Bukang Zhou

Origin of the high performance of perovskite solar cells with large grains

Due to excellent carrier transport characteristics, CH3NH3PbI3 film made of large single crystal grains is considered as a key to improve upon already remarkable perovskite solar cell (PSC) efficiency. We have used a simple and efficient solvent vapor annealing method to obtain CH3NH3PbI3 films with grain size over 1 μm. PSCs with different grain size films have been fabricated and verified the potential of large grains for improving solar cells performance. Moreover, the larger grain films have shown stronger light absorption ability and more photon-generated carriers under the same illumination. A detailed temperature-dependent PL study has indicated that it originates from larger radius and lower binding energy of donor-acceptor-pair (DAP) in larger grains, which makes the DAP is easily to be separated and difficult to be recombine.

Mechanical behavior and failure modes of aluminum/bamboo sandwich plates under quasi-static loading

Experimental investigations have been made on the quasi-static mechanical behavior and failure modes of aluminum/bamboo sandwich plates. Thermosetting epoxy resin and thermoplastic Polybond resin were used to bond the aluminum sheets and the bamboo. Tensile, compressive and flexural properties were evaluated. The effects of bond conditions on the mechanical behavior and failure modes were examined. The thermoplastic Polybond resin resulted in a stronger interface bond than the thermosetting epoxy resin. The improvement of the interface bond led to significant increases in compressive and flexural properties. The tensile properties were found to be insensitive to the interface bond. The dominant failure mechanisms affected by the interface bond dictated the mechanical properties of the sandwich plates in individual loading conditions.

Reformed bamboo/glass fabric/aluminium composite as an ecomaterial

A super-hybrid (natural composite/fibre-reinforced composite/metal hybridization) ecomaterial, reformed bamboo/glass fabric/aluminium (RB/GF/Al) was developed. The addition of a sparse glass fabric/epoxy resin layer between reformed bamboo and aluminium proved to be effective in increasing the compressive, tensile strength of the composite material. In particular, the interfacial shear strength between the reformed bamboo and aluminium was improved, and was the transverse tensile strength. These were the major shortcomings of normal bamboo and reformed bamboo/aluminium composites. The good recyclability of reformed bamboo and aluminium make RB/GF/Al an environmentally friendly material. Extensive use of such an ecomaterial instead of wood would save natural forest resources.

Optical and electrical simulations of silicon nanowire array/Poly(3-hexylthiophene):Phenyl-C61-butyric acid methyl ester hybrid solar cell

A coupled three-dimensional optoelectrical simulation was used to evaluate the optical and electrical designs requirements for improving the efficiency of Si nanowire array (NWA)/Poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) solar cells. Due to the addition of organic coating layers, better light absorptions are observed for the Si NWA/P3HT:PCBM structure at the wavelengths both below and above the absorption cutoff wavelengths of P3HT:PCBM. Optimized optical designs are obtained by maximizing the ultimate photocurrent under AM1.5G illumination. Furthermore, from the electrical simulation of the solar cell at optimized photo-absorption structure, the proposed Si NWA/P3HT:PCBM hybrid cell exhibits a promising character that is insensitive to exciton recombination at the optimized coating thickness of 80 nm. An efficiency of 9.4% can still be obtained even at high bimolecular recombination rates.

Absorption enhancement of GaInP nanowires by tailoring transparent shell thicknesses and its application in III-V nanowire/Si film two-junction solar cells.

A non-absorbing transparent shell is proposed to be coated on the outer surface of the core photoactive GaInP nanowire array (NWA) of the III-V nanowire (NW)/Si film two-junction solar cell. Interestingly, the diluted (at the filling ratio of 0.25) GaInP NWA with core / transparent shell structure can absorb more light than that in bare denser (at the filling ratio of 0.5) NWA. This allows for less source material consumption during the fabrication of III-V NWA/Si film two-junction cell. Meanwhile, the condition of current matching between the top III-V NWA and Si film sub cell can be easily fulfilled by tailoring the coating thickness of the transparent coating. Beyond the advantages on light absorption, the surface passivation effects introduced by the addition of some transparent dielectric coatings can reduce the surface recombination rate at the top NWA sub cell surface. This facilitates the effective extraction of photo-generated carriers and enhances output stability of the top NWA sub cell. From electrical simulation, a power conversion efficiency of 29.9% can be obtained at the optimized coating geometry.

