This paper presents a decent polymer material for fabricating optically-induced dielectrophoretic (ODEP) devices, which can manipulate microparticles or cells by using moving light patterns. A thin film of a bulk-heterojunction (BHJ) polymer, a mixture of regioregular poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester, is used as a light-activated layer. When illuminated by a projected light beam, the photo-induced charge carriers created by the electron transfer of excitons at a donor/acceptor interface in the BHJ layer, disturbs the uniformly-distributed electric field applied on the ODEP devices. A negative DEP force is then generated by virtual electrodes defined by the optical images from a computer-programmable projector to manipulate microparticles, thus providing a functionalized platform for particle manipulation. The effect of the polymer thickness and composition on the magnitude of the generated DEP force has been extensively investigated. The maximum particle drag velocity and the force applied on 20.0 μm diameter polystyrene beads are measured to be approximately 202.2 μm/s and 38.2 pN, respectively, for a device with a 497.3-nm thick BHJ layer. The lifetime of the developed device is also explored (~5 hours), which is sufficient for applications of disposable ODEP devices. Therefore, the BHJ polymer may provide a promising candidate for future ODEP devices capable of nanoparticle and cell manipulation.
©2009 Optical Society of America
The manipulation of microparticles and cells by using dielectrophoretic (DEP) forces has been extensively explored in the past few decades . Briefly, non-uniform alternating electric fields are used to induce motion in polarized particles or cells. Since most biological cells and macromolecules behave as dielectric particles when under the influence of an external alternating electric field, DEP forces have been shown to have many useful applications in biology such as particle separation, levitation, manipulation and characterization . However, fabrication of conventional DEP devices usually requires complicated photolithography and delicate thin-film deposition and etching processes to pattern micro-electrodes [3,4]. Recently, advancements in the design of optically-induced dielectrophoresis (ODEP) devices, upon which “virtual” electrodes are generated by a programmable presentation projector, have become a viable alternative approach to manipulating the microparticles and cells [5,6]. The ODEP chip consists of a top indium-tin-oxide (ITO) glass substrate and a bottom ITO glass substrate with a photoconductive layer (usually amorphous silicon). The top and bottom ITO glasses are supplied with an alternating-current (AC) voltage. Before the light source illuminates the photoconductive layer, it has high electrical impedance. After the light hits the photoconductive layer, electron-hole pairs are excited and thus the impedance of the amorphous layer is decreased by four to five orders of magnitude. Hence, the applied voltage will drop across the liquid layer and change the distribution of the electric field, thus inducing a non-uniform electric field distribution between the top and bottom ITO glasses. The particles/cells are then induced with a DEP force when they are exposed to this non-uniform electric field. In this manner, the particles or cells can be manipulated by the optical image illuminated on the photoconductive layer. These ODEP devices can manipulate microparticles and cells using moving light patterns, without any physical contact. More importantly, the projected virtual electrodes can be easily modified using computer software, without additional micro-fabrication processes. In addition to particle/cell manipulation, a number of ODEP devices have been demonstrated for manipulation of nano-wires , particle counting and sorting  DNA manipulation  and cell lysis .
As mentioned above, the typical configuration for the ODEP devices has two ITO glass substrates, with one of them deposited with a thin layer of amorphous silicon as the photoconductive layer. In addition, the non-uniform electric field caused by the photo-induced charge carriers generates attractive or repelling forces (positive or negative DEP forces) and therefore is used to manipulate the microparticles/cells. However, the fabrication of the amorphous silicon layer on the ITO glass substrate usually requires a plasma-enhanced chemical vapor deposition process, which is still a relatively costly, high-temperature, and precision-manufacturing procedure. Accordingly, there exists a need to replace the amorphous silicon layer with an appropriate photoconductive or light- activated materials to simplify the fabrication of ODEP chips.
This study reports an innovative application of the bulk-heterojunction (BHJ) [11,12] polymer, as the light-activated layer for the fabrication of the ODEP devices. Mixtures of regioregular poly(3-hexylthiophene) (P3HT) [13,14] and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) have been commonly used as a light-activated material to generate the charge carriers in high-performance, polymer BHJ photovoltaic cells, due to their efficient electron transfer and the effective charge separation processes of the photo-induced excitons at the donor/acceptor interface . Accordingly, the fabrication of an ODEP device using the P3HT:PCBM thin film instead of using amorphous silicon could be an attractive alternative. This decent material has unique advantages of a high optical extinction coefficient in the wavelengths of visible light and the charge carriers have a small lateral diffusion length for precise motion control by a projected image. The BHJ layer can be spin-coated at a relatively low temperature for mass production of the ODEP chips. It is also feasible to fabricate the ODEP device on large-area and flexible polymer substrates through a continuous roll-to-roll coating process. In addition, various conjugated polymers with different band-gap structures can be fine-tuned, if necessary, by replacing the P3HT with PCBM as the light-activated layer to optimize the specific spectrum response of ODEP devices. The operating lifetime of these BHJ ODEP devices can be also improved when a chemically inert insulation layer is deposited on the surface of the light-activated layer to reduce oxidation or degradation of the conjugated molecules.
