Corey V. Hoven

A modular molecular framework for utility in small-molecule solution-processed organic photovoltaic devices

We report on the design, synthesis and characterization of light harvesting small molecules for use in solution-processed small molecule bulk heterojunction (SM-BHJ) solar cell devices. These molecular materials are based upon an acceptor/donor/acceptor (A/D/A) core with donor endcapping units. Utilization of a dithieno(3,2-b;2′,3′-d)silole (DTS) donor and pyridal[2,1,3]thiadiazole (PT) acceptor leads to strong charge transfer characteristics, resulting in broad optical absorption spectra extending well beyond 700 nm. SM-BHJ solar cell devices fabricated with the specific example 5,5′-bis{7-(4-(5-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine}-3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene (6) as the donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor component showed short circuit currents above −10 mA cm−2 and power conversion efficiencies (PCEs) over 3%. Thermal processing is a critical factor in obtaining favorable active layer morphologies and high PCE values. A combination of UV-visible spectroscopy, conductive and photo-conductive atomic force microscopies, dynamic secondary mass ion spectrometry (DSIMS), and grazing incident wide angle X-ray scattering (GIWAXS) experiments were carried out to characterize how thermal treatment influences the active layer structure and organization.

Electron injection into organic semiconductor devices from high work function cathodes

We show that polymer light-emitting diodes with high work-function cathodes and conjugated polyelectrolyte injection/transport layers exhibit excellent efficiencies despite large electron-injection barriers. Correlation of device response times with structure provides evidence that the electron-injection mechanism involves redistribution of the ions within the polyelectrolyte electron-transport layer and hole accumulation at the interface between the emissive and electron-transport layers. Both processes lead to screening of the internal electric field and a lowering of the electron-injection barrier. The hole and electron currents are therefore diffusion currents rather than drift currents. The response time and the device performance are influenced by the type of counterion used.

Chemically fixed p-n heterojunctions for polymer electronics by means of covalent B-F bond formation

Widely used solid-state devices fabricated with inorganic semiconductors, including light-emitting diodes and solar cells, derive much of their function from the p-n junction. Such junctions lead to diode characteristics and are attained when p-doped and n-doped materials come into contact with each other. Achieving bilayer p-n junctions with semiconducting polymers has been hindered by difficulties in the deposition of thin films with independent p-doped and n-doped layers. Here we report on how to achieve permanently fixed organic p-n heterojunctions by using a cationic conjugated polyelectrolyte with fluoride counteranions and an underlayer composed of a neutral conjugated polymer bearing anion-trapping functional groups. Application of a bias leads to charge injection and fluoride migration into the neutral layer, where irreversible covalent bond formation takes place. After the initial charging and doping, one obtains devices with no delay in the turn on of light-emitting electrochemical behaviour and excellent current rectification. Such devices highlight how mobile ions in organic media can open opportunities to realize device structures in ways that do not have analogies in the world of silicon and promise new opportunities for integrating organic materials within technologies now dominated by inorganic semiconductors.

Real-time X-ray scattering studies of film evolution in high performing small-molecule–fullerene organic solar cells

We have studied the influence of the formulation additive 1,8-diiodooctane (DIO) on the structural evolution of bulk heterojunction (BHJ) films based the small molecule donor 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-5-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2) and phenyl-C71-butyric-acid-methyl ester ([70]PCBM). Real-time, in situ, grazing-incidence X-ray scattering experiments allow us to characterize the development of crystalline order via diffraction and phase separation via small angle scattering. The performance of p-DTS(FBTTh2)2 based solar cells exhibits a distinct optimum with respect to volume fraction of DIO in the coating solution, unlike many polymer–fullerene systems that exhibit plateaus in performance above a certain additive volume fraction. Increasing the DIO volume fraction increases the crystallinity of p-DTS(FBTTh2)2 and dramatically increases the phase separation length scale even at small DIO amounts. These results suggest that the existence of an optimal DIO amount is a consequence of the phase separation length scale and its relationship to the optimal length for exciton dissociation. The effects of DIO on the time evolution of the drying films indicates that it acts as both a solvent and a plasticizer for p-DTS(FBTTh2)2, controlling its nucleation density and promoting its crystal growth.

Direct measurement of electric field screening in light emitting diodes with conjugated polyelectrolyte electron injecting/transport layers

Electroabsorption spectroscopy was used to directly probe the electric fields in a polymer light emitting diode that utilizes a conjugated polyelectrolyte electron transporting/injection layer. The electric field in the emitting layer was found to be negligible at applied biases greater than the built-in field of the device. Holes injected at these biases accumulate at the emitting layer/conjugated polyelectrolyte interface and screen the field from the emitting layer to the conjugated polyelectrolyte layer. In conjunction with mobile ions that redistribute the field in the conjugated polyelectrolyte layer, this leads to greatly improved electron injection from high work function cathodes.

Photocurrent hysteresis by ion motion within conjugated polyelectrolyte electron transporting layers

The change in photocurrent characteristics of polymer light-emitting diodes containing cationic conjugated polyelectrolyte electron transport/injection layers depends on the nature of the counterions and the rate of bias sweep. We attribute this feature to the motion of counteranions and differences in the rates of ion migration.

CHAPTER 5:Design and Synthesis of Small Molecule Donors for High Efficiency Solution Processed Organic Solar Cells

Solution processed small molecule bulk-heterojunction solar cells have emerged as one of the most promising approaches to the development of organic photovoltaic technology. The design of these solar cells is based on an active layer that is comprised entirely of discrete molecular systems. To date, the best performance has been realized using donor–acceptor type active layers, employing small molecule donors and fullerene acceptors. Through rational molecular design strategies, improvements in the small molecule donor architecture have resulted in solar cells reaching power conversion efficiencies in excess of 10%. This chapter briefly describes the evolution of small molecule donors for use in organic solar cells and details key materials properties. The bulk of the chapter highlights the synthesis and structure of several important classes of small molecule donors that have enabled the fabrication of highly efficient devices, including diketopyrrolopyrrole, isoindigo, porphyrins, and donor–acceptor type oligothiophene-based materials.