Roy Murray

Analysis of voltage and temperature dependent photocurrent collection in p3ht/pcbm solar cells

Current-voltage (J-V) analysis of poly (3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) blend organic solar cells (OSC) at various temperatures has been recorded and analyzed. The photovoltaic parameters extracted from a lumped circuit analysis completely describe the illuminated J-V data from far reverse bias to beyond the open circuit voltage (Voc). A simple model for carrier collection, previously applied to inorganic thin film solar cells, has been used to describe the voltage dependence of the photocurrent, JL(V), with only one adjustable parameter, Lc/D, the ratio of the carrier collection length to the active-layer thickness. Measured J-V curves of a variety of OSCs with varying thickness and blend ratios are closely fit with this model. Effect of resistive and collection losses has been quantified and removed, allowing the intrinsic junction behavior to be uncovered. The temperature dependence of Voc is linear with negative temperature coefficient, dVoc/dT,∼ −1 mV/K. The v...

Biofunctionalization of PEDOT films with laminin-derived peptides

UNLABELLED Poly(3,4-ethylenedioxythiophenes) (PEDOT) have been extensively explored as materials for biomedical implants such as biosensors, tissue engineering scaffolds and microelectronic devices. Considerable effort has been made to incorporate biologically active molecules into the conducting polymer films in order to improve their long term performance at the soft tissue interface of devices, and the development of functionalized conducting polymers that can be modified with biomolecules would offer important options for device improvement. Here we report surface modification, via straightforward protocols, of carboxylic-acid-functional PEDOT copolymer films with the nonapeptide, CDPGYIGSR, derived from the basement membrane protein laminin. Evaluation of the modified surfaces via XPS and toluidine blue O assay confirmed the presence of the peptide on the surface and electrochemical analysis demonstrated unaltered properties of the peptide-modified films. The efficacy of the peptide, along with the impact of a spacer molecule, for cell adhesion and differentiation was tested in cell culture assays employing the rat pheochromocytoma (PC12) cell line. Peptide-modified films comprising the longest poly(ethylene glycol) (PEG) spacer used in this study, a PEG with ten ethylene glycol repeats, demonstrated the best attachment and neurite outgrowth compared to films with peptides alone or those with a PEG spacer comprising three ethylene glycol units. The films with PEG10-CDPGYISGR covalently modified to the surface demonstrated 11.5% neurite expression with a mean neurite length of 90μm. This peptide immobilization technique provides an effective approach to biofunctionalize conducting polymer films. STATEMENT OF SIGNIFICANCE For enhanced diagnosis and treatment, electronic devices that interface with living tissue with minimum shortcomings are critical. Towards these ends, conducting polymers have proven to be excellent materials for electrode-tissue interface for a variety of biomedical devices ranging from deep brain stimulators, cochlear implants, and microfabricated cortical electrodes. To improve the electrode-tissue interface, one strategy utilized by many researchers is incorporating relevant biological molecules within or on the conducting polymer thin films to provide a surface for cell attachment and/or provide biological cues for cell growth. The present study provides a facile means for generating PEDOT films grafted with a laminin peptide with or without a spacer molecule for enhanced cell attachment and neurite extension.

Study of the Nanoscale Morphology of Polythiophene Fibrils and a Fullerene Derivative

Nanoscale blending of electron-donor and electron-acceptor materials in solution-processed bulk heterojunction organic photovoltaic devices is crucial for achieving high power conversion efficiency. We used a classic blend of poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM) as a model to observe the nanoscale morphology of the P3HT fibrils and PCBM nanoclusters in the mixture. Energy-filtered transmission electron microscopy (EFTEM) clearly revealed a nanoscopic phase separation. Randomly connected and/or nonconnected P3HT fibrous networks and PCBM domains, revealed by 2-dimensional micrographs, were observed by collecting electron energy loss spectra in the range of 19-30 eV. From EFTEM images, the average length and the diameter of P3HT fibrils were found to be approximately 70 ± 5 and 15 ± 2 nm, respectively. Combining the EFTEM, selected area electron diffraction, and X-ray diffraction results, the number and spacing of the ordered chains in P3HT fibrils were determined. There were 18 ± 3 repeating units of P3HT perpendicular to the fibril, ∼184 layers of π-π stacking along the fibril, and ∼9 layers of interchain stacking within the fibril. These conclusive observations provide insight into the number of molecules found in one instance of ordered-plane stacking. This information is useful for the calculation of charge transport in semicrystalline polymers. Using cross-section samples prepared with a focused ion beam technique, the vertical morphology of each phase was analyzed. By collecting 30 eV energy loss images, the phase separation in the P3HT/PCBM system was distinguishable. A higher P3HT concentration was observed at the top of the cell, near Al contact, which could possibly cause loss of carriers and recombination due to a mismatch in the P3HT and Al energy bands.

