Meng-Huan Jao

Nanoparticle-tuned self-organization of a bulk heterojunction hybrid solar cell with enhanced performance.

We demonstrate here that the nanostructure of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PCBM) bulk heterojunction (BHJ) can be tuned by inorganic nanoparticles (INPs) for enhanced solar cell performance. The self-organized nanostructural evolution of P3HT/PCBM/INPs thin films was investigated by using simultaneous grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence wide-angle X-ray scattering (GIWAXS) technique. Including INPs into P3HT/PCBM leads to (1) diffusion of PCBM molecules into aggregated PCBM clusters and (2) formation of interpenetrating networks that contain INPs which interact with amorphous P3HT polymer chains that are intercalated with PCBM molecules. Both of the nanostructures provide efficient pathways for free electron transport. The distinctive INP-tuned nanostructures are thermally stable and exhibit significantly enhanced electron mobility, external quantum efficiency, and photovoltaic device performance. These gains over conventional P3HT/PCBM directly result from newly demonstrated nanostructure. This work provides an attractive strategy for manipulating the phase-separated BHJ layers and also increases insight into nanostructural evolution when INPs are incorporated into BHJs.

Achieving a high fill factor for organic solar cells

Organic photovoltaics (OPVs) have developed rapidly in the last decades due to their potential for providing cost-efficient, low-energy consumption, and environmentally friendly renewable energy sources. Some research reports have focused on the device physics of organic photovoltaics that governs open circuit voltage (Voc) and short circuit current (Jsc) to improve their performance. In this review, we focus on the third parameter, fill factor (FF), that is equally important in determining the power conversion efficiency. We discuss the mathematical calculation of the FF and the relationship between the FF and equivalent circuit model elements, namely, shunt resistance, series resistance, and diode ideal factor. In order to provide a strategy toward a high FF for OPVs from the viewpoints of device design and material synthesis, we review important device features and BHJ features that have a large impact on the device FF, including preventing shorting, buffer layer design, domain size or purity, gradated BHJ structures, π–π stacking distance or direction, etc. We hope this article can provide a comprehensive insight into elements controlling the FF of OPVs and give a valuable direction for better device and material design.

Facile synthesis of wurtzite copper–zinc–tin sulfide nanocrystals from plasmonic djurleite nuclei

The present research demonstrates a facile one-pot heating process without injection to synthesize an important light harvesting quaternary nanocrystal: wurtzite copper–zinc–tin sulfide (w-CZTS). High quality w-CZTS nanocrystals can be easily obtained by mixing all the precursors and simply heating to the reaction temperature. The nano-crystal formation mechanism is thoroughly investigated and resolved by X-ray diffraction spectroscopy (XRD), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). It starts with the nucleation of plasmonic djurleite Cu1.94S, subsequent growth of CZTS–Cu1.94S heterostructures and inter-diffusion of cations and then finally leads to single phase and single crystal w-CZTS nanocrystals. The mechanism of nanocrystal formation can be applied universally regardless of the type of zinc and tin precursor for high quality w-CZTS nanocrystals. The in-depth interpretations of the reaction mechanism of this process significantly advance the current knowledge of multi-component nanocrystal formation. The developed method is scalable for high throughput and low cost w-CZTS suspensions which await practical photovoltaic applications.

Diketopyrrolopyrrole-based oligomer modified TiO2 nanorods for air-stable and all solution processed poly(3-hexylthiophene):TiO2 bulk heterojunction inverted solar cell

