Shuangshuang Liu

Carbon Quantum Dots/TiOx Electron Transport Layer Boosts Efficiency of Planar Heterojunction Perovskite Solar Cells to 19%

In planar n-i-p heterojunction perovskite solar cells, the electron transport layer (ETL) plays important roles in charge extraction and determine the morphology of the perovskite film. Here, we report a solution-processed carbon quantum dots (CQDs)/TiO2 composite that has negligible absorption in the visible spectral range, a very attractive feature for perovskite solar cells. Using this novel CQDs/TiO2 ETL in conjunction with a planar n-i-p heterojunction, we achieved an unprecedented efficiency of ∼19% under standard illumination test conditions. It was found that a CQDs/TiO2 combination increases both the open circuit voltage and short-circuits current density as compared to using TiO2 alone. Various advanced spectroscopic characterizations including ultrafast spectroscopy, ultraviolet photoelectron spectroscopy, and electronic impedance spectroscopy elucidate that the CQDs increases the electronic coupling between the CH3NH3PbI3-xClx and TiO2 ETL interface as well as energy levers that contribute to electron extraction.

Novel porphyrin-preparation, characterization, and applications in solar energy conversion

Porphyrins have been demonstrated as one of the most efficient sensitizers in dye-sensitized solar cells (DSSC). Herein, we investigated a series of porphyrin sensitizers functionalized with various π-spacers, such as phenyl for LD14, thiophene for LW4, thiophene-phenyl for LW5, and 2,1,3-benzothiadiazole (BTD)-phenyl for LW24. Photo-physical investigation by means of time-resolved fluorescence and nanosecond transient absorption spectroscopy revealed an accelerated inner charge transfer in porphyrins containing the BTD-phenyl π-spacer. Implementation of an auxiliary electron-deficient BTD unit to the porphyrin spacer also results in a broad light-harvesting ability extending up to 840 nm, contributing to an enhanced charge transfer character from the porphyrin ring to the anchoring group. When utilized as a sensitizer in DSSCs, the LW24 device achieved a power conversion efficiency of 9.2%, higher than those based on LD14 or LW5 porphyrins (PCE 9.0% or 8.2%, respectively) but lower than that of the LW4 device (PCE 9.5%). Measurements of transient photovoltage decays demonstrate that the LW24 device features the up-shifted potential band edge of the conduction band of TiO2, but involves serious charge recombination in the dye/TiO2 interface. The findings provide insights into the molecular structure and the charge-transfer characteristics for designing efficient porphyrin sensitizers for DSSC applications.

Pyrene-conjugated porphyrins for efficient mesoscopic solar cells: the role of the spacer

With a view to broadening the porphyrin light-harvesting cross section and improving mesoporous solar cell power conversion efficiency, pyrene-conjugated porphyrin dyes with various π-spacers between the porphyrin chromophore and carboxylic acid have been designed and synthesized in this study, including tailoring with phenyl (LW17), thiophene (LW18), and 2-phenylthiophene (LW19). These porphyrins show stepwise red-shifted absorption spectra and consistently decreased oxidation potential when the spacer changes from phenyl to an electron-rich unit of thiophene, and to an elongated spacer of 2-phenylthiophene, respectively, for LW17, LW18, and LW19 dyes. A mesoporous solar cell based on LW18 dye can achieve an overall power conversion efficiency of 8.7% under full sunlight (AM 1.5G, 100 mW cm−2) irradiation. The result reveals that both photocurrent and photovoltage can be effectively tuned by changing the spacers. Detailed investigation with transient photovoltage decay measurements provides general information on factors affecting the principal photovoltaic parameters.

