Systems containing interfaces between dissimilar materials can exhibit lower thermal conductivity than their pure constituents, with important implications for thermal management and thermoelectric energy conversion. However, the heat transfer processes at such interfaces, in particular those between organic and inorganic materials, remain for the most part uncharacterized. We use vacuum thermal evaporation to grow archetypal multilayer thin films of copper phthalocyanine (CuPc) and Ag or Al, and measure their thermal conductivity as a function of interface density. We observe large thermal boundary resistance values (7.8×10−8 m2 K/W for CuPc/Ag and 2.0×10−8 m2 K/W for CuPc/Al), attributable to acoustic mismatch, heat carrier mismatch, and weak bonding.

We measure the room temperature thermal conductance of interfaces between an archetypal organic semiconductor copper phthalocyanine (CuPc) and several metals (aluminum, gold, magnesium, and silver) using the 3−ω method. The measured thermal boundary conductance (TBC) scales with bonding strength at the CuPc-metal interface, a correlation that is supported by molecular dynamics (MD) simulation, allowing the extrapolation of the effective interface Young's modulus. The trend in modeled interface modulus is in agreement with that deduced from adhesion tests, e.g., approximately 2 GPa for CuPc-gold and CuPc-silver interfaces, comparable to the van der Waals interaction strength of the materials. Using MD simulations in which the effects on thermal transport can be studied as a function of interfacial bond strength only, we isolate the relative contribution of acoustic mismatch and interface bond strength to TBC. Furthermore, measurements and modeling of organic/organic (e.g., CuPc/C60) interfaces reveal that the TBC of this system is not as sensitive to bonding strength as the CuPc/metal system, due to a larger overlap in the phonon density of states in the low frequency regime, despite the weak bonding between organic layers.

In organic photovoltaic (OPV) cells, photocurrent generation relies on exciton diffusion to the donor/acceptor heterojunction. Excitons that fail to reach the heterojunction are lost to recombination via quenching at the electrodes or relaxation in the bulk. Bulk recombination has been mitigated largely through the use of bulk heterojunctions, while quenching at the metal cathode has been previously circumvented through the introduction of exciton blocking layers that “reflect” excitons. Here, we investigate an alternative concept of a transparent exciton dissociation layer (EDL), a single layer that prevents exciton quenching at the electrode while also providing an additional interface for exciton dissociation. The additional heterojunction reduces the distance excitons must travel to dissociate, recovering the electricity-generating potential of excitons otherwise lost to heat. We model and experimentally demonstrate this concept in an archetypal subphthalocyanine/fullerene planar heterojunction OPV, generating an extra 66% of photocurrent in the donor layer (resulting in a 27% increase in short-circuit current density from 3.94 to 4.90 mA/cm2). Because the EDL relaxes the trade-off between exciton diffusion and optical absorption efficiencies in the active layers, it has broad implications for the design of OPV architectures and offers additional benefits over the previously demonstrated exciton blocking layer for photocurrent generation.

We have investigated the effects of N on the electronic properties of Si-doped GaAs1−xNx alloy films and AlGaAs∕GaAsN modulation-doped heterostructures. For bulk-like alloy films, the electron mobility is independent of free carrier concentration and arsenic species, and decreases with increasing N composition. Thus, N-related defects are the main source of scattering in the dilute nitride alloys. For AlGaAs∕GaAsN heterostructures, gated and illuminated magnetoresistance measurements reveal a two-dimensional electron gas mobility which increases with carrier concentration to a constant value. Thus, in contrast to the long-range ionized scattering sources which are dominant in N-free heterostructures, N-induced neutral scattering sources are the dominant source of scattering in AlGaAs∕GaAsN heterostructures. Finally, a decrease in free carrier concentration with increasing N composition is apparent for bulk-like films, while the free carrier concentration is independent of N composition in modulation-doped heterostructures. Since N and Si atoms are spatially separated in the modulation-doped heterostructures, N–Si defect complexes in the bulk GaAsN layers are likely acting as trapping centers.

We have used rapid thermal annealing to investigate the influence of N interstitials on the electronic properties of GaAsN alloys. Nuclear reaction analysis reveals an annealing-induced decrease in the interstitial N concentration, while the total N composition remains constant. Corresponding signatures for the reduced interstitial N concentration are apparent in Raman spectra. Following annealing, both the room-T carrier concentration, n, and the mobility increase. At higher measurement-Ts, a thermally activated increase in n suggests the presence of a trap near GaAsN conduction band edge with activation energy 85±15 meV. The annealing-induced increase in n suggests the association of the trap with interstitial N.

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Successful thermal management in nanostructured devices relies on control of interfacial thermal transport. Recent measurements have revealed poor thermal transport across interfaces between two dissimilar materials, e.g. organic semiconductors and metals. In such systems, the interfacial thermal conductance, G b , is dominated by the strength of interfacial bonding, but existing analytical models still fail to accurately predict G b , especially for organic-metal interfaces.Growing interest in this research area calls for comprehensive understandings of interfacial thermal transport across hybrid material interfaces. Here we demonstrate that spatial nonuniformity has to be assessed in the calculation of G b for interfaces with partial coverage or incommensurate growth that is characteristic of these interfaces. The interface between copper phthalocyanine (CuPc) and F.C.C metals (Ag, Al, Au) exhibits a six-fold difference between the metal's (~ 4 Å) and organic molecule's (~ 25 Å) lattice constant. Molecular dynamics simulations reveal the spatial variation of G b , and a model is developed that considers the spatial variations in phonon transmission, successfully predicting G b for many organic-metal interfaces.

Strain effect analysis on the thermoelectric figure of merit in n-type Si/Ge nanocomposites J. Appl. Phys. 111, 054318 (2012); 10.1063/1.3693307High efficiency semimetal/semiconductor nanocomposite thermoelectric materialsIn organic semiconductors, the Wiedemann-Franz law is often violated, potentially enabling independent control over electrical and thermal conductivities, as observed here with the organicmetal nanocomposites. This effect is attributed to the interface between metal particles and organic matrix materials impeding thermal transport. Thermal conductivity (k th ) can be decoupled from electrical conductivity (r e ) in the composite of an archetypal organic semiconductor (Copper Phthalocyanine, CuPc) and silver, with thermal boundary conductance as low as 13 MW/m 2 K at the interface. We show that k th decreases with volume fraction occupied by silver nanoparticles (x Ag %) in the dilute limit, reaching a minimum value at a concentration x Ag %ðminÞ ¼ 18%, while r e exceeds that of the pure organic semiconductor. Further modeling indicates that ZT values of organic-inorganic nanocomposites can be potentially enhanced 10 fold around x f %ðminÞ, compared to ZT of the pure compounds. These findings suggest a novel pathway for the future design of organic thermoelectric materials. V C 2013 AIP Publishing LLC. [http://dx.