Diketopyrrolopyrrole-based oligomer was synthesized and used to modified TiO2 nanorods. The surface modified TiO2 was employed in the fabrication of air-stable and all solution processed poly(3-hexylthiophene):titanium dioxide nanorods (P3HT:TiO2 nanorods) bulk heterojunction (BHJ) inverted solar cells. The oligomer (copolymerized 4,5-diaza-9,9′-spirobifluorene with diketopyrrolopyrrole (PZFDPP)) was synthesized by Stille coupling reaction. The PZFDPP was coated on TiO2 nanorods by refluxing the TiO2 nanorods in oligomer containing solution at low temperature (70 °C). A concentration gradient profile of polymer/nanocrystals (P3HT/TiO2 nanorods) BHJ was observed for the first time by X-ray photoelectron spectroscopy (XPS) technique together with in situ ion sputtering, showing that the TiO2-rich region and P3HT rich region are aggregated adjacent to electron transport layer (ETL) and hole transport layer (HTL) respectively. The obtained depth profile indicates the inverted device structure is more suitable for polymer/inorganic nanocrystals BHJ solar cells. Furthermore, instead of using an energy consuming process for ETL layer deposition, the PZFDPP modified TiO2 nanorods were used to deposit the ETL layer by spin coating. The surface features and properties of deposited TiO2 ETL that was coated by PZFDPP were systematically investigated. The developed photovoltaic device shows a promising power conversion efficiency (PCE) of 1.2% benefited from improved electron mobility in P3HT:TiO2 BHJ film and across the ETL/active layer interfaces. Moreover, the device is extremely stable stored in ambient condition without encapsulation (less than 10% loss over 1000 h test). The results of this work demonstrate the successful development of highly efficient and air-stable polymer/inorganic nanocrystal hybrid BHJ inverted solar cells based on chemically modified nanocrystals which significantly extend the current knowledge of device fabrication.

Synthesis, optical and photovoltaic properties of bismuth sulfide nanorods

Bismuth sulfide (Bi2S3) nanorods exhibit a low band gap, a high absorbance coefficient and good dispersity. In this study, the synthesis conditions of Bi2S3 nanorods were systematically investigated to obtain nanorods of a desired dimension, with high aspect ratios and good crystallinity. The as synthesized Bi2S3 nanorods, 37.2 nm in length and 6.1 nm in width, have a low band gap of ∼1.4 eV with a conduction band and valence band of −3.8 eV and −5.2 eV, respectively. The nanorods were blended with poly(3-hexylthiophene) (P3HT) at a weight ratio of 1:1 to form a light harvesting P3HT:Bi2S3 hybrid film. The incorporated Bi2S3 nanorods can not only contribute light harvesting but also lead to a more ordered structure of the P3HT phase and a more efficient π–π* transition. Surface potential mapping of the hybrid film, measured by Kelvin probe force microscope (KPFM), shows a significantly negative shift (−34 mV) under white light illumination, which indicates carrier dissociation and the accumulation of negative charge on top of the hybrid film. The photovoltaic characteristics of the devices were also observed for those based on the P3HT:Bi2S3 hybrid film. This novel P3HT:Bi2S3 hybrid material provides a new candidate for the fabrication of low-cost and environmentally friendly polymer/inorganic hybrid solar cells.

Insights into solvent vapor annealing on the performance of bulk heterojunction solar cells by a quantitative nanomorphology study

Bulk heterojunctions (BHJ) represent the most promising structures for high efficiency polymer solar cells and their morphologies can be finely tuned by post-treatments such as thermal annealing and solvent vapor annealing. Though extensive studies have shown improved power conversion efficiencies by tuning the treating parameters of both treatments, substantial knowledge of how the BHJ morphologies evolve with various solvent vapors related to photovoltaic characteristics and differ from those with thermal annealing is still limited. Herein we employed simultaneous grazing incidence wide and small angle X-ray scattering (GIWAXS/GISAXS) to systematically investigate the changes in morphology of a poly(3-hexylthiophene)/C61-butyric acid methyl ester (P3HT–PCBM) BHJ manipulated by solvent vapor annealing using different solvents. Solvents with different solubility, i.e. non-solvent, poor solvent and good solvent were studied. Distinctive morphologies were quantitatively resolved among these solvent vapor-annealed BHJs and their evolutions during processing are interpreted. The resolved morphologies can clearly explain the subtle variations in photovoltaic characteristics of open circuit voltage (Voc), short circuit current (Jsc) and fill factor (FF) related to the working mechanism of the BHJ, i.e. carrier generation, carrier transportation and recombination. This work provides fundamental new insights into how the BHJ morphologies and photovoltaic characteristics can be flexibly tailored by solvent vapor annealing using various kinds of solvent vapors.