Current-voltage analysis of annealing effects of poly(3-hexlythiophene) and phenyl-C61-butyric acid methyl ester organic solar cells

Abstract. The field of organic photovoltaics (OPV) has progressed rapidly. With new materials and methods being briskly developed, the characterization of OPV also needs to be updated. Current-voltage (JV) analysis of poly(3-hexlythiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) OPV devices yield valuable insight into the internal physics of devices. A simple lumped circuit model, previously used to analyze various inorganic thin-film PV and more recently applied to OPV, has been used to study annealing parameters. To investigate the change in the lumped circuit model parameters, we carried out an annealing study of P3HT:PCBM blend OPV devices. We annealed devices at various temperatures and before and after evaporating the contact. We characterized and quantified the effect of thermal annealing by studying how the model parameters changed. We found that all parameters studied reacted favorably to annealing, including device resistances and parameters measuring recombination. While studying the resistances in unannealed and annealed devices, a barrier was found around the flat band voltage. This barrier disappeared upon annealing, indicating that it was due to material characteristics related to the crystallinity or the phase separation. The data were used to better characterize annealing effects.

A comparative study on the morphology of P3HT:PCBM solar cells with the addition of Fe 3 O 4 nanoparticles by spin and rod coating methods

Our previous study presented up to 20% power conversion efficiency (PCE) enhancement of poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells under the Fe3O4 nanoparticles (NPs) self-assembly (SA) effect by spin coating. Fe3O4 NPs (about 11 nm hydrodynamic diameter) form a thin layer at the top interface of the light absorbing active layer, which results in the generation of PCBM rich region improving the charge transport (Zhang et al. Sol Energ Mat Sol C 160:126–133, 2017). In order to investigate the feasibility of this Fe3O4 NPs SA effect under large-scale production condition, a smooth rod was implemented to mimic roll-to-roll coating technique and yield active layers having about the same thickness as the spin-coated ones. Small angle neutron scattering and grazing incidence X-ray diffraction were employed finding out similar morphologies of the active layers by these two coating techniques. However, rod-coated solar cell’s PCE decreases with the addition of Fe3O4 NPs compared with the one without them. This is because PCBM rich region is not created at the top interface of the active layer due to the absence of Fe3O4 NPs, which is attributed to the weak convective flow and low diffusion rate. Moreover, in the rod-coated solar cells, the presence of Fe3O4 NPs causes decrease in P3HT crystallinity, thus the charge transport and the device performance. Our study confirms the role of spin coating in the Fe3O4 NPs SA effect and enables researchers to explore this finding in other polymer nanocomposite systems.

Current voltage analysis of silver nanoparticle doped organic photovoltaic devices

Organic photovoltaic devices utilizing surfactant free thermally evaporated metallic nanoparticles to aid in light absorption were fabricated. These devices were characterized with a number of methods, including current voltage (JV) testing, UV-Vis spectroscopy, SEM, and AFM. The JV results were further analyzed using a simple lumped circuit model combined with a model to relate the carrier collection efficiency with the lifetime-mobility product. These findings indicated that while these nanoparticles did in fact increase the absorption, they suffered from high recombination that led to a decrease in the efficiency. The devices with nanoparticles located close to the electrodes had the highest recombination. Sandwiching the nanoparticles in the active layer proved useful in decreasing recombination and creating a well-functioning device.