2. Operating Principle of BHJ ODEP device
The DEP force is generated from the BHJ polymer when it is illuminated by a light source. Figure 1(a) shows a schematic illustration of the ODEP chip consisting of a sandwiched structure, including a top ITO glass, a liquid layer containing particles and a bottom ITO glass deposited with thin films. An AC voltage is applied between the top and the bottom ITO glass to produce a uniform electric field across the chip initially. Figure 1(b) shows a schematic illustration of the mechanism for the generation of photo-excited charge carriers through an electron-transfer process of excitons at the P3HT/PCBM interface . The dissociated charge carriers generated by illustration disturb the uniform applied AC electric field on ODEP devices. As a result, a non-uniform electric field is generated in the illumination area in the liquid layer. This induces electric dipoles in the particles, thus generating attractive or repelling DEP forces . As the light patterns are projected onto the polymer thin film, they work in a similar manner as the microelectrodes in a chip, and are thus referred to as “virtual electrodes”. The illuminated area induces a negative or a positive DEP force to repel or to attract the polystyrene beads depending on the driving frequency of the applied voltage, the conductivities of the beads and the fluid medium, and the permittivity of the beads and the medium . In this study, we use a repulsion force to confine the beads, which are moved around by using different patterns of illumination.
It is assumed that the interactive force between the particles is relatively small. For this case, the DEP force Fcan be presented as Eq. (1). As a result, an indirect method based on Stokes’ law  can be used to estimate the ODEP force while the ODEP force was balanced by the viscous force during the motion.
As shown in Fig. 1(a), the bottom ITO glass (RITEK Corp., 15Ω/□) is deposited with a light-activated layer and a water/oxygen-proof layer. The light-activated layer is composed of different concentrations of the P3HT:PCBM mixture and the water/oxygen-proof layer is made of lithium fluoride (LiF). It is then bonded with a top ITO glass by using a double-sided tape as a spacer. The thickness of the tape is 30.0 μm, thus forming a gap between the top and the bottom glass substrates for particle flow in the ODEP device. Two holes with a diameter of 1 mm are mechanically drilled into the top ITO glass as the fluidic inlet and outlet prior to bonding.
Prior to fabrication, the bottom ITO glass substrate is sequentially cleaned by ultrasonic agitation in a detergent, deionized water, acetone and isopropyl alcohol. Then a Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Baytron P, Bayer AG, Germany) layer is spin-coated at 4000 rpm to modify the ITO surface [19,20] and baked at 150 °C for 30 min. PEDOT:PSS is a commonly used conductive polymer as an interfacial buffer layer to smoothen the roughness of ITO glass substrate. Then, the following protocol is implemented in a nitrogen-filled glove box to form the light-activated layer. A mixed solution of regioregular P3HT (Rieke Metals, Inc., USA) and PCBM (Nano-C, Inc., USA) at a ratio of 1:1 is dissolved in 1,2-dichlorobenzene  with a concentration ranging from 3 wt% (30 mg:30 mg/ml) to 5 wt% (50 mg:50 mg/ml). It is prepared by stirring them for at least 24 hours at room temperature. Then the solution is filtered to prevent the aggregation of particles. The light-activated layer is formed by spin-coating the mixed solution with different concentrations in a “slow solvent vapor treatment” process, which involves keeping the film in a dichlorobenzene vapor-filled Petri glass dish to slowly evaporate away the solvent for four hours. Then LiF layer is thermally deposited in vacuum (10−6 torr) onto the P3HT:PCBM film and the PEDOT:PSS layer and the LIF film have a thickness of 30.0 nm  and 5.0 nm, respectively.
