John A. Carr

Enhanced charge separation in organic photovoltaic films doped with ferroelectric dipoles

A key requirement for realizing efficient organic photovoltaic (OPV) cells is the dissociation of photogenerated electron-hole pairs (singlet-excitons) in the donor polymer, and charge-transfer-excitons at the donor–acceptor interface. However, in modern OPVs, these excitons are typically not sufficiently harnessed due to their high binding energy. Here, we show that doping the OPV active-layers with a ferroelectric polymer leads to localized enhancements of electric field, which in turn leads to more efficient dissociation of singlet-excitons and charge-transfer-excitons. Bulk-heterojunction OPVs based on poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester are fabricated. Upon incorporating a ferroelectric polymer as additive in the active-layer, power conversion efficiencies increase by nearly 50%, and internal quantum efficiencies approach 100% – indicating complete exciton dissociation at certain photon energies. Similar enhancements in bilayer-heterojunctions, and direct influence of ferroelectric poling on device behavior show that improved dissociation is due to ferroelectric dipoles rather than any morphological change. Enhanced singlet-exciton dissociation is also revealed by photoluminescence lifetime measurements, and predicted by simulations using a numerical device model.

The identification, characterization and mitigation of defect states in organic photovoltaic devices: a review and outlook

In any microelectronic device, fundamental physical parameters must be well understood for successful electronic optimization. One such prominent parameter is energetic trap states, which are well-known to plague amorphous or otherwise impure semiconducting materials. Organic semiconductors are no strangers to such states and their electronic properties are evidently tied to these defects. Herein, this article discusses the identification, characterization and mitigation of bandgap residing trap levels in organic photovoltaic devices. A compilation of select studies to date is given and a general outlook is proposed. Organic photovoltaic materials are depicted as multiple trap-level systems with a seemingly continuous distribution of electronic states throughout the bandgap. Some elucidations as to the origins of these electronic states as well as recent works centered on defect removal are also presented.

On accurate capacitance characterization of organic photovoltaic cells

Capacitance measurements, widely used to characterize numerous semiconductor properties, have been recently adopted to characterize organic photovoltaic (OPV) devices. It is known that certain challenges are associated with capacitance measurements. Of upmost importance is the employment of a proper measurement model (series or parallel). Owing to larger capacitive impedances and low series resistances, the parallel model is typically employed in inorganics. However, we find that for characteristically thinner organic films, a hybrid model should be used. We highlight the inconsistencies in OPV literature due to indiscriminate usage of parallel model and show how proper model selection can rectify any artifacts.

On the identification of deeper defect levels in organic photovoltaic devices

Defect levels play a significant role in altering organic photovoltaic (OPV) performance, affecting device aspects such as recombination, carrier transport, and Fermi-level pinning. In the ongoing effort to optimize the promising OPV technology, the identification, characterization, and potential mitigation or enhancement of such defect states remain important regions of interest. Herein, low frequency admittance spectroscopy is coupled with a high frequency, point-by-point capacitance versus voltage measurement to reveal a previously unknown deep-defect distribution in poly(3-hexylthiophene) based OPVs. The capacitance models of Cohen and Lang, Walter et al. and Kimmerling are employed alongside a trap-free dark current model to give good characterization and substantiation to the discovered band. Repetitions of the measurements on devices with and without a fullerene acceptor show the measured distribution to contain acceptor-like traps spatially located in the polymer bulk. The findings presented here ...

Amplitude-modulated sinusoidal microchannels for observing adaptability in C. elegans locomotion

In this paper, we present a movement-based assay to observe adaptability in Caenorhabditis elegans locomotion behavior. The assay comprises a series of sinusoidal microchannels with a fixed wavelength and modulating (increasing or decreasing) amplitude. The channel width is comparable to the body diameter of the organism. Worms are allowed to enter the channel from the input port and migrate toward the output port. Within channel sections that closely match the worms natural undulations, the worm movement is relatively quick and steady. As the channel amplitude increases or decreases along the device, the worm faces difficulty in generating the propulsive thrust, begins to slow down and eventually fails to move forward. A set of locomotion parameters (i.e., average forward velocity, number and duration of stops, range of contact angle, and cut-off region) is defined for worm locomotion in modulated sinusoidal channels and extracted from the recorded videos. The device is tested on wild-type C. elegans (N2) and two mutants (lev-8 and unc-38). We anticipate this passive, movement-based assay can be used to screen nematodes showing difference in locomotion phenotype.

Microfluidic bioassay to characterize parasitic nematode phenotype and anthelmintic resistance.

