Mingtai Wang

Chemically Binding Carboxylic Acids onto TiO2 Nanoparticles with Adjustable Coverage by Solvothermal Strategy

This paper presents a solvothermal strategy for chemical modification of TiO(2) nanoparticles with carboxylic acids. Solvothermal reaction between the TiO(2) nanoparticles and carboxylic acid molecules in an autoclave at 100 degrees C provides carboxylic acid-modified TiO(2) particles with a modification efficiency much higher than the conventional immersion method. TiO(2) nanoparticles were prepared by hydrolysis of titanium isopropoxide in nitric acid solution; the modified nanoparticles were characterized by powder X-ray diffraction pattern, scanning electron microscopy, absorption and Fourier transform infrared spectra, and thermogravimetric analysis. Results show that the binding form of the modifier molecules on TiO(2) surface is in a bidentate chelating mode, the crystalline phase composition and morphological structure of the preformed TiO(2) nanoparticles are not affected by the solvothermal reaction, and the surface coverage of the modifier molecules can be adjusted by the weight ratio of modifier/TiO(2) during feeding. It is evident that the reaction processes in the solvothermal strategy involve the formation of double hydrogen bondings between carboxylic acid molecule and TiO(2) at the same Ti site and the coordination at solvothermal temperature by dehydration from the hydrogen bondings. The solvothermal strategy for modifying TiO(2) nanoparticles is expected to find potential applications in many fields; for example, our results demonstrate that the photovoltaic performance of the TiO(2) nanoparticles can be improved by the solvothermal modification even with an insulating modifier and controlled by the modifier coverage.

Triple junction polymer solar cells

Similar to single and double junction polymer solar cells, triple junction devices can also be fabricated from all-solution processing. Although single and double junction polymer solar cells have exhibited an efficiency of 11.1% and 10.6% respectively, the triple junction structure shows promise in significantly increasing the device efficiency. In this work, the efficiency prediction for triple junction polymer solar cells and their dependence on subcell bandgaps, cutoff absorption wavelengths and active layer thicknesses are calculated and reviewed. Recent developments of triple junction polymer solar cells including intermediate layer materials, device performance evolution and future direction are presented. Also low bandgap polymers that are currently used or can potentially be used in triple junction solar cells are reviewed. This review provides researchers with a deep understanding and guidance in developing high performance triple junction polymer solar cells.

Improved Open-Circuit Voltage in Polymer/Oxide-Nanoarray Hybrid Solar Cells by Formation of Homogeneous Metal Oxide Core/Shell Structures

Incorporation of vertically aligned nanorod/nanowire arrays of metal oxide (oxide-NAs) with a polymer can produce efficient hybrid solar cells with an ideal bulk-heterojunction architecture. However, polymer/oxide-NAs solar cells still suffer from a rather low (normally, < 0.4 V) open-circuit voltage (Voc). Here we demonstrate, for the first time, a novel strategy to improve the Voc in polymer/oxide-NAs solar cells by formation of homogeneous core/shell structures and reveal the intrinsic principles involved therein. A feasible hydrothermal-solvothermal combined method is developed for preparing homogeneous core/shell nanoarrays of metal oxides with a single-crystalline nanorod as core and the aggregation layer of corresponding metal oxide quantum dots (QDs) as shell, and the shell thickness (L) is easily controlled by the solvothermal reaction time for growing QDs on the nanorod. The core/shell formation dramatically improves the device Voc up to ca. 0.7-0.8 V depending on L. Based on steady-state and dynamic measurements, as well as modeling by space-charge-limited current method, it is found that the improved Voc originates from the up-shifted conduction band edge in the core by the interfacial dipole field resulting from the decreased mobility difference between photogenerated electrons and holes after the shell growth, which increases the energy difference between the quasi-Fermi levels of photogenerated electrons in the core and holes in the polymer for a higher Voc. Our results indicate that increasing Voc by the core/shell strategy seems not to be dependent on the kinds of metal oxides.

