cells (Pero-SCs) are realized by tuning the band gap of the perovskite to resolve the trade-off between the transparency and efficiency of the photo-absorber. We synthesized wide-bandgap MAPbI3−xBrx perovskite, and the transparency and efficiency of the corresponding semi-transparent Pero-SCs were investigated systematically by varying the I : Br ratio and thickness of the perovskite film. Increasing Br content widened the bandgap of perovskite (i.e., blue shift of the absorption edge), and led to an increase in the average visible transmittance (AVT). This strategy allowed for high AVTs, and concomitantly achieved high power conversion efficiencies. Meanwhile, increasing the Br content could facilitate formation of perovskite films with large grains that were highly crystallized. Compared with the narrow-bandgap perovskite, the wide-bandgap perovskite showed advantages for obtaining semi-transparent Pero-SCs with thick perovskite films (>200 nm) and high (20%) transparency.

Room-Temperature and Aqueous Solution-Processed Two-Dimensional TiS2 as an Electron Transport Layer for Highly Efficient and Stable Planar n–i–p Perovskite Solar Cells

In this study, a room-temperature and aqueous solution-processed two-dimensional (2D) transition-metal dichalcogenide TiS2 was applied as an electron transport layer (ETL) in planar n-i-p perovskite solar cells (Pero-SCs). Upon insertion of the 2D TiS2 ETL with UV-ozone (UVO) treatment, the power conversion efficiency (PCE) of the planar Pero-SCs was optimized to 18.79%. To the best of our knowledge, this value should be the highest efficiency to date among those PCEs of the n-i-p Pero-SCs with room-temperature-processed metal compound ETLs. More importantly, the n-i-p Pero-SCs with the UVO-treated 2D TiS2 as an ETL also show extremely high stability, where the average PCE remained over 95% of its initial value after 816 h storage without encapsulation.