Haiwu Zheng

Effect of energy level matching on the enhancement of photovoltaic response about oxide/Zn2SnO4 composites

Four oxide/Zn2SnO4 composites (WO3/Zn2SnO4, TiO2/Zn2SnO4, α-Fe2O3/Zn2SnO4 and SnO2/Zn2SnO4) were synthesized via a sol–gel route with the molar ratio 4 : 1 of (W, Ti, Fe and Sn)/Zn. All composites present similar absorption characteristics to their one component (oxide or Zn2SnO4). The surface photovoltage spectroscopies for composites show stronger photovoltaic effects than each component before compounding under zero and the same positive bias. We also find that the separation efficiency of photogenerated charges will increase by degrees with decreasing the difference of conduction band edges between two components under the same conditions. These results are attributed to the energy level matching, and we adopt the overlapping of wave functions to explain the micro-process and physical mechanism of the separation of photogenerated charges in the composite system.

Factors on the separation of photogenerated charges and the charge dynamics in oxide/ZnFe2O4 composites

Four oxide/ZnFe2O4 composites (WO3/ZnFe2O4, α-Bi2O3/ZnFe2O4, TiO2/ZnFe2O4 and SnO2/ZnFe2O4) were synthesized with 2 : 1, 4 : 1, 6 : 1 and 8 : 1 molar ratios of (W, Bi, Ti and Sn)/Zn. From the energy band structure, all composites can satisfy the energy level matching between their components (oxides and ZnFe2O4). For WO3/ZnFe2O4 and α-Bi2O3/ZnFe2O4, almost all of the molar ratios have stronger photovoltaic responses than each component before compounding, but the other two composites present the opposite result. Due to the different charge dynamics, the photovoltaic characteristics of four composites with molar ratio 4 : 1 did not satisfy the “inverted region effect”. Time-resolved fluorescence spectroscopy provided direct evidence of energy transfer between components. Then we discussed the factors affecting the separation of photogenerated charges and the charge dynamics from surface area, band gap, conduction band edge, energy level matching and overlapping of wave functions. The research results also provide a way to select composite systems as photoanodes for dye-sensitized solar cells.

Room-temperature ferromagnetism in Cu-implanted 6H-SiC single crystal

200 keV Cu+ ions were implanted into 6H-SiC single crystal at room temperature with fluence of 8 × 1015 cm−2. No ferromagnetism (FM)-related secondary phase was found by the results of high-resolution x-ray diffraction and x-ray photoelectron spectroscopy. Positron annihilation lifetime spectroscopy results indicated that the main defect type was silicon vacancy and the concentration of it increased after Cu implantation. The room-temperature ferromagnetism was detected by superconducting quantum interference device. First-principles calculations revealed that the magnetic moments mainly come from the 2p orbitals of C atoms and 3d orbitals of Cu dopant. The origin of the FM has been discussed in detail.

Photoelectric properties and charge dynamics for a set of solid state solar cells with Cu4Bi4S9 as the absorber layer

In this work, many single-crystalline, orthorhombic phase Cu4Bi4S9 nanoribbons were fabricated by a facile solvothermal method. Four types of three-dimensional solar cells prepared with In2O3, ZnO, TiO2 and SnO2 thin films as the working electrodes, In2S3 as the buffer layer and Cu4Bi4S9 as the inorganic dye and hole collector were tested using standard techniques. Under the same conditions, the I–V characteristics and photocurrent spectra of the cells were systematically measured. It is shown that the four types of cells all have good photoelectric properties. However, the In2O3 and TiO2 cells show better performance than the other two cells, and the highest efficiencies are about 6.2% (In2O3), 5.5% (TiO2), 4.8% (ZnO) and 3.9% (SnO2). The overall efficiencies were also compared with various oxide film thicknesses and the best performance was for films of 6 μm thickness for each type of cell. Using steady state and electric field-induced surface photovoltage spectroscopy, we further studied the charge dynamics and the transport mechanism in the sensitized electrodes. It is suggested that the matching degree of energy levels between two conduction bands (Cu4Bi4S9 and the oxides) has an obvious effect on the transport of the photogenerated charges. For good performance, we should pay more attention to alternative materials with matching degrees of energy levels, besides the optical stability, film structure, substrate thickness and the carrier mobility for solid state solar cells in the future.