The experimental setup is schematically shown in Fig. 1(a). The liquid is a mixture of polystyrene beads and a solution consisting of deionized (DI) water with 1% fetal bovine serum (FBS), which is used to prevent the adhesion of polystyrene beads onto the chip surface. An AC voltage (100 kHz, 24 Vpp) is applied across the top and the bottom ITO glass by a function generator (Model 195, Wavetek, U.K.) connected to a power amplifier. Commercially available computer software (FLASH, Macromedia, USA) is used to generate the images illuminated on the ODEP chip and a charge-coupled-device (CCD, SSC-DC80, Sony, Japan) camera is used to acquire images of particle manipulation. The illumination source is a polysilicon thin-film transistor (TFT) micro-lens projector (ViewSonic PJ1172, Japan) with a spatial resolution of 1024 x 768 pixels. The programmable optical images generated from the projector are focused onto the BHJ layer through a 50X objective lens (Nikon, Japan).
3. Results and discussion
3.1 The effect of the PCBM layer
In order to quantify the induced DEP force applied on the polystyrene beads, a fixed-width (28.0 μm), line-shaped light segment is illuminated on the ODEP chip and moved with an increasing velocity to drag the beads with a diameter of 20.0 µm. The white light with an intensity of 7.8 W/cm2 is used in our studies. The maximum drag velocity of the beads is determined when they cannot follow the movement of the line-shaped light segment. Accordingly, the induced DEP force, which is balanced by the drag force of the fluid, can be calculated from Stokes’ law . Figure 2 presents the results of the measured maximum drag velocity and the calculated induced DEP force for ODEP devices made of P3HT-only and P3HT:PCBM as the light active layer. It is found that the maximum drag velocities of the ODEP devices increase with the thicknesses of the light-activated layers for P3HT-only and P3HT:PCBM films. Since the absorbance of the incident light is proportional to the thicknesses of the light-activated layer, the increased drag velocities (and hence the induced DEP forces) can be attributed to the generation of higher concentrations of photo-induced charge carriers in the thicker polymer layers. The maximal drag velocities are measured to be 10.3, 20.2, and 34.2 µm/s for the ODEP devices with P3HT thicknesses of 173.1, 227.3, and 295.2 nm, respectively.
The drag velocity (and the induced DEP force) is remarkably enhanced when an electron acceptor, PCBM, is mixed with the P3HT film to form a BHJ layer, which effectively facilitates the electron transfer of the photo-induced excitons and the generation of the charge carriers (positive and negative polarons) in the light-activated layer. The excessive photo-induced charge carriers in the P3HT:PCBM film disturb the uniform electrical field applied on the ODEP devices, thus resulting in the enhancement of the induced DEP force, as measured by the drag velocities, when compared to those in the P3HT-only films. The maximum drag velocities are measured to be 70.2, 104.1, 168.3 and 202.2 µm/s for the ODEP devices with a total P3HT:PCBM thickness of 177.2, 277.1, 339.2, and 497.3 nm, respectively. This is about a five-fold increase as compared to those P3HT-only devices with light-activated layers of similar thicknesses.
The maximum induced DEP force is estimated based on Stokes’ law and presented in Fig. 2. The DEP force applied on the spherical beads is calculated from the drag force exerted by the fluid on the particle, i.e., F = 6πrηv, where r is the radius of the spherical bead, η is the dynamic viscosity of the fluid (1.002 × 10−3 Ns/m2 at 20°C for water). As shown in Fig. 2, the induced DEP forces applied on the polystyrene beads increase with the thicknesses of the light-activated layers for both P3HT-only and P3HT:PCBM films. The induced DEP force is calculated to be 38.2 pN for the ODEP device with a P3HT:PCBM thickness of 497.3 nm. It is concluded that the application of the donor/acceptor BHJ polymer as light-active layers is essential important to the performance of these ODEP devices.
3.2 UV-Vis absorption spectra of P3HT:PCBM layer
Figure 3 illustrates the UV-Vis spectra of the P3HT:PCBM films prepared from different concentrations of P3HT:PCBM solutions at a spin speed of 600 rpm. All films are prepared under the slow solvent vapor treatment process. The intensities of the UV-Vis spectra of the polymer films are found to increase with the concentration of P3HT:PCBM solutions (1-5 wt%) because of the increased thicknesses. Therefore, a higher induced DEP force should be observed when increasing the concentration.
Figure 3 also shows that the absorption peaks are located at around 335 nm and 500-550 nm, which correspond to the absorption bands of the PCBM and the P3HT, respectively . There is no obvious chemical shift in the peak positions of the UV-Vis spectra for the P3HT:PCBM films, which suggests that the morphologies of the polymer films prepared under the slow solvent vapor treatment process  are quite stable although they are prepared from different concentrations of polymer solutions. In addition, the peak around 610 nm in Fig. 3, associated with the order degree in the intermolecular chains of the microcrystalline domains for P3HT, implies a high level of molecular ordering during formation of the P3HT:PCBM films.
