The Sellinger research group focuses on the design, synthesis and characterization of organic and hybrid based semiconductor materials for application in solar cells, white lighting, and thin film transistors.

Below is a list of ongoing projects in the Sellinger group:

1. Electron transporting organic semiconductors for organic photovoltaics

-Dr. Tommaso Giovenzana , Dr. Andrew Higgs and Dr. Bogyu Lim

Currently some of the most efficient organic solar cells are based on bulk heterojunction (BHJ) devices that use phenyl-Cx-butyric acid methyl ester [PCBM, x=61 or 71] as the electron accepting material, shown in Figure 1. PCBMs are derivatives of fullerenes [C60 or C70] and are soluble in a variety of organic solvents. The fullerene core of PCBM readily accepts electrons from a wide range of organic donor materials and exhibits relatively high electron mobilities that helps to achieve power conversion efficiencies (PCE) of >8%. Nonetheless, it is a non-ideal material for photovoltaic applications due to its high cost, weak absorption in the visible spectrum, and excessively deep lying LUMO level, which results in lower open-circuit voltages.

Our group is interested in designing, synthesizing and characterizing alternative acceptor materials that exhibit favorable electron-transporting and processing properties, but that also absorb strongly in the solar spectrum. Of particular interest in this regard is a new class of small molecule acceptor materials based on 2-vinyl-4,5-dicyanoimidazole (Vinazene) as recently reported by our group. Vinazene derivatives, specifically EV-BT shown in Figure 1, are soluble in standard organic solvents, have good film-forming properties and exhibit high optical densities.

An added advantage is the simple high-yield chemical syntheses which, coupled with their inherent chemical flexibility, offers considerable scope for tuning the HOMO and LUMO levels and thereby optimizing photovoltaic performance. We are continuing to prepare and test new acceptor materials that have provided very promising results.

Figure 1: Structures of PCBM-C60 and EV-BT

2. Organic dyes and hole conductors for solid state dye sensitized solar cells (ssDSSCs)

Dr. Tommaso Giovenzana , Mr. William Nguyen, and Mr. Tomas Leijtens

Nanostructured dye sensitized solar cells (DSSCs) have shown promise as low cost alternatives to conventional solar cells. For example, top power conversion efficiencies (PCE) of over 11% have been reported for DSSCs using liquid electrolyte hole conducting solutions based on the iodide/tri-iodide redox couple. Although this efficiency is quite high and approaching what is required for commercial solar cells, problems with scaling this technology to high manufacturing volumes, primarily due to electrolyte leakage concerns, have led researchers to focus on ssDSSC that use solid state hole conductors. Current state-of-the-art ssDSSCs have PCEs of 6.5%.

From a materials perspective, the key components to ssDSSC are the dyes, hole conductors and titania.

Our group is working on new all-organic dyes for increasing the absorption of the solar spectrum into the near infrared (>700nm), increasing the dye extinction coefficient, building in features to reduce charge recombination, and new linker moieties for stronger bonding to the titania.

With regard to hole conductors, our group is designing materials paying close attention to properties such as:

  • Morphology: Crystallization is believed to prevent effective pore filling, reduce hole transport due to crystal boundaries, and reduce dye regeneration by reducing the intimate contact between the hole conductor and the dye. Therefore the material must be amorphous.
  • Solubility: Enhanced solubility is necessary to optimize infiltration into the titania from solution based coating processes.
  • HOMO level: Enhanced electron donating properties for HOMO levels -(4.8-5.2 eV) that may enhance electron transfer to the oxidized dye.
  • Polar properties: Increased polarity may provide better wetting and thus enhanced infiltration properties and contact to the dyes attached to the titania surface.
  • Size: A relatively small molecular size of <3 nm could help to provide better infiltration into the dye covered titania network.

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