Qiquan Qiao

Electrospun carbon nanofibers as low-cost counter electrode for dye-sensitized solar cells.

Electrospun carbon nanofibers (ECNs) have been explored as an electrocatalyst and low-cost alternative to platinum (Pt) for triiodide reduction in dye-sensitized solar cells (DSCs). The results of electrochemical impedance spectroscopy (EIS) and cyclic voltammetry measurements indicated that the ECN counter electrodes exhibited low charge-transfer resistance (Rct), large capacitance (C), and fast reaction rates for triiodide reduction. Although the efficiency (η) of ECN-based cells was slightly lower than that of Pt-based cells, their short circuit current density (Jsc) and open circuit voltage (Voc) were comparable. The ECN-based cells achieved an energy conversion efficiency (η) of 5.5 % under the AM 1.5 illumination at 100 mW cm(-2). The reason for lower cell performance using the ECN electrode was because of its lower fill factor (FF) than that of Pt-based cells, probably caused by high total series resistance (RStot) at ∼15.5 Ω cm2, which was larger than that of ∼4.8 Ω cm2 in the Pt-based devices. Simulated results showed that the fill factor (FF) and η could be substantially improved by decreasing RStot, which might be achieved by using thinner and highly porous ECNs to reduce the thickness of the ECNs counter electrode.

Conjugated polymer–inorganic semiconductor hybrid solar cells

Polymer–inorganic semiconductor hybrid solar cells have attracted extensive research and attention as a promising approach to achieve cost effective solar energy. Power conversion efficiencies exceeding 3% have been achieved for polymer–inorganic hybrid solar cells. However, these efficiencies are still lower than those of polymer-fullerene solar cells, which have recently reached up to 8.13%. In this article, we review the recent developments including device operation mechanism, cell structures, polymer and inorganic materials, and various approaches to improve cell performance. In addition, the dependence of power conversion efficiency on the polymer bandgap and the lowest unoccupied molecular orbital (LUMO) using several typical inorganic acceptors including TiO2, ZnO and CdSe are presented and may provide guidance for the engineering of donor polymers.

Dye-sensitized solar cells based on low cost nanoscale carbon/TiO2 composite counter electrode

A dye-sensitized solar cell based on low cost nanoscale carbon/TiO2 composite counter electrode was fabricated and its photovoltaic performance (η = 5.5%, AM 1.5, 91.5 mW cm−2) was comparable to that from platinum counter-electrode devices (η = 6.4%, AM 1.5, 91.5 mW cm−2) made at similar conditions.

Composite of TiO2 nanofibers and nanoparticles for dye-sensitized solar cells with significantly improved efficiency

A composite made of electrospun TiO2 nanofibers and conventional TiO2 nanoparticles is an innovative type of photoanode, which noticeably improves the harvesting of light without substantially sacrificing the attachment (uptake) of dye molecules for convenient fabrication of dye-sensitized solar cells with significantly improved efficiency.

Strategic review of secondary phases, defects and defect-complexes in kesterite CZTS–Se solar cells

Earth abundant kesterite copper-zinc-tin-sulfide–selenide (CZTS–Se) is considered as cost-effective material for next generation solar cells. However, current CZTS–Se solar cells have much lower efficiency than CIGS solar cells. Rapid progress in achieving the target efficiency in CZTS–Se solar cells is hindered by the narrow phase stability of the quaternary phase, Cu2ZnSn(SxSe1−x)4, and the existence of other competitive and complex secondary phases and defects. This resulted in structural inhomogeneity, local fluctuation of open circuit voltage and high carrier recombination that finally lead to poor device performance and repeatability issues. The higher performance of off-stoichiometric CZTS materials, copper-poor and zinc-rich, and their inherent association with secondary phases and defects force the scientific community to investigate them together. This work aims to provide a comprehensive review for optimum growth conditions to achieve efficient kesterite CZTS–Se material under different conditions, complementary characterization techniques to detect unwanted phases, defects and defect-complexes and various approaches to reduce the secondary phases, defects and defect-complexes for higher performance in CZTS–Se solar cells. Understanding and addressing the structural inhomogeneity, control growth and material characterization are expected to yield closer performance parity between CZTS–Se and CIGS solar cells.