With increasing resistance to anti-parasitic drugs, it has become more important to detect and recognize phenotypes of resistant isolates. Molecular methods of detecting resistant isolates are limited at present. Here, we introduce a microfluidic bioassay to measure phenotype using parameters of nematode locomotion. We illustrate the technique on larvae of an animal parasite Oesophagostomum dentatum. Parameters of sinusoidal motion such as propagation velocity, wavelength, wave amplitude, and oscillation frequency depended on the levamisole-sensitivity of the isolate of parasitic nematode. The levamisole-sensitive isolate (SENS) had a mean wave amplitude of 135 μm, which was larger than 123 μm of the levamisole-resistant isolate (LEVR). SENS had a mean wavelength of 373 μm, which was less than 393 μm of LEVR. The mean propagation velocity of SENS, 149 μm s-1, was similar to LEVR, 143 μm s-1. The propagation velocity of the isolates was inhibited by levamisole in a concentration-dependent manner above 0.5 μm. The EC50 for SENS was 3 μm and the EC50 for LEVR was 10 μm. This microfluidic technology advances present-day nematode migration assays and provides a better quantification and increased drug sensitivity. It is anticipated that the bioassay will facilitate study of resistance to other anthelmintic drugs that affect locomotion.

Scanning Angle Raman Spectroscopy of Poly(3-hexylthiophene)-Based Films on Indium Tin Oxide, Gold, and Sapphire Surfaces

Interest in realizing conjugated polymer-based films with controlled morphology for efficient electronic devices, including photovoltaics, requires a parallel effort to characterize these films. Scanning angle (SA) Raman spectroscopy is applied to measure poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM)-blend morphology on sapphire, gold, and indium tin oxide interfaces, including functional organic photovoltaic devices. Nonresonant SA Raman spectra are collected in seconds with signal-to-noise ratios that exceed 80, which is possible due to the reproducible SA signal enhancement. Raman spectra are collected as the incident angle of the 785 nm excitation laser is precisely varied upon a prism/sample interface from approximately 35 to 70°. The width of the ∼1447 cm(-1) thiophene C═C stretch is sensitive to P3HT order, and polymer order varied depending on the underlying substrate. This demonstrates the importance of performing the spectroscopic measurements on substrates and configurations used in the functioning devices, which is not a common practice. The experimental measurements are modeled with calculations of the interfacial mean square electric field to determine the distance dependence of the SA Raman signal. SA Raman spectroscopy is a versatile method applicable whenever the chemical composition, structure, and thickness of interfacial polymer layers need to be simultaneously measured.

Unidirectional, electrotactic-response valve for Caenorhabditis elegans in microfluidic devices

We report a nematode electrotactic-response valve (NERV) to control the locomotion of Caenorhabditis elegans (C. elegans) in microfluidic devices. This nonmechanical, unidirectional valve is based on creating a confined region of lateral electric field that is switchable and reversible. We observed that C. elegans do not prefer to pass through this region if the field lines are incident to its forward movement. Upon reaching the boundary of the NERV, the incident worms partially penetrate the field region, pull back, and turn around. The NERV is tested on three C. elegans mutants: wild-type (N2), lev-8, and acr-16.

Deep defects and the attempt to escape frequency in organic photovoltaic materials

Trap states are well-known to plague organic photovoltaic devices and their characterization is essential for continued progress. This letter reports on both the deep trap profiles and kinetics of trap emission, studied through temperature dependent capacitance measurements. Three polymer based systems relevant to photovoltaics, namely, P3HT:PC60BM, PTB7:PC70BM, and PCDTBT:PC70BM were investigated. Each polymer showed a markedly different deep trap profile, varying in shape from a nearly constant density of states to a sharp Gaussian. In contrast, the frequency of trap emission was similar for each—ca. 108−109 Hz—indicating a universal value and similar trapping mechanisms despite the differences in energetic distribution. The latter result is important in the light of range of conflicting values reported, or higher value (1012 Hz) typically borrowed from crystalline inorganic materials.

Tailoring Nanoscale Morphology of Polymer:Fullerene Blends Using Electrostatic Field

To tailor the nanomorphology in polymer/fullerene blends, we study the effect of electrostatic field (E-field) on the solidification of poly(3-hexylthiophene-2, 5-diyl) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) bulk heterojunction (BHJ). In addition to control; wet P3HT:PC60BM thin films were exposed to E-field of Van de Graaff (VDG) generator at three different directions-horizontal (H), tilted (T), and vertical (V)-relative to the plane of the substrate. Surface and bulk characterizations of the field-treated BHJs affirmed that fullerene molecules can easily penetrate the spaghetti-like P3HT and move up and down following the E-field. Using E-field treatment, we achieved favorable morphologies with efficient charge separation, transport, and collection. We improve; (1) the hole mobility values up to 19.4 × 10-4 ± 1.6 × 10-4 cm2 V-1 s-1 and (2) the power conversion efficiency (PCE) of conventional and inverted OPVs up to 2.58 ± 0.02% and 4.1 ± 0.40%, respectively. This E-field approach can serve as a new morphology-tuning technique, which is generally applicable to other polymer-fullerene systems.