Chloroform and chlorobenzene are other commonly-used solvents to prepare the P3HT:PCBM solutions. The solubility of P3HT:PCBM in chloroform and chlorobenzence is comparable to that of dichlorobenzene. However, the boiling point of chloroform and chlorobenzence is relatively low. The polymer films prepared from P3HT:PCBM chloroform and chlorobenzene solutions may dry very easily during the spin-coating process, which are not suitable for the “slow solvent vapor treatment” of the P3HT:PCBM films to well control the polymer morphology. The studies for the influence of different P3HT:PCBM thin-film morphologies on the performance of ODEP devices are currently in progress.
3.3 The thickness effect
The thickness of the light-activated layer plays an important role on the generation of the induced DEP force. Figure 4 presents the maximum drag velocity and the induced DEP force on the polystyrene bead for the P3HT:PCBM films prepared from different concentrations of polymer solutions. The thickness of the P3HT:PCBM film ranges from 115.2 nm to 497.3 nm, which are prepared from 3%, 4%, and 5% solutions, respectively. As shown in Fig. 4, both the maximum drag velocity and the induced DEP force increase with the thicknesses of the P3HT:PCBM film for all cases. Since the morphologies of the polymer films prepared from different concentrations of solutions are very similar based on the UV-Vis spectra as illustrated in Fig. 3, the increasing drag velocities and induced DEP forces can be attributed to the higher absorption of the incident light in the thicker P3HT:PCBM films which results in the generation of larger concentrations of charge carriers. For instance, the drag velocity and the induced DEP force for polystyrene beads with a diameter of 20.0 µm are measured to be 38.1 µm/s and 7.2 pN, respectively, for the ODEP device with a P3HT:PCBM film with a thickness of 120.1 nm. This can be increased by approximately five times to 202.2 µm/s and 38.2 pN when the thickness of the P3HT:PCBM film increases to 497.3 nm. It is then concluded that the thickness of the P3HT:PCBM film is the most critical parameter for the generation of the induced DEP force even though the film is prepared from different initial concentrations.
3.4 Lifetime of the polymer-based ODEP devices
The long-term operation of the polymer-based ODEP devices is explored in this section. Figure 5 illustrates the relationship between the maximum drag velocity and the continuous operating time of the polymer-based ODEP devices. The induced DEP force is also plotted in the same figure. Initially, the maximum drag velocity for a polymer-based ODEP device with a 308.3-nm P3HT:PCBM layer (4 wt%, 600 rpm) without a LiF layer as the insulation layer is measured to be 118.2 µm/s. It drops to half of this value after 50 minutes of continuous operation. The polymer-based ODEP device eventually fails to operate after 100 minutes. The moisture and oxygen inside the fluid tend to react with the conjugated molecules in the P3HT:PCBM layer. The defects and traps at the P3HT:PCBM layer due to the oxidation or the degradation of the conjugated molecules result in the quenching of photo-excited molecules. The reduction in the total amount of the photo-excited charge carriers at the polymer layer is responsible for the decline of the maximum drag velocity and the associated induced DEP force.
Oxidation or degradation of the conjugated molecules is alleviated by the addition of an inert insulation layer of a 5.1 nm LiF film, which is deposited on the surface of the P3HT:PCBM layer. As shown in Fig. 5, the maximum drag velocity for the ODEP device with a 308.3 nm P3HT:PCBM layer capped with a 5.1 nm LiF layer is measured to be 140.2 µm/s, which is about a 15% increase when compared to a device without the LiF layer. Presumably, the LiF layer prevents direct contact to the P3HT:PCBM layer with the fluid. Electron transfer of the photo-excited carriers in the conjugated molecules to the fluid at the top surface of the P3HT:PCBM layer is then blocked by the insulating LiF layer. Furthermore, the local concentration of the photo-excited charge carriers is increased such that a larger DEP force is generated. Since the diffusion of, and any reaction with, the moisture and the oxygen into the P3HT:PCBM layer is partially prevented by the LiF layer, the trap and defect sites created by the oxidation of the conjugated molecules are then reduced and a longer operating lifetime is observed. Similarly, the maximum drag velocity for an ODEP device with a 497.0-nm (5 wt%, 600 rpm) P3HT:PCBM layer capped with a 5-nm LiF layer is measured to be 202.0 µm/s, initially. It drops to half of this magnitude after 200 minutes of continuous operation. After that, the maximum drag velocity and the induced DEP force remain stable at around 78.0 µm/s and 14.8 pN, respectively, after 350 minutes of continuous operation. This operating lifetime is sufficient for a disposable ODEP device to manipulate particles or cells for most biomedical analysis protocols. Thus, the application of the LiF layer is critical to increasing the maximum drag velocity and also to extend the operating lifetime. Additionally, the LiF layer is relatively robust during operation and cannot be dissolved in the fluid, which may reduce the risk of the reactions between the light-activated layer and the charge-carrying particles in future biological applications.
