Chih-Min Chuang

Quantitative Nanoorganized Structural Evolution for a High Efficiency Bulk Heterojunction Polymer Solar Cell

We have developed an improved small-angle X-ray scattering (SAXS) model and analysis methodology to quantitatively evaluate the nanostructures of a blend system. This method has been applied to resolve the various structures of self-organized poly(3-hexylthiophene)/C61-butyric acid methyl ester (P3HT/PCBM) thin active layer in a solar cell from the studies of both grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence X-ray diffraction (GIXRD). Tuning the various length scales of PCBM-related structures by a different annealing process can provide a flexible approach and better understanding to enhance the power conversion of the P3HT/PCBM solar cell. The quantitative structural characterization by this method includes (1) the mean size, volume fraction, and size distribution of aggregated PCBM clusters, (2) the specific interface area between PCBM and P3HT, (3) the local cluster agglomeration, and (4) the correlation length of the PCBM molecular network within the P3HT phase. The above terms are correlated well with the device performance. The various structural evolutions and transformations (growth and dissolution) between PCBM and P3HT with the variation of annealing history are demonstrated here. This work established a useful SAXS approach to present insight into the modeling of the morphology of P3HT/PCBM film. In situ GISAXS measurements were also conducted to provide informative details of thermal behavior and temporal evolution of PCBM-related structures during phase separation. The results of this investigation significantly extend the current knowledge of the relationship of bulk heterojunction morphology to device performance.

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.

Bi-hierarchical nanostructures of donor–acceptor copolymer and fullerene for high efficient bulk heterojunction solar cells

Solvent additive processing has become the most effective method to tune the nanostructure of donor–acceptor (D–A) type copolymer/fullerene bulk heterojunctions (BHJs) solar cells for improving power conversion efficiencies. However, to date qualitative microscopic observations reveal discrepant results on the effects of solvent additives. Here, we present quantitative evolution of bi-hierarchical nanostructure of D–A copolymers and fullerenes by employing grazing-incidence small/wide angle X-ray scattering (GISAXS/GIWAXS) techniques and [2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]/[6,6]-phenyl-C71-butyric acid methyl ester (PCPDTBT/PCBM) BHJ as model materials. An accurate GISAXS model analysis is established herein for revealing the distinctive bi-hierarchical nanostructures from molecular level to a scale of hundreds of nanometers. The mechanisms of hierarchical formation and mutual influence between PCPDTBT and PCBM domains are proposed to correlate with photovoltaic properties. These results provide a comprehensive interpretation in respect to previous studies on the nanostructures of D–A copolymer/fullerene BHJs. It is helpful for optimum structural design and associated synthesis improvement for achieving high efficiency BHJ solar cells.

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.

High intensity fluorescence of photoactivated silver oxide from composite thin film with periodic array structure

We have fabricated a composite thin film that exhibits intense photoactivated fluorescence of silver oxide at 522 and 529nm under the irradiation of a 488nm laser. This film consists of a silver coated polymeric periodic array on indium tin oxide glass substrate. By adjusting the column diameters and lattice constants of the array to coincide with the excitation wavelength, an order increase in fluorescence intensity was obtained due to the surface plasmon polariton resonance of silver. This composite film has many potential applications in highly efficient optoelectronic devices.

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.

Manipulation of nanoscale phase separation and optical properties of P3HT/PMMA polymer blends for photoluminescent electron beam resist.

A novel photoluminescence electron beam resist made from the blend of poly(3-hexylthiophene) (P3HT) and poly(methyl methacrylate) (PMMA) has been successfully developed in this study. In order to optimize the resolution of the electron beam resist, the variations of nanophase separated morphology produced by differing blending ratios were examined carefully. Concave P3HT-rich island-like domains were observed in the thin film of the resist. The size of concave island-like domains decreased from 350 to 100 nm when decreasing the blending ratio of P3HT/PMMA from 1:5 to 1:50 or lower, concurrently accompanied by significant changes in optical properties and morphological behaviors. The lambda(max) of the film absorption is blue-shifted from 520 to 470 nm, and its lambda(max) of photoluminescence (PL) is also shifted from 660 to 550 nm. The radiative lifetime is shorter while the luminescence efficiency is higher when the P3HT/PMMA ratio decreases. These results are attributed to the quantum confinement effect of single P3HT chain isolated in PMMA matrix, which effectively suppresses the energy transfer between the well-separated polymer chains of P3HT. The factors affecting the resolution of the P3HT/PMMA electron beam resists were systematically investigated, including blending ratios and molecular weight. The photoluminescence resist with the best resolution was fabricated by using a molecular weight of 13 500 Da of P3HT and a blending ratio of 1:1000. Furthermore, high-resolution patterns can be obtained on both flat silicon wafers and rough substrates made from 20 nm Au nanoparticles self-assembled on APTMS (3-aminopropyltrimethoxysilane)-coated silicon wafers. Our newly developed electron beam resist provides a simple and convenient approach for the fabrication of nanoscale photoluminescent periodic arrays, which can underpin many optoelectronic applications awaiting future exploration.

Manipulation of luminescence from CdSe nanoparticles by three-dimensional photonic crystal

We have demonstrated that the luminescence properties of CdSe nanoparticles can be manipulated by self-assembled silica photonic crystal. When the emitting wavelength of nanoparticles matches the stop band of the photonic crystal, the photoluminescence of the CdSe nanoparticles can be greatly enhanced by up to five times. By changing the collection angle of photoluminescence measurements, the photoluminescence intensity of CdSe embedded in photonic crystal can also be controlled.

Nanolithography made from water-based spin-coatable LSMO resist

A dual functional and water soluble spin-coatable lanthanum strontium manganese oxide (LSMO) resist has been developed that consists of lanthanum nitrate, strontium nitrate, manganese nitrate, polyvinyl alcohol and water. Energetic nitrates plus polyvinyl alcohol fuel promote autoignition and produce nanopatterns (<60 nm) upon mild electron beam exposure (< 2m C cm −2 ). The formation of cubic perovskite LSMO has been confirmed by micro-IR spectroscopy, elemental analysis, x-ray diffraction and transmission electron microscopy. The patterned LSMO film can be developed using nontoxic and environmentally friendly pure water, and the resist can fabricate active magnetic patterns directly by electron beam exposure. The spin-coatable LSMO resist can be fabricated into either positive or negative patterns easily by varying the electron doses. It can change from negative resist to positive resist and then finally negative resist with the increase of electron dose. The positive and negative dual functional mechanism of spin-coatable LSMO resist is reported. A resist with simultaneous positive and negative capabilities patterning will benefit the direct writing technology of an electron beam. The active magnetic characteristics and high refractive index of the material are useful for the direct fabrication of magnetic and optical devices. (Some figures in this article are in colour only in the electronic version)