Thermal Annealing Effects on the Absorption and Structural Properties of Regioregular Poly(3-Hexylthiophene) Films

Regioregular poly(3-hexylthiophene) (P3HT) was synthesized by a Grignard metathesis method. The annealed and fast cooled P3HT films were studied by UV-vis absorption spectroscopy, atomic force microscopy, and grazing incidence X-ray diffraction (GIXRD) measurements. It is demonstrated that thermal annealing at around the onset temperature of the melting point of the polymer results in the annealed film having the highest interchain and intrachain orders while preserving the original morphological features. However, the film that has the highest or lowest crystallinity does not have the strongest or weakest absorption. The lack of clear correlation between the absorption spectra and GIXRD data is explained by a quasi-ordered phase model, in which crystalline, amorphous, and quasi-ordered chains interconvert into each other depending on annealing temperature.

Analytical model for the photocurrent-voltage characteristics of bilayer MEH-PPV/TiO2 photovoltaic devices

The photocurrent in bilayer polymer photovoltaic cells is dominated by the exciton dissociation efficiency at donor/acceptor interface. An analytical model is developed for the photocurrent-voltage characteristics of the bilayer polymer/TiO2 photovoltaic cells. The model gives an analytical expression for the exciton dissociation efficiency at the interface, and explains the dependence of the photocurrent of the devices on the internal electric field, the polymer and TiO2 layer thicknesses. Bilayer polymer/TiO2 cells consisting of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and TiO2, with different thicknesses of the polymer and TiO2 films, were prepared for experimental purposes. The experimental results for the prepared bilayer MEH-PPV/TiO2 cells under different conditions are satisfactorily fitted to the model. Results show that increasing TiO2 or the polymer layer in thickness will reduce the exciton dissociation efficiency in the device and further the photocurrent. It is found that the photocurrent is determined by the competition between the exciton dissociation and charge recombination at the donor/acceptor interface, and the increase in photocurrent under a higher incident light intensity is due to the increased exciton density rather than the increase in the exciton dissociation efficiency.

Cu2ZnSnS4 quantum dots as effective electron acceptors for hybrid solar cells with a broad spectral response

High-purity Cu2ZnSnS4 quantum dots (CZTS-QDs) with a size of 3–5 nm and a band gap of 1.67 eV are synthesized by a facile solvothermal method using simple chemicals in ethanol solvent. The CZTS-QDs have an ionization potential (IP) of −5.96 eV and an electron affinity (EA) of −4.33 eV, which are almost not changed after removal of capping molecules on them. Due to the favorable IP and EA positions with respect to those of poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV), the CZTS-QDs act as effective electron acceptors for hybrid solar cells based on polymer/CZTS blends with MEH-PPV as the polymer. CZTS-QDs and MEH-PPV form type-II heterojunctions to enable the solar cells to have a promising open-circuit voltage of 0.63 V, and the efficient charge separation for neutral excited states produced either on the polymer or on the CZTS-QDs makes the solar cells have a wide spectral response extending to 900 nm. It is revealed that removal of capping molecules on the quantum dots mainly leads to a reduced polymer exciton diffusion effect on the electron transport dynamics due to the formation of wider CZTS charge transport channels and an increased short-circuit current (Jsc) in the solar cells, where the enhanced Jsc dominantly correlates with the increased charge transfer and collection efficiencies due to the improved charge transport property in CZTS channels.

Nonconjugated Polymer Poly(vinylpyrrolidone) as an Efficient Interlayer Promoting Electron Transport for Perovskite Solar Cells

The interfaces between perovskite layer and electrodes play a crucial role on efficient charge transport and extraction in perovskite solar cells (PSCs). Herein, for the first time we applied a low-cost nonconjugated polymer poly(vinylpyrrolidone) (PVP) as a new interlayer between PCBM electron transport layer (ETL) and Ag cathode for high-performance inverted planar heterojunction perovskite solar cells (iPSCs), leading to a dramatic efficiency enhancement. The CH3NH3PbI3-xClx-based iPSC device incorporating the PVP interlayer exhibited a power conversion efficiency (PCE) of 12.55%, which is enhanced by ∼15.9% relative to that of the control device without PVP interlayer (10.83%). The mechanistic investigations based on morphological, optical, and impedance spectroscopic characterizations reveal that incorporation of PVP interlayer promotes electron transport across the CH3NH3PbI3-xClx perovskite/Ag interface via PCBM ETL. Besides, PVP incorporation induces the formation of a dipole layer, which may enhance the built-in potential across the device, conjunctly promoting electron transport from PCBM to Ag cathode and consequently leading to significantly improved fill factor (FF) from 58.98 to 66.13%.