Insights into collaborative separation process of photogenerated charges and superior performance of solar cells

ZnO nanowires/Cu4Bi4S9 (ZnO/CBS) and ZnO nanowires/CBS-graphene nanoplates (ZnO/CBS-GNs), as well as two types of solar cells were prepared. The photovoltaic responses of CBS-GNs and ZnO/CBS-GNs can be improved with incorporation of GNs. The transient surface photovoltage (TPV) can provide detailed information on the separation and transport of photogenerated carriers. The multichannel separation process from the TPVs indicates that the macro-photoelectric signals can be attributed to the photogenerated charges separated at the interface of CBS/GNs, rather than CBS/ZnO. The multi-interfacial recombination is the major carrier loss, and the hole selective p-V2O5 can efficiently accelerate the charge extraction to the external circuit. The ZnO/CBS-GNs cell exhibits the superior performance, and the highest efficiency is 10.9%. With the adequate interfaces of CBS/GNs, GNs conductive network, energy level matching, etc., the excitons can easily diffuse to the interface of CBS/GNs, and the separated electrons and holes can be collected quickly, inducing the high photoelectric properties. Here, a facile strategy for solid state solar cells with superior performance presents a potential application.

Comprehensive Insights into Charge Dynamics and Improved Photoelectric Properties of Well-Designed Solar Cells

Here, Zn2SnO4 nanorods/Cu4Bi4S9 (ZTO/CBS) and ZTO nanorods/CBS-graphene nanosheets (ZTO/CBS-GNs), as well as two types of bulk heterojunction (BHJ) solar cells with high flexibility were fabricated on stainless steel meshes (SSMs). The excellent photovoltaic responses of CBS-GNs and ZTO/CBS-GNs with incorporation of GNs were determined using surface photovoltage spectroscopy (SPS). The signals of time-resolved fluorescence response (TFR) and transient surface photovoltage (TPV) can provide more detailed information for transition, separation, and transport of photoinduced carriers. Besides, the ZTO nanorods/CBS-GNs cell exhibits the superior performance and the highest efficiency is 11.2%. The multichannel separation process from the TPVs indicates that the macro-photoelectric signals can be attributed to the photogenerated charges separated at the interface of CBS/GNs, rather than CBS/ZTO. The multi-interfacial recombination is the major carrier loss with electrical impedance spectroscopy (EIS) and the hole selective NiO can efficiently accelerate the charge extraction to the external circuit. The comprehensive signals of SPS, EIS, TFR, and TPV provide insights into transition, separation, recombination and shifting of carriers. Importantly, the BHJ flexible solar cells with high efficiency and facile, scalable production present a potential for application.

Photochemical charges separation and photoelectric properties of flexible solar cells with two types of heterostructures

Photochemical charges generation, separation, and transport at nanocrystal interfaces are central to energy conversion for solar cells. Here, Zn2SnO4 nanowires/Cu4Bi4S9 (ZTO/CBS), ZTO nanowires/CBS-reduced graphene oxide (ZTO/CBS-RGO), and bulk heterojunction (BHJ) solar cells were measured. The signals of steady state and electric field-induced surface photovoltage indicate that RGO with high electron mobility can evidently improve the photovoltaic response. Besides, ZTO/CBS and ZTO/CBS-RGO cells exhibit the excellent performance and the highest efficiencies of 1.2% and 2.8%, respectively. The internal relations of photoelectric properties to some factors, such as film thickness, direct paths, RGO conductive network, energy level matching, etc., were discussed in detail. Qualitative and quantitative analyses further verified the comprehensive effect of RGO and other factors. Importantly, the fine bendable characteristic of BHJ solar cells with excellent efficiency and facile, scalable production gives the as-made flexible solar cells device potential for practical application in future.