Water-soluble polythiophene∕nanocrystalline TiO2 solar cells

We report the characteristics of polymer∕nanocrystalline solar cells fabricated using an environmentally friendly water-soluble polythiophene and TiO2 in a bilayer configuration. The cells were made by dropping the polymer onto a TiO2 nanocrystalline film and then repeatedly sweeping a clean glass rod across the polymer as it dried. The devices showed an open circuit voltage of 0.81 V, a short circuit current density of 0.35mA∕cm2, a fill factor of 0.4, and an energy conversion efficiency of 0.13%. The water-soluble polythiophene showed significant photovoltaic behavior and the potential for use in solar cells.

A review of polymer multijunction solar cells

Polymer solar cells are one of the most promising prospects for widespread renewable energy due to their low cost, light weight, and mechanical flexibility. However, to date, low efficiencies (7.9%) of these devices inhibit their application. New materials and device designs are needed to increase the efficiency and make this technology available for large-scale applications. A polymer multijunction solar cell made of two or more subcells in series, parallel, or other special connections offers a potential solution to the losses in the current polymer single-junction solar cells. In this article, the recent developments in polymer multijunction photovoltaic materials, cell structures, and device modelling are reviewed. In addition, the current challenges that need to be addressed to achieve siginificantly higher efficiency are discussed.

A non-doped phosphorescent organic light-emitting device with above 31% external quantum efficiency.

The demonstrated square-planar Pt(II)-complex has reduced triplet-triplet quenching and therefore a near unity quantum yield in the neat thin film. A non-doped phosphorescent organic light-emitting diode (PhOLED) based on this emitter achieves (31.1 ± 0.1)% external quantum efficiency without any out-coupling, which shows that a non-doped PhOLED can be comparable in efficiency to the best doped devices with very complicated device structures.

Characteristics of water-soluble polythiophene: TiO2 composite and its application in photovoltaics

We have studied the characteristics of composites of an environmentally friendly water-soluble polythiophene sodium poly[2-(3-thienyl)-ethoxy-4-butylsulfonate] (PTEBS) and TiO2. We observed that the ultraviolet-visible absorption spectrum of low molecular weight PTEBS is redshifted possibly due to the formation of aggregates. Cyclic voltammetry reveals the values of highest occupied molecular orbitals and lowest unoccupied molecular orbitals for PTEBS. A factor of 7 in photoluminescence quenching indicates that the exciton dissociation and charge separation occur successfully at the PTEBS:TiO2 (1:1 by weight) interface. This enhances the possibility that the separated charges will reach the electrodes before recombining. Scanning electron micrograph images show how the PTEBS and TiO2 are interconnected and form paths to the electrodes to improve charge transport. Photovoltaic devices with TiO2:PTEBS composite achieved an energy conversion efficiency of η=0.015%, a short circuit current of JSC=0.22mA∕cm2, ...

Ultrathin FeSe2 Nanosheets: Controlled Synthesis and Application as a Heterogeneous Catalyst in Dye‐Sensitized Solar Cells

Two-dimensional (2D) semiconducting nanosheets have emerged as an important field of materials, owing to their unique properties and potential applications in areas ranging from electronics to catalysis. However, the controlled synthesis of ultrathin 2D nanosheets remains a great challenge, due to the lack of an intrinsic driving force for anisotropic growth. High-quality ultrathin 2D FeSe2 nanosheets with average thickness below 7 nm have been synthesized on large scale by a facile solution method, and a formation mechanism has been proposed. Due to their favorable structural features, the as-synthesized ultrathin FeSe2 nanosheets exhibit excellent electrocatalytic activity for the reduction of triiodide to iodide and low charge-transfer resistance at the electrolyte-electrode interface in dye-sensitized solar cells (DSSCs). The DSSCs with FeSe2 nanosheets as counter electrode material achieve a high power conversion efficiency of 7.53% under a simulated solar illumination of 100 mW cm(-2) (AM 1.5), which is comparable with that of Pt-based devices (7.47%).