Highly efficient non-fullerene polymer solar cells enabled by novel non-conjugated small-molecule cathode interlayers

The interface strategy has been identified as an effective process to optimize the overall performance of polymer solar cells (PSCs). Herein, three novel non-conjugated small molecules comprising amino cations and sulfonate anions, as well as various core atoms of oxygen, sulfur, and sulfone, were successfully synthesized and employed as cathode interlayers (CILs) for non-fullerene PSCs. A large improvement of the device performance was observed, in which the solution processed sulfur-based CIL shows excellent cathode modification ability and device properties with the highest power conversion efficiency (PCE) of 11.30%. Compared with the vacuum deposited Ca, the non-conjugated small-molecule CILs could significantly increase the charge transport and collection capabilities, decrease the work function (WF) of the Al counter electrode, and reduce the series resistance and charge recombination at the interface. Most importantly, these simple water/alcohol soluble CILs are of great significance and suitable for the low-cost and large-area preparation of PSCs.

Sequential Deposition Route to Efficient Sb2S3 Solar Cells

We report a facile two-step sequential deposition method to prepare Sb2S3 thin films, where antimony acetate and thiourea are utilized as antimony and sulfur sources, respectively. The sequential deposition of two precursor materials followed by swift annealing at mild temperature leads to high-quality Sb2S3 films. The detailed reaction mechanism is revealed on the basis of structural and compositional analyses. By optimizing the concentration of thiourea and annealing temperature, uniform and flat Sb2S3 thin films are obtained with either sulfur-deficiency or sulfur richness. Finally, a planar heterojunction solar cell based on the as-prepared Sb2S3 film delivers a high power conversion efficiency of 5.69%, which is a top value for planar heterojunction Sb2S3 solar cells fabricated by a solution approach. This research provides a convenient and low-cost approach for the deposition of Sb2S3 films for efficient solar cell applications.

Growth of Compact CH3NH3PbI3 Thin Films Governed by the Crystallization in PbI2 Matrix for Efficient Planar Perovskite Solar Cells

As a convenient preparation technique, a two-step method, which is normally done by spin-coating CH3NH3I onto PbI2 film followed by a thermal annealing, is generally used to prepare solution-processed CH3NH3PbI3 films for planar perovskite solar cells. Here, we prepare the compact CH3NH3PbI3 thin films by the two-step method at a low temperature (<80 °C) and investigate the effects of PbI2 crystallization on the structure-property correlation in the CH3NH3PbI3 films. It is found that the importance of the crystallization in PbI2 matrix lies in governing the transition from the (001) plane of trigonal PbI2 to the (002) plane of tetragonal CH3NH3PbI3 in the rapid reaction process for atoms to coordinate into perovskite during spin-coating, which actually determines the morphology and the type of vacancy defects in resulting perovskite; a better crystallized PbI2 film has a much stronger ability to react with CH3NH3I solution and produces larger CH3NH3PbI3 grains with a higher crystallinity. The CH3NH3PbI3/TiO2 planar solar cell derived from a better crystallized PbI2 film exhibits significantly improved performance and stability as the result of the higher crystallinity inside the perovskite film. Moreover, it is demonstrated that the crystalline PbI2 film matrix subjected to the annealing after a slow heating process prior to contacting CH3NH3I solution is more effective for CH3NH3PbI3 formation than that with a direct annealing history. The results in this paper provide a guide for preparing high-quality CH3NH3PbI3 thin films for efficient perovskite solar cells and CH3NH3PbI3 interfacial films over the layers susceptible to temperature.