July 20, 2011 -  Day 2 of the 3 day Organic Microelectronics & Optoelectronics Workshop VII spent the morning on PV technologies, returning in the afternoon to organic semiconductors. John Benner of NREL opened with an overview of PV economics and technology. Silicon PV is well on its way to $0.70/watt at the module level, but CdTe is still setting the pace for low cost, projected to be under $0.50/W shortly. Alta Devices has a 28.2% cell efficiency on a thin-film device now in pilot production. Multi-band cells are commercially available at >40% efficiency with the ability to fine tune to the local solar spectrum. CPV systems producing 60kW are operating in the field at the 27% system efficiency level, which is a significant milestone for commercialization and capitalization. Installation permit fees are a significant non-technical contributor to the expense of installing systems with any technology.

Jeff Peet of Konarka explained charge carrier recombination in bulk heterojunctions (BHJ) cells. Konarka's OPV technology is polymer cells cast from sol gel solutions. They've opened a new R2R production facility in New Bedford, MA in the old Polaroid plant. One target market is building-integrated semi-transparent OPV window coatings that can be colored and printed with designs. Many of the San Francisco bus stop kiosks have Konarka flex cells mounted on the red kiosk ripple top. Using semiconducting films thicker than 200nm typically leads to a loss of fill factor. A new material (internal label ZZ115) maintains a high fill factor at thicknesses of 200-400nm, which enables a broader flat band for EQE. The material has been shared with NIST and three other collaborators to characterize the material and understand why it works so well.

Decomposition of a high-energy singlet into two low energy triplets was explained by Priya Jadhav of MIT, and elucidated with several device examples. This singlet fission is an example of a multi-exciton generation process that will potentially allow OPV cells to exceed the Shockley-Quiesser limit. An applied magnetic field is found to increase the photocurrent by 5% in a DPT-C₆₀ device and decrease photocurrent in a Tetracene-C₆₀ device.

Organic Electronics Workshop 2011

Day 1: TFTs, FETs, and a seeing microphone

Day 2: Pushing organic PV performance

Day 3: OLEDs, OTFTs, OPV, and futile resistance

Moritz Riede of Technische Universität Dresden described a class of OPV structures based on the p-i-n concept, in which an intrinsic PV active layer is placed between the n-doped electron transport layer and the p-doped hole transport layer. In collaboration with IAPP, Heliatek, and BASF, they were able to demonstrate an efficient multi-frequency tandem device. Use of a bias light is important for measuring current in tandem cells; without it, all you can measure is the current of the limiting device in the pair. The closing slide declared Dresden to be the heart of Organic Electronics Valley -- I don't know what it sounds like in German, but in English this phrase does not roll off the lips very readily.

High performance tandem OPV cells were presented by Yang Yang of UCLA. Mitsubishi Chemical has an as-yet uncertified OPV efficiency record of 9.3%. A heavily doped PEDOT PSS layer is as an interconnecting layer (ICL) in a tandem device, giving an absorption range of 300nm to 850nm with an uncertified published 7.0% efficiency at 0.5 sun, but 7.8% at 0.1 sun. An inverted tandem structure with a MoO₃/Al/ZnO ICL has produced an uncertified 9.5% efficiency. Addition of 80nm Au particles to the ICL has been seen to enhance tandem cell efficiency an additional 15%-20% by plasmonic coupling.

The afternoon semiconductor session kicked off with Rachel Segalman of UC Berkeley speaking on controlled crystallization and self-assembly in conjugated copolymers. Several rod-coil systems were identified in which a reversible order-disorder transition occurred as a function of temperature, allowing calculation of the energy associated with such self-assembly. Induced orientation of some of these same systems can be achieved by cooling in an external magnetic or electric field. P3EHT (ethyl hexyl thiophene) exhibits a linear degree of crystallization with time, but a discontinuous jump in mobility about 30min into the crystallization cycle; work to understand this is underway.

Ingo Hörselmann of Ilmenau Technical University (Germany) simulated the cutoff frequency ƒ_T of OFETs. Shorter channel length and higher mobility each correlate to a higher ƒ_T. As expected, S/D overlap in turn contributes to a reduction in ƒ_T.

Stanford's Alberto Salleo described disorder and trapping in OFETs for the purpose of elucidating those microstructural properties that impede electron transport, using the polycrystalline disorder factor g to rank materials from crystalline to amorphous. PBTTT is often cited as a highly crystalline polymer semiconductor. This is true through one orientation, but not along the π-π stacking direction in which electron transport occurs. This disorder creates electronic traps, with g=7.3% producing ~100meV tail of traps.

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Michael A. Fury, Ph.D, is senior technology analyst at Techcet Group, LLC, P.O. Box 29, Del Mar, CA 92014; e-mail mfury@techcet.com.