Zbyslaw R. Owczarczyk

Large n- and p-type thermoelectric power factors from doped semiconducting single-walled carbon nanotube thin films

Lightweight, robust, and flexible single-walled carbon nanotube (SWCNT) materials can be processed inexpensively using solution-based techniques, similar to other organic semiconductors. In contrast to many semiconducting polymers, semiconducting SWCNTs (s-SWCNTs) represent unique one-dimensional organic semiconductors with chemical and physical properties that facilitate equivalent transport of electrons and holes. These factors have driven increasing attention to employing s-SWCNTs for electronic and energy harvesting applications, including thermoelectric (TE) generators. Here we demonstrate a combination of ink chemistry, solid-state polymer removal, and charge-transfer doping strategies that enable unprecedented n-type and p-type TE power factors, in the range of 700 μW m−1 K−2 at 298 K for the same solution-processed highly enriched thin films containing 100% s-SWCNTs. We also demonstrate that the thermal conductivity appears to decrease with decreasing s-SWCNT diameter, leading to a peak material zT ≈ 0.12 for s-SWCNTs with diameters in the range of 1.0 nm. Our results indicate that the TE performance of s-SWCNT-only material systems is approaching that of traditional inorganic semiconductors, paving the way for these materials to be used as the primary components for efficient, all-organic TE generators.

Integrating theory, synthesis, spectroscopy and device efficiency to design and characterize donor materials for organic photovoltaics: a case study including 12 donors

The remarkable improvements in the power conversion efficiency of solution-processable Organic Photovoltaics (OPV) have largely been driven by the development of novel narrow bandgap copolymer donors comprising an electron-donating (D) and an electron-withdrawing (A) group within the repeat unit. Given the large pool of potential D and A units and the laborious processes of chemical synthesis and device optimization, progress on new high efficiency materials can, and has been, slow with a few new efficient copolymers reported every year despite the large number of groups pursuing these materials. In this paper we present an integrated approach toward new narrow bandgap copolymers that uses theory to guide the selection of materials to be synthesized based on their predicted energy levels, and time-resolved microwave conductivity (TRMC) to select the best-performing copolymer–fullerene bulk heterojunction to be incorporated into complete OPV devices. We validate our methodology by using a diverse group of 12 copolymers, including new and literature materials, to demonstrate good correlation between (a) theoretically determined energy levels of polymers and experimentally determined ionization energies and electron affinities and (b) photoconductance, measured by TRMC, and OPV device performance. The materials used here also allow us to explore whether further copolymer design rules need to be incorporated into our methodology for materials selection. For example, we explore the effect of the enthalpy change (ΔH) during exciton dissociation on the efficiency of free charge carrier generation and device efficiency and find that ΔH of −0.4 eV is sufficient for efficient charge generation.

Photobleaching dynamics in small molecule vs. polymer organic photovoltaic blends with 1,7-bis-trifluoromethylfullerene

Two organic photovoltaic (OPV) donor materials (one polymer and one small molecule) are synthesized from the same constituent building blocks, namely thiophene units, cyclopentathiophene dione (CTD), and cyclopentadithiophene (CPDT). Photobleaching dynamics of these donor materials are then studied under white light illumination in air with blends of PC70BM and the bis-trifluoromethylfullerene 1,7-C60(CF3)2. For both the polymer and small molecule blends, C60(CF3)2 stabilizes the initial rate of photobleaching by a factor of 15 relative to PC70BM. However, once the small molecule:C60(CF3)2 blend bleaches to ∼80% of its initial optical density, the rate of photobleaching dramatically accelerates, which is not observed in the analogous polymer blend. We probe that phenomenon using time-resolved photoluminescence (TRPL) to measure PL quenching efficiencies at defined intervals during the photobleaching experiments. The data indicates the small molecule donor and C60(CF3)2 acceptor significantly de-mix with time, after which the blend begins to bleach at approximately the same rate as the neat donor sample. The work suggests that perfluoroalkylfullerenes have great potential to stabilize certain OPV active layers toward photodegradation, provided their morphology is stable.

Improving photoconductance of fluorinated donors with fluorinated acceptors

This work investigates the influence of fluorination of both donor and acceptor materials on the generation of free charge carriers in small molecule donor/fullerene acceptor BHJ OPV active layers. A fluorinated and non-fluorinated small molecule analogue were synthesized and their optoelectronic properties characterized. The intrinsic photoconductance of blends of these small molecule donors was investigated using time-resolved microwave conductivity. Blends of the two donor molecules with a traditional non-fluorinated fullerene (PC70BM) as well as a fluorinated fullerene (C60(CF3)2-1) were investigated using 5% and 50% fullerene loading. We demonstrate for the first time that photoconductance in a 50:50 donor:acceptor BHJ blend using a fluorinated fullerene can actually be improved relative to a traditional non-fluorinated fullerene by fluorinating the donor molecule as well.

Triphenylamine-based star-shaped absorbers with tunable energy levels for organic photovoltaics

A new family of soluble low-bandgap star-shaped molecules based upon triphenylamine (TPA) and containing benzothiadiazole (BTD) acceptor moieties have been designed and synthesized for organic photovoltaic (OPV) applications. The design results in the lowest unoccupied molecular orbital (LUMO) being spatially distributed on the periphery of the molecule, allowing facile photo-induced electron transfer to the fullerene phenyl C61 butyric acid methyl ester (PCBM). Photoluminescence quenching studies indicate efficient quenching of excitons, while time-resolved microwave conductivity experiments demonstrate effective separation of charges. Adjunct electron-withdrawing moieties allow tuning of the LUMO level. Theoretical calculations indicate three derivatives with LUMO levels in varying proximity to that of PCBM, which allows empirical testing of the theorized need for a 0.3 eV LUMO offset to ensure efficient charge transfer to PCBM. Design, characterization and bulk heterojunction device results for the new materials will be presented.

Oligomeric dithienopyrrole-thienopyrroledione (DTP-TPD) donor-acceptor copolymer for organic photovoltaics

A new donor-acceptor copolymer system based upon a dithienopyrrole (DTP) donor moiety and a thienopyrrolodione (TPD) accepting moiety has been designed and synthesized for organic photovoltaic (OPV) applications. The TPD accepting moiety has recently gained significant attention in the OPV community and is being incorporated into a number of different polymer systems. In contrast, the DTP donor moiety has received only limited attention, likely due in part to synthetic difficulties relating to the monomer. In our hands, the bis(trimethyltin)-DTP monomer was indelibly contaminated with ∼5% of the mono-destannylated DTP, which limited the Stille polymerization with the dibromo-TPD monomer (>99% pure) to produce material with Mn ∼ 4130 g/mol (PDI = 1.10), corresponding to around eight repeat units. Despite this limitation, UV-visible absorption spectroscopy demonstrates strong absorption for this material with a band gap of ∼1.6 eV. Cyclic voltammetry indicates a highest occupied molecular orbital (HOMO) energy level of −5.3 eV, which is much lower than calculations predicted. Initial bulk heterojunction OPV devices fabricated with the fullerene acceptor phenyl C61 butyric acid methyl ester (PCBM) exhibit Voc ∼ 700 mV, which supports the deep HOMO value obtained from CV. These results suggest the promise of this copolymer system.