Lydia Helena Wong

Laminated Carbon Nanotube Networks for Metal Electrode-Free Efficient Perovskite Solar Cells

Organic-inorganic metal halide perovskite solar cells were fabricated by laminating films of a carbon nanotube (CNT) network onto a CH3NH3PbI3 substrate as a hole collector, bypassing the energy-consuming vacuum process of metal deposition. In the absence of an organic hole-transporting material and metal contact, CH3NH3PbI3 and CNTs formed a solar cell with an efficiency of up to 6.87%. The CH3NH3PbI3/CNTs solar cells were semitransparent and showed photovoltaic output with dual side illuminations due to the transparency of the CNT electrode. Adding spiro-OMeTAD to the CNT network forms a composite electrode that improved the efficiency to 9.90% due to the enhanced hole extraction and reduced recombination in solar cells. The interfacial charge transfer and transport in solar cells were investigated through photoluminescence and impedance measurements. The flexible and transparent CNT network film shows great potential for realizing flexible and semitransparent perovskite solar cells.

A cuprous oxide–reduced graphene oxide (Cu2O–rGO) composite photocatalyst for hydrogen generation : employing rGO as an electron acceptor to enhance the photocatalytic activity and stability of Cu2O

Photocorrosion, that causes rapid deactivation of Cu(2)O photocatalysts, was addressed by incorporating this oxide in a composite with reduced graphene oxide which acts as an electron acceptor to extract photogenerated electrons from Cu(2)O. Cu(2)O-rGO composite engineering also allows enhancing significantly photocatalytic activities of Cu(2)O for H(2) generation.

Copper molybdenum sulfide: a new efficient electrocatalyst for hydrogen production from water

A new inorganic solid state electrocatalyst for the hydrogen evolution reaction (HER) is reported. Highly crystalline layered ternary sulfide copper-molybdenum-sulfide (Cu2MoS4) was prepared by a simple precipitation method from CuI and [MoS4]2− precursors. In aqueous solution and over a wide pH range (pH 0 to 7), this Cu2MoS4 showed very good catalytic activity for HER with an overvoltage requirement of only ca. 135 mV and an apparent exchange current density of 0.040 mA cm−2 (Tafel slope of ca. 95 mV per decade was found irrespective of the pH value). This Cu2MoS4 catalyst was found to be stable during electrocatalytic hydrogen generation. Therefore, it represents an attractive alternative to platinum.

Solvent additives and their effects on blend morphologies of bulk heterojunctions

Controlling the blend morphology is one of the ways to achieve high power conversion efficiency in organic bulk heterojunction (BHJ) photovoltaic devices. One simple yet effective method is “solvent additive” approach, which involves the addition of a small fraction of high boiling point solvent into the blend of donor/acceptor dissolved in another host solvent. Even though this method has been successfully applied in a number of polymer/fullerene BHJ devices, the selection rule of the choice of additive and the host solvent has yet to be fully established. In this work, we performed a systematic study of the effect of alkyl lengths of alkanedithiol additives on the nanoscale phase separation of P3HT:PC61BM blends and consequently, the power conversion efficiency (PCE) of the devices. The extent of the additive-induced phase separation is related to the additive boiling point and the degree of interaction between the additive and fullerene, as evident from grazing incidence X-ray diffractometry (GIXRD) and scanning transmission X-ray microscopy (STXM) data. We found that both the boiling point and the degree of interaction are correlated and should be considered simultaneously in the selection of the appropriate solvent additives. Lastly, PCE as high as 3.1% can be achieved in an optimally phase-separated blend due to an improvement in the charge dissociation and a decrease in bimolecular recombination.

A simple spiro-type hole transporting material for efficient perovskite solar cells

We developed a cost-effective spiro-type 4,4′,4′′,4′′′\-(2H,2′H,4H,4′H-3,3′-spiro-bi[thieno[3,4-b][1,4]dioxepine]-6,6′,8,8′-tetrayl)tetrakis(N,N-bis(4-methoxyphenyl)aniline) hole transporting material (PST1) for perovskite solar cells (PSCs) that works efficiently even without a cobalt dopant. The PST1 is obtained by employing facile synthetic routes and tends to crystallize in the solid state. An X-ray diffraction study of PST1 revealed a unique quasi-spiro molecular configuration and found multiple CH/π and π–π intermolecular contacts. For the first time, the crystal structure of 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) is also studied for comparison. The device based on PST1 exhibited a PCE of 13.44%, and a comparable 12.74% PCE was achieved for its undoped form, which paves the way for developing new low cost hole transporting materials and final industrialization of perovskite solar cells.

A new insight into controlling poly(3-hexylthiophene) nanofiber growth through a mixed-solvent approach for organic photovoltaics applications

One dimensional (1-D) nanostructures of conjugated polymers, such as nanofibers, offer the possibility of directed charge transport and improved absorption due to better chains ordering. Poly(3-hexylthiophene) (P3HT) nanofibers can be synthesized by utilizing its interaction with marginal solvents. This work explores the effect of different poor solvents in driving P3HT chain self-assembly into nanofibers and also the effect of a small amount of good solvent in such a poor solvent system in controlling the nanofiber morphology. The organic photovoltaic (OPV) devices based on the blend of P3HT nanofibers and PCBM showed an improved short circuit current when anisole was used compared to p-xylene. Surprisingly, the presence of a small amount of good solvent such as chlorobenzene (CB) in anisole resulted in a higher degree of crystallinity and thinner nanofibers compared to purely anisole system. These are evident from the absorption, scattering and morphology data. The presence of CB delayed crystallization, which is evident from the synchrotron small angle X-ray scattering (SAXS) measurements. This modification of fiber morphology with CB addition into P3HT/anisole results in an improved power conversion efficiency (PCE) of 2.3%; an improvement of more than 50% compared to the pure anisole system. Our investigation provides a new insight into self-assembly of polymers in a mixed solvent system, paving the way to new approaches of controlled self-assembly of organic nanofibers.

