Craig E. Murphy

The nature of in-plane skeleton Raman modes of P3HT and their correlation to the degree of molecular order in P3HT:PCBM blend thin films.

The nature of main in-plane skeleton Raman modes (C=C and C-C stretch) of poly(3-hexylthiophene) (P3HT) in pristine and its blend thin films with [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) is studied by resonant and nonresonant Raman spectroscopy and Raman simulations. Under resonant conditions, the ordered phase of P3HT with respect to its disordered phase is identified by (a) a large shift in the C=C mode peak position to lower wavenumber (~21 cm(-1) shift), (b) a narrower fwhm of the C=C mode (~9 cm(-1) narrower), (c) a larger intensity of the C-C mode relative to the C=C mode (~56% larger), and (d) a very small Raman dispersion (~5 cm(-1)) of the C=C mode. The behavior of the C=C and C-C modes of the ordered and disordered phases of P3HT can be explained in terms of different molecular conformations. The C=C mode of P3HT in P3HT:PCBM blend films can be reproduced by simple superposition of the two peaks observed in different phases of P3HT (ordered and disordered). We quantify the molecular order of P3HT after blending with PCBM and the subsequent thermal annealing to be 42 ± 5% and 94 ± 5% in terms of the fraction of ordered P3HT phase, respectively. The increased molecular order of P3HT in blends upon annealing correlates well with enhanced device performance (J(SC), -4.79 to -8.72 mA/cm(2) and PCE, 1.07% to 3.39%). We demonstrate that Raman spectroscopy (particularly under resonant conditions) is a simple and powerful technique to study molecular order of conjugated polymers and their blend films.

Optoelectronic and Charge Transport Properties at Organic−Organic Semiconductor Interfaces: Comparison between Polyfluorene-Based Polymer Blend and Copolymer

We report detailed studies of optoelectronic and charge transport properties at the organic-organic semiconductor interfaces formed between polymer chains (interchain) and within a polymer chain (intrachain). These interfaces are fabricated using poly(9,9-di-n-octylfluorene-alt-N-(4-butylphenyl)diphenylamine) (TFB [f8-tfb]) (electron-donor) and poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT [f8-bt]) (electron-acceptor) conjugated polymers, by blending them together or by covalently attaching them via a main polymer backbone (copolymer). For optoelectronic properties, when a bulky and twisted tfb molecule is incorporated into a rigid F8BT conjugated backbone, it disturbs the conjugation of F8BT polymer, leading to a blue-shift in the lowest absorption transition. However, by acting as an effective electron donor, it assists the formation of an intrachain singlet exciton that has a strong charge-transfer character, leading to a red-shifted and longer-lived emission than that of F8BT. An extremely efficient and fast energy transfer from tfb donor to bt acceptor is observed in the copolymer (<1 ps) compared to transfer from TFB to F8BT in the blend (tens of ps). This efficient energy transfer in the copolymer is found to be associated with its low fluorescence efficiency (40-45% vs 60-65% for blend) because of the migration of radiative singlet excitons to low-energy states such as triplet and exciplex states that are nonemissive or weakly emissive. The presence of molecular-scale tfb-f8-bt interfaces in the copolymer, however, does not hinder an efficient transport of charge carriers at high drive voltages. Instead, it provides a better balance of charge carriers inside the device, which leads to slower decay of the device efficiency and thus more stable light-emitting diodes with increasing voltage than the blend devices. These distinctive optoelectronic and charge transport properties observed at different organic-organic semiconductor interfaces will provide useful input for the design rules of conjugated polymers required for improved molecular electronics.

Phase-separated thin film structures for efficient polymer blend light-emitting diodes.

We report laterally and vertically phase-separated thin film structures in conjugated polymer blends created by polymer molecular weight variation. We find that micrometer-scale lateral phase separation is critical in achieving high initial device efficiency of light-emitting diodes, whereas improved balance of charge carrier mobilities and film thickness uniformity are important in maintaining high efficiency at high voltages. The optoelectronic properties of these blend thin films and devices are strongly influenced by the polymer chain order/disorder and the interface state formed at polymer/polymer heterojunctions.

Wire-bar coating of semiconducting polythiophene/insulating polyethylene blend thin films for organic transistors

Organic blend thin films consisting of semiconducting poly(3-hexylthiophene) (P3HT) and insulating high-density polyethylene (HDPE) have been fabricated by novel application of a large area wire-bar coating technique in air. The microstructure of P3HT:HDPE blend films reveals a strong structural dependence on initial composition. Preferential segregation of P3HT toward the film surface is observed for all blend compositions, while P3HT (or P3HT-rich) columnar structures enclosed by HDPE (or HDPE-rich) lamellar matrix is distinctive for 50:50 (by weight) blends. The transistors fabricated with P3HT:HDPE blend films show a clear field effect behavior, exhibiting charge carrier mobilities up to 5 × 10−2 cm2/Vs, comparable to the values reported in spin-coated similar blends and of neat P3HT devices. The wire-bar coated blend films and devices are highly repeatable and spatially uniform over large areas (few cm by few cm), demonstrating the suitability of this technique for manufacturing of large area organic...

