Nanoscale energy transport in organic hybrid materials : combined experimental investigation and molecular dynamics simulation

Abstract

With the rapid development of organic semiconductors, which provides the optimized molecule packing and higher carrier mobility, thermal properties of organic semiconductors are playing significant roles in the performance and lifetime of organic electronics. In this thesis, I focus on investigating the nanoscale energy transport in organic hybrid materials by experiments and molecular dynamics (MD) simulations. The findings can provide useful information for thermal management of organic electronics and thermoelectric applications. Firstly, we employ silver nanoparticles (Ag NPs) to modify the thermal conductivity of the small molecule organic semiconductor, dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (DNTT). The thermal conductivity of the Ag-DNTT hybrid thin film measured by differential 3-ω method initially decreases and then increases when the Ag volume fraction increases from 0% to 32%. By applying the effective medium approximation to fit the experimental results of thermal conductivity, the extracted thermal boundary resistance (TBR) of the Ag-DNTT interface is 1.14±0.98×10-7 m2-K/W. Finite element simulations of thermal conductivity show good agreement with experimental results and effective medium approximations. Additionally, after the thermal annealing of DNTT, the grain size and in-plane crystallinity increase while the cross-plane crystallinity keeps relatively constant. We demonstrate the cross-plane thermal conductivity is independent of the thermal annealing temperature, and high annealing temperatures will reduce the space-charge-limited current (SCLC) mobility and increase the field effect mobility. Secondly, the thermal transport in DNTT crystal is further simulated by using non-equilibrium molecular dynamics (NEMD). We find that the thermal conductivities of DNTT have a strong dependence on the crystal size and orientation direction. The bulk thermal conductivities of DNTT along the a*, b* and c* directions are 0.73, 0.33 and 0.95 W/m-K, respectively. The TBR across different interfaces are calculated as 7.000.26, 6.150.13 and 3.200.09 10-9 m2-K/W for the a* - b*, a* - c* and b* - c* interfaces. Moreover, DNTT vacancy defects have the negative effect on the thermal conductivity of DNTT. Our results indicate that the phonon boundary and defect scatterings have crucial effects on the thermal conductivity of organic semiconductors. Thirdly, the interfacial thermal transport across graphene and DNTT is investigated by MD simulation. The average TBR of graphene and DNTT is 4.880.12 10-8 m2-K/W at 300 K. We find that TBR of graphene-DNTT heterostructure reduces by 83.4% after the hydrogenation of graphene and graphene vacancy can also benefit the interfacial thermal transport. The reduction of TBR mainly attributes to the coupling enhancement of the graphene and DNTT phonons as evaluated from the phonon density of states (DOS). Lastly, we apply NEMD to investigate the modification of the thermal boundary conductance (TBC) between gold (Au) and DNTT by self-assembled monolayers (SAMs, CnH2n+1S). We conclude that the SAMs can act as the phonon channel to bridge the phonon DOS of Au and DNTT to facilitate the interfacial thermal conductance between Au and DNTT. For the binary SAMs with different chain lengths at the interface, at the short SAM ratio of 80%-90%, the TBC of Au-binary SAMs-DNTT drops to the minimum point due to the phonon void scattering.