David C. Coffey

Photovoltaic Charge Generation in Organic Semiconductors Based on Long-Range Energy Transfer

For efficient charge generation in organic solar cells, photogenerated excitons must migrate to a donor/acceptor interface where they can be dissociated. This migration is traditionally presumed to be based on diffusion through the absorber material. Herein we study an alternative migration route--two-step exciton dissociation--whereby the exciton jumps from the donor to acceptor before charge creation takes place. We study this process in a series of multilayer donor/barrier/acceptor samples, where either poly(3-hexylthiophene) (P3HT) or copper phthalocyanine (CuPc) is the donor, fullerene (C60) is the acceptor, and N,N-diphenyl-N,N-bis(3-methylphenyl)-[1,1-bisphenyl]-4,4-diamine (TPD) acts as a barrier to energy transfer. By varying the thickness of the barrier layer, we find that energy transfer from P3HT to C60 proceeds over large distances (∼50% probability of transfer across a 11 nm barrier), and that this process is consistent with long-range Förster resonance energy transfer (FRET). Finally, we demonstrate a fundamentally different architecture concept that utilizes the two-step mechanism to enhance performance in a series of P3HT/CuPc/C60 devices.

Charge Generation Measured for Fullerene−Helical Nanofilament Liquid Crystal Heterojunctions

The helical nanofilament (HNF) liquid crystal phase is an ordered architecture exhibiting interesting properties for charge transport. It is a small molecule self-assembly of stacked and twisted crystalline layers, which form alignable organic nanorods with half the surface area of the filaments consisting of aromatic sublayer edges. HNFs mixed with an electron acceptor generate an intriguing network for photoinduced electron transfer (PET). In this work, we characterize the structure of the HNF phase as processed into thin films with transmission electron microscopy (TEM) and X-ray diffraction (XRD). Additionally, we measure the flash-photolysis time-resolved microwave conductivity (TRMC) in samples where the HNF phase is fabricated into heterojunctions with the fullerenes C60 and PC60BM, prototypical electron acceptors for organic photovoltaics. Two distinct microstructures of the thin films were identified and compared for PET. A near-unity charge generation yield is observed in a bilayer of HNFs with C60. Moreover, the HNF phase is shown to be 10× better at charge generation than a lamellar structuring of the same components. Thus, the HNF phase is shown to be a good charge-generation interface.

Template Guided Structuration of Polymer Films

The ability to control polymer film structure has wide-ranging applications in the fields of optical coatings, electronics, organic photovoltaics, and biofilms. Structured polymer films can be fabricated using three broad strategies: direct deposition (such as with ink-jet printing), selective etching (such as with photolithography), and template directed structuring. Each strategy currently possesses advantages, disadvantages, and opportunities for improvement. As the methods to fabricate structured polymer films have improved, so has the associated scientific understanding of the rules governing these methods. The focus of this chapter, template guided structuring, has arisen in the past two decades to encompass several exciting techniques. In this chapter the methods and principles behind these discoveries are introduced.

Detecting free carriers in organic photovoltaic systems: Time-resolved microwave conductivity

So-called organic photovoltaic devices have seen certified power conversion efficiencies increase from 2.5% in 2001[1] to 8.3% in 2011[2]. Close inspection of the strategies employed to realize this impressive improvement in performance reveal a common approach of synthesizing new donor polymers and molecules, fullerene acceptors and, in some cases, new device architectures. It is questionable as to whether this approach will result in a similar four-fold level of improvement over the next ten years, and it is this question that motivates the work described here.

Transient Microwave Conductivity Studies of Poly (3-alkyl thiophene)s and Blends with PCBM

Using flash photolysis, transient microwave conductivity we report some preliminary results on two polythiophene derivatives and compare the results with the ubiquitous poly(3-hexylthiophene). The data provide an insight into the efficiency of exciton dissociation into free charge carriers; a result that is of importance to bulk heterojunction, photovoltaic solar cells that are a construct of a blend of polymers of this type with the soluble fullerene, [6,6]-phenyl-C61-butyric acid methyl ester, (PCBM).