High-mobility solution-processed copper phthalocyanine-based organic field-effect transistors

Abstract Solution-processed films of 1,4,8,11,15,18,22,25-octakis(hexyl) copper phthalocyanine (CuPc6) were utilized as an active semiconducting layer in the fabrication of organic field-effect transistors (OFETs) in the bottom-gate configurations using chemical vapour deposited silicon dioxide (SiO2) as gate dielectrics. The surface treatment of the gate dielectric with a self-assembled monolayer of octadecyltrichlorosilane (OTS) resulted in values of 4×10−2 cm2 V−1 s−1 and 106 for saturation mobility and on/off current ratio, respectively. This improvement was accompanied by a shift in the threshold voltage from 3 V for untreated devices to -2 V for OTS treated devices. The trap density at the interface between the gate dielectric and semiconductor decreased by about one order of magnitude after the surface treatment. The transistors with the OTS treated gate dielectrics were more stable over a 30-day period in air than untreated ones.

Surface and subsurface morphology of operating nanowire:fullerene solar cells revealed by photoconductive-AFM

The 3D nanometer scale phase separated morphology of organic solar cells crucially affects performance. We demonstrate that photoconductive atomic force microscopy can provide both surface and subsurface information in operating organic solar cells providing direct correlation between 3D film morphology, local nanoscale optoelectronic properties and device characteristics. P3HT nanowire:PCBM bulk-heterojunction working devices were investigated. The macroscopic solar cell performance improvements upon thermal annealing, such as an increase in the short circuit current, the open circuit voltage and the fill factor, are consistent with observed enrichment of PCBM at the air interface and increased nanowire crystallinity. PC-AFM is able to directly resolve the associated changes in charge transport and collection at the local scale, with an estimated depth resolution of at least 20 nm inside the film.

Understanding the relationship between molecular order and charge transport properties in conjugated polymer based organic blend photovoltaic devices

We report a detailed characterization of the thin film morphology of all-polymer blend devices by applying a combined analysis of physical, chemical, optical, and charge transport properties. This is exemplified by considering a model system comprising poly(3-hexylthiophene) (P3HT) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT). We show that the interactions between the two conjugated polymer components can be controlled by pre-forming the P3HT into highly ordered nanowire structures prior to blending with F8BT, and by varying the molecular weight of the F8BT. As a result, it is possible to produce films containing highly ordered P3HT with hole mobilities enhanced by three orders of magnitude over the pristine blends. Raman spectroscopy under resonant excitation conditions is used to probe the molecular order of both P3HT and F8BT phases within the blend films and these morphological studies are complemented by measurements of photocurrent generation. The resultant increase in photocurrent is associated with the enhanced charge carrier mobilities. The complementary analytical method demonstrated here is applicable to a wide range of polymer blend systems for all applications where the relationships between morphology and device performance are of interest.

A metrology perspective on the dark injection transient current method for charge mobility determination in organic semiconductors

Charge mobility is a key parameter for understanding the performance of organic semiconductor devices and materials. A range of techniques is available that can measure charge mobility with varying accuracy and precision. In this paper we analyze the dark injection transient current (DITC) method from a metrology perspective. We carried out a systematic study of the sensitivity of single carrier analogues of organic light-emitting diodes (OLEDs) to small changes in electrical input and environmental conditions. We observed that the experimental results depend strongly on the previous history of the device under test, with both long term and short term effects in evidence. Our findings demonstrate the need for caution in interpreting the results of single experiments to determine the charge mobility of OLEDs and the difficulty of associating uncertainty statements with the results of charge mobility measurements.

Quantitative nanoscale surface voltage measurement on organic semiconductor blends.

We report on the validation of a method based on Kelvin probe force microscopy (KPFM) able to measure the different phases and the relative work function of polymer blend heterojunctions at the nanoscale. The method does not necessitate complex ultra-high vacuum setup. The quantitative information that can be extracted from the topography and the Kelvin probe measurements is critically analysed. Surface voltage difference can be observed at the nanoscale on poly(3-hexyl-thiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blends and dependence on the annealing condition and the regio-regularity of P3HT is observed.

Modelling of organic field-effect transistors for technology and circuit design

As organic field-effect transistors (OFETs) are preparing to take a key role in the flexible and low cost electronics applications, there is a pressing need for predictive device models to support technology optimization and circuit design. This paper focuses on the specific OFET features that challenge current modelling approaches. The presented modelling techniques range from the fundamental semiconductor equations to compact device model representations as required for implementation in advanced TCAD and EDA commercial tools. The models are verified with measured characteristics of advanced OFET device structures.