Yeng Ming Lam

The origin of high efficiency in low-temperature solution-processable bilayer organometal halide hybrid solar cells

This work reports a study into the origin of the high efficiency in solution-processable bilayer solar cells based on methylammonium lead iodide (CH3NH3PbI3) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). Our cell has a power conversion efficiency (PCE) of 5.2% under simulated AM 1.5G irradiation (100 mW cm−2) and an internal quantum efficiency of close to 100%, which means that nearly all the absorbed photons are converted to electrons and are efficiently collected at the electrodes. This implies that the exciton diffusion, charge transfer and charge collection are highly efficient. The high exciton diffusion efficiency is enabled by the long diffusion length of CH3NH3PbI3 relative to its thickness. Furthermore, the low exciton binding energy of CH3NH3PbI3 implies that exciton splitting at the CH3NH3PbI3/PC61BM interface is very efficient. With further increase in CH3NH3PbI3 thickness, a higher PCE of 7.4% could be obtained. This is the highest efficiency attained for low temperature solution-processable bilayer solar cells to date.

Organic Photovoltaic Devices Using Highly Flexible Reduced Graphene Oxide Films as Transparent Electrodes

The chemically reduced graphene oxide (rGO) was transferred onto polyethylene terephthalate (PET) substrates and then used as transparent and conductive electrodes for flexible organic photovoltaic (OPV) devices. The performance of the OPV devices mainly depends on the charge transport efficiency through rGO electrodes when the optical transmittance of rGO is above 65%. However, if the transmittance of rGO is less than 65%, the performance of the OPV device is dominated by the light transmission efficiency, that is, the transparency of rGO films. After the tensile strain (∼2.9%) was applied on the fabricated OPV device, it can sustain a thousand cycles of bending. Our work demonstrates the highly flexible property of rGO films, which provide the potential applications in flexible optoelectronics.

Perovskite-based solar cells: impact of morphology and device architecture on device performance

Organic–inorganic metal halide perovskites have recently shown great potential for application in solar cells with excitingly high performances with an up-to-date NREL-certified record efficiency of 20.1%. This family of materials has demonstrated considerable prospects in achieving efficiencies comparable to or even better than those of thin film solar cells. The remarkable performances thus far seem not to be limited to any specific device architecture. Both mesoscopic and planar cells showed good device performance and this eventually leads to the inevitable comparison between both architectures. Regardless of device architecture, device performance is highly dependent on the film morphology. The factors influencing the film morphology such as the deposition method, material composition, additives and film treatment will be discussed extensively in this review. The key to obtaining good-quality film morphology and hence performance is to essentially lower the energy barrier for nucleation and to promote uniform growth of the perovskite crystals. A comparison of the material selection for various layers as well as their corresponding impact on the perovskite film and device behavior in both device architectures will be presented.

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.

Solution processed transition metal sulfides: application as counter electrodes in dye sensitized solar cells (DSCs)

A solution processed method for fabricating transition metal sulfides on fluorine doped tin oxide (FTO) as efficient counter electrodes in iodine/iodide based solar cells has been demonstrated. Conversion efficiencies of 7.01% and 6.50% were obtained for nickel and cobalt sulfides, respectively, comparable to the conventional thermally platinised FTO electrodes (7.32%). A comparable charge transfer resistance of Ni(3)S(2) and Co(8.4)S(8) to conventional Pt was found to be a key factor for such high efficiencies. Cyclic voltammetry, Kelvin probe microscopy, Electrochemical Impedance Spectroscopy, and Tafel polarization were performed to study the underlying reasons behind such efficient counter electrode performance.

Printable photo-supercapacitor using single-walled carbon nanotubes

A printable, all solid-state photo-supercapacitor (PSC) incorporating both organic photovoltaic (OPV) and supercapacitor (SC) functions has been demonstrated utilizing a single-walled carbon nanotube network, enabling a thinner (< 0.6 mm) and lighter (< 1 g) device architecture, which leads to a 43% reduction in device internal resistance as compared to external wire connected OPVs and SCs.

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.

Crystallization of Carbon Nanotube and Nanofiber Polypropylene Composites

A variety of semicrystalline isotactic polypropylene composites containing carbon nanotubes and nanofibers were produced by melt and solution techniques. The effect of the nanofillers on the crystallization process was investigated by transmission electron microscopy, scanning electron microscopy, and differential scanning calorimetry. Under the processing conditions applied in this study, the surfaces of the carbon nanomaterials act as nucleation sites in bulk samples and highly oriented composite films. This conclusion is confirmed by the calculation of Avrami exponents and, in particular, by direct microscopy evidence.

Synthesis of Two‐Dimensional Transition‐Metal Phosphates with Highly Ordered Mesoporous Structures for Lithium‐Ion Battery Applications

Materials with ordered mesoporous structures have shown great potential in a wide range of applications. In particular, the combination of mesoporosity, low dimensionality, and well-defined morphology in nanostructures may exhibit even more attractive features. However, the synthesis of such structures is still challenging in polar solvents. Herein, we report the preparation of ultrathin two-dimensional (2D) nanoflakes of transition-metal phosphates, including FePO4, Mn3(PO4)2, and Co3(PO4)2, with highly ordered mesoporous structures in a nonpolar solvent. The as-obtained nanoflakes with thicknesses of about 3.7 nm are constructed from a single layer of parallel-packed pore channels. These uniquely ordered mesoporous 2D nanostructures may originate from the 2D assembly of cylindrical micelles formed by the amphiphilic precursors in the nonpolar solvent. The 2D mesoporous FePO4 nanoflakes were used as the cathode for a lithium-ion battery, which exhibits excellent stability and high rate capabilities.

Micellar poly(styrene-b-4-vinylpyridine)-nanoparticle hybrid system for non-volatile organic transistor memory

We present organic field-effect transistor (OFET) memories where in-situ synthesized gold (Au) nanoparticles in self-assembled polystyrene-block-poly-4-vinylpyridine (PS-b-P4VP) block copolymer nanodomains successfully functioned as charge storage elements. Both p-type (pentacene) and n-type (perfluorinated copper phthalocyanine) OFET based memories are reported, which have stable large charge capacity and programmable-erasable properties due to charge confinement in the embedded Au nanoparticles. Optical excitation has been utilized to demonstrate photogenerated minority carrier trapping in the Au nanoparticles for efficient erasing operations. The memory devices can hence be written and read electrically and erased optically, resulting in large memory windows (∼9–11 V), high on/off ratio between memory states (103–105) and long retention times (>1000 s). The results clearly indicate the utility of the block copolymer-nanoparticle approach for OFET based memories.