17% efficient printable mesoscopic PIN metal oxide framework perovskite solar cells using cesium-containing triple cation perovskite

Fully printable perovskite solar cells (PSCs) based on an inorganic metal oxide architecture have attracted tremendous attention due to its feature of showing principally high stability. However, fully printable PSCs show a lower power conversion efficiency (PCE) than the thin film PSCs owing to the thick mesoscopic layers that pose an obstacle to charge collection. Herein, the triple cation perovskite Cs0.05(FA0.4MA0.6)0.95PbI2.8Br0.2, for the first time, is introduced in fully printable PSCs on the basis of a mesoporous metal oxide TiO2/Al2O3/NiO layered framework with a carbon counter electrode. We found that partial replacement of FA/MA by Cs could increase the bandgap and exciton binding energy of Csx(FA0.4MA0.6)1−xPbI2.8Br0.2 perovskite. An optimal efficiency of 17.02% can be obtained using Cs0.05(FA0.4MA0.6)0.95PbI2.8Br0.2 as the light absorber under AM 1.5G 100 mW cm−2 light illumination, which, to the best of our knowledge, represents the highest efficiency observed to date for fully printable PSCs using a carbon counter electrode. Detailed investigations with nanosecond transient absorption spectroscopy and transient photovoltage/photocurrent decay measurements revealed that the presence of Cs in perovskite compounds can increase the charge carrier lifetime along with diffusion length, benefiting charge transport in thick mesoscopic layers. Furthermore, the Cs0.05(FA0.4MA0.6)0.95PbI2.8Br0.2-based PSCs exhibit good stability with a retention of over 90% initial PCE after 1020 h in dark conditions at 85 °C.

Engineering NiS/Ni2P Heterostructures for Efficient Electrocatalytic Water Splitting

Developing high-active and low-cost bifunctional materials for catalyzing the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) holds a pivotal role in water splitting. Therefore, we present a new strategy to form NiS/Ni2P heterostructures. The as-obtained NiS/Ni2P/carbon cloth (CC) requires overpotentials of 111 mV for the HER and 265 mV for the OER to reach a current density of 20 mA cm-2, outperforming their counterparts such as NiS and Ni2P under the same conditions. Additionally, the NiS/Ni2P/CC electrode requires a 1.67 V cell voltage to deliver 10 mA cm-2 in a two-electrode electrolysis system, which is comparable to the cell using the benchmark Pt/C||RuO2 electrode. Detailed characterizations reveal that strong electronic interactions between NiS and Ni2P, abundant active sites, and smaller charge-transfer resistance contribute to the improved HER and OER activity.

Efficient Planar Perovskite Solar Cells with Improved Fill Factor via Interface Engineering with Graphene

Organic-inorganic hybrid lead halide perovskites have been widely investigated in optoelectronics both experimentally and theoretically. The present work incorporates chemically modified graphene into nanocrystal SnO2 as the electron transporting layer (ETL) for highly efficient planar perovskite solar cells. The modification of SnO2 with highly conductive two-dimensional naphthalene diimide-graphene can increase surface hydrophobicity and form van der Waals interaction between the surfactant and the organic-inorganic hybrid lead halide perovskite compounds. As a result, highly efficient perovskite solar cells with power conversion efficiency of 20.2% can be achieved with an improved fill factor of 82%, which could be mainly attributed to the augmented charge extraction and transport.

Highly Efficient Perovskite Solar Cells with Gradient Bilayer Electron Transport Materials

Electron transport layers (ETLs) with suitable energy level alignment for facilitating charge carrier transport as well as electron extraction are essential for planar heterojunction perovskite solar cells (PSCs) to achieve high open-circuit voltage ( VOC) and short-circuit current. Herein we systematically investigate band offset between ETL and perovskite absorber by tuning F doping level in SnO2 nanocrystal. We demonstrate that gradual substitution of F- into the SnO2 ETL can effectively reduce the band offset and result in a substantial increase in device VOC. Consequently, a power conversion efficiency of 20.2% with VOC of 1.13 V can be achieved under AM 1.5 G illumination for planar heterojunction PSCs using F-doped SnO2 bilayer ETL. Our finding provides a simple pathway to tailor ETL/perovskite band offset to increase built-in electric field of planar heterojunction PSCs for maximizing VOC and charge collection simultaneously.