A novel strategy for surface treatment on hematite photoanode for efficient water oxidation

In this paper, we report a novel strategy for surface treatment of hematite nanorods for efficient photo-driven water oxidation. This is the first report describing the growth of Sn treated hematite from α-FeOOH nanorod arrays in one step without substantially altering morphologies. With this treatment the photocurrent density increased from 1.24 for pristine hematite nanorods to 2.25 mA cm−2 at 1.23 V vs. RHE (i.e. 81% improvement). The increase in photocurrent density was also accompanied by improved incident-photon-to-current efficiencies and oxygen evolution. The photocurrent improvement is mainly attributed to a reduced electron–hole recombination at the hematite–electrolyte interface through the formation of FexSn1−xO4 layer at the hematite nanorod surface as shown by XPS, HRTEM, EDAX line scan analyses and PEC measurements.

Perovskite–Hematite Tandem Cells for Efficient Overall Solar Driven Water Splitting

Photoelectrochemical water splitting half reactions on semiconducting photoelectrodes have received much attention but efficient overall water splitting driven by a single photoelectrode has remained elusive due to stringent electronic and thermodynamic property requirements. Utilizing a tandem configuration wherein the total photovoltage is generated by complementary optical absorption across different semiconducting electrodes is a possible pathway to unassisted overall light-induced water splitting. Because of the low photovoltages generated by conventional photovoltaic materials (e.g., Si, CIGS), such systems typically consist of triple junction design that increases the complexity due to optoelectrical trade-offs and are also not cost-effective. Here, we show that a single solution processed organic-inorganic halide perovskite (CH3NH3PbI3) solar cell in tandem with a Fe2O3 photoanode can achieve overall unassisted water splitting with a solar-to-hydrogen conversion efficiency of 2.4%. Systematic electro-optical studies were performed to investigate the performance of tandem device. It was found that the overall efficiency was limited by the hematites photocurrent and onset potential. To understand these limitations, we have estimated the intrinsic solar to chemical conversion efficiency of the doped and undoped Fe2O3 photoanodes. The total photopotential generated by our tandem system (1.87 V) exceeds both the thermodynamic and kinetic requirements (1.6 V), resulting in overall water splitting without the assistance of an electrical bias.

Understanding the Synthetic Pathway of a Single-Phase Quarternary Semiconductor Using Surface-Enhanced Raman Scattering: A Case of Wurtzite Cu2ZnSnS4 Nanoparticles

Single-phase Cu2ZnSnS4 (CZTS) is an essential prerequisite toward a high-efficiency thin-film solar cell device. Herein, the selective phase formation of single-phase CZTS nanoparticles by ligand control is reported. Surface-enhanced Raman scattering (SERS) spectroscopy is demonstrated for the first time as a characterization tool for nanoparticles to differentiate the mixed compositional phase (e.g., CZTS, CTS, and ZnS), which cannot be distinguished by X-ray diffraction. Due to the superior selectivity and sensitivity of SERS, the growth mechanism of CZTS nanoparticle formation by hot injection is revealed to involve three growth steps. First, it starts with nucleation of Cu(2-x)S nanoparticles, followed by diffusion of Sn(4+) into Cu(2-x)S nanoparticles to form the Cu3SnS4 (CTS) phase and diffusion of Zn(2+) into CTS nanoparticles to form the CZTS phase. In addition, it is revealed that single-phase CZTS nanoparticles can be obtained via balancing the rate of CTS phase formation and diffusion of Zn(2+) into the CTS phase. We demonstrate that this balance can be achieved by 1 mL of thiol with Cu(OAc)2, Sn(OAc)4, and Zn(acac)2 metal salts to synthesize the CZTS phase without the presence of a detectable binary/ternary phase with SERS.

In situ photo-assisted deposition of MoS2 electrocatalyst onto zinc cadmium sulphide nanoparticle surfaces to construct an efficient photocatalyst for hydrogen generation

We reported herein a facile and scalable preparation process for MoS(2)-decorated Zn(x)Cd(1-x)S hybrid photocatalysts for hydrogen generation. Zn(x)Cd(1-x)S nanopowder was first prepared from commercially available precursors employing a solution based process. MoS(2) hydrogen evolution reaction catalyst was then loaded onto the Zn(x)Cd(1-x)S nanopowder via a photo-assisted deposition process which employed mild conditions (room temperature, atmospheric pressure and visible light illumination). Thus, this process represents an important advantage in the large scale production of semiconductor/MoS(2) hybrid photocatalysts in comparison to the conventional method relying on thermal decomposition of (NH(4))(2)[MoS(4)] precursor at high temperature and under H(2)S pressure. The best Zn(0.2)Cd(0.8)S/MoS(2) 3% showed two hundred-and-ten times (210 times) faster hydrogen generation rate on visible light illumination compared with that obtained for un-treated Zn(0.2)Cd(0.8)S. That was the most impressive catalytic enhancement ever recorded for a semiconductor photocatalyst decorated with a noble metal free electrocatalyst.