Xiangyang Liu

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.

Enhancement of photogenerated charges separation in α-Fe2O3 modified by Zn2SnO4

The composite of α-Fe2O3/Zn2SnO4 was synthesized via a sol–gel route. Photoelectric properties are investigated by surface photovoltage spectroscopy and electric field-induced surface photovoltage spectroscopy. The photovoltaic response for the composite with the molar ratio 4 : 1 of α-Fe2O3 to Zn2SnO4 without bias is similar to that for the pristine α-Fe2O3 under positive bias. The results show that the modification of Zn2SnO4 can significantly improve the surface photovoltaic response of α-Fe2O3. The enhanced photogenerated charges separation could be ascribed to the good contact and energy level matching between α-Fe2O3 and Zn2SnO4. The high electron mobility and low rate of electron–hole recombination in Zn2SnO4 are also responsible for the improved photoelectric properties of α-Fe2O3.

Surface photoelectric properties of AgNbO3 photocatalyst

The surface photoelectric property of AgNbO3 photocatalyst particles is investigated by surface photovoltage spectroscopy (SPS) and electric field-induced surface photovoltage spectroscopy (EFISPS). In the SPS, two response peaks appear separately in the visible and the ultraviolet light regions. The EFISPS results indicate that the peak in the visible light region could be enhanced and decreased by using a positive and negative external bias, respectively. This phenomenon suggests that the visible light photocatalytic activity could be tuned by the external voltage bias.

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.

Competition of carrier separation and recombination for an optimized electrode configuration for flexible thin-film solar cells

The inhibition of the rapid recombination of photochemical charges at heterointerfaces and the promotion of transfer and extraction are central to photoelectric conversion in solar cells. With the use of Cu4Bi4S9 nanoribbons-graphene sheets (CBS-GSs) as a hybrid photosensitive layer and the growth of ZnO nanowires on Zn2SnO4 nanowires (ZTO-ZnO) for the electron transfer layer, a flexible solar cell ZTO-ZnO/CBS-GSs was prepared on a stainless steel mesh. In this study, time-resolved fluorescence spectroscopy (TFS) and transient surface photovoltage (TPV) were used to describe the transition and transport process for the photogenerated carriers. With a high fill factor (0.76) and improved electron mobility for the ZTO-ZnO nanostructure, a high photoelectric conversion efficiency of 11.6% was obtained, which was evidently higher than that of a ZTO/CBS-GSs cell (9.2%) and a ZnO nanoparticles/CBS-GSs cell (3.9%). The efficient dissociation of photogenerated carriers at CBS-GSs interfaces, rapid transfer of free electrons in the ZTO-ZnO system, and fast extraction of holes from the selective NiO layer with an optimized architecture led to a superior performance. The charge recombination at the interface can be determined by electrochemical impedance spectroscopy (EIS). Active exploration via inevitable competition between charge separation and recombination (TFS, TPV, EIS, etc.) can provide insights into the entire dynamic process and a separation mechanism for photoinduced carriers, which can also promote the application of flexible thin-film solar cells.