Ti/Au Cathode for Electronic transport material-free organic-inorganic hybrid perovskite solar cells

We have fabricated organic-inorganic hybrid perovskite solar cell that uses a Ti/Au multilayer as cathode and does not use electron transport materials, and achieved the highest power conversion efficiency close to 13% with high reproducibility and hysteresis-free photocurrent curves. Our cell has a Schottky planar heterojunction structure (ITO/PEDOT:PSS/perovskite/Ti/Au), in which the Ti insertion layer isolate the perovskite and Au layers, thus proving good contact between the Au and perovskite and increasing the cells’ shunt resistance greatly. Moreover, the Ti/Au cathode in direct contact with hybrid perovskite showed no reaction for a long-term exposure to the air, and can provide sufficient protection and avoid the perovskite and PEDOT:PSS layers contact with moisture. Hence, the Ti/Au based devices retain about 70% of their original efficiency after 300 h storage in the ambient environment.

On-Demand Fabrication of Si/SiO2 Nanowire Arrays by Nanosphere Lithography and Subsequent Thermal Oxidation

We demonstrate the fabrication of the large-area arrays of vertically aligned Si/SiO2 nanowires with full tunability of the geometry of the single nanowires by the metal-assisted chemical etching technique and the following thermal oxidation process. To fabricate the geometry controllable Si/SiO2 nanowire (NW) arrays, two critical issues relating with the size control of polystyrene reduction and oxide thickness evolution are investigated. Through analyzing the morphology evolutions of polystyrene particles, we give a quantitative description on the diameter variations of polystyrene particles with the etching time of plasma etching. Based on this, pure Si NW arrays with controllable geometry are generated. Then the oxide dynamic of Si NW is analyzed by the extended Deal-Grove model. By control, the initial Si NWs and the thermal oxidation time, the well-aligned Si/SiO2 composite NW arrays with controllable geometry are obtained.

Synthesis and morphology control of diluted Si nanowire arrays by metal-assisted chemical etching and thermal oxidation based on nanosphere lithography

Well-separated silicon nanowires with good periodicity and lower porosity density are fabricated using thermal oxidization and HF acid etching after forming silicon nanowire (Si NW) arrays with high diameter-to-pitch ratio by metal-assisted chemical etching based on nanosphere lithography. The factors responsible for the special morphology features of Si NW during the oxidation process are understood by thermal oxidation process simulation. A high-temperature oxidation process is proposed to facilitate the alleviation of necking and the acceleration of oxidation of NWs. Furthermore, to reduce the tapering trend, an oxygen diffuse barrier layer is proposed to be pre-deposited on the top of Si NWs before the high-temperature thermal oxidation treatment. By using these methods, a periodic array of Si NWs with a large pitch and small diameter is created. This approach can substantially reduce porosities and surface defects on the outer surface of Si NWs.

Synthesis of Si/SiO2 core–shell nanowire arrays and broadband anti-reflection effects in diluted Si nanowire arrays by adjusting dielectric shell thickness

A low filling ratio and enhanced absorption is needed to enable the full potential of Si nanowire (NW) arrays for optoelectronic applications. In this paper, we report a versatile, scalable fabrication technique that uses nanosphere lithography (NSL) patterning for the synthesis of vertically aligned Si and Si/SiO2 NW arrays. The optical reflection of the NW arrays can be substantially suppressed by the addition of the transparent shell. Meanwhile, by the finite-difference time-domain (FDTD) simulation, we find that the absorption enhancement in the core Si NW can be obtained by adding the transparent shell. The special absorption enhancement of the Si NW arrays with a core-shell structure can be theoretically understood by modal analysis. The absorption in such Si NW array structures is very sensitive to the thickness of transparent coating. By the addition of a SiO2 shell layer, the absorption in the inner Si NW array can be substantially enhanced. Furthermore, significant absorption enhancement and broadband anti-reflection effects can be achieved by the diluted Si NWs combined with the single dielectric shell.