Hierarchical i–p and i–n porous heterojunction in planar perovskite solar cells

A hierarchical pore network in planar CH3NH3PbI3 perovskite is demonstrated herein. Quantitative characterizations by grazing incidence small angle X-ray scattering (GISAXS) with modeling and complementary microscopic observations provide insight at various length scales. It is a pore structure comprised of nano-scaled primary pores aggregating into meso-scaled fractal networks within the perovskite layer. Its structural evolution and mechanistic interpretation are explored with respect to different preparation methods/steps. The time-of-flight secondary ion mass spectrometer (TOF-SIMS) results suggest the infiltration of hole transporting materials (HTM) or electron transporting materials (ETM) deposited on top at different length scales. The inter-penetrating perovskite/HTM or perovskite/ETM form i–p or i–n one-sided porous heterojunctions, respectively, over the typically regarded planar-stacked heterojunction. They show distinctive photovoltaic characteristics and behaviors in which the large i–n interfaces at the nanoscale lead to highly efficient, hysteresis-free and reliable solar cell devices. The morphology–performance correlation is helpful for associated design of device architecture and processing toward higher efficiency and stability.

Synthesis and Characterization of Wurtzite Cu2ZnSnS4 Nanocrystals

Copper–zinc–tin–chalcogenide (CZTSSe) with earth abundant elements has attracted increasing attention due to large absorption coefficient and band gap of ~1.5 eV which is near the optimum band gap of single-junction photovoltaic devices. In this study, we used commercially available precursors to produce wurtzite Cu2ZnSnS4 nanocrystals by simple solvothermal synthesis. Different from the typical kesterite or stannite phases of CZTS, the nanocrystals synthesized in this study are in wurtzite phase with hexagonal crystal cell. The n-dodecanethiol was used to control the reactivity of metal ions, leading to the controlled size of CZTS nanoparticle by simply varying the reaction time. Furthermore, the as synthesized CZTS nanocrystals have novel wurtzite crystal structure. As a result, a red shift of absorption band edge between the CZTS nanoparticles with different size was obtained. Our study provides an extending method of CZTS nanocrystal ink preparation awaiting for further photovoltaic device application.

Low temperature and rapid formation of high quality metal oxide thin film via a hydroxide-assisted energy conservation strategy

Metal oxide thin films made from a sol–gel solution process are promising candidates for stable, low cost, and high performance electronic devices. Reducing the thermal budget required for their crystallization process can relax the fabrication limitation and expand their possible applications. We show that with the addition of an adequate amount of tetramethylammonium hydroxide (TMAOH) in the precursor solution, the activation energy of the sol–gel reaction can be reduced by about 50%. Using this strategy, not only can the required thermal treatment time and temperature of the sol–gel reaction be significantly reduced but also the quality of the film can be improved. The enhanced reaction rate can be ascribed to the presence of hydroxyl anions, which facilitate the formation of the metal hydroxide and the subsequent metal oxide. Additionally, the strategy developed here can be applied to multiple kinds of metal oxides. By this method, the processing temperature can be lowered by at least 50 °C and the time can be shortened by half for the fabrication of electronic devices such as thin film transistors and photovoltaics. Our results open up a new paradigm to fabricate highly crystalline metal oxide thin films quickly at an energy saving low temperature using the solution process.

Enhancing the efficiency of perovskite solar cells using mesoscopic zinc-doped TiO2 as the electron extraction layer through band alignment

Lead halide perovskite-structured solar cells (PSCs) have drawn great attention due to a rapid improvement in their photoelectric conversion efficiency in recent years. In this study, we have enhanced photovoltaic performance by using mesoscopic zinc-doped TiO2 (meso-Zn:TiO2) as the electron extraction layer. Zn:TiO2 nanoparticles (Zn:TiO2 NPs) with various zinc doping levels were synthesized by combining sol–gel and hydrothermal methods. The synthesized Zn:TiO2 NPs were used to fabricate electron extraction layers by a screen-printing method. We systematically investigated the surface morphology, crystal structure, contact angle, charge carrier dynamics, electron mobility, and electrical conductivity of various meso-Zn:TiO2. Furthermore, photo-assisted Kelvin probe force microscopy (KPFM) was used to analyze the surface potential of perovskite films coated with various meso-Zn:TiO2 to understand the electron extraction behavior under the illumination of light at various wavelengths. Moreover, the energy levels of various meso-Zn:TiO2 were estimated by ultraviolet photoelectron spectroscopy (UPS) and UV-vis absorption spectroscopy. We discovered that the 5.0 mol% meso-Zn:TiO2 exhibited the optimal band alignment with perovskite. Finally, the average power conversion efficiency (PCE) of PSCs with meso-Zn:TiO2 was enhanced from 13.1 to 16.8%, and such fabricated PSC yielded a champion PCE of 18.3%.