Advances in Organic-Based Electronic Devices

Promising results are reported for memory devices and solar photovoltaic cells.

February 1, 2008

Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio

A program of research has addressed multiple topics in the field of electronic and optoelectronic devices based on polymers and other organic substances, with special attention to memory devices and solar photovoltaic cells. The accomplishments of this research are summarized as follows:

Polymer/metal nanocomposite memory devices.

Applied Potential and Current Response of an experimental polymer/metal nanocomposite memory device were measured over several cycles of writing (W), erasure (E), and reading (R).

Electrically, these devices are characterized as bistable in that they can be switched between a high-electrical-conductance and a low-electrical conductance state. A basic device of this type comprises a polymer/ metal nanocomposite film sandwiched between two metal electrodes. In one successful experiment, the electrodes were made of aluminum, and the polymer/metal nanocomposite comprised 8 hydroxyquinoline, polystyrene, and gold nanoparticles capped with 1 dodecanethiol. The gold nanoparticles were prepared in a two-phase arrested-growth process and had sizes ranging from 1.6 to 4.4 with an average of 2.8 nm. The device was fabricated in a relatively simple process: The first layer of aluminum was deposited on a glass substrate by thermal evaporation in a vacuum. The polymer/ nanoparticle composite was deposited by spin coating with a 1,2-dichlorobenzene solution of the aforementioned polymeric and nanoparticle ingredients. The second layer of aluminum was deposited on the polymer/nanoparticle composite by thermal evaporation in a vacuum.

The figure presents measurement data from an experiment that demonstrated switching between the high- and low-conductance states, which could be taken to represent binary "1" and "0," respectively. The device was written, read, and erased repeatedly: A potential of 5 V was applied to write "1"; that is, to switch the device to the high-conductance state. The state was read by applying a lower potential (1.1 V). The current during the reading of the "1" state was 10-7 A. The "1" state was erased by applying a potential of -2.3 V, which switched the device to the low-conductance "0" state. Upon application of the reading potential of 1.1 V, the device responded at a "0"-state current of the order of 10-9 A. These write-read-erase cycles demonstrate that the device could be used as a nonvolatile memory device. Other measurements, not illustrated in the figure, have been interpreted as signifying that reading and writing can be accomplished by pulses as short as 25 ns.

•Polymer/virus/metal nanocomposite memory devices.

A device of this type is similar to the polymer/ metal- nanocomposite memory device described above, except that in this device, the nanocomposite layer between the electrodes consists of a polymer matrix within which are embedded viruses conjugated with metal nanoparticles. In one successful experiment, the nanocomposite consisted of a polyvinyl alcohol matrix with embedded tobacco mosaic viruses to which platinum nanoparticles had been attached by electroless deposition from a platinum- ion solution. A device containing this nanocomposite exhibited switching to the "1" state at a potential of about 3 V and switching to the "0" state at a potential of about -2.4 V.

•Accurate measurement and characterization of organic solar cells.

This body of research addresses the need for improved means of accurately measuring the performances of organic solar cells and using the measurements to accurately characterize the cells according to international norms that, heretofore, have seldom been followed in practice. A simple method of accurately determining the efficiencies of organic solar cells was developed. Different kinds of test cell/reference cell combinations were used to calculate spectral mismatch factors under a standard reference spectrum. Also, the importance of choosing a suitable reference cell for intensity calibration of light sources was demonstrated.

•Effect of self-organization in polymer/ fullerene bulk heterojunctions on the performances of polymer photovoltaic cells.

Some background information on polymer photovoltaic (PV) devices is prerequisite to a meaningful description of this body of research. Of all PV devices, those based on the concept of a bulk heterojunction (BHJ) of donor and acceptor components have exhibited the highest efficiencies reported thus far. Among the most promising donor materials is regioregular poly(3-hexylthiophene) or [RR-P3HT]. Highly efficient solar cells based on BHJs of P3HT with [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) have been demonstrated. In one recent study, it was found that the efficiencies of solar cells based on P3HT/PCBM can be increased significantly by suitably controlling the rates of growth of the active layers during fabrication of these cells: In particular, it was found that the slow growth of the active layer induced self-organization in the polymer chains, as a result of which there were significant improvements in absorbance and the carrier mobility was observed. This concludes the background information.

In the body of research reported here, the mechanisms underlying the increases in efficiency in P3HT/PCBM-based cells were investigated in more detail. The structural effect of self-organization by slow growth was examined by atomic force microscopy. Transport properties were studied by fitting current-versus-voltage characteristics measured in the dark to the corresponding characteristics predicted by a space-charge-limited-current. Single-carrier (hole only and electron only) devices were fabricated for use in determining the charge-carrier mobilities in the active layers. The photocurrent behaviors of PV devices under reverse bias were examined on the basis of Onsager's theory of ion-pair dissociation in weak electrolytes; reasonable fits of the theoretical to the experimental photocurrent- versus-bias-potential data were obtained by varying the parameters involved in the model. The effect of the growth rate on the dissociation efficiency of electron-hole pairs under short-circuit conditions, and the consequent effect on device performance, were examined.

This work was done by Yang Yang of the University of California Los Angeles (UCLA) for the Air Force Research Laboratory.