3.5 Manipulation of polystyrene beads
Figure 6 shows images taken under an optical microscope demonstrating successful manipulation of 20.0 μm diameter polystyrene beads using the developed polymer-based ODEP chip. An AC voltage with a frequency of 100 kHz and a magnitude of 24.0 Vpp is applied across the top and the bottom ITO glass substrates to generate an electrical field. The illuminated optical images are white lights with an intensity of 7.8 W/cm2. Initially, three polystyrene beads are randomly distributed on the substrate as shown in Fig. 6(a). A circular pattern is illuminated to generate a negative DEP force. Then, the three polystyrene beads are moved to, and concentrated at, the upper left side of the substrate (Fig. 6(b)) by gradually moving and shrinking the size of the light circle. The light circle is able to carry the three polystyrene beads around the four corners of the substrate as shown in Figs. 6 (c)-(f). These results demonstrate the capability of the developed polymer-based ODEP platform for the collection and transportation of microparticles. A similar approach can be used to manipulate cells .
The ODEP device using a donor/acceptor BHJ polymer thin film instead of amorphous silicon as the light-activated layer is a very attractive approach. First, the conjugated molecules (P3HT used in this study) have a higher absorption intensity (with an extinguishing coefficient of 9.6x105 cm−1 at 530 nm for π-π* electronic transition) than that of amorphous silicon (with an extinguishing coefficient of 4.8x102 cm−1 at 570 nm for band-band transition) in the visible region. The maximum drag velocity for polystyrene beads (20 µm in diameter) in an ODEP device made of 1000-nm amorphous silicon is measured to be approximately 260.0 µm/s in our previous study . However, an ODEP device made with a 500-nm BHJ polymer layer, under the same operating conditions presented in this study, has a maximum drag velocity of 202.2 µm/s. Optimistically, an ODEP device made with the P3HT:PCBM film could generate a larger induced DEP force than that of a device made with an amorphous silicon layer, with the same thickness. Second, the carrier mobility of the P3HT:PCBM film (~electron mobility = 7.7x10−5 cm2V−1s−1, hole mobility = 5.1x10−5 cm2V−1s−1)  is much lower than that of amorphous silicon layer (~1.0 x10−2 cm2V−1s−1) . Additionally, the diffusion length of the excitons in the conjugated molecules is usually limited to ~10 nm. Therefore, the lateral diffusion of photo-induced charge carriers and excitons should be confined to the illuminated region on the P3HT:PCBM film. This is an important advantage in the design of polymer-based ODEP devices for precise and effective optical manipulation of the microparticles. Third, various conjugated polymers with different band-gap structures can be used, if necessary, instead of P3HT with PCBM as the light-activated layer. The optical response of the ODEP devices can then be fine-tuned and optimized to the specific illuminating wavelength for the generation of a maximum induced DEP force.
In this study, a polymer-based ODEP device is demonstrated by manipulating microparticles. The optical manipulation of the polystyrene beads is achieved by using a computer program to change the position of projected images onto the ODEP device. The generation of the photo-induced charge carriers at the donor/acceptor interface disturbs the initial uniform electric field applied on the devices, thus inducing a negative DEP force to manipulate the polystyrene beads. The mixture of the electron acceptor, PCBM, with P3HT to form the BHJ interface in the light-activated layer is critical to the generation of photo-induced charge carriers and the resulting magnitude of the induced DEP forces. Additionally, the LiF layer partially inhibits the oxidation or degradation of the conjugated molecules during illumination and significantly extends the operating lifetime. The maximum drag velocity and the induced force on the polystyrene beads (20.0 µm in diameter) in this study are measured to be 202.2 µm/s and 38.2 pN, respectively, for an ODEP device with a 497.3-nm thick P3HT:PCBM layer. Further development of this polymer-based ODEP device may provide a new approach for future biological applications.
The author G. B. Lee would like to thank the National Science Council (NSC) in Taiwan (NSC 96-2120-M-006-008) for their financial support. The author T. F. Guo would like to thank the NSC in Taiwan (NSC 96-2113-M-006-009-MY3) and the Asian Office of Aerospace Research and Development (AOARD-09-4055) for financially supporting this research.
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